Study On Nitrocarburizing Technology,Microstructure And Properties Of Ductile Iron Dense Bar For Piston Pump Cylinder

Taking horizontal continuous casting LZQT600-3 ductile iron dense bars(HCCDIBs)as the research object,aiming at the requirements of the microstructure and properties of the dedicated HCCDIBs for plunger pump cylinder.The characteristics of the microstructure and propertyies of different parts(Surface,1/2R,Center)of HCCDIBs were studied.Image J software was used for quantitative statistical analysis of as-cast microstructure(graphite characteristics,pearlite number and interlamellar spacing).In order to improve the hardness and wear resistance of the material working surface of the plunger pump cylinder.Orthogonal experimental design method was used to nitrocarburize on three parts of LZQT600-3 HCCDIBs surface.By comparing and analyzing the evolution of the co-infiltration parameters on the thickness of the surface infiltration layer,the depth of the effective hardened layer,surface hardness and wear resistance between the evolution law of the experiment,the optimal parameters of the experiment were determined.The main conclusions are as follows:(1)The matrix microstructure of LZQT600-3 HCCDIBs is composed of ferrite and pearlite.From the surface,1/2R to the center,the size of the graphite ball increase from 7 to 6;the spheroidization level reduce from ??? level to ??? level;Statistics results show that the number of graphite balls decreases and the average shape factor of graphite balls decreases gradually.The average diameter of graphite balls increases from 29.86 ?m to 31.03 ?m;The pearlite content in the matrix increase from 65%to 75%,and the spacing of pearlite lamellar tends to increase.(2)Due to the largest content of pearlite in the center and has a high wear resistance,at the same time,due to the largest spacing of the pearlite in the center,the corrosive liquid is more likely to enter the matrix for corrosion.Therefore,the wear resistance of the LZQT600-3 HCCDIBs from the surface,1/2R to the center gradually improves,and the corrosion resistance is gradually deteriorated.(3)The permeation layer on the surface of the nitrocarburized sample of LZQT600-3 HCCDIBs is composed of a compound layer composed of ?-Fe2-3(N,C)phase and ?'-Fe4N phase and a diffusion layer composed of ?-Fe phase.(4)The effect of nitrocarburizing parameters on the thickness of infiltration layer thickness of LZQT600-3 HCCDIBs is in the following order of primary and secondary:co-infiltration time t>gas ratio s>co-infiltration temperature t,the optimal scheme is 580?×6 h and 4:1(NH3:CO2).(5)The effect of nitrocarburizing parameters on the surface hardness of LZQT600-3 HCCDIBs is in the following order of primary and secondary:co-infiltration time t>co-infiltration temperature T>gas ratio s,the best scheme is 560?×4 h and 4:1(NH3:CO2).(6)The effect of nitrocarburizing parameters on the effective hardened layer depth of LZQT600-3 HCCDIBs is as follows order of primary and secondary:co-infiltration temperature T>gas ratio s>co-infiltration time t,the best scheme is 540?×2 h and 4:4(NH3:CO2).(7)The effect of nitrocarburizing parameters on the comprehensive friction and wear performance of LZQT600-3 HCCDIBs is in the following order of primary and secondary:co-infiltration temperature T>gas ratio s>co-infiltration time t,the best scheme is 580?×4 h and 4:1(NH3:CO2).(8)Through the comprehensive equilibrium method,the optimal conditions of the four indexesi are comprehensively compared and analyzed,and a better scheme is obtained:540?×6 hand4:3(NH3:CO2).