Heat Treatment and its Applications for
Rehabilitating Steel Bridges in Indiana
Amit Varma and Matt Lackowski
School of Civil Engineering
Purdue University
11/12/2006
OUTLINE



Heat treatment and its relation to heat
straightening
Heat straightening survey findings
Feasibility study report
Heat Treatment






Introduction
Steel Metallurgy
Steel Micro-Constituents
Effects of Plastic Deformations
Effects of Heat Treatment
Implications
Introduction



Heat treatment is used to change the metallurgy
and the related material, structural, and surface
properties of steel
Different heat treatment processes have different
effects on the resulting microstructure and
structural properties including strength, fracture
toughness, and ductility of steel.
Heat treatment has commonly been used during
the steel manufacturing process to improve the
quality or properties of steel, but it can also be used
to repair damage or rehabilitate older or used steel.
Steel Metallurgy: LC vs. HSLA steel

Low-carbon steels contain up to 0.25% carbon along with
various other elements


High-strength low-alloy (HSLA) steels have lower carbon
contents (0.05-0.25% C)



manganese (up to 1.65%), sulfur (up to 0.05%), phosphorous (up
to 0.04%), silicon (up to 0.60%), and copper (up to 0.60%)
Manganese contents up to 2.0%, and small quantities of
chromium, nickel, molybdenum, copper, nitrogen,
vanadium, niobium, titanium and zirconium added in
various combinations
High-Strength Low-Alloy steels provide better mechanical
properties and/or greater resistance to atmospheric
corrosion, better formability, and weldability.
The microstructure of low-carbon and HSLA steel is
similar in terms of its micro-constituents

The difference is that HSLA steels typically have smaller
grain sizes.
Steel Metallurgy: Phase Transformation
Low-carbon and HSLA steels contain less than 0.8%
carbon, which classifies them as hypoeutectoid steels
Hypoeutectoid steels consist of pearlite and proeutectoid ferrite
1000

Ac3 Temp. Line

  FCC structure






 BCC structure
800
1500


0.022
0.76



Ac1 Temp.
Line

600
1000
Pearlite

400
(Fe)
Composition (wt% C)
1
Temperature ( F)

Temperature ( C)

Steel Metallurgy: Phase Transformation

At high temperatures exceeding the Ac3
temperature, the hypoeutectoid structure is in a
stable -austenite phase

As the steel cools below the Ac3 temperature:



Proeutectoid ferrite will nucleate and grow at the
austenite grain boundaries
The amount of proeutectoid ferrite formed will
continue to grow and the austenite will increase in
percent carbon content.
At the Ac1 temperature (approximately 1340F):

The remaining austenite will transform into pearlite. It
consists of both ferrite and cementite (Fe3C) layers
Steel Metallurgy: Alloy Effects

In HSLA steels, the addition of different alloying elements
changes:




the physical and structural properties
the eutectoid temperature (Ac1)
the temperature that defines stable austenite (Ac3)
Empirical equations relate the effects of alloying elements
to the critical transformation points


Manganese (Mn) and nickel (Ni) are distinguished as
austenite-stabilizing elements that lower the eutectoid
temperature expanding the temperature range over which
austenite is stable
Silicone (Si), chromium (Cr), molybdenum (Mo), arsenic
(As), niobium (Nb), and tungsten (W) are distinguished as
ferrite stabilizing elements which raise the eutectoid
temperature of the steel and reduce the range over which
austenite is stable
Steel Metallurgy: Grain Size


The grain size focuses on the size of the a-ferrite grains.
HSLA steels usually have smaller (fine) graints
Fine grains are preferred over coarse (large) grains
because:




Higher strength
Slightly increased ductility
Better fracture toughness
Finer-grained steels have more grain boundaries that act
as barriers to dislocations



A higher density of grain boundaries will produce higher yield
and tensile strengths
Plane strain fracture toughness normally increases with a
reduction in grain size when composition and other
microstructure variables are maintained constant
Decreasing the grain size significantly decreases the transition
temperature governing the ductile to brittle fracture
Steel Micro-Constituents


When steel is cooled from temperatures higher than the
phase transformation temperatures (Ac1 or Ac3), a
number of micro-constituents form depending on the
cooling rate
Two types of time-temperature-transformation (TTT)
diagrams are used to predict the formations of different
micro-constituents in steel after cooling below the Ac1
temperature


