Oxy–acetylene practice — applications of a neutral flame: A neutral flame (approximately equal oxygen and acetylene) is generally suitable for welding which of the following materials?

Introduction / Context:
A neutral flame is produced when oxygen and acetylene are supplied in roughly equal proportions, eliminating excess oxygen (oxidising) or excess acetylene (carburising). Selecting the correct flame type is crucial to minimise oxidation, decarburisation, or carburisation of the base metal during welding.

Given Data / Assumptions:

• We consider common workshop welding of steels, cast irons, and coppers.
• No special fluxes or proprietary gas mixes are assumed.
• Joint preparation and preheat requirements may still apply.

Concept / Approach:
A neutral flame neither adds carbon to the weld pool nor scavenges it, making it a safe default for many ferrous and non-ferrous metals. Mild and low-alloy steels, cast irons (with appropriate preheat and technique), and copper can be welded with a neutral flame to control metallurgical side effects. Some special cases may use slightly oxidising (for copper deoxidation) or slightly carburising (for aluminium bronze), but the general-purpose, exam-level answer remains the neutral flame for all three materials listed.

Step-by-Step Solution:
Define neutral flame: balanced oxygen and acetylene, with a well-defined inner cone and no feather.Assess material response: steel and cast iron dislike excessive oxidation; copper oxidises readily but can be welded neutrally with flux.Conclude that a neutral flame is broadly applicable to steel, cast iron, and copper.

Verification / Alternative check:
Welding handbooks list neutral flame as the baseline for steels and as acceptable for cast iron and copper with appropriate fluxes and preheat management.

Why Other Options Are Wrong:
Restricting to only one metal ignores standard practice.

“None of these” contradicts well-known applications of neutral flame.

Common Pitfalls:
Using an oxidising flame on steels leading to brittle welds; failing to preheat cast iron sufficiently; not using deoxidising flux on copper, which increases porosity risk.

Final Answer:
all of these