lesson17ch8_castironv4讲义

1、1Materials for mechanical EngineeringCast iron(2004-11-22)Dr. Liuzhang OuyangSchool of Mechanical EngineeringSouth China university of technology2The iron-carbon phase diagram showing the eutectic and eutectoid reactions.White Ironsiron-carbon phase diagram3Iron-iron carbide phase diagram1538 ATempe

2、rature C14951394 N912 GF1148K727RT QDLBJL+Fe3C L+ EC +Fe3C real lineHP MS +Fe3C real lineFeFe3C 6.69A3Acm0.22 0.772.114.30.090.170.53 +G broken line +G broken line4Cast IronsnProduced from pig ironncreated in blast furnacenhigh carbon content (2-4%)nexcellent casting propertiesnRefined in cupola融铁炉融

3、铁炉npig iron and scrap 废料nlimestone石灰石used to remove impuritiesnholding furnace 保温炉holds melt before castingnCarbon can exist asngraphite (flakes)ncementitenCooling affects structurenslow cooling, graphite appears as flakesnfast cooling forms cementite5一、一、 石墨化石墨化 铸铁中的碳以石墨的形式析出的过铸铁中的碳以石墨的形式析出的过程，程， 称

4、为石墨化。称为石墨化。石墨化程度石墨化程度 ，G数量数量 ，基体中，基体中P数量数量 所以，石墨化程度决定了铸铁的组织：所以，石墨化程度决定了铸铁的组织：1、石墨形态对铸铁性能的影响、石墨形态对铸铁性能的影响石墨相当于钢基体上的裂纹或空洞，减少基体有效截面积，引石墨相当于钢基体上的裂纹或空洞，减少基体有效截面积，引起应力集中。片状石墨引起严重应力集中，团絮状和球状作用起应力集中。片状石墨引起严重应力集中，团絮状和球状作用轻。轻。 From a microstructural view point, cast irons can regarded as steels embedded with

5、a carbon-rich phase, which can be either graphite or cementite (Fe3C). Graphite offers no strength nor toughness and acts like voids in the iron; cementite is very hard and brittle.变质处理后，石墨片细化，割裂作用减轻，强度提高。变质处理后，石墨片细化，割裂作用减轻，强度提高。蠕墨铸铁，端部钝，强度、塑性提高。可锻、球状铸铁中石蠕墨铸铁，端部钝，强度、塑性提高。可锻、球状铸铁中石墨对基体裂作用小，强度、塑性提高墨对基

6、体裂作用小，强度、塑性提高常用铸铁材料常用铸铁材料6For conventional cast irons,the austenite then decomposes into pearlite at the eutectoid temperature.However, in grey cast irons, if the cooling rate through the eutectoid temperature is sufficiently slow, then a completely ferritic matrix is obtained with the excess carbon

7、 being deposited on the already existing graphite.White cast irons are hard and brittle; they cannot easily be machined.conventional cast irons7按石墨形态按石墨形态分类分类 a. 灰口铸铁灰口铸铁片状片状flakeGreyb. 可锻铸铁可锻铸铁 团絮状团絮状flocculeMalleablec. 球墨铸铁球墨铸铁球状球状Spheroidal graphited. 蠕墨铸铁蠕墨铸铁蠕虫状蠕虫状vermiform铸铁的组织和分类铸铁的组织和分类Types

8、of Cast IronnWhiteheart; Blackheart; PearliticAlloy cast irons8Fe-C合金的五大元素：合金的五大元素：C, Si, P, Mn, SC是是G的形成元素，的形成元素，C% ， G化化 ；碳当量：碳当量：CE=C+1/3(Si+P)原因：原因：SiSi能抑制能抑制FeFe3 3C C的形成，促进的形成，促进FeFe3 3C C分解分解 Si: 是强烈促进是强烈促进G化的元素，化的元素，Si% , G化化 。 合金元素合金元素Cr、W、Mo、V 与与C亲合力强，阻碍石墨化亲合力强，阻碍石墨化合金元素合金元素Al、Cu、Ni、Co增加增加

