Baffling crack in stainless overlay on mild steel turbine shaft 1

[calculor]

Approximately 4 months ago we prepared a WPDS for replacement of a stainless overlay on a turbine shaft which consisted of 1035 mild steel. Based on Lincon's recommendation we used a 410 stainless steel. The shaft was machined to remove the existing overlay and an MPI was completed to indentify any cracking or inclusions. The procedure required preheating the base to approximately 500F and the overlay was applied about the circumference of the shaft using SAW method, the overlay is between .5" to .75" thick. The firm that completed the work did not compile any QA reports noting interpass temperature, electrode speed etc. The overlay was completed approximately a month and half ago and recently the shaft was delivered to the site and partially installed in the vertical direction.Approximately 3 days ago a crack was observed in the overlay, the crack was approximately .25" thk and extended the full length of the overlay on the shaft. Has anybody encountered a situation similar to this? Does anybody know why this crack may have occurred at this point in time? If this crack was attributed to residual stress would we not have seen the crack within 2-3 days after completion of the procedure? Incidently the fabricator insists he has followed the WPDS to the letter.Appreciate any information.

 

[johnc01]

If the contractor that completed the work insists that he followed procedure, then i would respectfully suggest that you insist that he demonstrates compliance to approved procedure by objective documented evidence. Simply saying verbally, i followed procedure without having any documentation to substantiate that claim does not carry any validity.I find it extraordinary in this day and age of ISO 9000 and the emphasis placed on quality systems/quality management and the importance of traceability that you can find a company who does not have some sort of a quality system in place.
My own view is that if he cannot or will not produce documentation to substantiate his claim, then i believe there is a distinct possibility that they did not follow the procedure and that is the reason why they cannot produce documentation, and if they did not follow procedure then chances are that is why you had the failure.

Hopefully one of our more experienced welding or metallurgical engineers on this page can give an opinion as to the criticality of not applying preheat, incorrect interpass temperature, incorrect welding wire,incorrect wire speed etc and to how that may relate to the weld crack.

I would suggest that any repairs that are to be done, be done with an approved repair procedure, coupled with an inspection and test plan where you can put hold points to allow your company to witness procedure being complied with, that way at least you will not have to take only their word that they complied with procedure

 

Not sure of your service requirements but the choice of 410 ss for the overlay material was a bad choice. Need more information to determine root cause of cracking but expect hydrogen induced from poor control of welding flux.
Bob

 

[Rich2001]

I have to agree with the above comments. My first thought is that hydrogen induced cracking is what occured.

The flux used in SAW, is not called Crack in a Bag for nothing. Examine the control of the flux, how is it handled, stored, temperature, etc. you might find your answer there.

With the high C, Cr and Mn content of the materials used, cracking does not come unexpectedly.

 

[PARRON]

This is an hydrogen crack problem. I'm agree with the above opinions. After weld, keeping the preheat temperature, you should perform a soacking heat treatment at 300ºC per 8 hs. By very careful with the cooling rate, no more than 40ºC/h. The NDT shall be performed 72 hs after cooling down. MT + UT is the better method

 

[dar]

PREHEAT, INTERPASS, POSTWELD SOAK, PROPER CARE OF FLUX TO AVOID MOISTURE, CORRECT FILLER METAL AND SLOW CONTROLED COOLING RATE. These are all areas where if not properly executed can result in cracking. I also agree that a delayed NDT inspection is essential. Investigate these areas and assure they are followed during repair.