Surface Hardening - Case Hardening

Hardening of Metals

In materials science, hardness is the ability to withstand surface indentation (localized plastic deformation) and scratching. Hardness is probably the most poorly defined material property because it may indicate resistance to scratching, abrasion, an indentation, or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.

Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. To improve the hardness of the pure metal, we can use different ways, which include:

Surface Hardening – Case Hardening

Case hardening or surface hardening is the process in which the hardness of an object’s surface (case) is enhanced while the inner core of the object remains elastic and tough. After this process surface hardness, wear resistance, and fatigue life are enhanced. This is accomplished by several processes such as a carburizing or nitriding process by which a component is exposed to a carbonaceous or nitrogenous atmosphere at elevated temperature. As was written, two main material characteristics are influenced:

Hardness and wear resistance is significantly enhanced. In materials science, hardness is the ability to withstand surface indentation (localized plastic deformation) and scratching. Hardness is probably the most poorly defined material property because it may indicate resistance to scratching, abrasion, an indentation, or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.
Toughness is not negatively influenced, and toughness is the ability of a material to absorb energy and plastically deform without fracturing. One definition of toughness (for high-strain rate, fracture toughness) is a property that is a material’s resistance to fracture when a crack (or other stress-concentrating defects) is present.

The case-hardening process involves infusing additional carbon or nitrogen into the surface layer for iron or steel with low carbon content, which has poor to no hardenability. Case hardening is useful in parts such as a cam or ring gear that must have a very hard surface to resist wear and a tough interior to resist the impact that occurs during operation. Further, the surface hardening of steel has an advantage over hardening (that is, hardening the metal uniformly throughout the piece) because less expensive low-carbon and medium-carbon steels can be surface hardened without the problems of distortion and cracking associated with the through hardening of thick sections. An atomic diffusion from the gaseous phase introduces a carbon- or nitrogen-rich outer surface layer (or case). The case is normally 1 mm deep and harder than the material’s inner core.

Classification of Case Hardening Methods

Case hardening by surface treatment can be further classified as diffusion or localized heating treatments. Diffusion methods introduce alloying elements that enter the surface by diffusion as solid-solution or hardenability agents that assist martensite formation during subsequent quenching. During this process, the alloying element concentration is increased at a steel component’s surface. Diffusion methods include:

Carburizing. Carburizing is a case hardening process in which the surface carbon concentration of a ferrous alloy (usually low-carbon steel) is increased by diffusion from the surrounding environment. Carburizing produces hard, highly wear-resistant surface (medium case depths) of product with an excellent capacity for contact load, good bending fatigue strength, and good resistance to seizure.
Nitriding. Nitriding is a case hardening process in which the surface nitrogen concentration of a ferrous is increased by diffusion from the surrounding environment to create a case-hardened surface. Nitriding produces hard, highly wear-resistant surface (shallow case depths) of product with a fair capacity for contact load, good bending fatigue strength, and excellent resistance to seizure.
Boriding. Boriding, also called boronizing is a thermochemical diffusion process similar to nitrocarburizing in which boron atoms diffuse into the substrate to produce hard and wear-resistant surface layers. The process requires a high treatment temperature (1073-1323 K) and long duration (1-12 h) and can be applied to a wide range of materials such as steels, cast iron, cermets, and non-ferrous alloys.
Titanium-carbon and Titanium-nitride Hardening. Titanium nitride (an extremely hard ceramic material) or titanium carbide coatings can be used in the tools made of this kind of steel through a physical vapor deposition process to improve the performance and life span of the tool. TiN has a Vickers hardness of 1800–2100 and a metallic gold color.

Localized heating methods for case hardening include:

Flame hardening. Flame hardening is a surface hardening technique that uses a single torch with a specially designed head to heat the metal, which is then cooled rapidly, generally using water. This creates a “case” of martensite on the surface, while the inner core of the object remains elastic and tough. It is a similar technique to induction hardening. The carbon content of 0.3–0.6 wt% C is needed for this type of hardening.
Induction hardening. Induction hardening is a surface hardening technique that uses induction coils to provide a very rapid means of heating the metal, which is then cooled rapidly, generally using water. This creates a “case” of martensite on the surface. The carbon content of 0.3–0.6 wt% C is needed for this hardening.
Laser hardening. Laser hardening is a surface hardening technique that uses a laser beam to provide a very rapid means of heating the metal, which is then cooled rapidly (generally by self-quenching). This creates a “case” of martensite on the surface, while the inner core of the object remains elastic and tough.

