| The family of maraging stainless steels is characterized by very low carbon contents and substitutional alloying elements, which produce age hardening in iron-nickel martensites. They have found a wide range of applications such as the case of rocket motor, the shell of missile and some important parts of airplane. A new type of maraging stainless steel 12Cr-9Ni-4Mo-2Cu (wt%), termed Sandvik 1RK91, was recently developed by AB Sandvik Steel. The material possesses high strength, good resistance against overaging together with high ductility and high corrosion resistance. Its main application is as parts in electric shavers, but it also found use in various demanding applications in medical field.The present alloy melted is a 00Cr12Ni9Mo4Cu2 maraging stainless steel. After solution treatment in the range 950-1200℃ and various cold working, the specimens were then aged at various temperature. By means of optical microscopy, hardness testing, X-ray diffractometry(XRD), Scanning electron microscopy (SEM) and Energy dispersive spectrometer (EDS), the effects of the solid solution temperature, coldking and aging on the microstructure and hardness of 00Cr12Ni9Mo4Cu2 maraging stainless steel were studied and the optimal heat treatment of 00Cr12Ni9Mo4Cu2 maraging stainless steel has been determined.The results show that the matrix consists of lath martensite after solution treatment at various temperatures. When subjected to higher temperature solution treatment, the grain size increases and more martensite was transformed. Furthermore, it is found that the δ ferrite exists in net along the grain boundary after lower temperature solution treatment and changes to spherical shape inside the grain. The amount of δ ferrite was also reduced gradually with the increase of solution temperature. The composition of δ ferrite, as measured with EDS, showed a depletion of nickel compared to chemical composition of the investigated steel, inversely, chromium, molybdenum and titanium were absolutely opposite. All elements of the matrix under solution state were found almost to be consistant with the chemical composition except copper. There is noobvious effect of solution treatment in temperature range of 950-1200℃ on the mechanical properties for the steel under solution state, cold working state and aging treatment state. The increment of the hardness is almost the same with various solution temperatures. Thus, the optimal solution temperature is considered to be in the range 1050-1100℃.Cold working before ageing treatment is contributed to improve the strength. The results showed that the hardness increased almost linearly with the increase of cold working. This is partly attributed to the stress induced martensitic transformation, as evidenced with XRD. Ageing after cold working can substantially increase the hardness of the alloy. The hardness increases significantly already during the first few minutes of the ageing treatment and continues to do so up to 24 hours of aging, softening only being observed at 550℃ and above. The softening could be explained by reversion from martensite to austenite, as evidenced by XRD. Furthermore, the composition of δ ferrite under aging treatment, as measured with SEM and EDS, showed almost the same as the chemical composition aging at 450℃ and parts of δ ferrite grow together when aged at higher temperature. Thus the optimal ageingtmcnt was considered to be 2 hours at 450℃. |
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