Arc Welding of Specific Steels and Cast Irons

Arc Welding of Heat-Resistant Low-Alloy Steel
2. Welding metallurgy and weldability
Heat-resistant low-alloy steels are weldable but to a lesser degree than ordinary steels
because of greater self-hardenability and crack susceptibility. The following sections discuss
these problems and preventive measures.
2.1 Hardenability of welds
Alloying with chromium and molybdenum causes such greater hardenability that the steel
can be hardened by cooling in still air from its austenitizing temperature. This hardenability
is often called “self-hardenability.” The degree of self-hardenability of Cr-Mo steels can be
presumed with the Time-Temperature-Transformation (TTT) Curves. Fig. 2.1 shows a TTT
curve for 2.25Cr-lMo steel, while Fig. 2.2 shows a TTT curve for 5Cr-0.5Mo steel. The two
cooling curves shown with 1 and 2 in these figures refer to the lowest and the highest
cooling rates in common arc welding. Fig. 2.1 clearly shows that the microstructure of
2.25Cr-1Mo steel is transformed from austenite to a mixture of martensite and bainite. As for
5Cr-0.5Mo steel, Fig. 2.2 clearly shows that the steel is transformed from austenite to
martensite completely, which means 5Cr-0.5Mo steel has greater self-hardenability when
compared with 2.25Cr-lMo steel.
Temperature (℃)
Time
Fig. 2.1 — A Time-Temperature-Transformation (TTT) curve for
2.25Cr-1Mo steel
Ac 3 : 871℃, Ar 3 : 821℃, A: Austenite, C: Cementite, M: Martensite
Ac 1 : 804℃, Ar 1 : 721℃, B: Bainite, F: Ferrite
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