8.1 Name TWO factors that should be taken into account during oxy-acetylene welding to ensure high quality welding - NSC Mechanical Technology Welding and Metalwork - Question 8 - 2018 - Paper 1

HAZ stands for Heat Affected Zone, which is the area of the base metal that has not melted but has had its microstructure altered due to the heat generated during the welding process.

8.3.1 Spatter:

1. Too High Welding Current: An excessive amount of current can cause excessive spattering.
2. Disturbance in the Molten Weld Pool: This can occur due to wind or other factors affecting the stability of the weld pool.

8.3.2 Undercutting:

1. Too Fast Travel Speed: Moving too quickly while welding can result in not enough material being deposited, creating a groove.
2. Electrode Too Big: Using an electrode that is too large for the joint can lead to improper welding conditions.

8.3.3 Incomplete Penetration:

1. Welding Current Too Low: Inadequate current can lead to insufficient melting of the base material, resulting in incomplete penetration.
2. Insufficient Root Gap: A gap that is not large enough can restrict material flow and penetration into the joint.

8.4.1 Transverse Cracks:

1. Pre-heating the Base Metal: This can reduce the risk of cracks developing due to thermal stress.
2. Using Clamping Devices: Properly securing the components can prevent movement and reduce stress during cooling.

8.4.2 Centre-line Cracks:

1. Ensure that Width-to-Depth Ratio is 1:1: This helps maintain the structural integrity of the weld.
2. Decrease the Current: Lowering the current can help mitigate excessive penetration that leads to cracking.

A non-destructive test does not harm the welded joint and allows for the assessment of the weld's integrity without causing damage. In contrast, a destructive test involves testing methods that will damage or alter the welded joint, hence it is used to evaluate the ultimate strength of the materials or joints.

During an ultrasonic test, typical defects that can be identified include internal flaws such as porosity and cracks within the welded joint.

1. Slag Inclusion: This occurs when slag becomes trapped in the weld metal, leading to weakened joints.
2. Lack of Fusion: This defect arises if the weld metal did not fuse properly with the base metal.

A machinability test is conducted to determine the hardness and strength of the welded joint. This helps ensure that the material can be effectively machined after welding without causing excessive wear on cutting tools.

1. Uniformity of the Surface: The weld surface should be smooth without irregularities or inconsistencies.
2. No Visible Defects: There should be no signs of porosity or cracks visible on the surface of the weld.