MECHANISMS IN FATIGUE-STRENGTH IMPROVEMENT OF THERMOMECHANICALLY MANUFACTURED AUTOMOTIVE SUSPENSION SPRINGS

In a previous work, application of thermomechanical treatment (TMT) to suspension spring samples led to high increases in fatigue strength ( up to 40 %). For the purpose of a better comprehension of these result s, further investigations were carried out with major attention to the mechanical properties of quenched and tempered steel 50CrV4 and a Nb- microalloyed variant of this steel, respectively. First, the samples w ere ground prior to presetting, stress peening and subsequent dynamic testing. This was done to work out the influence of the surface state on the benefits of TMT. An extended program of materials testing was c arried out with particular attention on the crack initiation and growt h features of thermomechanically treated steel compared to conventiona l heat treatment (CHT). One of the results was that grinding of the sp ring surface causes further improvement of fatigue strength. On the ot her hand, the advantages of TMT compared to CHT are reduced as a resul t of surface grinding. The reason for this result is an overproportion al increase in fatigue strength of CHT specimens due to grinding. An e xplanation imaginable for both the improvements caused by TMT and the reduction of these improvements after grinding could be the change in fracture mechanical properties. In particular TMT causes a significant ly retarded crack initiation in the finite life range and an increased permissible stress intensity without crack formation.