Investigation Of Carbonitride In Nitrogen-combining 18Mn-4Cr-0.4C Non-magnetic Steel

The carbonitride in nitrogen-combining 18Mn-4Cr-0.4C non-magnetic steel has a serious detrimental effect on the production quality of steel. The production defect of 18Mn-4Cr-0.4C non-magnetic steel with 0.1 % and 0.03 % nitrogen is analyzed. The defect of 18Mn-4Cr-0.4C-0.1N non-magnetic steel is mainly penetrating crack at the edge of billet. Corrosion pittings distribute on the surface of 18Mn-4Cr-0.4C-0.03N steel, meanwhile there is also some cracks around the edge of billet. It is found that the carbonitride precipitates along grain boundary near the defect of these two non-magnetic steels. Furthermore during smelting of nitrogen alloying steel, the fluctuation of nitrogen easily causes the formation of air bubble at high nitrogen area, resulting in the presence of corrosion pittings on the surface. The isothermal precipitation behavior of nitrogen-combining carbides in 18Mn-4Cr-0.4C-0.1N and 18Mn-4Cr-0.4C-0.03N non-magnetic steel with the range from 800 to 950 °C is investigated by using metallography, x-ray diffractometry (XRD) and electron probe microanalyzer (EPMA). respectively, to establish the isothermal precipitation -dissolution kinetic curves of nitrogen-combining carbides. The results show that the nitrogen-combining carbide formed along austenitic grain boundary. With increasing the treatment time, the amount of precipitated nitrogen-combining carbides increases and discontinuous precipitation congregates to chain. The precipitation dissolves into austenite with further increasing the treatment time. Aging 8.3～120 min at 800 ～ 950 ℃ the precipitation phase of 18Mn-4Cr-0.4C-0.1N non-magnetic steel is M₂₃(C. N)₆and M₇(C. N)₃, but only M₇(C. N)₃ phase precipitates from 18Mn-4Cr-0.4C-0.03N non-magnetic steel. The decrease of microhardness of the isothermal aged steel matrix with the precipitation indicates that precipitation of C and N atoms leads to the decrease of matrix microhardness. and dissolution of carbonitride makes the microhardness of matrix recover. The critical temperature of isothermal precipitation and dissolution of carbonitride in these two...