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Carbon steel case hardening color

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Nashanas

Nashanas

Petroleum
Joined
Apr 23, 2021
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54
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NL
Hello,

I have a question about the physical appearance of carbon steel after it is hardened with inudction hardening vs carburising. The steel grade is C45.

The reason for question is we have parts with indication of induction hardening to 52 HRC on design. I control it with a portable hardness tester, but one of my colleagues was saying that it is enough to see the part visually, if it appears balck and the black layer can not be removed by abrasive paper it is hardened, if the black layer goes away by abrasive paper it is not hardened enough.

I find this a bit strange criteria for QC of hardening process. Maybe he is confusing it with carburising process?
 
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It is actually not that strange, it's just not very quantifiable.

I used to do a lot of macro etching on steel welds and I could pretty reliably tell you which pipe had the higher carbon content by how dark the HAZ etched.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
 
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The problem with induction is that changes in weather (temp and humidity) will change the appearance of the surface.
And carb colors can change with the cool down process which has no impact on the actual case.
It might work for a short time span, but I wouldn't trust it overall.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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Nashanas
it depends on the requirement of the product.
commercial or aerospace , makes a difference.
for aerospace requires hardness testing
edit also add if required for safety reasons, under loads
visual will not suffice.
 
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