Tuesday, December 30, 2008

[MW:1431] RE: 1430] Re: PWHT Requirement for clad plate vessel

Hi Israr,

 

Regarding the heat treatment for lined vessel you can refer ASME SEC. VIII Div. 1 section UCL 34 and for examination of stainless lining refer UCL 36.

 

 


From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of Ali Asghari
Sent: December 30, 2008 1:54 PM
To: materials-welding@googlegroups.com
Subject: [MW:1430] Re: PWHT Requirement for clad plate vessel

 

Hi
please reply these question.
how much the thickness of lining?
what is the material of shell?

 


From: israr <ahmedisrar.nm@gmail.com>
To: Materials & Welding <materials-welding@googlegroups.com>
Sent: Thursday, December 25, 2008 1:44:00 PM
Subject: [MW:1422] PWHT Requirement for clad plate vessel


Dear Friend,
i have a vessel of 70 mm thick with internal lining of ss 316L plate.

I am not getting how to carryout heat treatment after welding of
internal lining,
I have suggested to use clad plate but there may chances to open clad
during heat treatment.
please help.

Regards
Israr Ahmed
Sr. Engineer QA/QC



This e-mail, including any attached files, may contain confidential and privileged information for the sole use of the intended recipient.  Any review, use, distribution, or disclosure by others is strictly prohibited.  If you are not the intended recipient (or authorized to receive information for the intended recipient), please contact the sender by reply e-mail and delete all copies of this message.


--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to materials-welding@googlegroups.com
To unsubscribe from this group, send email to materials-welding-unsubscribe@googlegroups.com
For more options, visit this group's bolg at http://materials-welding.blogspot.com/
The views expressed/exchnaged in this group are members personel views and meant for educational purposes only, Users must take their own decisions w.r.t. applicable code/standard/contract documents.
-~----------~----~----~----~------~----~------~--~---

[MW:1430] Re: PWHT Requirement for clad plate vessel

Hi
please reply these question.
how much the thickness of lining?
what is the material of shell?


From: israr <ahmedisrar.nm@gmail.com>
To: Materials & Welding <materials-welding@googlegroups.com>
Sent: Thursday, December 25, 2008 1:44:00 PM
Subject: [MW:1422] PWHT Requirement for clad plate vessel


Dear Friend,
i have a vessel of 70 mm thick with internal lining of ss 316L plate.

I am not getting how to carryout heat treatment after welding of
internal lining,
I have suggested to use clad plate but there may chances to open clad
during heat treatment.
please help.

Regards
Israr Ahmed
Sr. Engineer QA/QC


--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to materials-welding@googlegroups.com
To unsubscribe from this group, send email to materials-welding-unsubscribe@googlegroups.com
For more options, visit this group's bolg at http://materials-welding.blogspot.com/
The views expressed/exchnaged in this group are members personel views and meant for educational purposes only, Users must take their own decisions w.r.t. applicable code/standard/contract documents.
-~----------~----~----~----~------~----~------~--~---

Saturday, December 27, 2008

[MW:1429] Re: ASME B31.3-2006 Ed. Fig. 328.5.2.c

Gap should be given before fit up so that during welding there is shrinkage or expansion of the pipe and also RT should be performed in order to check wether there are any defects or cracks caused during  welding on the weld.If it is a pressure part it is compulsory to perform RT.
 
 
 
with regards
Durrga Prasad


From: tong tong <tong2je@gmail.com>
To: materials-welding@googlegroups.com
Sent: Friday, December 26, 2008 7:05:31 PM
Subject: [MW:1428] Re: ASME B31.3-2006 Ed. Fig. 328.5.2.c

Purpose of the gap is to allow the expansion of piping and fitting during welding. If there are no gap between piping and fitting may be can cause distortion. For my opinion, RT is not suitable for fillet weld but may be your client just want to ensure whether the gap is there or not.

Regards,
SBS

On Thu, Dec 25, 2008 at 5:34 PM, Jivan Dhamane <jivanadhamane@yahoo.co.in> wrote:
Dear Friends
 
As per ASME 3.31.3 gap is given at the time of fit-up and offcurse after weding that gap will be less than 1.5 mm because of  shrinkage due to welding heat. the meaning of RT is nothing but radiography test and i think your client might have asked you to perfom Rt to check whether 1.5mm gap has been provided or not
not rejected.

Regards,
 
Jivan Dhamane


--- On Wed, 24/12/08, T. Rizal H. <aatkho@gmail.com> wrote:
From: T. Rizal H. <aatkho@gmail.com>
Subject: [MW:1419] Re: ASME B31.3-2006 Ed. Fig. 328.5.2.c
To: materials-welding@googlegroups.com
Date: Wednesday, 24 December, 2008, 11:35 PM


1.5 mm gap is set before welding, it is to accomodate shrinkage after welding. Of course, gap will be less than 1.5 after welding, but it is not rejected.  What's the meaning after RT?, it is strange to perform RT on the fillet weld.  Regards,  Tex  2008/12/24  <nozhan@cimtas.com.tr>: > > > (Embedded image moved to file: pic14310.jpg) > After welding, less than 1.5 mm gap is the cause to reject the weld after > RT? > If yes, in accordance which criteria? > > Thanks... > > > > Nebi ÖZHAN > > e-mail:nozhan@cimtas.com.tr > > >    --  Regards,  T. Rizal Hidayat   


Unlimited freedom, unlimited storage. Get it now



--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to materials-welding@googlegroups.com
To unsubscribe from this group, send email to materials-welding-unsubscribe@googlegroups.com
For more options, visit this group's bolg at http://materials-welding.blogspot.com/
The views expressed/exchnaged in this group are members personel views and meant for educational purposes only, Users must take their own decisions w.r.t. applicable code/standard/contract documents.
-~----------~----~----~----~------~----~------~--~---

Friday, December 26, 2008

[MW:1428] Re: ASME B31.3-2006 Ed. Fig. 328.5.2.c

Purpose of the gap is to allow the expansion of piping and fitting during welding. If there are no gap between piping and fitting may be can cause distortion. For my opinion, RT is not suitable for fillet weld but may be your client just want to ensure whether the gap is there or not.

Regards,
SBS

On Thu, Dec 25, 2008 at 5:34 PM, Jivan Dhamane <jivanadhamane@yahoo.co.in> wrote:
Dear Friends
 
As per ASME 3.31.3 gap is given at the time of fit-up and offcurse after weding that gap will be less than 1.5 mm because of  shrinkage due to welding heat. the meaning of RT is nothing but radiography test and i think your client might have asked you to perfom Rt to check whether 1.5mm gap has been provided or not
not rejected.

