Charles Becht IV
Becht Engineering Co., Inc.,
22 Church Street,
P.O. Box 300,
Liberty Corner, NJ 07938
e-mail: chuck@becht.com
New Weld Joint Strength
Reduction Factors in the Creep
Regime in ASME B31.3 Piping
The 2004 edition of ASME B31.3, Process Piping Code, includes strength-reduction
factors that apply to welds operating in the creep regime for the material. These factors
apply to allowable stresses for sustained loads, such as weight and pressure. This paper
describes the background for the rules. 关DOI: 10.1115/1.2140291兴
Keywords: weld strength, creep, ASME B31.3, weldment creep rupture
Introduction
Welds have been known to perform worse with respect to creep
life than the base material in some circumstances. The difference
in material properties between the base metal, heat affected zone,
and weld metal can create a metallurgical notch, and the creep
properties of the weld can be inferior to the base metal.
These effects at the weld joint can result in reduction of longterm strength of welded pipe, relative to the base material. This
change to ASME B31.3 Process Piping Code 关1兴 addresses this
concern. The change requires consideration of the creep strength
of welded joints, either by using presumptive weld joint strengthreduction values or more applicable data, if available. This approach provides necessary flexibility, considering the limited
amount of available weldment creep rupture test data and the fact
that the code presently provides no control on weldment design,
with respect to long-term creep life and no control over weld
metal selection other than ambient temperature tests.
There is little in the way of ASME B31.3 Code requirements
with respect to the metallurgy of the weld. This is not atypical, as
many other codes are the same in this regard. It is essentially left
to the designer to make a proper selection. The only requirements
tested in qualifying the weld procedure specification, per ASME
Boiler and Pressure Vessel Code, Section IX 关2兴, are the ambient
temperature tensile strength of the weld joint and passing a bend
test, showing sufficient room temperature ductility. The Section
IX requirements are adopted by reference by ASME B31.3. Neither of these tests provides an indication as to the suitability of the
weld joint for creep.
Weld Joint Strength Reduction Factor
A weld joint strength-reduction factor W is introduced in the
2004 edition of ASME B31.3 for welds in the creep regime. The
intent of the weld joint strength reduction factor is for it to be the
ratio of the creep rupture strength of the weldment to that of the
base material. It is based on the expected 100,000 hr creep rupture
strength. Of course, since 100,000 hr tests are not practical to run,
the creep data are extrapolated. Such strength-reduction factors
have been developed for high temperature nuclear applications,
and are provided in the Boiler and Pressure Vessel Code 共BPVC兲,
Section III, Subsection NH 关3兴.
The weld joint strength-reduction factors apply to longitudinal
and spiral welds in design for internal pressure, and to girth welds
Contributed by the Pressure Vessels and Piping Division of ASME for publication
in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received July 19, 2005;
final manuscript received October 10, 2005. Review conducted by G. E. Otto Widera.
Paper presented at 2005 ASME Pressure Vessels and Piping Conference 共PVP2005兲,
July 17, 2005–July 21, 2005, Denver, Colorado, USA.
46 / Vol. 128, FEBRUARY 2006
in the design for sustained longitudinal forces and moments, such
as due to weight and internal pressure. The effect of these rules is
expected to be as follows:
1. For seamless pipe, there is no effect in the required wall
thickness for internal pressure.
2. For longitudinally welded pipe, a greater wall thickness will
be required at elevated temperature unless creep tests are
performed that show the weldment has a comparable creep
rupture strength to the base material.
3. Although the weld joint factor on girth welds could theoretically require an increase wall thickness, this is considered to
be a highly unlikely event, for the following reasons:
a.
b.
The longitudinal bending stresses need to be limited
to less than the Code allowable in the creep regime to
avoid excessive sagging of the pipe due to creep. See
Becht 关4兴 for a discussion of long-term creep deflection of pipe and a method of evaluating it.
If longitudinal stresses are high, the solution is almost
always to provide additional support, not to increase
the pipe wall thickness.
The factor is also applied to welded fittings, such as clamshell
elbows. Note that if welded fittings are used with a seamless piping system, then the required fitting thickness could become a
governing consideration.
The weld joint strength-reduction factor is not applied to occasional loads, since they are anticipated to be of short duration, so
the creep properties are not a consideration.
The weld joint strength-reduction factor is not applied to the
allowable stress range for displacement stresses SA because these
stresses are not sustained. The displacement stresses relax over
time. The allowable stress criteria for displacement stress range is
designed such that the piping system will self-spring so that the
highest level of displacement stresses only occurs at the hot condition once over the lifetime of the piping system.
