Abstract
Charpy type impact test, hardness measurement, optical and transmission electron microscopy were performed with Fe–P alloys (P, 0.05, 0.1, 0.5 and 1.0 mass%) containing various amount of carbon (0.001 to 0.18 mass%). The fracture surface was examined by Auger electron spectroscopy to measure the degree of segregation of phosphorus and carbon. Specimens were quenched into oil from 1123 K or 1173 K.
Phosphorus increases the ductile-brittle transition temperature both in intergranular and transgranular fracture mode, and carbon tends to suppress the intergranular fracture. The optimum carbon concentration to improve the toughness is 0.01%. Carbon above this concentration causes the formation of pearlite nodules during quenching and the toughness decreases.
Discussion was made on the mechanisms proposed to explain the effect of carbon to suppress the intergranular fracture. Any theory which assumes an interaction between carbon and phosphorus atoms at grain boundaries seems to have difficulties. Although more researches are needed, the hypothesis considering carbon by itself to improve the grain boundary cohesion seems to be promising.
