On Reliability of Stainless Maraging Steel

The investigation results on the problem of the reliability of high-strength low-carbon maraging steel products have been generalized. The influence of a method for remelting on the reliability behavior is shown and the ways for the reliability behavior improvement are suggested. The study of the reasons for decreasing KCU during heat treatment shows that in addition to the precipitation of phases causing brittleness at cooling, chromium zones at heating, and formation of chemical and structure inhomogeneity in the two-phase region, the main reason is the remelting method with the parameters which predetermine the variation in grain size in the structure, a small number of interstitial elements (IE), retained austenite in the structure, and lower level of KCU of the steel prepared by VAR both after quenching and after TST. Shows influence of the quenching temperature on the amount of retained austenite and level of impact strength (KCU), of the time of aging on the work of the crack development (KCV) at the temperature of maximal development of brittleness in steel 08Cr15Ni5Cu2Ti and on the position of brittleness transition temperature prepared by ESR and VAR. After cooling down to the liquid nitrogen temperature, the VAR-steel is less liable to brittle fracture after maximal strengthening aging and more reliable after 1.5h-aging (KCV is twice as much as that in ESR-steel despite the low KCU level). The science-based regimes are developed for stamped semi-finished items from steel 08Cr15Ni5Cu2Ti allowing guaranteeing the proper quality and reliability of functioning of the items made from them.

Plasma Ion Nitriding of Low Carbon Stainless Maraging Steel

Low carbon stainless maraging steel (0.03%C-12%Cr-10Ni-0.6Mo-0.2Ti) is being used widely for various components of the aerospace engines. To improve the wear resistance of the steel various surface hardening processes are being utilized to improve the surface hardness above 900HV. In this present research, plasma nitriding was carried out at two different temperatures of 450 °C and 475 °C for the holding duration of 10 hrs. Temperature of the nitrding process was ensured below the ageing temperature (500 °C) of the steel to avoid lowering of mechanical properties. Effect of plasma nitriding parameters on the surface hardness, case depth, microstructure and phases present in the nitrided layer were investigated in detail using microhardness analysis across the nitrided layer, X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM). It was observed that increase in nitriding temperature increased the surface hardness and case depth. In addition, the presence of Fe3N and Fe4N phases in the nitrided layer were observed using X-ray diffraction technique.

Optimization of Aging Cycle of Stainless Maraging Steel Using Dilatometric and Differential Scanning Calorimetric Analysis to Improve its Strength

04X13H5M5К9Л is a stainless maraging steel with high strength and excellent toughness. This alloy has been selected for making Impeller casting which is subjected to localized stresses reaching as high as 700 MPa at the tip of its vanes. The impeller rotates at 19000 RPM for 253 seconds.This alloy comprises of austenitic – martensitic dual phase at room temperature, in which martensite exhibits Body Centered Cubic (BCC) structure with moderate hardness (HRC 30) and high toughness. The required strength is achieved by precipitation of second phase particles in soft martensite matrix at suitable aging temperature. There is always a chance of reversion of martensite to austenite on heating either for extended period or at higher temperature. Thus optimization of aging temperature and time becomes highly critical keeping in mind the functional stress requirements of the component. The optimization has been done using Differential Scanning Calorimetric (DSC) analysis, dilatometric studies and experimental iterations of heat treatment temperatures and aging time.Different combination of strength and ductility could be achieved by varying heat treatment parameters. Martensite start and finish temperatures and austenitic reversion temperatures were established through DSC and dilatometric studies. Effect of subzero temperature on properties and microstructure of this steel is also presented here. Scanning Electron Microscopy and Energy Dispersive Spectroscopy were also carried out to analyze phases in different heat treatment conditions.