FIELD OF THE INVENTION
The present invention relates to a testing setup to determine the stress
corrosion cracking characteristics of thin walled stainless steel tubes in chloride
media by inducing the known residual stress.
The invention further relates to a process of producing steel tubing samples with
known residual stress.
BACKGROUND OF THE INVENTION
For evaluating the effect of residual stress on Stress Corrosion Cracking (SCC)
characteristics of thin walled tubing, the stresses applied on the samples should
be within the elastic range only. For making such sample setups, the guidelines
as defined standard in G39 are used. However in G39, the design of samples
mainly pertains to sheets and strips. No guide lines are mentioned for tube
samples, which means that if SCC characteristics of the tubes are to be
evaluated then the samples have to be made from sheets from which the tubes
are to be manufactured. Thus the results obtained from the sheets may not be
representative for the tube manufactured out of these sheets. Also if the samples
are made from sheets, thickness of which is in the range of 0.5 to 0.9 mm, the

calculated sample size is too large for carrying out effective testing in a
laboratory. Accordingly, for using in Heat Exchanger/ condenser, the level of
residual stress in the pipes should be such that the pipes should not be subjected
to stress corrosion cracking especially when being used in chloride media. Thus,
the tubes need to be tested for SCC under the known induced stresses to
determine the appropriate extent of residual stresses, which may be allowable in
the tube.
OBJECTS OF THE INVENTION
Accordingly, it an object of the present invention to propose a testing setup with
known residual stress to determine the stress corrosion cracking characteristics
of thin walled stainless steel tubes in chloride media to prevent stress corrosion
cracking.
Another object of the invention is to propose a process of producing steel tubing
samples with known residual stress.
SUMMARY OF THE INVENTION
Accordingly, there is provided in one aspect of the invention a process of
producing steel tubing samples provided with known residual stress to predict
stress corrosion cracking characteristics in chloride content media, wherein the
stresses induced are in the elastic range. Instead of U bend specimens, C-ring

specimens (tube pieces having a slit) are used. Known stresses within the elastic
limit are introduced in the samples using the testing setup of the invention, to
determine the optimum residual stress to be induced in a particular sample.
According to another aspect of the invention, there is provided a process of
producing steel tubing samples with known residual stress.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Tube sample having a slit produced according to the invention.
Figure 2: Testing device configured according to the invention.
Figure 3: Outside Diameter measurement before and after stressing the Tube
sample according to the inventive process.
Figure 4: Straining and unstraining of the tube samples in a micrometer to
estimate the optimum strain (deflection) which can be induced.
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.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The steel tubing samples (TS) are selected from a lot, having no prior residual
stress. This is ensured by taking samples from a lot-size having plurality of tubes,
which is only annealed, without any straightening or any other cold working
operation. The absence of residual stress is also ensured by measurement of
residual stresses, prior to sampling.
The samples (TS) in this case are tube pieces having a slit (C - ring) as shown in
Figure 1. A small tube of 15-35 mm dia and of approx 20 mm length is taken and
slitted
Slitting of tubes in to C-ring specimen is made in such a way that no further
stresses is introduced during slitting. Sample preparation by EDM wire cut
machine is preferred.
Initial diameter of the tube is measured. The point of measurement is to be
marked (Figure 3).
Before making actual samples, some samples are strained and unstrained in a
micrometer to estimate the maximum amount of elastic strain a sample can with
stand (Figure 4). This information is further used in deciding the amount of
strain

require to prepare the samples. The samples uses at this stage are to be
discarded and not to be used for testing.
The stress, which is used to produce strain, is calculated in a known method.
The testing device consists of two stainless steel plates (SSP) fixed by nut and
bolt arrangement (FA).
The sample (TS) to be tested is placed in between the plates (SSP) such that the
slot (SL) is at 90 degrees to the two opposite points of contact as shown in
Figure 3.
In addition, to avoid any galvanic contact between the stainless steel plates
(SSP) and tube (TS), pieces of oxidized Zircaloy-4 (AP) placed in between as
shown in Figure 3.
A known amount of stress level is induced into the sample (TS) by tightening the
both bolts (FA) equally. This is ensured by measuring the size of slot (SL) with a
distance measuring microscope. Final diameter of the tube (TS) is measured at
the marked point (Figure 3). The amount of tightening is measured by
measuring the deflection produced in the tube (TS) by tightening the plates
(SSP). From these geometries, the developed stress is calculated. (The samples
should not have more strain as explained in SI. No.5)

The SCC testing is done on a constant strained C-ring specimen that is
configured for testing SCC susceptibility under known amount of applied stress
levels.
These samples are successfully used to study the effect of residual stresses on
SCC susceptibility of austenitic stainless steel tubes.

WE CLAIM :
1. A process of producing steel tubing samples with known residual
stress, comprising the steps :
- providing steel tubing samples having zero residual stress;
- slitting the tube samples in the form of C-ring without allowing
inducement of any stress on the samples;
- marking the initial diameter of the tube sample after measurement;
- estimating the maximum amount of elastic strain of the tube
sample based on strained and unstrained values of identical tube
samples measured in a micrometer;
- Straining and measurement of deflection thus induced;
- calculating the corresponding stress value which produce the
elastic strain (DEFLECTION) of the tube sample.
2. A testing device to determine the induceable residual stress in thin
walled stainless steel tubes used in chloride media to prevent stress
corrosion cracking , comprising :

- at least two stainless steel plates disposed at a spaced-apart
position to allow disposition of the tube-sample to be tested, the
slit of the tube located at 90° in respect of the two opposite points
of contact;
- a plurality of pieces of oxidized Zircaloy-4 interposed between the
tube sample and the stainless steel plates, and
- a fastening means for releasably fastening the tube sample onto
the testing device which is gradually rotatable to tighten the tube
sample,
wherein the deflection produced on the tube sample corresponding
to tightening of the fastening means, and wherein the residual
stress is calculated corresponding to the value of said deflection.

The invention relates to a testing setup to determine the effect of residual
stresses in thin walled stainless steel tubes used in chloride media on the
stress corrosion cracking CHARACTERISTICS, comprising at least two stainless
steel plates disposed at a spaced-apart position to allow disposition of the tube
sample to be tested, the slit of the tube located at 90° in respect of the two
opposite points of contact; pieces of oxidized Zircaloy-4 interposed between the
tube sample and the stainless steel plates, and a fastening means for releasably
fastening the tube sample onto the testing device which is gradually rotatable to
tighten the tube sample, wherein the deflection produced on the tube sample
corresponding to tightening of the fastening means, and wherein the residual
stress is calculated corresponding to the value of said deflection.