Tempered Martensite Embrittlement?

[amyh33]

How can I tell if the steel I am working with is susceptible to Tempered Martensite Embrittlement? According to ASM Metals Handbook Desk Reference, (online - Carbon and Alloy Steels; Service Characteristics of Carbon and Alloy Steels; Embrittlement of Steels) tempering in the region of 200 C - 400 C reduces toughness because of potential for tempered martensite embrittlement. The steel is 0.51% C, 1 % Cr, .93% Mn, .23% Si and is hardened to 60Rc on outside surface. This temper is after finish grinding and prior to chrome plating and is done to relieve residual stresses without impacting hardness. Several recommendations have been to temper 200 C for 1 hour per thickness of part, but after reading about the Tempered Martensite Embrittlement, I am hesitant to specify 200 C. How critical is the post-temper cooling rate?

 

[TVP]

If you temper at 200 +/- 5 C, you should not have any problems with tempered martensite embrittlement. This phenomenon is less critical for surface hardened components. Cooling rate from 200 C is not a significant process variable.

 

[unclesyd]

Do you give this material a bakeout after chrome plating?

 

[amyh33]

Thanks for your inputs. Yes, the plated parts are "baked" at approximately 200 C for hydrogen release soon after they are removed from the plating bath.

 

[unclesyd]

amyh33,
For your information:
Along with tempered martensite embrittlement you may also hear the term "Blue Brittleness". This occurs in some steels when in the temperature range of 400°F-700°F. It can also occur when the metal is cold and has been worked in the temperature range of 400°F-700°F.

Most of the steels used today don't have the problem.

The tempered martensite embrittlement usually is found by notched bar tests. The tendency to embrittle can be controlled to a great extent by heat treating procedures.