Lzqt600-3 ductile iron profile with horizontal continuous casting was selected as the research object. The as cast microstructure and properties of different parts (edge, 1 / 2R, center) of the profile were studied, and the influence of horizontal continuous casting process on the microstructure and properties of the sample was obtained. In order to obtain the piston pump cylinder body material with stable neutral energy in actual production, the lzqt600-3 ductile iron profile was nitrocarburized by orthogonal design experiment. The variation rules of the nitriding parameters (holding temperature T, holding time t, NH3: CO2 gas ratio s) on the thickness, surface hardness, effective hardened layer depth and wear resistance of the carburizing layer were explored, and the optimal experimental parameters were determined. Finally, the surface hardness and effective depth of hardened layer of the samples prepared by the optimal parameters were tested

(1) From the edge, 1 / 2R to the center of lzqt600-3 ductile iron profile, the size of graphite balls increases in turn, and the spheroidization level decreases in turn; the number of graphite balls is 318, 312 and 303 / mm2; the average shape factor of graphite balls is 0.724, 0.672 and 0.660; the average diameter of graphite balls is 29.86, 26.78 and 31.03 μ m, respectively. The amount of pearlite from the edge, 1 / 2R to the core is 64.36%, 65.90% and 70.84%, respectively. The pearlite lamellar spacing is 274, 286 and 316 nm.

(2) The friction coefficients of lzqt600-3 ductile iron profile from the edge, 1 / 2R to the center are 0.565, 0.612 and 0.611 respectively, and the wear rates are 3.373, 1.337 and 1.679, respectively. The wear degree of 1 / 2R is close to that of the center, which is better than that of the edge.

(3) Lzqt600-3 ductile iron profile has the best corrosion resistance at the edge, 1 / 2R is slightly lower than the edge, and the core is the worst.

(4) The compound layer of nitrided layer of lzqt600-3 ductile iron profile is composed of oxide layers (OLS), compound layers (CLS) and diffusion layers (DLS). The number of active nitrogen atoms increases with the increase of nitriding time, and the phase transition of α – Fe (n) → γ – Fe (n) → γ′ – Fe4N occurs. When γ′ – Fe4N reaches the supersaturation state, the phase transition of γ′ – Fe4N → ε – fe2-3 (n, c) occurs. Therefore, the permeating layer is composed of compound layer composed of ε phase and γ′ phase and diffusion layer composed of α – Fe phase.

(5) The influence of nitriding parameters on the thickness of carburizing layer in three parts of lzqt600-3 ductile iron profile is as follows: nitriding time t > gas proportion s > nitriding temperature T. The results show that the increase of nitriding time and gas ratio has a positive effect on the thickness of the sample, and the thickness of the surface layer determines the wear resistance, hardness and other properties of the workpiece. Therefore, the level with the largest thickness of the infiltration layer under various factors should be selected. The optimal scheme is 580 ℃ × 6 h, NH3: CO2 4:1.

(6) The influence of nitriding parameters on the surface hardness of lzqt600-3 ductile iron profile is as follows: nitriding time t > nitriding temperature T > gas ratio S. with the extension of the nitriding time, the number of active atoms in the surface layer increases, and the hardness of the layer increases. Therefore, we should choose the surface hardness level under various factors. The optimal scheme is 560 ℃ × 4 h, NH3: CO2 is 4:1.

(7) The influence of nitriding parameters on microhardness (effective hardening depth) of lzqt600-3 ductile iron profile is as follows: temperature T > gas ratio s > time t. In the actual working condition, with the continuous wear of the workpiece, the thickness of the carburized layer decreases, and the effective hardening depth and the hardness of the compound layer should be the maximum. Therefore, the maximum effective hardening depth level should be selected under various factors. The optimal scheme is 540 ℃ × 2 h, NH3: CO2 4:4.

(8) The influence of nitrocarburizing parameters on the comprehensive score of friction and wear properties of lzqt600-3 ductile iron profile is as follows: temperature T > gas ratio s > time T. the optimal scheme is 580 ℃ × 4 h, NH3: CO2 is 4:1.

(9) The results show that there are obvious furrows on the surface of as cast and nitrocarburized samples after friction and wear tests. With the increase of testing time, the as cast samples change from slightly adhesive wear to more serious adhesive wear. The nitrocarburized samples are affected by both adhesive wear and abrasive wear. However, due to the existence of hardening phase on the surface, the as cast and nitrocarburized samples have obvious furrows Compared with the as cast sample, the friction coefficient and wear rate of the as cast sample are the highest, which means that the wear resistance of the sample after infiltration is obviously improved.

(10) Through the comprehensive balance method, the optimal conditions of the four indexes were compared and analyzed, and the optimal scheme was obtained as follows: 580 ℃ × 4 h, NH3: CO2 4:1.