| Auto industry's rapid development greatly promote the plastic mould industry's prosperity.At the same time,the requirement for the plastic mould industry becomes more and more strict.In the process of producing polyvinyl chlorides and other plastic,specially,corrosive gases such as HCl,HF and SO2 generated and seriously deteriorate the mold cavity,which makes the mould failure.Therefore,it is required that the die steel should possess not only high hardness,good polishability,but also good corrosion resistance.Nitrogen addition can enhance the comprehensive properties,such as corrosion resistance and hardness,of die steel.In foreign,M340 is regarded as the typical high nitrogen plastic mould steel,which is belonging to the martensitic steel.The corrosion resistance could be greatly changed by heat treatment.Therefore,it is significant to investigate the effect of heat treatment on the corrosion resistance for the improvement of die life,promoting the development and application of high nitrogen plastic mould steel.The intention of this paper includes following parts:The high nitrogen plastic mould steel 55Cr18Mo1VN has been produced by using pressured induction melting in Northeastern University,China.Then the ingot was forged and rolled into plates for the experiment use.The optimal method of high temperature homogenization,normalizing and spheroidizing annealing has been obtained for 55Cr18Mo1VN.The microstructure,elemental distribution of 55Cr18Mo1VN different heat-treated sample was investigated by X-ray diffraction,electron probe microanalyser analysis,trans-mission electron microscopy and optical microscope.The corrosion resistance was evaluated using electrochemical measurements(including OCP,DP and EIS).In addition,the analysis of passive film for the quenched sample was carried out by X-ray photoelectron spectroscopy.The conclusions could be obtained as follows.(1)The optimal technology of high temperature homogenization for high nitrogen plastic mould steel 55Cr18Mo1VN is 1270?×8h air-cooled,it can significantly eliminate the component inhomogeneity during metallurgical process and obtain uniform microstructure;(2)The optimal process of spheroidizing annealing is 890×5h?20?/h×700??700?×3h?20?/hx600??X600??air-cooled,it is able to obtain the small size carbonitride and the carbonitride distributes evenly.Besides,almost all the carbonitride exist sphere.(3)Pitting corrosion initiated preferentially around the coarse M23C6 carbides,because the degree of Cr-depleted zone around Cr23C6 was severer than that around Cr2N.Therefore,the addition of nitrogen leads to generate small size nitride,which increases the corrosion resistance of plastic mould steel.(4)Corrosion resistance of quenched 55Cr18MolVN was improved with increasing austenitizing temperature.On the one hand,the volume fraction of Cr-rich precipitates decreased,inducing more uniformly elemental distribution and enhancing content of C,N and Cr in the matrix.With the increase in austenitizing,the passive film was thickened and Cr2O3 in the inner layer of passive film was enriched.The higher content of nitrogen in solid solution at higher austenitizing temperature contributed to the increased intensities of CrN and NH3,leading to the decrease in pH value in the pit and promoting the repassivation of 55Cr18Mo1VN.(5)The corrosion resistance of tempered 55Cr18Mo1VN increased firstly and then decreased with the increasing of tempering temperature.Besides,the corrosion resistance is worst at 500?.The reason for that is a large number of nano-sized carbides and nitrides precipitated during the tempering,which leaded to the connection of Cr-depleted zone,and then,deteriorated the corrosion resistance of 55Cr18Mo1VN stainless steel. |
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