Case-hardening steels
Case-hardening steels are designed to achieve high surface hardness while maintaining good internal toughness. These steels are essential for mechanical components that require wear resistance and the ability to withstand heavy loads, such as gears and transmission shafts.
01Case-Hardening Steels: A Technical Guide for Industry Professionals
Case-hardening steels represent a specialised category of metallurgical materials specifically designed to deliver high performance through the heat treatment of case-hardening steels.
These materials, characterised by a low carbon content and an optimised chemical composition, are the preferred technological solution for mechanical components subjected to severe surface stresses, combining extreme surface hardness with core toughness.
02Definition and Characteristics of Case-Hardening Steels
Case-hardening steels are iron-carbon alloys specifically formulated with a low carbon content, generally not exceeding 0.20%, designed to respond optimally to surface carbon enrichment processes.
This category of steel has the distinctive feature of retaining a tough core after hardening and tempering while developing an extremely hard surface layer through the thermochemical process of case hardening.
The distinctive characteristic of these materials lies in their ability to develop a gradient of mechanical properties from the core to the surface, ensuring the optimal combination of surface hardness (62-64 HRC) and core toughness.
This configuration is achieved through the controlled diffusion of carbon into the surface zone of the component, which typically reaches 0.7-0.9% while keeping the core at a low carbon content.
03Optimal Chemical Composition
The chemical composition of case-hardening steels is designed to optimise hardenability and the mechanical properties of case-hardening steels.
The alloying elements present include manganese up to 1.2%, nickel up to 3.5%, chromium up to 1.5% and molybdenum up to 0.50%. These elements help improve core hardenability and dimensional stability during heat treatment.
Manganese promotes hardenability and mechanical strength, while nickel significantly improves core toughness, which is particularly important for applications requiring resistance to mechanical shock.
Chromium contributes to the formation of stable carbides during case hardening, while molybdenum increases strength and hardness, making the steel suitable for high-strength and wear-resistant applications.
04Microstructure and Phase Transformations
The microstructure of case-hardened steels shows a characteristic gradient from the surface to the core.
After hardening and tempering, the case-hardened layer exhibits a tempered martensitic structure with finely dispersed carbides, while the core retains a lower-carbon structure with superior toughness.
The transition zone shows a gradual change in composition and microstructure that ensures the absence of mechanical discontinuities.
05Classification of Case-Hardening Steels According to International Standards
5.1European Standard EN ISO 683-3:2022 (formerly EN 10084)
The classification of case-hardening steels in Europe is governed by the EN ISO 683-3:2022 (formerly EN 10084) standard, which sets out the technical delivery conditions for non-alloy and alloy case-hardening steels.
This standard defines the minimum requirements for the steel production process, the chemical composition, hardness, hardenability and the technological characteristics agreed upon with the manufacturer.
The standard prescribes specific checks, including grain size verification and visual and dimensional inspection.
For steels ordered without hardenability requirements, the hardness requirements indicated in Table 1 of the EN ISO 683-3:2022 (formerly EN 10084) standard must be verified, whereas for steels ordered with hardenability requirements (+H), additional hardenability checks according to the Jominy test are required.
5.2ASTM and JIS Standards
The international equivalents of case-hardening steels include the ASTM A304 standard for the North American market and JIS G 4052 for the Japanese market.
The JIS system uses designations such as SNCM420, which corresponds approximately to AISI/SAE 4320, demonstrating the international harmonisation of specifications for these materials.
Comparative Table of Designations
| EN designation | AISI/SAE | JIS | C (%) | Typical applications |
|---|---|---|---|---|
| 20MnCr5 | 5120 | - | 0.17-0.22 | Medium-load gears |
| 16NiCr4 | 3115 | - | 0.13-0.19 | Precision gears |
| 18CrNiMo7-6 | - | SNCM420 | 0.15-0.21 | High-stress components |
| 17NiCrMoS6-4 | - | - | 0.14-0.20 | High-machinability components |
06Heat Treatment for Steel Case Hardening
6.1Case-Hardening Processes (Gas, Liquid, Solid)
Gas, liquid and solid case hardening comprises three distinct methods for surface carbon enrichment.
Gas carburising, now the most widely used method for high-volume production, consists of heating the parts to 850-950 °C in an endothermic carrier gas (mainly CO, H₂ and N₂) enriched with a hydrocarbon such as methane (CH₄); the CO transfers carbon to the surface (2CO → C + CO₂) and the methane regenerates CO by reacting with the CO₂ formed (CH₄ + CO₂ → 2CO + 2H₂).
