When hard steel is heated to bright rednce reduce and then allowed to cool slowly, Its hurdness harthess decreases and softwere increases ,The process is called

Options

A. case hundening
B. tempering
C. quenching
D. nitriding

Answer

d

Explanation

When steel is heated in the atmosphere of dry `NH_(3)` at `500 - 600^(@)C` for about `3` to `4`days a hard coating of iron nitride is produced at the surface .The process ,therefoore called natriding.
Case hardeving is the process of producing a this coating of hardent steel on the surface of the mild steel .It is done by heating by the mild steel with charcoal and then plunging into oil .This produces thim coating of hardened steel on the surface .Such a steel becomes resistence to wear and tear .
Hard steel can be fi=urtgher hardened by heating steel to red heat (bright redness`850^(@)C)` fololowed by cooling it suddenly by pluining cold water or oil this process , known as quenching , maj=kes the steel extremily hard but brittle .It has very low clasticity.
The process can be explained as follows : above `850^(@)` structure is face -centered cubic and the carbon is present in solid solution. Rapid cooling does not allow sufficient time for the carbon to separate as comentite. `Fe_(3)C` , and instead carbon is deposiled the preselce of C otoms makes it impossible for the iron to adopt are true body centered structureand a distorped modification which is hard but brittle is the result .
Annealing is a process of heating steel to bright to redness `(850^(@)C)` and then cooling it very slowly .This treament makes the steel soft and ductile and thus this type of steel is uused in fabrication process oif the steel is heated to a temperature between `230^(@)C and 300^(@)C ` and alowed to cool slowly , it retains most of its jhardness but the brittleness disappears .the process known as tempereing . Thus , temperitring is a process of heating the steel to a temperature below redness and then cooling it slowly .During temperig the surface of the steel takes on cetrains colors , being yellow at `239^(@)C` and blue at `300^(@)C` at in between the colour is brown .These colors are due to the formation of a very thing oxide film on the surface of the steel , the thickness of the film determinn the final color . Watch springs are blue , because temperature at `300^(@)C ` removes all brittlness and producess extremely taugh and spring steel .
During 'tempering' microscotivcally small area of cementile are formed and this help to relivev the strain present in quenchal steel without algnificantly allecting the hardness of the steel
Finaly we can conclude that the hardiness and elasticity of the steel can be changed by being the steel at sdifferent temperature and then allowing to cool it in different ways methods `(5),(3) and (2)` are then to steel is provide heat breatments the surface treatment to the steel.