Development of the Weld Joint Strength-Reduction Factor
It is not feasible at this time to specify weldment strengthreduction factors that are specific to the base material, weld material, heat treatment, weld configuration, etc. There are myriad
possible combinations a user may wish to specify and a general
lack of available weldment creep rupture data. As a result, a rather
pragmatic approach was taken. Weld joint strength-reduction factors were specified, based on available data, to be used in the
absence of more applicable data. This recognizes the weld may be
a limiting consideration in creep life, but deals with it in a very
general manner.
Copyright © 2006 by ASME
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Fig. 2 Low carbon welds with higher carbon base material
„see Table 1…
Fig. 1 Basis for weld factors „see Table 1…
Weld joint strength-reduction factors were specified for use at
temperatures above 510° C 共950° F兲 and are based on consideration of the effects of creep. The designer may determine the weld
joint strength-reduction factor for the specified weldments based
on creep rupture test data. This is encouraged to develop factors
specific to the base-material–weld-material combinations used in
the design. However, a simplified factor was provided for use by
the designer, in the absence of more applicable data. Because it is
impractical at this time to establish factors for specific materials, a
general factor was used. The factor varies linearly from 1.0 at
510° C 共950° F兲 to 0.5 at 815° C 共1500° F兲.
The factor was based on weld joint strength-reduction factors
that were developed for high-temperature nuclear codes, for a few
specific base-material–weld-material combinations.
The weld joint strength-reduction factors that are to be used in
the absence of more applicable data were based on those provided
in ASME BPVC Section III, Subsection NH 关3兴. The development
of these factors is described by Corum 关5兴. Weld metal, base
metal, and cross-weld creep rupture data had been used to develop
weld joint strength-reduction factors in Subsection NH for limited
material combinations. The basic weld joint strength-reduction
factors provided in ASME B31.3 were developed based on evaluating the factors provided in Subsection NH, after adjusting them
as appropriate for comparison to the ASME B31.3 stress basis.
The two considerations were that the factors for 100,000 hr durations were used, and the factors were adjusted as was done in
Code Case N-253 关6兴.
The factors in Code Case N-253 adjusted so that they were 1.0
if 80% of the expected minimum stress to rupture of the weldment
exceeded the allowable stress of the base material. The factors
from the Code Case, which are based on 100,000 hour durations,
are plotted in Figs. 1 and 2 共see also Table 1兲.
Figure 1 shows the data for 2-1 / 4 Cr-1Mo, 800H, 9Cr-1Mo,
and stainless steel with at least 0.04% minimum carbon content
weld metal as well as the basic weld joint strength reduction factor W provided for use in the absence of more applicable data.
Figure 2 shows data for Type 304 and 316 base material, required
by Subsection NH to have a minimum 0.04% carbon content,
welded with lower creep strength low carbon 共⬍0.04% 兲 weld
metal. Note that the sudden drop in the weld joint strength reduction factor for Type 316 stainless steel between 510° C 共950° F兲
and 538° C 共1000° F兲 is due to a shift from tensile property to
creep property control of the allowable stress. The weld joint
strength reduction factor is 1.0 when tensile properties govern the
allowable stress.
The data in Fig. 2 were considered to represent a poor choice of
weld material for high-temperature service and were not included
in the data used to develop the weld joint strength reduction factors. Selection of an appropriate weld material for elevated temJournal of Pressure Vessel Technology
perature service remains the responsibility of the designer. The
weld joint strength reduction factors will not protect against poor
material selection, but are a step toward addressing creep effects
in high-temperature welds.
Review of Fig. 1 shows that the approximate factor, which varies linearly from 1.0 at 510° C 共950° F兲 to 0.5 at 815° C 共1500° F兲
provides a fairly good representation of the available data. One
could argue that worse behavior can certainly occur 共e.g., see Fig.
2兲. However, in this circumstance, the new rules are at least more
conservative than the prior rules. One could also argue that better
behavior can be achieved in some circumstances. In this case, the
better behavior can be recognized if creep rupture tests of weldments are performed to develop specific factors for that weldment.
The designer can use other factors, based on creep tests. The
Code states that the tests should be full thickness cross-weld
specimens with test durations of at least 1000 hr. Full thickness
tests are required unless the designer otherwise considers effects
such as stress redistribution across the weld.
