Surface Hardening of Steels

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Introduction to Surface Hardening Technologies

Surface hardening represents a comprehensive process that encompasses numerous techniques specifically developed to enhance the wear resistance of steel components while maintaining the softer, tougher interior characteristics essential for impact resistance. This unique combination of surface hardness and core toughness proves invaluable in applications such as cams and ring gears, where components must possess extremely hard surfaces to resist wear while retaining tough interiors capable of withstanding operational impacts.

The advantages of surface hardening over through hardening extend beyond performance characteristics to include economic benefits. Surface hardening allows manufacturers to utilize less expensive low-carbon and medium-carbon steels effectively, avoiding the distortion and cracking problems commonly associated with through hardening of thick sections. This approach provides both cost savings and improved manufacturing reliability across diverse industrial applications.

Classification of Surface Hardening Methods

Surface hardening techniques can be organized into two distinctly different approaches, each offering unique advantages and applications. The first category involves methods that create intentional buildup or addition of new layers to the substrate material. The second category encompasses techniques that modify surface and subsurface regions without intentional buildup or dimensional increases.

Table 1. Engineering methods for surface hardening of steels, organized by treatment approach and specific technique

Layer additions	Substrate treatment
Hardfacing Fusion harcifacing Thermal spray Coatings Electrochemical plating Chemical vapor deposition (electroless plating) Thin films (physical vapor deposition, puttering, ion plating) Ion mixing	Diffusion methods Carburizing Nitriding Carbonitriding Nitrocarburizing Boriding Titanium-carbon diffusion Toyota diffusion process Selective hardening methods Flame hardening Induction hardening Laser hardening Electron beam hardening Ion implantation Selective carburizing and nitriding Use of arc lamps

Layer Addition Methods

The first group of surface hardening methods includes thin films, coatings, and weld overlays, commonly referred to as hardfacing applications. Layer addition techniques encompass hardfacing through fusion welding and thermal spray processes, various coating methods including electrochemical plating and chemical vapor deposition, and advanced thin film applications such as physical vapor deposition, sputtering, and ion plating.

Films, coatings, and overlays generally become less cost-effective as production quantities increase, particularly when complete surface coverage is required. The fatigue performance of these surface treatments may present limitations depending on the bond strength achieved between the substrate and added layer. Fusion-welded overlays typically provide strong metallurgical bonds, though primary surface-hardened steels used in wear applications under fatigue loading conditions typically include heavy case-hardened steels and flame or induction-hardened materials.

Despite these considerations, coatings and overlays demonstrate effectiveness in specific applications. Tool steel applications, for example, benefit significantly from titanium nitride and aluminum oxide coatings, which provide not only exceptional hardness but also chemical inertness that reduces wear and prevents chip welding to tool surfaces. Overlay techniques prove particularly effective when selective hardening of large surface areas is required.

Substrate Modification Methods

The second category of surface hardening methods divides into diffusion processes and selective hardening techniques. Diffusion methods fundamentally modify surface chemical composition through the introduction of hardening species such as carbon, nitrogen, or boron. These processes enable effective hardening of complete part surfaces and are typically employed when large quantities of components require surface treatment.

Selective surface hardening methods provide localized hardening capabilities and generally involve transformation hardening achieved through controlled heating and quenching cycles. However, certain selective hardening approaches, including selective nitriding, ion implantation, and ion beam mixing, rely exclusively on compositional modification rather than thermal transformation.

Understanding Diffusion-Based Surface Hardening

Surface hardening through diffusion processes involves chemical modification of surface regions through thermochemical treatment. Heat application becomes essential to enhance the diffusion of hardening species into surface and subsurface areas of treated components. The fundamental nature of diffusion processes creates predictable relationships between treatment parameters and achieved results.

Diffusion Kinetics and Case Depth Relationships

The depth of diffusion exhibits time-temperature dependence that follows established mathematical relationships. Case depth can be approximated using the equation:
Case depth ≈ K √Time,
where K represents the diffusivity constant. This constant depends on several critical factors including temperature, steel chemical composition, and the concentration gradient of the specific hardening species being diffused.

Temperature effects on the diffusivity constant follow exponential relationships with absolute temperature, making temperature control crucial for achieving consistent results. Concentration gradients depend on surface kinetics and chemical reactions specific to each particular process, influencing the final hardening characteristics achieved.

Process Variations and Characteristics

Diffusion hardening methods encompass several variations based on the hardening species employed, such as carbon, nitrogen, or boron, and the process methods used to transport these species to component surfaces. Process variations involve handling hardening species in different forms including gases, liquids, or ionic states. These variations naturally produce differences in typical case depths and achievable hardness levels.