Regards,
 
Jivan Dhamane


--- On Wed, 24/12/08, T. Rizal H. <aatkho@gmail.com> wrote:
From: T. Rizal H. <aatkho@gmail.com>
Subject: [MW:1419] Re: ASME B31.3-2006 Ed. Fig. 328.5.2.c
To: materials-welding@googlegroups.com
Date: Wednesday, 24 December, 2008, 11:35 PM


1.5 mm gap is set before welding, it is to accomodate shrinkage after welding. Of course, gap will be less than 1.5 after welding, but it is not rejected.  What's the meaning after RT?, it is strange to perform RT on the fillet weld.  Regards,  Tex  2008/12/24  <nozhan@cimtas.com.tr>: > > > (Embedded image moved to file: pic14310.jpg) > After welding, less than 1.5 mm gap is the cause to reject the weld after > RT? > If yes, in accordance which criteria? > > Thanks... > > > > Nebi ÖZHAN > > e-mail:nozhan@cimtas.com.tr > > >    --  Regards,  T. Rizal Hidayat   


Unlimited freedom, unlimited storage. Get it now


--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to materials-welding@googlegroups.com
To unsubscribe from this group, send email to materials-welding-unsubscribe@googlegroups.com
For more options, visit this group's bolg at http://materials-welding.blogspot.com/
The views expressed/exchnaged in this group are members personel views and meant for educational purposes only, Users must take their own decisions w.r.t. applicable code/standard/contract documents.
-~----------~----~----~----~------~----~------~--~---

[MW:1426] NDT


Dear All,
 
                I have tank with shell course thickness of 23.4mm.Shell course above and below this is greater than 10mm.As per API 650 sect 6.1.2.2 if i cover 100% T joints RT with 10x24 film is it required to take spot RT as per section 6.1.2.2 a.Or this 100% T oints will cover the rquirements.
 
waiting for u r reply
 
regds
 Nilesh Parsekar,
 QC CO-ORDINATOR
(Dp Mech,API 510,API 570,AWS-CWI,NDT LEVEL2)
 Belleli Energy
 QATAR


--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to materials-welding@googlegroups.com
To unsubscribe from this group, send email to materials-welding-unsubscribe@googlegroups.com
For more options, visit this group's bolg at http://materials-welding.blogspot.com/
The views expressed/exchnaged in this group are members personel views and meant for educational purposes only, Users must take their own decisions w.r.t. applicable code/standard/contract documents.
-~----------~----~----~----~------~----~------~--~---

[MW:1425] Inspecting inaccessible piping -LRUT

Source: HP Nov2008

--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to materials-welding@googlegroups.com
To unsubscribe from this group, send email to materials-welding-unsubscribe@googlegroups.com
For more options, visit this group's bolg at http://materials-welding.blogspot.com/
The views expressed/exchnaged in this group are members personel views and meant for educational purposes only, Users must take their own decisions w.r.t. applicable code/standard/contract documents.
-~----------~----~----~----~------~----~------~--~---

Heat Treatment - What Is It?

Heat Treatment - What Is It?
       

J.G. Gillissie

October 1981   

Category: Design/Fabrication 

Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (4 printed pages)

A short time ago during a joint review of an ASME Certificate Holder, I found myself asking the question, "Do you use heat treatment?"

The immediate answer was, "Oh yes."

I have asked the same question many hundreds of times in a like number of fabricators' shops, knowing full well that my question was all-inclusive and covered a number of processes. Ninety-five times out of any hundred the answer I got was a straight "yes" or "no." Once in a blue moon the company representative would explain that he uses only stress relieving of weldments for those material P-numbers and material thicknesses as required by the Code sections to which he is fabricating.

When I get an answer like that I think to myself, "This gent knows what he's talking about." At the same time, I have a deep suspicion that HE is thinking to HIMSELF, "This clown probably doesn't know the difference between stress relieving and stealing third base."

Generalized questions usually get generalized answers. As an example, if somebody asks me the question, "Do you travel?", my answer would probably be, "Yes." If I were asked, "How do you travel?", my answer would possibly be, "By airplane, train and automobile but not by bicycle, pogostick or horseback."

Get the point? There is a difference. There is nothing wrong with the term, "heat treatment," but it is a generalized term covering various processes. Heat treatment in any of its forms is used to achieve a desirable improvement in the characteristics of material or to regain those characteristics which may have been adversely affected by production processes such as welding/bending/forming etc.

Let's take a short look at some of the most frequently used processes of heat treatment, those which the Authorized Inspector may encounter in boiler and pressure vessel fabrication shops.

STRESS RELIEVING (postweld heat treatment)

This is by far the most frequently used form of heat treatment which will confront the authorized inspector. As a result of welding processes used to join metals together, the base materials near the weldment, the deposited weld metal and, in particular, the heat affected zones transform through various metallurgical phases. Depending upon the chemistry of the metals in these areas, hardening occurs in various degrees, dependent mainly upon carbon content. Again, this is particularly true in the heat affected zone (HAZ) adjacent to the weld metal deposit where the highest stresses due to melting and solidification result. Stress relieving, as the name implies, is designed to relieve a proportion of these imposed stresses by reducing the hardness and increasing ductility, thus reducing danger of cracking in the vessel weldments.

The Code sections contain requirements for stress relieving, specifying rate of heating and cooling above 800oF and requiring a holding temperature, usually one hour per inch of thickness of the material. The holding temperatures vary with the P-numbers of the material which in turn are based on alloy content. As an example, P-1 through P-4 require 1100-F holding temperature, P-1 being carbon steels, P-3 being carbon steels alloyed in relatively small percent with molybdemum, manganese and vanadium. P-4 steels are the nickel steels, chrome-molys and nickel- chrome-molys. P-5, P-6 and P-7 high alloy steels generally require a higher holding temperature ranging up to 1350oF. Some of the special steels now listed in the Code sections call for even higher temperatures.

Following the holding (soaking) time, controlled cooling down to 800oF or lower is vitally important. Many high carbon steels are subject to surface cracking if cooled too rapidly.

QUENCHING AND TEMPERING

Oriented toward carbide steels such as carbon-moly, this process is designed to enhance toughness as well as controlling yield strength and ultimate tensile strength of steel. The steel is heated to above its upper critical temperature and quickly immersed in fresh water or brine to achieve rapid setting of the desired metallurgical structure. Oil quenching is sometimes used. The usual practice is to quench until cooling reaches around 800oF, quickly followed by a tempering period in a fired furnace in order to soften the martensitic structure and achieve the desired mechanical properties in the material including a desired measure of ductility. The tempering process is, in effort, a stress relieving process.

NORMALIZING AND TEMPERING

This process is used for virtually the same purposes as quenching and tempering. It differs in that normalizing is accomplished by cooling in air in place of fast quenching in a liquid. Air normalizing, much slower than liquid quenching, may be used by itself or the material may be subjected to a controlled furnace tempering process in order to better control desired mechanical properties.

Steel manufacturers will furnish material in either of the above conditions when so specified on the purchase order or as required by the material specification.

