The development of new materials in engineering has led to the problem of improvement of old methods in order to weld them satisfactorily. For obtaining good results under various circumstances, we will study below the proper welding procedure to be used for various metals and also the proper shielding atmosphere, proper fluxing material and filler metal desired.
1. Ingot Iron:
Due to its purity, uniformity of grain structure and freedom from gas-forming metalloids, ingot iron has excellent welding characteristics and it can be welded by any or the welding processes. Further as it has low carbon content it does not harden after welding has been carried out. Therefore heat treatment before or after welding is also not needed.
2. Wrought Iron:
It can be readily welded by any of the commonly used welding processes. The slight amount of slag present in the wrought iron has a fluxing action and protects the metal during heating, thereby producing good results.
3. Low Carbon Steels (Carbon upto 0.30%):
These are easily welded by all the welding processes and the resultant welds and joints are of extremely high quality.
4. Medium Carbon Steels (Carbon from 0.30 to 0.50%):
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These can be welded by the various fusion processes. The technique and materials used are dictated by the metallurgical characteristics of the base metal. In some instances pre-heating and subsequent heat treatment may be required to produce the desired weld quality, especially in steels containing more than 0.40% carbon.
5. High-Carbon Steels (Carbon more than 0.50%):
As these steels contain high carbon content these are more difficult to weld. These are therefore, welded for very few applications, such as for repairing cracked or worn parts of high-carbon steel.
With proper care, the following types of welding processes can be utilised for high carbon steels:
i. Gas welding,
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ii. Shielded metal arc welding,
iii. Submerged arc welding,
iv. Resistance welding,
v. Pressure gas welding.
6. Tool Steels (Carbon: 0.80 to 1.50%):
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Compared to other carbon steels, tool steels are relatively difficult to weld. Gas welding and pressure gas welding processes are recommended for welding of tool steels. In gas welding, proper welding rods having such carbon content which will ensure the carbon percentage of deposited metal at least equal to the base metal should be used.
Preferably carburising or excess acetylene flame should be used for securing strong welds. Proper flux as used for cast-iron welding is also essential. It is also necessary to pre-heat the steel and anneal it after welding.
7. Galvanised Steels:
A coating of zinc is applied to the steel to protect it against corrosion. Zinc has a melting point of about 400°C and is volatile when heated to about 920°C. Therefore, when galvanised iron or steel is heated to welding temperature (1500°C), zinc vaporises and cloud of white zinc oxide fumes is formed.
This is dangerous for the workers and, therefore, the welding should be carried out in well ventilated location. Generally fusion welding is carried out for galvanised steel by the shielded metal-arc process. It can also be brazed with the carbon arc without causing any injury to the galvanised coating. The filler metal used can be either copper, silicon or manganese alloy.
8. Cast Steels:
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The weld ability of cast steel is comparable to that of wrought steel with variations according to the chemical composition of the steel. Usually arc welding, gas welding, brazing and pressure welding are the processes used for welding the cast steels.
9. Cast Irons:
Carbon in cast iron is present in either free or chemically combined form. During welding both free and combined carbon go into solution in the molten metal. If it is suddenly cooled, a large amount of combined iron is retained which results in formation of hard metal in the weld.
Thus to reduce this effect rate of cooling is reduced by preheating the cast iron. Every care is required to prevent breakage due to uneven expansion or contraction of casting. The composition of filler metal should be same as that of base metal.
(i) Grey Cast Iron:
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Generally gas welding is used for it due to lack of ductility in the metal. First the metal is preheated to about 500°C and then welding carried out by neutral flame using backhand technique. Proper filling material having low melting point and containing practically no sulphur and phosphorus is due.
On completion of welding the entire casting is covered with suitable high temperature insulating material to prevent chilling effects. Other methods of welding are shielded metal, arc welding using special electrodes, carbon arc welding, thermit welding, braze welding and brazing.
(ii) Malleable Iron:
For this, gas welding, shielded metal arc welding and braze welding processes are used.
10. Low Alloy Steels:
In welding of low alloy steels, the small amount of alloy elements commonly affect the metallurgical response principally because of their influence upon the austenite transformation of the metal during cooling. Also these are more susceptible to the cold cracking in the heat-affected zone when subjected to some welding processes.
The metallurgical behaviour of the alloying elements during welding influences various properties of the steels. The microstructure of steel also has a definite influence on the behaviour of metal in the heat-affected zone adjacent to the weld. Thus in order to develop the required mechanical properties in welded low-alloy steels, proper preheats and post- heat treatments are essential. Sometimes stress-relieving operation is also essential.
The various methods of welding for various alloys are:
i. Gas Welding:
a. Hot rolled steels,
b. Heat treated steels.
ii. Shielded Metal Arc:
Hot rolled steels.
iii. Inert Gas Metal Arc:
a. Hot rolled
b. Heat treated steels.
iv. Submerged Arc:
a. Hot rolled
b. Heat treated steels.
v. Atomic Hydrogen:
a. Hot rolled
b. Heat-treated low-alloy tensile steels.
vi. Thermit Welding:
Nearly all low alloy steels.
