| The development degree of moulds is one of the important measures of a country's manufacturing level because it is the mould which is used to fabricates or processes various parts.Moulds are usually divided into hot work,cold work,plastic and special moulds according to different circumstances where they works.The demand and output of plastic molds are the highest,the yield of the steel used to produce plastic moulds reaching 50%of the total output of mould steels in 2017,showing great Prospects.Since many plastics release HCl,HF or other corrosive gases which can corrode or erode the moulds during the production process,so it is necessary to use the martensitic stainless steel possessing good mechanical and corrosion propertites as a corrosion-resistant plastic mould steel.At present,the domestic 4Cr13 plastic mold steel can not meet the production requirements of new types of plastics.Therefore,the objects of this project are to design a new kind of corrosion-resistant plastic mould steel and to study the strengthening mechanism by adding nitrogen or molybdenum.For starters,simulation and calculation were carried out to study the effect of carbon,molybdenum and nitrogen on phase transformation and precipitates using Thermo-calc software.The results showed that the carbon and nitrogen could not only stabilize the austenite phase,but also reduced the precipitation temperature of the coarse M23C6 phase and that the effect of nitrogen is stronger than that of carbon.Additionally,molybdenum could significantly lower the ??? transition temperature and remarkably increase the precipitation temperature of M23C6,though it could promote the precipitation of fine ? phase and M6C phase at the temperature between 300?500?.What is more,the calculation results provided a theoretical basis for the analysis of the subsequent design of chemical components and the analysis of the microstructures after heat treatments.Secondly,five kinds of experimental steel ingots with different compositions were smelted and refined using Vacuum Induction Melting followed by Electroslag Remelting.Then the microstructures of all ingots were observed by Optical Microscope and the secondary dendrite spacing was measured.It was found that nitrogen could narrow the secondary dendrite spacing i.e.dendritic segregation could be inhibited,while molybdenum showed the double-effects of inhibiting and promotes dendrite segregation depending on the cooling rate.Next,the ingots were annealed at 1250?,followed by forging at 1200?.They were then spheroidizingly annealed,austenitized at 1020?,followed by rapid cooling in oil,and finally twice tempered at 250?,followed by cooling in air.The analysis of the microstructures was next performed on heat-treated specimens using Optical Microscope(OM)and Scanning Electron Microscope(SEM).It was found that nitrogen could significantly refine the prior austenite grains and promot the precipitation,refinement and dispersion distribution of carbides.Although molybdenum could slightly promote the refinement of grains and precipitation of carbides,it resulted in the generation of ?-ferrite,and this also caused some of the carbides to increase in size and distribute along the grain boundaries.In this paper,Rockwell hardness measurement,impact test and tensile test at room temperature were carried out on the experimental steel after tempering.The results showed that nitrogen could improve the value of hardness,tensile strength and impact energy.To be more specific,the hardness of 3#experimental steel with 0.1 mass%nitrogen was up to 52.43 HRC,with tensile strength of 3#reaching 1909 MPa,and impact energy reaching 19.6 J.However,as the nitrogen content increased,the plasticity first rose and then decreased,and the ductility of 2#with 0.05 mass%nitrogen was the strongest.In addition,the ductility and impact toughness of samples with molybdenum were significantly reinforced.Specifically,the impact energy of 5#V-notched sample with 1mass%molybdenum reaches 20.2J,but the strength and hardness decrease considerably.Salt spray corrosion test was carried out with reference to DIN 50021 SS,and the electrochemical corrosion studies consisting of measuring the open circuit potential(OCP),the Potentio-dynamic Polarization Curve(PPC)and the Electrochemical Impedance Spectroscopy(EIS)were conducted in 3.5 mass%NaCl+HCl solution(pH=4)to research the corrosion resistance of nitrogen and molybdenum.The results showed that the addition of nitrogen and molybdenum significantly improved the corrosion resistance becausse the area ratio of salt spray corrosion area decreased significantly,the OCP value,corrosion potential,pitting potential and the impedance of passivation film increased significantly.The corrosion resistance of 4#steel both with 0.1%mass nitrogen and 1mass%molybdenum is the best,and the pitting potential,specifically,reached 142.7 mVSCE,with the passivation film impedance reaching 9.52×104?. |
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