As a cautionary note; alloyed steel mechanical properties are ultimately determined by the tempering process and if the materials are subsequently welded during fabrication, subsequent stress relieving temperature, if used, should not exceed that of the tempering process, otherwise mechanical properties of the material may be adversely affected.

SOLUTION HEAT TREATMENT (solution annealing)

While the Code sections state that heat treatment of austenitic stainless steel (P-8) is neither required nor prohibited, this refers to postweld stress relieving. There are certain processes to which this material may be subjected. These are performed almost exclusively by the material manufacturers due to the fact that temperature ranges and holding time are critical and require careful controls, otherwise damage to the material can result from either too high or too low a furnace temperature. Material manufacturers have the metallurgical staffs to determine requirements.

In solution heat treatment the material is subjected to a high heat, around 2000oF, and rapidly cooled in liquid in order to achieve an evenly distributed solution of carbon and austenite in the metallurgical structure of the material.

STABILIZING HEAT TREATMENT

Everything said in the first paragraph under solution heat treatment also applies to stabilizing heat treatment. In the latter process the material is cooled slowly in order to bring as much carbon as possible out of solution and into evenly distributed concentrations apart from the austenite.

Both solution heat treatment and stabilizing heat treatment are used to reduce susceptibility to intergranular stress corrosion and embrittlement also to increase high temperature creep strength.

PREHEATING

While most of us do not look upon preheating as a form of heat treatment, its use can contribute substantially in reducing hardness in all three constituents of a weldment; the parent metal, the weld metal deposit and the heat affected zone. As a weldment cools, it goes through various transformations in which molecules rearrange themselves. If cooling is rapid, this rearrangement is arrested resulting in entrapment of stresses and hardening of the material with coincident loss of ductility which is the highly desirable ability of the material to bend elastically, under stress.

Preheating of the weldment area achieves better weld penetration and slows the cooling process, thus allowing added relief of stresses and reduced hardening of the materials.

The ASME Code sections take cognizance of the foregoing, in some cases allowing exemption from postweld stress relieving PROVIDED preheating of a specified temperature is used.

Here again, a word of caution is in order. Preheat, like any other heat treatment, must be carefully planned and used. Specific written procedures should be provided for each individual use. Misuse, such as light surface heating, can do more harm than good. A soaking heat and maintenance of interpass temperature throughout the weldment - and beyond, are recommended.

In all cases, high chrome-moly steels should be preheated prior to welding and postweld stress relieved at around 1400oF.

In summary, the authorized inspector (or ANI) is not assigned the duty of being an authority on metallurgy of all the various ferrous and nonferrous materials used in boiler, pressure vessel or piping system fabrication. The various Code sections do, however, require that results of heat treatment be made available to him for his review in order that he may assure himself that temperature readings and holding (soaking) time conform with Code requirements. Only a diligent study of Code requirements will enable him so make this decision.

As previously mentioned, heat treatments which will confront the AI-ANI are for the most part preheating and postweld heat treatment, that is, stress relieving.

Some points to remember:

Post weld heat treatment is designed to return a metal as near as possible to its prefabrication state of yield, ultimate tensile and ductility.

The rate of temperature rise, holding time at temperature and rate of cooling are vitally important. For this reason, furnace thermocouples must measure metal temperature, not furnace atmospheric temperature.

Heat treatment of any type must be a planned, systematic action. Poorly performed heat treatment can result in far more harm to material than any good which may result.

Test coupons must be subjected to the identical conditions as the vessel or part in order to obtain meaningful tensile and toughness (Charpy) test results.

The foregoing is a short generalization. Specific requirements are found in ASME Section II "Material Specifications" and in the "Material Tables", of the various Code sections.

Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.   

Low Voltage Short Circuiting-GMAW

Low Voltage Short Circuiting-GMAW
       

M.J. Houle

January 1985  

Category: Design/Fabrication 

Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (3 pages)

The National Board has been frequently asked to give some guidance to gas metal arc welding (GMAW or commonly called MIG) when welding in the low voltage short circuiting (GMAW-S) mode.

GMAW-S is normally a solid wire (ASME SFA-5.18) gas shielded welding type process which uses semiautomatic or automatic equipment. It is similar to the spray arc or globular arc transfer GMAW processes, the flux cored arc welding (FCAW) process, and is closely related to the submerged arc welding (SAW) process. Although, the GMAW-S has unique features.

All of the related processes are very high energy processes which transfer weld metal across a continuous electric arc and generally carry a large molten weld puddle. GMAW-S is a low energy process which also generates its heat from an electric arc but the weld metal is transferred only partially across an arc and partially when the filler metal touches the base metal and the arc is short circuited. This short circuiting occurs 20 to 200 times per second which results in a small molten weld puddle.

When the arc is short circuited, the molten weld puddle is able to freeze more quickly than when welding with a continuous arc. This gives GMAW-S a unique ability to weld out of position, to weld thin base metals and to weld open butt root passes without backing and without "blowing through." One bad characteristic, however, is that the quick freeze puddle has a tendency to "cold lap" when not carefully deposited with the correct technique by a skilled welder. This bad characteristic is often why the process is written out of purchase specifications.

Because of the same bad characteristic, ASME Section IX applied special variables when the GMAW-S process is used. When qualifying a welding procedure, the variable QW-403.10 limits the base metal thickness qualified to a maximum of 1.1 times the test coupon thickness for procedure qualification test coupons less than l/2 inch thick. The variable QW-404.32 also limits the deposited weld metal thickness range to a maximum of 1.1 times the deposited thickness of the procedure qualification test coupon for deposits less than 1/2 inch thick.

The welding technique employed when welding the GMAW-S process is also unique. There are techniques for groove root pass and for fillet welding that will produce soundly deposited welds. But the technique is so different from the spray, globular or pulsed modes of transfer, that Section IX applies a special variable for the GMAW-S process for welder performance qualification. When qualifying a welder, the variable QW-409.2 becomes an essential variable which requires requalification of the welder if he changes from spray arc, globular arc or pulsating arc to the low voltage short circuiting arc, or vice versa. The variable QW-404.32 also limits the deposited weld metal thickness for performance qualification as noted above.

The quick freeze puddle which has a tendency to cause cold lap also affects the performance qualification testing requirements as outlined in Section IX, QW-304. This variable alternatively allows welders to be qualified by radiographic examination in lieu of the mechanical tests prescribed in QW-302 for some few processes. A welder may be qualified by radiographic examination when welding with most modes of the GMAW process, but when qualifying with the GMAW process using the short circuiting arc mode, performance qualification by radiography is not permitted. The reason for this is that cold laps are such tight defects that they are very difficult to detect by radiography.

