The Effect of Residual Stress on the Impact Strength of Steel
Abstract
The processes of metal working, metal removal, and metal finishing are known to cause varying degrees of plastic deformation below the worked surface of a material, resulting in increases in hardness of the deformed layers, changes in the microstructure, and changes in the surface stress and internal stress states of the material.Much research has been concerned with the effects of these changes on the fatigue life of materials, but research with respect to their effects on other material properties has been relatively limited. This investigation attempted to establish whether or not the residual stresses induced by various methods of metal processing could be shown to have some effect on the impact strength of high-strength steel.Research consisted of preparing the tensile surface of an experimental form of a Charpy-type specimen by six different processing methods each at two levels of use. Processes included were milling, planning. grinding, electro- discharge machining, shot peening and burnishing. Specimens were prepared from two different lots of 4140 steel and tested on an instrumented Charpy testing machine under both single impact and, at a reduced load, under repeated impact. Preparation and testing was done using a planned statistical factorial experimental procedure and results were analyzed by use of analysis-of-variance and a statistical test for contrasts between methods.Specimens were examined for typical residual stress patterns to verify the stresses induced by each process used, and metallographic examination of the prepared surfaces and the subsurfaces were also conducted.Results indicated that the processes examined had a significant difference in effect on the impact strength and that those processes inducing compressive residual stresses in the material were the significantly different processes among those tested. The effects also varied with the two lots of material used, the materials alone having a significant effect on the impact strength. Methods of preparation and lots of material were also significant in the repeated impact tests but did not support the same conclusions as for single impact.
Degree
Ph.D.
Subject Area
Industrial engineering
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