FIELD OF INVENTION
The present invention relates to determination of residual stresses in thin walled
welded tubes for example, tubes with a diameter between 15-35 mmv, and a
thickness between 0.5-1.0 mm, primarily adapted for condenser/ Heat exchanger
applications. More particularly, the invention relates to a process of determining
residual stress in thin walled welded tubes adapted in manufacturing of heat
exchangers.
BACKGROUND OF THE INVENTION
It is known in the art that thin walled (between 0.5-1.0 mm thickness) welded
stainless steel tubes are adapted for construction of heat exchangers. These
tubes are produced under prescribed specification according to which the
residual stresses shall be between ± 4 Kg/mm2• Residual stresses are generated
during manufacturing of the tubes. Tubes stresses may be longitudinal or
circumferential, depending on the method of straightening. Straightening of the
tubes by the known roller straightening process produces positive circumferential
residual stresses. The residual stress of the tubes is measured as per gUidellnes
stipulated in ASTM E1928, presumably considering that the tubes are seamless.
A need therefore exists to develop a standard procedure for an accurate
measurement of circumferential residual stresses in thin walled welded tubes to
determine that the tubes maintain a residual stress between ± 4 Kg/mm2•
OBJECTS OF INVENTION
It is therefore an object of the present invention to propose a process of
determining residual stress in thin walled welded tubes adapted in manufacturing
of heat exchangers, which further enables to measuring the circumferential

stresses present in the tubes to avoid stress corrosion cracking when exposed to
harsh environmental conditions.
The nature of the invention, its objective and further advantages residing in the
same will be apparent from the following description made with reference to
non-limiting exemplary embodiment of the invention represented in the
accompanying drawings.
SUMMARY OF THE INVENTION
According to the inventive method, for measurements of circumferential residual
stresses in thin walled welded tubes, a longitudinal slit is made along the entire
length of the tube after measuring the initial outside diameter at four different
points. After slitting the tube, the change in outside diameter is measured at the
same four points and the average value is considered. Circumferential residual
stress is finally calculated by adapting a formula mentioned in ASTM E1928.
This test is performed after the completion of all manufacturing processes.
Testing is done on one tube selected from a lot of tubes manufactured from a
single coil.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a sample of tube showing a weld line according to the
invention.
Figure 2 - Shows a line marked at 1800 to the weld line on the sample
according to the invention.

Figure 3 - Shows four points marked at 90° to the weld line on the tube
sample according to invention.
Figure 4 - Shows the initial outside diameter of the tube sample (top view
before slitting) according to invention.
Figure 5- Shows a wooden mandrel fitted inside the tube sample in
accordance with the invention.
Figure 6 - Shows a tube sample held in a vise in accordance with the
invention.
Figure 7- Shows a longitudinally slitted tube sample according to the
invention.
Figure 8 - Shows the final outside diameter of the tube sample (top view after
slitting) according to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The inventive process is implemented by sequentially carrying-out the following
steps:-
1. A sample (S) of 100mm length is cut from a full-size tube with least
deformation being developed during the cutting step (Fig-1).
2. Marking a line (L) on the sample (S) with a felt pen or marker at 180 degree
from the line (WL) weld (Fig. 2).

3. Marking four points (1,2,3,4) along the longitudinal surface of the tube
sample (5) at 90 degree from the weld line (WL) on a first side (Fig.3).
4. Determining an average outside diameter (Do) of the tube sample (5) by
averaging the initial outside diameters measured at the four points (IODMP).
5. Determining the average thickness (t) of the tube by measuring thickness at
both ends of the tube (s) (Fig. 4)
6. Longitudinally slitting the sample (5) at the marked line (L) by disallowing
introduction of any additional stress in tube. For this following activities are
performed.
A. Providing a wooden mandrel (WM) with a diameter slightly smaller than the
internal diameter (ID) of the tube (5), so that the stresses are only on the
mandrel, the mandrel (WM) being insertable in the tube (5) (Fig. 5).
B. Holding the tube (5) in a vise in such a way that no compressive stresses is
introduced at the tube ends (Fig. 6).
C. Cutting the sample (5) longitudinally along the marked line (L) by a Hacksaw
(Fig.?)
7. Measuring the average diameter (Dr) at each location (1,2,3,4) mentioned in
step (4).
Note : After slitting, the stiffness of the tube (5) becomes vetr less. As an
ordinary vernier caliper or micrometer likely to generate inaccurate values, one
of the following measures is adapted:

i) Non contact methods
ii) Micrometers with electrical contacts
iii) Any other method which allows measurement of diameter accurately
without any substantial force.
Calculation

The average value of at least three samples are measured. If the difference
between the measured values is more than 20%, more number of samples shall
be taken.
Acceptance Norms:
Acceptance norms shall be as per relevant prescribed specification of the full-size
tubes.

Retesting:
1. If any sample fails, then new samples from two more tubes shall be taken
and tested for RSM.
2. In case of discrepancy in measured value, the samples may be slitted by EDM
to minimize the errors caused by deformaiion during manual slitting.


WE CLAIM
1. A process of determining residual stress in thin-walled welded tubes adapted
in manufactuiing of heat-exchangers, comprising the steps of:
- cutting a sample piece of welded metal tube from a full-size tube length, the
sample piece being substantially free from cutting-deformation;
- marking a line at 1800 from the weld line of the sample piece;
- locating four different points on a first side along the longitudinal surface of
the tube sample at 900 from the weld line;
- measuring initial outside diameter at said four located points and obtaining an
average initial outside diameter;
- measuring thickness at both ends of the tube sample and obtaining an
average thickness;
- longitudinally slitting the sample piece along the marked line simultaneously
disallowing introduciion of additional stress;
- measuring the final outside diameter at said four locations of the slitted tube
sample, and determining the average final outside diamete;; and
- calculating the residual stress of the tube by adapting a relationship of:
Residual stress

Where, E = Modules of elasticity
µ = Poisson ratio
Df = average final diameter after slitting
t = average thickness of the tube
2. The process as claimed in claim 1, wherein the step of slitting the tube
comprises:
- providing a wooden mandrel with a diameter slightly smaller than the internal
diameter of the tube enabling the mandrel to be inserted in the tube;
- holding the tube in a vise and disallowing introducing any compressive stress
at the tube ends, and
- cutting the sample longitudinally along the marked line by a hacksaw.

3. The process as claimed in claim 1, wherein the measurement of diameter of
the tube after slitting is conducted by one of the non-contact method,
micrometer with electrical contact, and any known force-less measurement
method.
4. A process of determining residual stress in thin-walled welded tubes adapted
in manufactuiing of heat-exchangers as substantially herein described with
reference to the accompanying drawings.

The present invention relates to a process of determining residual stress in thinwalled
welded tubes adapted in manufacturing of heat-exchangers, comprising
the steps of: cutting a sample piece of welded metal tube from a full-size tube
length, the sample piece being substantially free from cutting-deformation;
marking a line at 180o from the weld line of the sample piece; locating four
different points on a first side along the longitudinal surface of the tube sample
at 90° from the weld line; measuring initial outside diameter at said four located
points and obtaining an average initial outside diameter; measuring thickness at
both ends of the tube sample and obtaining an average thickness; longitudinally
slitting the sample piece along the marked line simultaneously disallowing
introduction of additional stress; measuring the final outside diameter at said
four locations of the slitted tube sample, and determining the average final
outside diameter; and calculating the residual stress of the tube by adapting a
relationship of: