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Surface Gas Nitriding Treatment of 2Cr13 Steel

2Cr13 steel is a high-chromium martensitic stainless steel with high fatigue strength and good comprehensive properties. It is mainly used for parts with high plasticity requirements and impact loads, such as steam turbine blades, large axial compressor blades, and valve bodies. ﹑ Fasteners, shaft pump sleeves, bearings, corrosion-resistant cutting tools, etc. However, due to the low carbon content of 2Cr13 steel and poor wear resistance and corrosion resistance, in some occasions with high requirements for wear resistance and corrosion resistance, it must be surface strengthened before it can be used.

The grain size after quenching and tempering treatment is an important factor affecting the surface quality of 2Cr13 martensitic stainless steel after nitriding, and the grain size is determined by the vacuum quenching temperature. The microstructure is refined by reducing the vacuum quenching temperature. 1030°C is the critical quenching temperature of this steel.

The matrix structure of the 2Cr13 steel plate sample after gas nitriding is tempered sorbite with excellent comprehensive mechanical properties (sufficient strength and high toughness), which is composed of fine granular carbides and supersaturated ferrite. The local area organization of the matrix retains the martensitic lath-like morphology. This is because the high chromium in 2Cr13 stainless steel strongly hinders the aggregation and growth of carbides during high temperature tempering, and delays the recovery and recrystallization of ferrite. Because the ferrite is not fully recrystallized, the microstructure inherits the substructure and morphology of the martensitic lath, which is a normal structure.

The wear resistance of the nitrided layer mainly depends on the thickness, strength and microstructure of the nitrided layer. There is a clear boundary between the nitrided layer and the substrate, and the interface is neat and changes slowly, which shows that the bond strength between the nitrided layer and the substrate is very strong. The nitriding layer includes a compound layer and a diffusion layer, and the thickness is about 165um, wherein the thickness of the compound layer is about 10um.

After gas nitriding, the microhardness of the sample increases significantly; while the microhardness before nitriding is only 420 HVo.1. In addition, the hardness between the compound layer and the substrate changes gradually, which means that the nitrided layer is not easy to peel off from the substrate. Therefore, the slow change of the hardness is very beneficial to its friction and wear performance.

The figure below shows the volume wear of the sample when the friction pair of the non-nitrided 2Cr13 steel pin and the nitrided 2Cr13 steel disc are ground under atmospheric conditions. It can be seen from the figure that the wear amount of the nitriding disc is only 21% of that of the non-nitrided pin. The SEM observation shows that the wear surface of the non-nitrided 2Cr13 pin is severely worn, with a large number of wear debris and serious cracks, while the nitrided 2Cr13 pin is severely worn. No cracks appeared on the worn surface of the disc, as shown in the figure. It can be seen that the wear resistance of 2Cr13 steel has been significantly improved after gas nitriding. The reason for the improved wear resistance is the increase in hardness after nitriding and the residual compressive stress after nitriding.

In conclusion

1) The microhardness of 2Cr13 steel after gas nitriding increases significantly, and the highest hardness reaches 1051 HVol.1, which is about 2.5 times of the hardness before nitriding.

2) The nitrided layer of 2Cr13 steel has high wear resistance, and the core has good toughness. When the non-nitrided pin and the nitrided disc are worn against each other, the surface of the non-nitrided 2Cr13 pin is severely worn, and there are a large number of wear debris and serious cracks, but no cracks are found on the nitrided wear surface.

Selection of nitriding equipment: The RVN series vacuum nitriding furnace produced by SIMUWU is an excellent choice for completing this type of process. Its process performance can fully meet the needs of such hot processing, with good temperature control accuracy, temperature uniformity and gas quenching uniformity, and the production is convenient and fast.