Microstructure evolution of medium carbon low alloy steel during ingot breakdown process

High-strength steels used for the manufacturing of large size components such as turbine shafts are produced by ingot casting, followed by forging, quenching, and tempering operations to breakdown the as cast structure and to optimize the properties. The first step of the deformation process, called ingot breakdown, is carried out well above the paraequilibrium Ae3 temperature (temperature at which ferrite to austenite transformation accompanied by interstitial diffusion reaches equilibrium, and therefore produces major microstructural changes. Moreover, the microstructure developed during the ingot break down process has a determining influence in the design of the subsequent thermomechanical treatments to achieve the desired properties in the wrought product. The main aim of this thesis is to investigate and understand the mechanisms involved in the evolution of microstructure during break down of as-cast structure of a medium carbon low alloy steel used as a die material in the transport industry. In addition, constitutive behavior of the as-cast structure was modelled and the material model best describing the behavior of the investigated steel during the ingot breakdown process of very large ingots was determined for the first time. The developed material model was implemented in a finite element (FEM) code. The first part of this study provides the details of the development of a precise model in order to predict the flow behavior of the material under different deformation conditions. The microstructure evolution from as-cast to wrought condition needs to be accurately quantified and the optimum material model and constants need to be identified. The model which can better anticipate the flow curves during the deformation of an as-cast structure is determined. Specifically, Hansel-Spittel and Arrhenius constitutive models were developed using hot compression tests and then incorporated in the FEM code Forge NxT 1.0®. The simulation results thus generated were further utilized to analyze the adiabatic heating and force vs. time analysis. To determine and quantify the accuracy of the developed models, a comparison of the Arrhenius and Hansel-Spittel model is done. The accuracy and the reliability of both models were compared in terms of correlation coefficient (R) and the average absolute relative error (ARRE). The values of R and ARRE for Arrhenius model are 0.978 and 1.76%, whereas, for the Hansel Spittel model these values are 0.972 and 3.17 % respectively. Stress-strain predictions by both equations reveal that Hansel-Spittel model is unable to predict the dynamic softening of the material and also proved insignificant in predicting the curves of other range of deformation parameters. Finite Element Modeling (FEM) was used by integrating the two models in Forge NxT 1.0® software for the design and optimization of the ingot breakdown processes. The simulation results showed good agreement with the theoretical and simulated values of adiabatic heating using both models, whereas only Arrhenius model was able to accurately predict the force vs. time values. The overall results indicate that the Arrhenius model is more accurate and efficient in predicting the deformation behavior of the cast structure than the Hansel-Spittel model. The second part of this study analyzes the occurrence of dynamic transformation during ingot break down process. This investigation focuses on fundamental mechanisms responsible for the evolution of the cast microstructure towards a wrought one. As-cast homogenized medium carbon steel with a grain size of ~ 200 μm was used for the investigation. Hot deformation was carried out using Gleeble 3800 thermomechanical simulator at a temperature 1150 °C and 1200 °C under strain rates of 0.25, 1 and 2 s-1. The selected hot deformation temperatures were 450-500 °C above Ae3. The double differentiation technique was employed to analyze stress-strain curves and a combination of optical, SEM, and Electron Back Scattered Diffraction (EBSD) technique were used for microstructure characterization. Kernel average misorientation (KAM) was used to measure internal misorientation to separate out the fraction of dynamically transformed (DT) ferrite grains. Results indicated that with the increase in the deformation temperature, ferrite fraction increased, whereas it decreased with increase in strain rate. The investigation on the effect of strain rate on the occurrence of DT ferrite and its growth was determined using pipe diffusion coefficient. It was found that the strain rate has significant role in the microstructure evolution in DT. At higher strain rate, Widmanstatten ferrite plates were observed and at lower strain rates, quasi polygonal ferrite was observed. The change in morphology was related to variation of carbon diffusivity and diffusion distance w.r. t strain rate. A mechanism for this transformation is proposed and shown schematically. The third part of this study focusses on the effect of the addition of Chromium (Cr) on the dynamic transformation (DT) of austenite to ferrite at temperature above Ae3 up to 430 °C in as-cast medium carbon low alloy steel. Calculating the driving force and the barrier energy to the transformation, the results pointed to a fact that although Cr increases the driving force of the transformation, the barrier energy is also increased with its addition. To further understand the fundamental behind this at the atomistic level, diffusion analysis was performed by considering stress due to deformation. It was found that, with the increase in stress, the diffusivity of Cr, increased. Upon comparing the diffusion results with other alloys like carbon and silicon, it was found that the diffusivity and the diffusion distance of Cr was significantly lower than that of carbon and silicon. This finding gave an insight that with the addition of elements like Cr, the austenite matrix becomes stronger in terms of work done in shear and dilatation energy which is related to the sluggish diffusion of Cr and thereby, results as a barrier to the transformation. The forth part of this study presents the results of the investigation on the effect of double hit deformation on Metadynamic Recrystallization (MDRX) kinetics. Ingot breakdown process often consists of several successive deformation steps with high interpass times, during which MDRX occurs. Two-stage isothermal compression tests were carried out at 1150 °C and 1200 °C with strain rates of 0.25-2 s-1 and interpass times of 5-25 s. Based on the experimental results, a material model for MDRX is proposed. The constitutive model was implemented in Forge NxT 1.1® software to simulate the multistage compression. The predicted results from the material model were found to be consistent with the numerical analysis and experimental results, which indicated that the proposed kinetic equations can give precise softening behavior for hot deformed as-cast medium carbon low alloy steel.