Two-Step Continuous Cooling Heat Treatment Applied in a Low Carbon Bainitic Steel

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.

Microstructure and Wear Properties of a Low Carbon Bainitic Steel on Plasma Nitriding at Different N2-H2 Gas Mixtures

“…The steel to be plasma nitrided was a DIN 18MnCrSiMo6-4 steel which is a continuous cooling bainitic steel. The material's microstructure obtained directly after hot rolling and controlled cooling is composed of pro-eutectoid ferrite and granular bainite 7,8,22 . The material's chemical composition and mechanical properties are given in Table 1.…”

“…Continuous cooling bainitic steels has an increasing use in industrial processes owing to its excellent combination of yield strength and toughness [1][2][3][4] . The new generation of continuous cooling bainitic steels allows the formation of different bainitic morphologies, such as granular bainite, degenerate upper bainite, and lower bainite, and may contain other phases such as retained austenite and ferrite [5][6][7][8][9] . Unlike quenched and tempered steels, which need further heat treating to obtain properties suitable for their applications 10,11 , continuous cooling bainitic steels already present high levels of strength and toughness after hot rolling or forging, presenting the potential to shorten processing routes 5,6,8,12 .…”

Numerical and Experimental Study of an Industrial Case for Grain Size Evolution in Bainitic Steel in Controlled Hot Forging and its Influence on Mechanical Performance

“…The boundary conditions of the process are listed in Table 3, according to the industrial environment and the steel properties. The thermal conductivity and heat capacity were taken from Castro et al 21 inverse heat transfer analysis for this steel. The friction factor µ was set at 0.3, corresponding to the use of graphite and water in the closed die hot forging condition 14 .…”