The ASME Code has essential variables directly related to the GMAW-S mode of welding, but has no clear definition of "What is GMAW-S?" Industry can easily determine when they are in the GMAW-S mode by checking the electrical arc characteristics on an oscilloscope. This instrument will clearly indicate when the arc is short circuiting. But the average Authorized Inspector and small shop does not have an oscilloscope, so some characteristics for which to look are presented in the following paragraphs.

The GMAW process can change from the various arc modes to the short circuiting mode and vice versa by a change in amperage, voltage, shielding gas, filler wire diameter or any combination of these factors. The GMAW-S process is usually characterized by fine wire, that is, the 0.030 inch through 0.045 inch diameter filler metal. But GMAW-S has been used in production on larger diameter filler metals. The process is commonly used for welding of carbon, low alloy and stainless steels, but a wide variety of other metals can be welded using GMAW-S. The GMAW-S process is usually shielded with carbon/dioxide (CO2 ) or a mixture of argon and carbon/dioxide. Unfortunately, globular arc and spray arc may also use similar shielding gas. The high percentage argon, argon-oxygen, helium and their mixtures are used with the spray arc mode and not the short circuiting arc mode.

The voltage range used is generally the best guide to GMAW-S. A low voltage is one of the reasons that GMAW-S does short circuit. If the shielding gas is CO2 or an argon CO2 mixture, the filler metal diameter is 1/16 inch diameter or less and the voltage is 22 volts or less, the GMAW process is most likely in the short circuiting mode. The short circuiting mode is actually one wherein the short circuiting mode and globular mode are both occurring at random. As the voltage increases from 17 volts to 22 volts the arc short circuits less frequently, and the arc increases its globular transfer. Voltages above 25 usually indicate a true globular arc or spray arc mode.

Some characteristics to look for are the ability of the GMAW-S process to weld "out-of-position" and to weld open root groove welding without the use of backing. The pulsed arc mode is easily distinguished from GMAW-S. The spray arc or globular arc generally are not used out-of-position and are not used for open root groove welding without the use of backing.

The process was originated as a sheet metal joining process which did not require skilled welders to operate. This theory was not true. A well-trained skilled welder is required to properly deposit sound weld metal with the GMAW-S process. The use of the process has expanded to plate applications 1/2: inch thick and greater in both groove and fillet weld applications. It is used to handle out-of-position welding, root pass welding without backing, root pass welding to keep the inside clean and flux free, and for clean smooth tack welding.

In the final analysis, it is the ASME Code user who is responsible for determining the mode of arc transfer being applied to Code welds. The Code user must apply the Code variables and restrictions when using the GMAW-S process and must document the variables, including the mode of arc transfer on the WPS, PQR and WPQ forms.

This explanation of the ASME Code variables is the opinion of the National Board and not an official ASME Code definition. The National Board does, however, encourage those with expertise in this area to work with the appropriate ASME Code committee to help clarify and define this process.

Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.   

Failure Avoidance in Welded Fabrication


Failure Avoidance in Welded Fabrication
       

Alan W. Pense
Professor, Associate Dean, College of Engineering and Applied Science
Lehigh University, Bethlehem, PA

Category: Design/Fabrication 

Summary: The following article is a part of the National Board Technical Series. This article was originally published in the October 1988 National Board BULLETIN. (9 printed pages)

Failure avoidance is the responsibility of designers, materials engineers, fabricators, inspectors, and owners all working together.

Abstract

In spite of well developed code and other failure avoidance procedures, failures of pressure vessels and other welded components sometimes occur. Experience with failure analysis shows that many problems could be avoided by better communication between engineers of different disciplines early in the production process. With a vessel failed in hydrostatic testing as an example, the interaction between engineers that might have prevented the failure are explored and the benefits of improved interaction stressed.

Introduction

In most welded structures, a variety of controls are in existence that provide a high level of failure avoidance. For example, codes for design and fabrication of structures, inspection and inspector qualification requirements, materials specifications, and owner operational instructions all contribute in a significant way to the control of failure. Behind these codes and requirements are the combined experience of all of the engineering groups involved in their creation and periodic revision, without which they could not exist. Thus, failure avoidance has been an ongoing activity in the engineering community for a long time.

However, welded structures do fail on occasion, and pressure vessels are no exception to this fact. When this happens, there may be serious consequences, almost always in terms of financial loss, and sometimes in terms of loss of human life. Welded structures have special characteristics, some of which tend to make them more susceptible to propagating, and thus more catastrophic fractures.

For example, they are more monolithic, and discontinuities from one part of the structure can propagate by a number of mechanisms into other parts of the structure without encountering interrupting open interfaces. Another characteristic of many welded structures is residual stresses, and the effects are difficult to assess. In pressure vessels, their effects may be diminished, but not eliminated, by postweld heat treatment. Yet another consideration is the fact that welded structures contain joints that are metallurgical composite and their behavior is complex for this reason.

An important result of all of these factors, and one especially so for failure avoidance, is that there are many interactions between parts of the structure considered more critical and those considered less so in an engineering sense. While this is true to some degree of all welded structures, it is especially true of pressure vessels. There are, in fact, virtually no inconsequential parts of a pressure vessel from the failure avoidance viewpoint.

Experience with failure analysis also demonstrates 1,2,3 that some of the problems leading to failure are common to many failed structures, and thus, with some knowledge and foresight, should be avoidable. Moreover, although the author's background is with weld related problems, these failures have not necessarily been due to the execution of the welds, but many different causes.

Using as an illustration the failure of a pressure vessel, which occurred during hydrostatic testing, this paper is intended to reinforce the fact that failure avoidance is a chain of interrelated responsibilities. It is not the job of one link in the chain, but all the links working together. As failures go, this was a "good" one, occurring during the last of the many checks and balances in the system. There were no injuries. It is an instance in which, in spite of some lapses, the failure avoidance system ultimately worked. However, it can also serve as a warning that omissions and communication failures can occur, even when engineers think they are doing a good job, and the results can have serious consequences.

Engineers Working Together

Good welded fabrication is ultimately a team effort involving the designer, the materials engineer, the fabricator, the inspector and the owner. These individuals may not work for the same company, and thus may not feel the corporate responsibility that they should. In fact, the current legal system may discourage this. But even if they are employed by one company, this is no assurance that good team action will result. A fabrication engineer working for a large company once told me that sometimes he felt that the design drawings for welded fabrication were "thrown over the wall in the dark of night" from the company design group. While this may be an exaggeration, it was clear that he felt their input was not solicited as it should have been.

Indeed, if various engineers don't work together, the result is often less than, not more than, the sum of the whole. It is the author's experience that designers sometimes design structures which cannot easily be built using materials that are incompletely specified. To compound the problem, the fabricator attempts from these designs to build these structures but in such a way that they cannot really be inspected. Finally, "inspection" completed, they are turned over to an owner who then operates them in ways that the designer did not anticipate, the materials cannot tolerate, and for which the fabrication or inspection was not appropriate. This will not result in failure avoidance.

