Effects of Microstructure on the Low-Temperature Toughness of an X80 × D1422 mm Heavy-Wall Heat-Induced Seamless Bend

Abstract

The relationship between the microstructure and the low-temperature toughness of an X80 × D1422 mm heavy-wall heat-induced seamless bend was investigated, including the influence of microstructure on crack initiation and crack propagation. Using optical microscopy, scanning electron microscopy, transmission electron microscopy, and electron backscattered diffraction, the microstructure and crystallographic orientation characteristics were studied. An instrumented impact tester was used to investigate the impact toughness. The results showed that during the hot bending process, there was a difference in the induction heating temperature and the cooling rate results in the uneven microstructure of the inner surface, center position, and outer surface of the bend. The center position was mainly composed of granular bainite and exhibited the best combination of strength and toughness. The ductile–brittle transition temperatures of the inner surface, center position, and outer surface were −88, −85, and −60 °C, respectively. In the process of impact deformation, the non-uniformly distributed strain concentration regions are likely to cause uneven distribution of plastic deformation and the nucleation of microcracks. The high ratio of high-angle grain boundaries and the smaller effective grain size of the inner surface and center position lead to higher crack growth absorption energy. The low crack propagation energy of the outer surface is attributed to the fact that the high-angle grain boundary does not effectively deviate or arrest the crack propagation, and multiple microcracks are connected to one another and cause fracture failure.

Avaliação da possibilidade de aplicação de tratamentos térmicos de múltiplas normalizações como mecanismo de refino microestrutural e incremento de propriedades mecânicas em aços API-OCTG

“…Em relação ao grau K55, quando normalizado, a sua microestrutura é constituída por ferrita e perlita. Quanto mais refinada for a microestrutura, melhor será a relação entre os indicadores de resistência mecânica e a tenacidade ao impacto [3,[10][11][12][13][14]. Em relação ao grau T95, quando temperado, ele é constituído 1 Introdução Nos últimos anos, o segmento industrial de óleo e gás vem aumentando significativamente a sua produção, impulsionando assim os investimentos em tecnologia para a extração e transporte dos hidrocarbonetos.…”