Carburizing Process – Types Of Carburizing heat treatment Process

Surface Hardening or Case Hardening:

The process by which the surface of an object or part can be made hard and leaving the interior as it is, is called as surface or case hardening process. In many engineering applications, it is required that a steel being used should have a hard surface so that it can resist wear and tear. At the same time it should have soft and tough interior so that steel is able to absorb any shocks, impact, etc. This is possible only when the surface is hard while rest or interior of the metal is left as soft and ductile. This type of treatment is called surface hardening or case hardening and is applied to gears, ball bearings, railway wheels etc.

See also: Introduction To Heat Treatment and Objectives Of Heat Treatments

Carburizing Process:

The process of inducing of additional carbon to low carbon steels in order to give it a hard surface is known as carburizing. The surface is made hard only upto a certain depth using the any one of below:

pack carburizing and gas carburizing
pack carburizing and gas carburizing

i) Pack or Solid Carburizing:

The article to be carburized is placed in a carburizing box and surrounded by solid carbonaceous materials—usually a mixture of charcoal, leather, horn and barium carbonate as catalyst. The boxes are sealed with clay to exclude air and are placed in an oven or furnace where they are heated to a temperature between 900-950°C for several days depending upon the extent of carburizing action desired. In this way, carbon from the carburizing compound soaks or diffuses into the surface of the hot- steel. After carburizing, steel is reheated to a temperature just above its critical point followed by quenching in water, brine or oil. This hardens the skin and at the same time refines the core. This steel is also given a second heat treatment at a lower temperature range i.e. 750-770°C, in order to improve the ductility and impact resistance of the core and case.

ii) Liquid Carburizing:

The articles to be carburized are heated in a container filled with a molten salt, such as sodium carbonate. If only selected portions of the components are to be
carburized, then the remaining portions are covered by copper plating.

Advantages:

(a) Very little deformation of article.
(b) Time saving process.
(c) Greater depth of penetration possible.
(d) Selective carburizing is possible if needed.
(e) Ease of carburizing a wider range of products
(f) Uniform heating
(g) Parts leave the bath with a clean and bright finish. There is no scale as there in pack hardening.

iii) Gas Carburizing:
In gas carburizing, the article is heated and surrounded by a hydrocarbon gas (such as methane, ethane, carbon monoxide, etc) in the furnace.

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