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Effect of Carbon content on Steel Hardness
1
Carbon wt %
10xx Plain Carbon steels
11xx Resulfurized for
machinablity
12xx Resulfurized and
rephosphorized
Manganese
13xx Mn 1.
15xx Mn 1.00 - 1.
Nickel
23xx Ni 3.
25xx Ni 5.
Nickel Chromium
31xx Ni 1.25 Cr 0.65-0.
32xx Ni 1.75 Cr 1.
33xx Ni 3.50 Cr 1.50-1.
34xx Ni 3.00 Cr 0.
Chromium Molybdenum
41xx Cr 0.50-0.95 Mo 0.12-0.
Nickel Chromium
Molybdenum
43xx Ni 1.82 Cr 0.50-0.80 Mo
47xx Ni 1.05 Cr 0.45 Mo 0.20 –
86xx Ni 0.55 Cr 0.50 Mo 0.
Nickel Molybdenum
46xx Ni 0.85-1.82 Mo 0.
48xx Ni 3.50 Mo 0.
Chromium
Effects of Alloying Elements
on Steel
2
- Manganese contributes to strength and hardness; dependent upon the
carbon content. Increasing the manganese content decreases ductility and
weldability. Manganese has a significant effect on the hardenability of steel.
- Phosphorus increases strength and hardness and decreases ductility and
notch impact toughness of steel. The adverse effects on ductility and
toughness are greater in quenched and tempered higher-carbon steels.
- Sulfur decreases ductility and notch impact toughness especially in the
transverse direction. Weldability decreases with increasing sulfur content.
Sulfur is found primarily in the form of sulfide inclusions.
- Silicon is one of the principal deoxidizers used in steelmaking. Silicon is
less effective than manganese in increasing as-rolled strength and
hardness. In low-carbon steels, silicon is generally detrimental to surface
quality.
- Copper in significant amounts is detrimental to hot-working steels. Copper
can be detrimental to surface quality. Copper is beneficial to atmospheric
corrosion resistance when present in amounts exceeding 0.20%.
- Nickel is a ferrite strengthener. Nickel does not form carbides in steel. It
remains in solution in ferrite, strengthening and toughening the ferrite
phase. Nickel increases the hardenability and impact strength of steels.
- Molybdenum increases the hardenability of steel. It enhances the creep
strength of low-alloy steels at elevated temperatures.
Classification of Metal Alloys
Metal Alloys
Steels
Ferrous Nonferrous
Cast Irons
<1.4wt%C 3-4.5 wt%C
Steels
<1.4 wt% C
Cast Irons
3-4.5 wt% C
Fe
3
C
cementite
1600
1400
1200
1000
800
600
400
0 1 2 3 4 5 6 6.
L
austenite
+Fe
3
C
ferrite
+Fe
3
C
L +Fe
3
C
(Fe)
C
o
, wt% C
Eutectic:
Eutectoid:
727°C
1148°C
T (°C)
microstructure: ferrite,
graphite/cementite
Cast Iron
Wide range of applications (including pipes, machine and car parts,
such as cylinder heads, blocks and gearbox cases) due to:
low melting point,
good fluidity,
relatively easy to cast,
excellent machinability,
resistance to deformation,
and wear resistance
Cast iron tends to be brittle, except for malleable cast irons, so
shaping these by deformation is very difficult.
It is resistant to destruction and weakening by oxidization (rust).
Cast iron coated
with durable
porcelain enamel
distributes heat
slowly and evenly.
8
Grey Cast Iron
Grey cast iron is named after its grey
fractured surface that occurs when the
graphitic flakes deflect a passing crack and
initiate many new cracks as the material
breaks.
graphite flakes surrounded by -ferrite or
pearlite matrix
weak & brittle in tension (the graphite
flake tips are sharp; act as stress raisers)
stronger in compression
excellent vibrational dampening
wear resistant
Carbon content: 3.0 – 4.0 wt%
Silicon content: 1.0 – 3.0 wt %
Modifying silicon content and cooling rate
affects microstructure.
Casting shrinkage is low
grey
grey
Nodular (Ductile) Cast Iron
Adding Mg and/or Cerium to grey iron
before casting produces a distinctly
different microstructure and mechanical
properties.
graphite forms nodules not flakes
Normally a pearlite matrix
Photo (nodular) shows ferrite matrix
that was heat treated for several hours
at 700˚C.
Castings are stronger and much more
ductile than grey iron.
grey
nodular
nodular
White Cast Iron
White cast iron is named after its
white surface when fractured due
to its carbide impurities that allow
cracks to pass straight through;
the crystalline fractures are shiny
compared to the dull gray
fractures of graphite irons.
< 1 wt% Si, rapid cooling rates
pearlite + most of the carbon
forms cementite, not graphite.
very hard and brittle;
thickness may result in
nonuniform microstructure from
variable cooling; white iron
develops from faster cooling;
slower cooling rate yields grey
iron.
limited applications; used as
intermediate to produce malleable
cast iron.
13
Fe-C True Equilibrium
Diagram
Graphite
formation
promoted by
Cementite decomposes to ferrite + graphite
Fe
C 3 Fe () + C (graphite)
Variety of Cast Iron Microstructures
G
f
, graphite flake
G
r
, graphite
rosettes
G
n
, graphite
nodules
P, pearlite = +
cementite
, ferrite
