.state the process of case hardening by gas nitriding
. state the process of case hardening by nitriding in a salt bath
In the nitriding process, the surface is enriched not with carbon, but with nitrogen. There are two systems in common use, gas nitriding and salt bath nitriding.
Gas nitriding
The gas nitriding process consists of heating the parts at 500°c in a constant circulation of ammonia gas for up to 100 hours.
During the gas nitriding process, the parts are in an externally heated gas tight box, fitted with inlet and outlet bores for the ammonia gas which supplies the nitrogen. At the completion of the soaking the ammonia is still circulated untill the temparature of the steel has fallen to about 150°c, when the box is opened, and the cooling completed in air. Nitriding causes a film to be produced on the surface but this can be removed by a light buffing.
Nitriding in salt bath
Special nitriding baths are used for salt bath nitriding. This process is suitable for all alloyed and unalloyed types of steel, annealed or not annealed, and also for cast iron.
Process
The completely stress relieved work pieces are pre heated (about 400°c) before being put in the salt bath (about 520 to 570°c). A layer 0.01 to 0.02 mm thick is formed on the surface which consists of a carbon and nitrogen compound. The duration of nitriding depends on the cross section of the workpiece (half an hour to three hours). (It is much shorter than gas nitriding ). After being taken out of the bath, the workpieces are quenched and washed in water and dried.
Avavtages
The parts can be final machined before nitriding because no quenching is done after nitriding, and, therefore, they will not suffer from quenching distortion.
In this process, the parts are not heated above the critical temparature, and, hence warping or distortion does not occur.
The hardness and wear resistance are exceptional. There is a slight improvement in corrosion resistance as well.
Since the alloy steel used are inherently strong when properly heat treated, remarkable combinations of strength and wear resistance are obtained.👍👍
FLAME HARDENING
In this type of hardening, the heat is applied to the surface of the workpiece by specially constructed burners. The heat is applied yo the surface very rapidly and the work is quenched immediately by spraying it with water. The hardening temparature is generally about 50°c higher than that for full hardening.
The workpiece is maintained at the hardening temparature for a very short period only, so that the heat is not conducted more than necessary into the workpiece
Steels used for surface hardening will have a carbon content of 0.35% to 0.7%
The following are the advantages of this type of hardening.
-The hardening devices are brought to the workpiece.
-it is advantageous for large workpiece
-short hardening time
-great depth of hardening
-easily controlled
-small distortion
-low fuel consumption
The following are the disadvantages
-not suitable for small workpieces because of the danger of hardening through
-the workpieces must be stress relieved before hardening
INDUCTION HARDENING
This is a production method of surface hardening in which the part to be surface hardened is placed within an inductor coil through which a high frequency current is passed. The depth of penetration of the heating becomes less, as the frequency increases.. The depth of hardening for high frequency current is 0.7 to 1.0mm. The depth of hardening for medium frequency current is 1.5to 2.0mm. Special steels and unalloyed steels with a carbon content of 0.35 to 0.7% are used.
After induction hardening of the workpieces, stress-reliving is necessary
The following are the advantages of this type of hardening
-the depth of hardening, distortion in width and the temparature are easily controllable
-the time required and distortion due to hardening are very small
-the surface remains free from scale
-this type of hardening can easily be incorporated in mass production
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