Q5: Surface heat treatment

Induction heating - incorrect, as the steel only has a carbon content of 0.2 wt% the maximum hardness achievable is <500 HV. Try again.

Gaseous carburising at 950°C for ˜ 4 hours - correct, (note that the high temperature required may result in some degree of distortion on quenching)

Gaseous carburising at 950°C for ˜ 16 hours - incorrect, this gives a case depth of 1.2 mm. Try again.

Gaseous nitriding at 525°C for 32 hours - incorrect, whilst this does give a case depth of 0.6 mm and a surface hardness of >550 HV the use of nitriding to surface harden gears is not common as carburising is the preferred process for bulk production, note that the production time would be very long and therefore relatively uneconomic. Try again.

d) and e) correct - Correct.

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NOTEPAD

Three most important properties for an engineering steel for a gear in an automotive gear box: Surface Hardness, Wear Resistance and Fatigue Resistance

Technique used to determine the hardenability of a steel: Jominy End Quench

Correct statement (s) regarding the measure of hardness and hardenability of a steel:
Increasing the Mn content increases the hardenability of the steel but not the maximum hardness achieved.
+ Controlled additions of alloying elements such as Cr, Ni, B and Mo can be used to increase the hardenability of a steel such that martensite can be obtained on air cooling of a 10mm bar.

Correct statement (s) regarding surface hardness techniques:
Nitriding grades of steel must contain strong nitride forming elements such as Cr, Al and / or V.
+ Induction hardening is a versatile technique allowing a range of compositions to be hardened to a range of case depths with little distortion produced.

Heat treatment procedure to obtain >500HV and case depth of 0.6 mm for a 0.2 % C, 1.1% Mn, 1.1% Cr steel: Gaseous carburising at 950°C for 4 hours
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