Material with minimal work hardening

[oneeyedwitchdoctor]

Hello

I am looking for a material that will fail without work hardening during operation. The application is a failsafe piece that can break to prevent damage to other more expensive equipment. The operation is fairly rough and ready with shock loading and potential impact. Based on that the "weak point" is designed to fail in tension I thought a more ductile failure was preferred compared to shearing a pin or the like. The problem is that the current weak point is work hardening by 70-80%. The material is AISI 4140 18-22 HRC. Does anybody have any recommendations for a material that will work harden less? Space is not an issue so if the material is not as strong as 4140 I can just design it with a larger cross sectional area. Some other relevant information is that the operating temperature can be up to 177C and there may be some nasty chemicals present, such as H2S.

Thanks in advance

 

[EdStainless]

Steels work harden the least of the common engineering materials.
In general the more ductile a material is the more that it will strain harden.

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Plymouth Tube

 

[CoryPad]

4140 at 20 HRC meets NACE guidelines for sour service but has been so overtempered that it strain hardens a lot. If you need to meet NACE guidelines, then you may try stainless steel 17-4 H1150 which would be better. If your budget allows, titanium alloy Beta C or the Ni-Co-Cr-Mo alloy MP35N offer essentially zero strain hardening.

 

[oneeyedwitchdoctor]

Thanks for the replies.

With respect to NACE, it is not a requirement, but the equipment could be used in a sour environment and it will be in tension so i am using NACE more as a guideline than a requirement. I think MP35N etc is on the expensive side for a part that we will break from time to time. It seems that as the design is at the moment (weak point failing in tension) I will need to use something that is either extremely expensive or does not satisfy NACE.

CoryPad, are you familiar with 4140 30-36 HRC and whether the UTS can be used to accurately determine an expected failure load. Or do you have any recommendations for non-NACE materials that will strain harden less that I could coat, or isolate from the sour environment (lower cost materials, bearing in mind that this is a disposable part for temporary use only)?

 

[CoryPad]

Your example is a good one, that material and condition could be used for this application.