FIELD OF INVENTION
The present invention relates to a method for determination of the fusion/melting temperature of all welding and brazing fluxes employing heating microscope. More particularly the invention relates to a method of determining a melting profile of a flux.
BACKGROUND OF THE INVENTION
In a welding/brazing process a flux plays a major role by protecting the welding environment from various problems like oxidation of metal due to the ingress of atmospheric oxygen and nitridation, sustaining a uniform arc condition without any interruption. During a welding process the melting/fusion behavior is a main factor for selecting a flux type. When the arc is generated the flux melts and protects the weld metal and upon cooling the flux forms a solid phase commonly known as weld slag. After the slag formation it is removed by mechanical means like chiseling, chipping which involves reasonable manual work and often it is a time consuming process. Here the slag detachability for the particular flux is a deciding factor for determining the effective slag removal from the weld metal. It is observed that in many of the multipass welding process the incomplete removal of slag led to slag inclusions in subsequent welds which later resulted in many weld failures and degraded weld quality. So the characterization of slag detachability is to be studied for the selection of fluxes.

Many workers have studied various properties of fluxes e.g. Canadian patent publication CA2533365C discloses composition of flux that produces less than 7ml/100g of diffusible hydrogen in the weld metal during submerged arc welding.
Canadian patent publication CA2525350C discloses a welding flux for pipe welding or one-side welding applications.
Studies of wettability between solid and liquid phase using heating microscope has been reported by C.K.Schoff in "Modern approach to wettability. Theory and Applications" edited by M.E.Schrader and G.L Loeh p.375,1992.
The wettability of intermetallic substances by liquid alloys has been investigated using heating microscope by W.F.Gale et al J.Mater. Sci 32 (1997) 4931-4940.
The heating microscope has been used to study the wetting/non wetting transition temperature of feldspar melts on silica sands by T.F Lee et al. (Ceram. Bull. 61 (1982) 737-740.
The US patent US7653511B2 discloses a method for assessing the performance of a flux in a soldering process by monitoring the activity of the flux via its electrical conductance measurement.

Several characteristics of welding fluxes viz, activity, corrosivity, clean-ability, volatility, viscosity and flammability are cited in different literature and data for the above are available. But fusibility of flux are not available. Hence the fusion temperature data from the start of the sintering to completion of fusion is found to be very useful for future application. Hence determination of fusion temperatures of fluxes using heating microscope is a new and novel idea.
The present invention proposes a method for characterizing the melting/expansion behavior of fluxes using heating microscope and the correlation of the fusion behavior with slag detachability.
The aim of the invention is to provide a tool/technique for the determination of melting profile (finger print of fusion process of the flux) from which all the above required characteristic of flux are obtained.
The welding fluxes consists of mineral oxides and fluorides and have wide melting/fusion temperatures. Information about softening stage to welding stage is important to assess the characteristics of the fluxes with respect to shaping the weld bead, slag detachability, weld finish etc. Heating Microscope enables to find out the range of temperature and the shape of the flux mass that remains after heating.

OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method for determination of the fusion/melting temperature of all welding and brazing fluxes employing heating microscope which is capable of determining a melting profile of a flux to bring a correlation of the fusion behavior with slag detachability.
Another object of the invention is to propose a method for determination of the fusion/melting temperature of all welding and brazing fluxes employing heating microscope which is able to provide a complete fusion/melting profile of flux that includes start of temperature of sintering, Deformation temperature (DT), Softening temperature (ST), Hemisphere temperature (HT) and flow temperature (FT) of flux and to obtain dilatometric curves.
SUMMARY OF THE INVENTION
The present invention is in the general field of welding and more particularly directed to welding flux. This invention relates to the determination of melting profile of fluxes. Factors which influence welding temperature are the melting temperature of flux, its fluidity and its wetting ability. The slagging-off (Slag Detachability) characteristics of the fluxes are also important in promoting effective joining of the weld metal at relatively low temperature. From the data of melting temperature of flux,

