Mee1005 Materials Engineering and Technology

MEE1005 Materials Engineering and
Technology
Dr. Ashish Kumar Saxena, Assistant Professor, CIMR
ashishkumar@vit.ac.in
L3.4 Surface treatment

Dr. Ashish Kumar Saxena
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Surface Hardening Process
Surface Hardening
 Many engineering must be very hard to resist surface indentation or wear and yet posses adequate toughness to resist impact damage Surface Hardening is a process by which a steel is given a hard, wear resistant surface, while retaining a ductile but tougher interior Surface hardening is usually done for the following reasons To improve wear resistance To improve resistance to high contact stresses To improve fracture toughness and impact strength To improve fatigue resistance To improve corrosion resistance

Dr. Ashish Kumar Saxena
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Components that usually require surface hardening include:
Gears Bearings
Valves
Cams shafts Hand tools
Rolls
Shafts Machine tools
Bearing races
Surface Hardening Process

Dr. Ashish Kumar Saxena
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Surface hardening methods include:
Surface hardening techniques can be classified into two major categories. Processes that change the surface chemical composition (casehardening or thermochemical processes. Processes that do not change the surface chemical composition (selective surface hardening or local thermal surface hardening
Surface Hardening Process Casehardening or thermochemical processes
1. Carburising
2. Nitriding
3. Carbo-nitriding
4. Cyaniding
• Selective surface hardening or local thermal surface hardening
1. Induction hardening
2. Flame hardening
3. Laser hardening
4. Electron beam hardening

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Carburising is a hardening process in which carbon is introduced into the surface layer of the steel. The steel is heated in contact with a substance that has a high carbon content. The steel is held at a temperature above the UCT (850
–950 C) fora suitable period of time. Then quenched rapidly to produce a hardened surface layer or case over a softer and tougher core. The steel is then tempered to the desired hardness
Case Hardening Carburising

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Carburising is done on low C steel (< 0.25 %)
• The carburising time varies between 4 – 70 hours
• The length to time the steel is left in the furnace determines the depth of
carburising
• Case depths ranging from 0.08 mm - 6.4 mm maybe specified, depending on the service requirements of the product The carburising process does not harden the steel, it only increases the carbon content to a desired depth below the surface
Case Hardening Carburising
Carburising Processes
Pack Carburising
Gas Carburising
Liquid Carburising

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Pack Carburising
• In pack carburising, the steel piece is packed in a steel container and completely surrounded with charcoal
• The Charcoal is treated with BaCO3, which promotes the formation of COCO reacts with low carbon steel surface to form atomic C, which diffuses into the steel
• Quenching is difficult in pack carburising, usually the part is allowed to cool slowly and then hardened and tempered.
Carburising time 8 to hours
Carburising depth no limits (<1.3mm)

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Gas Carburising
Liquid Carburising
• Caburising is done with carbonaceous gases, such as methane, ethane, natural gas or propane at around C The advantage of gas carburising is that the steel can be quenched directly from the carburising temperature
• Caburising is done with liquid salts, which contain cyanide compound such as NaCN
• Shorter carburising time compared to pack and gas carburising
• Environmental hazards of the salts are used

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• In this process, nitrogen is diffused onto the surface of the steel being treated The reaction of nitrogen with the steel causes the formation of very hard iron and alloy nitrogen compounds The resulting nitride case is harder than tool steels or carburized steels The advantage of this process is that hardness is achieved without the oil, water or air quench As an added advantage, hardening is accomplished in a nitrogen atmosphere that prevents scaling and discoloration Nitriding temperature is below the lower critical temperature of the steel and it is set between 500 – 600 C. The nitrogen source is usually Ammonia (NH At the nitriding temperature, the ammonia dissociates into Nitrogen and Hydrogen.
Nitriding

Any Question ? Dr. Ashish Kumar Saxena
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