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9.1 State TWO factors that affect distortion and residual stress in a welded joint - NSC Mechanical Technology Welding and Metalwork - Question 9 - 2023 - Paper 1
Question 9
9.1 State TWO factors that affect distortion and residual stress in a welded joint. 9.2 Describe shrinkage in a welded joint. 9.3 State TWO factors that affect the... show full transcript
Worked Solution & Example Answer:9.1 State TWO factors that affect distortion and residual stress in a welded joint - NSC Mechanical Technology Welding and Metalwork - Question 9 - 2023 - Paper 1
Step 1
State TWO factors that affect distortion and residual stress in a welded joint.
Answer
There are several factors that contribute to distortion and residual stress in a welded joint:
- Expansion and Contraction: When metal is heated during welding, it expands; if this expansion is restricted, distortion can occur. Conversely, when the metal cools and contracts, this can also introduce stresses if not properly accounted for.
- Applied Stress: If the applied stresses from the welding process cause movement, distortion will occur. However, if the applied stress does not lead to movement, then residual stress will remain in the joint.
Step 2
Describe shrinkage in a welded joint.
Answer
Shrinkage refers to a form of plastic deformation that occurs when metal contracts as it cools after being welded. This contraction can lead to dimensional changes in the welded joint, potentially resulting in misalignment and internal stresses.
Step 3
State TWO factors that affect the grain size of cold-worked steel.
Answer
The grain size in cold-worked steel is influenced by several factors:
- Prior Amount of Cold Work: The greater the amount of deformation, the finer the grain size tends to become due to work hardening.
- Temperature and Time of the Annealing Process: The temperature and duration of any annealing that follows cold working can significantly affect grain growth, impacting overall material properties.
Step 4
Describe the difference between cold working and hot working of steel.
Answer
Cold working and hot working are two different processes of deforming steel:
- Cold Working: This process takes place at temperatures below the recrystallisation temperature (AC1) of steel. It enhances strength through strain hardening but may lead to increased brittleness if too much deformation is applied.
- Hot Working: This process occurs above the recrystallisation temperature (AC1), allowing for significant deformation without hardening, which retains more ductility in the steel.
Step 5
State FOUR methods used to reduce distortion.
Answer
To minimize distortion during welding, the following methods can be employed:
- Avoid Over-welding: Excessive welding can induce too much heat and consequently more distortion.
- Apply Intermittent Welding: Using intermittent welds instead of continuous ones can help control heat input.
- Place Welds Near the Neutral Axis: Positioning welds properly reduces the moment that leads to distortion.
- Use Jigs and Fixtures: Proper clamping and support can help maintain the alignment of components during the welding process.
Step 6
Identify the types of distortion in FIGURES 9.6.1 and 9.6.2.
Answer
The types of distortion in the provided figures are as follows:
- FIGURE 9.6.1: This illustrates longitudinal distortion, where the weld causes elongation or shortening along the length of the joint.
- FIGURE 9.6.2: This depicts angular distortion, where welding causes misalignment in angles, leading to undesirable geometric configurations.
Step 7
Explain the result when metal is cooled rapidly. Refer to the internal and external areas of the metal.
Answer
When metal is cooled rapidly, significant temperature gradients can occur between the internal and external areas. The external surface cools and contracts quicker than the interior, setting up internal stresses. These stresses may lead to cracking or warping in the material, particularly if the cooling rates exceed the material’s thermal shock resistance.