Abstract
One of the main limitations in application of nanostructured carbide-free bainite as a construction material is the difficulty of joining. This research presents a structural characterization of welded joints of medium carbon 55Si7 grade steel after the welding process with a regeneration technique as well as post welding heat treatment (PWHT). The hardness distribution of the welded joint with regeneration exhibit an overall decrease in hardness when compared to the base material and a significant decrease in hardness was observed in the heat-affected zone (HAZ). Unfavorable hardness distribution was caused by the presence of diffusion-type transformations products (pearlite) in the HAZ and bainite degradation processes. On the other hand, welding with the PWHT promotes the achievement of a comparable level of hardness and structure as in the base material. However, a slight decrease in hardness was observed in the weld zone due to the micro-segregation of the chemical composition caused by the indissoluble solidification structure. Based on the structural analysis, it was found that steel with relatively low hardenability (55Si7) should be welded using PWHT rather than a regeneration technique.
Highlights
Nanostructured (NB) carbide-free (CFB) bainitic steels have been widely developed for many years due to high mechanical properties
The regeneration technique consists of controlled cooling after the welding process and isothermal annealing at the temperature of the designed bainitic transformation and at a time allowing for the completion of the bainitic transformation
The focus was primarily on the Low-Temperature Heat-Affected Zone (LTHAZ), while this paper describes in detail all existing zones (LTHAZ, High-Temperature heat-affected zone (HAZ), Fusion Zone)
Summary
Nanostructured (NB) carbide-free (CFB) bainitic steels have been widely developed for many years due to high mechanical properties. It should be highlighted that this is an additional technological process that cannot always be applied (e.g., due to dimensions of elements) and generates higher costs Another method to increase the strength of welded joints is the regeneration technique proposed by Fang et al [12,13,14]. This method involves synchronously welding and impacting processes, which introduce compressive and shear stresses at the same time during the welding process Another method to reduce regeneration time is a refinement of austenite grains [16] because, in high carbon steels, the transformation time will be shortened [18]
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