Generally good experience with the strength of carbon-steel
weldments relative to carbon-steel base material indicated that
there was no need to use the strength reduction factor at temperatures below 510° C 共950° F兲 for carbon steel. This is independent
Table 1 Key to Weldment Designation „see Figs. 1 and 2…
304 A
304 B
304 C
316 A
316 B
316 C
800H A
800H B
2-1 / 4 Cr
9Cr
304 stainless steel welded with SFA 5.22 E 308T and E
308LT; SFA 5.4 E 308 and E 308; and SFA 5.9 ER 308
and ER 308L
304 stainless steel welded with SFA 5.22 EXXXT-G
共16-8-2 chemistry兲; SFA 5.4 E 16-8-2; and SFA 5.9 ER
16-8-2
304 stainless steel welded with SFA 5.22 E 316T and E
316LT-1, -2, and -3; SFA 5.4 E 316 and E 316L; and
SFA 5.9 ER 316 and ER 316L
316 stainless steel welded with SFA 5.22 E 308T and E
308LT; SFA 5.4 E 308 and E 308L; and SFA 5.9 ER 308
and ER 308L
316 stainless steel welded with SFA 5.22 EXXXT-G
共16-8-2 chemistry兲; SFA 5.4 E 16-8-2; and SFA 5.9 ER
16-8-2
316 stainless steel welded with SFA 5.22 E 316T and E
316LT-1, and -2; SFA 5.4 E 316 and E 316L; and SFA
5.9 ER 316 and ER 316L
800H welded with SFA 5.11 ENiCrFe-2 共Inco A兲
800H welded with SFA 5.14 ErNiCr-3 共Inco 82兲
2-1 / 4 Cr-1Mo 共60/ 30兲 welded with SFA 5.28 E 90C-B3;
SFA 5.28 ER 90S-B3; SFA 5.5 E 90XX-B3 共⬎0.05C兲;
SFA 5.23 EB 3; SFA 5.23 ECB 3 共⬎0.05C兲; SFA 5.29 E
90T1-B3 共⬎0.05C兲
9Cr-1Mo welded with standard 9Cr-1Mo filler wire and
modified 9Cr-1Mo filler wire GTA, SMA and SA
welding processes
FEBRUARY 2006, Vol. 128 / 47
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of other concerns, such as the potential for graphitization, which
may limit use of carbon steel at elevated temperatures.
Looking into the Future
One of the intents of including these rules is to encourage the
improvement of technology with respect to weldment design at
elevated temperatures. In circumstances where the default weld
joint strength-reduction factors are found to be unconservative, a
lower value can be used or perhaps, preferably, the weld design
would be changed. If there are weldments for which this curve is
conservative, creep test data can be developed to justify higher
strength-reduction factors 共up to 1.0兲. Should this data be made
available to the ASME B31.3 Section Committee, factors may be
put in for specific weldment designs. For example, it has been
suggested that a factor of 1.0 may be applicable to normalized low
chrome welds. Should data be made available, it could be so
specified for all users. This is a practical approach to the problem,
considering the availability of creep test data on weldments, and
the likelihood of gathering sufficient data to do anything different,
in the foreseeable future.
Increased focus on this issue of elevated temperature design
will lead to further improvement in technology. This is in contrast
to ignoring a real concern, due to lack of sufficient data to rigorously address it.
48 / Vol. 128, FEBRUARY 2006
Conclusion
Weld joint strength-reduction factors are included in the 2004
edition of ASME B31.3 关1兴. These are applied to the allowable
stresses for sustained loads of long duration. The designers may
develop their own weld joint strength-reduction factors based on
creep testing of weldments or use default values, which are specified for use in the absence of more applicable data. It is hoped that
the requirement for weld joint strength-reduction factors will lead
to improved technology in this area.
References
关1兴 ASME, 2004, ASME B31.3 Process Piping Code, ASME, New York.
关2兴 ASME, 2004, Welding and Brazing Qualifications, ASME Boiler and Pressure
Vessel Code, Section IX, ASME, New York.
关3兴 ASME, 2004, Class 1 Components in Elevated Temperature Service, ASME
Boiler and Pressure Vessel Code, Section III, Division 1, Subsection NH,
ASME, New York.
关4兴 Becht, C., and Chen, Y., 2000, “Span Limits for Elevated Temperature Piping,”
ASME J. Pressure Vessel Technol., 122共2兲, pp. 121–124.
关5兴 Corum, J. M., 1990, “Evaluation of Weldment Creep and Fatigue StrengthReduction Factors for Elevated-Temperature Design,” ASME J. Pressure Vessel Technol., 112共4兲, pp. 333–339.
关6兴 ASME, 2004 Construction of Class 2 or Class 3 Components for Elevated
Temperature Service, Section III, Division 1, Code Case N-253, ASME Boiler
and Pressure Vessel Code, ASME, New York.
Transactions of the ASME
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