Introducing steels

1
Introducing steels

• EF 420 Lecture 5
• John Taylor

2
Pig iron production

• Blast furnace Charge
• Ore Iron oxide or carbonate impurity
• Limestone Calcium carbonate, flux.
• Coke
• Charge flows downwards
• Air blown through tuyeres moves up furnace
• Iron oxide reduced to pig iron by coke

3
Pig iron composition

• Fe with 4 C, 0.4 Si, 0.3 Mn, 0.025 S, and up
  to 1.5 P
• Too impure to be of commercial use
• Cast into sand moulds as pigs
• Stored molten in torpedoes and transported to
  steel works

4
Steel

• An alloy of iron with up to 1.5 carbon and other
  elements
• Extremely wide range of strengths is available
  (100MPa to 2000MPa)
• Allotropy of iron is the key to producing high
  strength
• Strength depends mostly on carbon content and
  heat treatment
• Other alloy elements determine hardenability

5
Other alloy elements

• Hardenability increasing
• Mn, Cr, Ni, Mo, V, W, B
• Deoxidants
• Mn, Al, Si,
• Micro alloys - grain boundary pinning
• Ti, V, Nb, Al

6
Steelmaking

• Oxidation of carbon in pig iron to CO gas
• Obsolete processes used air
• Bessemer open hearth
• Modern processes use pure oxygen
• BOS (basic oxygen steelmaking) shown
• Some steel is produced in electric arc furnaces
  from scrap and DIR

Oxygen in
Argon in
7
Ladle metallurgy
Cored Wire
Lance

• Ladle treatments used to further refine steel for
  high quality
• Melt is vigorously stirred by argon
• Powders gas injected into melt through the
  lance
• Cored wire is added
• Further decarburisation
• Inclusion shape control
• Alloying

Argon stirring
8
Deoxidation degassing

• Steel picks up excess oxygen during steelmaking
• Al, FeMn or FeSi added to form oxides, which
  float as a slag
• Oxygen, hydrogen and nitrogen can also be removed
  by vacuum degassing
• Molten steel streams into mould in a vacuum
• Gases evaporate

Mould
Vacuum
9
Ingot casting

• Traditional, batch, low productivity method
• Small quantities
• Special steels
• Magnets,
• Tool steels,
• Stainless
• Non-ferrous metals

Killed
Semi-killed
Rimming
10
Continuous casting

• Structural and pressure equipment steel
• Killed steel
• High productivity for large quantities
• Used for all steel production in Australia

11
Remelting and refining
Consumable electrode

• Produces super quality steel
• The impure ingot is used as a consumable
  electrode in a vacuum electric arc furnace (VAR
  steel)
• It can also be remelted by induction heating in a
  vacuum (VIM steel)
• It can also be remelted by passing current
  through a slag bath ( Electroslag refined or ESR
  steel)

Transformer
Slag pool
Water cooled Moveable mould
ESR steel
12
Processing structural steel

• Slab is hot rolled (over 950C) to final shape
  (plate or section)
• Slow cooling as-rolled
• Reheat to 910C and air cool normalised
• TMCR (final rolling between 930 and 870C)
• Quench and temper

13
Forms of delivery

• Steel castings, forgings
• Sheet, strip, bar, billets, ingots
• Plate, tube, pipe, rolled sections
• Reinforcing bar
• Rod, wire, cable, chain
• Springs
• Fasteners - nails, screws, bolts, studs, nuts

14
Classification by composition

• Steel - Carbon between 0.05 and 1.5 C
• Soft Iron - Less than 0.05 C
• Cast Iron - More than 1.5 C
• Carbon and carbon-manganese steels
• Low alloy steels (lt5 Alloy)
• High alloy steel

15
Classification by application

• Structural steel
• Pressure vessel steel
• Piping
• Deep drawing steel
• Engineering steels
• Ultra high strength steels
• Stainless steels
• Tool steels

16
Allotropy of iron
17
Phase

• Physically distinct and mechanically separable
  portion of a material, often with boundaries
• The atomic structure of a phase is uniform
• If in equilibrium, the composition is uniform
• Gas - only one phase. Different gases mix
• Liquid - multiple phases can occur
• Oil and water separate into 2 phases
• Solids - can have multiple phases in fine mixtures

18
Equilibrium phase diagram
Pressure

• Shows phases present over a range of
  compositions, temperature and pressure
• A point in the diagram represents the phases
  present under a set of conditions

