Mech 473 Lectures
Professor Rodney Herring
Quenched Steels
A quenched steel is composed of:
•
Martensite – dark needles
•
Retained austenite – light matrix regions in micrograph
Quenched Steels
A quench steel is subject to residual stresses caused by:
•
Uneven contraction due to sudden hardening of the exterior
•
Expansion of the interior, which later transforms to
martensite
•
The transformation expansion of austenite to martensite is
greater than the thermal contraction.
Quenched Steels
If the stress exceeds the yield point, then the plastic
deformation causes warping.
If the stress exceeds the fracture stress then quench cracks will
occur at the surface
martempering or marquenching – the austenite is cooled to
just above Ms and held until the temperature equalizes in the
steel. Subsequent quenching permits all the steel to transform
to martensite at about the same time.
This prevents quench cracks to form at the surface due to the
surface transforming to martensite before the interior of the
steel, which puts the surface in tension above the yield strength
and the center in compression.
Martempering
Properties of Quenched Steels
Martensite and retained austenite are both metastable phases and
break down to form more stable constituents when a steel is
aged, ie., held for a long period at low temperature (few
hundred of degrees to room temperature).
As the phases formed during ageing involve changes in specific
volume, quenched martensitic steels exhibit dimensional
changes over a period of time, which renders them unsuitable
for a large number of applications.
In addition, although martensitic steels are harder than pearlite
steels, they are also considerably more brittle.
Properties of Quenched Steels
The combination of dimensional instability and brittleness results
in increased quench cracking, which again makes quenched
steels unsuitable for structural engineering applications.
Adding alloying elements that lower Ms (C > Mn > Mo > Cr > Si >
Ni > Cu ) increase the incidence of warpage and quench
cracking because of the increase in retained austenite.
High carbon steels are also more susceptible to quench cracking
because of the increased hardness of the martensite.
Quenched steels are thus given a heat treatment, known as
tempering (~200 oC), to eliminate the dimensional changes
and induce toughness.
Tempering of Quenched Steels
The martensitic reaction in carbon steels is NOT reversible at low
temperatures.
When a quenched steel is heated at a temperature below the
eutectoid,
it will be converted to a mixture of intermediate metastable
phases (eg. e-carbide, bainite), or,
if the temperature is high enough, it will transform to stable
ferrite plus cementite.
(pearlite)
(bainite)
Hardness Changes During Tempering
Hardness is increased at temperatures from RT to 100 oC due to
clustering of carbon at defects and the “coherent”
precipitation of e-carbide.
The hardness drops above 200 oC due to a lowering of the
carbon content in the martensite by the precipitation of
rods of cementite, but in contrast to e–carbide, this
precipitation is “not coherent”.
Hardness Changes During Tempering
The hardness falls further at higher tempering temperatures
but at a reduced rate due to two parallel processes:
1) Spheroidization and later particle growth of
cementite
2) Recovery, and later recrystallization, of ferrite
(formed from martensite) occurs.
Hardness Changes During Tempering
Precipitation of e–Carbide during Tempering
In the image below, freshly quenched martensite is not visible as they have tilted
surface facets, which are under oblique incident illumination in the
microscope.
Fine dispersion of e–carbide formed by tempering below 100 oC are not resolved
in the light microscope, but cause the martensite needles to become
darkened.
Tempered martensite at 500x mag
Formation and Growth of Cementite during Tempering
Rod-like structures of
cementite after 1 hr at 250 oC
Spheroidized cementite
formed after 1 hr at 500 oC
Globular particle growth of
cementite after 12 hrs at 675 oC
Effect of Carbon Content and Tempering Temperature on
Hardness of Martensite
The hardness values plotted
against carbon content
refer to the hardness of
quenched martensite
after tempering for 1 hr
at various temperatures.
These hardness changes
cover a wider range of C
composition than seen in
the earlier slide.
