Photo AI
State TWO factors responsible for causing residual stress in welds - NSC Mechanical Technology Welding and Metalwork - Question 9 - 2021 - Paper 1
Question 9
State TWO factors responsible for causing residual stress in welds. 1. Heat present in the weld. 2. Qualities of parent metal, filler rod or electrode.
Worked Solution & Example Answer: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
-
Heat present in the weld: The heat generated during the welding process can lead to thermal expansion and contraction, causing residual stresses.
-
Qualities of parent metal, filler rod or electrode: Variations in the physical and chemical properties of the materials involved can influence the amount and pattern of residual stress.
Step 2
State THREE factors that affect the cooling rate in a welded joint.
Answer
-
Size of work piece: Larger pieces take longer to cool due to their mass.
-
Weld thickness: Thicker welds retain heat longer, thus slowing the cooling rate.
-
Thermal conductive properties of parent metal: Materials with high thermal conductivity will dissipate heat more quickly, affecting cooling rates.
Step 3
Describe FOUR effects of cold working on steel.
Answer
-
Increased hardness: Cold working elongates and distorts the crystal structure, leading to greater hardness.
-
Decreased ductility: The process makes steel less ductile, meaning it is more prone to fracture under stress.
-
Increased strength: The process also increases tensile strength due to the grain structures being elongated.
-
Referred to as work hardening: The overall effect on the steel is often termed work hardening, which is essential for strengthening the material.
Step 4
State THREE effects of welding speed on distortion.
Answer
-
Increase in welding speed increases distortion: Faster welding speeds can lead to larger flame zones which expand and contract rapidly, resulting in more distortion.
-
Causes more residual stress: When the welding speed is high, the current distribution becomes more localized, potentially causing greater residual stresses.
-
Causing more distortion: The combination of fast cooling and uneven heating can lead to further distortion.
Step 5
State THREE quenching media used in the heat treatment of steel.
Answer
-
Water: Commonly used for quenching due to its high heat transfer capability.
-
Oil: Used to slow down the cooling process compared to water, reducing the risk of cracking.
-
Brine: A saltwater solution that enhances cooling rates compared to water alone.
Step 6
State THREE methods used to reduce distortion.
Answer
-
Do not over-weld: Limiting the amount of weld material can help reduce stresses that lead to distortion.
-
Use back stepping: This technique helps balance thermal contraction in the welded structure.
-
Heating metal before welding (pre-heating): This reduces the temperature gradient during the welding process, which can minimize distortion.