MEE1005 Materials Engineering and Technology Dr. Ashish Kumar Saxena, Assistant Professor, CIMR ashishkumar@vit.ac.in L3.4 Surface treatment
Dr. Ashish Kumar Saxena2 Surface Hardening ProcessSurface 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 3 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 4 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 hardeningSurface Hardening Process Casehardening or thermochemical processes 1. Carburising2. Nitriding3. Carbo-nitriding4. Cyaniding• Selective surface hardening or local thermal surface hardening 1. Induction hardening2. Flame hardening3. Laser hardening4. Electron beam hardening
Dr. Ashish Kumar Saxena 5 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
Dr. Ashish Kumar Saxena 6 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 ProcessesPack Carburising Gas Carburising Liquid Carburising
Dr. Ashish Kumar Saxena 7 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)
Dr. Ashish Kumar Saxena 8 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
Dr. Ashish Kumar Saxena 9 • 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 10
Share with your friends: |