Low temper embrittlement and brittle-to-ductile transition in 0.2 mass% carbon steels

Low temperature temper embrittlement of low carbon steels was investigated. 0.2 mass % carbon steels with fully martensitic structure were employed. The specimen was tempered at 473 K, 623 K and 873 K for 7.2 ks. Impact absorbed energy tested at room temperature shows the lowest value for the specimen tempered at 623 K while the value of Vickers hardness decreases with the increasing in the temperature tempered. Temperature dependence of impact absorbed energy was also measured, exhibiting the brittle-to-ductile transition temperature was highest in the specimen tempered at 623 K, which is low temperature temper embrittlement. Inter-granular fracture was observed in the specimens tempered at 623 K and 873 K, which indicates that tempering at not less than 623 K tends to increase the BDT temperature due to the inter-granular fracture. The temperature dependence of yield stress was also measured, showing that the athermal stress in the specimen tempered at 873 K was drastically decreased, which tends to decrease the BDT temperature. These opposite temperature dependences against BDT temperature induce the low temper embrittlment.