材料科学与工程基础5(上课.pptx

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1、Chapter 5 Engineering Alloys5.1 IntroductionMetals and alloys are the most important engineering materials in industry,which account for about 90%of the worlds production of metals.Because of:Good strength,Toughness,Ductility,Low cost Major types:Ferrous alloy（黑色合金）（黑色合金）:based on ironNonferrous all

2、oy（有色合金）（有色合金）:based on the other metals5.2 Production of Iron and Steel5.2.1 Production of Pig Iron(生铁生铁)Most pig iron is extracted from iron ores in a large blast furnace(高炉高炉),and then is usually transferred to steel-making furnace in the liquid state.5.2.2 Typical reaction:Fe2 O3(iron ores 铁矿石铁矿

3、石)+3CO(coke 焦炭焦炭)2Fe +3CO2Production of Iron and SteelProperties of pure iron:low strength and ductilityTensile strength:180 230 MN/m2Yield strength at 0.2%offset:100-170 MN/m2Hardness(Hb):50 80 kgf/mm2Percent elongation at fracture:30-50%Therefore,it is rare to use pure iron in industry.5.3 Steel m

4、aking and processing of major steel products The majority of steels contain less than 0.5%of carbon.Most commonly process for converting pig iron into steel is basic process(氧气吹顶法氧气吹顶法).Pig iron and 30%steel crap is fed into refractory furnace to which oxygen lane is inserted.Oxygen reacts with liqu

5、id bath to form iron oxide.FeO+C Fe+CO Slag forming fluxes are added.Carbon content and other impurities are lowered.Molten steel is continuously cast and formed into shapes.5.4 The iron iron carbide phase diagram (铁碳合金相图铁碳合金相图)The components for iron iron carbide phase diagram (Fig.9.6)The componen

6、ts for iron iron carbide phase diagram (Fig.9.6)*Pure Fe *Fe3 C(渗碳体渗碳体):intermetallic compound,up to 6.67 wt%c in the systemBasic properties for Fe3 C:High hardness with brittle featureTensile strength:30 MN/m2Hardness(Hb):800 kgf/mm2Percent elongation at fracture:0 Solid phases in the phase diagram

7、:*-ferrite:高温铁素体高温铁素体 BCC structure An interstitial solid solution of carbon in iron.It has a greater lattice constant.*-ferrite:铁素体铁素体 BCC structureAn interstitial solid solution of carbon in the BCC iron crystal lattice.Carbon is only slightly soluble in ferrite *-austenite:奥氏体奥氏体 FCC structure An

8、 interstitial solid solution of carbon in iron.*cementite(Fe3 C):渗碳体渗碳体 +Fe3 C:mixture of eutectic reaction ledeburite莱氏体莱氏体 Liquid(4.3%C)(2.08%C)+Fe3 C(6.67%C)*+Fe3 C:mixture of eutectoid reaction,Pearlite 珠光体珠光体 (0.8%C)(0.02%C)+Fe3 C(6.67%C)Point T C C wt%MeaningA 1538 0 melting T of pure FeB 1495

9、 0.53 Lb for peritectic reactionC 1148 4.30 eutectic point(共晶点共晶点)D 1227 6.67 melting T of Fe3 C E 1148 2.08 max.C in -FeG 912 0 -Fe -FeH 1495 0.09 max.C in -Fe J 1495 0.17 peritectic point(包晶点包晶点)N 1394 0 -Fe -FeP 723 0.02 max.C in -Fe S 723 0.8 eutectoid point(共析点共析点)Q 650 0.005 max.C in -Fe Point

10、s in the phase diagram:Invariant reactions Peritectic reaction:Liquid(0.53%C)+(0.09%C)(0.17%C)Eutectic reaction:Liquid(4.3%C)austenite(2.08%C)+Fe3C(6.67%C)Eutectoid reaction:Austenite(0.8%C)Ferrite(0.02%C)+Fe3C(6.67%C)14950C11480C7230C0.8%CEutectoid SteelHypoeutectoidSteelHypereutectoidSteelLess tha

11、n 0.8%More than 0.8%5.5 The cooling process of typical composition in iron-iron carbide phase diagram1)Eutectoid plain-carbon steel(共析普通碳素钢共析普通碳素钢):0.8%C,cooled from 750 C Fig.9.7 on page 375 Final phases:Pearlite2)Hypoeutectoid plain-carbon steel(亚共析普通碳素钢亚共析普通碳素钢):0.4%C,cooled from 900 C Fig.9.9 on

