Testing Flat SS 30114: breaking out of the 2" gage length

[Christian62]

We test thousands of flat Stainless steels (>50 variations) and the only one that breaks out of the extensometer's 2" gage length are some types of 301 SS. They break at the beginning of the reduce section; where the necking or shoulder joins the centre part of the mechanical test sample. This is an annealed material.
Is there an explanation ?
Thank you.

 

[gdodd]

You may be work hardening the gauge length portion of the sample during sample preparation. Try a different process to manufacture the test piece or conduct microhardness tests on the edge or adjacent to it and comparing results with the region that is breaking.

 

[TVP]

301 undergoes rapid work hardening, so if you are introducing any deformation into the reduced section, as gdodd mentioned, then this could be the reason for the unwanted breakage. mcguire is an expert on stainless steel and probably can add more to this discussion.

 

[mcguire]

Thanks, TVP, I had missed this thread. You're right that this would be associated with 301's extremely high work hardening rate due to meta-stable austenite transforming to martensite in the gage area. The adjacent shoulder would be relatively constrained from plastic flow geometrically and the initiation of failure there would result from the relatively embrittling stress state not permitting the flow that strengthens the material in the gage section.
There is probably a subtle thermal component to the phenomenon also. The gage section would heat up with the deformation,also suppressing the strengthening transformation at the shoulder as heat flows there.
I have to admit I never saw this in our test labs, but maybe our technicians did.

 

[Christian62]

Well,

My reply is very late but... tada! I have (I think) the answer:
(Sorry it's a little long)
I did a LOT of tests (based on your info.. thank you) and after a lot of failures (our ration of good was low) I finally decided to change the tool (knife) that was machining our samples. I moved from Rapid steel knife to Carbide milling inserts. This change forced us to really increase the speed of rotation and lateral movement. The technician was now forced to be sharp and remove the carbides from the samples as soon as the mill was hitting the end of its run (the shoulder). Guess what? No more defect, guess again? This procedure works also with rapid steel blades..
So at the end of the run the tool were creating small defects that initiated for this type of steel some rapid failure at the shoulders. Since we stack 3/4" high of flat samples of different types, when we mill, the technician would not see any difference between the 301 and the others. But 301 doesn't react to these small defects like the others... ha ha!
So again thank you very much you are really good.

 

[TVP]

Christian62,

Thanks for the update. Just for your information, in English-speaking countries, we use the term HIGH SPEED STEEL (HSS) for tool steel alloys that contain Molybdenum, which are often used for milling and drilling.