Metallographic diagram of 45 steel (soft nitriding treatment after quenching and tempering)

Microstructure Characteristics of High-quality 40Cr Steel after Nitrogen Treatment

High-quality 40Cr steel is one of the most popular materials used in the engineering industry. Nitrogen treatment can improve 40Cr steels mechanical properties significantly, such as tensile strength, yield strength, and elongation rate. For high-quality 40Cr steel after nitrogen treatment, the microstructure produces a fine ferrite grain structure and a granular carbide precipitates in the grains, which greatly improves its fatigue resistance, wear resistance, and creep resistance. Additionally, element segregation should be as little as possible to improve fatigue properties. In this study, the microstructure characteristics of high-quality 40Cr steel after nitrogen treatment were analyzed.

The high-quality 40Cr steel samples were cut into rectangular test pieces and then subjected to a nitrogen treatment process. The temperature of the nitrogen treatment was 800℃ and the holding time was 30 minutes. Then, the specimens were cooled rapidly in oil to room temperature. Thus, the microstructures of high-quality 40Cr steel after nitrogen treatment were subsequently analyzed under a metallurgical microscope and analyzed by electron microprobe (EPMA).

The metallurgical microscope images of the high-quality 40Cr steel after nitrogen treatment show that the ferrite grains have an acicular shape and have a small average sizes of approximately 8.3µm. The microstructure is composed of ferrite and granular carbide inclusions which have a size distribution of approximately 0.1 to 1.7µm. The carbon content in the ferrite grain boundary and the matrix of the martensite are 0.37–0.40wt% and 0.23–0.25wt%, respectively. The EDX analysis suggests that Ni, Mn and Si are the alloying elements of this steel which should be as little as possible to enhance its fatigue properties.

The EPMA results reveal that V, Cr, C, Ni and Mn are the main elements in the matrix while Mo and Si are relatively low in content. There are also traces of other elements, such as N, P, O, Cu, and Al, which can be assumed to be incorporated during the nitrogen treatment and could affect the mechanical properties by influencing the grain structure and impurity levels.

The microstructure of high-quality 40Cr steel after nitrogen treatment can be characterized as ferrite acicular grains with granular carbide inclusion distribution. The amount of Ni, Mn, and Si should be as low as possible to increase its fatigue properties, and traces of other elements, such as Mo and N, should also be taken into consideration for better results. With the microstructures and elements after nitrogen treatment, this kind of 40Cr steel can be further combined with other methods to be further improved to better meet the needs of engineering applications.

45 steel (annealed and tempered nitriding treatment) metallographic picture

45# steel is a medium carbon steel, containing 0.45%~0.6% carbon and 0.17%~0.4% silicon. It is the most commonly used quenched and tempered steel. In order to improve the surface hardness and wear resistance of 45# steel, it was annealed and tempered, and then nitrided.

The nitriding treatment process of 45# steel can be divided into five steps: preheat treatment, nitriding treatment, quenching treatment, intermediate tempering and surface tempering. In the first step, the preheat treatment of 45# steel was carried out to eliminate internal stress and improve the plasticity of the steel surface. In the second stage, the nitriding treatment process can be divided into three parts: alkaline solution treatment, nitriding treatment and nitriding oxidation. The alkaline solution treatment can reduce the hardness and friction of the surface layer of steel. The nitriding treatment can increase the surface hardness and wear resistance of steel through the diffusion of nitrogen. The nitriding oxidation can decarburized the surface layer of steel, so as to further improve the corrosion resistance. In the third step, quenching treatment was performed after nitriding treatment to enhance the surface hardness and wear resistance of 45# steel. In the fourth and fifth steps, tempering was performed to eliminate internal stress and improve the toughness of 45# steel.

The metallographic picture of the 45# steel after nitriding treatment shows that the surface layer is a combined nitrided layer and diffusion layer. The nitrided layer is located outside the diffusion layer, with a thickness of about 0.4 μm and a hardness of about 1100HV. The diffusion layer is located between the nitriding layer and the substrate, with a thickness of about 3.2 μm and a hardness of about 450HV. The main hard phase of the nitrided layer is M3N and compound nitride, while the main hard phase of the diffusion layer is Fe2-3N. The results show that the nitriding treatment of 45# steel has the effects of increasing surface hardness and wear resistance, improving corrosion resistance, and increasing fatigue strength and fatigue life.