Heat treatment is the unsung hero of industrial metallurgy, transforming the performance of materials through controlled temperature cycles. Among these treatments, case hardening and tempering play distinct and critical roles. While often used together, they serve different purposes: one improves surface hardness, while the other modifies internal strength and ductility.
This article explores the difference between case hardening and tempering, focusing on the intricacies of case hardening heat treatment, including carburising processes, material compatibility, and how companies like Metal Technology Engineering deliver industry-leading results.
What Is Case Hardening?
Case hardening is a heat treatment process that hardens only the surface layer (or “case”) of a metal component while keeping the inner core soft and ductile. This results in a part that is both wear-resistant on the outside and tough and impact-resistant on the inside.
The process is particularly valuable for components that experience surface wear but must also endure impact or torque, such as:
- Gears
- Bearings
- Sprockets
- Cams
- Low-carbon steel components
Carburising: A Type of Case Hardening Heat Treatment
Carburising is the most widely used form of case hardening heat treatment. It involves diffusing carbon atoms into the surface of a metal, typically low-carbon steel, to increase its surface hardness. The process is performed after machining, allowing for the manufacture of complex parts from relatively inexpensive, easily machinable steels.
How Carburising Works
- Carbon Introduction: The steel component is placed in a carbon-rich environment. In Metal Technology Engineering’s case, molten cyanide salt baths with a carbon potential of 0.6 to 0.8%.
- Heat Application: The component is heated from 880°C to 900°C, allowing carbon atoms to diffuse into the surface.
- Time Control: The part is held at this temperature for a controlled period (from 1 to 60 hours), determining the depth of the case (typically 0.1 to 3 mm).
- Quenching: After carburising, parts are either cooled slowly for later treatment or quenched immediately in water, oil, or salt to harden the carbon-rich surface.
- Tempering and Cryogenic Treatment (optional): To reduce internal stress and convert any retained austenite, components may be tempered and cryogenically treated before a final tempering cycle.
What Is Tempering?
Tempering is a separate heat treatment process applied after hardening. It involves heating a previously hardened metal to a lower temperature, typically between 150°C and 700°C, and then cooling it.
Tempering:
- Reduces brittleness introduced during hardening
- Increases toughness and ductility
- Improves dimensional stability
- Retains adequate hardness for performance
Where case hardening is about increasing surface hardness, tempering is about balancing that hardness with internal toughness, especially important after quenching, which can create internal stresses.
Key Differences Between Case Hardening and Tempering
| Aspect | Case Hardening | Tempering |
| Purpose | Hardens the surface while preserving a tough core | Reduces brittleness and internal stress after hardening |
| Process type | Carburising (carbon diffusion) followed by quenching | Reheating and cooling post-hardening |
| Temperature range | 880°C to 900°C (for carburising) | 150°C to 700°C |
| Effect | Wear-resistant exterior, tough interior | Balanced toughness and hardness |
| Application timing | Applied before or during initial hardening | Always applied after hardening or quenching |
| Common uses | Gears, cams, bearing surfaces | Quenched tools, springs, structural parts |
Metal Technology Engineering: Leaders in Case Hardening Heat Treatment
Metal Technology Engineering specialises in molten salt bath carburising, a highly controlled and versatile method of case hardening. Our facilities include:
- Five carburising furnaces, with four in operation and one on standby for redundancy
- Salt baths that allow for tight control over carbon potential
- Customisable cycle durations, from 1 to 60 hours
- Quench options in water, oil, and salt to meet a wide range of material and dimensional specifications
- Post-treatment tempering and cryogenic processing to maximise performance
Our jobbing capability and flexible furnace design enable us to accommodate a wide range of materials, shapes, and production volumes, all while maintaining the highest quality standards.
Benefits of Case Hardening Heat Treatment
- Increased surface hardness for superior wear resistance
- Maintains core toughness and shock resistance
- Enhances fatigue strength and lifespan of components
- Improves resistance to pitting and contact stress
- Allows complex machining before hardening
This makes it an ideal process for high-load, high-friction components across various sectors, including automotive, industrial, agricultural, and aerospace.
FAQs: Case Hardening Heat Treatment
What is the best steel for case hardening?
Low-carbon steels such as 1018 or 1020 are commonly used because they are easy to machine and can develop a hard outer case through carburising.
How deep can a carburised case be?
At Metal Technology Engineering, we can produce case depths ranging from 0.1 mm to 3 mm, depending on the treatment time and temperature.
Is case hardening permanent?
Yes, the hardening effect is permanent, forming a hardened outer shell that cannot be reversed through normal wear.
Why temper a component after case hardening?
Tempering reduces internal stress caused by quenching, helps stabilise dimensions, and improves the overall mechanical properties of the part.
Can all metals be case hardened?
Not all. Case hardening is typically used on low-carbon steels. High-carbon steels may become too brittle, and non-ferrous metals usually don’t respond to carburising.
What’s the difference between case hardening and surface coating?
Case hardening modifies the steel’s actual surface through atomic diffusion, whereas coatings sit on top of the material and can wear off more easily.
Engineering Strength Where It Matters
Case hardening and tempering are not interchangeable; they are complementary. Case hardening heat treatment imparts a hardened shell to components, while tempering fine-tunes the internal structure for enhanced performance and reliability.
We combine deep technical knowledge with world-class facilities to deliver precision-engineered, wear-resistant components for demanding applications.
Ready to enhance the performance and lifespan of your components? Contact Metal Technology Engineering today for expert case hardening heat treatment services.
