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Cast Irons

Cast Irons. Charpter VI Cast Irons. 第 5 章 铸 铁. Introduction of Cast Iron.

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Cast Irons

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  1. Cast Irons Charpter VICast Irons 第5章 铸 铁

  2. Introduction of Cast Iron • The term cast iron arises from the method by which the iron is produce. Pig iron（生铁）, the product of the blast furnace（鼓风炉，高炉）with about 3 to 4% carbon, is remelted in a furnace and properties of the iron modified by the addition of other materials. The resulting iron is then cast. • Cast iron have often significant amounts of silicon, as well as smaller amounts of other elements. • The range of carbon content gives the materials a high fluidity. Also the materials when solidifying show no significant volume contraction. 1

  3. Grey and White Cast Iron • The iron-carbon phase digram shows that a temperature of 1148℃ a eutectic occurs for alloys with 4.3% carbon. • Between 2.0 and 4.3% carbon, increasing the carbon percentage reduces the temperature at which solidification occurs. 2

  4. Grey and White Cast Iron • Consider the cooling of an iron with between 2.0 and 4.3% carbon from the liquid state. • (1) When solidification starts to occur, the result is formation of austenite in the liquid. • (2) At 1148 ℃ the solidification is complete. • (3) With further cooling, austenite plus Cm can form. • However, if there is very slow cooling, the Cm is not stable and graphite flakes can form. Moderate and slow cooling rate favor the formation of graphite. The solidification rate also affects the type of matrix formed in gray cast iron. 3

  5. Grey and White Cast Iron • Moderate cooling rates favor the formation of a pearlitic matrix, whereas slow cooling rates favor a ferritic matrix. • At 727℃ the remaining austenite changes to alpha ferrite and the result at room temperature is a structure of pearlite plus graphite flakes.(Fig. 3.21) 4

  6. Grey and White Cast Iron • This type of cast iron is known as grey iron because of the grey appearance of its freshly fractured surface. • Graphite has negligible tensile strength. The strength of grey iron increases as the amount of free graphite is reduced and as the fineness of graphite flakes is increased. • An increase in the carbon content up to 4.3% carbon, the eutectic value, lowers the melt point of iron and, in doing so, favours the production of iron with graphite rather than Cm because the cooling from the liquid state is from a lower temperature and hence the cooling rate is slower. 5

  7. Grey and White Cast Iron • Some elements when included with the carbon in the iron affect the formation of graphite. • (1) Carbon Gray cast irons usually contain 2.5 to 4 percent C.Gray cast iron is formed when the carbon in the alloy exceeds the amount that can dissolve in the austenite and precipitates as graphite flakes. • (2) Silicon and phosphorus Gray cast irons usually contain 1 to 3 percent Si. Since silicon is a graphite stabilizing element in cast irons, a relatively high silicon content is used to promote the formation of graphite. 6

  8. Grey and White Cast Iron • (2) Silicon and phosphorus • Silicon and phosphorus affect the composition of the eutectic point and are considered to have a carbon equivalence in the formation of graphite. • To produce a fully ferritic matrix in a gray iron, the iron is usually annealed to allow the carbon remaining in the matrix to deposit on the graphite flakes, leaving the matrix completely ferritic. • The combined effect of carbon, silicon and phosphorus is represented by the value of the carbon equivalent where: carbon equivalent% = %C + one-third(%Si + %P) [1] 7

  9. Grey and White Cast Iron • The carbon equivalent determines how close the composition of an iron is to the eutectic value of 4.3% carbon and hence how likely grey iron is to be produced. • For example, if there is 1.5% silicon and 0.3% phosphorous with an iron containing 3.5% carbon, then the carbon equivalent is 4.1% and thus the iron is very close to the eutectic point. • A fast cooling rate gives a structure with solidifies at 1148℃ to give austenite and cementite which then, at 727℃, gives a structure of Cm and pearlite. This structure is known as white iron because of the white appearance of its freshly fractured surface. 8

  10. Grey and White Cast Iron • White iron, because of its high Cm content, is hard and brittle. This makes it difficult to machine and hence of limited use. • The main use is where a wear-resistant surface is required. (用途：炼钢原料、轧辊、 犁铧、球磨机的磨球) • The tendency of cast iron to solidify with the carbon present in the form of Cm rather than graphite increases as the carbon percentage, or carbon equivalent percentage, is reduced and as the cooling rate increase. The iron is said to chill（冷淬）if such changes occur, Chilled structure are hard and brittle. 9

