HARDNESS TESTING FOR NITRIDED COMPONENT 1

[Gurjjeet]

Hello Everyone

We have a Vickers hardness tester(Some Chinese made). We are having trouble with checking a part from our supplier which is pre-hardened 4140 with Nitriding uoto 52-55 HRC & case depth of 0.5mm.

My first question is if we have the specs right? With relation to hardness vs case depth, is it achievable?

Our supplier says that using 5kg load will break into the case depth & will give us a lower reading. He is suggesting 0.5kg which is very hard to read given the diamond impression is very small. Please suggest what load should we use to check?

Thanks

 

[tbuelna]

Gurjjeet,

The 0.5mm depth of your nitrided case is definitely achievable with 4140, but the hardness at the case surface will likely be a bit harder than an Rc55 equivalent (thin cases like nitrided ones are usually specified with a superficial hardness scale like 15N in the Rockwell system). Nitrided cases are very hard at the surface but the hardness tends to fall off very quickly thru the case depth.

If all you want is a surface hardness, your Vickers tester should work OK. A 5kg load sounds appropriate.

Make sure the nitride white layer has been removed before testing.

Hope that helps.
Terry

 

[TVP]

Gurjjeet,

If the 4140 was previously quenched & tempered prior to nitriding, then a surface hardness measurement using HV5 should be no problem. As long as the core after quenching & tempering has a strength of ~ 800-1100 MPa, and the nitride depth (Nht, diffusion zone) is ~ 0.5 mm, then it should support even HV10. Lower loads like HV0.5 are usually only specified/required when a part is not hardened prior to nitriding, such as with low or medium carbon steel shafts. By the way, I agree with Terry that the surface hardness could be considerably higher than 52-55 HRC for 4140-- probably more like 58 minimum.