Case-Hardening Steels

1. Case-Hardening Steels: Technical Guide for Industry Professionals

Case-hardening steels represent a specialized category of metallurgical materials specifically designed to achieve high performance through case hardening heat treatment for steels.

These materials, characterized by a low carbon content and an optimized chemical composition, constitute the preferred technological solution for mechanical components subjected to severe surface stresses, combining extreme surface hardness with core toughness.

2. Definition 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 steels has the peculiarity of maintaining a tough core after quenching and tempering while developing an extremely hard surface layer through the thermochemical case hardening process.

The distinctive characteristic of these materials lies in their ability to develop a gradient of mechanical properties from core to 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 area of the component, which can reach concentrations of 0.8-1% while maintaining the low carbon content of the core.

3. Optimal Chemical Composition

The chemical composition of case-hardening steels is designed to optimize 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, particularly important for applications requiring resistance to mechanical shocks.

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.

4. Microstructure and Phase Transformations

The microstructure of case-hardened steels presents a characteristic gradient from surface to core.

The case-hardened layer, after quenching and tempering, has a tempered martensitic structure with finely dispersed carbides, while the core maintains a lower carbon content structure with higher toughness.

The transition zone shows a gradual change in composition and microstructure that ensures the absence of mechanical discontinuities.

5. Classification of Case-Hardening Steels According to International Standards

5.1. European Standard UNI EN 10084

The classification of case-hardening steels in Europe is regulated by the UNI EN 10084 standard, which establishes the technical delivery conditions for non-alloy and alloy case-hardening steels.

This standard defines the minimum requirements for the steel production process, chemical composition, hardness, hardenability and technological characteristics agreed with the manufacturer.

The standard prescribes specific controls including verification of grain size, visual and dimensional inspection.

For steels ordered without hardenability requirements, the hardness requirements indicated in Table 1 of the UNI 10084 standard must be verified, while for steels ordered with hardenability requirements (+H), additional hardenability controls according to the Jominy test are required.

5.2. ASTM 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 can be replaced by ASTM 4320, demonstrating the international harmonization of specifications for these materials.

Comparative Table of Designations

6. Case Hardening Heat Treatment for Steels

6.1. Case Hardening Processes (Gas, Liquid, Solid)

Gas, liquid, and solid case hardening comprise three distinct methodologies for surface carbon enrichment.

Gas case hardening, now more widely used for large-scale production, involves heating the parts to 850-950°C in a gas stream consisting essentially of methane (CH₄) which, in contact with carbon dioxide and oxygen, releases CO molecules.

Case hardening temperatures vary from 850°C to 950°C depending on the steel composition, the required depth and the type of process (gas, liquid, solid). Gas case hardening typically operates at 920-930°C for carbon-manganese steels.

Liquid case hardening involves immersing the parts in molten salt baths such as sodium cyanide, alkali and barium carbonates and chlorides. Sodium cyanide reacts with atmospheric oxygen and carbon dioxide, releasing carbon monoxide which releases carbon atoms into the part.

Solid case hardening, less used in modern industry, involves the use of solid carbonaceous mixtures in boxes or closed containers, with process temperatures and times similar to the other methods but with greater difficulties in controlling the atmosphere and treatment uniformity.

6.2. Process Parameters and Control

The case hardening depth of steels is controlled through specific time and temperature parameters. The diffusion depth of carbon and the actual associated case hardening (ECD) can range from a few tenths of a millimeter to several millimeters.

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 limit hardness of 550 HV. Recent studies have shown that samples with case hardening depths of about 0.55 mm, 0.9 mm and 1.9 mm were obtained, with surface hardness between 640 HV1 and 760 HV1.

6.3. Post-Case-Hardening Heat Treatments

The post-case-hardening thermal cycle may involve direct quenching from the case hardening temperature or cycles with double quenching for medium or large components.

Direct quenching involves immersion in oil or a suitable quenching fluid starting from the case hardening temperature, followed by tempering at 150-200°C.

For critical components requiring high quality and reliability, the double quenching cycle allows the properties of the core and surface to be optimized separately, performing a first quenching for the core and a second quenching at the correct temperature for the surface, tempering the core at the same time.

7. Limitations of the Case Hardening Process:

The case hardening process of steels has limitations that are essential to know when working with this treatment:

• Inevitable distortions: deformations linked to phase transformations
• Selective decarburization: risk in unmasked areas
• Intergranular oxidation: critical atmosphere control
• Formation of massive carbides: for surface C concentrations >0.9%
• Hydrogen embrittlement: particularly critical in Ni steels
• Long process times: 6-24 hours for significant depths
• Mandatory masking: for areas not to be case hardened

8. Mechanical Properties and Performance Characteristics

8.1. Surface Hardness and Case Hardening Depth

The mechanical properties of case-hardening steels are characterized by high surface hardness that can reach 62-64 HRC, ensuring excellent resistance to mechanical wear. Surface hardness varies depending on the diffused carbon content and the subsequent quenching and tempering conditions.