the flux can be classified as low melting point material or high melting point material. The lower the flow temperature, better will be the weld bead formation.
Full slag coverage of the weld is also important in insulating the weld from the atmosphere to reduce the cooling or chilling rate which is of most important in the welding process. After the weld has been fully cooled, however the slag should be friable in order to reduce the labour cost in removing the slag.
Hence melting temperature of the flux, its fluidity, wetting ability, slag detachability, dilatometric (expansion shrinkage) coverage are some of important and very useful characteristics of the flux.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig 1: shows the fusion behaviour of four different type of fluxes with the help of a graph plotted with percentage cross sectional area and temperature in degree centigrade.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
The invention provides the technique for the determination of fusion/melting characteristic of welding flux using heating microscope which provides complete fusion/melting profile of flux that includes, start of temperature of sintering,

Deformation temperature (DT), Softening temperature (ST), Hemisphere temperature (HT) and flow temperature (FT) of flux and dilatometric curves. The welding flux may be Stainless Steel (SS) welding flux, Manganese silicate based flux, Alumina based flux, brazing flux or any other flux. The fusion/welding profile lies from ambient to 1500»C. The dilatometric curves may be.both expansion, shrinkage and expansion cum shrinkage as detailed in fig.1. The heating microscope can also be used for the determination of melting behavior of a flux system and thereby phase diagram for the flux system at different temperatures with varying flux compositions can also be formulated.
Deformation Temperature (DT) is the temperature at which the flrst sign of deformation or the rounding of edges of the flux pellet is observed.
Softening Temperature (ST) is the temperature at which the flux pellet has fused
down to a shherical lump in which the height is equal to the width of the base of the
flux pellet.
Hemispherical Temperature (HT) is the temperature at which the flux pellet has fused to a hemispherical lump at which the height is one half the width of the base of the flux pellet.
How temperature (FT) is the temperature at which the flux pellet spreads out in a flat layer over the sample holder.

Various types of fluxes are used in electrode coating of shielded metal arc welding, submerged arc welding and brazing. A novel method for determining the fusion characteristics of the fluxes has been identified by using Heating Microscope. Various samples of the fluxes have been collected from the actual welding consumables and studies have been carried out. Heating Microscope is an instrument used for determining the melting/fusion temperatures of fuel ashes namely Initial Deformation Temperature (DT), Softening Temperature (ST), Hemispherical Temperature (HT) and Flow Temperature (FT). This microscope is used for determining the fusion characteristics of welding fluxes and brazing fluxes.
The heating microscope is a testing system designed to determine the high-temperature characteristics of materials. Its method of measurement is based on thermo-optical analysis.
Small-scale, compact and technically optimized tube furnaces have been designed for use in the heating microscope.
As the samples are positioned in the confined space of the furnace tube with exceptional accuracy, each sample is exposed to exactly the same temperature field -as a result, repeatability of the measurement result of a test series is better.

All components of the heating microscope are arranged on an Optical bench: this reliably ensures exact alignment of the lamp, furnace and camera, which is essential for precise image analysis.
The heating microscope software determines the deformation, sphere, hemi-sphere and flow temperatures.
Based on quantitative measurement of the test object height and width, dilatometric curves can be viewed on screen or printed out. These curves can be used to derive information on sintering or blistering or even on direction-dependent shrinkage behavior. The wetting behavior of melts on various substrates can also be characterized by measurements of the contact angles.
The following welding flux with different applications was selected for verification of the new idea of determination of fusion/melting temperature of fluxes.
1. S.S welding flux -A, B, C
2. Alumina based flux - D, E
3. Manganese silicate based flux - F, G
4. Brazing flux
Each flux material was grounded to powder and pellets (Specimen) of shape (truncated cone of 4 mm height, diameters of 3 mm - bottom and 1.5 mm - top) is prepared by uni-axial pressing of flux powder in a die without using any binder. The pellet is then air dried.

