FIELD OF INVENTION

The present invention generally relates to a method for evaluation of solidification cracking susceptibility in stainless steels using shielding gas. In particularly, it relates to determination of susceptibility in austenitic stainless steels through a critically controlled supply of shielding gas using different variants of shielding gas or mixture of through an optimum heat input by exposing the weld metal to both thermal and hydraulic pressure.
BACKGROUND OF THE INVENTION
The fabrication of the components of a boiler is carried out through a welding process for producing coils like super heaters, re-heaters, economizers, water walls etc. Amongst the various available welding processes, the arc welding processes is the most preferred option for the resulting consistent quality and good integrity. The problem associated with welding of the austenitic stainless steel is cracking of the steel and is usually termed as solidification cracking. It mainly happens immediately after the welding stage when the solidification for liquid metal takes place. This type of cracking phenomenon can be controlled by a change in the amount of heat input. Thus, one of the methods to control the amount of heat input in the welding process is change of shielding gas or a change in mixing ratio of the shielding gas done through the gas tungsten arc welding (GTAW) process. However, it was found that currently there is no method or a procedure to optimize the variations in shielding gas as mentioned above in order to avoid or reduce the cracking tendency of the steel during the welding process. Hence, to address this issue a novel method was developed to test and evaluate the susceptibility of solidification cracking in austenitic stainless steels during the process of welding.
US patent No: US6,204,477 discloses a method for the elimination of solidification cracking in a single material 312 stainless steel by using a shielding gas mixture which consists of Ar-5% nitrogen gas by volume and the balance being an inert gas or a mixture of inert gases. The concentration of nitrogen is kept at a sufficient level to avoid solidification cracking during the arc welding process. However, this reference doesn’t provide for any method or procedure to test steel materials for solidification cracking susceptibility.
US patent No: 20050028897A1 discloses a set process related to a welding, repair welding or cladding process of metallic alloys which are sensitive to hot cracking. It uses a first heat source, which is directed to the parts of the metallic alloy forming the metallic pool on the parts of the metal or metallic alloy. The first heat source is followed by one or more additional heat sources directed on the parts of the metallic alloy at a distance and in the same direction and at the same speed as the first heat source. The additional heat sources are directed on the solidification region of the metal pool generated by the first source of heat. The power of the additional heat sources is set so as to reduce the local cooling rate of the solidification region of the melt pool thereby reducing tensile stresses in order to avoid hot cracks during the whole process. However, it does not describe any testing method or a procedure to optimize the shielding to avoid solidification cracking
WIPO patent No: WO 2014181458 discloses a welding method through which welding carried out for the prevention and reduction of solidification cracking during the welding step without essentially requiring a prior examination of the construction method during the hot-melt welding whenever a change occurs in the material or the shape of the product. However, this reference doesn’t give any information on the aspect of controlling and optimizing the welding process.

OBJECTIVE
One object of the present invention is to provide a standardized testing and evaluation method to determine the solidification cracking susceptibility in austenitic stainless steel using shielding gas.
Another object of the present invention is to determine the threshold level of cracking during solidification in the welding process.
Still another object of the present invention is to compare the variants of shielding gas and gas mixtures to determine the optimum heat input and composition of the shielding gas.
SUMMARY OF THE INVENTION:
The present invention herein relates to a novel method for testing and evaluating the solidification cracking susceptibility of austenitic stainless steels through an automatic arc welding process over a curved die by using shielding gas, wherein the flowing shielding gas is varied so as to control the heat input during the welding process. The complete testing procedure includes fixing of the flat test specimen in the assessment setup with the ends clamped on both the sides followed by the fastening of a removable curved die block so as to bend the specimen around the die. Further, bending the test specimen through a mechanical force to create a bending load using a hydraulic power pack ram and then moving the die block at varying speed so as to bend the specimen to the desired curvature. Subsequently, the testing method is accomplished by removing the specimen and then cleaning it with acetone by using an ultrasonic cleaning machine. The examination of the surface of the specimen for presence of cracks is carried out and the required parameters are recorded. This test method is conducted in an apparatus which consists of a set-up for holding a flat austenitic stainless steel specimen through clamps at the ends. A curved die which is placed under the flat austenitic stainless steel specimen to bend the specimen and a torch mounted on a linear slide for welding the flat austenitic stainless steel specimen. An automatic welding machine is used for carrying out the linear welding process. In order to further understand the characteristics and technical contents of the present subject matter, a description relating thereto will be made with reference to the accompanying drawings. However, the drawings are illustrative only but not used to limit scope of the present subject matter.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system or methods in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:
Fig. 1 illustrates three dimensional plan view of the equipment set-up consisting of different machines, in accordance with an embodiment of the present subject matter, wherein
1a: Flat austenitic stainless steel sample specimen
1b: Curved die
1c: Clamps
1d: Welding torch
1e: Welding machine
1f: Shielding gas mixture
1g: Gas cylinders
1h: Hydraulic power pack ram
The figures depict embodiments of the present subject matter for the purposes of illustration only. A person skilled in the art will easily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.

