Reduction of temper embrittlement of 2·25Cr–1Mo steels by rare earth additions

An investigation has been conducted into the suppression of temper embrittlement of 2·25Cr–1 steels by the addition of rare earth elements to scavenge phosphorus, which is the commonest embrittling impurity in these steels. Fracture surfaces were examined by scanning electron microscopy and Auger electron spectroscopy, and the compositions of inclusions determined by energydispersive spectroscopy and Auger electron spectroscopy. It was found that the shift in Charpy V-notch transition temperature after step cooling was indeed smaller in rare earth containing alloys. Hmrever, in the re-toughened condition, the Charpy V-notch upper-shelf energy was lower and the transition temperature higher in these alloys than in rare earth free alloys. This correlates with the increase in the volume fraction of inclusions caused by rare earth additions. The presence of rare earths, and occasionally phosphorus, in many of the inclusions was confirmed.