12、 page 377 Final phases:+pearlite3)Hypereutectoid plain-carbon steel(过共析普通碳过共析普通碳素钢素钢)1.2%C,cooled from 950 C Fig.9.11 on page 379 Final phases:Fe3 C+pearliteExample 1 A 0.80%C eutectoid plain-carbon steel is slowly cooled from 750to a temperature just slighly below 723.Assuming that the austenite is

13、 completely transformed to and cementite:(1)Calculate the weight percent eutectoid ferrite formed.(2)Calculate the weight percent eutectoid cementite formed.Example 2A 0.4%C hypoeutectoid plain-carbon steel is slowly cooled from 940to a temperature just slightly below 723(1)Calculate the weight perc

14、ent proeutectoid ferrite present in the steel(2)Calculate the weight percent eutectoid ferrite and weight percent eutectoid cementite present in the steelExample 3A hypoeutectoid plain-carbon steel was slow-cooled from the austenitic region to room temperature contains 9.1%eutectoid ferrite.Assuming

15、 no change in structure on cooling from just below the eutectoid temperature to room temperature,What is the carbon content of the steel?ProblemA hypereutectoid plain-carbon steel contains 10.7wt%eutectoid Fe3C.What is its average carbon content in weight percent?Fig.EP 9.3 P381Slow Cooling of Plain

16、 Carbon Steel Eutectoid plain carbon steel:If a sample is heated up to 7500C and held for sufficient time,structure will become homogeneous austenite.Below eutectoid temperature,layers of ferrite and cementite are formed.Pearlite.Slow Cooling of Plain Carbon Steel(Cont.)Hypoeutectoid plain carbon st

17、eel:If a sample of 0.4%C is heated up to 9000C,it gets austenitized.Further cooling gives rise to and pearlite.Pearlite Slow Cooling of Plain Carbon Steel(Cont.)Hypereutectoid plain carbon steel:If a 1.2%C sample is heated up to 9500C and held for sufficient time,it entirely gets austenitized.Furthe

18、r cooling results results in eutectoid cementite and pearlite.Heat treatment is the way to improve the components and/or structures of metallic materials.*Annealing(退火）退火）*Normalizing(正火正火)*Quenching(淬火淬火)*Tempering(回火）回火）5.6 Heat treatment of metallic materialsAnnealing(退火退火):Definition:金属加热到适当金属加热

19、到适当温度保温一段时间，以适温度保温一段时间，以适当速度冷却到室温的热处当速度冷却到室温的热处理过程。理过程。full annealing(完全退火完全退火)process annealing:stress relief (低温退火低温退火,去应力退火去应力退火 )1)full annealing(完全退火完全退火)full annealing temperature for hypereutectoid steel:above Accm(+40)temperature Structure:pearlite(珠光体）珠光体）+cementite （渗碳体）（渗碳体）for hypoeutect

20、oid steel:above Acs temperature Structure:pearlite(珠光体）珠光体）+ferrite（铁素体）（铁素体）purpose:increase of ductility,decrease of hardness and strength2)process annealing:stress relief (低温退火低温退火,去应力退火去应力退火)temperature below the eutectoid temperature.Which is often referred to as a stress relief,partially softe

21、ns cold-worked low-carbon steels by relieving internal stresses from cold working.For hypoeutectoid steels with less than 0.3%C below eutectoid temperature,550-650*Normalizing(正火正火)The steel is heated in the austenitic region and then cooled in still air.ACs(orAccm)+3050 The purpose of normalizing:(

22、1)To refine the grain structure(2)To increase the strength of the steel(3)To reduce compositional segregation in castings or forgings and thus provide a more uniform structure*Quenching(淬火淬火)1.Definition and Destination of quenching Definition:A sample of a heated metallic materials is rapidly coole

23、d to a settled temperature in water or oil.(加热的钢或合金，在一设定温度保温一段时间，以获得加热的钢或合金，在一设定温度保温一段时间，以获得不同要求的高温相，然后快速冷却以获得远离平衡相组不同要求的高温相，然后快速冷却以获得远离平衡相组织的过程。）织的过程。）For example:plain-carbon steel in the austenitic condition be quenched in waterthe forming of martensite(马氏体马氏体):A interstitial solid solution(间隙固溶体

24、间隙固溶体)of carbon in BCC iron.Metastable phase consisting of supersaturated solid solution of C in BCC or BCT iron.Caused by rapid cooling of austenitic steel into room temperature(quenching).Ms temperature of martensite start.Mf temperature of martensite finish.Microstructure of Fe C Martensites Lath

