ML20116F493
| ML20116F493 | |
| Person / Time | |
|---|---|
| Site: | Catawba  |
| Issue date: | 10/11/1984 |
| From: | AFFILIATION NOT ASSIGNED |
| To: | |
| References | |
| OL-I-161, NUDOCS 8505010111 | |
| Download: ML20116F493 (6) | |
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,I Concern #1 Page 1
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Violation of Interpass Temperature INVESTIGATION RESULTS Backaround It was alleged by several Welders that their Foreman had placed pressure on them to sacrifice quality for production causing the Welders to willfully exceed weld interpass temperature on stainless steel socket welds.
~Interpass temperature control is an ASME Section IX. supplementary essential variable for various processes, but is not applicable since notch toughness requirements are never specified for austenitic stainless steels (austenitic stainless steels do not undergo a ductile to brittle transition as do carbon and low alloy steels).
Interpass temperature is employed to minimize the occurrence of weld heat affected zone sensitization and is required per Duke Nuclear Guide 1.31, paragraph 4.0.
Test Shop Fabricated Socket Welds In order to assess the influence of interpass temperature on stainless steel i
socket welds eight weldsents were fabricated in the Catawba Test Shop.
The weldingcondIt1onsforeachweldsentareshowninTable1.
As can be seen in this table, two welding conditions were used to make the weldsents, the highest energy input permitted by Field Weld Data Sheet L-213 with 350'F of maximum interpass temperature and greater than 750*F interpass temperature.
The two inch Schedule 160 specimen (number 7) was welded by Welder #248, the other seven were welded by Test Shop personnel.
For welds 1, 3 and 5, where the interpass temperature requirement was exceeded, Test Shop personnel duplicated conditions used by Welder #248 on weld number 7.
The surface conditions after welding for welds 7 and 8 are shown in Figure 1.
Exceeding interpass temperature resulted in a poorer surface appearance and a surface condition that would require extra cleaning prior to final inspection.
The surface conditions shown in Figure 1 are representative of the other welds produced for this investigation.
ASTM A-262 Practice A - Evaluation of Test Weldsents Practice A of ASTM Standard A-262 was employed to determine if sensitization due to welding occurred in the piping material of the welds. The evaluation of sensitization by A-262-A involves comparing microstruct' ural features revealed by the method to standards given in the specification (See Appendix A). The more severe the exposure of the material to sensitizing conditions the more degraded the microstructure becomes.
The' degree of sensitization seen in a material is dependent on its carbon content, with higher carbon materials exhibiting more severe sensitization for a given sensitizing exposure.
(SA 240 Type 304 max carbon allowed is 0.08 wt%).
Sensitization is manifested as a localized corrosive attack at the grain boundary regions.
The etching procedure of A-262 Practice A selectively attacks the grain boundary regions at sensitized austenitic stainless steel.
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.A section of each weldsent was removed and subjected to A-262 Practice A.
The heat affected zone (HAZ) of the weld was examined and the region with i
the great'est degree of sen'sitization documented with photomicrographs.
All the test weldsents exhibited either a step structure or a dual structure (reference Figures 1 through 3, Appendix A).
The sensitization condition revealed by Practice A are shown in Figure 2.
Figure 2A shows the nicrostructures of specimens 5 and 6, the 1 inch Schedule 160 weldments.
Both these weldsents exhibit a dual structure, considered acceptable by Practice A.- Likewise, Figure 28 shows tt.e structures of welds 7 and 8; both exhibit a dual structure. The other four weldsents considered acceptable by i
-tractice A, also exhibited dual structure conditions and as with those shown in Figure 2.
No difference in the degree of sensitization noted between weldsents where interpass temperature requirements were met and weldments l
where interpass requirements were exceeded.
5 Thus it appears requirements"a'nd'that over the range of welding conditions where FWDS h
in the case where these requirements are deliberately i
exceeded (by a large margin), the degree of sensitization is not worsened, i
but the range over which sensitization occurs is increased.
In effect, violating energy input and interpass conditions broadened the sensitized region, but did not make it more severe. This would also be true for i
smaller diameter thinner wall piping where physical limitations such as 1
thermal conductivity would similarly broaden the range of sensitization and i
ot the severity of sensitization.
In summary, the accepted welding practice for welding austenitic stainless i
steels at Catewha is to restrict interpass temperatures to 350'F.
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if cases have occurred as alleged, test results indicate that the severity of sensitization is not increased.
Further, U.S. Nuclear Regulatory Guide 1.44 (Appendix B) recognizes tnat sensitization is a natural consequence of welding austenitic stainless steels but only atypical conditions using very high heat input could result in stress corrosion cracking of the heat i
affected zone of the weld. Test results indicate that an interpass temperatere requirement of 350*F is conservative and exceeding it does not necessarily result in unacceptably sensitized material, provided the nominal carbon content of the material is less than 0.0XX, the highest carbon content of the test materials.
EPR Evaluation of Test Socket Welds In order to further define the degree of sensitization, caused by exceeding interpass temperature, J A Jones' Applied Research Laboratory employed the Electrochemical potentiokinetic reactivation technique on the test coupons.
l The report of this effort is attached as Appendix C.
Their conclusions were:
1)
That for these test coupons,itization.interpass temperature did not appear to influence the degree of sens 2)
Test results were consistent with ASTM A-262 Practice A results.
