Q1
Section A
Complete both questions in this section in the BLUE answer booklet
You are using an AISI 4340 (Fe-0.4wt% C+ alloying additions) steel to produce forged/machined
ground anchors for a large mobile phone mast.
(a) The steel is supplied in the normalised condition. What fraction of the steel do you expect
to be austenite, martensite and pearlite? Justify each answer.
(5 marks)
(b) Upon examination you find the steel is mostly bainite and martensite with a small amount
of proeutectoid ferrite. Determine the range of possible cooling rates this material might
have experienced? Would the presence of these microconstituents cause you any concern
considering the steel will be hot-forged?
(5 marks)
(c) The steel needs to be processed in the following manner:
(i) Hot forge to the basic shape.
(ii) Substantial machining to create threads.
(iii) Heat treatment and cooling to create a 100% martensite microstructure.
(iv) Tempering to modify the toughness.
Sketch a time-temperature history that you would use for this process. Focus on specify-
ing the temperatures and the required cooling rates for each stage. Clearly indicate where
you have had to use your judgement to estimate a value.
(5 marks)
(d) You decide that the forging should be a dual-phase steel consisting of 50% ferrite and
50% martensite in order to improve the damage resistance of the anchor. What single
change could be made to the above process to produce this desired microstructure? Fully
explain your reasoning.
(5 marks)/n8-iron
(BCC)
Temperature, T (°C)
Ferrite
a (BCC)
1600
1400
1200
1000
800
600
400
200
0
Melting point
/of pure Fe
1534°C
L+8
Peritectic
point
0
Fe
Austenite
Y (FCC)
910°C
0.8
α+Y
0.035
1
L+Y
Liquid, L
2.1
Eutectoid
point
2
Eutectic
point
3
Ferrite, & + Fe,C
Austenite. Y + Fe₂C
723°C
4.3
wt% C
4
1147°C
Compound,
Cementite
Fe₂C
5
6
7/nTemperature (C)
900
800
700
600
500
400
300
200
100
0
10⁰
M₂
M₁
B.
Rate (C/s) 20 8
10¹
10²
10³
time (s)
Figure Q1
4
AISI 4340
0.33 0.08 0.023 0.006
104
105/nFORMULAS
Emax = VjEj + (1 - V₁) Em
1
[r+rs(f − 2)]¹/2
-
agel
Fig: 1
Fig: 2
Fig: 3
Fig: 4
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