| 1RK91(12Cr-9Ni-4Mo-2Cu)maraging stainless steel,which contains lower than 0.01wt%carbon,possesses strength of 3000MPa together with high ductility, good fracture toughness,good resistance against overaging and high corrosion resistance after solution at 1000～1200℃followed by a ageing treatment in the range 400～500℃.The good mechanical properties are resulted from the fine precipitate in the martensite substrate during the ageing.The steel has been successfully used in surgical needles and bistoury.Titanium is the major constitutional element of the precipitate.Previous researches have made much works to identify the precipitates, but there is no report about the effect of Ti concentration on the microstructure and hardness of this alloy.Regarding to this background,the present work was conducted to investigate systematically the effect of Ti concentration on the microstructure and hardness,in a hope that it can help the application of the alloy on surgical instrument.The alloys(00Cr12Ni9Mo4Cu2)with various Ti concentration were melted with a vaccum conductive furnace.After solution treatment,the specimens were then cold rolled and aged at various temperature and time.By means of optical microscopy, hardness testing,X-ray diffractometry(XRD),scanning electron microscopy(SEM) and energy dispersive spectrometer(EDS),the effects of cold working and aging on the microstructure and hardness of alloy were studied and the optimal heat treatment process was obtained.The results show that the grain size of the parent anstenite decreased with Ti concentration after solution treatment at 1100℃.Lath martensite dominates in the matrix and a few residual austenite exists.The amount of residual austenite increases with Ti concentration.The alloys with lower Ti concentration transform to austenite more completely.It is found that the,δferrite exists in the alloy with 1.5wt%Ti concentration.Furthermore,the composition ofδferrite,as measured with EDS, showed a much lower nickel concentration and higher chromium,molybdenum and titanium concentration,compared to chemical composition of the investigated steel. Vicker's hardness showed that cold rolling of 75%resulted in the increase of△HV(72～103)and the hardness of the alloys both before and after cold rolling increased with increasing Ti concentration.The hardening effect of the subsequent ageing is much profound than that of the cold rolling,which gave an HV increase of 165～300.After aging at 450～500℃,the hardness increases with Ti concentration and 1 wt%Ti concentration can result in a hardness increment of about 115 HV.The hardness for all the alloys increased significantly during the early 5min aging,then increased slowly with increasing ageing time and finally reach the max at about 48 hour.The preferred aging temperature of the alloys with lower Ti concentration(0～1.0Ti)and higher Ti concentration(1.0～1.5Ti)is 450℃and 475℃,respectively. |
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