FIELD OF THE INVENTION
The present invention relates to a method for evaluating susceptibility of stress
corrosion cracking in Boiler grade steels to determine ultimate weldability of the
materials.
BACKGROUND OF THE INVENTION
Boiler components are fabricated using welding for producing coils like super
heaters, re-heaters, economizers, water walls. Out of the various available
processes, the arc welding processes are preferred for consistent quality and good
integrity. The problem associated with the ferritic grade materials are the weldability
of the materials due to environmental cracking resulting from a combination of
stress corrosion and Hydrogen induced cracking. There is no standard method
available to assess this environmental Stress corrosion cracking issue related to
ferritic steels immediately after welding. To address this cracking a new method is
developed to assess susceptibility for environmental cracking on a tube material
immediately the after welding.
US Patent publication US 2012/0103 105 Al describes a testing device for stress
corrosion cracking. It provides a device for stress corrosion testing of materials in
corrosive medium.
US Patent 3,710,616 describes an apparatus and a method for stress crack
resistance testing of plastic articles wherein a weighted rod stresses the plastic
article in contact with a stress crack agent and an electrically conductive liquid.

US Patent 4107979 teaches method and device for measurement of environmental
stress cracking of a given material. It is measured by a method comprising the steps
of deforming a test piece for producing stress and placing the deformed test piece in
a test environment and assessing time of failure.
US Patent 4335615 describes a motor-driver equipment for testing of materials for
stress corrosion cracking consisting of a microprocessor and a memory. The present
invention is about a method for assessing the environmental stress corrosion
cracking for welded tube materials.
US patent 4217180 discusses a method of determining susceptibility of alloys to
stress corrosion cracking. Test parts are anodized to form an oxide coating and
cracking of the oxide coating during service conditions helps to assess the
susceptibility of the cracking. In our invention, combination effect of welding and
cracking environment are involved for assessing of weldability characteristics of the
boiler grade tube material. This anodizing method is only suitable for virgin
materials, not for welded tubes.
OB3ECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for assessing
weldability of boiler grade tubing based on environmental stress corrosion cracking.

SUMMARY OF THE INVENTION
Accordingly, there is provided a method for assessing weldability properties of an
ferritic alloy steel to determine the environmental stress corrosion cracking (ESCC)
susceptibility. The ferritic grade weldments are susceptible to ESCC immediately
after welding. Diffusible hydrogen present in the weldment enhances the
susceptibility for the cracking tenancy along with stress and corrosive environment.
The hydrogen atoms diffused out after cooling of weldment. This hydrogen
recombines to form molecular hydrogen. This process results in the build-up of
molecular hydrogen, with an increase in pressure within the weldment. This
continues until the pressure build-up is sufficient to initiate fracture. A number of
factors such as pH, volume of hydrogen diffused, volume-fraction, and the shape of
inclusions / defects present, and the surrounding microstructure influence the
cracking tendency. The ESCC assessment method is evolved to assess the cracking
tendency. The present method can be applied immediately after welding to assess
stress corrosion cracking in welded boiler grade tubes.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Implant sample and Holder tube
Figure 2 - Assembling of Implant sample inside the Holder tube
Figure 3 - Welding position, placement of stain gauge at the implant sample and
loading of the setup.
Figure 4 - Cross sectioning of test sample for micro analysis

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Boiler grade ferritic steel are susceptible to environmental stress corrosion cracking
in fabrication welding which results due to diffusible hydrogen present in the
weldment, stress and corrosion environment. To assess the susceptibility of
different materials, following procedure is developed and the steps involved are
detailed herein.
According to the invention, an implant holding specimen (1) is prepared by turning,
and the implant hold test tube is further prepared by milling and drilling, wherein
heating and deformation in the material during machining is avoided. The dimension
of the implant hold test tube is given in Table 1. A general arrangement of holding
an implant sample (1), and a test tube (2) are shown in Figures 1 and 2. The
implant specimen (1) is selected so as to accommodate in the test bead it is shown
in Figure 3. Normally, an 8 mm diameter implant specimen (1) is used. However, for
small weld beads a 6 mm implant sample may be used. Test sample (1) is
assembled inside the test tube (2) through inserting holes (5, 6).
Preheat - When the test requires a preheat, both the backing plate and the implant
specimen is preheated by any suitable method. Prior to any welding, the
temperature of the backing plate and the implant is checked using a calibrated
thermocouple. Where the test demands a specific temperature of preheat, welding is
not commenced until the required temperature is achieved. The temperatures of the
backing plate and the implant in the test area shall not differ by more than 5 °C.
Deposition - Run on and run off plates are attached for welding. Welding can be
done with help of water circulation environment to accelerate the testing. Weld bead
(7) is deposited in flat position in one direction and in a single pass. Fusion

