In order to avoid too many times of Nitrocarburizing on different parts of lzqt600-3 ductile iron profile, the difficulty of the experiment is increased. The orthogonal test method is used to design the experiment content. When there are many factors influencing the experiment, it belongs to the situation of multiple factors and multi levels. Due to the nature of orthogonality, some points with the characteristics of “uniform dispersion, integrity and comparability” are selected from the original comprehensive test.

Since its establishment, orthogonal experiment has been paid attention to and used because it is scientific, innovative and efficient. The orthogonal experiment was designed for three factors and three levels, and a total of 9 groups of processes were needed. The three factors are the time t, the temperature T and the gas ratio s respectively. In L9 (34) orthogonal table design, the factors of nitriding time t, temperature T and gas ratio s were successively arranged into the first three columns of L9 (34) orthogonal table, and then the nitrocarburizing test schedule was obtained by the actual level value of each factor.