Carburizing – Advantages and Application

Carburizing is a case hardening process in which the surface carbon concentration of a ferrous alloy (usually low-carbon steel) is increased by diffusion from the surrounding environment. Carburizing produces hard, highly wear-resistant surface (medium case depths) of product with an excellent capacity for contact load, good bending fatigue strength, and good resistance to seizure. Carburizing is usually used for low-carbon steels, heated to a temperature sufficient to render the steel austenitic, followed by quenching and tempering to form a martensitic microstructure. So that a high-carbon martensitic case with good wear and fatigue resistance is superimposed on a tough, low-carbon steel core, in its earliest application, parts were placed in a suitable container and covered with a thick layer of carbon powder (pack carburizing). Today, the steel piece is exposed, at an elevated temperature (usually above 850°C), to an atmosphere richydrocarbonarbon gas, such as methane (CH4). In gas carburizing, commercially the most important variant of carburizing, the source of carbon is a carbon-rich furnace atmosphere produced either from gashydrocarbonsrbons, for example, methane (CH₄), propane (C₃H₃), and butane (C₄H₁₀), or from vaporhydrocarbonrbon liquids. Heat enhances the diffusion of carbon into the steel surface and subsurface regions. The depth of diffusion (case depth) follows a time-temperature dependence such that:

Case depth ∝ D . √Time

where the diffusivity factor, D, depends on temperature, the chemical composition of the steel, and the concentration gradient of carbon at the surface. In terms of temperature, the diffusivity factor increases exponentially as a function of absolute temperature. This diffusion rate increases greatly with increasing temperature; the rate of carbon addition at 925°C is about 40% greater than at 870°C. The depth of any carburized case is a function of time and temperature.

Nitriding

Nitriding is a case hardening process in which the surface nitrogen concentration of a ferrous is increased by diffusion from the surrounding environment to create a case-hardened surface. Nitriding produces hard, highly wear-resistant surface (shallow case depths) of product with a fair capacity for contact load, good bending fatigue strength, and excellent resistance to seizure. In contrast to carburizing, nitriding nitrogen is added into ferrite instead of austenite. Therefore nitriding does not involve heating into the austenite phase field and a subsequent quench to form martensite. A temperature is significantly lower, and a range of 500 to 550 °C is typically used. These processes are most commonly used on low-carbon and low-alloy steels, and they are also used on medium and high-carbon steels, titanium, aluminium, and molybdenum. The most significant hardening is achieved with a class of alloy steels (nitralloy type) that contain approximately 1% Al. Typical applications include the production of machine components, shafts, axles, gears, crankshafts, camshafts, cam followers, valve parts, extruder screws, die-casting tools, or forging dies.

Carbonitriding

Carbonitriding is a hardening heat treatment that introduces carbon and nitrogen in the austenite of steel conducted from 1073 K to 1173 K. This treatment is similar to carburizing in that the austenite composition is changed. High surface hardness is produced by quenching to form martensite. Carbonitriding is often applied to inexpensive, easily machined low carbon steel to impart the surface properties of more expensive and difficult-to-work grades of steel without the need for drastic quenching, resulting in less distortion and reducing the danger of cracking the work. The surface hardness of carbonitrided parts ranges from 55 to 62 HRC. Carbonitriding (around 850 °C / 1550 °F) is carried out at temperatures substantially higher than plain nitriding (around 530 °C / 990 °F) but slightly lower than those used for carburizing (around 950 °C / 1700 °F) and for shorter times. It is often performed on power transmission parts, such as gear teeth, cams, shafts, and bearings, submitted to structural and surface fatigue operating conditions.

Boriding

Boriding, also called boronizing is a thermochemical diffusion process similar to nitrocarburizing in which boron atoms diffuse into the substrate to produce hard and wear-resistant surface layers. The process requires a high treatment temperature (1073-1323 K) and long duration (1-12 h) and can be applied to a wide range of materials such as steels, cast iron, cermets, and non-ferrous alloys. The resulting surface contains metal borides, such as iron borides, nickel borides, and cobalt borides. As pure materials, these borides have extremely high hardness and wear resistance.

Their favorable properties are manifested even when they are a small fraction of the bulk solid. The properties of boride layers are usually superior to those formed by nitriding and carburizing, particularly in terms of their hardness. Most borided steel surfaces will have iron boride layer hardnesses ranging from 1200-1600 HV. Nickel-based superalloys such as Inconel and Hastelloy typically have nickel boride layer hardnesses of 1700-2300 HV. The hardness of the boride layer can be retained at higher temperatures than, for example, that of nitrided cases. On the other hand, both gas carburizing and plasma nitriding have the advantage over boronizing because those two processes offer reduced operating and maintenance costs, require shorter processing times, and are relatively easy to operate. Boriding is typically used for many high-performance applications such as automotive, machine tools, aerospace, hydraulic tools, agricultural and defense industries, etc.

Titanium-nitride and Titanium-carbide Coatings

High speed steel — High-speed steel (HSS) is a tool steel with high hardness, wear, and heat resistance. High-speed steel is often used in power-saw blades and drill bits.