The hardness distribution from surface to core follows a decreasing trend that reflects the carbon gradient obtained during case hardening. This configuration ensures a gradual transition from surface properties to core properties, avoiding critical stress concentrations.

8.2. Wear Resistance and Contact Fatigue

Case-hardened steels show 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 core toughness prevents the propagation of surface cracks.

Contact fatigue resistance is significantly improved by the presence of the hardness gradient, which distributes contact stresses over a larger volume compared to uniform hardening treatments.

8.3. Core Toughness

Core toughness in low-carbon case-hardening steels is maintained through control of the chemical composition and heat treatment parameters. Steels with a high nickel percentage are preferred to achieve high toughness, essential for withstanding dynamic stresses and mechanical shocks.

9. Most Widely Used Case-Hardening Steels

9.1. Non-Alloy Steels (C10, C15, C20)

Non-alloy case-hardening steels, designated according to the carbon content multiplied by 100, represent the basic 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.

9.2. Alloy Steels (20MnCr5, 18CrNiMo7-6, 20NiCrMo2-2)

20MnCr5 is the most widely used manganese-chromium case-hardening steel, available in soft-annealed condition for 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.

18CrNiMo7-6 is a high-hardenability alloy steel used for heavily stressed components, available in soft-annealed condition for 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 quenching.

20NiCrMo2-2, although not specifically mentioned in the sources consulted, typically represents a nickel-containing case-hardening steel for applications requiring high core toughness combined with good hardenability.

9.3. Special Steels for Critical Applications

Steels of the 17NiCrMoS6-4 series contain sulfur to improve machinability, also available with the addition of lead (17NiCrMoS6-4+Pb) to maximize machining speed. These grades are used for precision components produced in large series.

16NiCrMo12 represents a case-hardening steel with a high content of alloying elements for critical applications requiring high hardenability and strength.

10. Industrial Applications of Case-Hardened Steels

10.1. Automotive 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 the combination of high surface hardness and good core toughness to ensure reliability and durability over time.

Case-hardened steels are used to manufacture critical powertrain components, where wear and fatigue resistance are essential for vehicle performance and reliability.

10.2. Gears and Transmissions

Gears represent the main application of case-hardening steels, requiring high surface hardness to withstand tooth wear and core toughness to bear the transmitted loads. Case hardening makes it possible to obtain optimal hardness profiles to maximize the fatigue life of gears.

10.3. Bearings and Precision Components

In the aerospace industry, case-hardened steel is used to manufacture engine components, such as reduction gears, transmission shafts and bearings. The severe 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, molds and dies, where the combination of surface hardness and dimensional precision is critical.

11. Quality Control and Defect Analysis

11.1. Standard Tests and Controls

Quality control of case-hardened steels includes verification of case hardening depth according to ISO 2639, surface and core hardness checks, and microstructural verifications. The case hardening depth CHD, according to ISO 2639, 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.

Controls include fatigue tests to verify the strength of the finished component, dimensional checks to verify deformations induced by heat treatment, and metallographic checks to verify the microstructure and the absence of defects.

11.2. Typical Defects and Prevention

Typical defects in case-hardened steels include surface decarburization, internal oxidation, quench cracks and excessive deformations. Prevention requires strict control of the case hardening atmosphere, quenching parameters and component geometry.

12. Frequently 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 terms of carbon content and heat treatment approach. Case-hardening steels have a low carbon content (≤0.20%) and undergo surface enrichment followed by quenching, while quenched and tempered steels have a higher carbon content (0.25-0.60%) and are treated uniformly throughout the entire section.

Which case hardening process is most widely used in modern industry?
Gas case hardening is now the most widely used and is especially suited to large-scale production. It has the advantage of eliminating the solid carburizing agent and allowing direct quenching at the end of the operation without further heating.

How is the optimal case hardening depth determined?
The case hardening depth is determined by the operating stresses and the dimensions of the component. The depth can range from a few tenths of a millimeter to several millimeters, and is measured as CHD (depth with hardness 550 HV) according to ISO 2639.

What are the advantages of alloy steels over non-alloy steels for case hardening?
Alloy steels offer greater hardenability, allowing uniform properties to be obtained even in large 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 achieve good case hardening and to avoid excessively increasing core hardness, maintaining the toughness needed to withstand operating stresses while the surface acquires the required high hardness.