We manufacture rollers from AISA6820 steel with a hardness of 28HRC.

These are machined then further hardened by nitriding to a desired 56 HRC.

The depth of Nitrided material is about 0.4mm.

When we test the rolls with our Rockwell machine (Using B and C) we are only obtaining a hardness of 28 i.e. the core material.

Our supplier tells us that the depth of hard material at 0.4mm is too thin and that they test with a "non invasive" method obtaining the correct 56.

I have my doubts as some of the rolls are breaking up on the surface.

Is Rockwell C a suitable method for testing a 0.4mm case hardened surface?

Thanks in advance.

Hi dunlaw

to the Surface Engineering Forum.

Rockwell C can not be used to give a true value of surface hardness for a nitrided case of 0.4 mm (0.016") effective or total depth as your supplier has indicated. Rockwell B should not be used for the same reason, but also this scale is not suitable at this hardness level. Rockwell C method would need a minimum effective case depth of around 0.75 mm to give true surface hardness values. See link Test Piece Minimum Thickness for Rockwell Hardness Test which should also give a rough guide for coating and case hardened thickness. If you want to use a Rockwell hardness test method, you are really limited to using the Rockwell Superficial scales HR-15-N and at a pinch HR-30-N.

To be honest, if your supplier is telling you only, that the nitrided case is 0.4 mm thick and 56 HRC (so-called equivalent of a "non invasive" test), I think is poor practice. This situation tends to reinforce my arguments from previous message threads (most recent link). If converted or equivalent hardness values are to be quoted (particularly where the parts can not be tested with the quoted method), they should be backed up with data from the original tests. 56 HRC equivalent seems to be a low surface hardness for a nitrided case or is this in fact the hardness value used to determine the effective case depth? (Example: 56 HRC equivalent at 0.4mm depth).

The best method is to use a Vickers or Knoop hardness traverse on the cross-section through the nitrided case into the core. This will give you a hardness/depth profile. Plotting a graph hardness verses depth, you can determine the effective case depth at a specified hardness (possibly say 620 HV (~ 56 HRC equiv) at 0.4 mm depth) and the total case depth where hardness levels off to that of the core.



Hope that helps.