Critical level of intergranular fracture to affect the toughness of embrittled 2.25Cr-1Mo steels |
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Authors: | Islam M A Knott J F Bowen P |
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Affiliation: | (1) Materials and Metallurgical Engineering Department, Bangladesh University of Engineering and Technology (BUET), 1000 Dhaka, Bangladesh;(2) School of Metallurgy and Materials, The University of Birmingham, B15 2TT Edgbaston Birmingham, UK |
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Abstract: | 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 (σ
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 σ
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 °C to produce different proportions
of intergranular fracture. These unembrittled and embrittled steel specimens were tested to measure K (at −120 and −196 °C) and σ
F (at −196 °C). The experimental results and detailed fractographic observations show that the K and σ
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%. |
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Keywords: | brittle fracture embrittlement fracture stress fracture toughness segregation |
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