BULKHEADS AND WATERTIGHT DOORS

WELDING IS FUSION PROCESS OF JOINING METALS, BY APPLYING INTENSE THE

METAL WHICH IS TO BE JOINED MELTS AND IS CAUSES TO INTERMIX WITH A FILLER
METAL. UPON COOLING A METALLURGICAL BOND IS CREATED.

ADVANTAGES OF WELDING:
1. WELDING HELPS IN MAKING USE OF THE PREFABRICATION PROCESS. VARIOUS
PARTS OF A SUPERSTRUCTURE CAN BE FABRICATED AT ONE TIME, AND THEN
JOINED BACK LATER,
2. IT HELPS TO ACHIEVE AIR AND WATER TIGHTNESS,
3. STRONGER JOINTS ARE PRODUCED QUICKLY,
4. THE WEIGHT OF STRUCTURE IS LESS AS COMPARED TO RIVETTED ONES,
5. LESS MAINENANCE IS REQUIRED TO MAINTAIN INTEGRITY OF JOINTS,
6. FORMS SMOOTHER HULL STRUCTURES THUS RESULTING IN LESS RESISTANCE
AND HENCE REDUCING POWER AND FUEL CONSUMPTION.

THE WELDING PROCESS USED IN SHIP BUILDING IS KNOWN AS FUSION WELDING.

FUSION WELDING IS ACHIEVED BY MEANS OF A HEAT SOURCE WHICH IS INTENSE
ENOUGH TO MELT THE EDGES OF THE BASE METAL AS IT IS TRAVERSED ALONG THE
JOINT.
THE HEAT SOURCES MAY BE GENERATED IN NUMBER OF WAYS:
1. GAS WELDING: IT WAS THE FIRST PROCESS TO BE USED FOR FUSION
WELDING. IT USES OXYGEN WITH FUEL GAS AS ACETYLENE, WHICH GIVES A
FLAME TEMP OF ABOUT 3000. AN OXY-ACETYLENE FLAME HAS TWO DISTINCT
REGIONS, AN INNER CORE, IN WHICH THE OXYGEN FOR COMBUSTION IS
SUPPLIED BY THE TORCH AND A SURROUNDING ENVELOPE IN WHICH THE
OXYGEN FOR COMBUSTION IS DRAWN FROM THE SURROUNDING AIR. BY
VARYING THE RATIO OF OXYGEN AND ACETYLENE IT IS POSSIBLE TO OBTAIN
DIFFERENT TYPES OF FLAMES. IF THE OXYGEN IS MORE, AN OXIDIZING FLAME
IS OBTAINED, WHICH IS USED FOR WELDING MATERIALS WITH HIGH THERMAL
CONDUCTIVITY eg. COPPER BUT NOT STEEL AS STEEL MAY GET
DECARBURIZED AND THE WELD POOL DEPLETED OF SILICON. WITH EQUAL
AMOUNT OF OXYGEN AND ACETYLENE A NEUTRAL FLAME IS OBTAINED,
WHICH IS USED FOR WELDING STEEL AND MOST OTHER MATERIALS. WITH
LESS OXYGEN AND MORE ACETYLENE A CARBURIZED FLAME IS OBTAINED,
THE EXCESS ACETYLENE DECOMPOSES INTO CARBON, WHICH CAN GO INTO
THE MOLTEN METAL AND PRODUCE METALLURGICAL PROBLEMS. THE OUTER
ENVELOPE OF OXY-ACETYLENE FLAME BY CONSUMING THE OXYGEN
PROTECTS THE MOLTEN METAL FROM OXYGEN WHICH IF UNPROTECTED CAN
DIFFUSE INTO THE MOLTEN METAL AND PRODUCE POROSITY WHEN THE WELD
METAL COOLS. OXY-ACETYLENE WELDING IS A SLOWER PROCESS BECAUSE
THE TEMP OBTAINED IS LESS IN COMPARISON TO MELTING TEMPERATURE OF
THE METAL. THEREFORE THE PROCESS IS APPLICABLE TO ONLY THINNER
MILD STEEL PLATES (UPTO 7MM) USED IN VENTILATION TRUNKING, CABLE
TRAYS ETC.
2. ELECTRIC ARC WELDING: AN ELECTRIC ARC IS FORMED WHEN CURRENT IS
ALLOWED TO FLOW BETWEEN TWO ELECTRODES SEPARATED BY A SHORT
DISTANCE. ONE ELECTRODE IS THE WELDING ROD OR WIRE AND OTHER IS THE

