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H900 vs H950 difference in Results

We are a medical device manufacturer and have a condition where an instrument made out of 455C was heat treated to H900 and 50 HRC. The original spec was H950 and 48 HRC. Could this small difference make the metal brittle?

Steve Brown
Manufacturer - Logan, UT, USA
November 10, 2009

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First of two simultaneous responses --

I doubt if it would make a difference in most, but not all, applications. I do not have books with me, but foggy memory says that heating the parts at 450F for two hours with a slow cooldown should drop your Rc by 2 points.

James Watts
- Navarre, Florida
November 11, 2009

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Second of two simultaneous responses --

Marginally, in the same way that the hardness is marginally affected.

There is no technical reason why the parts cannot now be heat treated to H950. The numbers "900" and "950" refer only to the temperature in degrees F at which the parts were tempered. Because they were mistakenly tempered at too low a temperature, it is simple to re-temper them at the correct temperature.

Note that if instead the parts had been specified at H900 but were mistakenly done at 950, then you would have a problem. Under-tempering is easily fixed by further tempering; over-tempering can only be fixed by going back and repeating the hardening cycle then tempering at the correct temperature.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
November 11, 2009