The pellet is introduced at room temperature into the furnace of the microscope. As per the heating programme, the furnace was heated to 100oC at a constant heating rate of 10oC per min and then further heated to 1400»C with reduced heating rate of 8°C per min. The sintering/fusion test runs were performed in air. The images of the pellet were continuously captured using camera at pre-selected time intervals (10s), during the whole heating process. The dimensional changes of the pellet namely height, comer angle, area and shape factor were also measured from the stored images of the specimen. The area shrinkage/expansion % = (A0 - AT) 100 / A0 where A. and AT are the initial projected area of flux pellet and area after each interval of temperature T respectively were calculated from room temperature to final temperature of each run.


The ash fusion data can be correlated with welding technological characteristics viz slag detachability, side wall wetting and spreading. The above specific characteristics can be established combining with welding parameters viz current, voltage and speed.
The lesser the difference in Hemispherical (HT) and Flow Temperature (FT) better will be the slag detachability. Hence from the ash fusion data, following are the flux samples which will have better weld detachability than the remaining samples.
1. SS welding Flux-A
2. SS welding Flux-B
3. Alumina based flux - E
4. Manganese silicate based flux - G
Several characteristics of welding fluxes viz, activity, corrosivity, clean ability, volatility, viscosity and flammability are cited in different literature and data for the above available. But fusibility of flux are not available. Hence the fusion temperature data from the start of the sintering to completion of fusion is found to be very useful for future application. Hence determination of fusion temperatures of fluxes using heating microscope is a new and novel idea.

The data from the heating microscope is a useful parameter for investigating the individual flux types for their shrinkage/expansion behavior. These fluxes are tested under the same sample preparation procedure and same heating conditions. However the dilatometric curves as shown in Fig.1 for the fluxes show considerable variation in individual fusion properties viz. Expansion/shrinkage. In Fig.l sample 2 is having a lowest melting temperature and it has reasonable shrinkage than the other samples. Sample 1 & 3 behave in same manner such that both are melting at higher temperature but definitive final melting is occurring at different temperatures, where sample 3 is having highest melting profile with very less shrinkage/expansion. From sample 4 we can observe that the melting has started in low temperature region as that of a low temperature flux. But from the fusion profile expansion of the flux is observed over a range of temperature after that the sample collapses to flow temperature. These observations are used to identify the flux types into different categories such as high melting flux, low melting flux, and fusion with shrinking, fusion with expansion.

WE CLAIM
1. A method for determination of the fusion/melting temperature of all welding and brazing fluxes employing heating microscope, the said method comprising;
grinding each flux material taken as specimen to powder;
making pellets with said grounded powder;
drying the pellets in air; characterized in that the pellet is introduced at room temperature into the furnace of a microscope when the furnace is heated to 1000°C at a constant heating rate of 10°C per minute and then further heated to 1400°C with reduced heating rate of 8°C per minute, wherein the images of pellet is continuously captured with a camera at pre-selected time intervals of 10 seconds during the whole heating process, when the dimensional changes of the pellet namely height, corner angle, area and shape factor are measured from stored images of the specimen, wherein percentage area shrinkage and expansion are calculated after each interval of a predetermined temperature starting from room temperature to final temperature of each run of each sample, wherein dilatometric curves for different fluxes are prepared based on data of fusion temperature of fluxes and fusion properties like expansion/shrinkage obtained from the heating microscope for determination of fusion/melting characteristic of welding flux.
2. The method as claimed in claim 1, wherein the shape of pellets are of truncated cone of 4 mm height, 3 mm bottom diameter and 1.5 mm top diameter.

3. The method as claimed in claim 1, wherein the pellets are prepared by uni-axial pressing of flux powder in a die without employing any binder.
4. The method as claimed in claim 1, wherein initial projected area of flux Pellet and area after each interval of temperature "T" respectively are calculated from room temperature to final temperature of each run.