Martin Hunkel

Anisotropic phase transformation strain in forged D2 tool steel

Abstract Heat treatment of forged D2 tool steel produces anisotropic dimensional change. It has been observed that tools have a larger dimensional change along the forging direction than perpendicular to it. This paper investigates anisotropic dimensional change by means of dilatometry. The results show that anisotropic phase transformation strain is produced in forged D2 steel during heat treatment. The anisotropic transformation strain is the main reason for anisotropic distortion in heat treatment of forged D2 steel. Anisotropic transformation strain produced during martensite transformation increases with higher austenitising temperature and is little influenced by cooling rate. A suggested mechanism is that transformation induced plasticity is produced under the internal stresses caused by the anisotropic microstructure (carbide bands) in the steel.

Simulation of the distortion of cylindrical shafts during heat treatment due to segregations

A conventional low-alloy steel (SAE 5120, EN 20MnCr5) was extensively investigated by dilatometer experiments and at heat-treated cylindrical shafts to determine the three-dimensional anisotropic change in length and curvature during heat treatment due to segregations. The change in length due to the transformations during heating as well as during cooling can be detected as the main reason for distortion. Segregations are too small in relation to a whole part to simulate them for the entire part. This is the main problem with including segregations in a simulation model. Due to this a mean substitution had to be used. Segregations were taken into account by three-dimensional anisotropic transformation strains, which were implemented into the FEM-simulation program Sysweld ® . Thereby, heating as well as cooling - including phase transformations and mechanical behaviour - were considered, because anisotropy occurs in both cases and its overlapping leads to the final distortion.

Combined Neutron and X-Ray Diffraction Analysis for the Characterization of a Case Hardened Disc

The present work has been executed within the framework of the collaborative research center on Distortion Engineering (SFB 570) in order to evaluate the residual stress state of a disc after carburizing and quenching as well as to validate a simulation procedure. The combined use of X-ray and neutron diffraction analysis provided information about the residual stress states in the whole cross section. However, the stress free lattice spacing d0 for the neutron diffraction experiments is problematic and induces systematic uncertainties in the results and the application of a force balance condition to recalculate d0 might be a solution for improving the reliability of the results. Comparison of experimental results with simulation showed that an overall satisfying agreement is reached but discrepancies are still present.

Anisotropic Transformation Strain and Its Consequences on Distortion during Austenitization

The distribution of segregations, which is introduced in the continuous casting process and modified during succeeding manufacturing steps, is considered as an important “distortion potential carrier” for chemically banded steels. This article presents a recently developed mathematical model for integration of the effect of prior forming and cutting operations into heat-treatment simulations by considering “anisotropic transformation strain (ATS).” The model was justified experimentally by simulating the heating and austenitization of dilatometer specimens machined from the forged discs with distinct orientations with respect to the banded microstructure. After the verification, it is used in conjunction with former experimental work to demonstrate that the distribution of fiber flow is one of the important reasons of the dishing of carburized discs. The model provides promising results for process chain simulation to predict the heat-treatment distortion that cannot be predicted with currently available models.