.name four different types of surface hardening process
. the purpose of case hardening
. the purpose of carburising
. the process of gas carburising
Most of the components must have a hard, wear resisting surface supported by a tough, shock resisting core for better service condition and longer life. This combination of different properties can be obtained in single piece of steel by surface hardening.
Types of surface hardening
Case hardening
Nitriding
Flame hardening
Induction hardening
Surface hardening processes can be classified as
-processes in which the whole component is heated, Eg. Case hardening and nitriding
-processes in which only the surface of the component is heated
Eg. Flame hardening and induction hardening
Case hardening
Parts to be hardened by this process are made from a steel with a carbon content of 0.15% so that they will not respond to direct hardening.
The steel is subjected to treatment in which the carbon content of the surface layer is increased to about 0.9%.
When the carburised steel is heated and quenched, only the surface layer will respond, and the core will remain soft and tough as required.
The surface which must remain soft can be insulated against carburising by coating it with a suitable paste or by plating it with copper.
-case hardening takes place in two stages.
-carburising in which the carbon content of the surface is increased.
-heat treatment in which the core is refined and the surface hardened.
Carburising
In this operation, the steel is heated to a suitable temparature in a carbonaceous atmosphere, and kept at that temaprature until the carbon has penetrated to the depth required.
The carbon can be supplied as a soild, liquid or gas.
In all cases, the carbonaceous gases coming from these materials penetrate into the surface of the work piece at a temaprature of 880°- 930°.
Pack carburising
The parts are packed in a suitable metal box in which they are surrounded by the carburising medium.
The lid is fitted to the box and sealed with fireclay and tied with a piece of wire so that no carbon gas can escape and no air can either the box to cause decarburisation.
The carburising medium can be wood, bone, leather or charcoal, but an energiser, such as barium carbonate, is added to speed up the process.
Liquid carburising
Carburising can be done in a heated salt-bath. (Sodium carbonate, sodium cyanide and barium chloride are typical carburising salts). For a constant time and temparature of carburising, the depth of the case depends on the cyanide content.
Salt bath carburising is very rapid, but is not always suitable because it produces an abrupt change in the carbon content from the surface to the core. This produces a tendency for the case to flake.
This is suitable for a thin case, about 0.25mm deep. Its advantage is that heating is rapid and distortion is minimised.
Gas carburising
The work is placed in a gas tight container which can be heated in a suitable furnace, or the furnace itself may be the container.
The carburising gas is admitted to the container, and the exit gas is vented.
The gas such as methane or propane may be fed directly into the container in which the work is placed.
In a continuous gas carburising furnace, the carburising, quenching and Temaparing processes are carried out in sequence in the same closed furnace as they progress on a conveyer from one operation to the next.
Figure shows illustrates the appearance of the structure across its section produced by carburising.
Heat treatment
hAfter the carburising has been done, the case will contain about 0.9% carbon, and the core will still contain about 0.15% carbon. There will be a gradual transition of the carbon content between the case and the core.
Owing to the prolonged heating, the core will be coarse, and in order to produce a reasonable toughness, it must be refined.
To refine the core, the carburised steel is reheated to about 870° c and held at that temaprature long enough to produce a uniformity of structure, and is then cooled rapidly to prevent grain during cooling.
The temparature of this heating is much higher than that suitable for the case, and therefore an extremely brittile martensite will be produced.
The case and the outer layers of the core must now be refined.
The refining is done by reheating the steel to about 760°c, to suit the case, and quenching it.
Temparing
Finally the case is tempered at about 200°c to relieve the quenching stresses.
If the part is not required to resist shock, it is unnecessary to carry out the core refining operations in these conditions, a coarse martensite at the surface may not cause trouble, and so this part may be quenched directly after carburising.