Figure 1 Many engineers think of the design, materials selection, fabrication and inspection of a product, such as a pressure vessel, as occurring according to a chain such as that shown in fig. 1. This approach separates engineering functions neatly but is not the optimum for failure avoidance. The appropriate model is closer to that shown in fig. 2. This model has all engineers working together in an interactive systems approach to the product. It may be an idealization that cannot be fully realized, but it will inherently result in superior failure avoidance because it results in more informed design, more rational materials selection, more efficient fabrication, and more effective inspection.

Figure 2 Not only will this result in greater safety but should result in lower overall cost. For example, inefficient design must somehow be compensated with stronger or tougher materials, or lower overall stresses and therefore larger parts requiring more welding and inspection. Similarly, ineffectiveness in each of the other engineering disciplines must be compensated by the rest. Thus good failure avoidance procedures are nothing more than good engineering procedures and have many benefits.

The key component left out of fig. 2 is the owner, who must somehow be aware of these interactions, if not contribute to them, so that the operation of the product will be in accord with his and the designer's engineering expectations.

Design

The best approach to failure avoidance is good design to start with, including both design of the structure and of the welded joints. While many aspects of design will be governed by code requirements or company practice, the designer still exerts a significant influence on stress concentrations, particularly those close to and around welds. This is especially important in high cycle live loaded structures where the stress concentrations at weld toes, in association with defects inherent to those locations, may control structural performance. It is also important in pressure vessels from the brittle fracture viewpoint, and because creep and low cycle fatigue requirements are often involved. While the overall stress raising effects of typical welds are well known, design incorporating welds of inappropriate type or at inappropriate locations still occur.

Again, there are also occasional examples of welded details that are poorly designed from the fabrication viewpoint and that subsequently fail because they were difficult or even impossible to execute. A critical examination of these details by the fabricator, or the designer in consultation with the fabricator, would have revealed that they could not be executed as intended. Thus, a problem could have been predicted in advance and avoided. Most failures in structures originate at connections, and this is in spite of the fact that more time and effort is spent on their design, materials, fabrication, and inspection than most other parts of the structure. More time in this area is still time well spent, however.

Materials Selection

Materials selection may be considered a special subdivision of design because materials are often specified by designers and sometimes owners. However, it is a field that requires specialized training and thus is usually considered an independent engineering function. Regardless of who prepares the materials specification, one of the most common problems is failure to account for all the properties needed. For example, strength properties (which usually incidentally includes tensile ductility) are normally specified. On the other hand, toughness properties are seldom specified. Fabrication related properties, such as weldability or ductility for mechanical forming, are almost never specified.

In general, engineers have relied on materials to be "forgiving" in terms of ductility, toughness or weldability. That is, they have relied on their experience with respect to these unspecified but important properties for the materials they are accustomed to using rather than specifying them (which will undoubtedly increase their cost). For example, they may rely on indirect provisions in ASTM (American Society for Testing Materials) steel specifications, such as a steel being made to fine grain practice, as an indication of toughness. But unspecified properties critical to a given application are probably unknown to the materials supplier, and may not be provided, with a failure being the result. It is the author's experience that this is one of the more common causes of failures. The materials specifier must develop an awareness of all the properties required for a given application and identify them accordingly.

Fabrication

Good fabrication generally means attention to detail, and it, too, is essential for failure avoidance. As a minimum, this requires understanding of and adherence to the relevant codes. They represent the corporate experience of many segments of the industry and should not be taken lightly. Code requirements for welding documentation and control such as preheat, postheat, care of consumables, and operator skill levels need to be observed. The author has sometimes found codes and specifications treated as if they were desirable goals and not minimum performance standards. From the failure avoidance viewpoint, they are the starting point, not the end point, of control.

Effective fabrication also requires knowledge beyond what is in the relevant codes. There are scores of fabrication procedures, techniques, and practices which are not incorporated in codes but which are essential to effective fabrication. These must also be part of the fabricator's operating experience if failures are to be avoided. More than one failure can be traced to the use by a fabricator of a material or technique with which he was not really familiar and for which he did not get the needed background.

One of the special problems in fabrication is repair welding or, in an extreme case, retrofitting, of a finished product. Repair welding of a structure, such as a pressure vessel, is usually more difficult than initial welding, and requires more, rather than less, care. The vessel is more highly constrained, is more difficult to access, is harder to preheat and postheat than before, and sometimes is also harder to inspect. All this puts a premium on doing things right the first time and on special control of quality when repairs are to be done. The most difficult of repairs are field modifications. These have all of the problems indicated above and also the problem of the control of welding and inspection in the field.

Inspection

Fabrication is seldom perfect, and thus good inspection is also required for failure avoidance. Welds need to be planned so that they can be inspected, and then an inspection procedure established which accomplishes this to an adequate degree. Quality cannot be inspected in the attempt to make an inferior product a superior one by upgrading it through inspection. It is a risk at best and an expensive one as well. Appropriate inspection can verify the quality of a product and catch the occasional discontinuities that may occur. The current trend in codes is toward inspection requirements which are appropriate for service. This is a reasonable approach, and then there need not be any compromises on the quality expected or required.

Operation

The final step in failure avoidance is control of operation. No amount of careful design, good fabrication, and expert inspection will compensate for the failure to establish and control operating limits on equipment. Designs are usually conservative, but there are boundaries over which operations cannot go without major consequences. Overstressed pressure vessels will fracture, overloaded bolts fail and unprotected metals corrode. Operating equipment needs to be maintained and periodically inspected. Operators of equipment share with designers, fabricators and inspectors the responsibility for failure avoidance.

One recurring problem related to operation is owner modification which makes a structure such as a pressure containing component more failure prone. A number of causes may be identified. The first is modification which increases capacity in such a way that parts of the vessel are now overstressed. A second is modification which introduces stress concentrations through carelessly considered design changes. A third is poor quality workmanship which introduces discontinuities in the structure. Failure avoidance means that the same rules for repair mentioned above be applied for owner modifications.

Case Study: Vessel Failure In Hydrostatic Testing

Most of the above components are brought together in the case study described here. It occurred some years ago and some details are omitted. It is, however, not unique, as anyone who has worked on the analysis of failed pressure vessels and other structures can testify. There may be a tendency to think that the problems described "can't happen here." It is the author's opinion that they happen more often than engineers would like to think but do not come together in such a way as to cause failure. In this our system serves us well but should not be relied on to save us in all instances.

Design and Materials Selection

Although not pictured, a pressure vessel failed by a fast fracture that not only ran down a good portion of the length of the vessel but also severed it in half. The vessel was made of 20.6 mm (.81 in) thick C-Mn steel and was to be used for static liquefied gas storage. The length was about 28 m (92 ft) long and the diameter about 3.3 m (11 ft). It was designed according to Section VIII, Division I, of the (American Society of Mechanical Engineers)ASME Boiler and Pressure Vessel Code and had a capacity of 254 m3 (60,000 gal). The maximum design stress in the membrane was 140 MPa (20.25 ksi) which was to be one quarter of the steel tensile strength. The vessel had the two small set-on nozzles fillet welded into the shell.

