Rockwell Hardness Testing Question

I have a Rockwell C testing station and the calibration disc is right on.

Dealing with a disparity with the same product and trying to determine the hardness between an old lot and a newer lot. When I test the old lot the minor load goes to around zero and then when I come back with the major load it reads 15-18 C.
When I try to measure the new lot the indicator goes beyond zero starting around the dial again and when I apply the major load it reads 80, which of course its not. It has never actually reached the zero mark a part from the first load stopped at. How can something be less than zero?

Am I using the wrong scale for this.

jfbock said:

I have a Rockwell C testing station and the calibration disc is right on.

Dealing with a disparity with the same product and trying to determine the hardness between an old lot and a newer lot. When I test the old lot the minor load goes to around zero and then when I come back with the major load it reads 15-18 C.
When I try to measure the new lot the indicator goes beyond zero starting around the dial again and when I apply the major load it reads 80, which of course its not. It has never actually reached the zero mark a part from the first load stopped at. How can something be less than zero?

Am I using the wrong scale for this.

Click to expand...

Can someone with hardness testing experience help?

Thank you very much.

Stijloor.

What is the material? Its been awhile for my hardness testing expertise, but 15-18C is pretty low. Could be your second sample is even softer and off the scale.

Its steel, a formed ring.

JF - a few more details would help - what make and model of Rockwell tester, what grade of steel is the steel ring made of, what is its condition - annealed, as hot rolled, heat treated, etc.

As Tom says, 15 to 18 Rockwell C is low. At that C hardness level, I'd expect to be using HRBW scale.

My best guess with the info currently provided is that your new material is too soft to be tested on the C scale.

Agree with the others. This lot is definately softer than the old lot and that explains why the needle is going so far around on the initial loading.
If I were you I'd get a piece of material before heat treat and perform the exact same test. My guess is that it will do almost the exact same thing as the new lot.
Then I'd suggest doing a test of both the old and new lot's on a B scale.

One more thing, depending on the type of hardening this is (full vs case) there could be a problem with the case thickness. I fthe new lot is not thick enough the C penetrator may be punching right through.

Caveat - It's been a while since I've done any hardness testing, but have run into these problems in the past so I think I'm on firm ground here.

Peace
James

Might also want to check the steel chemistries to see if there is any evidence there of a source of the issue.

jfbock said:

I have a Rockwell C testing station and the calibration disc is right on.

Dealing with a disparity with the same product and trying to determine the hardness between an old lot and a newer lot. When I test the old lot the minor load goes to around zero and then when I come back with the major load it reads 15-18 C.
When I try to measure the new lot the indicator goes beyond zero starting around the dial again and when I apply the major load it reads 80, which of course its not. It has never actually reached the zero mark a part from the first load stopped at. How can something be less than zero?

Am I using the wrong scale for this.

Click to expand...

Yes, you might be using the wrong scale as 15HRC is pretty soft but none the less, it is on the ASTM E18 Rockwell C chart. My Mitutoyo chart only goes down to 20HRC and , in my opinion, for a good reason as readings that low tend to have more uncertainty. I would attempt the HRA scale (same setup as HRC but with 60kg load) HRA scale's mid range starts approximately where the HRC scale ends (around HRC15-20) your product should be in the HRA60s to HRA70s which is well within the middle range of the HRA scale. There is no correlation/conversion back to HRC at that range so, if the product is consistently that soft, I would switch to HRA.

My above suggestion is based on the assumption that there was absolutely no dirt between your failed (very soft) sample and the anvil and that the base of your anvil is rust/dirt-free and properly seated.

Hope this helps

Very helpful.. Thanks everyone. I am actually trying to get the customer to give me spec for the parts so I am just trying to prove one is softer than the other at this point.

Thanks again.

Hello,

In going through this thread, i made a couple of notes that will enable us to help you some more. It is only prudent that i mention now that i am not that familiar with the ASTM standards as we dont use them, but rather ISO. I am however pretty sure that the ASTM and ISO standards must be fairly (If not exactly) the same as ASTM.

So here goes:
1. What tester are you using? is this an auto 0 tester?
2. What is the hardness of your test block? you may have a had test block >50HRC, and that could very well read perfectly with a chipped diamond indenter but when you start testing things in the lower scale >20 <35 you may encounter the chip due to the depth of penetration of the diamond, and that could skew your readings as well.
3. According to ISO you cannot read reliable below 20HRC, and it is recommended to change to HRB (to be correct HRBW the W denoting a Tungsten Carbide Ball indenter)
4. Another factor that could also cause some havoc in your readings are the surface roughness of the components. are they both finished?
5. if all these are not causing the problems then send us a better description of your components in terms of sizing and thickness, as all thickness as well as the curvature of the component that you are testing.
5. as a previous poster wrote, you could go the HRA scale if you expect the survace hardening to be very thin. the other alternative is to go for superficial rockwell testing (HRN ranges). does your machine have the capabilty to perform superficial testing?

that is all for now...
Anton.