TITLE: A METHOD FOR DETERMINATION OF EFFECTIVENESS OF POST
WELD STRESS RELIEVING HEAT TREATMENT IN FERROMAGNETIC
MATERIALS THROUGH MEASUREMENT OF COERCIVE FORCE
FIELD OF INVENTION
The present invention relates to a Non destructive examination method for
evaluating the effectiveness of Post weld stress relieving Heat treatments of Pipe
joint welds say at Boiler erection sites in general and to a method for
determination of effectiveness of post weld stress relieving heat treatment in
ferromagnetic material through measurement of coercive force in particular. This
method is based on the difference in measurements of coercive force before and
after Post weld stress relieving heat treatment.
BACKGROUND OF INVENTION
Stress relieving Heat treatment is a very important process employed to improve
the mechanical properties of Heat Affected Zone & weld and to relieve the
residual stresses in parts fabricated / erected by welding. This process is carried
out by using either an oil/gas fired or electrical furnace in production shops
where the entire weldment is kept under uniform temperature condition
throughout the heat treatment operation.
However at erection sites, local post weld heat treatment is often applied by
wrapping heating coils around the circumference of weld joints covering the weld
and certain region close to the weld joint. In this context the effectiveness of
heat treatment becomes a matter of interest, especially in temper sensitive
materials.

At present there are many methods for determining the effectiveness of Heat
treatment such as Hardness measurements and Microstructure evaluation in
weld and HAZ (Heat Affected Zone) of weld joints. In addition residual stress
measurements using X ray diffraction method or Hole drilling strain gage method
are also carried out as a laboratory tool, to establish the Heat treatment
procedure. Hardness measurements require surface preparation; Microstructure
evaluation can be done only in labs. X ray diffraction method requires more
space and heavier equipments not much suitable for erection sites. Hole drilling
strain gauge method is a destructive method, requiring drilling equipment,
surface preparation and time consuming.
A new non-destructive evaluation method, using measurement of magnetic
coercive force values, before and after Post weld stress relieving is illustrated in
this presentation. This non-destructive testing method is based on the change in
coercive force values due to changes in submicroscopic imperfections in the
material. As high temperature heat treatment process brings about changes in
the micro and submicroscopic levels due to creep and plastic deformation, the
application of the coercive force to evaluate the change in structure becomes a
possibility.
Patent no.: JP2001252785 (A) provides a device and a method for non
destruction evaluation about the adequacy of a heat treatment after welding,
which is applied for a ferromagnetic weldment. This method uses an AC
magnetizing waveform. By detecting a phase difference between an exciting
waveform and a detected waveform, the heat treatment is evaluated. This is a
complicated method and evaluation is difficult.

Patent no.: JP59155743 (A) describes a method for measuring hardness of heat-
treated steel by x-ray diffraction using gaussian curve. This method involves X
ray source and is not suitable for erection sites.
Patent no.: US4129033 (A) describes a System for determining the level of heat
treatment. This method employs strain gauges for determining the heat
treatment level. This method is applicable only to a particular product process
line.
OBJECTS OF INVENTION
It is therefore an object of the invention to propose a non-destructive method for
evaluating the effectiveness of Post weld stress relieving Heat treatment of
welded parts at erection sites -
Another object of the invention is to propose a method using standard portable
coercive force measuring equipment for evaluating the effectiveness of Post weld
stress relieving Heat treatment of welded parts at erection sites

A further object of the invention is that no surface preparation is required for
taking coercive force measurements for evaluating the effectiveness of Post weld
stress relieving Heat treatment of welded parts at erection sites
A further object of the invention is to devise a simple and reliable method for
evaluating the effectiveness of Post weld stress relieving Heat treatment of
welded parts at erection sites

SUMMARY OF THE INVENTION
A standard portable Coercimeter (Magnetic structurescope) is used for the
proposed method. The coercive force measurements are taken before and after
the Post weld stress relieving Heat treatment around the pipe circumference in
weld & HAZ. The substantial changes in the readings show the effectiveness of
the Post weld stress relieving Heat treatment process.
By conducting laboratory experiments for stress relieving heat treatment, the
coercive values before and after stress relieving can be recorded for welds of
different material combinations as per requirement. The coercive values obtained
at actual jobs can be compared with standard values, to evaluate the
effectiveness of Heat treatment
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig 1 - Hysterisys Loop
Fig 2 - Equipment and Job setup
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The invention will now be described with help of accompanying drawings which
depict an exemplary embodiments of the invention. However there can be
several other embodiments, all of which are deemed covered by the description

