Dissimilar metal weld cracking in high temperature service

[Hang Suin]

Having to weld SS 304 tubes to SA-516 Gr 60 plate using E309 welding consumable, could anyone know which is the restriction to the highest temperature service since by my general knowledge I know that such a welding is vulnerable to cracking in high temperatures. Moreover I was wondering for what reason such a vulnerability occurs.

[wgadmin]

You could have a look to the ASME II, PART C, SFA-5.4, par. A7.10 E309, from which it is made clear that embrittlement or cracking can occur if such a dissimilar steel weld is to be subjected to a post weld heat treatment or to service above 370 deg C.

As far as the reason of such a vulnerability is concerned, it comes from two factors. The first one is the phenomenon of ''temperature-induced stress'' that happens because of the different coefficients of thermal expansion between ferritic structure (SA-516 Gr 60) and austenitic structure (SS 304, E309). The second factor is the great possibility of creep failure due to the carbon diffusion from the ferrite to the austenite. Such a diffusion occurs in high-temperature service and produces a carbon depleted low-alloy steel adjacent to the weldment making this area vulnerable to the creep.

[Ramin-Kondori]

Dear Hang Suin:

There are a few possible causes for this cracking:

If you see cracks in HAZ of SS-304 tubes, and your tubes are not of low carbon stainless steel (304L) then its because of carbide precipitation which leads to Cr-depletes spots and intergranular corrosion (or brittleness) and finally cracking. Cracks may be observed in weld metal too, that's because the carbon content of weld is high due to mixing with 516.

Sigma phase (chi or sigma) formation is another cause of cracking. These are intermetallic phases which are brittle.
475 embrittlement is another possible cause for cracking.

Solutions:

1) Discuss 304 issues with your designer or engineering department
2) Use 304L tubes instead of 304
3) Try to control the weld metal composition by welding process selection, joint design and welder technique. Less carbon steel in weld metal the better.
4) Low heat input is vital
5) No PWHT
6) Proper cleaning of weld and 304 HAZ can help (an especial local electro-chemical cleaning tool is available from Fronius - very intresting innovation)

If you could tell us more about the service conditions, we will be able to give more specific advise.

Regards
Ramin Kondori
IWE AT 0070