NABTEB FABRICATION AND WELDING ANSWERS
FABRICATION & WELDING OBJ!
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INSTRUCTION: ANSWER FIVE(5) QUESTIONS ONLY
(1a)
(i) Scriber
(ii) Center Punch
(iii) Steel Rule
(1b)
(i) Snips or Shears – hand tools used for cutting thin sheets of metal.
(ii) Hacksaw – used for cutting metal sheets and rods manually.
(iii) Guillotine Shear (or Bench Shear) – a machine tool used for cutting larger or thicker metal sheets accurately.
(1ci)
Soft soldering is a low-temperature metal joining process that involves melting a filler metal (solder) with a melting point below 400°C to bond components, typically using a soldering iron. It is commonly used in electronics and light metalwork.
(1cii)
The difference between soft soldering and hand soldering is that hand soldering is a common technique of soft soldering where a soldering iron with a pre-tinned tip is manually applied to heat the joint and melt the solder. Hand soldering is essentially a method of performing soft soldering by hand, often used in electronics for accessible and precise joints.
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(2a)
(i) Strength: The ability of a metal to resist breaking or deformation under applied forces.
(ii) Ductility: The ability of a metal to be drawn into wires without breaking.
(iii) Hardness: The resistance of a metal to indentation or scratching.
(2b)
(i) Painting
(ii) Polishing
(iii) Varnishing
(iv) Electroplating
(2ci)
A template is a pattern or gauge used to mark out or shape materials consistently and accurately in manufacturing or construction. It serves as a guide for cutting or shaping workpieces to the desired form.
(2cii)
(i) Rivets – Metal pins used to permanently join metal sheets or plates.
(ii) Welding – A process that fuses two pieces of metal together by melting their edges.
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(4a)
(i) Steel
(ii) Cast iron
(iii) Wrought iron
(4b)
Ferrous metals contain iron, while non-ferrous metals do not contain iron. This difference affects their properties: ferrous metals are usually magnetic, have high tensile strength, and are more prone to rust and corrosion, whereas non-ferrous metals are lighter, more resistant to corrosion, and generally not magnetic.
(4ci)
Copper is a pure metal with the chemical symbol Cu, and its composition is 99.9% pure copper, sometimes with small traces of oxygen or other elements.
(4cii)
Hardening is the process involves heating the entire metal component and then cooling it rapidly (quenching) so that the whole cross-section becomes uniformly hard and strong whereas Case hardening is the process hardens only the surface layer of the metal by infusing elements like carbon or nitrogen into the surface. The result is a hard, wear-resistant outer layer while the core remains softer and tougher, improving resistance to impact and fatigue.
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(5a)
(i) Cracks: These are serious defects that can cause failure of the weld, occurring as longitudinal, transverse, or crater cracks. They may be caused by factors like incorrect arc termination, high welding currents, or excessive joint restraint.
(ii) Porosity: This defect consists of gas bubbles trapped inside the weld metal, leading to weak spots. It is caused by contamination, moisture, or improper shielding gas.
(5b)
(i) Tensile Test: This test involves pulling a welded specimen until it breaks to measure the weld's strength and ductility. It determines how much load the weld can withstand before failure.
(ii) Bend Test: In this test, the welded joint is bent to a specified angle to check for cracks or defects in the weld and heat-affected zone. It assesses the weld’s ductility and soundness.
(5c)
(i) Improper welding technique
(ii) Contaminated base material
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(6a)
(i) Lower Cost and Maintenance:
AC welding machines are generally cheaper to purchase and maintain compared to DC machines, making them more affordable for general fabrication work.
(ii) Simpler Design and Greater Durability:
AC machines have fewer internal components (no rectifiers), making them more rugged and less prone to damage during rough handling or in outdoor environments.
(6b)
(i) Storage in a Dry Oven
Electrodes should be stored in a specially designed electrode drying oven at a controlled temperature (usually between 100°C to 350°C depending on the electrode type). This prevents moisture absorption from the atmosphere, which can cause hydrogen-induced weld defects such as porosity and cracking. The oven keeps the electrodes dry and ready for use.
(ii) Sealing in Airtight Containers or Packaging
After drying, electrodes can be kept in sealed airtight containers or vacuum-sealed packaging to prevent them from absorbing moisture during storage or transportation. This method is especially important for basic or low-hydrogen electrodes that are highly sensitive to moisture.
(6c)
(i) Classification by coating type: Electrodes are classified based on the type of flux coating they have, such as cellulose, rutile, or basic coatings, which affect arc stability, penetration, and weld appearance.
(ii) Classification by welding current and polarity suitability: Electrodes are categorized according to whether they are suitable for AC or DC welding, and whether they work best with electrode positive or electrode negative polarity.
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COMPLETED
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November 27, 2024, 09:47:22 am
