Masters Theses
Date of Award
8-1992
Degree Type
Thesis
Degree Name
Master of Science
Major
Metallurgical Engineering
Major Professor
Anthony J. Pedraza
Committee Members
D. H. Lowndes, C. R. Brooks
Abstract
Stainless steel specimens were analyzed by X-ray diffraction (XRD) in the as-received conditions and cold rolled condition and after excimer laser treatments. The sin2ψ method was employed to measure the macro-residual stresses. The contribution to line broadening from mosaic structure and defects present in the stainless steel specimens were obtained using the Fourier analysis method developed by Warren and Averbach. Stainless steel films were sputter-deposited on stainless steel substrates. Diffraction patterns showed that when the passive oxides layer present in the substrate was not removed the film had a body-centered cubic crystal structure. If the oxides layer is removed, then the films grow from the substrate, and have a face-centered cubic structure(FCC). Thermodynamic calculations suggest that the stable phase in austenitic stainless steel should be BCC rather than the usual FCC. It is reasonable to assume that if no oxide layer is present the films should grow in a homoepitaxial manner reproducing the FCC crystal structure. If an oxide lAyer is present, then nucleation of the stable BCC can take place as observed. Tensile residual stresses were produced in the surface of 20% cold rolled specimen. In the 40% cold rolled and 60% cold rolled specimens the residual stresses were compressive in nature. In the rolling operation the metal sheet is subjected to the compressive stress by sqeezing action of the rolls and the shear stress in the surface due to frictional force between rolls and metal sheet. That is, the surface is deformed more than the center, producing the surface compressive residual stresses. However, the tensile residual stress produced in 20% cold rolled specimen indicates that the center was elongated more than the surface. Stainless steel specimens in the annealed and with 20%, 40% and 60% cold rolling were irradiated with a XeCl excimer laser at various energy densities. The X-ray lines of annealed specimens were broadened after laser irradiation. On the other hand the line broadening of the X-ray lines for all the cold rolled specimens was reduced. The laser processing in metals produced two effect; first, annealing owing to the heating process, and second, plastic deformation due to the extremely steep thermal gradient. Fourier analysis of the line broadening in annealed specimens showed that at lower energy densities the broadening was mostly due to the generation of single dislocations (lattice strain broadening) and for higher energy density the mosaic structure (size effect broadening) was the major contribution to the line broadening of annealed specimen. The mosaic boundaries, also called cell boundaries, originate from dislocation aggregation and interactions after laser irradiation. In the cold rolled specimens the size effect on the line broadening decreased after laser irradiation. The single dislocation density did not change during the laser processing of the cold rolled specimens. Residual macro-stresses are always tensile, independently of the initial state of the specimen (viz. annealed or cold rolled). Moreover, the magnitude of the stresses measured is almost the same for annealed or cold rolled specimens irradiated with an energy density of 4J/cm2. The line width of silicon single crystals did not change after laser processing. This result indicates that very little or no plastic deformation takes place during laser processing of the silicon. In agreement with that result no macro residual stresses were detected in the irradiated silicon.
Recommended Citation
Park, Jaewon, "Residual stresses in stainless steel produced by cold rolling and laser processing. " Master's Thesis, University of Tennessee, 1992.
https://trace.tennessee.edu/utk_gradthes/12239