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Monday, 29 February 2016

Temper Brittleness (Embrittlement)

Embrittlement is a loss of ductility of a material, making it brittle. Various materials have different mechanisms of embrittlement. Hydrogen embrittlement is the effect of hydrogen absorption on some metals and alloys. It is more susceptible to BCC and HCP structured metals as compare to FCC structured metals. As little as 0.0001 weight percent of hydrogen can cause cracking in steel. 
  1. Alloy steels containing nickel, manganese, and chromium when cooled slowly from tempering temperature of about 350°C to 550°C becomes brittle in impact. However, they, usually show normal ductility in the standard tension test. If these steels are quenched in oil or water from the above temperature, they remain tough in impact. The embrittlement, produced during slow cooling may be due to the separation of some brittle phase. This phase may be soluble above 350°C and hence its separation suppresses during rapid cooling, eliminating the embrittling effect.
  2. Addition of about 0.5% molybdenum also eliminates temper embrittlement.
  3. Steels produced at about 350°C appear blue in color and hence the brittleness observed at 350°C is called Blue brittleness.

     

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