Issue with tensile test of 61HRC tool steel 3

[Jeff37]

Hi All,

I have a strange requirement to tensile test a tool steel at 61HRC with spec limits of 2700MPA UTS, 1% elong & 2% ROA and it's not going well and our lab has no experience of testing material this hard. To start with, the threads on the tensile piece are fracturing, making it very hard to get a sensible stress strain curve, which is making me think that tapered samples are a must for this sort of test. When we do manage to coax the machine into running the test, it's fracturing in a brittle manner well below the requird UTS, typically at the taper where the tensile gauge length meets the threaded portion, and often in two places at once. This is making me think that the issue here could be the sample rather than the material, and we should be looking at the tensile specimen (surface finish / tensile geometry etc.). Does anyone have any experience tensile testing things this hard? Are we missing a trick?

Thanks for any help.

 

[Screwman1]

Your specimen needs to be totally free of surface defects to reduce the impact of notch sensitivity at that hardness level. It will be difficult to get repeatable results trying to pull tensile at Rc61.
You may have to come up with a larger thread size to reduce the thread stress level and prevent the cracking.
Who designs a tensile loaded part up into the brittle fracture zone? I hope I don't ever have to be around this part when it is in service.

 

[swall]

When we tensile tested heat treated 300M bars, we used button head bars rather than threaded. Of course, you need to obtain the appropriate grips.

 

[CoryPad]

Move away from threads, make sure to have spherical seating. The attached image is from ASTM E8.

 

[jwhit]

I'm in agreement with button head or spherically seated bars, also, any little nick is going to be a stress raiser, so I would recommend a ground and polished finish. Visually check for any defects or surface anomaly that might influence your test. Put mildly the requirement does seem kind of unusual for material at this hardness level.

 

[mrfailure]

For something this hard, would it be better to use waterjet machining to cut out flat dogbone samples?

 

[CoryPad]

No, flat specimens have sharp corners which act as stress raisers. It is much better to use cylindrical specimens.

 

[Jeff37]

Thanks guys. Don't worry Screwman. It's not used in tension - just a hoop we have to jump through to certify it.

 

[EdStainless]

ground and polished samples.
either spherical ends or flat button ends in gimbaled mounts.
And I hope that your machine is stiff, my hunch is that if fracture is over 50% of your rated machine load then you are probably bending the sample.

We used to test high tensile wire, we had to use optical strain gages because the elongations were so small.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[tbuelna]

Are the threads produced prior to quenching the specimen? If so, did you perform any NDI on the specimen to check for cracks in the thread roots after HT? You might consider form grinding the threads after heat treat to preclude the possibility of quench cracks forming in the thread roots.