2.1 The Iron-Carbon Phase Diagram
Key Takeaways
- The iron-carbon phase diagram predicts microstructure based on temperature and carbon content
- Steel is classified by carbon content: low-carbon (0.05–0.15%), mild (0.15–0.30%), medium (0.30–0.45%), high (0.45–0.75%)
- Critical temperatures A1 (~1,333°F) and A3 mark phase transformation boundaries
- Faster cooling rates produce harder, more brittle microstructures (martensite)
- Preheat slows cooling rates to prevent martensite formation and cracking
- The critical cooling range is approximately 1,500°F to 900°F (800°C to 500°C)
2.1 The Iron-Carbon Phase Diagram
The iron-carbon phase diagram is the foundation of welding metallurgy. It shows the phases (crystal structures) that exist in steel at various temperatures and carbon contents. Understanding this diagram is essential for predicting how steel will behave during and after welding.
Steel Classification by Carbon Content
| Classification | Carbon Content | Typical Applications | Weldability |
|---|---|---|---|
| Low-carbon steel | 0.05–0.15% C | Sheet metal, wire, nails | Excellent |
| Mild steel | 0.15–0.30% C | Structural shapes, plate, pipe | Good |
| Medium-carbon steel | 0.30–0.45% C | Axles, gears, shafts | Fair (preheat often required) |
| High-carbon steel | 0.45–0.75% C | Springs, cutting tools | Poor (preheat + PWHT required) |
| Very high-carbon steel | 0.75–2.0% C | Files, dies, razors | Very poor (specialized procedures) |
Critical Temperatures
| Temperature | Name | Significance |
|---|---|---|
| A1 (Lower critical) | ~1,333°F (723°C) | Pearlite begins to transform to austenite on heating |
| A3 (Upper critical) | Varies with carbon content (~1,670°F for 0.15% C) | Ferrite fully transforms to austenite on heating |
| Acm | Varies | Cementite dissolves in austenite on heating |
| Ar1, Ar3 | Same as A1/A3 but on cooling | Austenite transforms back — but lower than equilibrium due to cooling rate |
Phases in Steel
| Phase | Crystal Structure | Properties |
|---|---|---|
| Ferrite (α-iron) | BCC (body-centered cubic) | Soft, ductile, magnetic; stable below A3 |
| Austenite (γ-iron) | FCC (face-centered cubic) | Soft at high temp, non-magnetic; stable above A3 |
| Cementite (Fe₃C) | Orthorhombic | Very hard, brittle; iron carbide compound (6.67% C) |
| Pearlite | Layered ferrite + cementite | Moderate strength and ductility; stable mixture |
| Martensite | BCT (body-centered tetragonal) | Very hard, brittle; forms from rapidly cooled austenite |
| Bainite | Ferrite + carbides | Intermediate properties between pearlite and martensite |
Why This Matters for Welding Inspectors
The cooling rate through the critical temperature range (approximately 1,500°F to 900°F / 800°C to 500°C) determines the final microstructure of the heat-affected zone (HAZ):
- Slow cooling → ferrite + pearlite → soft, ductile (usually acceptable)
- Moderate cooling → bainite → stronger, still tough (often acceptable)
- Fast cooling → martensite → very hard, brittle (risk of cracking)
For the Exam: The key concept is that faster cooling rates produce harder, more brittle microstructures. This is why preheat (slowing the cooling rate) is used to prevent martensite formation and cracking in susceptible steels.
What microstructure forms when austenite is rapidly cooled (quenched)?
What is the approximate carbon content range for mild steel used in structural applications?
The A1 (lower critical) temperature for carbon steel is approximately: