X線による塑性変形後の低炭素鋼残留応力に関する研究

抄録

The data of stress determined by X-rays agree well with that obtained by the mechanical method in the macroelastic range. But the meaning of the residual lattice strain that is to be determined by X-rays as to the residual stress in metals subjected to plastic deformation is not yet sufficiently known. Especially in steel, the residual stress on the surface of the specimen after uniaxial plastic deformation shows higher compressive stress than in the interior by the so-called surface effect¹⁾ or the in homogeneity of strain hardening²⁾, and the residual compressive stress is distributed uniformly over the cross section of the specimen. Moreover, there are some problems of“Gefügespannung”^{4), 5)}due to the difference of the strength of ferrite phase and cementite phase³⁾, the wave length of X-rays and the diffraction plane dependency etc. For the application of stress measurement by X-rays in the determination of the residual stress to the plastically deformed metal, it is important to elucidate these problems.
After removal of load in each stage of uniaxial tensile deformation, in this paper, the residual stresses of the surface of several low carbon steel plates (contained 0.04%C, 0.06%C, 0.16%C, 0.21%C, 0.30%C) and the stress distribution over the cross section of specimens by electrolytic etching were determined by X-rays. The chemical composition and heat treatment of each specimen are shown in Table 1. The lattice strain was determined by sin²φ method from the photographic film of the diffraction line of (211) plane using CrKα₁. Since the average ferrite grain size in the finished specimens was about 0.02-0.04mm, the diffraction line become spotty. To avoid this effect a specimen holder consisting of horizontal and vertical oscillation mechanism was prepared (Fig. 2).
The residual stresses of the surface of the specimens above 0.06%C were all compressive and increased with increasing plastic deformation (Fig. 3). These compressive residual stresses were approximately proportional to the true stress obtained by mechanical test⁶⁾. The proportional constant was nearly 0.2 (Fig. 6). From these results, on the contrary, it is shown that the residual stress of the surface can be estimated from the applied mechanical stress, but the surface stress of the steel up to 0.04%C is low and not proportional to the strain.
The residual stress distribution over the cross section of each specimen was shown in Fig. 7. The residual stress of the specimens up to 0.16%C was higher on the surface or in the vicinity of the surface than in the interior. The residual stress distribution of specimens above 0.21%C was almost uniform. The difference of these distributions is considered not only to be affected by the carbon content but also by the plastic strain⁷⁾.