Class: Senior Secondary School 2 (SS2 / SSS2)
Term: 1st Term
Week: 8
Age: 16 years
Duration: 45 minutes
Subject: Metal Work
Curriculum Theme: Pre-Vocational Studies
Previous Lesson: Oxy-Acetylene Welding Equipment.
Topic: Oxy-Acetylene Welding Flame
Subject Matter: Welding flames (neutral flame, oxidizing flame, carbonizing flame), backfire and flashback, welding methods (leftward welding, rightward welding)

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

Define the three types of oxy-acetylene welding flames.
Explain the uses of each welding flame type.
Differentiate between backfire and flashback.
Describe leftward and rightward welding methods.

Affective Domain:

Appreciate the importance of correct flame adjustment for quality welds.
Recognize the safety precautions associated with backfire and flashback.

Psychomotor Domain:

Adjust an oxy-acetylene torch to produce neutral, oxidizing, and carbonizing flames.
Perform basic leftward and rightward welding operations.

Work safely and cooperatively with others during practical demonstrations.

Reference Materials

The following resources were used in planning this lesson:

Senior Secondary Schools Education Curriculum
State Unified Scheme of Work
Metal Work for Senior Secondary Schools Textbook

Instructional Materials

The teacher will teach this lesson with the aid of:

Oxy-acetylene welding equipment (torch, cylinders, regulators)
Spark lighter
Filler rods
Base metals
Safety goggles
Welding gloves

Rationale for the Lesson

Understanding different welding flames and methods is important for pupils to safely and effectively join metals. This knowledge enables pupils to select the correct flame for specific welding tasks and improve their practical welding skills.

Prerequisite/Previous Knowledge

Pupils should have basic knowledge of metalwork hand tools, safety practices in the workshop, and the principles of gas welding.

Lesson Content/Board Summary

Oxy-Acetylene Welding Flame

Types of Oxy-Acetylene Welding Flames

Oxy-acetylene welding uses a mixture of oxygen and acetylene gases to create a high-temperature flame. The ratio of these gases can be adjusted to produce different types of flames, each with specific characteristics and uses.

The following are the three main types of oxy-acetylene welding flames:

Neutral Flame: This flame has an equal ratio of oxygen and acetylene. It is characterized by a clear, well-defined inner cone, a luminous white feather, and a pale blue outer envelope. It is the most commonly used flame for general welding of mild steel, cast iron, stainless steel, and copper, as it causes minimal oxidation or carburization.
Oxidizing Flame: This flame has an excess of oxygen compared to acetylene. It is characterized by a shorter, sharper inner cone and a more intense hiss. The outer envelope is also shorter and bluer. Oxidizing flames are used for welding brass and bronze, as the excess oxygen helps to form a protective oxide layer that prevents zinc fuming. It is not suitable for steel as it causes oxidation and brittleness.
Carbonizing (Carburizing/Reducing) Flame: This flame has an excess of acetylene compared to oxygen. It is characterized by a longer, feathery inner cone (acetylene feather) and a less defined outer envelope. Carbonizing flames are used for hardfacing operations, welding high-carbon steels, and some non-ferrous metals. The excess acetylene adds carbon to the weld metal, which can increase hardness but also make the metal brittle if not controlled.

Backfire and Flashback

These are dangerous occurrences that can happen during oxy-acetylene welding if safety procedures are not followed.

Backfire: This is a momentary popping sound at the torch tip, followed by the flame either extinguishing or reigniting. It occurs when the flame burns back into the torch tip.

Causes: Overheating of the torch tip, touching the tip against the workpiece, low gas pressure, incorrect tip size, or a loose tip.

Action: Immediately dip the torch tip in water or clean it, then relight.

Flashback: This is a more serious condition where the flame travels back beyond the mixing chamber into the hose and potentially into the gas cylinders. It is indicated by a high-pitched whistle, a burning smell, and smoke from the regulators.

Causes: Faulty or absent flashback arrestors, incorrect gas pressures, worn hoses, or a dirty torch nozzle.

Action: Immediately close the oxygen valve on the torch, then the acetylene valve on the torch, followed by the cylinder valves. Never continue welding after a flashback without checking all equipment.

Oxy-Acetylene Welding Methods

There are two primary methods for directing the torch and filler rod during welding:

Leftward (Forehand) Welding: In this method, the torch moves from right to left (for right-handed welders), and the filler rod precedes the torch. The flame points in the direction of welding. This method is generally used for thinner materials (up to 3mm) because it provides better visibility of the weld pool and allows for faster welding speeds.
Rightward (Backhand) Welding: In this method, the torch moves from left to right (for right-handed welders), and the filler rod follows the torch. The flame points back towards the completed weld. This method is preferred for thicker materials (above 3mm) as it provides better heat penetration, allows for slower cooling rates, and generally produces stronger welds with less distortion.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher greets the pupils and reviews the previous lesson. The teacher then introduces the topic by asking pupils if they know how metal parts are joined using heat.
Pupils’ Activity: Pupils respond to the teacher’s questions and recall previous knowledge.
Learning Point: Pupils are prepared for the new lesson on oxy-acetylene welding flames.

