Classification and selection of carburizing process

Carburizing is actually a kind of metal surface treatment. Most of the steels that are carburized are low-carbon steel or low-alloy steel. The specific method is to place the workpiece in an active carburizing medium, heat it to a single-phase austenite zone of 900-950 degrees Celsius, and keep it warm for a sufficient time, so that the active carbon atoms decomposed in the carburizing medium penetrate into the surface of the steel part, thereby obtaining high carbon on the surface, and the core still maintains the original composition.

What are the classifications of carburizing processes?

1. Direct quenching and low-temperature tempering structure and performance characteristics:

Cannot refine the grains of steel. The workpiece quenching deformation is large, the surface of the alloy steel carburized part has a large amount of residual austenite, and the surface hardness is low. Scope of application: simple operation, low cost, used to process parts that are not easily deformed and subjected to impact loads, suitable for gas carburizing and liquid carburizing processes.

2. Pre-cooling direct quenching, low temperature tempering quenching temperature 800-850℃ Organization and performance characteristics:

It can reduce the quenching deformation of the workpiece, the amount of residual austenite in the carburized layer can also be slightly reduced, the surface hardness is slightly increased, but the austenite grains do not change. Scope of application: simple operation, small oxidation, decarburization and quenching deformation of the workpiece, widely used in various tools made of fine grain steel.

3. One-time heating quenching, low temperature tempering, quenching temperature 820-850℃ or 780-810℃ Organization and performance characteristics:

For those with higher requirements for core strength, 820-850℃ quenching is used, the core is low carbon M, and the surface requires high hardness. 780-810℃ quenching can refine the grains. Scope of application: Suitable for carbon steel and low alloy steel workpieces after solid carburizing, coarse grain steel after gas and liquid carburizing, some workpieces that are not suitable for direct quenching after carburizing, and parts that need to be machined after carburizing.

4. Carburizing high temperature tempering, one heating quenching, low temperature tempering, quenching temperature 840-860℃ Organization and performance characteristics:

High temperature tempering decomposes M and residual A, and carbon and alloy elements in the carburized layer precipitate in the form of carbides, which is convenient for cutting and reducing residual A after quenching. Scope of application: Mainly used for Cr-Ni alloy carburized workpieces.

5. Secondary quenching and low temperature tempering organization and performance characteristics:

The first quenching (or normalizing) can eliminate the carburized layer network carbides and refine the core structure (850-870℃). The second quenching mainly improves the carburized layer structure. When the core performance requirements are not high, the material can be quenched between Ac1-Ac3. When the core performance requirements are high, it must be quenched above Ac3. Scope of application: Mainly used for important carburized parts with high mechanical properties, especially for coarse-grained steel. However, after carburizing, it needs to be heated twice at high temperature, which increases the deformation and oxidation decarburization of the workpiece, and the heat treatment process is more complicated.

6. Secondary quenching cold treatment low temperature tempering structure and performance characteristics:

Quenching at a temperature higher than Ac1 or Ac3 (core), the high alloy surface has more residual A, and cold treatment (-70℃/-80℃) promotes A transformation to improve surface hardness and wear resistance. Scope of application: Mainly used for high alloy steel workpieces that are not machined after carburizing.

7. Low temperature tempering structure and performance characteristics of induction heating quenching after carburizing:

It can refine the structure of the carburized layer and the area near the carburized layer. The quenching deformation is small, and the parts that are not allowed to harden do not need to be pre-proofed. Scope of application: various gears and shafts.