What is the Correct Test Method for Case Hardened Steel

[dunlaw]

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.

[Gordon]

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.

[mjdrogan]

The information above is very useful and relevant to me, but are there any BS EN ISO standards that define the thickness of the case hardening and what hardness tests should be used.

From what you have said and what I already believed Rockwell C is not suitable for 0.35mm nitrided steel surface hardness tests. instead Vickers hardness at 10kgf is far more suitable.

[Gordon]

Hi mjdrogan

to the Surface Engineering Forum.

Quote:The information above is very useful and relevant to me, but are there any BS EN ISO standards that define the thickness of the case hardening and what hardness tests should be used.

Not sure any of these standards would be particularly helpful in answering your specific question. Usually, this is driven more by customer requirements, part function and materials.

Total case depth I would define as the point/depth where the diffusion element (carbon, nitrogen, etc. depending on process) becomes equal to the core/original material. This may be indicated/specified/measured by:
* hardness traverse - the point where hardness becomes equal to core hardness
* metallography - the point where micro-structural changes equal the core.
* stepped layer chemical analysis

Effective case depth I would define as the point where the diffusion element or hardness reaches a certain specified minimum value.

* hardness traverse - point where minimum specified hardness is achieved.
* metallography - point where specified structural changes occur
* Fractured cross-section
* stepped layer chemical analysis

Value to use to determine effective case depth is usually chosen by customer requirements, part function, materials used and ultimate surface hardness.

Quote:From what you have said and what I already believed Rockwell C is not suitable for 0.35mm nitrided steel surface hardness tests. instead Vickers hardness at 10kgf is far more suitable.

Hardness traverses on cross-section is best done with low load Vickers hardness test or better with micro-hardness test with Vickers or Knoop.

Surface hardness test again Vickers or Knoop. Select load best suited to case depth. Slow and labour intensive as surface requires good quality finishing/polishing. Rockwell and Superficial Rockwell, select scale/load to suit case depth. Fast, simple, better for higher volume (if there is sufficient depth of hardening to support test).