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1、,Unit 2 Heat Treatment of Metals,What is heat treatment ？ Iron-Carbon Diagram Analysis Heat Treatment Techniques,What is heat treatment ？,The understanding of heat treatment is embraced by the broader study of metallurgy. Metallurgy is the physics, chemistry, and engineering related to metals from o
2、re extraction to the final product. Heat treatment is the operation of heating and cooling a metal in its solid state to change its physical properties. According to the procedure used, steel can be hardened to resist cutting action and abrasion, or it can be softened to permit machining.,Heat Treat
3、ment Application,May remove internal stresses May reduce grain size May increase toughness May produce a hard surface on a ductile interior,The analysis of the steel must be known because small percentages of certain elements, notably carbon, greatly affect the physical properties. Alloy steels owe
4、their properties to the presence of one or more elements other than carbon, namely nickel, chromium , manganese, molybdenum , tungsten, silicon, vanadium , and copper.1 Because of their improved physical properties they are used commercially in many ways not possible with carbon steels.,Iron-Carbon
5、Diagram Analysis,Those portions of the iron-carbon diagram near the delta region and those above 2% carbon content are of little importance to the engineer and are deleted.2 A simplified diagram, such as Fig.2.1, focuses on the eutectoid region and is quite useful in understanding the properties and
6、 processing of steel.,The key transition described in this diagram is the decomposition of single-phase austenite (Y) to the two-phase ferrite plus carbide structure as temperature drops. Control of this reaction, which arises due to the drastically different carbon solubility of austenite and ferri
7、te, enables a wide range of properties to be achieved through heat treatment .,The transition process At the upper temperatures, only austenite is present, the 0.77% carbon being dissolved in solid solution with the iron. When the steel cools to 727 (1341), several changes occur simultaneously . The
8、 iron wants to change from the fcc austenite structure to the bcc ferrite structure, but the ferrite can only contain 0.02% carbon in solid solution. 3,Fig.2.1 Simplified ion-carbon diagram,The rejected carbon forms the carbon-rich cementite intermetallic with composition Fe3C. In essence, the net r
9、eaction at the eutectoid austenite ：0.77%C ferrite：0.02%C cementite：6.67%C,Since this chemical separation of the carbon component occurs entirely in the solid state, the resulting structure is a fine mechanical mixture of ferrite and cementite. Specimens prepared by polishing and etching in a weak s
10、olution of nitric acid and alcohol reveal the lamellar structure of alternating plates that forms on slow cooling.,Heat Treatment Techniques,Hardening Tempering Annealing Normalizing Spheroidizing Surface Hardening,Hardening,Definition： Hardening is the process of heating a piece of steel to a tempe
11、rature within or above its critical range and then cooling it rapidly. Influencing factors: 1.the correct temperature is obtained. 2.the rate of heating is important. 3.The hardness obtained from a given treatment depends on the quenching rate, the carbon content, and work size.,Tempering,Steel that
12、 has been hardened by rapid quenching is brittle and not suitable for most uses. By tempering or drawing, the hardness and brittleness may be reduced to the desired point for service conditions. As these properties are reduced there is also a decrease in tensile strength and an increase in the ducti
13、lity and toughness of the steel.,The operation consists of reheating quench-hardened steel to some temperature below the critical range followed by any rate of cooling. The final structure obtained from tempering fully hardened steel is called tempered martensite. Tempering is possible because of th
14、e instability of the martensite, the principal constituent of hardened steel. Low-temperature draws from 300to 400 (150205), do not cause much decrease in hardness and are used principally to relieve internal strains.,Annealing,The primary purpose of annealing is to soften hard steel so that it may
15、be machined or cold worked. This is usually accomplished by heating the steel too slightly above the critical temperature, holding it there until the temperature of the piece is uniform throughout, and then cooling at a slowly controlled rate so that the temperature of the surface and that of the ce
16、nter of the piece are approximately the same.,Annealing effects,wipes out all trace of previous structure； refines the crystalline structure； softens the metal； relieves internal stresses previously set up in the metal.,The temperature to which given steel should be heated in annealing depends on its composition; for carbon steels it can be obtained readily from the partial iron-iron carbide equilibrium diagram. The heating rate should be consist
