Numerical Simulation Of Laser Transformation Hardening On The Inner Wall Of Pump Barrel And Study On The Mechanism Of Rapid Nitriding

During the oil extraction, the pump barrel suffer from sever corrosion and wear whichreduce their service life. Based on the service environment of pump barrel, this thesisproposed the laser transformation hardening-ion nitriding compound treatment to modify thesurface of inner wall of pump barrel. Using the commercial code SYSWELD to simulate thelaser transformation hardening (LTH) to get the temperature field, microstructure distributionand stress field. The differences of residual stress distribution between plate model and innerwall model were studied. The influence of scanning speed on the LTH was analyzed and theoverlapping ratio for the multi-passes was optimized. By using the optimized laser parameters,the LTH-ion nitriding compound modified layer was made on the inner wall of pump barrel.The microstructure and properties of compound layers were analyzed. The effects of LTH onthe ion nitriding were studied though the combination of simulation and experiment results.The3-D model of pump barrel was established to simulate the laser transformationhardening on the inner wall of pump barrel. The3-D Gaussian heat source was fitted by thetool of heat input fitting of SYSWELD. The change of thermal mechanical parameters withtemperature was considered. The results show that LTH is a rapid heating and cooling processand its value could reaches to104℃/s and103℃/s respectively. After cooling to roomtemperature, martensite is the main phase of hardened zone. Compressive stress exists inhardened zone and tensile stress exists in the heat affected zone; the value of compressivestress in the hardened zone of pump barrel is lower than that of plate model. With the increaseof scanning speed, the peak temperature is decreasing and maximum heat and cool rate isincreasing. The fraction of martensite and value of compressive stress in the hardened zone go down with the increase of scanning speed.During the process of multi-passes, the martensite of first track was tempered due to theheat effect of second track. With the increase of overlapping ratio, the tempered extent of firsttrack is intensified. After multi-passes treatment, the stress of first track was transformed fromthe compressive nature of solo pass to tensile nature of multi-passes while the residual stressof second pass is compressive nature. The heat affected zone of first track and second trackhave great compressive stresses which can not be avoid by the change of overlapping ratio.The phase of compound modified layer was made up of ’-Fe4N and-Fe3N. The results ofEPMA show that the diffusion depth of element N in the hardened zone is longer than that inthe non-hardened zone. The maximum hardness of compound modified layer is755.9HV0.2and the hardened depth is700μm. However the hardness and hardened depth of ion-nitridingare about643.9HV0.2and400μm respectively. The change of scanning speed has littleinfluence on the hardness of compound modified layer while the hardened depth increase withthe decrease of scanning speed; The wear experiment of substrate, LTH, ion-nitriding andLTH-ion nitriding shows that the compound modified layer owns lowest wear coefficient andweight loss. The corrosion experiment shows that the compound layer has the highest Ecorrandthe least Icorrwhich demonstrates that the compound layer has a better corrosion resistance.The grain size was fine after LTH which lead to the increase of grain boundaries andlarge amount of dislocations were produced in the hardened zone which all provide newdiffusion channels for the element N.