Probably you could reframe your question with more details, what
exactly you need, type of project you are involved , service and other
deisgn conditions, and the construction code, contrcat requirements.
As discussed by many members, CE is not a variable to look in normal
practice, and i am not aware of any BS or JIS standard (please specify
which standard you are reffering to)
rightly pointed out by some membrs CE is the parameter to be lloked
into even for PQR when you have SSC or Wet H2S cracking requirement,
and normally this comes from the contract not from the code.
Hardness is one of the important property associated with this and
depends mainly on C and CE
Hardness of Weld, HAZ depends on hardnabilty of welding consumable/
basemateril and cooling rate.
Thus, controlling the CE to a lower value reduces the likelihood of
hard regions in the as-welded condition under a given set of welding
conditions.
The cooling rate is affected by part configuration and welding
parameters. Carbon content restriction and CE limits
do not affect cooling rate.
NACE MR0175, contains the following paragraph :
"Welding procedure qualifications on carbon steels that use controls
other than thermal stress relieving to control the hardness of the
weldment shall also include a hardness traverse across the weld, HAZ,
and base metal to ensure that the procedure is capable of producing a
hardness of 22 HRC maximum in the condition in which it is used."
Some users interpret this paragraph that any procedure qualification
that includes hardness readings in the weld deposit, HAZ, and base
metal meets the intent of the paragraph above. However, the important
words in the above paragraph are not those regarding the hardness
traverse, but rather the words "use controls other than stress
relieving". Whereas the specific controls that may be required are
left up to the manufacturer, some type of extra control measures are
implied.
Unfortunately, the ASME codes don't require any correlation between
the carbon content or CE level of the PQR specimen and that of the
production base metal. In other words, it is perfectly acceptable
within the ASME codes to utilize a PQR specimen with 0.12% carbon and
a CE of 0.35 to qualify a non-PWHT procedure for welding any P-1
material. This is in spite of the fact that some P-1 materials have
maximum carbon contents of 0.35% and can produce CE values exceeding
0.60. Thus, it is logical that the "controls other than stress
relieving" mentioned in MR0175 would likely need to include some type
of correlation between the carbon content or CE level of the PQR
specimen and that of the production base metal.
The refinery industry's document, "NACE RP0472, Recommended Practice -
Methods and Controls to Prevent
In-Service Environmental Cracking of Carbon Steel Weldments in
Corrosive Petroleum Refining Environments,"
contains various alternative methods that can be followed to assure
that carbon steel weldments will be soft enough to resist cracking. To
summarize briefly, the document deals with two separate issues--weld
deposit hardness and base metal heataffected zone hardness.
There are three methods listed in RP0472 for control of base metal
heat-affected zone hardness:
* Chemistry control (specifically, control of the maximum carbon
equivalent);
* Weld procedure qualification hardness testing, including less-
restrictive chemistry control in conjunction with special
welding process controls; and
* Postweld heat treatment (PWHT).
The chemistry control method involves selection of filler metal
chemistry in conjunction with control of the base metal
carbon equivalent to such a low level that low weld deposit hardness
and HAZ hardness is virtually guaranteed regardless
of welding process parameters. NACE Committee Report 8X1944 states
that a CE of 0.43 is commonly specified for
base materials when this technique is employed.
The weld procedure qualification hardness testing method is a variant
of the chemistry control method. In this method, a
less restrictive maximum CE value may be chosen for production base
metal. A welding procedure qualification record
(PQR) test specimen is then created using actual production material
or a coupon of representative material with an actual CE corresponding
to the maximum CE value that is to be applied to the production base
material. Welding variables (such as preheat, current, voltage, travel
speed, interpass temperature, etc.) are controlled and monitored
closely during the creation of the procedure qualification specimen.
The PQR tests include a hardness traverse using either 10kg Vickers or
Rockwell 15N scale. Predefined hardness traverse diagrams are provided
for several weld geometries (See Figure 3 in RP0472). The resulting
welding procedure specification (WPS) then must contain certain
restrictions to ensure that the PQR specimen is actually
representative of production weldments.Those restrictions include:
* The procedure may only be used with the same base metal grade and
class. In other words, a procedure qualified on
A516 Grade 60 plate material could not be used to weld A516 Grade 70
plate material, even though all are within the same ASME Section IX P-
Number group.
* The actual CE of production material must be controlled to a value
less than or equal to that of the PQR specimen.
