Influence of deformation path on microstructure and damage evolution during the open die forging of high strength steels: experiments and FE analysis

Abstract

Large forged blocks of medium carbon, high strength tool steels are widely used in the forming industries to manufacture dies for applications such as plastic injection molding, hot deformation, and extrusion. The production of these components involves a sequence of casting, forging, quenching, and tempering operations. The primary goal of the forging process is to eliminate casting defects that may arise during solidification, such as segregation, shrinkage, porosity and break the as cast structure and convert it to a more homogeneous and isotropic material, wrought microstructure. It is essential to mitigate as many casting defects as possible during the forging stage. However, when the defects are large, it is not possible to fully eliminate them before or after forging through heat treatment alone, making it challenging to address material heterogeneity using heat treatment. Key parameters present during the forging process are strain rate, temperature, and strain significantly affecting both process efficiency and the quality of the final product. In addition to these parameters, factors like die and ingot geometry, die width ratio, and press capacity influence material flow and microstructure evolution during forging. This research specifically investigates the impact of die geometry by altering its design to create different deformation paths. The main objective of this thesis is to examine the effects of these deformation paths on microstructure evolution and damage development during the open die forging of high strength steels. The first step of this study was to develop a microstructure-based finite element (FE) model to examine the impact of deformation path on microstructure evolution, DRX fraction and DRX grain size, during the upsetting process of large medium carbon low alloy high strength steel ingot. To achieve this, hot compression tests were performed using the Gleeble 3800 thermomechanical simulator. The flow stress data obtained from these tests were corrected and used to establish both a material model and a microstructure model. These models were then integrated into the Forge® NxT 3.2 finite element simulation software using user routine. The FE predictions were validated by comparing them with experimental results from hot compression test, ensuring accurate predictions of flow stress and recrystallized grain size at the end of hot deformation. After validation, the FE model was scaled up to simulate the industrial upsetting process, enabling an investigation into the effects of deformation path on strain inhomogeneity and microstructure evolution in large forged ingots. An analysis of four die geometries was conducted to identify the optimal die shape for minimizing strain and grain size inhomogeneity across the ingot. The study found that the convex die caused the least deformation, while the concave die induced the highest deformation values at the ingot's center. Using the coefficient of variation, as a measure of heterogeneity, it was concluded that the vdie and concave die produced a more uniform grain size distribution compared to the flat and convex dies. The second part of study focused on investigation of the influence of deformation path, illustrated by die geometry, on damage evolution during the cogging of difficult to forge, medium carbon high alloy- high strength AISI H13 steel. Hot compression and tensile tests were performed using Gleeble 3800 thermomechanical simulator to develop the optimum material model which was then implemented in the finite element (FE) code Forge® NxT 3.2 using a developed user subroutine. Normalized Cockcroft and Latham damage criterion and maximum shear stress (Tresca’s) theory of failure were used to predict the damage and failure in the center axis of the shaft through FE analysis with three different die shapes: concave, flat, and convex. A comparative study between the three die geometries was conducted to quantify the effects of each of them on the sensitivity to central burst damage. FE model was validated using industrial data. The lowest and highest damage values were found to occur in the case of cogging with concave and flat die, respectively. The coefficient of variation (CoV) was employed as a measure of heterogeneity and it was found that the concave die provided more uniform deformation and most favorable results for the cogging compared to the flat and convex dies. The novel approach, application of concave die was successfully implemented at the industrial scale cogging. The third and last part of this study focuses on physical and numerical simulations of a multistep deformation process to investigate how deformation path influences microstructure evolution and the distribution of mechanical properties. Multi-step deformation experiments were conducted on high strength steel specimens using the Gleeble 3800 thermomechanical simulator, equipped with the MaxStrain® attachment. All tests were performed at a strain rate of 0.01 s⁻¹ and a temperature of 1150°C, with two different deformation paths considered. A total true strain of 0.84 was achieved over four steps, with approximately 0.21 strain applied in each step. The results were analyzed to examine the effect of varying deformation paths on microstructure evolution and hardness distribution. A finite element (FE) model was created using the Forge® NxT 3.2 FE code to simulate the multi-step deformation process. This FE model was validated against the measured average grain size and hardness distribution following the multi-step deformation experiments. Once validated, the FE model was able to predict strain distribution, dynamic recrystallization (DRX) volume fraction, and grain size evolution during the process. A comparative analysis of results from the two deformation paths was performed to identify the optimal path for achieving a uniform strain and grain size distribution. The Coefficient of Variation (CoV) was used to evaluate the heterogeneity of the hardness distribution. The findings indicated that concave anvils promoted a higher and more uniform strain distribution, which resulted in a more homogeneous grain size and hardness distribution

Date	3 Sept 2025
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Mohammad Jahazi (Supervisor)