Heat Treatment of Steels PDF

25/1/2021 Topic 6 Heat Treatment of Steels IC N H C TE...

25/1/2021 Topic 6 Heat Treatment of Steels IC N H C TE LY PO E R Learning Objectives O AP Students should be able understand: general principles of heat treatment G Time-Temperature-Transformation diagrams hardenability of steels N martempering and austempering processes the problems encountered in heat treatment SI carburising, nitriding & carbonitriding flame & induction hardening 1  25/1/2021 Classification of Processes IC Surface Heat Treatment N Diffusion Treatments Selective Hardening H Alters surface chemistry No composition change occurs C by introducing C and/or N TE Carburising Nitriding Carbonitriding Flame Hardening Induction Hardening LY PO E Diffusion Treatments R Principle O Surface hardness is obtained by diffusing an element into AP the steel. To achieve this, the element must be soluble in the steel at the temperature used. G When an element is diffused into the steel surface N  surface volume increases SI  Residual compressive stresses are induced at the surface This imparts fatigue resistance to the steel 12  25/1/2021 Diffusion Treatments Types of Processes IC  Ferritic ( < 600ºC ) N Ferrite dissolves very little C and N Less risk of distortion H C  Austenitic ( > 800ºC ) Austenite dissolves considerable amounts of C and N TE Problem of distortion LY PO E Carburising R O  Carbon is diffused into the steel surface when the steel is it contact with carbonaceous material. AP  Carbonaceous material may be in the form of solid, liquid or gas.  The process is applied to low carbon steels (< 0.2%C) G  Process N  Heat the steel to temperatures above 880ºC (usually > 925ºC) SI  Quench to develop a hard martensitic case (> 0.8%C) 13  25/1/2021 Nitriding Gaseous Nitriding IC  Used for hardening finish-machined and heat-treated steels.  Heat the components N at about 500ºC for 2 ~ 4 days in a H furnace through which ammonia gas C is circulated.  Nitriding reaction TE  2 NH3 2N + 3H2 Atomic nitrogen diffuses into the steel surface forming hard nitrides. No quenching is required. LY PO E Nitriding R Steels Used O  Plain carbon steels are not suitable as iron nitrides formed will diffuse to a considerable depth, and the AP surface hardness will be low.  In addition, a white layer forms. This is brittle, and so is detrimental to mating surfaces. G N SI 15  25/1/2021 Nitriding Steels Used  The process was designed for nitralloys. IC These alloys contain 0.3 ~ 0.5 %C, with Al, Cr, Mo. Some tool steels may also be gas nitrided. N H C TE L PO E Nitriding R Gaseous Nitriding O  Advantages AP  Use of low processing temperatures result in less likelihood of distortion  A very hard case is obtained (> HRc 70)  Nitrided cases give good corrosion resistance G  The process is clean, and simple to operate N  Disadvantages  Service temperatures must not exceed 500ºC, otherwise the surface hardness will be lost SI  Nitriding reduces the toughness  A “white layer” develops if the N potential is too high 16  25/1/2021 Carbonitriding The process is applied to mild steels and low-carbon IC alloy steels. Process N  C and N are simultaneously introduced into the surface  Heat the steel to 820 ~ 840ºC for up to 3 hours H  Quench in oil C TE LY PO E Carbonitriding R Advantages O Nitrogen stabilises the austenite AP  austenite transforms slowly  less risk of distortion G  The relative amounts of C and N can be varied N SI 17  25/1/2021 Selective Hardening Principles IC No composition change occurs. Surface hardness is the same as that for through N hardening (Quench and Temper). H It is produced by rapid application of heat to the steel surface for austenitisation, followed by quenching. C Rapid heating is required so that the core is not hardened. TE The higher the heating intensity, the shallower the case. LY PO E Selective Hardening R Steels Used O Applied to steels with 0.3 ~ 0.6 %C. AP This carbon level is required because of the need to form martensite. G Steels are to be heat-treated for core properties before processing. N SI 18  25/1/2021 Induction Hardening  The selected area to be IC hardened is surrounded by an inductor coil. N  A high frequency current is passed through the coil. H  This induces eddy currents in C the selected area, heating it up. TE  The surface is then spray quenched. LY PO E Induction Hardening R Process Control O AP  The pattern of heating depends on  shape of the coil  number of turns in the coil  operating frequency G  power input  nature of the workpiece N  Case depth depends on current frequency SI  higher frequency = thinner case 20