Resistance welding. Hot rolled steels and alloys having low hardenability.
11. Chromium Iron and Steels:
These contain chromium from 3—30% and are non-hardening type of steels. These steels require special protection against oxidation. Non-oxidising atmosphere is used for excluding air from the liquid weld metal.
Also there is a provision for suitable flux to assist in the removal of chromium oxide (extremely refractory material) from the liquid weld metal before it solidifies. Atomic hydrogen, inert gas and metal arc are the best processes for welding it, as these have proved to be very effective for preventing excessive oxidation.
Pre-heating, inter-pass temperature control and post-heating operations are also very essential for stress relieving and improving corrosion resistance. However, gas welding, resistance welding and pressure gas welding could also be employed.
12. Stainless Steel:
It can be welded satisfactorily by nearly all the processes. But under certain conditions, it tends to show what is commonly called weld decay, when it is heated within the temperature range of 470°C to 880°C or cooled slowly through this range. In this range the carbon is precipitated from solid solution mainly at the grain boundaries forming chromium-rich carbides.
The conditions is very much susceptible to corrosion. There are several ways to overcome this inter-granular corrosion ; one is the appropriate heat treatment wherever possible. Other method consists in adding the suitable amount of either titanum or columbium in the steel. These metals form their carbides, thus preventing the formation of chromium carbide when the metal is heated between 470°C and 480°C. Proper heat treatment after welding process is very essential.
Precautions for Welding Stainless Steel:
Welding of stainless steel may pause problems of war page and distortion, carbide precipitation, and matching up of chemical elements.
Stainless steel having lower thermal conductivity results in an uneven distribution of heat and distortion. This can be taken care of by lowering heat input by using small weld beads, using stringers instead of weaves, or using intermittent or back steeping beads.
In welding of stainless steel, carbon, because of its affinity for heat, precipitates into the weld zone and combines with the chromium atom to form chromium carbide which does not have shield effect as of pure chromium and thus results into oxidation or rusting.
This can be taken care of by using stabilized electrodes having columbium and titanium which form their carbides and chromium oxide remains free to shield the metal from further oxidation.
The matching up of chemical elements is essential to ensure that both weld and base metal have same properties and act in unison under load. This can be achieved by using same filler metal grade as the base metal to be welded.
13. Aluminium and its Alloys:
Aluminium alloys are generally available in two forms, i.e. non-heat treatable and heat-treatable. Strength in non-heat treatable alloys is developed by strain hardening and by alloying elements. In annealed or soft tempered type, the composition of the alloy wholly controls the strength whereas in the strain-hardened type, the degree of strain hardening is the controlling factor.
The alloying elements in the heat treatable aluminium alloys are dissolved in the aluminium at a high temperature by the process of solution heat treatment and maintained in solution by quenching from this temperature. Additional increase in strength is obtained by precipitation of a portion of the soluble elements of finely divided form.
The heat of welding decreases the strength of both types of aluminium alloys. The strength hardening effect and effect of precipitation treatment are adversely affected by the heat of welding. The extent of detrimental effect is directly proportional to temperature and time. Therefore proper post- heat treatment is very essential.
Also the proper selection of flux, filler metal and distortion care are equally important. Most of commercial welding processes like arc, gas, electric resistance, brazing and soldering can be used for welding of these alloys.
Welding of aluminium by GTAW-DCSP—helium method:
GTAW:
Direct current-helium welding with negatively polarised electrode is quite popular for welding 0.2 to 12 mm thick aluminium with single pass square-butt joints. The cleaning effect present in GTAW and GMAW is absent if DCSP is used due to negative polarity of electrode.
Oxide particles are therefore likely to be dragged and can cause flaws not usually detected by X-rays. To be sure that oxide skin is properly broken up and does not cause flaws in the joint, helium or a mixture of helium and upto 40% organ is used to protect the weld pool. This allows much higher arc voltages and therefore high concentration of heat which produces a small, fluid weld pool and localised tearing of the surface.
The high concentration of heat produced by GTAW- DCPS-helium welding offers the advantages of narrow heat affected zone, deep and uniform penetration, and minimum distortion of the workpiece. To ensure that edges are clean at the time of welding, the dressing should be done just before welding and during machining, grease free lubricants (industrial alcohol) should be used.
Manual welding calls for a high degree of skill in torch and weld pool control by controlling current by a foot peda. Pointed electrode tip results in good arcing and deep penetration. Blunt tip electrodes make welding easier and give more uniform results, but reduce the depth of penetration. The electrode workpiece distance must be kept less than 1 mm. Mechanised welding is preferred for uniform results.
14. Copper and its Alloys:
These can be welded by most of the commonly used methods such as gas welding, resistance welding, brazing and soldering.