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METAL TO BE WELDED. THE WELDING ELECTRODE OR THE WIRE IS
CONNECTED TO AN ELECTRIC SUPPLY AND AN ARC IS CREATED BY
MOMENTARILY TOUCHING THE METAL AND THEN WITHDRAWING IT TO
CREATE A SMALL GAP BETWEEN THE ELECTRODE AND THE METAL. THE HIGH
TEMPERATURE ARC WILL MELT THE EDGES OF THE METAL AND THE WELDING
ROD OR WIRE. THE CONSUMABLE WELDING ROD HAS A FLUX COATING WHICH
WHEN BURNS PROVIDES AN INERT GAS SHIELDING TO THE MOLTEN METAL
AND THE ELECTRODE. WHEN THE FLUX BURNS IT CONSUMES THE
SURROUNDING ATMOSPHERE GAS WHICH WOULD OTHERWISE BE ABSORBED
BY THE MOLTEN METAL, IT ALSO STABILIZES THE ARC AND PROVIDES A
PROTECTIVE SLAG FOR THE MOLTEN METAL.
3. TUNGSTEN INERT GAS WELDING (TIG): A WATER COOLED NON
CONSUMABLE TUNGSTEN ELECTRODE SHIELDED BY INERT GAS IS USED. A
HIGH FREQUENCY DISCHARE OR CURRENT SURGE IS USED TO IGNITE THE ARC
AS TOUCHING THE ELECTRODE WILL CAUSE CONTAMINATION OF THE
EXPENSIVE ELECTRODE. FILLER METAL WHICH COULD BE WASTE PIECES OF
MATERIAL TO BE JOINED MAY BE ADDED TO THE WELD POOL FOR THICKER
PLATES. AC IS PREFFERED OVER DC AS IT STABILISES THE ARC AND PREVENT
OVERHEATING. ARGON IS NORMALLY USED AS SHIELDING GAS FOR
ALUMINIUM AND STEEL. A CERAMIC DISC DIRECTS THE GAS IN A SWIRLING
FLOW. THE PROCESS IS NORMALLY USED TO WELD THIN PLATES LESS THAN
6MM. IT IS A CLEAN PROCESS AS THERE IS NO FLUX, IT CAN BE USED
EFFECIENTLY FOR MULTIPASS RUNS.
4. METAL INERT GAS WELDING (MIG): THIS PROCESS IS SOMEWHAT SIMILAR TO
TIG WELDING, DIFFERENCE IS THE ELECTRODE IS CONSUMABLE, BEING
WOUND ON A REEL AND FED CONTINUOUSLY AT A CONSTANT RATE. THE
INERT GAS IS CONTAINED IN CYLINDERS WHICH IS PIPED TO THE TORCH. THE
ARC LENGTH IS CONTROLLED BY THE POWER SOURCE WHICH HAS VARIABLE
INDUCTANCE IN THE CIRCUIT TO PREVENT CURRENT SURGES.POWDERED
FLUX MAY B E ADDED TO THE GAS AND THE ELECTRODE WIRE MAGNETISED
SO THAT THE FLUX CLINGS TO IT AS IT ENTERS THE WELD POOL. THIS METHOD
IS CONTINUOUS, GIVES A GOOD QUALITY WELD WITH NO NEED OF FINISHING
AND THE EQUIPMENT IS PORTABLE AND SUITABLE FOR OPEN AIR WORK.
5. SUBMERGED ARC WELDING (SAW): IN THIS PROCESS THE ELCTRODE IS
SUBMERGED UNDER A LAYER OF GRANULATED FLUX, POURED ALONG THE
WELD JOINT BY MEANS OF A TUBE. THE ELECTRODE IS A COPPER COATED
FILLER WIRE. THE ELECTRODE AND THE FLUX FUSE TOGETHERWITH THE
JOINT, FLUX PROVIDENG A PROTECTIVE BLANKET. ANY UNUSED FLUX CAN BE
RECOVERED AND REUSED. THE FLUX ALSO THERMALLY INSULATES THE ARC,
PROVIDING A HIGH CONCENTRATION OF HEAT AND MAKING THE PROCESS
EXTREMELY EFFICIENT, WITH HIGH DEPOSITS POSSIBLE IN QUICK TIME.
6. THERMIT WELDING: IN THIS PROCESS THERE IS A CRUCIBLE LOADED WITH
THERMIT MIXTURE. ON IGNITION THE IRON OXIDE IS REDUCED TO IRON AND
ALUMINIUM OXIDES. WHEN THE REACTION IS COMPLETE, MOLTEN METAL IS
TAPPED AND CAST INTO THE MOULD. IT IS ESSENTIALLY A CASTING PROCESS
AND THIS METHOD IS MAINLY USED IN JOINING OF LARGE STEEL CASTINGS
SUCH AS STERN FRAMES.

