Surface Hardening Process | A Complete Guide To Its Types and Techniques.

Surface hardening is defined as a process by which a metal is given a hard, wear-resistant surface while retaining a ductile but tougher interior.
It is a method used to improve the wear resistance of metal parts without affecting the softer and tougher interior of the part.
Surface hardening techniques can be classified into two major categories:
1. Case hardening or diffusion method.
2. Selective surface hardening method.
Case hardening :
it is a surface hardening process by diffusion that involves the chemical modification of a surface. They are processes that change the surface chemical composition.
Case hardening methods include Carburizing, nitriding, carbonitriding, and cyaniding.
★ Carburizing:
Carburizing is a surface hardening process in which carbon is introduced into the surface layer of steel.
It is the oldest surface hardening method in which steel is placed at a high temperature for several hours in a carbonaceous environment.
Carburizing methods include Pack carburizing, gas carburizing, vacuum carburizing, and liquid or salt bath carburizing.
★ Nitriding:
Nitriding is a surface hardening process that introduces nitrogen into the surface of a steel at a temperature range (500℃ to 550℃) in its ferritic condition.
The process methods for nitriding include gas, liquid, and plasma nitriding.
★ Carbonitriding:
Carbonitriding is a surface-hardening process by which carbon and nitrogen (via ammonia gas) permeate the surface layer of steel components. The process involves a temperature of around 850 ℃ followed by quenching in oil or gas solutions.
★ Cyaniding:
Cyaniding is similar to carbonitriding in that the process involves both the diffusion of carbon and nitrogen into the surface layers of the steel.
Cyaniding is carried out in a liquid bath of sodium cyanide (NaCN).
If the process is carried out in a gaseous atmosphere it is called carbonitriding.
SELECTIVE SURFACE HARDENING METHOD:
The selective surface hardening method is a process of hardening that does not change the surface chemical composition.
The methods of selective surface hardening include flame hardening, induction hardening, laser hardening, and electron beam hardening.
★ Flame hardening:
Flame hardening is a process of austenizing the surface of steel by heating it with an oxyacetylene or oxyhydrogen torch, and immediately quenching it with water or water-based polymer.
The result is a hard surface layer of martensite over a softer core with a ferrite-pearlite structure.
★ Induction hardening:
Induction hardening is a complex combination of electromagnetic heat transfer and metallurgical phenomena that can perform uniform surface hardening, and localized surface hardening on metal pieces.
It is a method of quickly and selectively hardening the surface of a metal part.
★ Laser hardening:
Laser hardening is the process of rapid heating of a material surface by a laser beam, a short hold at the target temperature, and an intensive cooling due to the high thermal conductivity of the material, resulting in very fine martensitic microstructure, even in steels with relatively low hardenability.
★ Electron beam hardening:
electron beam hardening uses a concentrated beam of high-velocity electrons as an energy source to heat and harden the selected surface area of ferrous parts.
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If only we had resources like this thirty years ago when I was doing my Mech Eng degree, I might have finished up with a decent overview of the field instead of general frustration & bewilderment.