Effects of Forming Route and Heat Treatment on the Distortion Behaviour of Case-Hardened Martensitic Steel type S156

Dimensional and shape changes which occur during the manufacturing of metallic components cause significant additional costs during subsequent processing. The control of dimensional changes during heat treatment is critical especially in case-hardened components requiring large stock removal and thick case layer in order to compensate high levels of distortions.

The main aim of the present work is to analyse the influence of manufacturing processes on the distortion behaviour of a conventional case-hardening steel, BS S156, in aerospace industry. Another aim is to gain understanding about the distortion behaviour of carburized components. A design of experiments (DoE) was carried out. C-ring shaped test pieces were machined from bar stock and pancake forging in order to compare the distortion behaviour of components machined directly from bar and forged parts, respectively. After each single heat treatment step (normalising, carburising, re-hardening, deep freezing and tempering) C-ring specimens were measured by coordinate measuring machine (CMM) to track dimensional changes. Additionally, specimens were retained for metallurgical and residual stresses analysis. A numerical model has been developed to predict the distortion behaviour of C-ring specimens. The model has been validated by experimental results and applied to identify key process variables affecting distortion during heat treatment.