Kok Ming,
It's been my experience time and again when trouble-shooting SCC in nickle-base materials there are three common denominators that seem to always come to the surface of the sensitization formula.
1. The weld interpass temperature of the WPS was not adhered to.
It should never be allowed higher then 300 degrees F during the weld application with a rapid cooling rate being desired. Beware, many times in reviewing nickle-base WPS's for approval, I've found the noted interpass temperature is well above 300 (usually 500) and the fact is missed by many reviewers. The interpass temperature is as critical to the success fo the weld as the proper filler material when welding nickle-based materials and if the noted interpass temperature is too high in the WPS, you are in trouble before you start.
2. Welders using too slow of a travel speed with a weave bead (carrying a large puddle) which promotes carbide precipitation while holding the interpass temperature too high for too long. A travel speed (regardless of the technique) that is just slow enough to facilitate complete fusion is what I've experienced will go a long way in helping minimize sensitization.
3. If the weldment contains base material that has been in service prior to welding- Insufficent cleaning is the third common denominator. Many think that all that is necessary for adequate cleaning is to grind, flapper wheel, or wire brush the surfaces or maybe use a chlorinated solvent. If the material has been in service none of the methods used alone is correct. The use of a chlorinated solvent should never be used-acetone is much better then grind. Again, if the materials have been in service, most people forget to clean the backside of the joint as well as they cleaned the side they are about to weld on.
To put the first two conditons in perspective and give insight to why they can happen. Welders know that regardless of the process, the hotter (to a point) the nickle-base material is, the better it welds for them! The puddle flattens and wets out nicely, they get better starts & stops, and the finished weld is cosmeticly appealing. But In short, what's good for the welder is not good for nickle-base material!
Hope I've been able to help.