The shell courses were submerged arc welded with the nozzles being manually welded. Low hydrogen consumables were used. The main seam welds were inspected by radiography, but the nozzle welds were inspected by surface techniques. No preheat was used unless the ambient temperature was below 10oC (50oF), and then a preheat of 27oC (80o F) was applied. No postweld heat treatment was used.

The material used had a yield strength requirement of 345 MPa (50 ksi) and a tensile strength requirement of 558-696 MPa (81-101 ksi). This was somewhat higher in strength than the fabricator was historically used to, but had adopted the steel for this application because it permitted fabrication of thinner, more economical vessels. There was no toughness specification for the material. Postweld heat treatment had been used in the past for vessels of this type but, in the current application, purchasers did not require it, so it was discontinued for economic reasons. A number of these vessels had been built in recent years without any problems prior to this failure. On the surface there was nothing to distinguish this vessel from the others.

Material Variability

A close examination of the events surrounding the failure revealed that this vessel was different from the others in several important respects. For example, the material was within but toward the top of the permissible composition limits for the specification. Both carbon and manganese contents were relatively high. In addition, molybdenum was present at 0.05%, the maximum level permitted, and there had been an addition of 0.05% vanadium which was neither specified nor prohibited for this grade. Discussions with the steel supplier revealed that there was some concern in the mill about producing material at the strength level required, and thus composition adjustments were being made to insure that the minimum tensile strength would be met in this type of plate. There was an indication that some material had to be rejected for this reason, and there was naturally an effort to avoid this in the future.

It turned out that one cause for concern on the part of the mill was the requirement that all tension test coupons be stress relieved prior to testing to simulate a postweld heat treatment. This had the potential for reducing their strength below the minimum, and there was an effort to compensate for this. For the material in these vessels there was no need for this requirement. However, the purchasing department had added this to the order because it had been their custom to do so even though it was not requested.

Tests by the supplier showed that, in fact, the steel from the failure initiation site and other parts of the vessel had an as-rolled tensile strength of 740 MPa (107.5 ksi). well over the specified limit, but the stress relief treatment, at 620oC (1150o F), reduced it to 680 MPa (98.7 ksi) which was within the specification. The vessel itself was not postweld heat treated, and thus the material was at its as-rolled strength. The material was about 100MPa (14.5 ksi) higher in tensile strength than most other heats of this steel.

The higher strength of the steel had significant implications with respect to toughness and weldability. The 20J (15 ft-lb) transition temperature for the steel was around 20oC (68oF) while most previous heats had a transition temperature closer to -10oC (16oF), although an occasional heat had a transition temperature of 5oC (40o F). The high level of strength and the level of carbon and alloys, although within specification, resulted in reduced weldability.

Fabrication Factors

The material variations discussed above had an effect on subsequent events. Examination of the fracture surfaces of the failed vessel indicated that the microscopic failure mechanism was brittle cleavage or quasi-cleavage, demonstrating that fracture occurred below the transition temperature for the material. The initiation site was established as being at the edge of a manual fillet weld that was around a nozzle. The weld was made in an unheated shop in which the temperature was above the minimum for preheat but probably below 15oC (59o F). The source of fracture was found to be a hydrogen induced code or delayed crack at the toe of a repair weld.

During or after fabrication, a visual inspection of the joint had shown some surface discontinuities and these were repaired by placing additional short passes, generally not more than 25mm (1 in) long, on the existing weld. The extent of subsequent inspection is unknown. The hardness of the heat affected zone in this location was over RC40, high enough to indicate martensite, a condition which in combination with excessive hydrogen in the arc atmosphere during welding provides both the potential for and presence of hydrogen induced (cold) cracking.4

Using a fracture mechanics model and the Charpy V-notch toughness data for the steel, it was possible to show that the size of the defect, in conjunction with the residual and applied hydrostatic test stresses, was sufficient to cause fracture of the vessel.

It is probable that the limited toughness and weldability of the steel in combination with excessive hydrogen in the welding arc made the normally effective welding procedures inadequate to prevent cracking, at least for the nozzle welds. However, it is also the author's opinion that the welding of this part of the vessel was considered less important than that of the main seams and did not get as much attention. The use of short repair passes on weldments is not good practice as the arc energy is quickly dissipated in the base metal and results in very high cooling rates in heat affected zone, promoting martensite formation and cracking when excessive hydrogen is present in the welding arc.

Inspection Considerations

Initially it should be noted that, while inspection of the vessel was in accord with typical practice, the inspection of the nozzles was at a different level than the main seams of the vessel. This was, in part, because the design of the nozzle would not permit meaningful inspection by other than surface techniques. Failure to obtain a good inspection of the nozzle welds was, without a doubt, a contributor to the failure.

A second contributor was the conditions of the hydrostatic test itself. The water used for the test was taken from a sump and was at a temperature of less than 15oC (59oF), and perhaps less than 10oC (50o F), which was the ground water temperature at that time of year. After reaching the test pressure, the vessel was left under pressure for a period of hours. It reportedly failed during the night when no one was present.

It was also reported that the vessel was pressurized to 2.79 MPa (400 psi) although the vessel specification only required a test pressure of 2.58 MPa (375 psi). This represents an overpressure of 10% or a test at 1.6 times the working pressure of the vessel. While this may not have contributed materially to the failure, it may have produced additional plastic strain in the vicinity of the nozzles where the fracture ultimately started.

Corrective Measures

In the aftermath of the vessel failure, all of the above information came to light. As a result, a series of preventive measures were undertaken. Some immediate action was required as a number of additional vessels made of the same heat of steel were in various stages of fabrication. The following steps were taken at once. (1) All nozzle welds were carefully inspected and all repairs made with a preheat of 121oC (250oF). Very short repair welds were prohibited. (2) Currently fabricated vessels were postweld heat treated at 620oC (1150o F) prior to hydrostatic testing. (3) Nozzle cutouts were to be used to sample material toughness, with the option being to heat the water for testing if the transition temperature of specific vessels was high.

The long term solutions incorporated some of the above measures but centered on better assessment of material properties, especially strength and toughness, actually needed in future vessels. The fabricator and steel supplier both participated. These measures did not necessarily include a toughness specification for the steel or continued postweld heat treatment of the vessels.

Summary

The case study cited above is intended to provide a practical focus for the points made before. While every detail is not reported, nor every potential problem illustrated, there are enough to demonstrate that potential gaps in the engineering process can exist. Perhaps bringing together the relevant engineers for a thorough discussion of the pros and cons of design stress levels, the materials available and their properties, the implications materials and design for fabrication and inspection, might have prevented this failure. Identifying the role of toughness, in the fabrication and testing, if not the operation of the vessel, would have been helpful in preventing the failure.