3)
The heat affected zone did not appear to be significantly different from unaffected base material conditions.
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j Results of the EPR evaluation confirmed A-262 Practice A results.
. Field Portable Meta 11ooraphy A number of Welders in the same crew had indicated that they had been pressured by their Foreman into violating interpass temperature on stainless
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steel welds. Since the principal consequence of violating interpass temperature is Heat Affected Zone sensitization, Duke Construction undertook i
to evaluate a sample of welds made b A field portable technique was developed employing A y these Welders.
j 262 Practice A.
All the Class A, B, and j
_,C welds made by this crew were identified as detailed in Appendix D.
From these welds it was determined that this crew had wel'ded on 6 critical i
systems (critical system is defined in Appendix E).
It was found that 360 2 l
inch and under socket welds were made by this crew.
Of these 360, 28 welds i
were selected for evaluation per ASTM A-262 Practice A.
These welds and the chemistry of the piping material is given in Table 2.
As can be seen in i
this table, most of the piping material is 0.40 carbon and above, and considered to have a potential for sensitization.
Details of the field l
portable test procedure are given in Appendix F.
Twenty-six o' f the welds exhibited a dual microstructure, a combination of a step structure and " ditching" at the grain boundaries (ditching is a i
localized attack of the grain boundary region principally caused by precipitation of chromium carbides at the grain boundary.
The precipitation i
of these carbides creates chromium depleted region; this region is there l
subject to more aggressive attack by the test solution).
This dual i
structure is censidered an acceptable condition and stainless steel piping i
systems with such conditions would not likely be susceptible to intergranular attack.
Three of the weldsents exhibited microstructures i
which would not be acceptable per ASTM A-262 Practice A.
The HAZ of these two weldsents exhibited a ditched structure.
It should be noted that A-262 Practice A is suitable only as an acceptance criterion and may not be used for rejection of stainless steel materials.
When non-acceptable T ructures are found per Practice A, other A-262 practices are required before rejectionisconsidered. All the other tests are for screening stainless steels for environments far more aggressive than any found in a Nuclear Power Plant.
Further, the tests are destructive in nature (unlike Practice A) requiring samples from the material to be evaluated.
Such tests would not be practical for field application since most of the power plant piping systems are in place and cutting into them for samples would invalidate hydrostatic testing.
Non-acceptable conditions were only found in three weldsents a-bo*h were piping material with a carbon content greater than 0.06' content stainless steels (generally greater than 0.04 c. u.05f..,
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)havea potential for exhibiting welding induced sensitization even if interpass control is imposed.
A heat of 4 inch Schedule 160 piping with 0.073% carbon i
content was entained and a series of weldsents made.
One weldsent was i
welded with a 72*F interpass temperature, a second with a 350*F interpass l
temperature, and a third with a 750*F interpass temperature.
A ditched
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structure was found in the 350*F and 750*F interpass welds but not in the 72*F interpass temperature welds.
Therefore, the presence of a ditched structure found on field welds does not indicate that interpass CC23084200010007
f Concern #1 Page 4 6
temperature was violated in making these welds.
It is app'arent from Test Shop specimen and results of field evaluation that there is no evidence to support the contention that interpass temperature i
was violated.
ASTM A-262 Practice A result of ditched microstructure was i
more dependent on the chemistry of the material than welding procedure, as no metallurgical differences were noted between specimens welded with interpass temperature control and those without.
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.JtESOLUTION While several Welders raised this concern, no evidence was found to support the contention that they were pressured into violating interpass temperature.
The accepted practice for interpass temperature control is to touch the weldment with the hand and if the hand can be held on the weld, it l
is cool enough to weld.
The required interpass temperature is 350*F and there are few if any Welders who could hold a hand on a weld at 350*F.
The concerns appeared to have been raised mostly on the basis of poor Craft supervisor interaction and not technical applications, as none of the Welders had concerns about quality of work at Catawba.
Therefore, this concern is not technically founded but may reflect a breakdown in
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supervisor-craft direction.
l ACTION ASSIGNMENT The supervisory / employee relation concern shall be evaluated and addressed as a part of Concern #
No additional technical action is required.
SUPNARY AND CONCLUSION No evidence was found to support the contention that interpass temperature was exceeded by Craft in making stainless steel socket welds.
Metallurgical evaluations of shop prepared weldsents revealed that the degree of sensitization was not worsened by exceeding interpass temperature and the i
degree of sensitization was influenced by the chemistry of the material.
l Metallurgical evaluation of field welds revealed that the degree of sensitization was influenced by the chemistry of the material welded and I
conditions observed (ditch microstructure in the weld HAZ of three welds) could be produced by welding within interpass temperature requirements.
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TABLE 1 ASTM A-262 Practice A Weldment No.
Pipe Size FWDS Voltage Current Interpass Results
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1 2" SCH 40 L-213 10 150 750'F Dual - Acceptable 2
2" SCH 40 L-213 10 150 350'F 3
1" SCH 40 L-213 10 140 750*F 0
4 1" SCH 40 L-213 10 140 350 F 0
5 1" SCH 160 L-213 12 180 750 F 6
1" SCH 160 L-213 12 180 350*F 7
2" SCH 160 L-213 12 180 750'F 8
2" SCH 160 L-213 12 180 350 F 9
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