Step 2: Explanation of Oxy-Acetylene Welding Flames

Time: 10 minutes
Teaching Skill: Explanation/Lecture
Teacher’s Activity: The teacher defines and explains the three types of oxy-acetylene welding flames: neutral, oxidizing, and carbonizing flames, along with their characteristics and specific uses. The teacher uses diagrams or charts to illustrate.
Pupils’ Activity: Pupils listen attentively, observe the illustrations, and take notes.
Learning Point: Pupils understand the different types of welding flames and their applications.

Step 3: Demonstration of Flame Adjustment

Time: 8 minutes
Teaching Skill: Demonstration/Practical
Teacher’s Activity: The teacher, using the oxy-acetylene equipment, safely demonstrates how to set up the equipment and adjust the torch to produce each of the three flame types (neutral, oxidizing, carbonizing). Safety precautions are emphasized.
Pupils’ Activity: Pupils observe the demonstration carefully, noting the visual differences and sounds of each flame type.
Learning Point: Pupils learn the practical skill of adjusting welding flames and the importance of safety.

Step 4: Explanation of Backfire and Flashback

Time: 7 minutes
Teaching Skill: Explanation/Discussion
Teacher’s Activity: The teacher defines backfire and flashback, explaining their causes, dangers, and the immediate corrective actions to take for each. The teacher stresses the importance of flashback arrestors.
Pupils’ Activity: Pupils listen, ask questions, and discuss the safety implications of backfire and flashback.
Learning Point: Pupils understand the risks of backfire and flashback and how to respond safely.

Step 5: Explanation of Welding Methods

Time: 7 minutes
Teaching Skill: Explanation
Teacher’s Activity: The teacher explains the two primary oxy-acetylene welding methods: leftward (forehand) and rightward (backhand) welding. The teacher describes the movement of the torch and filler rod for each method and their typical applications.
Pupils’ Activity: Pupils listen and take notes on the characteristics and uses of each welding method.
Learning Point: Pupils understand the principles and applications of leftward and rightward welding.

Step 6: Demonstration of Welding Operations

Time: 5 minutes
Teaching Skill: Demonstration/Practical
Teacher’s Activity: The teacher briefly demonstrates the basic torch and filler rod movements for both leftward and rightward welding on a base metal, emphasizing proper technique and safety.
Pupils’ Activity: Pupils observe the welding demonstrations, paying attention to the torch angle and rod manipulation.
Learning Point: Pupils gain visual understanding of the welding methods in practice.

Step 7: Evaluation/Review

Time: 5 minutes
Teaching Skill: Questioning/Assessment
Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

Define a neutral flame and state its primary use.
Differentiate between an oxidizing flame and a carbonizing flame.
What is backfire, and what immediate action should be taken?
Explain the difference between leftward and rightward welding methods.

Pupils’ Activity: Pupils answer orally and in writing.
Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 3 minutes
Teaching Skill: Summarizing
Teacher’s Activity: The teacher summarizes the key learning points on welding flames, backfire, flashback, and welding methods. The teacher then gives pupils a homework assignment to draw and label the three types of welding flames.
Pupils’ Activity: Pupils listen to the summary and copy down the homework assignment.
Learning Point: Pupils consolidate their understanding of the topic.

Lesson Keywords

Oxy-Acetylene – A gas welding process using a mixture of oxygen and acetylene.
Neutral Flame – A flame with an equal ratio of oxygen and acetylene, used for general welding.
Oxidizing Flame – A flame with excess oxygen, used for welding brass and bronze.
Carbonizing Flame – A flame with excess acetylene, used for hardfacing and high-carbon steels.
Backfire – A momentary burning back of the flame into the torch tip.
Flashback – A serious condition where the flame travels back into the hose or cylinder.
Leftward Welding – A welding method where the torch moves right to left, filler rod precedes the torch.
Rightward Welding – A welding method where the torch moves left to right, filler rod follows the torch.

Differentiation

For pupils who are struggling, the teacher will provide simplified diagrams and allow more supervised practice time with flame adjustment. Advanced pupils will be challenged to research specific applications of each flame type in different industries or explore advanced welding techniques.

Note for teachers using this lesson plan

Ensure all safety precautions are strictly followed during demonstrations involving oxy-acetylene equipment. Emphasize the dangers of flashback and backfire. Practical demonstrations should be conducted in a well-ventilated area with appropriate personal protective equipment. Encourage pupils to ask questions regarding safety and technique.