Abstract
Analysis of engineering steels by Auger electron spectroscopy has identified phosphorus, and to a lesser extent tin, as the impurities which commonly cause temper brittleness. Following a thorough thermochemical assessment of the feasibility of ‘gettering’ these residuals by forming stable microcompounds, the rare earth elements lanthanum and (to a lesser extent) cerium were selected as having the best promise because of their higher affinity for P and Sn than for the carbon of the steel.
Measurements on the ductile-brittle transition temperatures were carried out on two archetypal steels, 2¼Cr-1Mo and 3½Ni-Cr-Mo-V, in both high purity and commercial purity conditions. In the 2¼Cr-1Mo steel, P was shown to be very embrittling but Sn had little effect. The embrittling effect of as much as 0.02%P could be relieved by an addition of about 0.16%La. Concomitantly, there occurred the detrimental effect of a reduction of upper shelf energy in the ductile region. This reduction is thought to have arisen from the small laboratory-scale experiments and is not expected for industrial practice. The behaviour of the 3½Ni-Cr-Mo-V was similar to the 2¼Cr-1Mo except that both P and Sn were very embrittling, and the optimum La remedial additions were slightly lower. In both types of steel the phosphorus and tin temper brittleness can be fully removed by an optimum lanthanum addition, given by La = 8.7S + 2.3Sn + 4.5P, where the element symbols express concentration in wt-%. Tests with mischmetal showed an equivalently effective remedial action.
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