Table 2. Typical characteristics of various diffusion treatments

Process	Nature of case	Process temperature (°C)	Typical case depth	Case hardness (HRC)	Typical base metals
Carburizing Pack	Diffused carbon	815-1090	125μm-1.5mm	50-63*	Low-carbon steels, low-carbon alloy steels
Gas	Diffused carbon	815-980	75 μm-1.5mm	50-63*	Low-carbon steels, low-carbon alloy steels
Liquid	Diffused carbon and possibly nitrogen	815-980	50 μm-1.5mm	50-65*	Low-carbon steels, low-carbon alloy steels
Vacuum	Diffused carbon	815-1090	75 μm-1.5mm	50-63*	Low-carbon steels, low-carbon alloy steels
Nitriding Gas	Diffused nitrogen, nitrogen compounds	480-590	12μm-0.75mm	50-70	Alloy steels, nitriding steels, stainless steels
Salt	Diffused nitrogen, nitrogen compounds	510-565	2.5μm-0.75mm	50-70	Most ferrous metals. Including cast irons
Ion	Diffused nitrogen. nitrogen compounds	340-565	75μm-0.75mm	50-70	Alloy steels, nitriding steels, stainless steels
Carbonitriding Gas	Diffused carbon and nitrogen	760-870	75μm-0.75mm	50-65*	Low-carbon steels, low-carbon alloy steels, stainless steels
Liquid (cyaniding)	Diffused carbon and nitrogen	760-870	2.5-125μm	50-65*	Low-carbon steels
Ferritic nitrocarburizing	Diffused carbon and nitrogen	565-675	2.5-25μm	40-60*	Low-carbon steels
Other Aluminizing (pack)	Diffused aluminum	870-980	25μm-1mm	< 20	Low-carbon steels
Siliconizing by chemical vapor deposition	Diffused silicon	925-1040	25μm-1mm	30-50	Low-carbon steels
Chromizing by chemical vapor deposition	Diffused chromium	980-1090	25-50μm	Low-carbon steel < 30; High-carbon 50-60	High- and low carbon steels
Titanium Carbide	Diffused carbon and titanium, TiC compound	900-1010	2,5-12.5μm	> 70*	Alloy steels, tool steels
Boriding	Diffused boron. boron compounds	400-1150	12,5-50μm	40- > 70	Alloy steels, tool steels,Cobalt and nickel alloys

*  Requires quench from austenitizing temperature

The selection of appropriate diffusion methods depends heavily on the type of steel being treated, as different steel compositions respond differently to various hardening species and process conditions.

Table 3. Types of steels commonly used for various diffusion processes, showing the compatibility between steel grades and specific surface hardening techniques.

Diffusion substrates
Low-carbon steels	Alloy steels	Tool steels	Stainless steels
Carburizing Cyaniding Ferritic nitrocarburizing Carbonitriding	Nitriding Ion nitriding	Titanium carbide Boriding Salt nitriding Ion nitriding Gas nitriding	Gas nitriding Titanium carbide Ion nitriding Ferritic nitrocarburizing

Effective Case Depth Considerations

Understanding the distinction between total case depth and effective case depth represents a critical aspect of successful surface hardening implementation. The effective case depth typically measures approximately two-thirds to three-fourths of the total case depth achieved during treatment. This relationship becomes crucial when specifying treatment parameters, as the required effective depth must be clearly defined to ensure proper processing time and temperature selection.

Proper specification of effective case depth enables heat treatment facilities to process components for the correct duration at appropriate temperatures, ensuring that performance requirements are met consistently. This distinction becomes particularly important in applications where specific load-bearing capabilities or wear resistance characteristics must be achieved at predetermined depths below the surface.

Carburizing Processes and Applications

Carburizing represents one of the most widely used diffusion hardening processes, involving the introduction of carbon into low-carbon steel surfaces to create hard, wear-resistant cases while maintaining tough cores. Several carburizing methods are available, including pack carburizing, gas carburizing, liquid carburizing, and vacuum carburizing, each offering specific advantages for different applications.

Pack carburizing utilizes carbon-rich compounds in solid form, providing effective treatment for complex geometries but requiring longer processing times. Gas carburizing offers precise control over carbon potential and enables automated processing of large batches. Liquid carburizing, often called cyaniding when nitrogen is also introduced, provides rapid case formation but requires careful handling of hazardous materials. Vacuum carburizing eliminates oxidation concerns and provides exceptional surface cleanliness.

All carburizing processes require subsequent quenching from austenitizing temperatures to achieve maximum hardness, making distortion control an important consideration in process selection and part design.

Nitriding and Related Processes

Nitriding processes introduce nitrogen into steel surfaces to form hard nitride compounds without requiring subsequent quenching operations. Gas nitriding, salt bath nitriding, and ion nitriding represent the primary nitriding methods, each offering distinct advantages for specific applications.

Gas nitriding provides excellent dimensional stability and can treat complex geometries effectively, making it ideal for precision components. Salt bath nitriding offers rapid processing capabilities and good temperature uniformity but requires careful temperature control to prevent distortion. Ion nitriding, also known as plasma nitriding, provides precise control over surface composition and enables treatment at lower temperatures.

Carbonitriding and nitrocarburizing processes combine carbon and nitrogen introduction, creating cases with enhanced properties compared to single-element treatments. Ferritic nitrocarburizing operates at lower temperatures and provides excellent corrosion resistance along with moderate hardness increases.

Specialized Diffusion Processes

Advanced diffusion processes including boriding, aluminizing, siliconizing, and chromizing offer specialized surface properties for demanding applications. Boriding creates extremely hard boride compounds suitable for severe wear applications but requires careful control to prevent brittleness. Aluminizing provides oxidation resistance at elevated temperatures, while siliconizing offers both wear and corrosion resistance.

Titanium carbide diffusion processes create exceptionally hard surfaces suitable for cutting tools and wear-resistant applications. These specialized treatments typically require specific steel compositions and precise process control to achieve optimal results.

Process Selection and Implementation

Successful surface hardening implementation requires careful consideration of component requirements, steel composition, production volumes, and cost constraints. Layer addition methods work well for low-volume applications or when specific surface properties cannot be achieved through diffusion processes. Diffusion methods prove most cost-effective for high-volume production and when complete surface coverage is required.

Selective hardening techniques offer advantages when only specific areas require treatment, reducing processing costs and minimizing distortion risks. The choice between different surface hardening approaches ultimately depends on balancing performance requirements, production economics, and manufacturing capabilities.

Understanding the relationships between process parameters, steel composition, and final properties enables engineers to select optimal surface hardening methods for specific applications while ensuring consistent, reliable results in production environments.