Since the vessel eventually had to be, for the most part, scrapped, it is probable that the cost of such additional consultation would have been less than the replacement cost of the vessel. It would certainly have eliminated the cost and delay of the failure analysis and the outside "experts" consultants. It might even have identified a more economical design, a better material, a more efficient fabrication procedure and a more effective testing sequence, and thus a better and more economical vessel. One thing failures do accomplish is to get a lot of engineers' attentions and get them working together. A better approach might be to do so without the impetus of an engineering failure.

References

1.      Fisher, J.W., Pense A. W., and Robert, R. R., "The Failure of the Lafayette Street Bridge -- Influence of Designs," Prevention of Structural Failures, Materials/Metalworking Technology Series, American Society of Metals, 1978, pp 3-28.

2.      Roberts. R. R., and Pense A. W., "Basics of Failure Analysis of Large Structures," Civil Engineering, Vol. 52, No. 5 (May 1980), pp 64-67 (Part I) No. 7 (July 1980), pp 60-62 (Part II).

3.      Pense, A. W., Dias, R., and Fisher, J. W., "Examination and Repair of Bridge Structures," The Welding Journal, Vol. 63, No.4 (April 1984), pp 19-25.

4.      Interrante,C. G., Dalder, E. N. C. and Yeo, R. B. G., "Effect of Moisture on Cold Cracking of a Carbon-Manganese Steel," The Welding Journal, Vol. 48, No. 9 (September 1969).

Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.


       

Welding Symbols: A Useful System or Undecipherable Hieroglyphics?

Welding Symbols: A Useful System or Undecipherable Hieroglyphics?
       
Bill Green
Former National Board Consultant
Retired Professor Emeritus in Welding Engineering, The Ohio State University

Winter 1996  

Category: Design/Fabrication 

Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (6 printed pages)

Welding symbols, when properly applied to drawings and, as importantly, when correctly interpreted, offer a potentially convenient way of controlling the welding of a particular joint.

The need for consistency in both the application of welding symbols to engineering drawings, and the accurate interpretation by personnel directly involved in manufacturing or construction, led to the development of a standard for these activities. The current American standard for welding symbols was originated by the American Welding Society and approved by the American National Standards Institute as ANSI/AWS A2.4-93, Standard Symbols for Welding, Brazing and Nondestructive Examination.

Part A of this standard covers welding symbols, Part B deals with brazing symbols, and Part C describes symbols for specifying nondestructive examinations.

Welding Symbols
Although the basic symbol system is uniform, there is a need for some flexibility, as specific circumstances differ from one shop to another and field operations may involve entirely different situations. There are, therefore, often several ways to specify a given weld. Also, because cost is a consideration and is related to the specific equipment to be used, the details of joint geometry will vary from one manufacturer to another.

There is a considerable advantage to developing a shop standard for use with welding symbols. A shop standard for this purpose will establish the details that apply to all normal or standard joints. Welding symbols can therefore specify the welding without including all of the many possible details within the symbols. Simpler welding symbols are easier to add to a drawing and to interpret. Fewer errors are a benefit of this approach.

This article focuses on those welding symbols associated with typical applications with ASME Code items.

The Arrow
The first element of a welding symbol to consider is the arrow. The arrow is an essential part of every welding symbol and must point to the joint to be welded. The stem of the arrow should not be a horizontal line on the drawing. The side of the joint to which the arrow points is, by definition, the "arrow side" of the joint, and the opposite side of the joint is the "other side" of the joint (Figure 1).

Often only limited space is available on a drawing for welding symbols. To minimize the number of welding symbols required, it is permissible to use more than one arrow in a single welding symbol if each joint to which an arrow is pointing is to be welded in exactly the same way. Since a welding symbol specifies welding of only the joint to which an arrow is pointing, and a change of direction or change in geometry constitutes the end of a joint, a multiple arrow welding symbol can be very helpful, particularly around closed corners (Figure 2).

The Reference Line
Another essential part of all welding symbols is the reference line, which is a straight line, drawn horizontally on a drawing, and connected to the arrow. The arrow may be connected to either end of the reference line (Figure 1).

Information relating to the "arrow side" of the joint is placed below the reference line and information relating to the "other side" of the joint is placed above the reference line. These positional relationships exist whether the arrow is attached to the left or right end of the reference line, and do not change as the angle between the arrow and the reference line varies (Figure 1).

The sequence of operations necessary to produce a specified weld is not indicated by the normal single-reference-line welding symbol. For example, if welding is to be done on both the "arrow" and "other" sides of a joint (typically called a double weld), a single-reference-line welding symbol does not specify which side of the joint is to be welded first. In fact, the symbol does not specify completion of the welding from one side prior to the start of the welding on the opposite side. These details are normally left to the personnel interpreting the welding symbol, with the requirements of the completed weld specified by the symbol.

If the sequence of operations needs to be specified, a multiple-reference-line welding symbol may be used. Two or more reference lines may be connected to the same arrow, with the reference line closest to the arrow specifying the first operation, followed by the operations specified by the sequence of reference lines reading upward or downward from the arrow (Figure 3). It should be noted that operations other than welding, such as nondestructive examinations, can be specified by a multiple-reference-line symbol.

The Tail
A third element to be considered is the "tail" of the welding symbol. The tail is drawn as agreater-than (>) or less-than(<) symbol, connected at the end of the reference line opposite the arrow. Information for which there is no specific provision elsewhere in the symbol is placed to the left or right of the tail as appropriate.

Reference to the approved welding procedure specification (WPS) is an example of information appropriate in the tail of a welding symbol. Since a WPS can contain all of the details applicable to a specific joint, a welding symbol composed of an arrow, reference line, tail and applicable WPS designation would be sufficient to completely specify the welding of the joint.

The welding process to be used is often specified by entering process designation letters in the tail of the welding symbol. The American standard includes two lists of processes and their corresponding designation letters. Table 1 groups similar processes, such as arc welding, brazing, resistance welding, and thermal cutting, while Table 2 arranges the processes alphabetically.

Groove Welds
Additional information may be included in a welding symbol, even if also included in a WPS. For example, V-, U-, bevel-, or J-groove welds may be specified to provide increased weld size in a given joint, compared to that obtainable with a square-groove weld. The choice is usually made on the basis of cost for the completed weld. A groove-weld symbol may be added to a welding symbol, below the reference line, to specify a weld only on the "arrow side" of the joint (single weld); above the reference line, to specify a weld only on the "other side" of the joint (also a single weld); or weld symbols may be added both below and above the reference line, to specify a double weld (Figure 4).

