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Gas carburizing process for steel parts

After carburizing and quenching or nitriding, the wear resistance and fatigue strength of steel parts are improved at the same time. The improvement of wear resistance is due to the improvement of the chemical composition and hardness of the surface of the workpiece, and the improvement of fatigue strength is due to the residual compressive stress on the surface of the steel after the above treatment. The following discusses the internal stress distribution of steel parts after gas carburizing, vacuum quenching and nitriding treatment.

1.Internal stress in steel parts after gas carburizing and vacuum quenching

After the steel parts are quenched by gas carburizing and vacuum quenching, due to the difference in carbon content between the carburized layer and the interior, as well as the expansion and contraction of the carburized heating, cooling and vacuum quenching, and the expansion of the martensitic transformation , especially due to the difference in specific volume caused by the difference in carbon content between the surface and the core and the difference in the temperature of the martensitic transformation point (Ms), there will be residual stress in the steel.

The higher the carbon content, the lower the Ms point, the larger the specific volume change when martensite is formed, and vice versa. Due to these differences, the carburized layer has compressive stress. In addition, if there is retained austenite (its specific volume is smaller than that of martensite) in the carburized layer after vacuum quenching, the stress state in the surface layer will increase with the increase of retained austenite in the surface layer. Will be converted to the direction of the formation of tensile stress. Therefore, the actual stress distribution is related to the chemical composition of the steel, the size and shape of the workpiece, gas carburizing, vacuum quenching conditions and other factors.

(1)The influence of carbon content in steel. The picture above shows low carbon chromium nickel molybdenum steel (0.22%C, 0.59%Ni, 0.43%Cr, 0.19%Mo) and medium carbon chromium nickel molybdenum steel (0.38%C, 0,55%Ni, 0.48%Cr and 0, 24% Mo) steel cylindrical sample (φ5 mm) was quenched directly in oil after gas carburizing. From the comparison of the axial stress distribution, it can be seen that the surface compressive stress of medium carbon steel after gas carburizing and quenching is lower than that of low carbon steel after carburizing and quenching. This is because steel with high carbon content has greater internal hardening performance, and the difference in specific volume between the carburized layer and the core during quenching is small.

(2) The influence of steel size. It is generally believed that the larger the diameter of the steel piece, the greater the compressive stress on its surface after carburizing and quenching. Due to the larger thermal stress of steel parts with larger pore diameters during quenching, and the ratio of the area of the carburized layer to the internal area after carburizing is smaller. Therefore, after carburizing and quenching, a large compressive stress appears in the carbon layer. The picture above shows carbon-chromium-nickel-molybdenum steel (O.2%C, 0.67%Ni, 0.71%Cr, 0.11%Mo), made of cylindrical test bars with diameters of 12.7 mm, 19.05 mm, and 25.4 mm. Comparison of axial stress distribution after quenching in oil and tempering in low temperature vacuum at 150°C.

(3) The effect of carburized layer depth.Many experimental studies have proved that the greater the depth of the carburized layer, the larger the distribution depth of the surface compressive stress, but the smaller the value of the compressive stress, the shallower the carburized layer, the narrower the distribution area of the surface compressive stress , but the compressive stress value is larger. Moreover, the compressive stress gradually decreases from the surface to the interior, so that the change to the tensile stress is relatively sharp.

2.Internal stress of nitrided steel

There are residual stresses in steel parts after nitriding treatment, and the reason for this is relatively simple. During nitriding, because nitrogen atoms penetrate into the surface layer of the steel piece, the volume of the nitrided layer increases, so the surface layer produces compressive stress, and the interior presents tensile stress. The larger the diameter of the steel piece. After nitriding, the compressive stress of the surface layer also increases, while the internal tensile stress gradually decreases. The surface after nitriding has compressive stress, therefore, nitriding treatment can improve the fatigue performance of steel parts.

SIMUWU is a manufacturer specializing in the design, production, R&D and sales of vacuum furnaces. The company’s main business is vacuum heat treatment furnaces, vacuum sintering furnaces, vacuum brazing furnaces, gas nitriding furnaces, gas carburizing furnaces and other vacuum furnaces. Welcome to contact us .