GRAIN-BOUNDARY EMBRITTLEMENT IN IRON-ALLOYS, AN AES AND AP-FIM STUDY

The role of chromium in the intergranular fracture of high purity Fe-0 .2%P-Cr alloys containing small amounts of carbon was studied. The add ition of chromium to Fe-0.2%P alloys containing carbon less than its s olubility limit exerts no effect on grain boundary segregation of phos phorus, but reduces the susceptibility to intergranular fracture. This may be attributed to an increased grain boundary cohesion due to the segregation of chromium. On the other hand, chromium addition enhances grain boundary brittleness of Fe-0.2%P alloys which contain higher ca rbon exceeding its solubility limits. In these alloys grain boundary c ohesion will be decreased by a reduction in the degree of segregation of both chromium and carbon due to the precipitation of chromium-rich carbide in addition to an increase in the degree of segregation of pho sphorus along grain boundaries, Atom Probe FIM analyses about lath bou ndaries of tempered martensite in a Mn-P steel have revealed the coexi stence of manganese and phosphorus along lath boundaries, which strong ly indicate the promoting effect of manganese on the occurrence of tem per embrittlement. On the other hand, in a Mo-P steel lath boundaries are occupied by molybdenum and carbon atoms, and phosphorus atoms are not detected from lath boundaries but they are always detected in the matrix, indicating a suppressing effect of molybdenum.