Temper embrittlement was studied using laboratory heats of 2 1/4 Cr-1 Mo steel of a specified hardness and grain size. Particular amounts of P, Sn, Sb, Mn, and Si were added in various combinations. Charpy impact and hardness tests were performed and scanning electron microscopy, and Auger microprobe investigations were conducted. Specimens with both martensitic and bainitic microstructures were embrittled by step cooling and by long time ageing at 480 and 520°C prior to study. Only steels which contained added P showed any consistent and significant embrittlement. While neither Mn nor Si themselves acted as embrittling elements, they both strongly promoted P segregation and thus acted to raise the embrittlement susceptibility markedly. Evidence of the previously discovered [reference [7]] thermal history effect (due to reduction of the free-Mo content by carbide formation) on the embrittlement kinetics was obtained. The results on the bainitic and martensitic steels were essentially similar, the only difference being that the bainitic steels had higher transition temperatures in the unembrittled conditions. The 2 1/4 Cr-1 Mo steel can have a very low susceptibility to temper embrittlement if the P, Mn, and Si contents are kept sufficiently low.

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