Heat treatment: Purpose of heat treatment, Process of heat treatment, Annealing, Normalizing, Hardening, Tempering, Surface Hardening, Carburising, Nitriding, Hardenability

Purpose of heat treatment

Definition– It may be defined as heating and cooling operations applied to metals and alloys in solid state so as to obtain the desired properties. 

The object of this process is to make the metal better suited, structurally and physically, for some specific applications. Heat treatment may be undertaken for the following purposes.

(i) Improvement in ductility

(ii) Relieving internal stresses

(iii) Refinement of grain size

(iv) Increasing hardness or tensile strength and achieving changes in chemical composition of metal surface as in the case of case-hardening.

Also compress machinability, alteration in magnetic properties, modification of electrical conductivity, improvement in toughness and development of re-crystallized structure in cold-worked metal.

Normalizing,  Annealing, Hardening, Tempering, Case hardening, Carburizing, Cyaniding, Nitriding, Surface hardening, Induction hardening,  Flame hardening.

Heat treatment process


Process of heat treatment

Annealing

Annealing involves heating to predetermined temperature, holding at this temperature and finally cooling at a very slow rate. The temperature to which the steel is heated and the holding time are determined by various factors such as chemical composition of steel, size and shape and final properties required. The various purposes for this treatment are to

(i) Relieve interval stresses developed during solidification, machining, forging, rolling or welding.

(ii) Improve or restore ductility and toughness.

(iii) Enhance machinability.

(iv) Eliminate chemical non-uniformity.

(v) Refine grain size.

(vi) Reduce the gaseous contents in steel.

Normalizing

Normalizing is a process of heating steel to about 40-50 degree Celsius above upper critical temperature, holding for proper time and then cooling in still air or slightly agitated air to room temperature. After normalizing the resultant microstructure should be pearlite. This is important for some alloy steels which are air hardening by nature. Better dispersion of ferrite and cementite in the final structure results in enhanced mechanical properties. The grain size is finer and refinement of grain size. Rolled and forged steels possessing coarse grains due to high temperatures involved are subjected to normalizing. Normalized steels are generally stronger and harder than fully annealed steels.

Hardening

Hardening consists of heating to hardening temperature, holding at that temperature, followed by rapid cooling such as quenching in water oil or salt baths. High hardness developed by this process is due to phase transformation with rapid cooling. For plain carbon steels, it depends on carbon content. Hypoeutectoid steels are heated to about 30 – 50 degree Celsius above the critical temperature where as eutectoid and hyper eutectoid steels are heated to about 30 – 50 degree Celsius above the lower critical temperature.

Tempering

The process which consists of heating hardened steel below the lower critical temperature, followed by cooling in air or at any other desired rate, is known as tempering. This treatment lowers hardness strength and wear resistance of the hardened steel marginally. The higher the tempering temp, the more is the restored ductility and toughen the steel. Proper tempering treatment results is optimum combination of mechanical properties. Elastic properties is affected by this. Hardening followed by tempering will improve elasticity.

Surface Hardening

In order to process considerable strength to with stand forces acting on them and to withstand wear on their surface, the parts must be made of tough materials and provided with a hard surface by introducing carbon or nitrogen on its surface with core remaining soft. Surface hardening or case-hardening provides us a hand and wear resistant surfaces, close tolerance in machining parts and tough-core combined with a higher fatigue limit and high mechanical properties in core.

It is carried out by following operations

(a) carburising 

(b) Nitriding 

(c) Carbonitriding

(d) Cyaniding

(e) Induction hardening

 (f) Flame hardening

Carburising

It is the process of producing a hard surface on low carbon steel parts. There are three methods of carburising such as 

(a) pack or solid carburising 

(b) Gas carburising 

(c) Liquid carburing

Liquid carburising is performed in activated bath of calcium cyanamide, sodium or potassium cyanide and other controlling chemicals which govern the decomposition of the cylinders. The baths are operated at 815.5 degree Celsius to 898.85 degree Celsius produce a case of depth of 0.5mm in 90 minutes. The process extremely flexible and easily controlled. The reaction in the bath is 2Na2CO3 + SiC –Na2SiO3 + Na2O + 2CO + C.

Nitriding

The introduction of nitrogen into the outer surface of steel parts in order to give an extremely hard, wear resisting surface is called nitriding. It is provided by placing the article in ammonia vapour a temperature between 450 degree Celsius and 550 degree Celsius for 10 hours. The core should be brought to its original toughness before nitriding by quenching in oil from about 900 degree Celsius and tempering from about 600 degree Celsius to 650 degree Celsius. It is used for various automotives, airplane and diesel engine parts like cylinders, sleeves, liners etc.

Hardenability

It is defined as property of a steel to be hardened by quenching and determined the depth and distribution of hardness throughout a section obtained by quenching. 

The main factors affecting hardenability are

(a) Alloying elements

(b) Carbon content

(c) Grain size of steel

(d) The homogeneity of starting steel

(e) Homogeneity obtained in the austenite before quenching by increasing carbon content, hardness can be increased.


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