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방국수 · 안영호 · 정홍철
항복강도 390 MPa급 가공열처리강 대입열용접 열영향부 충격인성 및 연화현상 (Impact Toughness and Softening of the Heat Affected Zone of High Heat Input Welded 390MPa Yield Strength Grade TMCP Steel)
Received: 1 August 2018;  Accepted: 15 September 2018.  Published online: 11 October 2018.
The Charpy impact toughness of the heat affected zone (HAZ) of electro gas welded 390 MPa yield strength grade steel, manufactured by a thermo mechanically controlled process, was investigated. The effects of added Nb on the toughness of the steel and the factors influencing scatter in toughness are discussed in the present work. It was observed that adding Nb to the steel led to the deterioration of HAZ toughness. The presence of soluble Nb in the HAZ increased its hardenability and resulted in a larger amount of low toughness bainitic microstructure. Microstructural observations in the notch root area revealed the significant role of different microstructures in the area. In the presence of a larger amount of bainitic microstructures, the HAZ exhibited a lower Charpy toughness with a larger scatter in toughness. A softened zone with a lower hardness than the base metal was formed in the HAZ. However, theoretical analysis revealed that the presence of the zone might not be a problem in a real welded joint because of the plastic restraint effect enforced by surrounding materials.
Keywords: alloys, welding, toughness, impact test, niobium
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