LIQUID
SOLID
100 kPa
GAS
100C
0C
Temperature
19
Phases in iron
LIQUID
1535C
FERRITE (delta)
1390C
AUSTENITE (gamma)
AUSTENITE Face centred cubic (fcc)
910C
FERRITE (alpha)
FERRITE Body centred cubic (bcc)
20
Iron-carbon diagram
1535C
Delta ferrite
Liquid
1390C
Temperature
Austenite liquid
Austenite
1130C
1.7
4.3
910C
Austenite Fe3C
AF
Alpha ferrite
723C
0.83
Ferrite Fe3C
Percentage Carbon
0.02
21
Iron-iron carbide diagram
g
a g
g Fe3C
a
a Fe3C
22
Transformation to pearlite

• Eutectoid transformation
• Nucleation and growth process
• Controlled by diffusion of carbon
• More undercooling (more rapid cooling)
• Slower diffusion
• More nuclei
• More pearlite colonies of a finer lamellar spacing

23
Structure of pearlite
Pearlite (0.8 C average)
Cementite (6.67 C)
Ferrite (0.02 C)
24
Pearlite microstructure
Alternate plates of carbide and ferrite.
Normally the structure cannot be resolved.
Particularly slow cooling has been used.
10 mm
25
Iron-iron carbide diagram
A3
g
a g
g Fe3C
a
A1
a Fe3C
26
Hypoeutectoid steel

• Ferrite forms in austenite once cooled below A3
• Ferrite has low carbon, so as it forms the carbon
  level of the remaining austenite increases
• When the temperature falls below A1
• Carbon content of the remaining austenite is
  eutectoid level (0.83)
• The remaining austenite transforms to pearlite

27
Hypoeutectoid steel microstructure
Ferrite (white) pearlite (black)
Ferrite Austenite (yellow)
28
0.5 Carbon Steel
Pearlite islands surrounded by ferrite. The
ferrite shows the prior austenite grain boundaries
29
Annealing and normalising

• About 50C above Ac3 for hypoeutectoid steels
• Or 50C above Ac1 for hypereutectoid steels
• No cold work necessary
• Full Anneal - furnace cool (pearlitic structure)
• Normalise - air cool (finer pearlitic structure,
  higher strength)

30
Sub-critical or process anneal

• Hypoeutectoid steels only
• Must be cold worked
• Below Ac1 (500 to 600C)
• Removes effects of cold work by recrystallisation
  of ferrite.

31
Process annealed steel
Process annealed steel
Cold worked steel
32
Effect of pearlite on properties
UTS (Rm)
900 MPa
UTS
700 MPa
500 MPa
300 MPa
100
cementite
ferrite
pearlite
0
0
0.2
0.4
0.6
0.8
1.0
1.2
carbon
33
Effect of rapid cooling of austenite
34
Isothermal transformation

• Heating to austenite temperature
• Rapid cooling to transformation temperature
• Tin or lead bath
• Maintaining at transformation temperature
• Some transformations are time dependent
• Others are not

Temp Deg C
Time
35
Isothermal transformations

• If transformation is just below A3
• Few nuclei of the transformation products are
  present
• The energy to transform is low
• Coarse structure forms slowly
• As degree of undercooling increases
• More nuclei are created
• Transformation in more rapid
• Finer structure forms more rapidly

36
Isothermal transformation diagram
Austenite
A F P
Upper bainite
A B
Lower bainite
A M
Martensite
37
Formation of martensite

• High cooling rates - quenching
• Rapid cooling with water or oil
• Diffusion of carbon inhibited
• Neither ferrite nor pearlite have sufficient time
  to form
• Shear transformation
• Body centred tetragonal structure
• Carbon evenly distributed

38
Martensite
Martensite
Retained austenite
Martensite formed by shear transformation in an
austenite grain
39
Martensite
Martensite is created when austenite transforms
by a shear mechanism instead of by nucleation and
growth. The shear transformation products are
acicular. In this case, because of high carbon
(1.2) there is retained austenite.
40
Martensite properties

• Can be considered to be ferrite supersaturated
  with carbon
• Body centred tetragonal structure
• High strength (up to 2000 MPa) and hardness (900
  HV)
• Can be brittle
• Strength increases ductility reduces with
  increasing carbon content
• Effect of other alloy elements on properties is
  minimal (but not on hardenability)

41
Factors promoting martensite

• High cooling rate
• Increasing hardenability
• Hardenability is dependent on carbon and other
  alloy elements
• The desired cooling rate ensures transformation
  is to completely to martensite but avoids quench
  cracking

42
Cooling rate depends on

• Component thickness
• Component shape
• Equivalent cooling rates for shape variations
• Quenching medium
• Agitation is important
• Brine quench. Fastest cooling rates
• Water quench. Cheap and fast, spray or immersion
• Oil quench. Intermediate cooling rates. Flammable
• Air cool. Relatively slow, suits thin sections

43
Hardenability measurement

• Hardenability - the thickness of steel which will
  harden to martensite
• Length of an end-quenched bar which is hardened
• Jominy test
• Section thickness that can be through-hardened
• Low hardenability, only thin materials

