Oxy-fuel cutting of steel — when does the cut edge harden? During oxygen cutting (oxy-fuel), the cut edge of a steel plate can harden due to rapid heating and quenching. At approximately what carbon content does this edge-hardening become significant?

Introduction / Context:
Oxy-fuel cutting works by preheating steel to ignition temperature and then oxidizing iron with a jet of oxygen. The heat-affected zone (HAZ) experiences rapid thermal cycles. For certain carbon levels, the HAZ can transform to hard martensitic structures upon self-quenching, creating a brittle edge that is harder to machine or weld.

Given Data / Assumptions:

• Process: oxy-fuel cutting of plain carbon steels.
• Cooling: relatively rapid due to surrounding cold metal acting as a heat sink.
• Question: identify the approximate carbon content where hardening risk rises.

Concept / Approach:
Hardenability increases with carbon content. Steels above about 0.3% C have enough carbon to form significant martensite if heated above the austenitizing range and quenched quickly. In oxy-cutting, the thin HAZ can cool fast enough to produce martensite for C > ~0.3%, giving a hard rim that may require grinding or tempering.

Step-by-Step Solution:

Verification / Alternative check:
Shop practice flags oxy-cut edges on medium/high-carbon steels for post-heat or tempering; low-carbon steels (< 0.25–0.3% C) seldom show problematic hard rims.

Why Other Options Are Wrong:

• Less than 0.1% or 0.3%: carbon too low for significant martensite formation.
• Anywhere 0.1–1% regardless: cooling rate and composition both matter; not all compositions harden the same.
• Only above 1.2%: hardening begins far below this level.

Common Pitfalls:
Assuming all oxy-cut edges are equally hard; ignoring preheat/slow-cooling practices that reduce hardening, especially before welding.

Final Answer: