St. Petersburg Mining University (St. Petersburg, Russia):

E. I. Pryakhin, Dr. Eng., Prof., Head of Dept. of Material Science, E-mail: e.p.mazernbc@yandex.ru
D. M. Sharapova, Cand. Eng., Assistant

Localized heat applied during welding or rebuilding of parts can lead to a noticeable structural evolution in the heat affected zone of the high-strength low-alloy K65 steel. A comprehensive study that was carried out on model and full-scale specimens and looked at the structure and properties of steel in the heat affected zone after single and double heat cycles proved the above trend. Through metallographic and X-ray microstructural analysis, it was found how and in what conditions a brittle phase forms at coarse grain boundaries, the amount of which and the distribution around the grain is in direct relationship with the number of heat cycles and the heat input of the welding procedure. It was established that during the initial heat cycle the new phase around the grains tends to grow as the cooling rate goes down, which is consistent with the reduced toughness of steel in the heat affected zone during high heat input welding. When the HAZ is reheated, the undesirable structural changes become even more pronounced. The results of the study show that the steel in view does not provide enough workability and that a specification is required to limit the amount of heat input introduced during the initial and further heat cycles in welding.

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