Papers by Keyword: High Nitrogen Steel (HNS)

Abstract: The experiments of high nitrogen steels refining have been researched with high pressure and bottom-blown nitrogen in laboratory. We can gain high nitrogen nickel-free stainless steels that its content is more than 0.9%. The research indicates that the temperature influences nitrogen content greatly in the processes of smelting and concreting. The results are shown that saturated nitrogen content in steel increased slightly. When the temperature is as high as 1913K, the nitrogen content can reach 1.191%.

Abstract: This paper aims to study the effect of time and pressure on the nitrogen solubility limit through single factor experiment. The experiment is operated in the independent designed device whose main part is high temperature and high pressure reactor, Using high pressure and bottom blowing nitrogen in the furnace, it can gain high nitrogen nickel-free stainless steels whose nitrogen content is more than 1.1%.;The results show that there is solubility limit of nitrogen in 18Mn18CrN steel under certain condition ; The solubility limit of nitrogen in steel increases with the growth of pressure which ranges from 0.4MPa to 1.4MPa.

Abstract: The high nitrogen steel is alloy steel whose nitrogen content is more than solubility limit at atmospheric pressure[1]. Bottom-blowing high-pressure nitrogen smelting nitrogen steel has many advantages, in the paper, from the study of 18Mn18CrN,it can come to a conclusion that in the case of Temperature of 1853K, pressure of 1.0MPa, the bottom-blowing flow of 0.16m³ / h and temperature of 1873K, pressure of 1.4MPa, the bottom-blowing flow of 0.16m³ / h, With the bottom-blowing time of increased the nitrogen content in steel increases, but when the time is more the 10 minutes, the nitrogen content is saturated.

Abstract: Segregation and liberation occur easily in high nitrogen stainless steels, which can result in the formation of pores and the discarding of the steels. So it is very important to investigate the segregation and liberation theory during the solidification process of high nitrogen stainless steels. In the manuscript, the pouring and solidification progress of high nitrogen steels ingots were simulated through Pro Engineer and PROCAST software. The dynamic variation of temperature field and pressure field were obtained. With the results of PROCAST, the macrosegregation of 20kg weight ingot and 1.7 tons weight ingot were predicted using mathematical model of zone segregation. The predicted results show that 20kg weight ingot has no obvious macrosegregation and 1.7 tons weight ingot has obvious macrosegregation, which is consistent with measuring results. It shows that the accuracy of simulation and calculation to segregation of high nitrogen steels solidification process based on PROCAST software is high.

Abstract: The experiment studied the influence of slag system on nitrogen content in high nitrogen steels which refined by the method of high-pressure and bottom-blowing nitrogen under the conditions of 1.0Mpa. The results shows that nitrogen content are up to 1.0% without the over of slag, and after adding slag, nitrogen content are up to 1.39%; Alkalinity of protecting slag has little effect on the nitrogen content, however, there are very effective to increase nitrogen after using CaF2 instead of SiO2 ,and its nitrogen content are as high as 1.48%; the effect of increasing nitrogen are clear by adding no more than 0.5% pure aluminum, but when aluminum content are more than 0.5%, the nitrogen content in steel are not change significantly, meanwhile, if adding CaF2 slag system, it will be more effective increase nitrogen than only adding pure aluminum.

Abstract: The experimental results on high nitrogen steels refining with high-pressure and bottom-blowing nitrogen were analyzed by the theories of thermodynamics and kinetics in high nitrogen steels smelting. It was shown that nitrogen content in steel increases with the increase of the following factors, that is, pressure, alloying elements (Cr and Mn), bottom-blowing time and bottom-blowing flow. While the nitrogen content in steel decreases with the temperature increase, but it is not obvious. The nitrogen content in steel can also increase with the surface active elements (O and S) decreasing.

Abstract: To understand the reason of high work hardening in high nitrogen steel, a scale-bridging analysis consisting of in situ neutron diffraction, in situ electron backscattering diffraction measurement during tensile deformation and weak beam transmission electron microscopy was performed with particular attention to stress partitioning. This study demonstrates the contribution of stress partitioning, back stress due to dislocation pile-up at grain boundaries and dislocation interaction to work hardening at each strain level.

Abstract: The Fe-25Cr-1N alloy produced by solution nitriding possesses extremely high yield strength owing to the solid solution strengthening by nitrogen. However, it was found that the steel exhibited an insufficient elongation because of the brittle intergranular fracture caused during the uniform tensile deformation. This is due to the marked stress concentration at grain boundaries, which is derived from the grain coarsening caused during long time solution nitriding and the development of planar dislocation structure characteristic of high nitrogen austenitic steels. The most effective way to reduce the stress concentration at grain boundary during deformation should be grain refinement. In this study, grain refinement was attempted by the two-step heat treatment for the Fe-25Cr-1N(-Mn) alloy, and then the mechanical properties were investigated by means of tensile tests and fatigue tests. The two-step heat treatment resulted in the grain refinement of austenite to 20 microns in diameter. The intergranular fracture was greatly suppressed from 70% (as-solution-nitrided) to 10% (grain-refined) in area fraction by the grain refinement. In addition, elongation was markedly increased with local necking. The yield stress and tensile strength were also increased, and thus, the fatigue limit is also raised by more than 30%.

Abstract: Sliding wear mechanisms of the solution treated 18Cr-18Mn-2Mo-0.9N high nitrogen steel (HNS) were studied. Room-temperature dry sliding wear tests of the steel were carried out at various applied loads to explore the mechanism as a function of the load. The wear rate of the steel increased with the increase of the load; however, the increase rate was not constant. The rate increased slowly at low loads, rapidly at intermediate loads, and finally, the increase-rate became low again at high loads. Worn surfaces, their cross sections, and wear debris were examined, and phases of the heat-treated HNS as well as the wear debris were identified to find out the mechanism. The wear of the steel was found to be controlled by the tribo-oxidation, strain-induced phase transformation, and reverse transformation due to temperature rise on a wearing surface. The influence of each mechanism on the rate varied depending on the magnitude of the applied load.

Abstract: The effect of Cr2N precipitation on deformed microstructure in high nitrogen austenitic Fe-18Cr-18Mn-2Mo-0.9N steel was investigated with a particular emphasis on deformation twinning. Based on the crystallographic analysis in the stereographic projection, the orientation relationship between austenite (γ) matrix and Cr2N was determined to be Cr2N [110]γ //[1100] and Cr2N (111) γ //(0001) . The deformation twinning had {111} < 112 > crystallographic component similar to that of cellular Cr2N. The cellular Cr2N precipitates caused a different orientation dependence of deformation twinning: only one twinning system in the <111 > grain was activated almost parallel to the growth direction of Cr2N.