X-Ray Investigation of the Residual Stress of Metallic Materials

On the Residual Stresses due to Plastic Deformation under a Uniaxial and a Multiaxial Stress System

Abstract

It is pointed out by the authors as the result of studies on the residual stresses of stretched and quenched carbon steel specimens by using X-ray method: 1) that thermal residual stress in the (211) crystal plane represents the macroscopic residual stress itself, 2) that it is considered that the compressive residual stress obtained by using the Cr-Kα beams in stretched carbon steel specimen includes the macroscopic residual stress due to the surface effect, and also the microscopic one due to the anisotropy of plastic deformation.
It has been reported by many researchers that X-ray compressive residual stresses are observed on the plastically stretched carbon steel specimens. However, considering from the recently obtained experimental results, some contradictions to the discussions are often found since the past studies treated only the residual stress due to the uniaxial stretching, and scarcely dealt with the residual stresses caused by other processes. For this reason, the authors carried out experiments to examine the nature of X-ray residual stress in order to extend the method of X-ray stress measurement to practical use.
The crystal plane dependence of residual stress in stretched or quenched specimen was investigated, and the biaxial residual stresses in the thin-walled tubular specimens caused by plastic deformation due to the internal pressure were measured.
Iron and five sorts of carbon steels with carbon contents ranging between 0.06 and 1.10 percent were prepared as test materials. All the specimens were annealed after machining. Some of them were water-quenched from 600°C after being annealed, and these specimens include the residual stresses of purely thermal effect.
The X-ray diffraction apparatus used was a parallel beam type X-ray stress analyser, and the diffraction lines of Co-Kα and Cr-Kα beams from (310) and (211) crystal planes, respectively, were automatically recorded.
The results obtained in this study indicate interesting features as follows:
1) The crystal plane dependence of residual stress is not seen in the quenched specimens, and the thermal residual stress can be accurately measured by means of X-rays.
2) On the other hand, it is clearly observed in plastically deformed specimens. This result indicates that presence or absence of crystal plane dependence may be effected according to the difference of mechanism causing the residual stress.
3) In addition, for plastically deformed specimens, compressive residual stress tends to approach a constant value at the center of the specimen. It is important to note that even in the iron specimen which consists of sole ferrite structure, residual compressive stress was observed in the core of the specimen.
4) The plastic deformation under a uniaxial or multiaxial load stresses induces a uniaxial or a multiaxial residual stress system, respectively, of microscopic nature.
Based on these results, the authors suppose at present that the cause of X-ray compressive residual stress of microscopic nature lies in the anisotropy of plastic deformation at room temperature, consequently, the diverse values of residual stress will be observed in different crystal planes.