Materials Science II
Aluminium and Steel
An aluminium-4%copper alloy is aged at 120°
C for 1000h in which process four distinct precipitate phases are observed.
Describe the precipitation sequence and the reasons that they form. When
the ageing process takes place at 250° C
only two distinct precipitates are formed and transformation is complete
in less than one hour. Explain this behaviour illustrating your answer with
an appropriate TTT diagram.
Draw and describe the microstructure of a 1.1%C steel; (a) air cooled from
880° C, (b) quenched from
880°C,(c) as (b) but reheated for a moderately
long period at 750°C, quenched and tempered
for 1h at 400°C. What microstructure would
you prefer for a wood chisel? Give reasons.
Explain the prevention of the formation of stretcher strains in low carbon
steel during deep drawing operations.
What is meant by hardenability? By what experimental methods can it be
determined? Explain the effect of adding alloying elements on the hardenability
of low carbon steel illustrating your answer with an appropriate TTT diagram.
On tempering a quenched plain carbon steel it is found that the same hardness
is produced by a treatment of 1h at 645°
C or 10h at 585° C. What is the activation
energy of the tempering process?
On tempering a quenched steel which contains 2%Mo, 0.1%C its is found that
the same hardness is produced by treatments of 1h at
550° C and 1h at
625° C. Explain.
Describe each of the following illustrating your answer with appropriate
TTT diagrams; (a) Critical cooling rates for martensite formation,
(b) Ausforming, (c) Martempering.
Sketch the variation in temperature with distance away from the fusion zone
boundary in a weld between two 0.25% carbon steel plates and relate this
to the iron-carbon phase diagram. Hence describe the microstructures which
are observed in the heat affected zone around the weld.