Effect of Alloying Elements on Hardness of Tempered
Martensite
All elements other than C in solution slow down the softening of a
steel during tempering. The alloying elements may enter
either the ferrite or the carbide phase in a tempered steel
structure, i.e., “partitioning”.
Elements such as Al, Cu, Si, P, Ni, Zr, do not form stable carbide
phases and are thus found only in the ferrite phase.
Other elements such as Ti > V > Mo > W > Cr > Mn form stable
carbides, based on cementite, with the formula (Fe M)3C.
These precipitates are found in both the ferrite and carbide
phases.
Tetragonal Fe3C Structure
- carbon
(Ti, V, Mo, W, Cr, Mn)*
- Fe
* - these elements also
partition into the Fe3C
phase.
Effect of Alloying Elements on Hardness of Tempered
Martensite
A high temperature hardening effect is observed at higher
temperatures > 540 C, when carbide forming elements are
present, due to the formation of much harder carbides of the
type M6C23.
This results in the secondary hardening as seen with Mo.
Quench and Tempered Treatments
The mechanical properties of a high carbon steel that is quenched
and tempered to produce a fine dispersion of cementite in a
ferrite matrix are superior to the mechanical properties of a
rapidly cooled steel with a fine pearlite structure.
By selecting an appropriate tempering temperature, a wide range
of properties can be produced.
Quench and Temper Treatments
The effect of tempering temperature on the properties of a
quenched 1050 steel is shown below.
Classification of Carbon Steels (Not Stainless Steels)
There are four main groups of carbon steels, which can be defined
by their carbon concentration.
1. Low carbon steels (> 80% of the steel production)
< 0.20 %C Used in normalized condition for structural
purposes.
2. Medium Carbon Steels
0.25 – 0.55 %C Used in heat treated conditions for machine
parts.
3. Eutectoids Steels
0.6 – 0.8 %C Used in normalized condition for hard
wearing rails.
4. Tools Steels
0.9 – 2.2 %C Used in heat treated condition for tools and
dies.
Classification of Carbon Steels
(Not Stainless Steels)
This is where we start talking about specific steel alloys.
We will discuss these four broad categories over the next
couple of lectures.
Ferrous Alloy Charts
Plain-carbon steels are designated most commonly by the American Iron and Steel
Institute (AISI) or the Society of Automotive Engineers (SAE)
A four digit code is used:
If the first two digits are 10, the steel is plain carbon steel. Other designations,
ie., 20s, 30s, etc. refer to major alloying additions (see next slide).
The last two digits indicate the nominal carbon content of the steel in
hundredths of a percent.
AISI
Designation
for Steels
We will be
discussing the
properties of many
of these steels.
American Society for Testing Materials (ASTM)
Specifications
Steels for defined structural purposes (buildings, bridges, etc.) are
usually made to ASTM specifications, which use a system of
numbering that is unrelated to chemical composition,
mechanical properties, or applications.
The numbers simply refer to the chronological order in which the
steels were developed and approved for inclusion in the ASTM
system.
Group 1 Steels: Low Carbon Structural Steels
The bulk for the steels used for industrial processes are plain
carbon steels that contain less than 0.20 %C, with
microstructures of ~80% ferrite and ~20% pearlite.
In order to minimize processing costs, these steels are not heat
treated. It is not practical to heat treat very large sections
used for bridges, ships and buildings.
Their strength is thus controlled by:
1) The carbon content
2) The grain size (via the Petch-Hall relationship s = so + kd-1/2)
As these steels are susceptible to the ductile-brittle transitions at
room temperature, the Mn content is increased to improve
toughness.
Controlled Hot Rolling Programs to Obtain Fine
Grain Size
Special rolling procedures are used to obtain a very fine grain size,
~ 5 mm, in low carbon steels in the absence of a heat
treatment.
The final hot rolling pass is performed around 828 oC, at which
temperature recrystallization of austenite does not occur, so
that the austenite grains are left in a deformed state at 727 oC.
