8.5·Cast Irons 151 When discussing steels,we worked with the iron-cementite phase diagram.Now,cementite is actually a metastable com- pound which,when brought to its equilibrium state,decomposes into ferrite and graphite.(Graphite is one of the modifications of carbon,diamond being another.)The tendency for this disso- ciation increases for larger carbon concentrations and for slow cooling rates. Further,small additions of silicon(in the amount of about 2%) promote graphite formation.Thus,cast iron is generally a some- what different material than steel.It has important new me- chanical properties,as we shall see momentarily.Actually,even the phase diagram is slightly modified,as can be observed by comparing Figure 8.1 with Figure 8.7.The main features of the iron-graphite phase diagram are the absence of the cementite in- termetallic phase,a slightly higher eutectoid temperature (A 11C),a slightly higher eutectic temperature (A =6C),and an insignificant shift of the eutectic and eutectoid compositions to smaller carbon contents.Figure 8.7 also indicates,that below the eutectoid temperature,ferrite and graphite are intermixed,as is common in any two-phase region. Gray Cast Several types of cast irons are in use.Their names have been de- Iron rived from their appearances.Among them,gray cast iron is uti- lized extensively,mainly because it is the least expensive metal- lic material of all,but also because it has the ability to effectively damp mechanical vibrations.For these reasons,gray cast iron is frequently used for the body of heavy machines in which the vi- brations need to be reduced.A few more favorable properties are found in gray cast iron:It flows easily when in the liquid state, allowing castings of intricate shapes.Further,it shrinks little af- ter casting and has a high resistance toward wear.However,gray cast iron is hard and brittle and relatively weak in tension.The ductility is low,and it shatters easily when exposed to a blow. Thus,tools,especially hammers,should never be made out of gray cast iron,even though cheap tools are occasionally manu- factured in this way. Gray cast iron contains generally between 1 and 3 mass per- cent silicon.This causes the graphite to precipitate in the form of interconnected flakes,also called eutectic cells or clusters(quite similar in appearance to potato chips),imbedded in a ferrite ma- trix,see Figure 8.8(a).The flakes have nucleated on those points where they are interconnected.However,if the Si concentration is lowered or the cooling rate is increased,the decomposition of cementite into graphite and ferrite is incomplete and the graphite is then surrounded by cementite.8.5 • Cast Irons 151 When discussing steels, we worked with the iron–cementite phase diagram. Now, cementite is actually a metastable com￾pound which, when brought to its equilibrium state, decomposes into ferrite and graphite. (Graphite is one of the modifications of carbon, diamond being another.) The tendency for this disso￾ciation increases for larger carbon concentrations and for slow cooling rates. Further, small additions of silicon (in the amount of about 2%) promote graphite formation. Thus, cast iron is generally a some￾what different material than steel. It has important new me￾chanical properties, as we shall see momentarily. Actually, even the phase diagram is slightly modified, as can be observed by comparing Figure 8.1 with Figure 8.7. The main features of the iron–graphite phase diagram are the absence of the cementite in￾termetallic phase, a slightly higher eutectoid temperature (
11°C), a slightly higher eutectic temperature (
6°C), and an insignificant shift of the eutectic and eutectoid compositions to smaller carbon contents. Figure 8.7 also indicates, that below the eutectoid temperature, ferrite and graphite are intermixed, as is common in any two-phase region. Several types of cast irons are in use. Their names have been de￾rived from their appearances. Among them, gray cast iron is uti￾lized extensively, mainly because it is the least expensive metal￾lic material of all, but also because it has the ability to effectively damp mechanical vibrations. For these reasons, gray cast iron is frequently used for the body of heavy machines in which the vi￾brations need to be reduced. A few more favorable properties are found in gray cast iron: It flows easily when in the liquid state, allowing castings of intricate shapes. Further, it shrinks little af￾ter casting and has a high resistance toward wear. However, gray cast iron is hard and brittle and relatively weak in tension. The ductility is low, and it shatters easily when exposed to a blow. Thus, tools, especially hammers, should never be made out of gray cast iron, even though cheap tools are occasionally manu￾factured in this way. Gray cast iron contains generally between 1 and 3 mass per￾cent silicon. This causes the graphite to precipitate in the form of interconnected flakes, also called eutectic cells or clusters (quite similar in appearance to potato chips), imbedded in a ferrite ma￾trix, see Figure 8.8(a). The flakes have nucleated on those points where they are interconnected. However, if the Si concentration is lowered or the cooling rate is increased, the decomposition of cementite into graphite and ferrite is incomplete and the graphite is then surrounded by cementite. Gray Cast Iron