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9.1 State TWO factors responsible for causing residual stress in welds - NSC Mechanical Technology Welding and Metalwork - Question 9 - 2021 - Paper 1
Question 9
9.1 State TWO factors responsible for causing residual stress in welds. 9.2 State THREE factors that affect the cooling rate in a welded joint. 9.3 Describe FOUR e... show full transcript
Worked Solution & Example Answer:9.1 State TWO factors responsible for causing residual stress in welds - NSC Mechanical Technology Welding and Metalwork - Question 9 - 2021 - Paper 1
Step 1
State TWO factors responsible for causing residual stress in welds.
Answer
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Heat Present in the Weld: The heat generated during welding can create temperature gradients, leading to thermal expansion and contraction, which contributes to residual stress.
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Qualities of Parent Metal: Variations in the material properties of the parent metal can result in different levels of thermal expansion and contraction, thus influencing the residual stress levels.
Step 2
State THREE factors that affect the cooling rate in a welded joint.
Answer
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Size of Work Piece: Larger pieces have more mass and retain heat longer, thus affecting the cooling rate.
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Weld Thickness: Thicker welds absorb more heat and therefore cool at a slower rate compared to thinner welds.
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Thermal Conductive Properties of Parent Metal: Materials with higher thermal conductivity will diffuse heat more quickly, resulting in a faster cooling rate.
Step 3
Describe FOUR effects of cold working on steel.
Answer
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Increase in Hardness: Cold working leads to dislocation movements within the crystal structure, enhancing the hardness of the steel.
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Reduction in Ductility: As the steel is cold worked, its ability to deform without fracture diminishes, resulting in reduced ductility.
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Elongation of Crystal Structure: Cold working tends to elongate and distort the crystal structure of the steel, impacting its mechanical properties.
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Referred to as Work Hardening: The process of increasing hardness through the application of mechanical stress is commonly known as work hardening.
Step 4
State THREE effects of welding speed on distortion.
Answer
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Increase in Welding Speed: A faster welding speed can increase distortion due to rapid heat generation and uneven cooling.
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Larger Diameter Electrode Requires More Heat: Using a larger diameter electrode increases current and heat concentration, which can lead to more localized heating and distortion.
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Causing More Residual Stress: Variability in speed can cause differences in the cooling rates of weld regions, leading to enhanced residual stresses and greater distortion.
Step 5
State THREE quenching media used in the heat treatment of steel.
Answer
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Water: Commonly used due to its high heat capacity which allows for rapid cooling.
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Oil: Slower cooling rate than water, reducing the risk of cracking.
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Brine: A saltwater solution offering an even faster cooling rate than plain water.
Step 6
State THREE methods used to reduce distortion.
Answer
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Do Not Over-weld: Limiting the amount of weld material can minimize heating and cooling stresses.
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Place Welds Near the Neutral Axis: This positioning helps in balancing the forces and minimizing distortion.
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Use Pre-heating: Heating the metal before welding reduces the temperature differences and controls distortion.