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.