FIELD OF THE INVENTION
The present invention relates to a method for determining optimum cooling rate for
attaining toughness of hardenable steel during Quenching treatment using
Magnetic coercive force technique.
BACKGROUND OF THE INVENTION
Tubular components of boiler system are fabricated by bending the components to
produce coils like super heaters, re-heaters, economizers, water walls. Out of the
various available processes, the cold bending is preferred, because it is very fast and
can produce quality bends with good integrity. Impact testing of tubes as-bent using
the cold bending process is an important quality control step in the fabrication
procedure. As per ASME section I, the bends above predefined bend radius, bends
are required to be heat treated (quenching followed by tempering) for restoring the
toughness properties. In this case, the normalizing cooling quenching rates will
decide the toughness properties after tempering.
Magnetic Coercive Force (MCF) is a property of such ferromagnetic steels, which '
denotes the intensity of the applied magnetic field required to reduce the
magnetization to zero after it is magnetized to saturation. Magnetic Coercive Force
measures the resistance of a ferromagnetic material to becoming demagnetized. It is
the structure sensitive property and is affected by the presence of crystallog raphic
defects. This property is known to change with respect to mechanical and thermal
processing conditions of materials. By exploiting this magnetic coercive force, the
optimum cooling rate can be determined using magnetic coercive force technique.

EP 1308721A1 teaches a method for detectingmagnetic properties of a metal object,
in particular for detecting phase transformation of a hot rolled metal strip. .
EP2506003A1 describes methods and apparatus for inspecting platesor pipe walls of
magnetisable materialand in particular methods and apparatus for using magnetic
reluctance to gain data about the plate or pipe wall being inspected.
US patent 2211017 refers to a method ofthe heat treatmentof steel and in particular
relates to an arrangement for controlling heat treatment process to result in a
uniform predetermined standard hardness value.
US patent US 4335352 discusses the method of non-destructive measurement of the
properties of materials, and more specifically for ferrous materials magnetic
properties correspond to yield strength.
OBJECTS OF THE INVENTION
It is therefore an object of the invention is to propose a method for determining the
optimum cooling rate in normalizing treatment to obtain optimum toughness after
tempering.
SUMMARY OF THE INVENTION
According to the invention, an experimental method for determination of optimum
normalizing cooling rate to obtain adequate toughness after tempering. Creep
strength enhanced steels used for boiler applications due to high temperature creep
resistance combined with toughness properties. During fabrication these steels are
required to undergo cold bending and welding processes. To restore mechanical
properties, steels required to be renormalized and tempered after fabrication. The
optimum cooling rate in normalizing treatment will result in the best toughness
properties followed by tempering. To determine the optimum cooling rate, new
methodology was developed based on magnetic coercive force measurement;
magnetic property dependent on crystallography variations of the materials.

The present invention relates to a method of determining the optimum cooling rate
for obtaining suitable toughness properties. Further the invention teaches a method
of optimization of cooling rate to produce the toughness properties of tube bends
during post bending heat treatment. According to the invention the heat treatment
cooling rate is optimized to get the desired toughness properties using magnetic
coercive force method.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Magnetic coercive force measurement on a sample
Figure 2 -Sample graph for normalizing cooling rate vs. Toughness after temper for
SA 213 T23 steel.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
In this invention, a method for determining optimum normalizing cooling rate to
obtain adequate toughness after tempering is proposed. Creep strength enhanced
steels are widely used for boiler super heaters, re-heaters, economizers and water
walls. These steels were selected due to high temperature creep resistance
combined with best toughness properties. During fabrication, these steels are
required to undergo cold bending and welding processes. Such fabrication
techniques may alter mechanical properties. To restore mechanical properties, steels
required to be renormalized and tempered after fabrication. Normalizing helps in
refining the existing non-uniform structure, the rate of refining is dependent on
chemical composition, temperature of normalizing, soaking time and cooling rate
employed. Temperature and soaking time control can be obtained by equipment

employed for the heat treatment. Cooling rate may change with environment. The
optimum cooling rate will result in the best toughness properties followed by
tempering. To determine the optimum cooling rate, new methodology was
developed based on magnetic coercive force measurement.
Materials may contain a finite amount of dislocations, material defects like voids,
inclusions etc. spread through the bulk volume of the material. During the
application of an external magnetizing force, the initially randomly oriented magnetic
domains align themselves along the direction of the applied magnetic field by way of
movement of domain walls. These magnetic domain walls have to cross the locations
containing different crystal defects mentioned above. The existence of material
defects like voids, cracks, inclusions and grain boundaries etc. impede the free
movement of domain walls. So, the mobility of domain walls is reduced at such
locations in the material. If the domain walls were free to move without any
hindrance, then the material magnetizes and demagnetizes easily and shows a lower
coercivity. Whereas, if there happens to be more of crystal defects said earlier, then
they would be hindering the movement of domain walls during magnetization and
demagnetization cycle, thereby leading to higher magnetic coercivity.
In normalizing, fine grain structure is relatively stress free and the mechanical
properties generally show a decrease in overall hardness, altered ductility and
increased overall toughness. The changes in micro structure brought about in
normalizing are known to influence magnetic properties. In particular, the magnetic
coercive force property bears a very good correlation with the impact strength
property after tempering of the steel.
For instance, ASME grade of SA 213 T23 steel which contains 2% W and micro
alloying elements is used for boiler tubing applications. The optimization of
normalizing cooling rate is very important step to obtain toughness after tempering
treatment. To arrive at the optimum cooling rate which will give the best toughness,
it required to plan many experimental trials. This presently invented method helps to

reduce the number of trials and faster inspection method to avoid failure. A number
of specimens were normalized using gleeble (Thermo Mechanical Simulator) with
various cooling rates followed by tempering treatment in the furnace. In all the
specimens, magnetic coercive force was measured as shown in figure 1, in the as-
normalized and post tempering treatment conditions. These samples were subjected
to charpy impact energy measurements. The Authors have invented that, there is a
cut off magnetic coercive force value to obtain best toughness property. This trend
is indicated in figure 2.
Based on this figure 2, whenever, MCF value is exceeded more than the cut off
value, material will have the adequate toughness. For ASME grade of SA 213 T23
materials, the authors have invented that in the as-normalized condition if the MCF
exceeds 13 Amp/Cm, then the material will have a toughness of at least 50 Joules
after tempering treatment.

WE CLAIM :
1. A method for determining optimum cooling rate for attaining toughness of
hardenable steel during quenching treatment using Magnetic coercive
force technique comprising :
the MCF property of the material with toughness property;
linking the magnetic coercive force (MCF) value of a selected material
capable of attaining high toughness and applicable for boiler tubing;
wherein the MCF value of the selected material is measured under the
condition of as-normalized and post-tempering treatment stage, wherein the
material is subjected to charpy impact energy measurement, and wherein a
cut-off magnetic coercive force value is obtained which provides the best
toughness property.
2. The method as claimed in claim 1, wherein the process is applicable also
for Bainitic and Martensitic grade steels.

ABSTRACT

The invention relates to a method for determining optimum cooling rate for
attaining toughness of hardenable steel during quenching treatment using Magnetic
coercive force technique comprising the MCF property of the material with toughness
property; linking the magnetic coercive force (MCF) value of a selected material
capable of attaining high toughness and applicable for boiler tubing; wherein the
MCF value of the selected material is measured under the condition of as-normalized
and post-tempering treatment stage, wherein the material is subjected to charpy
impact energy measurement, and wherein a cut-off magnetic coercive force value is
obtained which provides the best toughness property