Abstract
The M-A (Martensite-Austenite) constituent is a microstructural compound occurring in the medium microstructures like the upper or lower bainite. During decomposition of coarse grain austenite a part of this austenite is decomposed into ferrite laths but the remaining part is enriched by carbon. The result of such decomposition is a complex microstructure composed of two kinds of cementite, two kinds of martensite, and residual austenite. We have investigated the effect of subsequent welding cycle upon decomposition of M-A constituent in the simulated underbead zone and in heat affected zone with intercritical temperatures in two types of steel, HT 80 and HT 100. The peak temperature of subsequent simulated welding cycles varied between 100 and 700 °C. The M-A constituent decomposes into ferrite-carbide aggregates, the carbides being cementite, Fe₃C. The decomposition starts at temperatures above 300 °C and full restoration of toughness was observed at temperatures above 500 °C. The kinetics of M-A decomposition is different in various M-A islands. The elongated M-A particles decompose easier. It seems that a secondary cementite can be also a product of the M-A decomposition. Various decomposition rates of M-A constituents indicate that the composition of M-A particles can differ.

Key words
Martensite-Austenite (M-A) constituent, welding cycle

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