ANALYSIS OF RESIDUAL STRESSES IN MAIN CRANKSHAFT BEARINGS AFTER INDUCTION SURFACE HARDENING AND FINISH GRINDING
INDUCTION SURFACE HARDENING
Â¢ Short Heat Treatment Times
Â¢ Good Repeatability
Â¢ Hardened Layer Quality
Â¢ Small or Negligible Subsequent Distortion
Â¢ Minimum Subsequent Product Surface Oxidation
Â¢ Good Possibilities for Automation
SURFACE INTEGRITY AND ITS LEVELS
Scientific discipline providing an integral assessment of the surface and surface layers
Measurement of Roughness and Analysis of Microstructure and Microhardness in the Thin Surface Layer
Studies of Residual Stresses in the Surface Layer and of the Mechanical Properties of the given material
Tests making clear the Behaviour of the given part under Operating Conditions
Important features on the curves are:
(a) the maximum value of the compressive residual stress in the hardened surface layer.
(b) the maximum value of the tensile residual stress in the hardened surface layer.
© the transition width from compressive to tensile residual stress in the hardened surface layer.
(d ) the transition steepness from a compressive to a tensile residual stress profile.
(e) the depth with a transition microstructure.
The last phase in the manufacturing of crankshafts is finish grinding in order to achieve the desirable condition at the surface and in the surface layer, i.e.:
1. The grinding residual stresses must be compressive or lowest tensile so that the favourable residual stress profile obtained by induction surface hardening of the surface layer is maintained.
2. There must be the smallest changes possible in the microstructure and thus also in the hardness and microhardness profiles in the heat-affected zone after finish grinding.
Microstructural changes depends on:
Â¢ Contact zone
The volume changes that may occur in the surface layer after steel grinding are thus as follows:
1. The formation of residual austenite contributes to the formation of tensile residual stresses due to finish grinding.
2. The lower content of cementiteâ€œcarbide phase results in the formation of ledeburite containing residual austenite, which contributes to the formation of tensile residual stresses due to finish grinding.
3. Tempering effects in the martensite microstructure also contribute to the formation of tensile residual stresses due to finish grinding.
4. Material plastic deformation due to finish grinding can produce compressive or tensile residual stresses depending on the type of grinding wheel and the grinding conditions.
Â¢ Induction surface hardening creates a very desirable residual stress state at the surface and residual stress distribution below the surface
Â¢ Residual stresses are always of a compressive nature and are usually present to the depth of the induction surface hardened layer
Â¢ Additional grinding of the induction surface hardened surface has an adverse effect on the stress state in the surface layer, since grinding always induces tensile residual stresses
Â¢ Correct selection of the machining conditions and the grinding wheel helps lessening the relative grinding tensile residual stress distributions which will help keep the favourable residual stress state after induction surface hardening