外文翻译材料的热处理.docx

1、外文翻译材料的热处理外文资料HEAT TREATMENT OF METALSThe understanding of heat treatment is embrace by the broader study of metallurgy .Metallurgy is the physics, chemistry , and engineering related to metals from ore extraction to the final product . Heat treatment is the operation do heating and cooling a metal

2、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 .With the proper heat treatment internal ductile interior . The analysis of the steel must be known because s

3、mall percentages of certain elements,notably carbon , greatly affect the physical properties .Alloy steels owe their properties to the presence of one or more elements other than carbon, namely nickel, chromium , manganese , molybdenum , tungsten ,silicon , vanadium , and copper . Because of their i

4、mproved physical properties they are used commercially in many ways not possible with carbon steels.The following discussion applies principally to the heat treatment of ordinary commercial steel known as plain-carbon steels .With this proves the rate of cooling is the controlling factor, produces t

5、he opposite effect .A SIMPLIFIED IRON-CARBON DAGRAMIf we focus only on the materials normally known as steels, a simplified diagram is often used . 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 delet

6、ed. A simplified diagram, such as the one in Fig . 2.1 focuses on the eutectoid region and is quite useful in understanding the properties and processing of steel.The key transition described in this diagram is the decomposition of single-phase austenite ()to the two-phase ferrite plus carbide struc

7、ture as temperature drop . Control of this reaction ,which arises due to the drastically different carbon solubilities of austenite and ferrite , enables a wide range of properties to be achieved through heat treatment .To begin to understand these processes , consider s steel of the eutectoid compo

8、sition , 0.77% carbon , being slow cooled along line in Fig .2.1 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, several changes occur simultaneously . The iron wants to change from the bcc austen

9、ite structure to the bcc ferrite Structure , but the ferrite san only contain 0.02% carbon in solid solution . The rejected carbon forms the carbon-rich cementite intermetallic with composition.In essence , the net reaction at the eutectoid is: Austenite ferrite +cementiteSince this chemical separat

10、ion of the carbon component occurs entirely in the solid state, the resulting structure is a fine mechanical mixture of ferrite and cementite . Speciments prepared by plolishing and etching in a weak solution lf nitric acid and alcohol reveal the lamellar structure lf alternating plates that forms o

11、n slow cooling . This structure is composed of two distinct phases, but has its own set of characteristic properties and goes by the name pearlite , because of its resemblance to mother-of-pearl at low magnification.Steels having less than the eutectoid amount of carbon(less than 0.77%)are known as

12、hypoeutectoid steels . Consider now the transformation of such a material represented by cooling along line y-y in Fig .2.1.At high temperatures , the material is entrirely austenite, but upon cooling enters a region where the stable phases are ferrite and austenite . Tie-line and lever-law calculat

13、ions show that low-carbon ferrite nucleates and grows, leaving the remaining austenite richer in carbon . At 727C (1341F),the austenite is of eutectoid compositon(0.77%carbon)and further cooling transforms the remaining austenite to pearlite. The resulting structure is a mixture lf primary or proeut

14、ectoid ferrite (ferrite that formed above the eutectoid reaction )and regions of pearlite.Hypereutectoid steels are steels that contain greater than the eutectoid amount of carbon. When such a steel cools, as in z-zof Fig .2.1 the process is similar to the hypoeutectoid case, except that the primary

15、 or proeutectoid phase is now cementite instead lf ferrite . As the carbon-rich phase forms, the remaining austenite decreases in carbon content, reaching the eutectoid composition at 727C(1341F).As before, any remaining austenite transforms to pearlite upon slow cooling through this temperature.It

16、should be remembered that the transitions that have been described by the phase diagrams are for equilibrium conditions , which can be approximated by slow cooling , With slow heating, these transitions occur in the revertse manner . However, when alloys are cooled rapidly ,entirely different result

17、s may be obtained , because sufficient time is not provided for the normal phase reactions to occur, In such cases , the phase diagram is no longer a useful tool for engineering analysis.HARDENINGHardening is the process of heating p piece of steel to a temperature within or above its critical range

18、 and then cooling it rapidly . If the carbon content of the steel is known, the proper temperature to which the steel should be heated may be obtained by reference to the iron-iron carbide phase diagram. However, if the composition of the t steel is unknown, a little preliminary experimentation may

19、be necessary to determine the range. A good procedure to follow is to heat-quench a number lf small specimens lf the steel at various temperatures lf the steel at various temperatures and observe the results, either by hardness testing or by microscopic examination. When then correct temperature is

20、obtained ,there will be marked change in hardness and other properties.In any heat-treating operation the rate of heating is important. Heat flows from the exterior to the interior of steel at a definite rate. If the steel is heated too fast, the outside becomes hotter than the interior and uniform

21、structure cannot be obtained. If a piece is irregular in shape, a slow rate is all the more essential to eliminate warping and cracking. The heavier the section, the longer must be the heating time to achieve uniform results. Even after the correct remperature has been reached, the piece should be h

22、eld at that temperature for a sufficient period of time to permit its thickest section to attain a uniform temperature.The hardness obtained from a given treatment depends on the quenching rate, the carbon content , and the work size, In alloy steels the kind and amount lf alloying element influence

23、s only the harden ability (the ability lf the workpiece to be hardened to depths ) lf the steel and does not affect the hardness except in unhardened or partially hardened steels .Steel with low carbon content will not respond appreciably to hardening treatments. As the carbon content in steel incre

24、ases up to around 0.60%,the possible hardness can be increased only slightly, because steels above the eutectoid point are made up entirely of pearlite and cementite in the annealed state. Pearlite responds best to heat-treating operations; any steel composed mostly of pearlite can be transformed in

25、to a hard steel .As the size of parts to be hardened increases ,the surface hardness decreases somewhat even though all other conditions have remained the same. There is a limit to the rate of heat flow through steel. No matter how cool the same . There is a limit to the rate lf heat flow through st

26、eel. No matter how cool the quenching medium many be ,if the heat inside a large piece cannot escape faster than a certain critical rate, there is a definite limit to the inside hardness. However, brine or water quenching is capable lf rapidly bringing the surface lf the quenched part to it own temp

27、erature and maintaining it at or close to this temperature. Under these circumstances there would always be some finite depth of surface hardening regardless lf size. This is not true in oil quenching , when the surface temperature may be high during the critical stages of quenching.TEMPERINGSteel t

28、hat 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

29、ductility and toughness of the steel . The operation consists lf reheating quench-hardened steel to some temperature below the critical range followed by any rate lf cooling . Although this process softens steel , it differs considerably from annealing in that the process lends itself to close contr

30、ol lf the physical properties and in most cases does not soften the steel to the extent that annealing would. The final structure obtained from tempering a fully hardened steel is called tempered martensite .Tempering is possible because of the instability of the martensite ,the principal constituen

31、t of hardened steel. Low-temperature draws, from 300to 400F(150-205C), do not cause much decrease in hardness and are used principally to relieve internal strains. As the tempering temperatures are increased, the breakdown of the martensite takes place at a faster rate, and at about 600F(315C) the c

32、hange to a structure called tempered martensite is very rapid. The tempering operation may be described as one lf precipitation and agglomeration or coalescence of cementite. A substantial precipitation lf cementite begins at 600F(315C),which produces a decrease in hardness. Increasing the temperatu

33、re causes coalescence lf the carbides with continued decrease in hardness. In the process of tempering, some consideration should be given to time as well as to temperature. Although most of the softening action occurs in the first few minutes after the temperature is reached, there is some additional reduction in hardness if