44
Hardenability determination

• Carbon equivalent formulae
• Bar diameter which will through-harden to 50
  martensite in centre, DI.
• Depends on cooling rate
• Critical diameter using ideal theoretical fastest
  possible quench (violently agitated brine) DC
• Depends on austenite grain size

45
Tempering

• Heating martensite to between 100 600C
• Softens toughens martensite
• Effects dependent on temperature and include
• Stress relief
• Epsilon carbide precipitates from martensite
• Cementite precipitates from martensite
• Epsilon carbide converts to cementite
• Retained austenite transforms to bainite

46
Effect of tempering

• Increases ductility and toughness
• Reduces hardness and strength
• Solid solution elements have little effect on
  tempering
• Ni, Si, Al, Mn
• Strong carbide formers raise tempering
  temperature for equivalent hardness
• Cr, Mo, V

47
Constituents of Steels

• Ferrite, Alpha (a)iron, Delta iron (d)
• Ferrite is the body-centred cubic (bcc) phase
  that pure iron exists as at temperatures up to
  910C. Pure iron can also exist as ferrite at
  high temperatures, between 1392C and the melting
  point of 1536C. The low temperature form is
  known as a ferrite and the high temperature form
  d ferrite, but the two forms are identical.
  Ferrite is soft and ductile when pure. Alpha
  ferrite can dissolve only 0.02 carbon, but may
  contain significant levels of other alloy
  elements. Pure ferritic steels are rare.

48

• Austenite, Gamma (g) iron
• Between 910C and 1392C pure iron exists as
  austenite, which has a face-centred cubic crystal
  structure. Austenite, also known as g, can
  dissolve much higher levels of carbon than
  ferrite, up to 2 at 1146C, so heating a carbon
  or low alloy steel to temperatures at which it is
  fully austenitic dissolves all the carbon. This
  is the key to heat treating these steels.
  Although this phase is stable only at high
  temperatures in low alloy and carbon steels, some
  alloy steels are austenitic at normal
  temperatures. Austenite is also relatively soft
  and ductile.

49

• Cementite, Iron carbide, Fe3C.
• Cementite is a compound of iron that can form a
  stable phase in steels and irons. It consists of
  one atom of carbon with three of iron, and has a
  carbon content of 6.67. As a pure phase,
  cementite is very hard (over 600 HB) and brittle.
  Ferritic steels contain carbon as cementite, and
  the strength and ductility of steel is affected
  by the amount and distribution of the cementite.

50

• Pearlite
• Pearlite is a two-phase mixture of ferrite and
  cementite arranged as alternating parallel
  plates. It always contains fixed amount of carbon
  (0.83 in carbon steel) and forms as a result of
  an eutectoid reaction when austenite is cooled.
  The spacing of the plates (lamellae) is very fine
  and is usually not seen on a light microscope,
  even with high magnifications. Pearlite is hard
  and strong, although not as ductile as pure
  ferrite or austenite, nor as strong as
  martensite. Its presence in steel confers
  strength.

51

• Martensite
• Martensite is a metastable phase formed when
  austenite is cooled so rapidly that carbide
  precipitation is suppressed. This occurs when
  carbon or low alloy steel is quenched (cooled
  rapidly by dipping in a fluid) from high
  temperature. The rate of cooling is dependent on
  the type of quenchant (brine, water, oil or air)
  and on the objects thickness. The minimum rate
  of cooling to transform to martensite depends on
  the alloy content of the steel, and is known as
  its hardenability. Martensite forms by an
  instantaneous process of microscopic shear. When
  steel is quenched to martensite it is in its
  hardest and strongest condition, although the
  ductility and toughness is often low. The
  strength and hardness are determined by the alloy
  content, in particular the carbon level. The
  ductility and toughness of martensite can be
  improved by tempering, which is heating in the
  range 150C to 700C. This allows some stress
  relief and carbide precipitation. Higher
  tempering temperatures result in more ductility
  at the expense of hardness.

52

• Bainite
• Bainite is a two-phase structure that can be
  formed in carbon steel by rapid cooling austenite
  to between 400C and 550C, followed by a time
  holding at this temperature. This is generally
  done by quenching into a bath of liquid tin or
  lead and holding the steel in this bath.
  Transformation to bainite is by precipitation of
  carbides in a very fine feathery or lath
  configuration. Some alloy steels can transform to
  bainite on continuous cooling, rather than by the
  rather impractical process used for carbon steel.
  The properties of bainite are similar to tempered
  martensite. Lower bainite (formed at lower
  temperatures) is extremely tough. Upper bainite
  (formed at higher temperatures) is a coarser
  structure and is not as hard or ductile as lower
  bainite. The matrix is ferrite in steel which is
  isothermally transformed but may be martensite in
  alloy steel which is continuously cooled.