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1) State the purpose of flux and describe its composition.
Flux composition and purpose
– composition of flux varies with different electrodes but it normally consists of composition purpose/effect
1) a combustible material like wood pulp,
– When this burns, carbon monoxide and hydrogen gases will be produced.
– This cloud of gas is called ‘gas shield’
– It keeps the air out during welding.
2) an arc stabilizer (e.g. Rutile, it is a natural ore of titanium oxide)
– Which helps maintain a steady electric arc
– To produce a hard brittle slag.
3) an asbestos
– it tends to combine with oxide and other impurities in the molten steel and float them out into a slag.
4) alloying elements (like manganese and carbon)
– they help in replenishing important constituents that may be burn out during welding.2) Describe briefly the different types of welds used in shipbuilding.
There are three main types of welds
A) Butt weld
B) Fillet weld
C) Tack weld
A) Butt weld
– plates are veed off to start. Angle between plates is about 60′ deg.
– plates are placed to maintain a small gap at the bottom of the ‘V’
– series of welding runs is made on the veed side.
– when the gap is completely filled on veed shide, a back run is made on the opposite side of the joint.
– thick plates can be doubled veed.
B) Fillet weld
– they are used for ‘T’ and lapped joints.
– the leg length is governed by the abutting part of the joint.
– throat thickness (T) is 70% of leg length (L).
– fillet weld can be continuous or intermittent.Full continuous fillet
– It is used when special strength is required at “T” joints.
– here vertical leg of the joint is veed. This help the weld metal penetrate right through.
Double continuous fillet
– it is used for oil and watertight joints.
– it can be single or double continuous fillet.
Intermittent fillet
– it is used for non watertight joints.
– it can be chain intermittent or staggered intermittent.
C) Tack weld
– it is used to hold the parts temporarily in place.3) State your understanding of the terms ‘single pass, multi pass and back run with reference to welding?
Single pass
– in welding process if only one pass is used to deposit the weld metal.
– it is called single pass.
– it can be the very first pass in the process.
– single pass welding is used in automatic welding processes where weld metal is deposited to complete the joint in one welding pass.Multi pass
– in welding process if more than one welding pass are used to ensure sufficient deposit of weld metal. – it is called multi pass welding.
– a back run is normally required to complete a weld.
– back runs are made on the reverse side of the joint.
Back run
– to complete the weld process which include the multi pass welding.
– a back run is made on the reverse side of the joint as mentioned in the figure.
– to do the back run, the joint must be cleaned out. Remove the slag if any.4) Describe the measures that taken to minimize distortion during welding.
Distortion during welding
– welding generate heats. That leads to expansion of metal.
– the weld metal contracts when it is cooled.
– the weld tried to pull the plate towards it on cooling.
– however the restraining action of the plate prevents the weld from contracting fully. – this causes structural deflection.
– the actual amount is difficult to predict.
Minimizing distortion during welding
– it was found that the distortion is more in the length than across of a metal.
– so to minimize the distortion it is important to weld butt (welding in athwart ship direction) first before the Seams (welding in fore and aft direction)
– to further minimize the distortion one can be used “the backstep” and “wandering” method of welding.5) Describe a gas cutting process that you are familiar with.
Gas cutting
Oxy-gas cutting
– in oxygas cutting process, a spot on the metal will be heated to ignition temperature (between 1400 F to 1600 F). This process is called preheating. Acetylene is used with oxygen to provide preheat flame. Propane or hydrogen could also be used.
– next, a jet of pure oxygen is directed at the heated metal. This oxidizes the metal rapidly. This rapid oxidation is called combustion. Thereby that part of the metal burn.
– the heat liberated by burning of iron or steel, melts the iron oxide formed by the chemical reaction and accelerates the preheating of the object.
– the molten material will be blown or washed out by the force of oxygen jet.
– the narrow slit formed in the metal as the cutting progresses. It is called kerf.6) Describe the electric arc welding.
– one wire from the electrical supply (from welding machine) is connected to the work.
– another wire is connected to the steel rod called electrode.
– the rod is allowed to touch the work and then withdrawn for a short distance.
– this creates an electric arc, the heat of which melts the electrode and the metal.
– this forms a pool of molten metal which later cools to form a solid joint.
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