Process Signature for Laser Hardening

Friedhelm Frerichs,Thomas Lübben,Yang Lu,Tim Radel

doi:10.3390/met11030465

Abstract

During many manufacturing processes for surface treatment of steel components heat will be exchanged between the environment and the workpiece. The heat exchange commonly leads to temperature gradients within the surface near area of the workpiece, which involve mechanical strains inside the material. If the corresponding stresses exceed locally the yield strength of the material residual stresses can remain after the process. If the temperature increase is high enough additionally phase transformation to austenite occurs and may lead further on due to a fast cooling to the very hard phase martensite. This investigation focuses on the correlation between concrete thermal loads such as temperature and temperature gradients and resulting modifications such as changes of the residual stress, the microstructure, and the hardness respectively. Within this consideration the thermal loads are the causes of the modifications and will be called internal material loads. The correlations between the generated internal material loads and the material modifications will be called Process Signature. The idea is that Process Signatures provide the possibility to engineer the workpiece surface layer and its functional properties in a knowledge-based way. This contribution presents some Process Signature components for a thermally dominated process with phase transformation: laser hardening. The target quantities of the modifications are the change of the residual stress state at the surface and the position of the 1st zero-crossing of the residual stress curve. Based on Finite Element simulations the internal thermal loadings during laser hardening are considered. The investigations identify for the considered target quantities the maximal temperature, the maximal temperature gradient, and the heating time as important parameters of the thermal loads.

Highlights

Manufacturing processes have an impact on the functional performance of components [1,2,3]
The target quantity is the depth of the 1st zero-crossing of the residual stress profile from pressure to tensile stress
Process Signature components for the target quantities “residual stress on the heated surface” and the “1st zero-crossing of the residual stress profile” are elaborated for laser hardening

Summary

Introduction

Manufacturing processes have an impact on the functional performance of components [1,2,3]. On the one hand these impacts are not always wanted Finishing processes such as grinding or hard turning affect the requested features and generate sometimes thermal effects which change the workpiece surface layer properties, e.g., residual stresses, microstructure, and hardness. High residual tensile stresses or large stress gradients may lead to reduced lifecycle performance because of easier crack initiation and propagation. If a surface hardening is wanted a selective heat treatment on the other hand can lead to an optimal surface hardness and residual stress state. These examples make clear that the most precise knowledge of plastic deformations and modified microstructures, which are generated by the considered processes, is necessary

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