An isothermal TTT diagram, which plots temperature vs.
logarithm of time for steel alloys of definite composition. It
is used to determine when transformation begins and ends
for an isothermal (constant temperature) heat treatment.
A continuous cooling transformation (CCT) diagram is a
plot of temperature vs. logarithm of time for a steel alloy of
definite composition. It is used to determine when
transformations begin and end when a previously
austenized alloy is cooled continuously at a specified rate.
Superimposed Continuous Cooling
Diagram of a AISI (0.12%C) Steel
1600
Ac3
A (Austenite)
Ferrite Line
1400
Ac1
Pearlite Line
A+F
Temperature (F)
1200
A+F+P
A = Austenite
B = Bainite
F = Ferrite
M = Martensite
P = Pearlite
1000
Bainite Line
A+F+B
800 Martensite
Line
1540 740
200
125
36
Cooling Rate (ºF/s)
600
M
400
0.1
M+B F+B+M
F+P+
1
B+M
F+P+B
10
F+P
Time (s)
Resulting Microconstituents
100
1000
10000
Steel Micro-Constituents: Martensite



Martensite is formed from austenized iron-carbon alloys
that are rapidly cooled or quenched to a relatively low
temperature
For alloys containing less than 0.6 wt% Carbon (LC and
HSLA), the martensite grains form as long and thin plates
that are aligned side by side and parallel to each other.
Martensite is the hardest, strongest, and the most brittle
form of micro-constituent for an iron-carbon alloy

It has extremely low ductility and fracture toughness

The formation of martensite during heat treatment of a
steel bridge needs to be strictly avoided.

Bainite is extremely difficult to form. It is usually only seen
in labs rather than as the results of manufacturing or heat
treatment process
Steel Micro-Constitutents: Pearlite

Pearlite


Consists of layers of -ferrite and cementite (Fe3C) layers
Cementite is much harder and more brittle than ferrite


Pearlite increases the hardness, yield strength, and ultimate
strength of steel but decreases the ductility and toughness
Dependent on the cooling period, the resulting pearlite
may be fine or coarse grained

Coarse-grained pearlite will form upon longer cooling
periods. Fine pearlite is harder and stronger, but more brittle.
Effects of Plastic Deformations on
Steel Microstructure

Plastic deformation can be produced by cold-working or
hot-working


Plastic deformations are produced by the movement of
individual crystal defects called dislocations (ASM 1973)
A very large number of dislocations exist in deformed metal
 The crystallographic plane along which the dislocation line
transverses is the slip plane
 A slip line represents a transfer of the material on opposite
sides of the slip plane
Due to random crystallographic orientations, the direction of
slip varies from one grain to another
Plastic deformations should result in slip planes and grain
elongation along the direction of extension.



The change in microstructure produced by both are
different. One involves elevated temperature effects.
Plastic Deformation and Microstructure
Plastic Deformation and Microstructure
Plastic Deformation and Microstructure
Heat Treatment and Microstructure

Heat treatment of damaged steel is different from heat
treatment of undamaged steel




The damaging process changes the microstructure by
forming several slip planes
These planes act as grain boundaries, and allow greater
effects of heat treatment
Additionally, some new processes like recovery,
recrystallization, and grain growth occur
As such there are several HT processes. The two that
are relevant to this study are:


Process Annealing
Normalizing Annealing
Heat Treatment and Microstructure

The most commonly used heat treatments include:




Soft or spheroidizing annealing, tempering, and isothermal
annealing which are used to improve the ductility of new steel
(formed by quenching)
Isoforming, low and high-temperature thermomechanical
treatments which are used to modify steels
Process annealing or normalizing annealing which are used
to relieve the effects of cold working, residual stresses, etc.
These heat treatment processes were reviewed to identify
their maximum heating temperature, heating duration,
time-temperature path, mechanical loads, etc.
Heat Treatment and Microstructure
Deformation (HTMT)
1600
Stable A
Ac3
1400
Ac1
Temperature (F)
Begin Transformation
Metastable A
End Transformation
F+A+P
Deformation ( Isoforming )
1200
F+P
1000
Deformation
(LTMT)
800
F+A+B
F+B
A = Austenite
F = Ferrite
B = Bainite
P = Pearlite
Martensite Line
Series1
Series2
Series3
Ms
Pearlite-Banite Line
600
0.0
0.1
1.0
10.0
100.0
1000.0
10000.0
Time (s)
Figure 320 Thermomechanical treatments on an isothermal transformation diagram
(Key to Metals 2004)
Heat Treatment and Microstructure

