Institute of Metals Division - X-Ray Line Broadening of Hardened and Cold Worked Steel

Warren and Averbach's multiple order, Fourier method was applied to solid specimens of SAE 1045 steel. The material was hardened to three hardness levels and specimens of each hardness were shot peened to three intensities. Particle size, root-mem-square strain, mean strain, md macro residual stress were determined for each. Microstrains caused by either hardening or peening varied markedly over microregions. In general, increasing the hardness or peening intensity increased the rms strain and decreased the particle size. Similar effects were observed by varying hardness and peening intensity independently. At a Rockwell hardness of C 50, peening had little effect on line broadening, but both mean strain and macro residual stress increased. Mean strain was independent of column length. Induced macro residual stress by peening was affected most by initial hardness, but not by specific peening intensity. The purpose of this work is to establish the state of residual strain and particle size by X-ray diffraction in hardened and cold-worked plain carbon steel and attempt to relate these X-ray measured parameters to fatigue performance. X-ray line broadening (and its micro implications), along with X-ray residual stress (and its macro implications), may be basic indicators of the inherent fatigue resistance of the material. X-ray line broadening is caused by variable lattice strains and small particle size. Both are a result of plastic deformation which accompanies hardening or cold working processes. Therefore, X-ray line broadening is a good measure of both. warren1 has recently reviewed the Warren and Averbach2,3 Fourier analysis of line broadening. Strain and particle size effects can be separated because broadening due to particle size is independent of order of the diffraction peaks, while broadening due to strain is not. Microstrain is obtained as a root-mean-square strain averaged over discrete distances within the crystals causing diffraction. These distances are quite small, varying from 10 to 10081, and may be interpreted as the gage length over which the strains are averaged. Because of their variable nature, the X-ray line is broadened. The plot of rms strain vs distance (or column length) within the specimen gives a measure of their variability. Small particles also cause X-ray line broadening as already mentioned. A particle is interpreted as a region within a crystal or grain which diffracts X-rays coherently. This region is one of given orientation and may be bounded by a small angle grain boundary, dislocations, a twin boundary, or a stacking fault. The present work covers measurement of X-ray line broadening and macro residual stress on an SAE 1045 steel, quenched and tempered to various hardnesses. Various amounts of cold working were produced by shot peening the surfaces to various Almen intensities. Particle size, root-mean-square strain, and mean strain were calculated from the Fourier coefficients of the X-ray diffraction profiles in a direction normal to the peened surface. No effort was made to separate the various causes of particle size broadening. Macro residual stresses were reported in directions parallel to the peened surface. I) X-RAY METHOD The Fourier coefficients were determined for the 110 and 220 X-ray profiles. They were corrected for instrumental broadening by the method of Stokes,4 using an annealed reference specimen of similar geometry presumed to be free from broadening due to small particles and distortions. The nth Fourier cosine coefficient, A,, is related to a particle size coefficient, Ap, and a distortional coefficient, ADn, by the equation: