2.1 The Iron-Carbon Phase Diagram

The Foundation of Welding Metallurgy

The iron-carbon (Fe-Fe₃C) phase diagram is the single most important tool in welding metallurgy. It maps which phases — distinct crystal structures — exist in plain-carbon steel as a function of temperature (vertical axis) and carbon content (horizontal axis). A Certified Welding Inspector does not have to draw the diagram, but must understand what it predicts: how a given steel behaves as the welding arc heats it past its critical temperatures and then cools rapidly back to room temperature. Every preheat requirement, every cracking concern, and every post-weld heat treatment decision traces back to this diagram.

Steel is iron alloyed with up to about 2.0% carbon. Above 2.0% carbon the alloy is cast iron, which is brittle and welded only with special procedures. The vast majority of structural and pressure work uses carbon and low-alloy steels below about 0.30% carbon, where weldability is good. As carbon content rises, the steel becomes harder and stronger but progressively harder to weld without cracking.

Steel Classification by Carbon Content

The eutectoid point sits at approximately 0.77% (often rounded to 0.80%) carbon at 1,333°F (723°C). A steel exactly at the eutectoid composition transforms entirely to pearlite on slow cooling. Steel below 0.77% C is hypoeutectoid (the structural-steel range); steel above 0.77% C is hypereutectoid.

Critical Temperatures

The critical (transformation) temperatures are the lines on the diagram where phase changes occur. They are labeled with the letter A (from the French arrêt, meaning arrest, because the temperature pauses during transformation). Inspectors must know A1 and A3 because they bound the austenitizing range — the temperatures to which the heat-affected zone (HAZ) is driven during welding.

The subscript c denotes heating (chauffage) and r denotes cooling (refroidissement). Because welding cools far faster than equilibrium, the actual Ar temperatures are suppressed — the faster the cooling, the lower the temperature at which austenite finally transforms, and the harder the resulting product.

Phases and Microstructures in Steel

A key exam fact: austenite (FCC) dissolves far more carbon (up to ~2.0%) than ferrite (BCC, only ~0.022%). When austenite cools slowly, the excess carbon precipitates as cementite in an orderly fashion, forming ferrite + pearlite. When it cools too fast to diffuse, carbon is trapped in solution, distorting the lattice into hard martensite.

Why Cooling Rate Decides the Outcome

The diagram is an equilibrium map — it assumes infinitely slow cooling. Real welds cool quickly, so the inspector must combine the phase diagram with the cooling rate to predict the actual HAZ microstructure. The decisive cooling range is roughly 1,500°F to 900°F (about 800°C to 500°C), often called the t8/5 (cooling time from 800°C to 500°C). What forms depends on how fast the steel passes through this band:

• Slow cooling → ferrite + pearlite → soft, ductile, usually acceptable
• Moderate cooling → bainite → stronger, still reasonably tough
• Fast cooling (quench) → martensite → very hard, brittle, crack-prone

This is the bridge between metallurgy and inspection: anything that speeds cooling (thick sections acting as a heat sink, low heat input, no preheat, cold ambient temperatures) pushes the HAZ toward martensite. Anything that slows cooling (preheat, higher heat input, maintained interpass temperature) keeps it soft.

For the Exam: The core principle is that faster cooling produces harder, more brittle microstructures. Preheat is applied specifically to slow the cooling rate through the 1,500–900°F range, suppressing martensite and the cracking it enables. Memorize A1 ≈ 1,333°F and that austenite is the non-magnetic FCC high-temperature phase.