2007年《机械工程材料》试题(双语考卷B及答案)

华南理工大学期末考试

《 机械工程材料 》试卷B---闭卷部分

4. 一律用中文答题

5. 本试卷共 5 大题，满分100分，考试时间120分钟。

题 号 一 二 三 四 五 总分

得 分

评卷人

1.In the Al-20%Cu alloy, does the first solid solution which forms on cooling contain ___c__

a)20% Cu

b)less than 20% Cu

c)more than 20% Cu

d)no Cu (i.e. pure Al)

2.In solidification, which of following mechanisms or steps best describing the process?___b___.

a)cooling and reheating

b)nucleation and growth

c)melting and cooling

d)none of the above

3.To ensure the extreme elasticity of spring made by 60Si2Mn steel, what microstructure should be

obtained after quenching and 450°C tempering. __d_

a)tempered bainite

b)tempered martensite

c)tempered sorbite

d)tempered troostite

4.For the equilibrium microstructure, the mechanical properties of steels are mainly influenced by the

amount, morphology and distribution of a

a)cementite

b)ferrite

c)martensite

d)pearlite

5.The grain size of solidified metals will not be affected by _c___

a)the cooling rate

b)the growth model of crystal

c)the artificially added impurity

6.Materials strengthening methods include ___a b c d__

a)solution strengthening

b)fine grain strengthening

c)work hardening

d)case hardening

7.Recrystallization process will not change the _b c__ of materials.

a)grain size

b)chemical composition

c)crystal structure

d)imperfection density

8.The purpose of cold treatment is to eliminate the ___b__.

a)stress

b)residual austenite

c)martensite

d)pearlite

9.Trace amounts of sodium are added to modify the microstructure and improve properties of Al-Si

die-castings, the role of sodium is to ___c_____.

a)increases the amount of primary Si

b)increases the eutectic temperature

c)refines the eutectic structure

d)none of the above

10.Engineering alloys can be broadly divided into___c___.

a)ferrous alloys and ceramic;

b)metals and non-ferrous alloys;

c)ferrous alloys and non-ferrous alloys;

d)metals and alloy.

Section 2 — Fill out the following blanks (12 points)

1.If some silicon is added (1 to 2%, maybe 3 %), the carbon will tend to graphitize, and there are various

forms the graphite can have. Then, the carbon can also be in the form of graphite ( 片状__ ;

__ 团絮; _蠕虫; Spheroidal cast iron)

2.固溶时效 is the main approach for the strengthening of aluminum alloys. It includes two processes:

固溶处理 and 时效处理。But only 加工硬化 could be used for the strengthening of pure aluminum and rust-prevention aluminum alloys.

3.Except Cobalt, the dissolution of most of alloying elements in austenite will shift the TTT curves to __右

___ direction, which means that the stability of overcooling austenite __增加______ and that the

hardenability __增加_____.

Section 3 — True or false (10 points)

1. ( 对 ) For crystallization process, heterogeneous nucleation is always easier to take place than

homogeneous nucleation;

2.( 对) Among carbon steels, the eutectoid steel has the best hardenability;

3.( 错) The quality of steels is dependent on the content of carbon and the alloying elements;

4.( 错) In FCC metals, the most closely packed atomic plane and direction are {111} and <111>;

5.( 错) Both α-Fe and γ-Fe have 12 slip systems, so have the same plasticity;

6. ( 对) The 20CrMnTi could have a surface hardness of HRC 60 after quenching, and the carburization

treatment is not necessary.

7. ( ×) For those steels with the same hardenability, the same depth of quench-hardening layer could be

obtained under any circumstance;

8.( 错 ) 1Cr13 steel and Cr12 steel have the similar Cr content, and therefore similar mechanical

properties and usages;

9. ( ×) After nitriding, the quenching and tempering heat treatment are needed to enhance the surface

hardness and wear-resistance of steels;

10.( × ) The so-called hot working means that the working temperature is higher than the recrystallisation

one and there is no work hardening;

Section 4 — Analysis and Calculation (43 points)

1. Answer the following questions (12 points):

(1)What is the crystal structure of γ-Fe phase? (1 point)

(2)Draw a schematic diagram of the unit cell of γ-Fe phase, and draw up the (110) and (111) crystal planes and

[122] direction. (5 points)

(3)What number of Fe atom is there in the unit cell of γ-Fe phase? Try to calculate the packing density of atoms

in the unit cell of γ-Fe phase. (6 points)

答：1. fcc 3. 4 个原子堆垛密度0.74

2.