Further, since the austenite grains are flattened by the rolling
deformation, the total austenite grain boundary area is
increased, leading to an increased number of ferrite
nucleation sites, which in turn decreases the ferrite grain size.
The effect of hot rolling to improve strength by lowering the
austenite grain size is illustrated in the next two slides for a 0.2
%C steel.
Grain Refining
Controlled Hot Rolling Programs to Obtain Fine Grain
Size
Ferrite
The additional increase in strength obtained by using alloying
elements is shown by the 0.2 %C HSLA steel (High-StrengthLow-Alloy (< 10% alloying additions), which was given the
same treatment.
Microalloyed Non-Heat Treatable Low Carbon Steels
In a hot rolled plain carbon steel, austenite can recrystallize at
temperatures as low as 760 oC, ie., at only 33 oC above the
eutectoid temperature.
To reduce the work involved, hot rolling is performed at
temperatures > 760 oC so recrystallization of austenite and
subsequent “grain growth is the norm in hot rolled plain carbon
steels”.
The time required to recrystallize austenite is greatly increased by
the addition of a little as 0.05 % of carbide forming elements
such as V, Ti and Nb.
Microalloyed Non-Heat Treatable Low Carbon Steels
The effect is shown for 0.1 %C steels rolled to 50 %RA at 954 oC with:
a) The addition of 2.0 %Mn, which is a non-carbide former
b) Additions of 0.03 %Nb + 0.00, 0.05, 0.20 %V, which are strong carbide
formers.
The carbide formers form fine particles of carbonnitrides – eg., NbCN –
which 1) pin the grain boundaries and 2) prevent recrystallization,
which 3) prevents grain growth.
Fe-0.1%C-2%Mn-0.03%Nb + …
Microalloyed Non-Heat Treatable Low Carbon Steels
Thus the alloying additions that are carbide formers can be used as
another tool by engineers to control the strength and ductility of
the steel.
In many instances, the steels are already strong enough for the
application. Ductility or fabricability is desired.
Thin Steel Sheets for Automobile Bodies
Automobile body stock is produced either from “rimmed steel” (not deoxidized) or
from “killed steel” (deoxidized with Al) – expressions used by industry.
The killing action also provides a method to control and reduce the grain size. It
gives lower strength but with increased elongation or ductility.
These general steels contain 0.05 – 0.15 %C and 0.2 – 0.6 %Mn with impurities
level of:
0.001 % (max) Si, 0.05 %(max) S and 0.02 %(max) P.
Thin Steel Sheets for Automobile Bodies
These steels have to be cold-formed into the desired shape and yet
retain a good surface finish, which requires a combination of low
yield and high tensile strength with a high degree of uniform
elongation.
The grain size is also critical (< ASTM 9 impairs cold forming and
> ASTM 7 gives an orange peel surface).
A sharp yield point means non-uniform elongation, which causes
surface roughness. So the sheet is tempered rolled to about 1%
reduction by “roller flexing”, which stretches each surface beyond
the yield point, before cold forming.
Tin Plate
The American automotive industry now extensively uses Snplated plain C sheet steel for autobodies to reduce corrosion.
Steel for tin plate contains 0.12 %(max) C, 0.2 – 0.6 %Mn, .01
%(max) Si, 0.05%(max)S, 0.02 %(max) P and 0.2 %(max) Cu
– essentially “The Tin Can” material.
The steel is first hot rolled and then cold rolled to the finished
gauge. After rolling it is process-annealed in coils and then
tempered-rolled and then electrolytically plated with tin.
Tin Plate
When a tin-can competition occurred with Al and plastic containers,
the cost of tin plate was reduced by annealing after rolling such
that a further 35% cold roll gave the final gauge, which enabled
thicknesses of 0.125 mm to be obtained, enabling it to have better
mechanical properties than Al and plastics.
