Distortion Potential of Cold Forming Prior to Induction Heat Treatment Processes

Cold forming of steel is known as a process of near net shape production with significant economic advantages compared to machining. The degree of cold working of the material however introduces high residual stresses into the workpiece which are released during subsequent heating processes e. g. by induction hardening. Those stress relief effects combined with plastic deformation during rapid heating and fast quenching as well as volume changes by transformation into martensite lead to distortion which causes losses in production costs.

The influencing parameters of the forming process (degree of deformation), the tool design (opening angle of extrusion tool), the lubrication conditions and the hardening parameters (surface hardening depth) on the distortion behavior after inductive case hardening have been analyzed by Design of Experiments (DoE) method. Diameter and length changes and changes in curvature and inclination angle of shouldered shafts made of SAE 1055 have been investigated for two different case depths.

The test results revealed the important distortion potential of the extrusion process and its parameters on the overall distortion after hardening. The significant influence of the induction hardening process on the size changes, not the shape changes, was proven.

How the initial microstructure of the raw material influences the surface hardness and hardness depth as well as the residual stress distribution in inductive surface hardened samples is demonstrated in a second investigation. The hardness response of pearlitic, spheroidize annealed, bainitic and martensitic quenched and tempered microstructure depends very much on the carbon solution in the very fast induction heat treatment process. This is of importance e. g. for contour hardening processes where only very short heating cycles have to be applied.