• Hard facing, coating or surface alloying.

Explanation: The purpose of surface hardening is to develop a hard surface with compressive residual stress, to improve its wear resistance, to increase its fatigue life and to avoid susceptibility to distortion and cracking.

• Surface diffusion & subsequent treatment.

Explanation: Shot peening technique is applicable to all metals and alloys that are amenable to plastic deformation. The part to be hardened is placed in a chamber where extremely fine hard particles moving at a high speed keep striking at its surface. The energy of the moving particles is high enough to cause local plastic deformation at its surface.

Explanation: BLanding gears of aircraft are subjected to shot peening to develop residual compressive stress on its surface. Even automotive gears, following carburizing, are subjected to hot peening to raise the value of compressive residual stress (to as high as 1000 – 1200 MPa particularly at depths of 30 – 40 microns. This help resist crack propagation during service as result of fatigue loading.

Explanation: Engineering components that are required to resist solid particle erosion, abrasion, fretting orcavitation are usually given a hard surface coating. This consists of a fine dispersion of hard metal carbides in a compatible metal matrix. Thermal spray is the most commonly used technique to apply such coatings on the component.

Explanation: The most commonly used coating materials are mixtures of chromium or tungsten carbides in either cobalt or nickel-chromium alloy matrix. Hard facing is also a commonly used technique to salvage worn out parts so that they could be reused.

Explanation: This is applicable only for steel. An induction coil is used to heat the component to be hardened. Only the surface gets heated. Its microstructure transforms into austenite from a mixture of ferrite and cementite, but the structure of the core remains intact as it remains cold all through the process.

Explanation: Once the process is complete the microstructure of the surface gets transformed into martensite while that at its core remains unaltered. Hardness of induction hardened steel component may often be higher than that in through hardened steel having identical composition. One of the main advantages of induction hardening is good surface finish and little distortion.

• Little distortion & good surface finish.

Explanation: The hardness of steel depends only on the concentration of carbon in steel. Therefore it may be enough to have high carbon only at the surface. This can be achieved by increasing the concentration of carbon in a component made of low carbon steel by allowing carbon to diffuse into it.

Explanation: A major limitation of pack carburizing is poor control over temperature & carburization depth. On completion of the process, the steel parts are cooled slowly. Direct quenching is not possible as the job is surrounded by carburizing mixture packed in a sealed box having high thermal mass. This can be overcome by using a gaseous or liquid carburizing medium.

Explanation: CH4 and CO are the most commonly used carburizing gas. It is usually mixed with decarburizing (H2 and CO2) and neutral gases (N2). This helps maintain close control over carbon potential. Gas carburization is done by keeping the samples at the carburizing temperature for a specified time in a furnace having a mixture of carburizing and neutral gas.

Explanation: The main purpose of case hardening is to harden the case. Therefore the component after case refining is heated to 30⁰C-40⁰C above the lower critical temperature. At this temperature, the case consists of austenite and globules of undissolved carbide. The structure of the core during this stage of heat treatment should have ferrite and austenite.

Explanation: One of the main reasons for case hardening is to develop compressive residual stress at the surface of the components that are subjected to fatigue loading. A general thumb rule is that the region that transforms last has a compressive stress. In the case of a carburized steel there is a large difference in the concentration of carbon at the surface and that at the centre. The difference is of large that although the surfaced on quenching cools faster it transforms to martensite later than the core. Since it transforms last it should be under compression. This is the reason why case hardened components have compressive residual stress.