  11. Grey and White Cast Iron • The microstructure of as-cast unalloyed white cast iron contains large amounts of iron carbides in a pearlitic matrix. • The large amount of iron carbides in their structure is mainly responsible for their wear resistance. • White cast iron serves as the raw material for malleable cast irons. Fig. 9.59 Microstructure of white cast iron. The white constituent is iron carbide.The gray areas are unresolved pearlite. 10

  12. Grey and White Cast Iron • Sulphur often exists in cast iron and has the effect of stabilising Cm, thus favouring the production of white iron rather than grey iron and hence increasing hardness and brittleness. The amount of sulphur in an iron has, therefore, to be controlled. • The addition of small amounts of manganese to an iron containing sulphur enables the sulphur to form manganese sulphide. The removal of the free sulphur has the effect of increasing the chance of grey iron being produced. 11

  13. Grey and White Cast Iron • A typical composition of a grey iron is 3.2 to 3.5% carbon, 1.3 to 2.3% silicon, 0.15 to 1.0% phosphorus, 0.1% sulphur and 0.5 to 0.7% manganese. This gives a carbon equivalent approaching 4.3%, the eutectic value. • A casting will often have sections of varying thickness. The rate of cooling of a part of a casting will depend on the thickness of the section concerned, the thinner the section the more rapidly it will cool. This means that there is likely to be a variation in the properties of different parts of a casting. • Thus, for example, a very thin section might be a white iron and so very hard and virtually unmachinable. 12

  14. Grey and White Cast Iron • A different section might be grey with white edges. A thicker section might be a fine-grained grey iron while an even thicker section might be, at least internally, a coarse-grained gray iron. Because the properties of gray iron depend on the rate of cooling and this is influenced by section thickness, the British Standard specification for grey iron are in terms of the properties of a test piece machined fron a specific size casting, namely a 30 mm diameter cast bar. • Grey cast irons are specified in British Standard by a grade number which gives the minimum tensile strength of the iron in the standard test piece. Table 3.11 gives some examples. 13

  15. Grey and White Cast Iron 14

  16. Grey and White Cast Iron • Fig. 3.22 shows the effect on the tensile strength of the different grades of section thickness. This has to be taken into account in considering the likely strengths at different thicknesses within a casting produced to a particular grade. 15

  17. 灰铸铁的组织、性能及牌号 • 组织：片状石墨分布在基体组织上（钢基体+片状石墨） • 基体组织：F, F+P, P • 性能: (1)F灰铸铁的强度、硬度和耐磨性都比较低，但塑性比较高。多用于制造低负荷不太重要的零件。 • (2)P灰铸铁强度和硬度高、耐磨性好，但塑性低（比F） • (3)F+P灰铸铁性能介于两者之间 • 用途： • 承受压力和要求消震好的场合 • 牌号：HT100，HT150，HT200，HT250，HT350 最小抗拉强度 16

  18. 灰铸铁的微观组织 17

  19. Properties of Grey Iron 1. Grey iron has excellent machinability since the graphite flakes act as chip breakers. • With coarse graphite flakes, grey iron is very good at damping out vibrations. • The network of graphite flakes also gives good thermal conductivty. • Gray iron has a wide range of strengths and hardness and in most cases are easy to machine. 2. Cast irons make excellent casting alloys since they are easily melted, are very fluid in the liquid state, and do not form undesirable surface films when poured. Cast irons solidify with slight to moderate shrinkage during casting and cooling. 阻尼，衰减 18

  20. Application of Grey Iron • (1) Cast irons can be alloyed to produce the parts with superior wear, abrasion, and corrosion resistance. • (2) Cast irons have relatively low impact resistance and ductility, and this limits their use for some applications. The wide industrial use of cast irons is due mainly to their comparatively low cost and versatile engineering properties. • (3)Typical uses are motor cylinders and pistons（活塞）, machine castings, crankcase（曲轴箱）and machine tool beds. • (4) 应用最广泛，拖拉机（占50%~70%）、机床（60%~90%） 19

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