(7)--3.10 问题与解答（双语）.ppt
3.10 Problem examples and solutions问题与解答Problem 1a.How is raw pig iron extracted from iron oxide ores?b.Write a typical chemical reaction for the reduction of iron oxide(Fe2O3)by carbon monoxide to produce iron.c.Describe the basic oxygen process for converting pig iron into steel.a.Raw pig iron is typically extracted from iron oxide ores in a blast furnace in which coke(carbon)acts as a reducing agent of the iron oxides.The pig iron produced contains approximately 4 percent carbon.b.The typical chemical reaction is:.c)In the basicoxygen process,pig iron is converted into steel through oxidation in a refractorylined converter.An oxygen lance insert provides pure oxygen to the bath of molten pig iron and scrap steel.The iron oxide formed then reacts with the carbon in the steel to form carbon monoxide:a.Why is the FeFe3C phase diagram a metastable phase diagram instead of a true equilibrium phase diagram?b.Define the following phases that exist in the FeFe3C phase diagram:(i)austenite,(ii)ferrite,(iii)cementite,(iv)ferrite.c.Write the reactions for the three invariant reactions that take place in the FeFe3C phase diagram.Problem 2 The phase diagram is a metastable phase diagram rather than a true equilibrium phase diagram because the compound iron carbide formed,cementite,is not a true equilibrium phase.While cementite is typically very stable,it can decompose into iron and carbon under certain conditions.Austenite,the interstitial solid solution of carbon in iron,has an FCC crystal structure and a relatively high maximum solid solubility of carbon(2.08 percent).An interstitial solid solution of carbon in the BCC iron crystal lattice,ferrite has a maximum solid solubility of carbon of 0.02 percent.Cementite,is a hard,brittle intermetallic compound with a carbon content of 6.67 percent.An interstitial solid solution in iron,ferrite has a BCC crystal structure and a maximum solid solubility of carbon of 0.09 percent.(c)The three invariant reactions that take place in the phase diagram are a peritectic reaction,a eutectic reaction,and a eutectoid reaction.Distinguish between the following three types of plaincarbon steels:a.eutectoid,b.hypoeutectoid,and c.hypereutectoid.Problem 3Eutectoid plaincarbon steel contains 0.8 percent carbon.Hypoeutectoid plaincarbon steel contains less than 0.8 percent carbon.Hypereutectoid plaincarbon steel contains more than 0.8 percent carbon.Distinguish between proeutectoid ferrite and eutectoid ferrite.Problem 4Proeutectoid ferrite is the ferrite formed during cooling from the upper transformation temperature to just above the eutectoid temperature,where as,eutectoid ferrite is produced during the eutectoid reaction at the eutectoid temperature of 723C.What is the normalizing heat treatment for steel and what are some of its purposes?Problem 5The normalizing heat treatment for steel consists of heating within the austenitic region and then cooling in still air.Some of the purposes for normalizing include:to refine the grain structure;to increase the strength of the steel,as compared to annealed steel;to reduce compositional segregation in castings or forgings and thus provide a more uniform structure.Problem 6(a)What alloying element and how much of it(weight percent)is necessary to make a stainless steel“stainless”?(b)What type of surface film protects stainless steels?(c)What are the four basic types of stainless steels?(a)A steel must contain at least 12 percent chromium to be“stainless”.(b)A surface film of chromium oxide protects stainless steels from certain corrosive environments.(c)The four basic types of stainless steels are ferritic,martensitic,austenitic and precipitationhardenable.Problem 7(a)What are the cast irons?(b)What is their basic range of composition?(c)What are some of the properties of cast irons that make them important engineering materials?(d)What are some of their applications?(e)What are the four basic types of cast irons?(a)Cast irons are a family of ferrous alloys intended to be cast into a desired shape rather than worked in the solid state.(b)These alloys typically contain 2 to 4 percent C and 1 to 3 percent Si.Additional alloying elements may also be present to control or vary specific properties.(c)Cast irons are easily melted and highly fluid and do not form undesirable surface films or shrink excessively;consequently,they make excellent casting irons.They also possess a wide range of strength and hardness values and can be alloyed to produce superior wear,abrasion,and wear resistance.In general,they are easy to machine.(d)Their applications include engine cylinder blocks and gear boxes,connecting rods,valve and pump casings,gears,rollers,and pinions.(e)The four basic types of cast irons are white,gray,ductile and malleable.Problem 8A 0.25 percent C hypoeutectoid plaincarbon steel is slowly cooled from 950C to a temperature just slightly below 723C.(a)Calculate the weight percent proeutectoid ferrite in the steel.(b)Calculate the weight percent eutectoid ferrite and weight percent eutectoid cementite in the steel.The weight percent proeutectoid ferrite just below 723C will be the same as that just above 723C.6.67%C0.25%C0.02%CThe weight percent total cementite and total ferrite are calculated based on the tie line shown to the right.Wt%eutectoid ferrite=96.5%70.5%=26.0%Wt%eutectoid cementite=Wt%total cementite=3.46%A plaincarbon steel contains 93 wt%ferrite and 7 wt%Fe3C.What is its average carbon content in weight percent?A plaincarbon steel contains 5.9 wt%hypoeutectoid ferrite.What is its average carbon content?