* The heat input used during production welding must not deviate from
the heat input used during creation of the PQR
specimen by more than 10% lower or 25% higher (alternatively, for the
shielded metal arc welding (SMAW) process,
the maximum bead size and the minimum length of weld bead per unit
length of electrode used in creation of the PQR
specimen can be imposed as a requirement in the WPS).
* Preheat and interpass temperatures must be at least as high as those
utilized in production of the PQR specimen.
* If preheat was not utilized for the PQR specimen, the maximum base
metal thickness of production weldments must
not be allowed to exceed the thickness of the PQR specimen.
* Other restrictions apply to fillet welds, submerged-arc welding
(SAW), gas metal arc welding (GMAW), flux-cored
arc welding (FCAW) processes, welding procedures involving bead-
tempering techniques and other techniques that are
sensitive to weld-bead sequence, and materials containing intentional
additions of microalloying elements such as Nb
(Cb), V, Ti, and B.
RP0472 cautions that the practice of postweld heat treating at lower
temperatures for longer times, as allowed by
some of the ASME Codes, should not be followed when the heat treatment
is being performed to reduce the hardness of
the weld deposit or the heat-affected zone.
Products procured for new oil and gas facilities, expansions, and/or
repairs often are governed by customer or contractor
piping specifications. These specifications may contain stipulations
requiring maximum CE values for all components
in the piping system, and these requirements are often imposed on
valves. The use of these special chemistry controls
allows the company and/or its contractors to perform welding on
components without PWHT and be reasonably sure that
weld heat affected zones will be soft enough to resist wet H2S
cracking. This is especially useful for components that will be
actually welded into the system, although it is also beneficial for
non-welding-end components that may eventually require welding to
repair erosion or corrosion damage.
There are several industry standards and documents available that
contain requirements and/or recommendations
for welding of carbon steels destined for sour service, including NACE
MR0175, NACE RP0472, and NACE Committee
Report 8X194.
The requirements listed in NACE MR0175 for carbon steels welded
without subsequent PWHT are not very specific.
NACE RP0472, in conjunction with Committee Report 8X194, documents
several methods that can be used to ensure
carbon steel weldments will be soft enough to resist cracking in wet
H2S environments. Those methods include hardness
testing of weld deposits, postweld heat treatment, control of base
metal CE to levels low enough to ensure low HAZ
hardness, and weld procedure qualification hardness testing in
conjunction with control of CE to intermediate levels and
detailed welding process controls to ensure low HAZ hardness.
Each of the hardness control methods has advantages and disadvantages.
The selection of the most robust and economically attractive method
depends upon several factors, including the available equipment,
available welding procedure specifications, product forms (castings,
plate, pipe, forgings, etc), and availability of heat treating
equipment.
Low CE requirements are an effective method for preventing hard spots
in welds. However, low CE requirements increase raw material costs and
lead time significantly, and have no beneficial effect on residual
stresses generated by welding. Weld procedure qualification hardness
testing, utilized in conjunction with intermediate CE requirements and
restrictive welding process controls, does not appear to be a
generally effective solution. Based upon the cost and lead time
factors and the very restrictive welding process control requirements,
this approach would probably be optimum only in very specific
situations.
Source: Welding of CS materials used in sour service by Donbush and
the valve magazine,
NACE Standard MR0175-2000, "Sulfide Stress Cracking Resistant Metallic
Materials for Oilfield
Equipment", NACE International, Houston, TX, 2000. NACE Standard
RP0472-1995, "Methods and Controls to Prevent In-Service Environmental
Cracking of Carbon Steel Weldments in Corrosive Petroleum Refining
Environments", NACE International, Houston, TX, 1995.
Shargay, C., "Overview of NACE International Standard RP0472", Paper
No. 417, NACE Corrosion/99, NACE International, Houston TX, 1999.
NACE Technical Committee Report 8X194, "Materials and Fabrication
Practices for New Pressure Vessels Used in Wet H2S Refinery Service",
NACE International, Houston, TX, 1994.
On Mar 19, 9:05 am, "mustak patel" <mustak_patel...@rediffmail.com>
wrote:
>
> Hi guys
>
> What is the tolerance given in the AMERICAN CODES for CE value for applicability of WPS and welder qualifications? Is it an essential variable? I think in British or japanese standards its given + 0.003 % Max.
>
> Appreceiate any feed back.
>
> Thanks and Regards
>
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