The general technique for doing a straight weld is to do small arcs side-to-side, pausing for a fraction of a second at the sides (shown bottom right). Another technique is to follow figure of 8 patterns all the way down (this gives slightly longer time at the sides). This pausing gives the weld pool a better shape.
The most important factor in Arc Welding is the set up also the correct size and type of rod has to be used to fit the job in hand. Other factors include: Arc length, current setting and moisture. These will be discussed in more detail in “Welding Effects on Microstructure”.
Process Applications
Brazing
Brazing is performed at low temperature at which the brazing alloy melts but not the base material. Brazing can be performed on different types of metal due to the nature of the bonding. It is more suited to joining different metals when the parent metals need to be kept intact since it does not melt them.
The negative aspects of brazing are that brazing alloys are relatively expensive in comparison to Arc rods, which already have flux on them. The different metals can cause sacrificial protection to occur thus corroding the more reactive metal. The process is time consuming and involves more than one stage in its process.
Gas Welding
Gas Welding is probably one of the earliest of modern welding history and also one of the most widely used processes. Of all the combustible gases Acetylene is the most widely used gas for welding mainly because the process is versatile, adapted to many different jobs, ease of production and transportation and that the equipment is relatively of low cost.
SMAW (Shielded Metal Arc Welding)
SMAW welding is widely used in the fabricating industries for the construction and repair of plain carbon and alloy steels. It is also used to weld surfacing type electrodes to make the base material wear resistant. SMAW welding has a lower equipment cost than other types of Arc welding like TIG/MIG as you do not need equipment like gas hose etc. SMAW can be used to weld in tight spaces with less difficulty. This process also deposits the filler at a faster rate than TIG/MIG. The equipment of the SMAW welding is easy to move and can be used out doors I conditions of up to medium wind.
The disadvantages of SMAW are that the cost of filler per weld is greater as one electrode deposits a small quantity of filler metal. You also need skin and eye protection from UV radiation emitted and rate of production is slow because you have to change the electrode and scrape the slag.
Procedure of SMAW
Aim
Using 6 mm thick low carbon steel plate as the parent material. Deposit a weld bead on each side of the parent using the shielded metal arc process. Vary the size of electrode in each case.
Equipment used
DC voltage/current power supply, 2.5 mm and 4 mm flux covered electrodes, 10mm thick low carbon steel plate, metal work table (earthed), gloves, protection helmet, electrode holder, pliers and stop clock.
Procedure
- Two welds are deposited one on each side of the plate. A stop clock is used to record the time taken to weld each side. This is done in pairs as one does the welding and the other records the time and then they swap places for the second weld. Different size of electrode is used during each welding procedure.
- The first weld is carried out using a 4mm electrode. The transformer is set at DC positive, 25V and 175A.
- Check the circuit to see that the voltage/current settings are correct and that the power supply is ON. Ensure the welding area is curtained off, to protect those who may be nearby. Check all your skin is covered. Check the worktable to see that it is earthed. Check Equipment for damage. Check that the electrode is secure in holder. Ensure that the electrode does not touch the worktable until the protection helmets are put on. Warn the person holding the timer when about to start.
- Scratch the rod on the bench to heat it, so it is easier to start.
- Check starting position
- Ensure that the angle of the electrode is about perpendicular to the plate surface and penetration or travel angle is about 45°.
- Begin the weld and start the stop clock.
- Adjust arc length and speed accordingly to get neatest weld.
- Once the weld is complete raise the electrode from the plate and stop the clock.
- Remove the helmets, and use a pair of pliers to take the steel plate in a sink to cool it down under cold water and then dry off the plate.
- This procedure is repeated by the other person using the 2.5 mm electrode with the transformer settings at 25V and 75A.
- Both times recorded in a table.
RESULTS
Using these results we can calculate the rate of heat input. The heat input rate is one of the most important variables in fusion welding as its variation affects the microstructure.
Heat input rate (q/v) (J/mm) is given by equation: Heat input rate = nVI/v.
n = Process Efficiency
V = Arc Voltage
I = Weld Amperage
v = Speed of Travel
The speed of travel (v) = d/t.
Weld 1
v = d/t
= 146/48
= 3.04 mm/sec
n= 0.75
V=25V
I=175
V=3.04 mm/s
Heat input rate = nVI/v
= (0.75*25*175)/3.04
= 1.079 KJ/mm
Weld 1
v = d/t
= 142/31
= 4.58 mm/sec
Heat input rate = nVI/v
= (0.75*25*75)/4.58
= 0.307 KJ/mm
Results Discussion
The results show that the greater the diameter of electrode the less time it takes to weld. This is shown by the speed of travel calculated. The 4mm diameter electrode has a speed of 3.04 mm/s where as 2.5mm diameter electrode has a speed of 4.58 mm/s. Weld 1 has a heat input rate of 1.079 KJ/mm where as weld 2 has 0.307 KJ/mm. Weld 1 has a greater heat input rate because of the higher current used and it took longer to make.
Welding Effects on Microstructure
Steel is an alloy of iron and carbon. Because steel contains carbon this means that you can alter its mechanical properties by heat transfusion. You can see changes in structure under microscope, usually due to effect of heat. If you heat up steel and put it in water it becomes hard and brittle. On the other hand a process called annealing that involves melting steel in a furnace and then allowing it to cool very slowly. Steel now becomes soft and ductile. These are the two extreme heat treatments of steel–maximum hard and maximum soft.
Figure A shows a typical microstructure of steel. Normally we see two types of grain. Ferrite appears white under microscope and is pure iron (mainly). Pearlite contains carbon constituent and is seen dark brown to black under microscope. Carbon present in steel is not just in elemental form, it is also present in layers of iron carbide. Any temperature above 200°c alters the Pearlite grains.
Temperatures involved in welding are way above 200°c and hence there is a significant change in structure. The weld pool is the only area that is ever liquid. In HAZ 1& 2 the structure is affected by heat from the weld. Diagrams HAZ 1 & 2 show these structures found in these zones.
In both heat affected zones 2 significant things occur:
1. The carbon diffuses away from weld pool towards the parent plate. This means HAZ 1 becomes depleted of carbon. It is deposited in HAZ 2, which becomes enriched with carbon. Normally this makes HAZ 1 becomes softer than HAZ 2.
2. In HAZ 1 there is grain growth (re-crystallisation), this is because HAZ 1 temperature is higher than HAZ 2 and is high enough to allow the grains to grow. The Grains become large and the soft. For these reasons HAZ is the weakest point in the weld.
Ideally we want to limit the size of HAZ and limit the amount of grain growth. Heat input has the biggest affect on this. Excessively large HAZ due to too much heat input results in large weakness. Opposite extreme is too little heat, in that case the weld doesn’t penetrate as well which is equally bad. You have to compromise some where in the middle – between too much or too little heat.
Flaws in the weld
At high temperature the flux on weld rod or electrode should break down and form slag and . Both the slag and form a protective seal that prevents the steel from oxidation because if you expose liquid steel to air it would oxidise instantly. Because slag is less dense than steel, it floats on the surface and provides physical barrier to atmosphere. The generated provides secondary barrier. Slag and halo of gas above it both prevent contacting liquid steel.
It is very common that slag and particles become trapped in weld pool due to insufficient heat. If weld pool then solidifies too fast it traps particles and bubbles. The bubbles get trapped the weld becomes porous and slag particles form holes filled with slag. This is very common in most welds but these don’t cause too much trouble. But if particles are too large or numerous it can significantly weaken the weld.
Three reasons normally due to which this occurs are: Welding current is set too low, Rate of travel is too fast and Arc length is too long then the welding rod drips rather than to penetrate on the parent plate.
Undercutting
Undercutting is a problem, normally happened due to too much heat input, rather than depositing a nice bead of weld metal it cuts deeps and can cut a hole though the parent plate easily. You end up with a trench along either side of the weld pool. This is undercut.
Two reasons why this is problem: The undercut reduces the thickness of the parent plate and the weld comes weak in that area. And due to its location at HAZ, which is already soft so undercut ca act as a crack initiator.
Too much heat is caused by three factors: Welding current set too high, Rate of travel too slow and Arc distance is too small or short – this means all the energy from the Arc is concentrated into a smaller area.
