Is Induction hardening a Better Alternative to Carburizing hardening?

Induction hardening was first used to increase the surface hardness of parts to meet wear resistance requirements. After decades of development, it has become the most widely used heat treatment technology, with a comprehensive technical and quality system in industries such as automotive, railways, shipbuilding, construction machinery, machine tools, and military.

Induction hardening is an important area of application for replacing carburizing and quenching. Due to its outstanding economic efficiency and high technical indicators, it has attracted great attention from the industry. In order to compare the two, the author would like to analyze them in the following aspects.

Economical

Advanced technology is to achieve performance that meets requirements at the lowest cost, and economy is the first factor to be considered in technology application.

1. Equipment investment

The investment in induction hardening equipment is relatively small. For example, for hardening medium-sized gears, a continuous gear carburizing line costs approximately 8 million yuan, and with auxiliary equipment such as quenching presses and slings, the total cost is approximately 15 million yuan. For the same production capacity, two induction hardening machines are required, each costing approximately 1 million yuan, only 10% to 20% of the cost of a carburizing machine. Compared to multi-purpose furnaces, a single induction hardening machine has the same production capacity as at least three multi-purpose furnaces, requiring only 50% of the investment (including auxiliary systems).

Equipment footprint and installation are also significant costs. Carburizing equipment requires significant floor space and high water, electricity, and gas requirements, resulting in significant investment and installation costs. Induction hardening equipment, on the other hand, occupies a smaller footprint, is easier to install, and is significantly less expensive.

2. Production operating costs and production rhythm

The low operating costs of induction hardening are also a key indicator of its promotional value. Statistics show that the energy consumption of induction hardening is about 20% of that of carburizing and quenching, the consumption of quenching medium is about 30%, the cost of equipment maintenance and spare parts is about 20%, and the emission of three wastes is also very low.

Induction hardening is a rapid heating process with a heating time of several seconds to tens of seconds, which results in a very fast production cycle. This has the advantage of reducing labor costs and work-in-progress rates.

3. Materials for heat-treated parts

Developed countries have a dedicated range of materials for induction hardening. However, specialized materials do not necessarily translate to higher costs; they are simply adjustments made to achieve better results. Induction hardening offers the widest range of materials, and due to its uniquely superior properties, lower-cost materials can be used to replace more expensive carburizing materials. Carburizing processes require high temperatures and long durations, requiring careful control of grain growth. Therefore, carburizing steels must contain a certain content of grain-refining alloying elements.

4. Processing after heat treatment

In the practice of carburizing and quenching, the carburized layer often wears away during the subsequent grinding process. This is because the carburized layer is relatively shallow, and deformation after heat treatment causes uneven wear. Compared to chemical heat treatments such as carburizing, induction hardening creates a deeper hardened layer, providing greater flexibility in subsequent processing and reducing the requirements for pre-heat treatment processes, resulting in lower processing costs and scrap rates.

Technical indicators

Carburizing and quenching forms a high-carbon martensite layer on the surface of a part, resulting in high hardness, a high carbide content, and excellent wear resistance. The core, however, is a low-carbon martensite structure, resulting in high surface compressive stress and high overall toughness. These characteristics make carburizing and quenching widely used in parts such as gears that require high wear resistance, fatigue strength, and contact fatigue strength. Induction quenching, characterized by rapid heating and cooling, significantly increases the material's grain size, achieving both ultra-high hardness and high toughness, thereby enhancing part performance.

1. Wear resistance

Carburized and quenched parts have high wear resistance due to their high surface hardness and carbides. Induction quenching can achieve high hardness at a lower carbon content, and wear resistance is also related to its microstructure.

Standard wear specimens were prepared by carburizing and quenching 20CrMnTiH3 and induction quenching 45 steel, both with hardnesses of 62 to 62.5 HRC. Tests were conducted on an M-200 wear tester, with the wear parts subjected to T10 quenching. After 1.6 million wear cycles, the carburized specimen lost 4.0 mg, while the induction-quenched specimen lost 2.1 mg. The mechanism that gives the induction-quenched specimen its superior wear resistance is worthy of investigation.

2. Strength

It's generally believed that strength is related to hardness, with the same hardness yielding the same strength. For specific parts, what other parameters are relevant? We tested standard dumbbell-shaped tensile specimens made by carburizing and quenching 20CrMnTiH3 and by induction quenching 45 steel, 40CrH, and 40MnBH. The diameter of the effective part of the specimens was 20 mm. The measured tensile strengths were 819 MPa, 1184 MPa, 1364 MPa, and 1369 MPa, respectively. The strength of the induction-quenched medium-carbon steel specimens was significantly higher than that of the carburized parts.

