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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 the welding process can lead to thermal expansion and contraction, resulting in residual stress within the metal.
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Qualities of parent metal: The chemical composition and physical properties of the parent metal can influence the degree of stress that develops post-welding.
Step 2
State THREE factors that affect the cooling rate in a welded joint.
Answer
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Size of work piece: Larger components may cool at a slower rate due to the greater volume of material.
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Weld thickness: Thicker welds retain heat longer, affecting the cooling rate.
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Thermal conductive properties of parent metal: Materials with higher thermal conductivity will dissipate heat more quickly compared to those with lower conductivity.
Step 3
Describe FOUR effects of cold working on steel.
Answer
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Increased hardness: Cold working alters the crystal structure, leading to an increase in hardness.
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Reduced ductility: The material becomes less ductile as the grains elongate and distort, making it less capable of deforming without fracturing.
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Enhanced tensile strength: The distortion of the crystal structure contributes to an increase in tensile strength, allowing it to withstand greater loads.
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Work hardening: The process introduces dislocations within the metal's structure, further strengthening it through work hardening.
Step 4
State THREE effects of welding speed on distortion.
Answer
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Larger flame in oxy-acetylene welding: An increase in welding speed results in a larger flame size, which can exacerbate distortion.
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Increased current: Faster welding speeds lead to more concentrated heat, which increases the current and can cause more localized heat impacts.
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Greater residual stress: Higher speeds can create more thermal gradients, resulting in greater residual stress due to uneven cooling.
Step 5
State THREE quenching media used in the heat treatment of steel.
Answer
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Water: Commonly used due to its availability and effective cooling capabilities.
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Oil: Provides a slower cooling rate than water, reducing the risk of cracking.
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Brine: A saltwater solution that enhances the cooling rate compared to plain water.
Step 6
State THREE methods used to reduce distortion.
Answer
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Avoid over-welding: Limiting the amount of weld metal can help in reducing the heat input and resultant distortion.
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Use back stepping: This technique entails moving the weld away from the previous weld to minimize the effects of heat accumulation.
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Preheat metals before welding: Heating the metal prior to welding can help in reducing thermal stress during the welding process.