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