Process annealing is applied to hypo-eutectoid steels (with
up to 0.3% Carbon) by heating it to temperatures in the
range of 500-650 C, which are below the Ac1 phase
transformation temperature.




The steel is held at these temperatures for the necessary time, and
cooled at a desired rate.
Process annealing is referred frequently as stress-relief or recovery
treatment since it partially softens cold-worked (inelastic deformed)
steels by relieving internal stresses (residual stresses)
This process does not cause any phase changes (i.e. changes to
austenite), but it induces recovery and recrystallization of the
damaged steel microstructure, which will slowly eliminate the
effects of damage (slip bands etc.)
Closely related to heat straightening with Tmax = 1200F

The holding time is the major difference.
Heat Treatment and Microstructure

Process annealing is closely related to heat straightening
because they both focus on relieving or repairing coldworked material, and similar temperature ranges are used
to achieve the desired outcome.



Process annealing requires temperature soaking (holding) times
from several minutes to hours.
Heat straightening involves subjecting the damaged material to
several heating cycles. In each heating cycle, the damaged
material spends some time between the temperature range of 500650C.
This time duration cumulates over several heats to subject the
damaged material to a reasonable duration of heating in the
process annealing (500-650C) temperature range.
Heat Treatment and Microstructure
a)
a)
Virgin
c)
A36-30-70-2
e)
A36-60-50-2
b)
d)
f)
Undamaged A588
b)
A588-20-25-1
A36-30-40-3
A36-60-25-3
A36-90-25-3
c)
A588-40-25-3
d)
A588-40-50-3
e)
A588-60-25-1
f)
A588-60-50-4
gure 321 Microstructures of undamaged and various damaged-repai red A36 Steel (240X) Figure 323 Microstructures of undamaged and various damaged-repaired A588 steel
Heat Treatment and Microstructure

Normalizing annealing subjects the steel to heating above
the phase transformation temperature (Tmax > Ac1)
followed by slow cooling




Heating above the Ac1 leads to the partial austenitization,
i.e., transformation of pearlite to austenite, and heating above
the Ac3 temperature leads to complete austenitization, i.e.,
transformation of both ferrite and pearlite to austenite.
The ferrite and pearlite microstructures reform as the
austenite microstructure steel is cooled slowly.
Normalizing annealing leads to considerable recovery and
recrystallization of the damaged microstructure, and results
in a refined grain sizes with uniform distribution.
This results in better structural properties and fracture
toughness values.
Heat Treatment and Microstructure

Heat straightening with overheating Tmax= 760 or 870oC leads to
normalizing annealing of the damaged steel.




Normalizing annealing requires temperature soaking (holding)
times from several minutes to hours (22).
Heat straightening repair involves subjecting the damaged material
to several heating cycles.
In each cycle, the steel is heated to the target temperature Tmax and
cooled continuously. The time spent above the Ac1 temperature
cumulates over several heating cycles to subject the material to
reasonable duration in the normalizing range.
Heat straightening with overheating can result in refined
microstructure producing better structural properties and fracture
toughess.
Heat Straightening with Microstructure
a)
Undamaged A36
c)
A36-60-50-3-1400
d)
A36-60-50-3-1600
e)
A36-90-25-1-1400
f)
A36-90-25-1-1600
Figure 327 Microstructures of undamaged and damaged-repaired overheated A36 Steel
(480X)
b)
A36-60-25
Recovery

Recovery is the relief of some of the stored internal strain
energy of a previously cold-worked metal by heat treatment.




The physical and mechanical properties of the cold worked steel
begin to revert to the properties that existed prior to cold-working
The rate of recovery is a thermally activated process that
decreases with increasing time and decreasing temperature
Early in the recovery process, some internal stresses are
relieved and the number of dislocations slightly reduces
As recovery proceeds, dislocation interaction results in an
increase of dislocation density as dislocation arrays are
formed.


