Week 5 Steels

Question 1

How does cooling change below the A3?

Answer 1

The austenite is unstable and begins to transform to ferrite. The carbon content at any temperature given by
the blue and green lines.
At 800°C, ferrite is C (~0.01wt%C)
and austenite is C (~ 0.4wt%C)
The austenite becomes progressively
enriched with C.
At 723°C, the composition of the austenite is the eutectoid composition and transforms to pearlite.

Question 2

What happens when fe3 is non equilibrium cooled?

Answer 2

It can cool either slowly- carbon diffuses out of the alpha matix to form carbide, medium rate- forming bainite or very fast cooling rate - much more jagged structure forming martensite.

Question 3

What do shapes and fields of the time temperature transformation curve depend upon?

Answer 3

Carbon content. This curve is only for ISOTHERMAL heat treatments. only austenite can form something else

Question 4

What are the phases for continuous cooling transformation curve for austenite?

Answer 4

for very fast - MArtensite
For medium- bainite
slow - ferrite+pearlite

Question 5

What are cast irons?

Answer 5

Cast irons are alloys of iron and carbon containing varying amounts of
manganese, silicon, sulphur and phosphorus. Specialised irons may also contain other alloy additions and /or be subjected to specific heat treatment cycles.

Question 6

How can carbon be present in cast irons?

Answer 6

Carbon may be present either in the uncombined state (graphite) or in the
combined state (cementite).

Question 7

How does carbon present itself in grey and white irons?

Answer 7

• White irons have no free carbon, are very hard and difficult to machine and thus
  possess good wear resistance.
  -Grey cast irons have free graphite present and are more readily machinable than
  white irons but possess poor tensile properties (although compression properties are good).

Question 8

How are white cast iron formed? what is the machinability like?

Answer 8

Fast cooling rate.
White irons have no free carbon, are very hard and difficult to machine and
thus possess good wear resistance.

Question 9

How are grey cast irons formed?

Answer 9

Grey cast irons have free graphite present and are more readily machinable than
white irons but possess poor tensile properties (although compression properties are good).
(Can also be graphite in ferrite with a faster cooling rate)

Question 10

How are ductile cast irons formed?

Answer 10

Pearlitic (moderate cooling) or ferritic (slow
cooling)
Additions of magnesium (Mg) or cerium (Ce)
change the graphite form to spheroidal.