is maintained to allow location of the notch coarse grain heat affected zone of the
implant specimen. This is schematically shown in Figure 3.
Implant loading -The implant specimen (1) is loaded under constant displacement
conditions with tensile stress produced on the sample (1) by tightening a screw (8)
placed at the end of the implant sample. The specimen (1) is slowly loaded, and the
specified load is reached within a time of 20 s to 90 s, and before the temperature
reaches 100°C or as soon as the holding temperature is reached.
Load on an orthogonal section of a notch root cylinder (14) can be calculated from
the strain measured using a strain gauge (10) placed on the center of the implant
sample (1) shown in Figure 3. The strain is divided by the modulus of elasticity in
order to determine the stress.
Stress = Strain / Modulus of Elasticity
Or
Based on length of extension (i.e. elongation of the implant sample (1)) of the
sample during tightening of the screw (8), theload can be calculated. The load F
applied on the cylinder is related to the orthogonal section of the notch root cylinder
(14).

Where σJ is related to the cross section at the notch, F is the load, d is the diameter
of the specimen (1).

Load may be chosen equal to yield strength or proof stress of the implant material.
Specimens is exposed to a test solution immediately after being stressed. The
testing solution shall consist of an acidified, H2S saturated aqueous environment.
Assessment of results -The implant may fracture while the load is maintained. In this
case, the load and the time to fracture are recorded. If no fracture occurs, cracks
which may have formed at the notch under the applied load can be detected using
the following method: Metallographic examinations at magnifications ranging from
200 to 500 X on 3 longitudinal sections located in the weld bead direction according
to Figure 4 (11, 12 & 13). The hardness of the HAZ of the implant sample is
determined using Vickers hardness in the HV10 scale.

WE CLAIM :
1. A method for assessing weldability of boiler grade tubing based on
environmental stress corrosion cracking, comprising the steps of:
preparing an implant holding specimen using turning, milling, and hole drilling
steps while minimizing heat generation and deformation in the sample in said
machining steps;
assembly the specimen inside a test tube using said drilled holes;
preheating the assembly to a required temperature before commencing
welding;
weld deposition in flat position of the specimen in unitary direction, with
water-circulating condition a single pass, for fast cooling of weldment and
acceleration testing by increasing the diffusible Hydrogen level in in heat
affected zones (HAZ) and weld metal;
producing a tensile stress by tightening a screw disposed at an end of the
specimen and Measuring applied loads using load measuring devices;
Selection of testing solution and exposing the specimen to a test solution
immediately after being stressed;
recording the time by which the fracture is assessed.
2. The method as claimed in claim 1, wherein the load is selected to be equal to
yield strength or proof stress of the implant material.
3. The method as claimed in claim 1, wherein the test solution consists of an
acidified and H2s saturated aqueous media.

4. The method as claimed in claim 1, wherein the strain is measured by a strain-
gauge placed on the centre of the implant specimen, and wherein load on an
orthogonal section of a notch-root cylinder is calculated from the measured
strain-value.
5. The method as claimed in claim 1, wherein the load can be alternatively
calculated from elongation of the implant sample during tightening of the
screw.

ABSTRACT

The invention relates to a method for assessing voldability of boiler grade tubing
based on environmental stress corrosion cracking, comprising the steps of :
preparing an implant holding specimen using turning, milling, and hole drilling steps
while minimizing heat generation and deformation in the sample in said machining
steps; assembly the specimen inside a test tube using said drilled holes; preheating
the assembly to a required temperature before commencing welding; weld
deposition in flat position of the specimen in unitary direction, with water-circulating
condition a single pass, for fast cooling of weldment and acceleration testing by
increasing the diffusible Hydrogen level in in heat affected zones (HAZ) and weld
metal; producing a tensile stress by tightening a screw disposed at an end of the
specimen and Measuring applied loads using load measuring devices; Selection of
testing solution and exposing the specimen to a test solution immediately after being
stressed; recording the time by which the fracture is assessed.