Carburizing heat treatment is widely used in various mechanical parts, mainly to improve fatigue and wear resistance. After carburizing treatment of the rock drilling tools, besides strengthening the surface, surface residual compressive stress on the fatigue crack expansion also plays a role in hindering. And high-quality rock drilling tools generally need to use the carburizing process, what is carburizing process?

Carburizing process refers to the chemical heat treatment process that the low carbon steel or low carbon alloy steel  need to be put into the carburizing medium, at 900℃-950℃ heating insulation, so that the activated carbon atoms seep into the steel surface to obtain a high carbon concentration. After proper quenching and tempering treatment, it can improve the surface hardness, wear resistance, and fatigue strength, making the core maintain good toughness and plasticity.

Carburizing can improve parts strength, impact toughness, and wear resistance to extend the service life of parts, so what are the methods and classifications of carburizing?

Carburizing method

Carburizing methods include solid carburizing, liquid carburizing, and gas carburizing.

However, along with the progress of technology, solid carburizing is gradually being eliminated. Liquid carburizing application is very few; only gas carburizing is the most widely used.

Gas carburizing is to install the in a sealed carburizing furnace, heat it to 900℃-950℃, drop into the furnace easily decomposed kerosene, benzene, methanol, propane, and other organic liquid or directly through gas, petroleum liquefied gas, natural gas and other artificial carburizing gas, through the reaction to produce activated carbon atoms, so that the surface of steel carburizing. Its advantages are high productivity, better labor conditions, and the carburizing process can be controlled by a computer. The quality and mechanical properties of the carburized layer are better.

Classification of carburizing process

1. Direct quenching and low-temperature tempering

Organization and performance characteristics: Cannot refine the grain of the steel. The quenching deformation of the workpiece is large, the surface of the alloy steel carburized part has a large amount of residual austenite, and the surface hardness is low. Simple operation and low cost.

Applications: It is used to deal with parts that are not subject to deformation and impact load. It is 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 workpiece quenching deformation. The volume of residual austenite decreased slightly, and the surface hardness increased slightly, but the austenite grain did not change. Simple operation, workpiece oxidation, decarburization, and quenching deformation of the workpiece.

Applications: Widely used in various tools for fine-grained steel manufacturing.

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

Organization and performance characteristics: For those with a higher requirement of heart strength, adopt 820-850℃ quenching. If the core is low carbon M and the surface hardness is high, quenching at 780-81℃ can refine the grain.

Applications: It is suitable for carbon steel and low-alloy steel workpieces after solid carburization, coarse-grained steel with gas or liquid carburization, and workpieces that are unfit for direct quenching after carburizing, and parts to be machining after carburizing.

4. Carburizing high-temperature tempering, one heating quenching, low-temperature tempering, quenching temperature 840-860 ℃

Organization and performance characteristics: High-temperature tempering causes decomposition of M and residual A, and the carbon and alloying elements in the infiltration layer separate in the form of carbide, which is convenient for cutting and reducing residual A after quenching.

Applications: Mainly used for Cr-Ni alloy carburizing workpieces.

5. Secondary quenching and low-temperature tempering

Organization and performance characteristics: The first quenching (or normalizing) can eliminate the carburizing layer network carbide and refine the core structure (850-870℃). The second quenching mainly improves the infiltration layer structure. When the performance of the core is not high, it is quenching between AC1-AC3 of the material, and for high-performance requirements, the quench should be above Ac3.

Applications: It is used for important carburized parts with high requirements on mechanical properties, especially for coarse-grained steel. However, after carburizing, it needs to be heated twice at a high temperature, which makes the workpiece deformation and oxidation decarbonization increase. The heat treatment process is more complicated.

6. Secondary quenching, cold treatment, low-temperature tempering

Organization and performance characteristics: When the temperature is higher than Ac1 or Ac3(core), there is more residual A on the surface of the high alloy, and the cold treatment (-70℃/-80℃) promotes the transformation of A to improve the surface hardness and wear resistance.

Applications: It is mainly used for high alloy steel workpieces without mechanical machining after carburizing.

7. Induction heating quenching and low-temperature tempering after carburizing

Organization and performance characteristics: It can refine the permeation layer and the tissue near the permeation layer. The quenching deformation is small and no hardening parts is allowed, no advanced seepage control is required.

Applications: All kinds of gears and shafts.

Carburizing is one of the most important processes of chemical heat treatment. Its rational design idea allows the steel surface layer to accept all kinds of loads maximally. through the carbon infiltration and other elements, rock drilling tools can achieve high surface hardness, high wear resistance, fatigue strength, and corrosion resistance. The process not only uses the low carbon steel or alloy steel to replace some of the more expensive high alloy steel but also can maintain the core of low carbon steel after quenching strength and toughness so that the workpiece can withstand impact load. Therefore, it has high practical value.