25、 martensite（板条状马氏体）（板条状马氏体）:Less than 0.6%C and consists of domains of lathe of different orientation.Plate martensite（片状马氏体）（片状马氏体）:More than 1.0%C and have fine structure of parallel twins.Lath typePlate typeLath Martensite 板条状板条状Plate Martensite 片状片状(或针状或针状)Martensite(Cont.)Transfer to martensite

26、 is diffusionless.No change of relative position of carbon atoms after transformation.Strength and hardness increases with carbon content.Strength is due to high dislocation concentration and interstitial solid solution strengthening.FCCBCCBCT2.Mechanism of quenching(Three steps)A:Surrounded by vapo

27、r(形成蒸汽膜包围形成蒸汽膜包围)B:boiling to form core(沸腾成核沸腾成核)C:liquid cooling(液体冷却阶段液体冷却阶段)3.Quenching medium of quenching(淬火介质淬火介质)(1)Water (2)Salt solution:NaCl CaCl2 (3)Basic water(碱水碱水):):5%-10%NaOH (4)Oil（常规，快速，等温）（常规，快速，等温）(5)Polymeric solution(高聚物溶液高聚物溶液)PVA（聚乙烯醇），（聚乙烯醇），PVP（聚乙烯吡咯烷酮，（聚乙烯吡咯烷酮，PAG(环氧乙烷环氧乙烷

28、和环氧丙烷的共聚物和环氧丙烷的共聚物)4.Methods of quenching (1)single stage quenching 单液淬火单液淬火(2)double quenching 双液淬火双液淬火(Ms-oil)(3)interrupted quenching 分级淬火分级淬火(4)isothermal quenching 等温淬火等温淬火(5)localized quench hardening局部淬火局部淬火Martempering(marquenching)（马氏体（马氏体等温淬火）等温淬火）Is a modified quenching procedure used for

29、 steels to minimize distortion and cracking that may develop during uneven cooling of heat-treated material.Process of Martempering:(1)Austenitizing the steel(2)Quenching it in hot oil or molten salt at temperature just slightly below the Ms temperature(3)Holding the steel in quenching medium until

30、the temperature is uniform throughout and stopping this isothermal treatment before the austenite-to bainite（贝氏体）（贝氏体）transformation begin(4)Cooling at moderate rate to room temperaturePearlite珠光体珠光体lower Bainite下贝氏体下贝氏体upper Bainite上贝氏体上贝氏体Advantages of MartemperingThe martempered steel has high im

31、pact energy values.Austempering（奥氏体等温淬火）（奥氏体等温淬火）Is an isothermal heat treatment which produces a bainite（贝氏体）（贝氏体）structure in some plain-carbon steels.Increasing the toughness and ductility.Process of Austempering:(1)Austenitizing the steel (2)Quenched in a molten salt bath (above the Ms temperatu

32、re)(3)held isothermally(austenite-to bainite)(4)Cool to room temperature in airAdvantages and disadvanges of Austempering Advantages(1)Improve ductility and impact resistance(2)Decrease distortion of quenched materialsDisadvantages(1)The need for special molten salt bath(2)The process can be used fo

33、r only a limited number of steels.Isothermal decomposition of Austenite.Several samples are first austenitized above eutectoid temperature and rapidly cooled in salt bath at a desired temperature,after various time intervals,and then quenched in water at room tremperature.Repeat procedureat progress

34、ivelowertemperaturesIT Diagrams for Noneutectoid Steels S curves of IT diagrams of noneutectoid steel is shifted to left.Not possible to quench from austenitic region to produce entirely martensite.Additional transformation line indicates start and formation of proeutectoid ferrite.Continuous Coolin

35、g-Transformation Diagram In continuous cooling transformation from martensite to pearlite takes place at a range of temperature.Start and finish lines shifted to longer time.No transformation below 4500C.*Tempering(回火回火):heating a steel at a temperature below the eutectoid point temperature（AC1）for

36、a certain time and then lower it to room temperature to make it softer and more ductile.the effect of tempering temperature on the shape of the carbide (200-700 cementitie)200-300 rodlike 400-700 spherical-like the effect of tempering temperature on the hardness of plain-carbon steel Fig.9.32 Effect

37、s of Tempering Hardness decreases as temperature increases above 2000C This is due to diffusion of carbon atoms from interstitial sites to iron carbide precipitates.5.7 The classification of iron iron carbide alloy materials*Major types on the basis of C concentration in metals:Iron:C 2.08 wt%Steel:

