Critical level of intergranular fracture to affect the toughness of embrittled 2.25Cr-1 Mo steels

In general, the low-temperature brittle fracture mode of unembrittled ferritic steel is transgranular cleavage. During temper embrittlement, impurity elements, such as sulfur (S), phosphorus (P), antimony (Sb), arsenic (As), and tin (Sn), segregate to prior austenite grain boundaries, which results in a decrease in the grain boundary cohesive strength. As a result, the brittle transgranular cleavage fracture mode changes to intergranular decohesion in association with the decrease in the critical fracture stress (sigma(F)) as well as the fracture toughness (K). However, the appearance of intergranular facets on the fracture surface does not cause a decrease in the K and sigma(F) values. In this work, quenched and fully tempered 2.25Cr-1Mo steel (in an unembrittled condition that exhibits almost 100% brittle transgranular cleavage fracture) has been embrittled for 24, 96, and 210 h at 520 degreesC to produce different proportions of intergranular fracture. These unembrittled and embrittled steel specimens were tested to measure K (at -120 and -196 degreesC) and sigma(F) (at -196 degreesC). The experimental results and detailed fractographic observations show that the K and sigma(F) values decrease with an increase in the area fraction of intergranular fracture, provided that the area fraction of the intergranular facet on the brittle fracture surface exceeded a certain critical level, approximately 20-22%.