| 摘要 |
PURPOSE:To square the surface hardness and the internal hardness of parts after their surface is treated by slightly carburizing when the parts made of low- carbon boron steel is hardened, by setting the carbon potential of the atmosphere gas to a level higher than the carbon content of the crude material. CONSTITUTION:Parts made of a low carbon boron steel composed of 0.15- 0.35wt% C, 0.60-1.30wt% Mn, <=0.50% Cr, <=0.04% Ti, <=0.04% Al, 5-30ppm B, and the rest of Fe, are hardened in a protective atmosphere. In this case, the carbon potential of the atmosphere gas is set to a level which is 0.04-0.25 higher than the carbon content of the crude material, and the gas is minutely carburized into the surface of the parts. In this way, lowering in the surface hardenability due to the deboronization phenomenon of the low-carbon boron steel can be compensated, and moreover, other improvements, such as improvement in the fatigue strength of the parts, prevention of dents, etc., are also contrived. This heat treatment method is applied for high strength bolts for buildings. |
