Pearlite Martensite Bainite

Transformation of Austenite

γ-Fe

PearliteAustenite

MartensiteAustenite

Furnace

Water Bucket

R. Martens

Martensite

Martensite

Hardness of Martensite

BainiteAustenite

Feathery appearance

Fe3CFerrite

T-T-T Diagram

T-T-T Diagram

Pearlite

Bainite

Martensite

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Mechanical Property

Case Hardening:Gears

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Spur GearsBevel Gears

Worm Gears

Helical Gears

Sproket

Fatigue Failure of Gears Tooth failed!

Scars on worm wheel surface!

Internal surface cracked!

Case Hardening of Gears

Carburizing 

Nitriding 

Carbo‐nitriding

Carburizing

Carburizing is a process of adding Carbon to the surface. This is done by exposing the part to a Carbon rich atmosphere at an elevated temperature and allows diffusion to transfer the Carbon atoms into steel.

Carburizing Process

Heat the gears to 850 ‐9500 C

Burn some carbonaceous gas(CH4 ,  C2H6 C3H8  in O2 )

2CO ↔ CO2 + C

CH4 + 3/2O2 ↔ CO +2H2O

↔ Diffuse ↔ Fe3C

•Gas carburizing

•Plasma carburizing

•Salt bath carburizing

•Pack carburizing

Carburizing

Carburizing: Case Depth

NitridingNitriding is a surface-hardening heat treatment that introduces nitrogen into the surface of steel at a temperature range (500 to 550°C), while it is in the ferrite condition. Thus, nitriding is similar to carburizing in that surface composition is altered, but different in that nitrogen is added into ferrite instead of austenite

Plasma Nitriding

Carbo‐nitriding ProcessCarbonitriding is a modified form of gas carburizing, rather than a form of nitriding. The modification consists of introducing ammonia into the gas carburizing atmosphere to add nitrogen to the carburized case as it is being produced.

Induction Hardening

Precipitation HardeningWidely used in aluminum‐copper alloys

Courtesy: Boeing

Precipitation Hardening Process

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Precipitation Hardening Process

Precipitation Hardening Microstructure

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