Case-hardening temperatures range from 850 °C to 950 °C depending on the steel composition, the required depth and the type of process (gas, liquid, solid). Gas carburising typically operates at 920-930 °C for carbon-manganese steels.
Liquid carburising involves immersing the parts in molten salt baths such as sodium cyanide, alkali and barium carbonates and chlorides. The sodium cyanide reacts with atmospheric oxygen and carbon dioxide, releasing carbon monoxide, which in turn releases carbon atoms into the part.
Solid (pack) carburising, less commonly used in modern industry, involves the use of solid carbonaceous mixtures in boxes or sealed containers, with process temperatures and times similar to the other methods but with greater difficulty in controlling the atmosphere and treatment uniformity.
6.2Process Parameters and Control
The case-hardening depth of steels is controlled through specific time and temperature parameters. The depth of carbon diffusion and the associated effective case depth (ECD) can range from a few tenths of a millimetre up to several millimetres.
The surface carbon concentration in case-hardened steels typically reaches 0.7-0.9%, rarely exceeding 0.85% to avoid the formation of brittle massive cementite.
The case-hardening depth (CHD) is defined as the vertical distance between the surface of the specimen and the layer showing a limiting hardness of 550 HV. Recent studies have shown that samples were obtained with case-hardening depths of approximately 0.55 mm, 0.9 mm and 1.9 mm, with surface hardness ranging between 640 HV1 and 760 HV1.
6.3Post-Case-Hardening Heat Treatments
The post-case-hardening thermal cycle may involve direct quenching from the case-hardening temperature or double-quenching cycles for medium and large components.
Direct quenching involves immersion in oil or a suitable quenching fluid from the case-hardening temperature, followed by tempering at 150-200 °C.
For critical components requiring high quality and reliability, the double-quenching cycle makes it possible to optimise the core and surface properties separately, performing a first quench for the core and a second quench at the correct temperature for the surface, while simultaneously tempering the core.
07Limitations of the Case-Hardening Process
The steel case-hardening process has limitations that it is essential to be aware of when working with this treatment:
- Unavoidable distortion: deformation linked to phase transformations
- Selective decarburisation: a risk in unmasked areas
- Intergranular oxidation: critical atmosphere control
- Formation of massive carbides: at surface C concentrations >0.9%
- Hydrogen embrittlement: particularly critical in Ni steels
- Long process times: 6-24 hours for significant depths
- Mandatory masking: for zones not to be case-hardened
08Mechanical Properties and Performance Characteristics
8.1Surface Hardness and Case-Hardening Depth
The mechanical properties of case-hardening steels are characterised by high surface hardness, which can reach 62-64 HRC, ensuring excellent resistance to mechanical wear. Surface hardness varies according to the diffused carbon content and the subsequent hardening and tempering conditions.
The hardness distribution from the surface to the core follows a decreasing trend that reflects the carbon gradient obtained during case hardening. This configuration ensures a gradual transition from the surface properties to those of the core, avoiding critical stress concentrations.
8.2Wear Resistance and Contact Fatigue
Case-hardened steels offer excellent resistance to adhesive and abrasive wear thanks to the tempered martensitic structure of the surface layer. The presence of finely dispersed carbides contributes to wear resistance, while the toughness of the core prevents the propagation of surface cracks.
Contact fatigue strength is significantly improved by the presence of the hardness gradient, which distributes contact stresses over a larger volume than uniform hardening treatments.
8.3Core Toughness
Core toughness in low-carbon case-hardening steels is maintained through control of the chemical composition and the heat treatment parameters. Steels with a high nickel content are preferred to achieve high toughness, which is essential for withstanding dynamic loads and mechanical shocks.
09Most Widely Used Case-Hardening Steels
9.1Non-Alloy Steels (C10, C15, C20)
Non-alloy case-hardening steels, designated according to their carbon content multiplied by 100, represent the base category for applications with standard mechanical requirements. C10 (0.10% C), C15 (0.15% C) and C20 (0.20% C) are used for components with simple geometries and limited case-hardening depths.
9.2Alloy Steels (20MnCr5, 18CrNiMo7-6, 20NiCrMo2-2)
The 20MnCr5 is the most widely used manganese-chromium case-hardening steel, available in a machinable annealed condition as hot-rolled, forged and cold-drawn bars with diameters from 16 to 500 mm. This grade offers a good compromise between hardenability and cost for medium-stress applications.
