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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 eff... 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
Two significant factors causing residual stress in welds include:
- Heat present in the weld: The intense heating during welding can cause thermal expansion and contraction, resulting in residual stresses.
- Qualities of parent metal: Differences in the metallurgical properties of the parent material and the weld material can introduce stresses.
Step 2
State THREE factors that affect the cooling rate in a welded joint.
Answer
Three primary factors affecting the cooling rate in a welded joint are:
- Size of work piece: Larger pieces retain heat longer, affecting the cooling rate.
- Weld thickness: Thicker welds cool more slowly than thinner ones due to the larger volume of material.
- Thermal conductive properties of parent metal: Materials with higher thermal conductivity cool quicker, while those with lower conductivity will retain heat.
Step 3
Describe FOUR effects of cold working on steel.
Answer
Four effects of cold working on steel include:
- Increased hardness: Cold working leads to strain hardening, which increases the hardness of the steel.
- Reduced ductility: As the steel is deformed, its ability to stretch without breaking diminishes.
- Altered microstructure: Cold working can create an elongated and distorted crystal structure in the steel, impacting its properties.
- Enhanced tensile strength: The processes involved in cold working can improve the tensile strength of the steel, making it more resilient.
Step 4
State THREE effects of welding speed on distortion.
Answer
Three effects of welding speed on distortion include:
- Increase in distortion: Faster welding speeds can cause greater distortion due to the larger heat-affected zone leading to more thermal expansion and contraction.
- More residual stress: High welding speeds tend to increase the current, resulting in more localized heating, which can create residual stresses post-weld.
- Uneven heating: If the welding speed is too high, uneven heating throughout the material can result, causing warping and distortion.
Step 5
State THREE quenching media used in the heat treatment of steel.
Answer
Three common quenching media used in the heat treatment of steel include:
- Water: Widely used due to its effective cooling rate, though it can lead to distortion due to rapid temperature changes.
- Oil: Slower cooling than water, providing better control and reducing the risk of cracking or warping.
- Brine: A mixture of salt and water, which increases the cooling rate compared to plain water, providing a balance of speed and control.
Step 6
State THREE methods used to reduce distortion.
Answer
Three effective methods to reduce distortion during welding include:
- Do not over-weld: Limiting the amount of weld metal can help to control heat input and reduce residual stress.
- Use back stepping: This technique helps to counterbalance the tendency for distortion by altering the sequence of welds.
- Pre-heating the metal: Heating the base metal before welding can reduce the temperature gradient, thus minimizing distortion.