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BUTT WELD: A BUTT WELD IS USED TO JOIN PLATES IN THE SAME PLANE.FOR
THINNER PLATES (UPTO 3MM) NO EDGE PREPARATION IS REQUIRED, THE EDGES ARE
BROUGHT CLOSE TO EACH OTHER AND A SQUARE BUTT WELD IS DONE. FOR THICK
PLATES (UPTO 18MM) DEEP PENETRATING ELECTRODES ARE USED OR THERE IS
EDGE PREPARATION IN THE FORM OF SINGLE “V” OR SINGLE “U”. FOR PLATES
THICKER THAN 18MM EDGE PREPARATION IS DONE BY A DOUBLE “V” OR A DOUBLE
“U”. A MACHINE EDGE IS SUPERIOR TO A FLAME CUT EDGE AND MUST BE USED FOR
BETTER AND EFFICIENT WELD QUALITY.
FILLET WELD: FILLET WELD IS USED TO JOIN PLATES IN DIFFERENT PLANES. THE
JOINTS MAY OR MAY NOT BE GROOVED. THE LATTER IS PREFFERED WHEN
STRENGTH IS NOT A CRITERIA.IN ORDER TO ACHIEVE FULL STRENGTH OF A PLATE
BY A SIMPLE FILLET WELD, THE FOLLOWING CONDITIONS MUST BE MET:
1. THE WELD MUST BE ON BOTH SIDES OF THE PLATE,
2. THE WELD MUST BE CONTINUOUS FOR THE FULL LENGTH OF THE PLATE,
3. THE LEG LENGTH MUST BE 75% OF THE CORRESPONDING PLATE THICKNESS.
IN ORDER TO PROVIDE STRENGTH, AN EDGE PREPARATION IN THE FORM OF SINGLE
“V” OR “U” IS USED. A DOUBLE “V” OR “U” WITH FULL PENETRATION IS USED FOR
THICKER PLATES.
OVERHEAD WELDING: OVERHEAD WELDING IS THE MOST DIFFICULT PROCESS IN
WELDING. TO RETAIN A COMPLETE CONTROL OF THE MOLTEN PUDDLE, A VERY
SHORT ARC AND REDUCED AMPERAGE IS RECOMMENDED. WHEN TOO LONG ARCS
ARE HELD, THE CHANCES OF MOLTEN METAL DRIPPING DOWN INCREASES. WHEN
THE ARC IS ROUTINELY SHORTENED AND LENGTHENED THE DROPPING OF MOLTEN
METAL IS PREVENTED.

IMPORTANCE OF FLUX IN WELDING

METALS AT VERY HIGH TEMEPERATURES TEND TO REACT WITH ELEMENTS
PRESENT I THE AIR-OXYGEN AND NITROGEN. WHEN THE METAL IN THE MOLTEN
POOL REACTS WITH SUCH ELEMENTS IT FORMS OXIDES AND NITRATES WHICH
DESTROY THE STRENGTH AND TOUGHNESS OF THE WELD JOINT. THEREFORE MANY
ARC WELDING TECHNIQUES INVOLVE SHIELDING THE MOLTEN POOL WITH GAS,
VAPOUR OR SLAG WHICH PREVENTS THE CONTACT OF MOLTEN POOL WITH AIR,
THIS IS CALLED AS ARC SHIELDING. FLUX PERFORMS THE SAME PURPOSE IN
ELECTRIC ARC WELDING:
1. IT STABILIZES THE AC AND HENCE MAKES AC TO BE USED SATISFACTORILY,
2. PROVIDES GAS SHIELDING TO THE ARC,
3. PROVIDES EASY STRIKING AND ARC STABILITY,
4. PROVIDES PROTECTIVE SLAG,
5. PROVIDES A GOOD WELD SHAPE,
6. PREVENTS ATMOSPHERIC CONTAMINATION,
7. GAS SHIELD CONSUMES THE SURROUNDING OXYGEN AND PROTECTS THE
MOLTEN METAL.

