ばね鋼の脱炭現象の速度論解析

抄録

The guideline as kinetic equation of decarburizing to prevent excess decrease in fatigue strength, which is expressed as temperature the quenching starts, has been proposed from the systematic investigation on decarburizing spring steels;

T_Q = 910 + 44.7^%Si - 30^%Mn - 11^%Cr - 15.2^%Ni + 31.5^%Mo - 203√^%C - (^%C - C_S) {1 - exp (-(3.29 × 10⁸t) / 2d exp (-(3.61 × 10⁴) / (T_h' + 273)))}

where ^%M[mass%] indicates content of element M in steel, d[m] initial particle size of austenite. Referring the guideline, quenching from above the evaluated temperature, T_Q can prevent excess decrease in fatigue strength the spring steel exposed to high temperature decarburizing atmosphere, whose temperature is T_h'[°C] and decarburizing time is t[s], because the quenching leads to form the quench microstructure without surface ferrite layer. In the case that the exposed atmosphere is excess CO₂ content, as same as the atmosphere of gas combustion furnace, the value of surface carbon content, C_S[mass%] can be employed to be 0.2. Moreover, if the cooling rate is grater than 80 °C/s corresponding to the cooling rate of oil quenching, the guideline can be used to every spring steels. Similarly if temperature rise is slow, the guideline can be acceptable with modification of the term of decarburizing time to t + 0.03t_r, where t_r[s] is the time period of temperature rise.