Flaws like these; porosity, slag accumulation and undercutting can only be prevent through experience.
Hydrogen Embrittlement
Hydrogen Embrittlement occurs when water is introduced to weld when welding. Energy from the arc is so intense that water doesn’t get a chance to evaporate. And it separates the hydrogen from the oxygen. The oxygen bubbles form tiny bubbles of slag (not too much trouble). Hydrogen remains as a bubble and as the weld pool contracts around the hydrogen it undergoes extreme pressure. This pressure can force cracks to run all the way up to the surface.
Preparation
The welds are cut in cross-section. It is not necessary although it’s because it is easier to prepare a small section of the weld then a large one and cross section gives most information about the weld s and heat affected zones (HAZ).
We use a resin-bonded aluminium oxide-cutting wheel. During the cutting it is important to keep the metal as cool as possible to minimise the damage to the microstructure. Microstructure is the grain structure seen under a microscope. During cutting lots of cooling water is used. Once weld is cut it is mounted into epoxy resin. This makes it easier to hold hazardous- sharp edges. Once mounted you need to grind the surface 2-3 mm to get to the true structure for analysis. No matter how carefully the steel is cut the damage is always done but grinding is essential. Grinding is done using silicon carbide grinding papers. Wet & dry sand paper, silicon carbide is much harder than usual sand paper and so is more suitable for grinding metal. 5 different grades of papers are used. Coarsest first then smoother and smoother papers are used. During grinding there is cooling water applied to reduce friction, stops more damage being done to the surface. After the final paper there should be a very fine scratch pattern on the surface. Fine scratches are removed by polishing. Cloth like velvet stuck to flat plastic base is used. Cloth above is not enough to polish so diamond paste is applied to the cloth. Diamond paste is very fine and abrasive with an average particle size of approximately 3 microns. Instead of cooling water blue lubricant is used to cool. Lubricant is alcohol based, hence evaporates quickly and therefore keeps the surface cool and prevents friction.
After the polishing we should have a perfect mirror finish with scratch-free surface. Before we can view the sample we must etch the surface with a dilute acid. Acid used for iron and steel is 2% Nitrol (commonly used for similar tasks). The acid is made up from 2% Nitric acid and 98% alcohol-dilute acid. It is applied for about 30 seconds using swabs of cotton wool soaked in it and then rinsed the acid and dry the sample. This is done only to etch the surface and not to corrode it. This will now reveal the microstructure.
Photographic Technique
The cross sections of the two welds are shown here. This technique is used to analyse the microstructure of the weld. The equipment used is a microscope and a monitor connected to it. The monitor displays a magnified image of what is seen under the microscope. We can use this to study various aspects of the welds such as the size of heat affected zones, angle of penetration and flaws like bubbles of carbon dioxide, oxygen, hydrogen, undercuts and cracks.
Observations
The first weld is a very wide weld of about 14mm wide. The metallographic examiner stated that this was a surprisingly well formed weld pool for such a wide weld. The shape of the weld was described as perfect although there are the initial signs of undercutting beginning to appear on the left. There is a small amount of spatter on the right had side as well. Two minute slag pores were detected close to the surface of the weld pool, but the metallographic examiner stated that these did not affect the strength of the weld to cause concern. The HAZ’s for this weld stated to be of correct depth and width for the weld.
The second weld was much poorer than the first. It is 5mm wide. There was a great deal of spatter attributed to this weld. It was very narrow and HAZ 1 was stated to be 400 microns deep. There was a slag pore found to be 200 microns wide, apart from that the weld pool was of perfect shape and low in porosity.
Conclusions
The only small problems with the first weld could have been attributed to the angle of the rod not being perpendicular to the plate and making the curving pattern too wide. Since the angle was not perpendicular the weld pool was slightly pushed to one side giving the first signs of undercutting. The arc length may have been slightly long thus a small degree of spatter was viewed.
The main problem elements with weld 2 were that the arc length was too high and the rate of travel was too fast. This caused a high degree of porosity and spatter. Also the skill of the welder was somewhat limited thus some overlapping did occur.
The HAZ’s of both weld were within specified depths and weld pools were of near perfect shape, thus the cross-section of both welds show that they were quite strong.