9、Fe自扩散能力，促进石墨化。自扩散能力，促进石墨化。S: 是阻碍是阻碍G化的元素，化的元素，S% , G化化 Mn：Mn 0.2 0.4%时，形成合金渗碳体时，形成合金渗碳体 (Fe,Mn)3C, G化化 。P：对石墨化影响不明显。对石墨化影响不明显。影响石墨化的因素影响石墨化的因素9Cast irons typically contain 2-4 wt% of carbon with a high silicon concentrations and a greater concentration of impurities than steels. The carbon equivalen

10、t (CE) of a cast iron helps to distinguish the grey irons which cool into a microstructure containing graphite and the white irons where the carbon is present mainly as cementite. The carbon equivalent is defined as:A high cooling rate and a low carbon equivalent favours the formation of white cast

11、iron whereas a low cooling rate or a high carbon equivalent promotes grey cast iron.carbon equivalent10V冷冷 ，G化化 ，Fe3C量量 ，G片细小、量少。片细小、量少。 V冷冷 ，G化停止，无石墨形成，化停止，无石墨形成，C Fe3C，即形成白口铸铁。即形成白口铸铁。G化是靠原子扩散进行的，需要温度、时间化是靠原子扩散进行的，需要温度、时间影响石墨化的因素影响石墨化的因素11During solidification, the major proportion of the carbon p

12、recipitates in the form of graphite or cementite.When solidification is just complete, the precipitated phase is embedded in a matrix of austenite which has an equilibrium carbon concentration of about 2 wt%. 1. On further cooling, the carbon concentration of the austenite decreases as more cementit

13、e or graphite precipitates from solid solution. solidification12Grey cast iron, Fe-3.2C-2.5Si wt%, containing graphite flakes in a matrix which is pearlitic. The speckled white regions represent a phosphide eutectic. Etchant: Nital 2%Grey Cast Iron (Flake Graphite)13Grey cast iron, Fe-3.2C-2.5Si wt%

14、, containing graphite flakes in a matrix which is pearlitic. The lamellar structure of the pearlite can be resolved, appearing to consist of alternating layers of cementite and ferrite. The speckled white regions represent a phosphide eutectic. Etchant: Nital 2%Grey Cast Iron (Flake Graphite)14Grey

15、cast irons are softer with a microstructure of graphite in transformed-austenite and cementite matrix. The graphite flakes, which are rosettes玫瑰形饰物in three dimensions, have a low density and hence compensate for the freezing contraction收缩, thus giving good castings free from porosity. graphite flake

16、s are rosettes in three dimensions15The flakes of graphite have good damping characteristics阻尼特性and good machinability because the graphite acts as a chip-breaker and lubricates the cutting tools. In applications involving wear, the graphite is beneficial because it helps retain lubricants. However,

17、 the flakes of graphite also are stress concentrators, leading to poor toughness. The recommended applied tensile stress is therefore only a quarter of its actual ultimate tensile strength. characteristics of flakes of graphite161、灰口铸铁的组织、灰口铸铁的组织 灰口铸铁的组织是由钢的基体组织基体组织与片状石墨片状石墨组成。钢的基体钢的基体：铁素体、铁素体珠光体、珠光

18、体，三种灰口铸铁：F灰口铸铁、F+P灰口铸铁、P灰口铸铁（如图所示）。其中珠光体灰口铸铁的强度最高，应用最广泛。 铁素体基体组织铁素体基体组织 铁素体基体铁素体基体+片状石墨片状石墨 X100铁素体基体铁素体基体+片状石墨片状石墨 X500 灰口铸铁灰口铸铁17Gray Cast IronnCooled slowlynAlloy of carbon, iron and siliconnSeven grades availablenAdvantagesnexcellent casting propertiesngood wear resistancengood machinabilityncan

