Abstract
The metallographic and microfractographical investigations of M-A constituent in the weld metal and weld HAZ of high strength low-alloyed steels are reviewed with particular reference to the internal structure of the M-A constituent and the role of M-A constituent in the crack nucleation and propagation. The M-A constituent may consist of retained austenite and martensite and two types of cementite: cementite precipitated from austenite and cementite produced during lath martensite self-tempering. Two types of martensite were detected in M-A constituent: lath martensite exhibiting high dislocation density and plate martensite in which martensite laths are internally twinned.
The M-A constituent, as second phase particles, is affecting the ductility and impact properties. In the ductile fracture temperature range M-A constituent exhibits appropriate straining ability. At higher strains, however, it fractures or even is fragmented and so initiate the dimple fracture formation. In the brittle fracture temperature range M-A constituent can contribute to cleavage fracture reinitiation.

Key words
microfractographical investigations, M-A constituent, weld metals, high strength low-alloyed steels, fracture

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