Coercive force (or Coercivity) is a magnetic characteristic, which may be
measured for any ferromagnetic material. The most common representation of
magnetic properties of a ferromagnetic material is its hysteresis loop. H
(magnetizing force) B (Flux density) are the coordinates. On a hysteresis loop,
the Coercivity (Hc) is the loop width on H axes. To understand the term coercive
force, hysteresis loop is of great help. Measuring the magnetic flux of a
ferromagnetic material while the magnetizing force is changed generates a
hysteresis loop. A ferromagnetic material, which has not been magnetized
previously or not gone through complete demagnetization, will follow a dashed
line as in the Fig 1, as the magnetizing force is increased. Higher the applied
current for magnetizing force (H), higher is the magnetic field (B). Beyond the
Saturation point , however the magnetizing force is increased, there will be a
little increase in the magnetic flux. This point is called as the magnetic saturation
point. When the magnetizing force (H) is made zero, still there exists some
magnetism in the material. This is called as the point of retention (Residual
Magnetism). This gives a measure of the residual magnetism present in the
material. Coercive force is the magnetizing force required to be applied in the
opposite direction to bring the residual magnetism to zero value.
The coercive force value Hc measured in (A/cm) Ampere per centimeter is very
sensitive to microscopic structural changes in the material. Hence by measuring
this value with a portable equipment we can evaluate the effectiveness of Post
weld stress relieving Heat treatment at erection sites also.
This method as shown in Fig 2 uses a portable battery operated standard
Coercive force-measuring meter (1). By magnetizing and demagnetizing the

material (2), it measures the coercive force in the material. The reduction in the
coercive force value indicates the Post weld stress relieving Heat treatment
effectiveness. We can compare the coercive force values with already recorded
standard values for various stress relieving parameters taken during lab tests for
the same material and conditions of the stress relieving process.
This coercive force value is different for different materials. This value is very
sensitive to submicroscopic changes in the atomic structure of the materials.
Hence stress relieving heat treatment of welded components effects a 50%
change in the value before and after the heat treatment. These values when
compared with standard values already measured in lab and recorded for the
same material under the predefined process conditions, results in the evaluation
of the effectiveness of Post weld stress relieving Heat treatment.
The method can be applied to ferromagnetic materials even with 2mm coating
on its surface. The method for determination of effectiveness of Post weld stress
relieving heat treatment in ferromagnetic materials through measurement of
coercive force comprises of a number of steps described herein. The area of the
job selected by the probe is magnetized. The selected area is then demagnetized
and the coercive force is measured. The measured values are then compared
with standard coercive force values for that material recorded in the laboratory
for the given stress relieving heat treatment parameters. The effectiveness of
Post weld stress relieving heat treatment is then determined or evaluated. The
determination or evaluation of said effectiveness is done through coercive force
measurement in a non destructive way and with portable equipment

WE CLAIM
1. A method for determination of effectiveness of Post weld stress relieving heat
treatment in ferromagnetic materials through measurement of coercive force,
comprising steps of
- magnetizing the area of the job selected by the probe;
- demagnetizing the selected area;
- measuring the coercive force;
- comparing with standard coercive force values for that material
recorded in the laboratory for the given stress relieving heat treatment parameters,
- determining the effectiveness of Post weld stress relieving heat treatment;
characterized in that the determination of said effectiveness is done through coercive
force measurement in a non destructive way and with portable equipment.
2. The method as claimed in claim 1, wherein the job can be carried out on the selected
area without special preparation of the surface.
3. The method as claimed in claim 1, wherein said effectiveness can be determined if
the surface is covered with even up to 2mm of coating.
4. The method as claimed in claim 1, wherein the change in coercivity of the weld
before and after the Post Weld Heat Treatment is by an extent of a minimum margin of
45%.

ABSTRACT

A method for determination of effectiveness of Post weld stress relieving heat
treatment in ferromagnetic materials through measurement of coercive force,
comprising steps magnetizing the area of the job selected by the probe
demagnetizing the selected area; measuring the coercive force; comparing
with standard coercive force values for that material recorded in the
laboratory for the given stress relieving heat treatment parameters,
determining the effectiveness of Post weld stress relieving heat treatment;
Characterized in that the determination of said effectiveness is done through
coercive force measurement in a non destructive way and with portable
equipment.