19、 be surface hardenedgraphite flakes embedded in ferrite matrix, cutting the continuity of the matrix and rendering the metal high brittleness.GraphiteFlakes18珠光体基体组织珠光体基体组织 珠光体基体珠光体基体+片状石墨片状石墨 X100 珠光体基体珠光体基体+片状石墨片状石墨 X500铁素体铁素体+珠光体基体组织珠光体基体组织 铁素体和珠光体基体铁素体和珠光体基体+片状石墨片状石墨 X100X50019如如HT100, 表示表示30mm试

20、样的最小抗拉强度值试样的最小抗拉强度值 100MN/m2牌号牌号显微组织显微组织性能性能基体基体石墨石墨HT100F+P粗片状粗片状铸造性能好，不用人工时铸造性能好，不用人工时效，减振性好效，减振性好HT150F+P较粗片状较粗片状HT200P中等片状中等片状强度较高，耐磨耐热，铸强度较高，耐磨耐热，铸造性较好，但要人工时效，造性较好，但要人工时效，减振性较好减振性较好HT250细细P较细片状较细片状HT300 索氏体或屈氏索氏体或屈氏体体细小片状细小片状高强度，高耐磨性，白口高强度，高耐磨性，白口倾向大，铸造性差，要人倾向大，铸造性差，要人工时效工时效（1） HT200、HT250、HT30

21、0多属孕育铸铁多属孕育铸铁（2）牌号越高，铸造性能越差，强度越高）牌号越高，铸造性能越差，强度越高（3）同一牌号，壁厚增加，抗拉强度下降）同一牌号，壁厚增加，抗拉强度下降灰口铸铁的牌号灰口铸铁的牌号20SAEUNSTensile StrengthG1800F1000418 (ksi) (140MPa)G2500F10005G3000F10006G6000F1001260 (ksi) (400MPa)Cheap to produce, excellent castability, high damping capacity, good metal-metal wear resistance wh

22、en lubricated, strength much higher in compression than in tension, brittle in tension.Grey Irons21（1）、去应力退火）、去应力退火 目的：消除铸造内应力，防止铸件开裂惑减少变形目的：消除铸造内应力，防止铸件开裂惑减少变形（2）、改善切削加工性的退火）、改善切削加工性的退火 目的：消除白口组织，改善切削加工性目的：消除白口组织，改善切削加工性（3）、表面淬火）、表面淬火 目的：提高表面的硬度和耐磨性，其方法有火焰淬火、目的：提高表面的硬度和耐磨性，其方法有火焰淬火、感应淬火和电接触淬火。感应淬火和

23、电接触淬火。热处理只能改变铸铁的基体组织，热处理只能改变铸铁的基体组织， 不能改变石墨的形态和分布。不能改变石墨的形态和分布。效果不大效果不大灰口铸铁的热处理灰口铸铁的热处理22Grey irons are by far the most produced among all cast irons. Grey irons are used primarily for their low cost and excellent castability. Typical applications include:engine cylinders汽缸汽缸, pistons, gear box casi

24、ng, transmission casing变速箱变速箱, machine tool bases, balance weight of large cranes起重机起重机, large diameter underground pipework管道工程管管道工程管.T h e y a r e u s e d a l w a y s u n d e r compressive loading conditions. They are unsuitable for taking tensile loads or bending loads.Grey ironMild steelStressSt

25、rainTensile stress-strain behaviour of grey cast ironGrey Irons - Application23Typical grey iron components24完全白口铸铁，经长时间石墨化退火（完全白口铸铁，经长时间石墨化退火（900980oC） 1牌号牌号 F可锻铸铁可锻铸铁KT3006 最低抗拉强度和延伸率最低抗拉强度和延伸率 P可锻铸铁可锻铸铁KTZ4505 最低抗拉强度和延伸率最低抗拉强度和延伸率2热处理热处理 a）铸造纯白口铸铁铸造纯白口铸铁 b）经长时间石墨化经长时间石墨化 c）控制不同冷速获得控制不同冷速获得F 和和P基体

26、的可锻铸铁。基体的可锻铸铁。可锻铸铁显可锻铸铁显微组织微组织可锻铸铁可锻铸铁25基体基体+团絮状石墨团絮状石墨 X100基体基体+团絮状石墨团絮状石墨 X500腐蚀后腐蚀后F+P基体基体+团絮状石墨团絮状石墨 X100F+P基体基体+团絮状石墨团絮状石墨 X500可锻铸铁腐蚀前石墨形态可锻铸铁腐蚀前石墨形态26Malleable irongraphite clusters converted by decomposing cementite in white iron, significantly improving toughness and ductility27ASTMUNS Yield

27、 strengthDuctility32510F3251032.5 (ksi)1035018F362003518Produced by heat treating white iron at elevated temperatures for long times.Such treatment decomposes cementite into ferrite and graphite. Consequently, these cast irons are stronger, tougher and much more ductile than grey irons, compatible t

28、o nodular irons. They have certain capacity to take shock loading, bending and tension. They are suitable for castings of thin thickness. They are expensive to produce, largely due to the heat treatment. Typical applications include gear box casing, transmission casing, differential casing. Malleabl

29、e Irons281、组织与性能：、组织与性能：(F, F+P, P) + 球状球状G G球对基体的割裂、应力集中减小到最低程度，所以，球对基体的割裂、应力集中减小到最低程度，所以， 机械性能大大提高。机械性能大大提高。铁素体基体铁素体基体珠光体基体珠光体基体石墨球石墨球球墨铸铁球墨铸铁29珠光体铁素体球墨铸铁珠光体铁素体球墨铸铁 在铸态下，球墨铸铁的基体组织一般为：铁素体珠在铸态下，球墨铸铁的基体组织一般为：铁素体珠光体少量自由渗碳体。经热处理后可获得基体为平衡组光体少量自由渗碳体。经热处理后可获得基体为平衡组织的球墨铸铁：织的球墨铸铁：QT40017 表示最低抗拉强度值是表示最低抗拉强度值

30、是400MPa和最低延伸率和最低延伸率172、球墨铸铁的牌号、球墨铸铁的牌号30（1）共析转变温度显著提高，并变成一个相当宽的温度范围，）共析转变温度显著提高，并变成一个相当宽的温度范围，而不是恒定的共析转变温度。而不是恒定的共析转变温度。（2）当在共析转变温度之上加热且球墨铸铁的基体组织完全）当在共析转变温度之上加热且球墨铸铁的基体组织完全奥氏体化后，若继续升高温度或延长保温时间，则奥氏体中的奥氏体化后，若继续升高温度或延长保温时间，则奥氏体中的含碳量将因石墨的溶入而增加。含碳量将因石墨的溶入而增加。（3）使）使C曲线右移，并形成两个曲线右移，并形成两个“鼻子鼻子”，这使淬火临界冷，这使淬火

31、临界冷却速度降低，淬透性增大，因此一般中小铸件可采用油淬。却速度降低，淬透性增大，因此一般中小铸件可采用油淬。（1）退火）退火获得塑性好的铁素体基体，改善切削性能和消获得塑性好的铁素体基体，改善切削性能和消除铸造应力。除铸造应力。原始铸态组织中存在自由渗碳体，采用高温退火；原始铸态组织中存在自由渗碳体，采用高温退火；原始铸态组织中不存在自由渗碳体，采用低温退火。原始铸态组织中不存在自由渗碳体，采用低温退火。球墨铸铁的热处理球墨铸铁的热处理31（2）正火）正火增加基体中珠光体的数量，细化基体组织，增加基体中珠光体的数量，细化基体组织，提高强度和耐磨性。提高强度和耐磨性。 完全奥氏体化，采用高温正

32、火；完全奥氏体化，采用高温正火； 不完全奥氏体化，采用低温正火。不完全奥氏体化，采用低温正火。 注：注：正火后要进行去应力退火。（正火后有较大内应力）正火后要进行去应力退火。（正火后有较大内应力）（3）调质）调质获得较高的综合力学性能。获得较高的综合力学性能。（4）等温淬火）等温淬火很高的强度，良好的塑性和韧性。很高的强度，良好的塑性和韧性。 这种工艺适合于综合力学性能要求高且外形又较为复这种工艺适合于综合力学性能要求高且外形又较为复杂、热处理易变形与开裂的零件，如：齿轮、凸轮轴。杂、热处理易变形与开裂的零件，如：齿轮、凸轮轴。32蠕虫状石墨性能介于片状和球状石墨之间。蠕虫状石墨性能介于片状和

