Volume 71, Issue 2

The back-face tilt of press-hardened crown wheels is investigated for continuous and ingot cast material. Two different geometries are studied, one with a flat back-face and one with a step. For two ingots, the original material position is tracked and coupled to back-face tilt, cross section hardness and chemical segregation. The chemical gradients are shifted from top to bottom of the ingot and are strongly coupled to the back-face tilt.

Bend straightening is a typical process step applied after heat treatment of e. g. axles and shafts. From a materials science perspective it is an elastic-plastic bending operation with a characteristic inhomogeneous deformation over the cross section of the component. Due to the local amount and distribution of plastic deformation alterations of residual stress fields result and phase transformations are possible. Consequently, bend-straightened components have inhomogeneous materials properties. In particular an anisotropy of strength exists which can be attributed to the position-dependent microstructure and sign of the residual stress state. In addition, a Bauschingereffect is possible which also has considerable impact on the deformation characteristic of the considered component. In the present paper the above mentioned aspects are demonstrated for quenched and tempered steel bars and shafts. Typical examples of residual stress states before and after straightening are presented as well as alterations of retained austenite distributions. Finally, the deformation behavior and strength of bend-straightened components is outlined and discussed.

Controlling distortion is of key importance during the case hardening process for the production of automotive and non-automotive metallic components. By effective control of distortion and the variation of distortion, significant costs in post heat treatment machining processes can be avoided. In some cases it is even possible to eliminate all post-machining. In other cases it may be possible to avoid the press-quenching of individual components, resulting in huge cost-benefits. A recently introduced new vacuum furnace design allows the treatment of small batches in a single layer of parts (“2D-treatment”) which allows for easy automated loading and unloading of the fixture-trays. By using the small batch concept, a continuous flow of parts can be established (“One Piece Flow”). There is no need to wait until enough parts are collected to build a large batch with multiple layers (“3D-batch”). This compact furnace unit can be implemented into the heart of the production chain and provides heat-treatment processes which can be fully synchronized with the green and hard machining-operations. When performing case hardening, the components are Low Pressure Carburized (LPC) at high temperatures (1050 °C) followed by gas quenching. The treatment in single layers offers an optimum in quality regarding: temperature homogeneity, quench homogeneity and distortion control. Typical components for this technology come from the automotive, aerospace and tool industry. The directly following contribution in this Journal (A. Schüler et al., p. 90–98)shows more results achieved with this technology on selected truck-components such as gears and sliding-sleeves.

A systematical approach regarding different distortion potentials in the process chain describes the influence on dimensional and shape changes of gears and sliding sleeves after case-hardening, like part geometry, cold and hot forming of blanks, carburizing concept and temperature profile, oil and gas quenching, as well as individual press and batch hardening. The results show an excellent potential of the new SyncroTherm® concept compared to the conventional case-hardening process for gears and sliding sleeves. Stable distortion characteristics even at elevated temperatures and without decreasing to hardening temperature as well as a good performance after two-dimensional batch quenching instead of the much more expensive individual press quenching were found in this study. Very sensitive part geometries are still a challenge. A clear limitation was found when processing cold formed blanks without annealing before soft machining.

Transformation plasticity is considered as a phenomenon limiting the actual stress for any specified strain rate under conditions of phase transformation. A one-dimensional model of a steel bar strained under martensitic transformation is provided. Discussion is limited to a series of accepted assumptions, such as athermal kinetics of martensite transformation, monotonic evolution of temperature and constant transformation plasticity parameter. An equivalent tangent modulus is deduced indicating the decisive parameters influencing the softening modes. The obtained model is utilized together with well-known transformation plasticity laws and martensite transformation kinetics to demonstrate an actual softening of steels under the martensitic transformation.