These arrays constitute the walls of new cells (subgrains) of
recovered steel and have lower energy configurations than the
dislocation tangles, which made up the cold-worked metal
Recovery constructs the dislocations in a more stable
arrangement forming small angle grain boundaries.
Recovery

The temperature of grain recovery correlates with the
recrystallization temperature, which relates to the melting
temperature of the same material according to Equation
 TGR = TR – 300 = 0.4TM – 300 [C]




Where, TGR = temperature of grain recovery,
TR = temperature of recrystallization
TM = melting point
Assuming a melting temperature of 1400C (2550F), the
temperature of grain recovery and the recrystallization
temperature is computed as 260C (500F) and 560C (1040F),
respectively

This means that both recovery and recrystallization occur when
heat straightening is conducted within limits.
Recovery and Recrystallization

It is difficult to draw a clear division between recovery and
recrystallization as the two processes often overlap

Recrystallization refers to the formation of a new set of strainfree grains within a previously cold worked material.

Formation and growth occur in a deformed matrix of new grains,
which are distortion-free and appreciably more perfect than the
matrix after polygonization (Gorelik 1981).

An annealing heat treatment such as process annealing is
necessary for recrystallization to occur.

During recrystallization, the restoration of mechanical and
physical properties is completed
Recrystallization

The most important factors that affect recrystallization are





Amount of prior deformation (damage)
Temperature and time
Initial grain size
Composition of the metal or alloy
The recrystallized volume in the material increases during
annealing due to the growth of the nuclei, where the rate is
described by two parameters known as the rate of
nucleation, N, and the rate of growth, G.

These processes above depend on the amount of prior cold
deformation
Recrystallization


Amount of deformation :
 Beginning from a critical deformation, an increase in plastic
strain causes N and G to increase and therefore the rate of
recrystallization to grow
 The size of the grains at the end of primary recrystallization is
smaller after greater deformations
 Elevated temperatures and slower heating shifts cr towards
greater values
Heating temperature:


The recrystallization behavior is sometimes specified in terms of
a temperature (TM) at which recrystallization just reaches
completion in 1 hour
The TM of low carbon steel is approximately 1000F


A greater prior deformation decreases the temperature for recrystallization
The grains that form at the end of primary recrystallization at typical heating
rates become noticeably coarser with a further increase in temperature
Recrystallization

Time of annealing


The average rate of growth G, in contrast to N is independent
of annealing time until the growing recrystallization nuclei
begin to collide
It has also been established by direct observations that the
rate of growth does vary with time.


When deformations slightly exceed the critical value, the two
processes of growth of initial grains and growth of
recrystallization nuclei can occur in parallel.
Initial grain size


A finer grain size increases the area of grain boundaries,
which increases the probability of nucleation sites to form
A finer initial grain size accelerates the process of primary
recrystallization.
Recrystallization

Rate of Heating


Rapid heating to the recrystallization temperature prevents
full recovery prior to recrystallization and causes a large
driving force for recrystallization to remain
With rapid heating, higher temperatures are reached before
recrystallization has time to begin, which facilitates nucleation
of new grains


Higher the heating rate, the higher the recrystallization
temperature (Totten and Howes 1997)
The rate of heating when heat straightening is not beneficial
for recrystallization.
Grain Growth

After recrystallization is complete the strain free grains will
continue to grow if the steel specimen is left at elevated
temperatures

Grain growth is referred to as the increase in average grain
size of a polycrystalline material. It generally follows
recovery and recrystallization

Extended annealing at a high temperature can cause a few
grains to grow to a very large size, which is known as
secondary recrystallization or abnormal grain growth

It appears that long holding periods would be required for
the full process of recrystallization.
Survey Results
Feasibility Analysis







Identifying heating and control equipment required for
conducting heat treatment in the field
The capital expenses for purchasing such equipment,
the extent of control available, and the power
requirements at the site
Quality control and techniques for inspecting the heat
treatment performed
Examining the need for bridge lane closures or traffic
redirection while heat treatment is being performed
Issues related to fracture and non-fracture critical
bridges
Evaluating the reduced strength of the bridge during
the heat treatment for rehabilitation
Anticipating the mechanical properties of the steel after
rehabilitation
Identifying heating and control equipment




