Weld HAZ Toughness and Its Improvement of Low Alloy Steel SQV-2A for Pressure Vessels (Report 1) : Effect of cooling time on microstructure and Charpy impact value in single thermal cycle(Materials, Metallurgy & Weldability)

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The microstructures and Charpy impact properties of weld HAZ of low alloy steel SQV-2A were investigated with particular reference to the effects of the formation of M-A constituent and its morphology. The weld HAZ thermal cycles having different cooling times Δt8/5 were simulated with the Gleeble 1500 thermal/mechanical simulator. The formation of M-A constituent and its morphology were examinated by TEM technique and two stage electrolytic etching method. For this steel, M-A constituent began to form at 20s of cooling time Δt8/5, and it coexisted with upper bainite and even lower bainite over a wide range of cooling time Δt8/5. With increasing in cooling time, the M-A constituent formed not only near the grain boundary but also in bulk region, and changed from elongated shape to massive shape. Both enlongated M-A constituent and massive M-A constituent seem to impair the HAZ toughness of SQV-2A steel. The M-A constituent acts as the initiation site of microcracking to induce the loss of absorbed energy of Charpy impact test, but the absorbed energy of Charpy impact test may be affected by not only the M-A constituent but also the matrix, because the matrix itself became brittle with increasing in cooling time.
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