38、C 2.08 wt%*Major types of steel:1)On C contents:low carbon steel:C 0.25 wt%medium carbon steel:0.25 wt%0.6 wt%2)On quality:plain-carbon steel:S 0.055 wt%(普通碳素钢普通碳素钢)P 0.045 wt%high quality steel:S 0.04 wt%(优质碳素钢优质碳素钢)P 0.04wt%super high quality steel:S 0.03 wt%(高级优质碳素钢高级优质碳素钢)P 0.035 wt%In addition

39、to C,steel also contains S,P,Si,Mn trace elements.S,P:disadvantages in steel Si,Mn in steel will improve its strength and hardness )3)On application:Carbon structure steel(碳素结构钢碳素结构钢)e.g.45Mn:C=0.45 wt%,Mn wt 0.8%Carbon tool steel (碳素工具钢碳素工具钢)e.g.T8:C=0.8 wt%碳素工具钢碳素工具钢 T8A:C=0.8 wt%高级优质碳素工具钢高级优质碳素工具

40、钢 4)On steel-making method:open-hearth furnace steel(平炉钢平炉钢)basic oxygen furnace steel(转炉钢转炉钢)5)Classification of plain-carbon steels in USA by a four digit AISI-SAE code AISI:American Iron and Steel Institute SAE:Society for Automotive EngineersFor example:1030:a plain-carbon steel with 0.3 wt%of C

41、Medium carbon steels:1020 1040High carbon steels:1060 1095 Plain carbon steelsTypical mechanical properties and applications of various kinds of plain carbon steels.The limitations of plain carbon steels in engineering applications:1.cant strengthened beyond about 690 MPa 2.They are not deep-hardena

42、ble 3.has low corrosion and oxidation resistance 4.Rapid quenching can obtain a fully martensitic structure,however,it leads to possible distortion and crack 5.Has poor impact resisitance at low temperature6)Classification of alloy steels in USA Low alloy steels:containing 1 to 4%of alloying element

43、s Alloy steels:may contain up to 50%of alloying elements.5.7.1 Low Alloy Steels and Alloy SteelsManganese steels:锰钢锰钢Chromium steels:铬钢铬钢Chromium molybdenum steels:铬钼钢铬钼钢Nickel molybdenum steels:镍钼钢镍钼钢Nickel chromium-molybdenum steels:镍铬钼钢镍铬钼钢 Ni:1.83 wt%,Ni:0.55 wt%Table 9.4 on page 394 Table 9.6 o

44、n page 402 Classification of Alloy Steels First two digits:Principle alloying element.Last two digits:%of carbon.Distribution of Alloying Elements Distribution depends upon compound and carbide forming tendency of each element.Effects of Alloying Element on Eutectoid Temperature Mn and Ni lower eute

45、ctoid temperature.They act as austenite stabilizing element.Tungsten,molybdenum and titanium raise eutectic temperature.They are called ferrite stabilizing elements.Mechanical Properties of Low Alloy Steels5.7.2 Cast iron 铸铁铸铁Normally contain 2 to 4%carbon and 1 to 3%silicon.Other alloying elements

46、may also be present to control or vary certain properties.Classifications:White cast iron 白口铁白口铁 Gray cast iron 灰口铸铁灰口铸铁Malleable cast iron 可锻铸铁可锻铸铁Ductile(nodular)cast iron 球墨铸铁球墨铸铁 White Cast iron Fractured surface appears white and crystalline.Low carbon(2.5 3%)and silicon(0.5 1.5%)content.Much o

47、f carbon forms iron carbide instead of graphite up on solidification.Excellent wear resistance.Iron CarbidePearliteGray Cast Iron Fractured surface appears gray.2.5 4%C and 1 3%Si(Promotes formation of graphite).Carbon exceeds the amount that can dissolve in austenite and precipitate as graphite fla

48、kes.Excellent machinability,hardness and wear resistance,and vibration damping capacity.GraphiteFlakesMalleable(可鍛）可鍛）Cast Iron 2-2.6%C and 1.1 1.6%Si.White cast iron is heated in malleablizing furnace to dislocate carbide into graphite.Irregular nodules of graphite are formed.Good castability,machi

49、nability,moderate strength,toughness and uniformity.Heat Treatment Heat treatment of white irons to produce malleable irons are Graphitization:Castings heated above the eutectoid temperature(9400C)and held for 3 to 20h depending on the composition and structure.white iron graphite and austenite.Cool

50、ing:Ferritic malleable iron:Fast cooled from 740-7600C and then slowly cooled.Pearlitic malleable iron:Slowly cooled up to 8700C and then air cooled.Tempered martensitic malleable iron:Casting cooled in furnace to a quenching temperature and homogenized and then quenched in agitated oil.Ductile（球磨）（