DETAILED DESCRIPTION OF THE INVENTION
It should be noted that the description and figures merely illustrate the principles of the present subject matter. It should be appreciated by those skilled in the art that conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present subject matter. It should also be appreciated by those skilled in the art that by devising various arrangements that, although not explicitly described or shown herein, embody the principles of the present subject matter and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the present subject matter and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. The novel features which are believed to be characteristic of the present subject matter, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. These and other advantages of the present subject matter would be described in greater detail with reference to the following figures. It should be noted that the description merely illustrates the principles of the present subject matter. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present subject matter and are included within its scope.
The present subject matter herein relates to a method for testing and evaluating the solidification cracking susceptibility in austenitic stainless steel using shielding gas. The entire set-up for the method of testing and evaluation so as to determine the threshold of solidification cracking susceptibility is placed according to the description as provided in the detailed description of the drawing.
To initiate the welding process, a gas or a gas mixture(1f) which is the shielding gas is introduced from a gas mixing machine located between gas tungsten arc welding (GTAW) machine(1e) and gas cylinders(1g). Upon the supply of this shielding gas to the welding machine the torch(1d) starts moving onto the linear slide and thus welding of the flat specimen(1a) is carried out with the simultaneous application of sudden hydraulic force by the hydraulic power pack ram (1h) at point B as depicted in Figure 1 to attain the desired radius of curvature of the specimen.
The augmented tangential strain for a given radius of curvature of the die block is calculated by using the following formula:
e = t X 100/ (2R+t)
where,
e = percent augmented tangential strain
t = specimen thickness
R = die block radius
and, the typical range of augmented tangential strain is 1, 2 and 3 percent
As depicted below, the following flowchart summarizes the testing method broadly in a step by step manner.

The steps of the flowchart for reference have been enumerated as follows:
S201: Setting of the complete device and its parts at the defined positions and according to the arrangement as shown in the Figure 1
S202: Placing the test specimen on the flat surface and clamping the ends with the fixed clamps
S203: Positioning of the curved die on the centre of the curved line of arc for the movement of die to be linear from position A to C
S204: Initiating the welding procedure by using the welding torch while the curved die moves on the centre line of arc
S205: Bending the flat specimen into the desired curvature using the curved die until the specimen attains the curved shape and the set radius of curvature
S206: Applying a bending force suddenly at point B during the linear movement of the curved die on the line of arc
S207: Removing the curved die and allowing it to cool down once the desired shape and radius of curvature has been attained in the test specimen
S208: Cleaning the prepared curved specimen with acetone using the ultra -sonic machine
S209: Examining the surface of the prepared specimen for the testing and evaluation of the solidification susceptibility by measuring the parameters
The object of this invention can be achieved by conducting the testing method in the manner as described earlier. Thus, the invention can be carried out in the following manner as described in detail in the following example.
Example 1:
The present subject matter relates to a test specimen in form of strip having 3 to 10 mm thickness(t) and a width of 50 mm which can be clamped in the test fixture at both the ends. The curved die block(1b) of the desired radius is fastened in the position shown in Figure 1 and it is easily removable. The arc of welding is initiated on the center line of the specimen, located approximately 20 mm from the specimen’s end location at point ‘A’ as shown in Figure 1. The bending force (F) is suddenly applied as the center of the arc passes through the point ‘B’, which is near the point of tangency between the curved surface of the die block and the fixed end of the specimen. The specimen(1a) is then bent downward until the specimen conforms to the radius of curvature of the top surface of the die block. The rate of arc travel remains constant from its point of initiation located at point ‘A’ to its point of termination in the runoff area located at point ‘C’. The bending load and the shielding gas flow are maintained for a duration of five minutes post the termination of the weld pass. The specimen(1a) is then removed from the fixture for examination. Three specimens must be tested under the same set of conditions consisting of a combination of an augmented tangential strain varying from 1%, 2% and 3% and a varied level of the heat input by changing the existing parameters. After cooling, the specimen has to be cleaned with acetone using an ultrasonic cleaning machine. The surface of the weld specimen is then examined for the presence of any cracks. Evaluation is carried at a magnification of 50X using a metallurgical microscope, and the following data is recorded for evaluation of the austenitic stainless steel specimens with 0.02 mm accuracy.
(1) Length of each crack
The total combined crack length is obtained by adding the lengths of each crack found in the weld metal and in the heat affected zone (HAZ) of each specimen. The total combined crack length produced in the weld metal and the heat affected zone (HAZ) gives the best quantitative index of the hot-crack sensitivity of the weld metal and the heat affected zone (HAZ) respectively, for a given welding procedure.
(2) Maximum crack length
The maximum crack length which is measured in each and every specimen is the quantitative index for preliminary screening of base metal, filler metal, or both at comparable levels of augmented tangential strain, for the given fixed parameter.
(3) Location of cracks (weld metal or HAZ)
Based on the location and the type of crack on the surface of the specimen its effect can be evaluated.
(4) Number of cracks
The total number cracks is recorded for each specimen.
(5) Cracking Threshold.
The cracking threshold which is the minimum augmented tangential strain required to cause cracking in a particular base metal with a given set of welding variables is recorded in terms of the tangential strain applied on the specimen.
On the basis of the above mentioned parameters and a record of the reading of the same for each specimen, a comparison has to be carried out for the different specimens. Further, on the basis of comparison of one shielding gas to other gases or a mixture of gases an optimized composition of shielding gas can be obtained for the welding of given austenitic stainless steel.
The present subject matter also consists of an apparatus and its structured arrangement has been shown in Figure 1. The following example i.e. Example 2 describes such an apparatus arranged to carry out the testing procedure as described in the description of the method above.
Example 2:
A welding machine torch(1d) is mounted on a linear slide and the torch has an inbuilt motor to facilitate the movement over the linear slide and its speed can also be controlled during its operation. This arrangement is mounted on the equipment setup at the bottom of the slide which is designed in such a way that it has the means to clamp(1c) the ends of the flat specimen(1a) on both the sides and a fixed curved die(1b) is placed below the it to bend the specimen around the die so as to obtain the desired radius once the welding torch(1d) reaches the center during the complete welding process. This concept is illustrated in the detailed drawings describing the Figure 1. Curved dies having different radii are used while conducting a series of tests whereby each specimen of the series has to be bent around a die(1b) having a smaller radius than the die used with the previous specimen of the preceding series and this is continued until the radius of the die bring used is sufficiently small enough to cause cracking in the flat specimen during the welding process. The bending force is applied with the use of hydraulic power pack ram (1h) so as to obtain a comprehensive shape of the specimen placed over and above the curved die. The die moves at a very high speed to bend the specimen to the desired fixed curvature. The above specification, examples, and data provide a description for the manufacture and use of the invention. It should be appreciated that many other alternative embodiments are possible. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.

