クロムモリブデン鋼の浸炭硬化層の機械的およびX線的弾性定数に及ぼす炭素濃度の影響

In order to determine the mechanical and x-ray elastic constants of carburized case layer in chrome molybdenum steel, three thin bend bar specimens with a thickness of about 0.3mm were cut from the carburized block after quenching by wire-electrical discharge machining. Carbon content and hardness gradients of block were measured before cutting. Thin bend bar specimens were subjected to four-point bending loads. The elastic strains were monitored using a strain gauge cemented on the tensile surface. Under each bending loads, the 2θ-sin^{<font size="-1">2</font>}<i>ψ</i> diagram from 211 plane of ferrite or martensite was also measured by x-ray. In the case of thin bend bar specimen, apparent Young's modulus determined from the slope in the applied stress versus monitored strain curve was relatively smaller than that of the ordinary bend bar specimen. In this study the monitored strain was corrected by the thickness of gauge base including glue. As a result, Young's modulus of case layer was larger than that of the interior, because of increased carbon content and resulting martensite transformation of carburized layer. Young's modulus of the interior at the total case depth was smaller than that of starting material. X-ray elastic constants, <i>E_{<font size="-1">X</font>}</i>/(1 + <i>ν_{<font size="-1">X</font>}</i>) and <i>E_{<font size="-1">X</font>}</i>/<i>ν_{<font size="-1">X</font>}</i>, of thin specimens were determined from the changes of the slope and intercept in sin^{<font size="-1">2</font>}<i>ψ</i> diagram with the applied stress. X-ray elastic constants of case layer were larger than that of starting material. The absolute value of stress constant, <i>K</i>, of case layer was also larger than that of starting material. It was found that up to 25 percent of the residual stress might be underestimated in our case if the stress constant, <i>K</i>, of starting material was used in x-ray stress measurement.