Anisotropies and inhomogeneities in the microstructure of the case hardening steel SAE 5120

Abstract

Distortion is an effect of inhomogeneous strain behavior. It can be initiated by plasticity due to thermal or transformation stresses (such as yielding, creep and transformation plasticity), phase transformations or strains during phase transformations. Inhomogeneities in microstructure and anisotropies constitute a distortion potential. Based on this knowledge several investigations were carried out on forged disks with a centered hole made of low alloy steel SAE 5120 (EN 20MnCr5) because both inhomogeneity and anisotropy have been observed in the microstructure of this material. Disks have been forged from continuous cast and hot rolled bar material. They show an inhomogeneous banded microstructure that is explicitly orientated in the direction of the two manufacturing processes. By forging, the initial microstructure of the bar material is influenced significantly: the segregation bands parallel to the rolling direction are deformed into a typical pattern. Besides this inhomogeneity in microstructure also the inhomogeneities of the bar material, such as center segregations from continuous casting, can still be observed in the deformed material. Gear blanks were machined from these disk blanks. It has been shown that the original position of the gear blank inside the disk blank influences the distortion [1]. The underlying reason remained unclear and investigations on this topic are discussed in this paper.

Process‐chain simulation for prediction of the distortion of case‐hardened gear blanks

“…Although the flow pattern was fairly similar to the practically observed one, it does not represent the material flow of the discs used in the experiments exactly. -The problem was idealised as an axisymmetric problem but recent findings of Eisbrecher et al [24] based on investigation of different sections of the forged discs reveals that this is not a very strong assumption. -There might be additional distortion potentials such as the local chemical composition dependent transformation behaviour as suggested by [25] considering the results of [24], which indicate non-homogeneous distribution of alloying elements throughout the cross-section of the gear blanks.…”

Modelling of phase transformations and transformation plasticity of a continuous casted steel 20MnCr5 incorporating segregations

“…The formation of the bands, kinetics and width Figure 2, depends strongly on the local manganese-and chromium-distribution. Gear blanks, which are forged from continuous casted and hot rolled bars, show similar segregation structures [6,7]. There is evidence that the distortion of discs made of the gear blanks is due to the segregations [8].…”

The Influence of Segregations after Forming on the Heat Treatment Result of Bevel Gears

“…Figure 1 shows the macrograph and the Mn-distribution of a hot forged gear blank with a punched hole made of 20MnCr5 (SAE 5120). [8] The Mn-distribution is determined by electronbeam microanalyzer. It is worth to note, that high definition measurements of a part like in Figure 1 needs about 48-72 h of measurement time with corresponding beam stability.…”

“…Figure shows the macrograph and the Mn‐distribution of a hot forged gear blank with a punched hole made of 20MnCr5 (SAE 5120) . The Mn‐distribution is determined by electron‐beam microanalyzer.…”