Lesson Notes By Weeks and Term v4 - SHS 1
WELDING TECHNOLOGY
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WELDING TECHNOLOGY
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Subject: Applied Technology
Class: SHS 1
Term: 2nd Term
Week: 15
Grade code: 1.4.2.LI.4
Strand code: 4
Sub-strand code: 2
Content standard code: 1.4.2.CS.1
Indicator code: 1.4.2.LI.4
Theme: METAL TECHNOLOGY
Subtheme: WELDING TECHNOLOGY
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This lesson introduces two fundamental methods of joining metals: Gas Welding and Electric Arc Welding. In Ghana, welding is a critical skill used everywhere from our local communities to large industries. We see its results in the burglar-proof bars on our windows, the frames of commercial vehicles ("trotros"), the construction of buildings, and the repair of farming equipment. Understanding the strengths and weaknesses of each welding method helps a technician or engineer choose the right tool for the job, ensuring safety, efficiency, and quality. This lesson will empower you to make informed comparisons between these two important technologies.
Welding is the process of permanently joining two or more pieces of metal by applying heat, pressure, or both. The heat melts the metal at the joint, and as it cools, the pieces fuse together to form a strong bond. Today, we will focus on two very common methods that use heat: Gas Welding and Electric Arc Welding. A. Gas Welding (Specifically, Oxy-acetylene Welding)
This is a fusion welding process that uses the heat from the combustion of a fuel gas (acetylene) mixed with pure oxygen. Working Principle: Acetylene gas and oxygen are stored in separate high-pressure cylinders. The gases flow through regulators and hoses to a welding torch. Inside the torch, they are mixed in a specific ratio and ignited at the nozzle. This creates an intensely hot flame (approximately 3,200°C). The welder directs this flame onto the edges of the metals to be joined, creating a molten pool. A separate 'filler rod' is often melted into this pool to add more metal and create a stronger joint. When the flame is removed, the molten pool cools and solidifies, joining the pieces together. Key Equipment: Oxygen cylinder (typically black or blue), Acetylene cylinder (typically maroon), pressure regulators, hoses (green for oxygen, red for acetylene), welding torch with nozzles, filler rods, and safety gear (goggles, gloves). Advantages of Gas Welding: High Portability: It does not require electricity. The entire setup (cylinders and torch) can be transported to any location, making it perfect for work on farms, in rural areas, or on construction sites without power. Greater Control: The welder has excellent control over the heat input and the rate at which the filler metal is added. This makes it ideal for welding thin or delicate metal sheets that could be easily burned through with an electric arc. Versatility: The same equipment can be used for other jobs besides welding, such as cutting steel (oxy-fuel cutting), brazing, and heating metal for bending. Lower Initial Equipment Cost: A basic oxy-acetylene setup can sometimes be cheaper to purchase than a high-quality electric arc welding machine. Disadvantages of Gas Welding: Slower Welding Speed: The heating process is less concentrated than an electric arc, so welding is significantly slower. This makes it less efficient for large projects. Not Suitable for Thick Metals: The flame cannot produce enough concentrated heat to effectively penetrate and weld thick metal sections (e.g., above 6mm). Large Heat-Affected Zone (HAZ): The heat from the flame spreads over a wider area of the metal. This can cause the metal sheet to warp or bend (distortion). Ongoing Cost & Safety: The continuous need to refill or rent gas cylinders can be expensive. Handling and storing highly flammable, high-pressure gases poses a significant safety risk if not done correctly. B. Electric Arc Welding (Specifically, Shielded Metal Arc Welding - SMAW or "Stick Welding")
This is the most common type of electric welding in Ghana. It uses an electric arc to generate intense heat for welding. Working Principle: A welding machine (power source) is connected to an electricity supply (mains or a generator). Two cables are connected to the machine: one with a ground clamp attached to the workpiece (the metal being welded), and another with an electrode holder. A consumable electrode (a metal rod covered in a special chemical coating called flux) is placed in the holder. The welder touches the tip of the electrode to the workpiece and then pulls it back slightly. This creates a continuous electric spark, or 'arc', between the electrode and the metal. This arc is extremely hot (over 5,000°C), instantly melting the tip of the electrode and the surface of the workpiece, creating a molten pool. The flux coating on the electrode also burns, creating a shielding gas that protects the molten weld pool from oxygen and nitrogen in the air, preventing a weak or porous weld. The burned flux forms a protective layer called 'slag' on top of the weld, which is chipped off after it cools. Key Equipment: Welding machine (transformer or inverter type), electrode holder, ground clamp, consumable electrodes ("sticks"), and safety gear (welding helmet with dark lens, leather gloves, apron). Advantages of Electric Arc Welding: High Strength on Thick Materials: It generates intense, concentrated heat, allowing for deep penetration. This creates very strong welds and is excellent for joining thick steel plates used in construction and heavy equipment repair. Faster Welding Speed: The high temperature allows for much faster metal deposition and travel speed compared to gas welding, making it more productive for larger jobs. Cost-Effective Consumables: Welding electrodes are relatively cheap and easy to find. Works Well Outdoors: The flux provides robust shielding, making it effective even in windy conditions where a gas shield (from other welding types like MIG) would blow away. Disadvantages of Electric Arc Welding: Requires Electricity: The biggest limitation. It cannot be used in locations without access to a stable power source or a powerful generator. More Skill Required for Thin Metals: The intense heat can easily burn through thin sheets of metal if the welder is not highly skilled. It is less suitable for delicate work compared to gas welding. Significant Clean-up: The process produces a lot of smoke, fumes, and 'spatter' (small droplets of molten metal). The slag must be carefully chipped off the finished weld, adding an extra step to the process. Not Suitable for All Metals: Standard SMAW is mainly used for steel and other ferrous metals. It is not easily used for non-ferrous metals like aluminium or copper.
Guided Practice (With Solutions)