We claim:

1. A method for testing and evaluating the solidification cracking susceptibility of austenitic stainless steels through welding of an austenitic stainless steel specimen (1a) over a curved die block (1b) using a shielding gas(1f) which is varied so as to control heat input during welding, wherein the method comprising:
fixing the austenitic stainless steel specimen (1a) in an assessment setup using clamps (1c) in such a way that ends of the austenitic stainless steel specimen (1a) are clamped on both the sides;
fastening the curved die block (1b) beneath the austenitic stainless steel specimen (1a) so as to bend the austenitic stainless steel specimen (1a) around the curved die block (1b);
bending the austenitic flat stainless steel specimen (1a) through a mechanical force generated by a hydraulic power pack ram (1h), wherein the bending includes moving the curved die block (1b) at varying speed towards the austenitic flat stainless steel specimen (1b) so as to bend the austenitic flat stainless steel specimen (1a) to a desired curvature;
removing the austenitic stainless steel specimen (1a) from the clamps (1c);
cleaning the austenitic stainless steel specimen (1a) with acetone by using an ultrasonic cleaning machine; and
examining the surface of the austenitic stainless steel specimen (1a) for presence of cracks.
2. The method as claimed in claim 1, wherein the method further comprising:
maintaining arc of movement of the curved die block (1b) on centre line of the austenitic stainless steel specimen (1a) in a linear manner.
3. The method as claimed in claim 1, wherein the examining comprising:
recording of the parameters such as the length of each crack, maximum crack length, location of cracks, number of cracks and cracking threshold.
4. The method as claimed in claim 3, wherein the examining further comprising:
comparing with subsequent types of the shielding gas(1f) or a mixture of gases(1f) for obtaining the optimized variant of the shielding gas(1f).
5. The method as claimed in claim 1, wherein the method further comprising:
testing and evaluating using curved die block (1b) of different radii.
6. The method as claimed in claim 5, wherein the method further comprising:
using the curved die block (1b) which is lesser than the radii of the preceding curved die block (1b) in the series of such tests until the curved die block (1b) radii is satisfactorily sufficient to cause cracking in the austenitic stainless steel specimen (1a).

7. An apparatus for testing and evaluating the solidification cracking susceptibility of an austenitic stainless steel, wherein the apparatus comprising:
a set-up for holding the austenitic stainless steel specimen (1a) through clamps (1c) at the ends;
a curved die block (1b) placed under the austenitic stainless steel specimen (1a);
a torch(1d) mounted on a linear slide for welding the austenitic stainless steel specimen (1a); and
an automatic welding machine (1e) for carrying out welding.
8. The apparatus as claimed in claim 7, wherein the automatic welding machine(1e) further comprises:
a gas mixing apparatus (1f) for the mixing of gases used for shielding in the welding process; and
a gas cylinder or a set of gas (1g) cylinders for the supply of shielding with a means control heat input during welding.
9. The apparatus as claimed in claim 7, wherein the apparatus further comprises:
the hydraulic power ram (1h) positioned below the curved die block (1b).