Study On Gas Nitriding Of Grey Cast Iron Used In Diesel Engine Cylinder Liner

Cylinder liner is one of the most important components in diesel engines. It issubjected to sever loading conditions such as high temperature, high combustion pressureand wear consumption. Especially, with the increasing power density and rotating speed ofdiesel engines, the requirement for higher resistance of wear and higher mechanicalstrength of cylinder liners have been increasing continuously. Nitriding can be used toeffectively improve the wear resistance, corrosion resistance and fatigue durability ofcylinder liners.The present work does a research on the gas nitriding of grey cast iron used in cylinderliners. In this work the influence of nitriding parameters on nitriding layer and themechanical properties inside the liner is investigated. The main purpose of the study is topropose an analysis methodology for gas nitriding of cylinder liner, which can determinethe appropriate process parameters of gas nitriding to meet the specifications on the surfaceand excellent mechanical properties inside the liner.The main research contents and conclusions are as follows.(1) In the first part of this work, a model for the gas nitriding process has beenproposed, taking into account the threshold value of nitriding potential. This model is basedon mass transfer and diffusion theory. Besides, the proposed model can be solvednumerically using MATLAB.(2) In the second part, a series of gas nitriding experiments on cylinder liners areconducted. At the same time, the calculated numerical results are compared with the testeddata in order to verify the numerical simulations. Afterwards, the effects of nitridingprocess parameters such as the nitriding temperature, time, as well as the nitriding potentialon nitriding layer are simulated. The results indicate that the numerical simulations,incorporating the nitriding potential’s threshold value, can accurately predict the case depthand nitrogen concentration profile. Furthermore, it is found that the nitriding layer increaseswith increasing nitriding time and temperature, whereas the nitrogen concentration on thesurface decreases with increasing temperature. Besides, the results also reveal that thenitriding potential makes little effect on the case depth, whereas it has great influence onnitrogen concentration on the surface and chemical composition of the compound layer.(3) In the third part of this work, the elements of Nb, V, Ti are added into the grey cast iron to obtain the NbVTi alloyed cast iron, in order to improve the mechanical propertiesof cylinder liner after gas nitriding. In addition, the microstructure and mechanicalproperties of NbVTi alloyed cast iron are studied by means of scanning electronmicroscopy (SEM) and mechanical property tests, under different nitriding times andtemperatures. The results show that with the increase of holding time or temperatureacicular bainite becomes thicker and shorter, graphite gets thicker and longer. The tensilefracture of NbVTi alloyed gray iron is quasi-cleavage fracture. Some ductile rupturecharacteristics appear in tensile fracture which lead to improve of plasticity and toughnesswith the increase of holding time or temperature. The tensile strength and the Brinellhardness (BH) of NbVTi alloyed gray iron decline with the increase of holding time ortemperature. However, after treating the value of tensile strength and Brinell hardness isimproved more than the one of common bainitic grey cast iron.The present research has investigated the effect of gas nitriding process parameters onnitriding layer on the surface and mechanical properties inside the liners. The analysismethodology proposed in the paper is of value for determining the gas nitriding process ofgrey cast iron used in cylinder liners.