2. Answer the following questions on the Fe-C equilibrium phase diagram (20 points)

(1) Complete the following Fe-C phase diagram, and label the phase fields with the proper symbol; (6

points)

(2) Write down the carbon content of P, S, E and C symbols in the phase diagram; (2 points) (3) Label the A1, A3 and Acm symbols in the Fe-C phase diagram; (3 points)

(4) Write down the reaction formula of eutectoid transformation corresponding to the PSK line, including

the phase transformation temperature; (3 points)

(5) For a carbon steel, the weight ratio of the pro-eutectoid ferrite to the ferrite in the pearlite is about 56/39,

try to calculate the carbon content using the level law; (6 points)

Fe 0

6.69 Fe 3C

T e m p e r a t u r e (°C )

T e m p e r a t u r e (°C )

P

S K

E

C

F

H

J

B

A

N

G

Q

D

C, wt%

3) γ→α+Fe 3C 727°C

4) 亚共析钢：α+P 共析钢： P 过共析钢：P+Fe3C

5）

设先共析铁素体为56，珠光体中的铁素体为39，则珠光体中的渗碳体为39*（6.69-0.77/0.77-0.02）=4.95 含碳量x为（56+39）/(4.95+56+39)=(6.69-x)/(6.69-0.02)

X=0.35

4.Alloying is the most effective way to improve the corrosion resistance of steels.

a)so what is the three principles for the addition of alloying elements? (3 points)

b)For the 1Cr18Ni9Ti austenitic stainless steel, analyze the role of alloying elements Cr, Ni and Ti; (3

points)

答：合金化的三个目的：

(1)使钢具有均匀化学成分的单相组织。

(2)减小两极间的电极电位，提高阳极的电极电位。

(3)使钢的表面形成致密的氧化膜保护层。

Cr：钝化，即提高阳极的电极电位

Ni：形成室温单相γ

Ti：防止晶间腐蚀（Ti优先与C形成TiC,使(Cr,Fe)23C6不出现，

防止晶界贫Cr）

华南理工大学期末考试

《 机械工程材料 》试卷A---开卷部分

Section 5----Answer the following questions (27 points)

1. There are three kinds of annealed carbon steels with the same size and shape, but different carbon content of 1.2%, 0.4% and 0.2%, respectively. Try to provide at least three methods to identify these carbon steels.

(6 points)

1. 金相分析，0.2%, 白色的铁素体多，0.45%，铁素体少，1.0% 无白色的铁素体

2. 硬度分析，从高到低分别为1.0%, 0.45% and 0.2%

2. 20CrMnTi steel is usually used to make automobile gear and carburizing is an essential heat treatment.

(1) What is the purpose of carburizing?

(2) What is the carburizing temperature range? At the carburizing temperature, what is the crystal structure and microstructure of 20CrMnTi? What maximum carbon amount could be carburized into the Fe lattice? (3 points)

(3) Describing the microstructure of the surface layer and the core of 20CrMnTi steel after carburizing, and the subsequent quenching plus low temperature tempering? (6 points)

答：1. 增加表面碳含量，从而提高材料表面的硬度和耐磨性

2. 920°C左右，FCC 结构的单相奥氏体， 2.11wt%

3. 渗碳：从表层到心部为过共析、共析和亚共析组织，表层组织含碳量升高，硬度和耐磨性提高。

淬火加低温回火：表层为M回+粒状碳化物+少量A′，心部淬透时为低碳回火马氏体，未淬透时为

索氏体+铁素体，进一步提高材料硬度和耐磨性能，低温回火使淬火引起的内应力消除，稳定组织，但仍保持高硬度。

3. To make a camshaft of a diesel engine, high surface hardness (HRC>50) and core toughness are necessary. (12 points)

1）Select proper materials for making the camshaft and formulate the hot-working and cold-working route. (4 points);

2）Point out the purpose, the temperature and the obtained structure of every heat treatment process; (6 points)

3）Selecting the replacing materials if the materials is out-of-stock; (2 points)

1） 40Cr

下料-------模锻-------- 正火----------机械粗加工--------- 调质处理----------机械精加工-------对轴颈进行高频淬火和低温回火--------------磨至尺寸

2）正火：得到珠光体，降低硬度，以便于机械加工

调质处理：回火索氏体，具有良好的综合性能，即较高的硬度和韧性

高频淬火+低温回火：得到回火马氏体，提高轴颈的表面硬度

3) 45#