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9.1 Describe the meaning of shrinkage in a welded joint - NSC Mechanical Technology Welding and Metalwork - Question 9 - 2019 - Paper 1
Question 9
9.1 Describe the meaning of shrinkage in a welded joint. 9.2 State FOUR factors that affect distortion and residual stress during welding. 9.3 Explain back-step we... show full transcript
Worked Solution & Example Answer:9.1 Describe the meaning of shrinkage in a welded joint - NSC Mechanical Technology Welding and Metalwork - Question 9 - 2019 - Paper 1
Step 1
Describe the meaning of shrinkage in a welded joint.
Answer
Shrinkage in welding refers to the physical contraction that occurs in a welded joint as the material cools from a molten to a solid state. This phenomenon is a type of plastic deformation, wherein the metal experiences a change in size due to the thermal contraction. Essentially, as the welded joint solidifies, the metal shrinks, which can lead to internal stresses and potential warping if not properly managed.
Step 2
State FOUR factors that affect distortion and residual stress during welding.
Answer
- Heat Input: The amount of heat introduced during welding can cause the material to expand; when it cools, it can distort.
- Cooling Rate: Rapid cooling can lead to greater stresses than slower cooling, affecting the overall distortion.
- Weld Joint Design: The geometry of the joint influences how heat is distributed and can lead to differences in distortion.
- Material Properties: Variations in material composition can also impact how the material reacts to heat and stress.
Step 3
Explain back-step welding as a method to reduce distortion by using a neatly labelled sketch.
Answer
Back-step welding is a technique used to minimize distortion during the welding process. In this method, the welder moves backward along the weld joint rather than progressing linearly. This allows the heat to dissipate more evenly, reducing the risk of deformation. The following sketch illustrates the direction of each bead segment in back-step welding:
Direction of each bead segment
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1 | 2 |
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3 | |
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Direction of weld
By applying this method, the heat-affected zones are minimized, leading to better control over distortion.
Step 4
State FOUR factors that affect the temperature at which cold-worked steel will recrystallise when heated.
Answer
- Prior Amount of Cold Work: The extent to which the steel has been deformed affects its recrystallization temperature. More extensive cold working typically lowers the recrystallization temperature.
- The Temperature and Time of Annealing Process: Higher annealing temperatures and longer times generally facilitate recrystallization.
- Composition of the Metal: The alloying elements present in the steel can significantly influence its recrystallization characteristics, as different elements alter the microstructure.
- The Melting Point: The melting point of the material sets a limit for recrystallization; steel will start to behave differently as it approaches this temperature.