why Surface is harder than core but no carburization? 1

[ht3jyx]

we did some normal thru hardening job on 8740. the surface is 1 or 2 point (RC) harder than core. microscope shows no sign of carburization. why this happen? what we can do?

thanks

 

[metengr]

The 1 or 2 HRc point spread in hardness is not that significant, in fact it can be within the margin of error in hardness testing.

How many hardness traverses did you perform for actual comparisons? Also, the surface will still cool more rapidly in comparison to the core for thru-hardening steels. If what you report is correct, it could simply be that the martensite adjacent to the surface contained less retained austenite, in comparison to the martensite formed toward the core.

 

[TVP]

I would tend toward the notion that the core is less hard than the surface due to less complete transformation, i.e. the surface is 100% martensite and the core is only ~ 90% martensite (other 10% may be bainite, ferrite, etc.). 8740 does not have that great of hardenability, and should not have much if any retained austenite. I also agree with metengr that hardness testing has sufficient variability that 1-2 HRC points on 1 sample is not uncommon. Sample preparation and test configuration can easily lead to amount of difference.

 

[ht3jyx]

understanding that 1 or 2 RC difference may be within tolerance. however we have had this variance constantly, i.e. surf is harder than core. therefore difficult to attribute the difference to test tolerance.

another thing is that the parts, washers, are only two coins' thickness. i am not sure if cooling transformation (i.e. surf cools faster than core) is an issue here.

any comments and ideas?

thanks

 

[unclesyd]

If you can get hold of the "End Quench Hardenability Curve" for 8740 you will see a couple of points different in thin sections.

 

[swall]

Is the hardness difference a problem for the product in question?