In order to repair a steel bridge girder to its proper shape
and maintain the steel’s material properties, it is
recommended that the steel be heat straightened and
then heat treated through two separate and distinct
processes
The processes are separated in this manner to minimize
working time in the field, to reduce overall costs, and to
maximize the steel shape integrity and steel properties
It is recommended that heat straightening practitioners
begin the heat straightening process as usual
In order to enhance the desirable steel properties, a heat
treatment process is then recommended
Identifying Heating and Control Equipment

In order to provide the heat treatment process, (one) flat
panel ceramic fiber heater will be needed, a control
device to regulate the temperature of the flat panel
ceramic fiber heater, and an electric generator to supply
the on-site power for this heater.

There are some regulations for the use of the flat panel
ceramic fiber heater that must be set in place to ensure
the safety of its operation:



Only one flat panel ceramic fiber heater be used per span
The heater should be moved to an adjacent heat
straightened area progressively until all of the heat
straightened steel bridge girder has been heat treated
This is done to prevent weakening of significant
portions of the bridge girder during treatment
Capital Expenses and Power Requirements

The power requirements at the site are derived from the
practical availability of electrical power by heat
straightening practitioners and by the flat panel ceramic
fiber heater device.

The recommended flat panel ceramic fiber heater size is:




40” X 16” X 2”
Under 20 lbs.
11.5 kW through a parallel series of a 240 volt connection.
The power requirement for the flat panel ceramic fiber
heaters is within the acceptable range of the heat
straightening practitioners.


Survey results indicate that the heat straightening
practitioners can easily provide an on-site generator
These generators can provide a significant level of electric
power to adequately run the heaters
Quality Control and Inspection

The heat energy from the flat panel ceramic fiber heater
will not be localized on the steel bridge girder as the
flame from the heat straightening torch



A flat panel ceramic fiber heater will have some heat
transfer through the section and length of the member
duerto conduction
This heat transfer will be more extensive as time goes by,
and must be evaluated
The flat panel ceramic fiber heater must maintain the
appropriate temperature as well as maintain the
appropriate distance (2’’-4”) between the surface of the
steel and the surface of the heater for the required time

The appropriate temperature, distance, and time have to
be designed prior to placement
Quality Control and Inspection

The effectiveness of the heat treatment must be
evaluated at a quantitative level.

The resulting micro-constituents and microstructure of the
steel must be examined and a variety of tests to determine its
material properties must be researched thoroughly
Transportation Management: Lane Closures

During the heat treatment, there may be a need to close the traffic
on the bridge close to the girder being treated.



The effects of heat treatment on the serviceability of the bridge should
be evaluated. The effects of heating on the strength and stiffness should
be considered in this evaluation.
The redundant nature of bridge design should allow for lane opening
The magnitude of dead load and traffic loading must be considered

It will not be necessary to close traffic under the bridge, but the
lanes used or head clearance available will have to be reduced to
allow adequate space for the heat straightening practitioners to
perform the job
 The main concern with allowing traffic under the bridge during
the heat treatment operation is the risk of falling objects (such as
the flat panel ceramic fiber heater)

Both of these risks present a dangerous situation for traveling
automobiles, and must be evaluated before major decisions
Fracture Critical Bridges



Whenever fracture of the bridge is a concern extra
precautions need to be made to reduce the bridges
susceptibility to this failure mode
A member that is non-redundant is deemed as fracture
critical because the fracture of this member during the
heat straightening or heat treatment process will force a
failure of the whole structure
In general, if the member satisfies the fracture critical
criteria for heat straightening than it should also be
acceptable for the same member to be heat treated.
Expected Properties after Heat Treatment

Upon completion of the recovery and recrystallization,
the steel material will regain it strength, ductility, and
fracture toughness.



The improvement in fracture toughness will be better than just for
heat straightening.
The yield strength and ultimate strength of the steel are
increased only slightly
Additional research is needed to verify these findings
Conclusion

Heat treatment in conjunction with heat
straightening will substantially benefit the
microstructure, material properties, and
fracture toughness of steels.