Complete Joint Penetration
Since many applications require welds providing complete joint penetration (CJP), there are several ways to specify this condition. One way is to use an arrow, a reference line, and add CJP in the tail of the symbol. This symbol specifies complete joint penetration with no detail as to how the final condition is to be achieved. Such a symbol may be appropriate when there is uncertainty as to what specific equipment will be available when the work is to go through the shop. Later, when equipment availability is known, it is a good practice to submit assembly drawings to engineering for final approval, with welding symbols containing all pertinent details.

A second way to specify complete joint penetration is to include a single groove-weld symbol or double groove-weld symbols (must be the same weld symbol on both sides of the reference line), without any dimensions to indicate depth of bevel or weld size. It should be noted that partial joint penetration can be specified by adding the depth-of-bevel dimension and the required weld size (in parentheses) to the left of the groove-weld symbol or both symbols of a double weld (Figure 5).

Inclusion of a backing weld symbol or back-weld-symbol, opposite a groove-weld symbol, specifies complete joint penetration if no depth-of-bevel or weld-size dimensions are added. Also, the inclusion of a backing symbol, opposite a groove-weld symbol, again without depth-of-bevel or weld-size dimensions, specifies complete joint penetration (Figure 6). Although there are provisions for specifying the root opening (gap), groove angle, and finish contours of welds, it is suggested that these details are best specified by a shop standard.

A discussion of groove welds should include at least mention of the scarf weld, a type of weld used on lighter gage material in small diameter piping, tube-to-tubesheet applications, and attachment of seats in smaller valves. This type of weld is intended for use with brazed joints, covered in Part B of the standard. There are two additional groove-weld types: the flare-bevel and the flare-V. These welds are used extensively in industries in which solid rounds, piping, and square and rectangular tubing are employed for structural purposes, rather than withstanding internal pressures typical of boilers and pressure vessels. The flare-type groove welds are therefore more common in the construction of buildings that house ASME Code items than in the manufacture of the items.

In a few cases there may be need to specify complete joint penetration plus a measurable reinforcement on the root side of a single-welded joint (welding from one side only). The addition of the "melt-through" symbol, on the opposite side of the reference line from a groove-weld symbol, identifies this requirement. The height of the root reinforcement may be specified by adding the appropriate dimension to the left of the melt-through symbol (Figure 7).

The use of consumable inserts has become more popular over the years, particularly in pipe joints, creating the need for a symbol to specify them. An open square, placed on the reference line opposite a groove-weld symbol, indicates the requirement for a consumable insert. The AWS consumable insert class must be added to the tail of the welding symbol, unless that information is specified in some other way (Figure 7).

Fillet Welds
Fillet welds are used extensively in industries including those producing boilers and pressure vessels. The fillet weld symbol is a right triangle placed on the reference line with the perpendicular leg always on the left. The dimension specifying the leg size of a fillet weld is placed to the left of the fillet weld symbol, and on the same side of the reference line.

Since the load-carrying capacity per pound of fillet weld is greatest for equal-leg fillet welds, they are used unless some geometry restriction requires the use of unequal-leg fillet welds. In these few cases, both leg dimensions, separated by a multiplication sign, are placed to the left of the fillet weld symbol. The order of the leg dimensions is not significant and the orientation of the weld must therefore be shown on the drawing.

In contrast to groove-type welds, fillet welds do not always extend for the full length of the joint. The length of a fillet weld, which has a length less than the length of the joint, is specified by placing the required dimension to the right of the fillet weld symbol. If the exact location of such a weld is critical, dimension lines, hatching, or detailing is necessary. The omission of a length dimension, of course, specifies a fillet weld for the full length of the joint.

Also, in contrast to groove welds, fillet welds are often specified as intermittent welds, meaning they are not continuous welds. For an intermittent weld, the segment length dimension is placed to the right of the fillet weld symbol, followed by a hyphen and pitch dimension. The pitch is the distance between centers of segments on one side of the joint.

Intermittent fillet welds can be specified on both sides of a joint by placing a fillet weld symbol both below and above the reference line. Intermittent fillet welds with the segments directly opposite across the joint are called chain intermittent fillet welds. If the segments of intermittent fillet welds are to be staggered in a symmetrical manner on both sides of a joint, the sequence of fillet size, fillet weld symbol, segment-length dimension, multiplication sign and the pitch dimension are offset, left to right, on one side of the reference line (Figure 8).

There are also provisions in the American standard for specifying the contours of fillet welds and even the method by which the contours are to be produced. As with groove welds, these details are best controlled through a shop standard.

Non-North American Company Standards
Increased interaction between domestic and non-North American companies has led to more frequent interpretation of drawings away from their country of origin. Fortunately, the American standard has included metric dimensions along with U.S. customary units for many years, which should minimize any associated problems.

The ISO standard for welding symbols, though very similar to the American standard, has some differences. The ISO standard provides for the specification of a fillet weld size as either the leg dimension (as with the American standard) or the throat dimension. Since with equal-leg fillet welds, the throat dimension is approximately 70 percent of the leg dimension, it is essential that this potential difference be recognized. Also, the ISO standard locates the pitch dimension of intermittent welds in the same location specified by the American standard. However, the pitch dimension, by definition of the ISO standard, is the clear distance between segments rather than the distance between centers of segments.

Brazing Symbols
Brazing symbols are similar to those used to specify welds. An arrow, reference line, and tail are used in the same way they are used in welding symbols. Because of the resulting mechanical properties, square- and scarf-groove symbols are more appropriate than the other groove symbols. The root opening or gap is also much more emphasized in brazing symbols compared to symbols specifying fusion welds. The approved WPS may be referenced in the tail of the brazing symbol. Current industry practices rely more on detail drawings to specify brazing applications than the use of brazing symbols.

Nondestructive Examination Symbols
The arrow, reference line, and tail used to specify a nondestructive examination (NDE), appear exactly the same as those used to specify welds and have the same significance. The various examination methods have been assigned designation letters which are placed below, above or both below and above the reference line to specify, respectively, examination on the "arrow side," "other side" or both sides of the part. Combinations of examinations may be specified by adding additional designation letters with an additional sign separating the methods. In those cases where there is no arrow or other side significance, or there is no preference from which side the examination is made, the designation letters are centered on the reference line.

The approved NDE procedure may be referenced in the tail of the NDE symbol. There is a provision that allows the number of examinations to be specified, and the field-examination and examine-all-around symbols may also be used.

Summary
Space does not permit complete coverage of the entire American standard. For more information, readers are referred to the ANSI/AWS A2.4-93, available from the American Welding Society, 550 NW LeJuene Road, Miami, FL 33126.

Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.


       

[MW:34820] RE: 34813] Clarification in Rate of heating and cooling.

Hello,   Please see the response below.   Regards.   P. Goswami, P. Eng, IWE.   From: materials-welding@googlegroups.com <materials-weld...