The role of microstructure in the temper embrittlement of low alloy steels.

A detailed investigation has been carried out to study the effects of heat treatment on the susceptibility to temper embrittlement (Ductile-Brittle transition temperature and low energy fracture characteristics) of eleven experimental steels.These experimental alloys represented a range of compositions related to engineering steels and corresponded to a nominal composition of 0.34 mass%C with alloying additions of Cr, Ni and Mo in varying combinations. These alloys were doped with P and Sb to study the effect of these additions on the susceptibility to temper embrittlement. These steels were investigated in the pearlitic, bainitic and tempered martensitic conditions to establish the role of microstructure.Heat-treated alloys were characterised by fracture studies including Izod impact testing and subsequent electron microscopy.Simultaneous Auger electron spectroscopy and energy dispersive X-ray analysis coupled with with Secondary Ion Mass Spectrometry were used to study the fracture surfaces and bulk compositions of the embrittled structures. It has been established that intergranular embrittlement in a quenched and tempered martensite microstructure was associated with the presence of P, whereas the initial intergranular embrittlement in a bainitic microstructure was associated with the segregation of Sb. It is suggested that the lower C activity produced in tempered martensite structures allows P migration to the grain boundaries causing intergranular embrittlement which was attributed to the development of M[7]C[3].Alloys in isothermally transformed bainitic condition showed that the predominant carbide precipitate was M3C which increased the C activity at the prior austenite grain boundaries with a resultant decrease in P concentration and consequently an absence of intergranular failure in the early stages of embrittlement. The increased C activity continued to prevent appreciable P segregation but was not sufficient to inhibit the co-segregation of Ni and Sb after extended ageing times when the bainitic alloys began to fail by intergranular fracture. After prolonged ageing increased Ni and Sb concentrations at the grain boundaries were associated with the formation of a fine grain boundary precipitate which was low in Cr. The tendency to fail by the low energy intergranular mode of failure was always greater in the tempered martensites, even when the bainites were significantly harder

Influence of welding procedure and PWHT on HSLA steel weld metals

The development of consumables for welding of high strength steels represents a continuous challenge taking into account the great variety of alloy systems. In this regard, there are many applications, such as offshore structures, where the welding is still preferably done by the SMAW process. In order to evaluate the properties of weld metals obtained by other processes with higher productivity, the present work presents a comparative analysis between the mechanical properties of high strength steel weld metals obtained by shielded metal arc (SMAW) and gas metal arc (GMAW) welding processes. Multipass welding by SMAW and GMAW processes was performed with preheating of 200 °C, in 750 × 150 × 19 mm plates. After welding, a post weld heat treatment (PWHT) at 600 °C for 1 h was performed and this condition was compared to the as welded one. Mechanical tests and metallographic examination by optical microscopy (OM), scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) were performed for mechanical and microstructural characterization. Thermodynamic calculations were also performed by using Thermo-Calc software, in order to evaluate the occurrence of carbides as a consequence of the PWHT.Although presenting impact toughness behavior slightly different due to the chemical composition and carbide precipitates, as predicted by computational simulation, both processes showed a good relationship between mechanical strength and impact toughness for all analyzed conditions, even after PWHT.Based on this scenario, it can be inferred that GMAW process can be applied as an interesting alternative for welding of high strength steels, once this process promotes a significant improvement in productivity with good quality. Keywords: Weld metal, Mechanical properties, Microstructure, Post-weld heat treatmen

Desempenho de metal de solda adequado para soldagem de componentes de ancoragem

O presente trabalho apresenta os resultados de um programa de pesquisa cujo objetivo principal é o desenvolvimento de consumíveis adequados para a soldagem de componentes de ancoragem fabricados em aços de alta resistência. Após avaliação de diferentes composições químicas, com base no balanço de Mn e Ni é feita uma análise detalhada das propriedades mecânicas e microestruturais da composição química que propiciou os melhores resultados para permitir uma conclusão decisiva. Foram soldadas juntas pelo processo eletrodo revestido utilizando consumíveis de 4,0 mm de diâmetro, com preaquecimento de 200 ºC, corrente contínua, posição plana e energia de soldagem nominal de 1,5 kJ / mm. Após a soldagem, foram realizados ensaios mecânicos (tensão, impacto Charpy-V, dureza) e metalográficos por microscopia ótica, microscopia eletrônica de varredura e EBSD em amostras removidas integralmente do metal de solda, tanto na condição de como soldado quanto após tratamento térmico pós-soldagem (TTPS) realizado a 600 ºC. Os resultados mostram que os metais de solda obtiveram propriedades mecânicas satisfatórias após o TTPS, sendo adequados para aplicação em componentes de ancoragem de acordo com a especificação IACS W22. Além disso, observou-se que a tenacidade ao impacto experimentou uma melhoria com o aumento do tempo de TTPS, o que é crucial para componentes com maiores espessuras. Palavras-chave: metal de solda, propriedades mecânicas, tratamento térmico, componentes de ancorage