welding issues in pressure parts 1

[Howardjames]

Hello, I am new in this forum. I have welding issues listed below. Is there any comment I will appreciate.

1. There are 5 weld passes to joint stub tube to header with materials of SA213-T12 and SA335-P12 in pressure parts of power plant. E8016 electrodes were used in the root pass, second pass and final pass. However the third and forth pass have E9016 electrodes instead. The temperature of preheat snd postweld heat treatment are 121C and 690C respectively. Are there any effect related to mechanical properity of the joint?

2. To tell E8016 and E9016 in weldment, PMI(positive material Identification) equipment was used to measure the surface chemical composition of final pass weld. Is there any experience of welding experts to do such a same thing in the world? Is there any recommendation to me how to tell different passes in weldment?

3. In lab, the weldment sample was taken through metallurgical analysis to show the microhardness of weldment as high as 350Hv. Is there any meanings for weldment?

 

[metengr]

1. No

2. At this point, I would not waste time or money to locate the 90xx deposition. You can remove it by excavating to a depth where the welders switched electrodes. Again, I would qualify a WPS with what you performed in production and leave it alone.

3. The 90 series weld deposit is higher strength in comparison to the 80 series deposit and will have higher hardness. Even at 350 HV hardness for a boiler application, I would not worry. If anything, the weld region will temper in service.

 

[Howardjames]

Thanks reply.
Actually, stub tube leakage was occurred in hydrostatic test. May be cold crack was a reason to fail. In question 3, sample taken to show 350Hv is higher than I expected. The microhardness of weldment is usually ranged in 180-230Hv after PWHT. The parent material of SA213 T12 is about 180Hv. Does 350Hv mean inproper PWHT process in weldment and the reason of crack?

 

[metengr]

It could be. Was this really subjected to PWHT? If so, the hardness is higher than I would expect. Better check again to determine if the stub attachment welds are PWHT.

 

[Howardjames]

Documents shown header with stubs are subject to PWHT in overseas. However, abnormal microhardness value shows PWHT procedure is not good as we expected. Can not sure other headers with stub weld have these issues or not. Can I measure surface hardness by Leeb equipment after grinding stub weld? In my experience, Leeb test have a bit variations depending on man skill, surface roughness...etc. Is there any suggection to make sure PWHT by other methods?

 

[davefitz]

Often headers with stubs are PWHT in a furnace in the shop, together with multiple other headers that also require the same PWHT. Unfortunately, some furnaces have large temperature unbalances and some parts of some headers might not have been properly PWHT'd. It might be useful to determine if that specific header was supplied with multiple monitoring thermocouples to prove correct PWHT. 350 Hv seems to be as welded and not PWHT.

"Nobody expects the Spanish Inquisition!"

 

[Howardjames]

Yes, you are correct. However, The PWHT records can not be trusted now. The headers with stubs are new one fabricated in shop. During hydrostatic test, some leakages were found in stub weldment. The problem is how to check Other headers with stubs have PWHT issues or not. Can we check by surface hardness in each stub weld or other methods?

 

[metengr]

Portable hardness testing will indeed be useful to check for PWHT. As long as the method is qualified to ensure consistency and proper review of hardness data.

 

[Howardjames]

Yes, We use portable Leeb rebound hardness tester to measure the surface hardness of stub weldment in header. However the measured hardness readings were varied from 150 Hb to 250Hb by different person at same position. I think the readings strongly affected by man's skill and the topography of weldment...etc.. Some hardness tester are measure by ultrasonic. Is ultrasonic type of tester more accurate than rebound type tester?

 

[metengr]

Howardjames
You are going to have some variability and surface preparation is critical. I would stay with the rebound tester and improve your test method (preparation and training). Keep in mind the variability could be the PWHT exposure range where the tempering was not fully achieved in certain locations due to temperature profiles. Again, the data should be reviewed by competent individuals.

 

[Howardjames]

We grid the surface up to #600. However the physical shape of stub tube weldment on header are complex due to 5 weld passes. Actually we also pick up high readings without respect to average reagings.

 

[Howardjames]

What is your opinion about root cause of leakage in stub tube weldment during hydrostatic test. Improper fillers(80 or 90series) have different preheat temperature. It sounds inadequate preheat temperature that 90 series and 80 series mixed passes in weldment only by preheat to 121C. Is improper filler used or PWHT issues the root cause of stub tube weldment leakage?

 

[metengr]

You have several factors that can lead to your problems of cracking stub tube to header weldments on hydrostatic testing - lack of preheat during fabrication, delayed cracking from hydrogen introduced during welding, and poor control of PWHT profiles. Attempting to zero in on one or more of the above as a root cause with the information you provided is difficult at best.

 

[weldstan]

Were the weldments heat treated in a furnace? I have seen a few, similar cases when PWHT was fully applied with resistance heating coils with poor T/C placement and preheat was inadequate. Poor choice of T/C placement if the item was furnace heat treated can also lead to your problrm.

Also I assume the electrodes were E8016-B2 and E9016-B3, which would require appropriate qualification.

 

[Howardjames]

Stub tube with header were heat treated in furnace. I wonder only 30 c in different of preheat tempature, 80 series preheat to 121c and 90 series need 30 c more, will result in serious delay cracking. It is difficult to tell preheat issue or PWHT issue is root cause of crack.

 

[weldstan]

How was preheat maintained? By torch? Actual preheat temp may have been much less. Had a similar case on a fired, P22 alloy heater where the manufacturer preheated the tube to header welds with a torch when the ambient shop temperature was less than 30 F. By the time the welder struck an arc the temp was closer to 70 F. After PWHT in a furnace, hardness exceeded 325 BHN. Had to PWHT twice more in the field reduce the weld hardness below 237 BHN to meet the contract.