33、球状石墨之间。蠕墨铸铁的热处理蠕墨铸铁的热处理 铸态时，基体中具有大量的铁铸态时，基体中具有大量的铁素体，通过正火可增加珠光体，提素体，通过正火可增加珠光体，提高强度和耐磨性。为消除自由渗碳高强度和耐磨性。为消除自由渗碳体或提高塑性，可以通过退火获得体或提高塑性，可以通过退火获得85以上铁素体基体的蠕墨铸铁。以上铁素体基体的蠕墨铸铁。五、特殊性能铸铁五、特殊性能铸铁1、耐磨铸铁：、耐磨铸铁： 高磷高磷磷共晶；急冷磷共晶；急冷白口；珠光体灰口白口；珠光体灰口2、耐热铸铁：、耐热铸铁： Al、Si、Cr氧化膜；单相氧化膜；单相F化；球墨化；球墨3、耐蚀铸铁：、耐蚀铸铁： Si、Cr、Al、Mo、C

34、u、Ni氧化膜，提高基体电位氧化膜，提高基体电位蠕墨铸铁蠕墨铸铁33White cast iron prior to heat treatment34 Ferritic malleable iron with graphite nodules and small MnS inclusions in a ferrite matrixFerritic malleable iron35Pearlitic malleable iron drawn to produce a tempered martensite matrixPearlitic malleable iron36Annealed duct

35、ile iron with a ferrite matrixAnnealed ductile iron37As-cast ductile iron with a matrix of ferrite (white) and pearliteAs-cast ductile iron38Normalized ductile iron with a pearlite matrixNormalized ductile iron39Typical properties of cast irons40When cast white on exterior and grey on interior.nProb

36、lems when machiningnMg sometimes added for strengthn1 2 % Si may be added for strengthC is good cathodebut improves mechproperties41White Cast IronnCooled quicklynno graphite formationnformation of cementitenwhite appearancenMalleable irons created from white ironsnspecial heat treatmentnThree types

37、 of malleable cast ironsnWhiteheartnBlackheartnPearliticcementite plates embedded in pearlite matrix, renders the metal high hardness and brittleness.42Intermediate product for producing malleable ironsAs abrasive wear resistant components, such as ball mill lining tiles, slurry pipe elbows, slurry

38、pump bodiesWhite IronsWhite irons contain relatively less C and Si. As a result cementite (Fe3C) is formed instead of graphite. This makes the alloy very hard and brittle, practically useless as structural materials. The high hardness, however, renders them high resistance to abrasive wear. White ir

39、ons are produced mainly for two purposes:43The chemical composition of the cast iron is similar to that of the grey cast iron but with 0.05 wt% of magnesium. All samples are etched using 2% nital. Spheroidal类似球体的graphite cast iron, Fe-3.2C-2.5Si-0.05Mg wt%, containing graphite nodules in a matrix wh

40、ich is pearlitic. One of the nodules结核is surrounded by ferrite, simply because the region around the nodule is decarburised as carbon deposits on to the graphite. Spheroidal Graphite Cast Iron44An illustration of the ductility of spheroidal graphite cast iron. Spheroidal Graphite Cast Iron45Sulphur

41、in cast irons is known to favour the formation of graphite flakes. The graphite can be induced to precipitate in a spheroidal shape by removing the sulphur from the melt using a small quantity of calcium carbide碳化钙,电石. Sulphur in cast irons46A minute addition of magnesium or cerium, which poisons th

42、e preferred growth directions and hence leads to isotropic growth resulting in spheroids of graphite. The calcuim treatment is necessary before the addition of magnesiumsince the magnesium also has an affinity具有亲和力for both sulphur and oxygen, whereas its spheroidising ability depends on its presence

