Abstract
In order to explore the effect of different the welding heat input on microstructure and impact properties in coarse grain zone in heat affected zone (CGHAZ) of ultra-low carbon bainitic steel welded joint. In this paper, Gleeble-3500 thermal simulation testing machine was used to simulate different heat input, to study the effect of linear energy on the microstructure and impact toughness of the CGHAZ of Q420qEN steel joint. The scanning electron microscope (SEM), oscillographic impact microscope and transmission electron microscope (TEM) were used. The results show that the microstructure of CGHAZ is mainly the lath bainite and granular bainite. When the welding heat input is 30kJ•cm-1, the CGHAZ grains have obvious growth phenomenon. With the increase of the welding heat input, the grain size also grows. Meanwhile, as the welding heat input increases from 18kJ•cm-1 to 30kJ•cm-1, the impact toughness of the CGHAZ of joint increases first and then decreases at -20°C.
Highlights
Ultra-low carbon Bainitic steel has important performance including the high strength, good impact toughness and weldability[1]
The results showed that the subsequent austempering heat treatment at a lower temperature, immediately after partial bainite formation at a higher temperature, would replace the coarse austenite/martensite areas with much refined bainite consisting nanoscale plates of bainitic ferrite and filmy austenite which leads to the refinement of the bainitic microstructure
The granular bainite is in ferrite matrix distribution of island, and the island group consists of martensite and retained austenite, known as the M-A island[14,15,16]
Summary
Ultra-low carbon Bainitic steel has important performance including the high strength, good impact toughness and weldability[1] At present, it is internationally praised as the environmental protection green steel in the 21st century, and it is widely used in oil and gas pipelines, heavy machinery, railway transportation and other aspects. The results showed that the subsequent austempering heat treatment at a lower temperature, immediately after partial bainite formation at a higher temperature, would replace the coarse austenite/martensite areas with much refined bainite consisting nanoscale plates of bainitic ferrite and filmy austenite which leads to the refinement of the bainitic microstructure This microstructural modification, in addition to the increased yield strength, causes a significant increase in the impact fracture toughness of the multi-step austempered steels[4]. This study will provide an experimental basis for the selection of welding technological parameters of steel Q420qEN
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