Re: Rockwell Vs. UTS

Posted by Gordon England on May 11, 2003 at 18:40:42:

In Reply to: Re: Rockwell Vs. UTS posted by Michael Doyle on May 09, 2003 at 15:23:43:

Hi Michael

I would expect the 304 to be harder than the 420 in this situation. I would stress that conversion comparison between the various hardness test methods is approximate (as is conversion to UTS) and becomes even less reliable for materials other than low alloy steels or where the properties have been dramatically altered by say cold working or as a thermal spray coating.

This can be further complicated by whether the hardness test is measuring microhardness or macrohardness (particle hardness or bulk hardness) and whether the material is homogeneous or heterogeneous in structure. Defining what is micro or macro is grey area, but generally the larger the load and indentation, the better the result for macrohardness and bulk property. UTS comparison in most tables are derived from comparative tests with brinell hardness on steel (best for bulk property). So you can see converting from rockwell C to Brinell then to UTS particularly for unlike materials will introduce even larger errors.

Wear resistance (abrasive) has a relationship with hardness. Again the correlation can be good between some very similar materials like carbon steels, but not necessarily when comparing different materials. Wear resistance is a very complex property with many different modes of operation.

Extreme Comparison:

Hardened steel 55HRC, 275 kpsi UTS, relatively homogeneous.

Thermal spray coating 13Cr steel, 28 HRC (equiv), 40 kpsi (~10kpsi transverse), relatively very heterogeneous, weak particle boundaries, hard and soft particulates, somewhat porous.

In many applications the coating shows equal or superior wear resistance.

As you can see these are complex relationships. Relating them to how a material is finally going to perform in service is not straight forward. Measuring properties like hardness, UTS, Young's Modulus etc.... provide useful guides and in specific materials/applications can provide the overriding factor in predicting performance, but invariably this can not be applied universally to other materials/applications. Sorry, I'm rambling on!

Are you looking at this subject for academic interest or for solving a specific technical problem?

Regards Gordon

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