Impurity element effects on the thoughness of 9Cr-1Mo steel

The effects of the trace impurity elements, phosphorus, sulfur, and silicon, on the impact toughness of a 9Cr-1Mo steel were investigated. The results of Charpy impact tests showed that the additions of all three elements produced a decrease in the upper shelf energy and an increase in the ductile-brittle transition temperature (DBTT). Above 0.04% P and 0.4% Si, the upper shelf energy decreases sharply. Additions of over 0.03% P or 0.4% Si cause an increase in the DBTT. Trace amounts of sulfur were found to have a much greater detrimental effect than phosphorus. Impurity segregation and delta ferrite content are used to explain the decrease in toughness produced by the addition of P and Si, respectively. Sulfur promotes non-metallic inclusions and intergranular fracture thus producing embrittlement. (orig.)