Metallurgy 1 (Heat Treatment Process)

 Heat treatment may be defined as operation or combination of operations involving heating and cooling of metal / alloy in solid steels to obtain desired conditions ( Relieve stress ) and properties ( like Machinability, ductility etc. )

Purpose of Heat Treatment:

1. To relieve stress created during cold working, welding, casting etc.
2. Improve Machinability.
3. Change grain size.
4. Improve ductility and
5. Homogenous structure.

Cooling rates also plays a important role. Slow cooling produces pearlitic structure and rapid cooling produces a Martensitic ( hard ) structure.

Different heat treatment process:

Annealing:

Metals are annealed to relieve internal stresses, soften them,make them more ductile, and refine their grain structures.Metal is annealed by heating it to a prescribed temperature,
holding it at that temperature for the required time, and then cooling it back to room temperature. The rate at which metal is cooled from the annealing temperature varies greatly. Steel must be cooled very slowly to produce maximum softness,This can be done by burying the hot part in sand, ashes, or some other substance that does not conduct heat readily (packing), or by shutting off the furnace and allowing the furnace and part to cool together (furnace cooling).
 Normalizing:
 

Ferrous metals are normalized to relieve the internal stresses produced by machining, forging, or welding. Normalized steels are harder and stronger than annealed steels. Steel is much tougher in the normalized condition than in any other condition. Parts that will be subjected to impact and parts that require maximum toughness and resistance to external stresses are usually normalized. Normalizing prior to hardening is beneficial in obtaining the desired hardness, provided the hardening operation is performed correctly. Low carbon steels do not usually require normalizing, but no harmful effects result if these steels are normalized. Normalizing is achieved by heating the metal to a specified temperature (which is higher than either the hardening or annealing temperatures), soaking the metal until it is uniformly heated, and cooling it in still air.

Hardening:

A ferrous metal is normally hardened by heating the metal to the required temperature and then cooling it rapidly by plunging the hot metal into a quenching medium, such as oil, water, or brine. Most steels must be cooled rapidly to harden them. The hardening process increases the hardness and strength of metal, but also increases its brittleness.

Tempering:

Steel is usually harder than necessary and too brittle for practical use after being hardened. Severe internal stresses are set up during the rapid cooling of the metal. Steel is tempered after being hardened to relieve the internal stresses and reduce its brittleness. Tempering consists of heating the metal to a specified temperature and then permitting the metal to cool. The rate of cooling usually has no effect on the metal structure during tempering. Therefore, the metal is usually permitted to cool in still air. Temperatures used for tempering are normally much lower than the hardening temperatures. The higher the

tempering temperature used, the softer the metal becomes. High-speed steel is one of the few metals that becomes harder instead of softer after it is tempered.

Case Hardening:

Case hardening is an ideal heat treatment for parts which require a wear-resistant surface and a tough core, such as gears, cams, cylinder sleeves, and so forth. The most common

case-hardening processes are carburizing and nitriding.During the case-hardening process, a low-carbon steel (either straight carbon steel or low-carbon alloy steel) is heated to a specific temperature in the presence of a material (solid,liquid, or gas) which decomposes and deposits more carbon into the surface of a steel. Then, when the part is cooled rapidly, the outer surface or case becomes hard, leaving the,inside of the piece soft but very tough.
For many applications, there is a need for a hard case and a soft tough core, which is shock resistant. No carbon can possess both these properties at the same time. Hence low carbon steel with desired core properties are chosen and Carbon / Nitrogen is added to the surface to provide a hardened case to a specified depth by using the following process

• Carburising.
• Nitriding,
• Cyaniding and
• Carbon Nitriding.

Also medium Carbon steel could be taken in normalized condition and case hardened by Induction and Flame Hardening.Carburizing:

This process is also called as cementation. Low carbon steel ( 0.2 % C ) is heated to 870 - 925 C in contact with gases or carbon for several hours. There are three types. They are Pack Carburizing, Gas Carburizing and Liquid Carburizing. This method is used for case hardening Gears, Camshafts and Bearing.

Nitriding:

It involves the addition of Nitrogen on certain types of steels and heating them and holding at a suitable temperature, in contact with ammonia or any other suitable medium. The steel should contain Aluminum or chromium to form hard nitrides.

In this the component to be case hardened is heat resistant container along with ammonia. It is then heated to a temperature of about 500^o C.

Cyaniding:

Both Carbon and nitrogen are introduced on the surface of steel by heating to a suitable temperature and holding the component in molten cyanide. Sodium cyanide is mostly used. This results in the formation of hardened Carbide - Nitride case. In this process Nitrogen provides hardening, but carbon responds to quenching process.

Carbo - Nitriding:

Both Carbon and Nitrogen are added to the surface of steel by using Gas atmosphere and not Molten Cyanide. The gaseous atmosphere contains the following

1. Carrier gas ( H₂, N₂ or CO )
2. Enriching gas ( Natural Gas )
3. Ammonia.

Flame Hardening:

The material is heated on the surface with flame. This is followed by quenching. Thus creating a hardened case and a soft core. Oxy acetylene flame is used and the steel should contain 0.3 % to 0.6 % of carbon.

Induction Hardening:

The material is heated in a alternative magnetic field followed by quenching.