Abstract
Thermo-mechanical treatments using continuous cooling after forging are an established method for producing bainitic steels, mainly because of the elimination of energy intensive additional heat treatment processes. The cooling is usually employed in an uncontrolled manner in the industrial sector, which can be detrimental to the resulting microstructural morphology and, consequently, to the final product properties. In this study, a new controlled two-step cooling route based on the principles of bainitic displacive growth was designed and applied in a 0.18C (wt-%) steel. Inverse finite element method was used on the cooling data to obtain the evolution of temperatures for the samples during cooling, allowing to assess point to point cooling rates. Investigations via X-ray diffraction, optical microscopy analysis and hardness testing revealed a variation of bainitic morphology, namely, the transition from granular bainite to lath-like bainite with relatively high hardness and constituents/phase boundaries than the pre-treated microstructure.
Highlights
There has been an increasing economic and ecological interest in the reduction of energy consumption
Granular bainite, which is the matrix of the material in its as-received condition, was characterized by Zajac et al.[22] as an aggregate of irregular ferrite interwoven with islands of different morphologies and chemical compositions dependent of carbon partitioning during cooling
Ferrite is surrounded with martensite-austenite constituents (MA), as shown in Figure 3c, because of the cementite precipitation suppression caused by silicon[23]
Summary
There has been an increasing economic and ecological interest in the reduction of energy consumption. One expressive example of this is the substitution of quenched and tempered steels by continuously cooling steels, which achieve their final microstructure right after manufacturing integrated thermomechanical processing (like hot rolling or forging)[1,2,3]. This change in the production chain results in a microstructural transition from tempered martensite to a wide array of bainitic microstructures in which the morphology of the microstructure and the resulting mechanical properties are closely related to the alloy’s chemical composition. This interest can be attributed to the difficulties regarding the understanding of the transformation’s thermodynamical aspects[7] as well as the wide range of industrial applications this microstructure provides[8,9,10]
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