Titanium nitride (an extremely hard ceramic material) or titanium carbide coatings can be used in the tools made of this kind of steel through a physical vapor deposition process to improve the performance and life span of the tool.

TiN has a Vickers hardness of 1800–2100 and a metallic gold color. A well-known use for TiN coating is for edge retention and corrosion resistance on machine tooling, such as drill bits and milling cutters, often improving their lifetime by a factor of three or more.

TiC is an extremely hard (Mohs 9–9.5) refractory ceramic material, similar to tungsten carbide. It is also an abrasion-resistant surface coating on metal parts, such as tool bits and watches mechanisms. Titanium carbide is also used as a heat shield coating for atmospheric spacecraft reentry.

For example, molybdenum high-speed steel – AISI M2 is the “standard” and most widely used industrial HSS. This type of steel may be coated with titanium nitride. According to the AISI classification system, molybdenum high-speed steels are designated as Group M steels. M2 HSS has small and evenly distributed carbides giving high wear resistance, though its decarburization sensitivity is a little bit high. It is usually used to manufacture various tools, such as drill bits, taps, and reamers. The carbon and alloy contents are balanced at sufficient levels to provide a high attainable hardening response, excellent wear resistance, high resistance to the softening effects of elevated temperature, and good toughness for effective use in industrial cutting applications.

Induction Hardening

Induction hardening is a surface hardening technique that uses induction coils to provide a very rapid means of heating the metal, which is then cooled rapidly and generally used later. This creates a “case” of martensite on the surface. The carbon content of 0.3–0.6 wt% C is needed for this type of hardening. Martensite is a very hard metastable structure with a body-centered tetragonal (BCT) crystal structure. Martensite is formed in steels when austenite’s cooling rate is so high that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe₃C). Induction hardening produces hard, highly wear-resistant surface (deep case depths) with good capacity for contact load and bending fatigue strength. Material has fair resistance to seizure.

Heating is accomplished by placing a steel part in the magnetic field generated by high-frequency alternating current passing through an inductor, usually a water-cooled copper coil. These so-called eddy currents dissipate energy and produce heat by flowing against the resistance of an imperfect conductor. With induction heating, the steel can be heated very quickly to red-hot at the surface before the heat can penetrate any distance into the metal. The surface is then quenched, hardening it, and is often used without further tempering. This makes the surface very resistant to wear, and the component’s core remains unaffected by the treatment. Its physical properties are those of the bar from which it was machined, while the hardness of the case can be within the range of 37/58 HRC. Induction surface hardened low alloyed medium carbon steels are widely used for critical automotive and machine applications which require high wear resistance. A common use for induction hardening is for hardening the bearing surfaces, or “journals”, on automotive crankshafts or the rods of hydraulic cylinders. The wear resistance behavior of hardened induction parts depends on hardening depth and the magnitude and distribution of residual compressive stress in the surface layer.

Flame Hardening

Flame hardening is a surface hardening technique that uses a single torch with a specially designed head to provide a very rapid means of heating the metal, which is then cooled rapidly, generally using water. This creates a “case” of martensite on the surface, while the inner core of the object remains elastic and tough. It is a similar technique to induction hardening. The carbon content of 0.3–0.6 wt% C is needed for this type of hardening.

A common use for induction hardening is for hardening large parts, such as gears and machine tool ways, with sizes or shapes that would make furnace heat treatment impractical. The gear will usually be quenched and tempered to a specific hardness first, making most of the gear tough. Then the teeth are quickly heated and immediately quenched, hardening only the surface. The wear resistance behavior of induction hardened parts depends on hardening depth and the magnitude and distribution of residual compressive stress in the surface layer.

Laser Hardening

Laser hardening is a surface hardening technique that uses a laser beam to provide a very rapid means of heating the metal, which is then cooled rapidly (generally by self-quenching). This creates a “case” of martensite on the surface, while the inner core of the object remains elastic and tough. The heat generated by the absorption of the laser light is controlled to prevent melting and is therefore used in the selective austenitization of local surface regions. The self-quenching phenomenon applies after removing the heat source from the interaction zone. Thermal energy absorbed by the surface layer is quickly distributed to the entire workpiece.

Martensite is a very hard metastable structure with a body-centered tetragonal (BCT) crystal structure. Martensite is formed in steels when austenite’s cooling rate is so high that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe3C). Laser hardening produces hard, highly wear-resistant surfaces (shallow case depths). Thin surface zones that are heated and cooled very rapidly result in fine martensitic micro-structures, even in steels with relatively low hardenability. Laser hardening is widely used to harden localized areas of steel and cast iron machine components. The main advantages are: the possibility of selective surface heat treatment of complex parts, minimal deformations of processed parts, and the process are fast, clean, and computer controlled.

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See above:
Metalworking