43、 in solution in the liquid iron.The magnesium is frequently added as an alloy with iron and silicon (Fe-Si-Mg) rather than as pure magnesium.magnesium or cerium in cast irons47 However, magnesium tends to encourage the precipitation of cementite, so silicon is also added (in the form of ferro-silico

44、n) to ensure the precipitation of carbon as graphite. The ferro-silicon is known as an inoculant接种菌接种菌. Spheroidal graphite cast iron has excellent toughness and is used widely, for example in crankshafts曲轴曲轴. The latest breakthrough in cast irons is where the matrix of spheroidal graphite cast iron

45、 is not pearlite, but bainite. This results in a major improvement in toughness and strength. The bainite is obtained by isothermal transformation of the austenite at temperatures below that at which pearlite forms.magnesium or cerium in cast irons48Nodular irongraphite nodules in ferrite matrix, si

46、gnificantly improving toughness and ductility of the metal49ASTMUNSTensile Yield Ductility60-40-18F3280060 (ksi)4018120-90-02 F36200120902Produced by inoculating (addition of RE or Mg to molten metal just before casting) grey iron. This treatment converts graphite flakes into granules. These irons a

47、re much stronger and tougher than grey irons. They are produced and used for high specification applications. They are more expensive than grey irons. Typical applications include:gears, crankshafts, pump bodies, pressure valves, rollers.Nodular Irons (ductile irons)50Typical nodular iron components

48、51 Graphite nodules in a ferritic matrix. Graphite nodules in a ferritic matrix. Spheroidal graphite cast iron usually has a pearlitic matrix. However, annealing causes the carbon in the pearlite to precipitate on to the existing graphite or to form further small graphite particles, leaving behind a

49、 ferritic matrix. This gives the iron even greater ductility. All samples are etched using 2% nital. Heat Treated Spheroidal Graphite Cast IronSome carbon deposited during tempering is also visible. Etchant: Nital 2%52The chemical composition of the cast iron is Fe-3.52C-2.51Si-0.49Mn-0.15Mo-0.31Cu

50、wt%. Austempered Ductile Cast Iron Ductile iron as-cast. Nodules of graphite, pearlite (dark islands) and ferrite (light background). Etchant: Nital 2%53The chemical composition of the cast iron is Fe-3.52C-2.51Si-0.49Mn-0.15Mo-0.31Cu wt%. Austempered Ductile Cast Iron Ductile iron as-cast. Nodules

51、of graphite, pearlite (dark islands) and ferrite (light background). Etchant: Nital 2%54Austenitised 950oC, austempered 350oC for 64 min.Colour micrographs are produced by first etching with 2% nital, followed by open air heat treatment of the metallographic sample at 270oC for 3 h. This oxidises th

52、e sample and produces interference colours which are phase dependent.55Austenitised at 950oC, austempered at 350oC for 64 min. The following images are of austempered ductile iron automobile components, provided by the Institute of Cast Metals Engineers. In order to avoid distortion, the crankshaft

53、for the TVR sportscar is rough-machined after casting, heat-treated to produce the bainitic microstructure, and then properly machined. It is reported to have excellent fatigue properties; its damping characteristics due to graphite reduce engine noise.56The Ford Mustang suspension arm was made from

54、 austempered ductile iron in order to reduce weight, noise and cost.It was designed using finite element modelling to optimise strength and stiffness. Auminium alloys were considered but rejected because the component would then occupy a much larger space because of their lower strength.The truck tr

55、ailer suspension arm was originally made from welded steel, for use on transportation across the rugged Australian Outback. These failed at the welds and were associated with distortions which led to accelerated deterioration of the tyres. The suspension made from the cast austempered ductile iron h

56、as proved to be much more robust.57TVR Tuscan Speed 6, high-performance sports car with an austempered ductile iron crankshaft. Austempered ductile iron suspension arm for a Ford Mustang CobraThe austempered ductile iron crankshaft for the TVR sportscar. A truck trailer suspension arm made from austempered ductile iron, Steele and Lincoln Foundry. 58