Although this treatment increases the yield point to within 7 MPa of
the UTS and the final measured tensile elongation is less than 1%,
the sheet is still sufficiently ductile to flange the ends of can bodies
without fracture.
The reason for this apparent contradiction is that the tensile
elongation is measured over a gauge length of 2 inches (50 mm),
but the stretching just before fracture is localized to a much
smaller length of material, which makes it possible.
If a gauge length of 0.01 inches (0.25 mm) was used, the measured
elongation would be 40 %.
(see next page)
Tin Plate
Note the yield and UTS are almost the same.
Zinc Plated Low Carbon Steels
(galvanized steel)
• If Tin cracks, it’ll establish an electrolytic cell with the steel
and it will corrode the steel faster than the steel will corrode by
itself since Tin is a more noble metal than steel.
• For some applications, a more effective steel coating is Zinc,
which is less noble than steel but more difficult to coat because
of its low vapor pressure.
• At one time, galvanized steel pipes were preferred for a home’s
water system, however, they were only good for 20-30 years and
homes last much longer.
• They have now been replaced by Copper, which lasts longer and
is more flexible.
Zinc Plated Low Carbon Steels
(galvanized steel)
In 1999, Dofasco started up its 72"-wide hot dipped galvanizing
line joint venture with Arcelor of France.
• This hot dip coating technology now produces 450,000 tons per
year of corrosion-resistant galvanized steel primarily for
automotive applications and with a particular focus on exposed
auto body panels.
• This product is a cost-effective replacement for
electrogalvanized material for exposed automotive applications.
• This is another reason why Dofasco is capturing the America
automobile market for steel and American/Canadian made cars
are improving over foreign made cars.
What else did Dofasco develop that revolutionized the steel
industry?
Heavy Steel Plates for Bridges and Tanks
Steels for thick plates and pressure vessels contain:
0.18 – 0.35 %C, 0.06 – 1.25 %Mn and impurity levels of:
0.15 – 0.35 %Si, 0.04 %(max) S and 0.035 %(max) P
The higher Mn level gives improved strength without lowering
ductility and Cu is sometimes added to improve corrosion
resistance.
These steels are usually given no heat treatment after hot rolling,
apart from a normalizing treatment for sections greater than 5 cm
in thickness.
Can you recall a normalizing heat treat?
A1 is the eutectoid transformation temperature.
A3 is the ferrite proeutectoid temperature of formation.
Acm is the cementite proeuctectoid temperature of formation.
Summary of simple heat treatments for hypoeutectoid
and hypereutectoid steels.
Heavy Steel Plates for Bridges and Tanks
These steels have only moderate strength, but their particular
combinations of strength, ductility, toughness and “weldability”
are required for fabricating massive structures, eg., for ship
building.
A lower ductile-brittle transition temperature for low temperature
applications is obtained by:
1) using higher Mn content killed steels,
2) performing extensive hot work, and by
3) normalizing.
The effect of carbon and
heat treatment on the
mechanical properties
of plain-carbon steels.
Heavy Steel Plates for Buildings, Bridges and Pressure
Vessels
Hence, for improved low temperature toughness, steels such as A516 contains
more Mn than A515 and is specified to be in the normalized condition with a
grain size < ASTM 5 (i.e., 16 grains per inch2 at 100x magnification).
Steel A662 contains more Mn and less carbon and retains its toughness down to
-60 oC.
“70” refers to the strength of the steel in kips per square inch (ksp), an old
strength designation still used in the USA.
Heavy Steel Plates for Buildings and Bridges
White grains are
ferrite and dark
regions are
pearlite colonies.
A516 Grade 70 Heavy Plate Steel
Ferrite and fine pearlite occur after normalizing from 900 oC.
The low carbon and alloy content of this steel also restricts the
microstructure to ferrite and pearlite in quenched sections that
are > 5 cm thick.
The End
(Any questions or comments?)