DISADVANTAGES OF WELDING
THE MAIN DISADVANTAGE IS THE DISTORTION EFFECT. BECAUSE OF THE INTENSE
HEAT SET UP IN THE ELECTRIC ARC, THE SURROUNDING METAL ALSO GETS HEATED
UP AND EXPANDS CAUSING EDGES BEING JOINED ACTUALLY COMING LITTLE
CLOSER TOGETHER THAN THEY WERE BEFORE GETTING HEATED. DUE TO THIS

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SOMETIMES THE PLATES GET DISTORTED AND GET BUCKLED. SOME OF THE
STRESSES CAUSING THIS WILL REMAIN IN THE JOINT AND ARE TERMED AS
RESIDUAL STRESSES. THICK PLATES OR THOSE WHICH ARE HELD FIRMLY SO THAT
THEY CAN’T MOVE MAY NOT BE ABLE TO BUCKLE OR TURN UP IN THIS WA. THE
STRESSES TRYING TO MAKE THEM DO SO WILL STILL EXIST AND WILL REMAIN IN
THE JOINT AS LOCKED UP STRESSES. DISTORTION CAN BE OF 3 TYPES:
TRANSVERSE, LONGITUDINAL AND ANGULAR.

FAULTS IN WELDING:
1. POROSITY: DUE TO THE PRESENCE OF DISSOLVED GASSES IN THE FORM OF
BUBBLES INSIDE THE WELD JOINT, CAUSED MAINLY BY OXYGEN, NITROGEN
AND HYDROGEN. CAN BE CONTROLLED BY CORRECT SELECTION OF THE
PARENT / FILLER METAL, WELDING PROCESS AND CLEAN AND DRY WORKING
ENVIRONMENT.
2. SLAG INCLUSION: SLAG GETS TRAPPED BETWEEN MULTI RUNS WELD WHEN
CLEANING IS NOT DONE PROPERLY BETWEEN THE RUNS.CHANCES OF SLAG
INCLUSION IS GREATER IF IT IS OF QUICK SOLIDIFYING TYPE. THIS CAN BE
AVOIDED BY PROPER CLEANING OF FLUX BEFORE STARTING THE NEXT RUN.
3. LACK OF FUSION: IT IS ONE OF THE MOST DANGEROUS FAULTS IN WELDING,
IT IS CAUSED DUE TO PRESENCE OF OIL, GREASE, DIRT OR RUST ON THE PLATE
SURFACE, OR DUE TO INSUFFICIENT CURRENT. THIS FAULT CAN BE AVOIDED
BY USE OF PROPER ELECTRODE, PROPER WELDING TECHNIQUE AND CURRENT,
PROPER EDGE PREPARATION AND CLEANING.
4. LACK OF PENETRATION: THIS FAULT IS CAUSED DUE TO GAP AT THE ROOT
OF THE WELD IF THICK PLATES ARE WELDED WITHOUT PROPER EDGE
PREPARATION. EXCESSIVE ROOT PENETRATION SHOULD ALSO BE AVOIDED AS
IT CAUSES STRESS CONCENTRATION. USE OF PROPER WELDING TECHNIQUE
AND CURRENT, PROPER ELECTRODE CAN AVOID THIS FAULT.
5. UNDERCUT: IT IS THE MELTING OR BURNING AWAY OF THE BASE METAL AT
THE TOE OF THE WELD. IT REDUCES THE THICKNESS OF THE METAL OR PIPE.
MAINLY DUE TO FAULTY WELDING TECHNIQUES.
6. CRACKS: THEY ARE GENERALLY LINEAR WITH HIGH LENGTH TO WIDTH
RATIO. THEY GENERALLY WEAKEN THE WELD. THEY CAN BE OF DIFFERENT
TYPES: RESTRAINTS, CRATER, HYDROGEN INDUCED, COLD OR HOT CRACKS OR
LAMELLAR CRACKS.