Surface Hardening – Case Hardening
Case hardening or surface hardening is the process in which hardness the surface (case) of an object 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, resistance to abrasion, resistance to 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. 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 that it is a property that is indicative of a material’s resistance to fracture when a crack (or other stress-concentrating defect) is present.
For iron or steel with low carbon content, which has poor to no hardenability of its own, the case-hardening process involves infusing additional carbon or nitrogen into the surface layer. Case hardening is useful in parts such as a cam or ring gear that must have a very hard surface to resist wear, along with a tough interior to resist the impact that occurs during operation. Further, the surface hardening of steel has an advantage over through 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. A carbon- or nitrogen-rich outer surface layer (or case) is introduced by atomic diffusion from the gaseous phase. The case is normally on the order of 1 mm deep and is harder than the inner core of material.
Carburizing – Advantages and Application
Carburizing is a case hardening process in which the surface carbon concentration of a ferrous alloy (usually a low-carbon steel) is increased by diffusion from the surrounding environment. Carburizing produces hard, highly wear-resistant surface (medium case depths) of product with excellent capacity for contact load, good bending fatigue strength and good resistance to seizure. Carburizing is usually used for low-carbon steels, which are heated to a temperature sufficient to render the steel austenitic, followed by quenching and tempering to form a martensitic micro-structure. 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 simply 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 rich in a hydrocarbon 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 gaseous hydrocarbons, for example, methane (CH4), propane (C3H3), and butane (C4H10), or from vaporized hydro-carbon 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. 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 case-hardened surface. Nitriding produces hard, highly wear-resistant surface (shallow case depths) of product with fair capacity for contact load, good bending fatigue strength and excellent resistance to seizure. In contrast to carburizing, in 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 range of 500 to 550 °C is typically used. These processes are most commonly used on low-carbon, low-alloy steels. 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 production of machine components, shafts, axles, gears, crankshafts, camshafts, cam followers, valve parts, extruder screws, die-casting tools or forging dies.
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