The Effect of Carbon Concentration On Mechanical Properties After Carburizing Heat Treatment

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Carburizing heat treatment is currently a chemical heat treatment method widely used in machinery manufacturing. It heats low-carbon steel in a carbon-rich medium to a high temperature (generally 900-950°C), so that activated carbon atoms can penetrate the surface of the steel to obtain a high-carbon layered structure. After carburizing and heat treatment, quenching and low-temperature tempering make the surface have high hardness, wear resistance and fatigue resistance, while the core still maintains sufficient strength and toughness.

After the parts are carburized and heat treated, the carburized layer should reach a certain surface carbon concentration, depth and concentration gradient in order to obtain the required mechanical properties. The surface carbon concentration has a greater impact on the mechanical properties of parts, and the impact on different mechanical properties is also different, such as:

1. With the increase of surface carbon concentration, the wear resistance also increases, as shown in Figure 1.

2. As the surface carbon concentration increases, the flexural strength and impact value decrease instead, as shown in Figures 2 and 3.

3. When the surface carbon concentration is 0.8-1.05%, the torsional strength reaches the maximum value, as shown in Figure 4.

4. When the surface carbon concentration is about 0.93%, the bending fatigue strength has the highest value, as shown in Table 1. This is because when the surface carbon concentration is greater than 1.1%, the unevenness of carbides in the carburized layer will increase, and large or networked carbides will easily form, which will increase the brittleness of the carburized layer and decrease the fatigue strength.

Surface carbon concentration（%）	0.8	0.93	1.15	1.42
Fatigue strength（kgf/mm2）	87	94	84	68

It can be seen that after carburizing heat treatment, the carbon content on the surface of the parts should be controlled between 0.7-1.05%. If it is too low, the wear resistance will decrease, and if it is too high, the brittleness will increase and the strength will not meet the requirements. For parts subjected to impact fatigue loads, the surface carbon concentration should be 0.7-0.9%; for parts subjected to bending fatigue loads, the surface carbon concentration should be 0.8-1.05%; for parts subjected to high contact fatigue loads and requiring high abrasion resistance For parts, the surface carbon concentration can be increased to 1.15%.

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