---
title: "Gas flames, arc welding and heat input"
subject: "Manufacturing and Production Processes"
chapter: 12
key: "semester_4/manufacturing_and_production_processes:oxyacetylene-flames-smaw-heat-input-numerical"
source: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/oxyacetylene-flames-smaw-heat-input-numerical
site: BCT Engineering
---

# Gas flames, arc welding and heat input

Manufacturing and Production Processes · Chapter 12: Material Joining Processes

- Asked in: year not given, 3+3+3 marks
- Video: [Gas Welding](https://www.youtube.com/watch?v=TaVus6RwdLc) — Pammi Kumari
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/oxyacetylene-flames-smaw-heat-input-numerical
- Whole chapter: https://www.bctengineer.com/md/semester_4/manufacturing_and_production_processes/chapter-12

## Question

(a) Describe the three types of oxyacetylene flame and their uses. (b) Explain the principle of shielded metal arc welding (SMAW) and the functions of the electrode coating. (c) In arc welding at 24 V and 200 A the travel speed is 5 mm/s. The heat transfer efficiency is 0.8. Find the heat input per unit length of weld, and the cross-sectional area of weld metal that can be melted if the melting factor is 0.5 and the unit melting energy of the steel is 10 J/mm$^3$.

## Short answer

Oxyacetylene flames are neutral (steel welding), carburising (hard facing, aluminium), oxidising (brass/bronze). SMAW uses flux-coated consumable electrode; coating shields gas, slag, stabilises arc, alloys/deoxidises. Heat input = 0.8x24x200/5 = 768 J/mm; melted area = 0.5x768/10 = 38.4 mm2.

## Answer

#### (a) Oxyacetylene flames

Acetylene burns with oxygen in a torch. The flame has an inner cone (about 3100 °C at its tip) and an outer envelope. The ratio of oxygen to acetylene decides the type of flame:

| Flame | O$_2$ : C$_2$H$_2$ | Appearance | Use |
|---|---|---|---|
| Neutral | about 1 : 1 | Clear, sharp inner cone | Welding steel, cast iron; most work |
| Carburising (reducing) | Excess acetylene | Feather around the inner cone | Hard facing, aluminium, Monel |
| Oxidising | Excess oxygen | Short, pointed inner cone | Brass, bronze (prevents zinc loss), cutting |

#### (b) SMAW

A consumable **flux-coated electrode** is held in a holder, and an arc (typically 20-30 V, 50-300 A) is struck between the electrode tip and the work. The arc heat melts the electrode core wire and the base metal, and the molten core wire is transferred to the weld pool as droplets. The coating burns and forms slag, which covers the weld. The current can be AC or DC (electrode negative or electrode positive).

**Functions of the coating:**
1. Produces a shielding gas (CO$_2$, H$_2$O) that protects the molten metal from oxygen and nitrogen in the air.
2. Forms slag that protects the cooling weld and slows its cooling.
3. Stabilises the arc (potassium and sodium compounds).
4. Adds alloying elements and iron powder (which increases deposition).
5. Acts as a deoxidiser that refines the weld metal, and helps shape the bead.

#### (c) Heat input and melted area

Arc power:

$$
P = VI = 24\times200 = 4800\ \text{W}
$$

Heat input per unit length of weld, with heat transfer efficiency $f_1 = 0.8$:

$$
H = \frac{f_1\,VI}{v} = \frac{0.8\times4800}{5} = 768\ \text{J/mm}
$$

The fraction of the transferred heat used for melting is $f_2 = 0.5$. Energy balance for melting:

$$
f_2\,H = U_m\,A_w
$$

$$
A_w = \frac{0.5\times768}{10} = 38.4\ \text{mm}^2
$$

**Answer:** heat input $= 768$ J/mm (0.768 kJ/mm); melted weld area $= 38.4$ mm$^2$.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which flame has excess acetylene and is used for hard facing and aluminium?
   A. Neutral flame
   B. Carburising (reducing) flame
   C. Oxidising flame
   D. Plasma flame
   Answer: B. Excess acetylene gives a carburising flame with a feather around the cone; neutral is for steel, oxidising for brass and bronze. (Key point: Oxyacetylene flame types)

2. Which is a function of the SMAW electrode coating?
   A. It conducts the main current instead of the core wire
   B. It forms shielding gas and slag and stabilises the arc
   C. It makes the electrode non-consumable
   D. It cools the weld pool quickly
   Answer: B. The coating burns to give shielding gas and slag, stabilises the arc, deoxidises and can add alloying elements; the slag slows cooling. (Key point: Coating functions)

3. For 24 V, 200 A, 5 mm/s and efficiency 0.8, what is the heat input per unit length?
   A. 960 J/mm
   B. 768 J/mm
   C. 614 J/mm
   D. 4800 J/mm
   Answer: B. H = f1 V I / v = 0.8 x 4800 / 5 = 768 J/mm. (Key point: Heat input per length)

4. With melting factor 0.5 and unit melting energy 10 J/mm^3, what area is melted when H = 768 J/mm?
   A. 76.8 mm^2
   B. 38.4 mm^2
   C. 153.6 mm^2
   D. 384 mm^2
   Answer: B. From f2 H = Um Aw, Aw = 0.5 x 768 / 10 = 38.4 mm^2. (Key point: Melted weld area)

## Related in Manufacturing and Production Processes

- [EDM principle and ECM metal removal rate](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aedm-ecm-principle-mrr-numerical) — Manufacturing and Production Processes, chapter 8
- [Weld metallurgy, HAZ and heat sources](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aweld-metallurgy-haz-defects-energy-sources) — Manufacturing and Production Processes, chapter 12

## Related in other subjects

- [Arc welding: SMAW, TIG and MIG](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Aarc-welding-smaw-tig-mig) — Workshop Technology, chapter 10
- [Resistance spot welding heat and efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Aresistance-spot-welding-heat-numerical) — Workshop Technology, chapter 10
- [Oxy-acetylene gas welding](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Agas-welding-flames-equipment) — Workshop Technology, chapter 10
- [Principle of heat treatment of metal](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aheat-treatment-principle) — Civil Engineering Materials, chapter 8, asked 5×