TEMPERED MARTENSITE EMBRITTLEMENT AND FRACTURE TOUGHNESS IN 4340 STEEL

Abstract

Abstract : Tempered martensite embrittlement (500 F embrittlement) was studied in 4340 steel by means of Charpy impact, tension, and fracture toughness tests carried out over a range of test temperatures. Embrittlement was shown in the impact tests by a minimum in room temperature impact properties for tempering temperatures ranging from 500 to 650 F, the same range for which the transition temperature is a maximum. No evidence of embrittlement was found in tension or room temperature fracture toughness tests. Embrittlement was noted, however, in fracture toughness tests carried out at -50 and -100 F, which indicates that low temperature testing will be necessary for proper materials evaluation. The plane strain fracture toughness (K sub Ic) of various heats of 4340 steel has been correlated with the weight percent sulfur and phosphorus in the steel. A mechanism for tempered martensite embrittlement is proposed. Certain impurity elements, such as phosphorus, which are more soluble in ferrite than in cementite, will segregate in the ferrite adjacent to the cementite shortly after the cementite precipitation. This transient enrichment of ferrite by impurity elements will be embrittling when the cementite is in a platelet or filmy form, and particularly so in the region of the prior austenite grain boundaries, where the impurity content may be higher than average.