Comparing the results of the two processes. The surface of the carburized and quenched specimen is high-carbon martensite, with a carburized layer of 1.25mm and a hardness of 62-63HRC, while the core is low-carbon martensite with a hardness of 32HRC. The surface of the induction-quenched specimen is medium-carbon martensite, with a hardened layer depth of 3.6mm and a hardness of 62HRC, while the core is tempered bainite with a hardness of 26HRC. It can be seen that there is a significant difference in the depth of the surface hardened layer obtained by the two treatments. Induction quenching produces a deeper hardened layer, resulting in greater part strength. Therefore, when discussing which strengthening process is better, it is necessary not only to analyze from a microscopic perspective, but also to consider from a macroscopic perspective.

3. Fatigue strength

After carburizing and induction quenching, the surface of the parts is effectively strengthened and large residual compressive stress is formed, both of which have high fatigue strength.

Gear parts with a module of 2.5 were selected for research. 20CrMnTiH3 was carburized and quenched to a carburizing depth of 1.2 mm; 45 steel and 42CrMo were induction hardened, with a tooth root hardening depth of 2.0 mm. The hardness was 61-63 HRC in all cases, and the gears were ground after heat treatment. Tests were conducted on a fatigue testing machine using the loading method shown in Figure 1. The median fatigue limit compressive loads for the bending of gear teeth made of three different materials and heat treatments were 18.50 kN, 20.30 kN, and 28.88 kN, respectively. The fatigue strength of the 42CrMo induction-hardened gear was 56% higher than that of the 20CrMnTiH3 carburized and quenched gear, showing a significant advantage. To analyze its mechanism, it is necessary to consider aspects such as the hardened layer structure, surface compressive stress level, core structure, and hardness.

4. Contact fatigue strength

For gear components, contact fatigue is a major failure mode. Lightly loaded gears have relatively low requirements for contact fatigue, but whether induction hardening can replace carburizing and quenching in specific heavily loaded gears is a crucial consideration. Our research in this area is not yet in-depth.

5. Quenching deformation

The carburizing process involves high temperatures and long times, resulting in significant quenching distortion. Subsequent grinding thins the surface layer, which is known for its highest strength and compressive stress, reducing component strength. Press quenching is increasingly being used in gear carburizing to minimize quenching distortion. Induction quenching produces relatively little distortion, and due to the thick quenching layer, grinding has a relatively minimal effect on the hardening depth.

Limitations of induction hardening

The induction hardening process has its own special application limitations, which are related to the objective laws of magnetic field distribution and require specific analysis for specific parts.

1. Complex cross-section parts

For example, a gearbox shaft, which includes multiple gears, steps, and bearings, would require induction hardening, a complex and cost-intensive process. Other parts with sharp corners in the hardened area are also difficult to induction harden and should be treated with carburizing or other chemical heat treatments.

2. Thin-walled parts

Carburizing and quenching can produce a very thin hardened layer, with a lower hardness in the core to ensure toughness. Induction quenching may cause brittle cracking due to through-hardening.

3. Small parts

Induction hardening requires loading and unloading, heating, and cooling for each part, making it uneconomical for very small parts. Carburizing and quenching can be performed in batches, resulting in high output and low cost.

4. Single piece production

Induction hardening requires different sensors to be made for different parts, which does not have economic advantages for smaller batch production.

Some suggestions for induction hardening instead of carburizing

In summary, there is still a lot of work to be done on the research of using induction quenching to replace carburizing and quenching for heavy-loaded gears. Other parts that need to improve wear resistance and strength can be replaced by using appropriate materials combined with induction quenching technology.

Material selection for induction heat treatment. Carburizing creates a high-carbon layer on the surface of the material, with a high carbide content, which is beneficial for wear resistance. Materials used for induction heat treatment are mostly medium-carbon steel and medium-carbon alloy steel. When replacing carburized materials for wear-resistant parts, materials with high carbon content can be selected, such as camshafts made of 80# steel, to achieve the desired performance.

The material requirements for induction hardening parts are lower than those for carburizing and quenching, and are often overlooked. The material selection process must not only consider whether it can be hardened to the required hardness and quenching depth, but also pay attention to the material's chemical composition, grain size, impurities, and other indicators. These factors also have a significant impact on the performance of the part, so induction hardening steel should be treated with the same high regard as carburizing steel.

Pay attention to preliminary heat treatment. The heating time of induction hardening is short, and the alloy composition does not have enough time to homogenize, so preliminary heat treatment is necessary. Induction hardening is usually surface hardening, and the core structure and other indicators must be guaranteed during the preliminary heat treatment process.