The 18CrNiMo7-6 is a high-hardenability alloy steel used for heavily stressed components, available in a machinable annealed condition as hot-rolled and forged bars with diameters up to 490 mm. The presence of nickel, chromium and molybdenum ensures high mechanical properties after case hardening and quench hardening.
Although not specifically mentioned in the sources consulted, 20NiCrMo2-2 is typically a nickel-bearing case-hardening steel for applications requiring high core toughness combined with good hardenability.
9.3Special Steels for Critical Applications
The 17NiCrMoS6-4 series steels contain sulphur to improve machinability, and are also available with added lead (17NiCrMoS6-4+Pb) to maximise the machining speed. These grades are used for precision components manufactured in large production runs.
The 16NiCrMo12 is a case-hardening steel with a high content of alloying elements for critical applications requiring high hardenability and strength.
10Industrial Applications of Case-Hardened Steels
10.1Automotive Industry
The industrial applications of case-hardened steels in the automotive industry include gears, transmission shafts, cams and pinions. These components are subjected to high contact stresses and require a combination of high surface hardness and good core toughness to ensure long-term reliability and durability.
Case-hardened steels are used to manufacture critical powertrain components, where wear resistance and fatigue strength are essential for the performance and reliability of the vehicle.
10.2Gears and Transmissions
Gears are the main application of case-hardening steels, requiring high surface hardness to resist tooth wear and core toughness to withstand the transmitted loads. Case hardening makes it possible to achieve optimal hardness profiles that maximise the fatigue life of the gears.
10.3Bearings and Precision Components
In the aerospace industry, case-hardened steel is used to produce engine components such as reduction gears, transmission shafts and bearings. The demanding operating conditions require materials capable of withstanding wear, fatigue and corrosion.
In the machine tool sector, case-hardened steel is used in the production of linear guides, spindles, dies and moulds, where the combination of surface hardness and dimensional accuracy is critical.
11Quality Control and Defect Analysis
11.1Standard Tests and Inspections
Quality control of case-hardened steels includes verification of the case-hardening depth in accordance with ISO 18203:2016 (formerly ISO 2639), surface and core hardness checks, and microstructural examinations. The case-hardening depth CHD, according to ISO 18203:2016, is defined as the distance from the surface to the point where the hardness measured with a Vickers indenter and a load of 0.3 kgf reaches 550 HV0.3.
The inspections include fatigue tests to verify the strength of the finished component, dimensional checks to assess the distortion induced by heat treatment, and metallographic examinations to verify the microstructure and the absence of defects.
11.2Typical Defects and Prevention
Typical defects in case-hardened steels include surface decarburisation, internal oxidation, quench cracks and excessive distortion. Prevention requires strict control of the case-hardening atmosphere, the quenching parameters and the component geometry.
12Frequently Asked Questions about Case-Hardening Steels
What is the main difference between case-hardening steels and quenched and tempered steels?
Case-hardening steels differ from quenched and tempered steels in their carbon content and their approach to heat treatment. Case-hardening steels have a low carbon content (≤0.20%) and undergo surface enrichment followed by hardening, whereas quenched and tempered steels have a higher carbon content (0.25-0.60%) and are treated uniformly throughout the cross-section.
Which case-hardening process is most widely used in modern industry?
Gas carburising is today the most widespread and is particularly suited to large-scale production. It offers the advantage of eliminating the solid carburising agent and the possibility of proceeding to direct quenching at the end of the operation without further reheating.
How is the optimal case-hardening depth determined?
The case-hardening depth is determined by the service stresses and the dimensions of the component. The depth can vary from a few tenths of a millimetre up to several millimetres, and is measured as CHD (depth at 550 HV hardness) according to ISO 18203:2016 (formerly ISO 2639).
What are the advantages of alloy steels over non-alloy steels for case hardening?
Alloy steels offer greater hardenability, making it possible to achieve uniform properties even in large cross-sections. Alloying elements such as nickel, chromium and molybdenum improve toughness, hardenability and strength respectively.
Why is the carbon content of case-hardening steels so low?
The low carbon content (≤0.20%) is necessary to ensure good case hardening and to avoid excessively increasing the core hardness, maintaining the toughness needed to withstand service stresses while the surface acquires the high hardness required.