Metallographic diagram of 45 steel (after quenched and tempered by 680°C gas nitrocarburizing)

Microstructure of 45 Steel (Nitrogen and Carbon Co-permeated After Annealing at 680 °C)

45 steel is one of the most commonly used steels in industry due to its excellent mechanical properties, such as good formability, welding ability, and yield strength. To improve its fatigue durability, there is an efficient way to anneal the steel at a special temperature in a nitrogen-carbon atmosphere. After annealing, the microstructure of the material can be examined to assess the effectiveness of the treatment.

This study aimed to analyze the microstructure of 45 steel (nitrogen and carbon co-permeated after annealing at 680 °C). Samples were cut from 45 steel plates, polished, etched, and then observed using optical microscopy. The findings showed a light gray background with a color between silver and gray, and bainite, martensite and ferrite in different sizes of grains. Bainite was observed in a light yellow color with a small grain size, martensite appeared as a dark blue color with a medium size grain, and ferrite exhibited a very small grain size distributed in the bainite and martensite.

Analysis of diffractograms showed that the steel was evenly distributed; no major concentration of any phase was observed. The diffractograms also revealed a single peak at the position corresponding to Goss texture, indicating a ferritic-bainitic microstructure. The texture found in this study was in agreement with those found in other studies, suggesting that nitriding was effective for a high degree of grain refinement.

The transmission electron microscopy (TEM) analysis showed a nano-size martensite structure in the 45 steel after nitriding. TEM images showed an inhomogeneous distribution of martensite and bainite in different crystal shapes, such as laths, rods, equiaxed grains, spheroids, and plates. The images also showed that nitriding had induced a high magnification of nano-size martensite. This microstructure is indicative of a uniform nanocrystalline grain size in the 45 steel, which results in improved fatigue strength and enhanced wear and corrosion resistance.

In conclusion, 45 steel (nitrogen and carbon co-permeated after annealing at 680 °C) was found to have a microstructure consisting of bainite, martensite, and ferrite grains with a Goss texture. TEM analysis revealed a uniform nanocrystalline grain size, which may improve the fatigue strength, wear resistance, and corrosion resistance of the material. Further study is necessary to further analyze the influence of nitriding on the mechanical and physical properties of 45 steel.

HG45 steel is a medium-carbon and low-alloy Bainite-martensite dual-phase steel, which has excellent comprehensive mechanical properties after tempering and nitrogen and carbon co-diffusion treatment.

The microstructure of HG45 steel is mainly dual-phase ferrite-martensite structure, and its microstructure change is much larger than that of ordinary steel. After 640℃ quenching, it has excellent mechanical properties such as impact toughness, stretch and flattening. After nitrogen and carbon co-diffusion treatment at 680℃, the component distribution of the alloy layer and the area are more uniform, and the carbide morphology changes from lath-like to acicular.

The microstructure of HG45 steel is mainly composed of martensite, martensite, bainite and retained austenite. According to the transmission electron microscope observation result of HG45 steel, it is found that high carbon martensite and its decomposition products of Bainite and carbon-rich acicular ferrite are mainly distributed in the alloy layer. The acicular ferrite distributed in the alloy layer forms continuous network along the grain boundary.

In addition, the microstructure of HG45 steel also has a certain amount of retained austenite. The retained austenite mainly includes carbon-enriched and carbon-deficient austenite. It has been found that the alloy layer of co-diffused steel is evenly distributed and the area and component distribution are more uniform than that of ordinary steel.

In summary, HG45 steel has excellent comprehensive mechanical properties after tempering and nitrogen and carbon co-diffusion treatment. Its main microstructure components are martensite, bainite and retained austenite. The area and component distribution of the alloy layer are more uniform.