Materials for engineering, 3rd Edition - (Malestrom)

Problems 225
7. Sketch on the same axes three hypothetical true stress–strain curves of
the shapes you would expect for a medium carbon steel which has been
quenched and then tempered at (a) 350°C (b) 450°C and (c) 550°C.
Construct a Considère tangent to each curve (Fig. 2.6). In the light of
your construction discuss the relationship between the tempering
temperature and the per cent elongation to tensile fracture for this material
(Fig. 3.24).
On your diagram construct another hypothetical true stress–strain
curve for a material which would be expected to exhibit both high
strength and high ductility.
8. Figure 3.16 indicates that α-brasses (containing up to 30% of zinc)
exhibit a progressive increase in both tensile strength and in the elongation
to fracture as the zinc content increases. This contrasts with the behaviour
of quenched and tempered steels (Fig. 3.24) where the tensile elongation
decreases as the strength rises. Can you give an explanation for this
difference in behaviour between brass and steel?
9. A welded joint is produced between two cold-rolled sheets of type 304
austenitic stainless steel. Sketch a diagram for this situation analogous
to that on the left-hand side of Figure 3.32. Discuss any metallurgical
changes that may take place in the heat-affected zone, and any problems
that might be encountered with this joint when the component is in
service in a corrosive environment. How might such problems be averted?
10. Give an account of the factors which give rise to an iron casting being
‘white’ or ‘grey’ in character. What methods are available for producing
ductile iron castings?
11. Give an account of the factors which determine whether an oxide film
has been formed on the surface of a metal or alloy protects it from
further oxidative attack. To what extent may alloys be designed to
exhibit oxidation resistance?
12. Two mild steel plates are to be rivetted together for subsequent use in
a structure to be in contact with seawater. Rivets made of four materials
are available, namely (a) stainless steel (b) copper (c) carbon steel and
(d) aluminium. Each of these types of rivet is tried, and each rivetted
structure is painted before being put into service.
Discuss the nature of any corrosion problems that may be encountered
in each of the four rivetted assemblies.
13. Extracting the data from Fig. 3.36, estimate the dimensional wear
coefficient (k) for a leaded α/β brass sliding against a hard stellite ring
(a) in the region of mild wear and (b) in the region of severe wear.
Discuss the mechanisms of wear in these two regimes.
[Answers: (a) 1.25 × 10 –5 ; (b) 3.17 × 10 –4 ]