Yield strength change in welded heat affected zone! 1

[Theanalyst]

Hello, I have a part that has three metal sheets made of 1075 steel laser welded together on the outer edges to form an enclosed volume of air that is compressible to take care of some pressure waves in a tube and regulate the dynamics. The question I have is how does the yield strength and poisson's ratio of the standard 1075 steel change in the heat affected zone of the weld? I am not sure what the melting temperature of this steel is that needs to be acheived to fuse the molten metal together. But the metal in the weld zone does get very hot and once it cools down, it becomes very brittle and hard, because of untempered martensite. Any help would be greatly appreciated. Thanks.

 

[Theanalyst]

Actually, I meant how do the Young's modulus and poisson's ratio change. Sorry about that. Thanks.

 

[EElmore]

Is this really the question you wanted to ask?

If you have to make bellows out of 1075 (shakes head) use a diffused pre-pass to preheat the area before the weld so that the weld metal and austinitized heat affected zone do not quench too fast.
Or, if you are going for production rate and have money for tooling you could build a resistance coil into your backup tooling to warm the part up before welding.

This will allow you to get the bellows together, then you still have to austinitize, quench and temper the parts to get the ductility and fatigue strength you need.

If you can not subsequently heat treat the assembly, run a couple of low power postweld laser passes to temper the weld and heat affected zone.

Life would be simpler if you could switch to a precipitation hardenable alloy such as A286.

 

[Theanalyst]

Hello, yes.. that was the question I intended to ask EElmore. I am simulating the assembly loading conditions for doing a stress analysis because we have had these 'bellows' failing left and right and some might have lasted for a maximum of 15mins after loading. The cyclic peak to peak pressure variation that these bellows have to handle is about 60 to 100PSI. Pressures applied in the failed cases have been about 600 to 1000PSI. I need to use separate material properties, mainly E and nu, for my linear static FEA study and thats the reason for the post. The air has to be sealed inside so there really is no other way than welding (can use O rings but rubber thats a mess) I need to come up with a design that moves the stress far away from the weld and be able to survive pressures varying anywhere from 1000 to maybe 3500PSI. Anyway, any more input would be greatly appreciated. If you have any more questions...shoot. Thanks, Sunny.

 

[EdDanzer]

From my experience designing highly stressed welded parts, your failure is from the material choice, not the welding process. High carbon steels are the worst choice in materials to weld. Rather than focus in FEA for the answer, have a failure analysis done on the material. Hardness should not change Young's modulus and Poisson's ratio enough to effect analysis.