lesson13-ch6-HeatTreatment_v1-讲义讲义.pptVIP

Today’s lecture: Heat Treatment of Metals Ageing or Precipitation Hardening Fine particles can also be formed that block dislocations from moving Involves Solution treatment to temperature above solvus line Quenching to room temperature to create supersaturated solution Heating to precipitation temperature (aging) for some time Ageing or precipitation treatment Surface Treatment Two major types of surface treatment are applied to steels: coatings and surface hardening. Coatings with elements such as tin, zinc, and aluminum produce intermetallic phases that are adherent and may also offer corrosion resistance. Tin cans are tin-coated steel in which the tin allows soldering. Galvanized guardrails are zinc-coated steel. Coatings of metals and oxides that are deposited in molten form (plasma-sprayed coatings) are used in high-temperature environments such as turbine engines. Surface hardening to improve wear or retain sharp edges can be accomplished in several ways: Surface Treatment rapid heating of just the surface followed by quenching as the heat is conducted away into the bulk can produce a thin layer of very hard martensite which would be brittle if it was not supported by a tougher structure beneath. This heating may be done by flame, induction, or laser. diffusion of carbon or nitrogen into the surface which alters the chemical composition and structure. Surface Hardening Thermochemical treatments to harden surface of part (carbon, nitrogen) Also called case hardening May or may not require quenching Interior remains tough and strong Carburizing of a low-carbon steel to produce a high-carbon, wear-resistant surface. Carburizing Low-carbon steel is heated in a carbon-rich environment Pack carburizing - packing parts in charcoal or coke - makes thick layer (0.025 - 0.150 in) Gas carburizing - use of propane or other gas in a closed furnace - makes thin layer (0.005 - 0.030 in) Liquid carburizing - molten salt bath containing sodium cyanide, barium chloride - th