The role of molybdenum in reducing temper embrittlement in steels.

The effect of two additions of molybdenum (2.5% and 5%) on the brittleness of an alloy based on Fe-8%Mn has been investigated. The intergranular brittleness exhibited by the as-quenched base alloy was eliminated by the addition of 2.5Mo possibly due to the interaction of molybdenum with embrittling agents Mn and N and the subsequent increase in solid solubility of these elements. A further addition of molybdenum did not significantly increase the toughness of the as-quenched Fe-8Mn-2.5Mo alloy in accordance with the theory of a limiting amount of molybdenum that may combat embrittlement. Aging quenched alloys at 450°C gave rise to an increase in DBTT and a change in brittle fracture mode to intergranular. The effect was most severe in the alloy with higher molybdenum (5%) due to precipitation of second phase particles both increasing the hardness of the matrix dramatically and denuding the matrix of molybdenum that would otherwise be free to combat embrittlement. A buildup of Mn and N at prior austenite grain boundaries was detected by AES, another factor contributing to embrittlement on aging. Aging quenched alloys at 525°C led to a more rapid change from cleavage to intergranular fracture for both molybdenum additions but toughening of the alloy containing the lower molybdenum content (2.5%). Although grain boundary segregation in this alloy meant that the preferred crack path for brittle fracture changed from transgranular to intergranular on aging, the buildup of reverted austenite at prior austenite grain boundaries acted as a crack blunter, slowing down crack propagation and thus increasing overall toughness. Embrittlement in the alloy containing 5% Mo was very severe due to more rapid precipitation. An investigation was also carried out on the aging of a commercial alloy based on 0.25C, 1.4Mn, 0.5Mo, 0.6Ni, 0.3Si doped with 0.4%Cu in one instance and 0.04%P in another. Phosphorus was seen to characteristically induce embrittlement on aging at 450°C and 525°C with an increase in the DBTT and a change in the brittle fracture mode from cleavage to intergranular. Aging of the base alloy at 450°C was also seen to be an embrittling treatment but the effects were not as severe as in the alloy containing phosphorus. The addition of copper is seen to retard this embrittlement on aging due to its grain refining ability.