Solidification, Phase Diagrams and the Iron-Carbon System

Learning outcomes

The learner should be able to describe nucleation, dendritic growth, grain formation and feeding; explain how cooling rate affects segregation and grain size; use a binary phase diagram qualitatively and apply the lever rule; identify key iron-carbon constituents; and connect solidification mechanisms with casting and weld discontinuities.

4.1 Freezing a liquid metal

Solidification begins when stable solid nuclei form in an undercooled liquid. Nuclei often form on mould walls, inclusions or added grain refiners because an existing surface reduces the energy barrier. Atoms then attach to the growing solid. Growth is frequently dendritic: branching crystals advance through the liquid and eventually meet neighbouring crystals to form grains.

The first solid and the last liquid may have different compositions. Solute is redistributed between them, creating microsegregation within dendrite arms and macrosegregation across larger regions. Diffusion during later heat treatment can reduce some microsegregation, but gross segregation and inclusions cannot be wished away by an attractive certificate.

Liquid metal contracts as it cools and usually contracts again when it freezes. If liquid cannot feed the final freezing regions, shrinkage cavities or dispersed shrinkage porosity form. Gas solubility can decrease during cooling, allowing pores. Oxide films, slag or other non-metallic material may be trapped. Hot tears can develop when restrained contraction strains a weak, partly solidified network.

4.2 Cooling rate and structure

Rapid heat extraction normally increases nucleation relative to growth, producing finer structure and reduced local solidification time. Slow cooling generally permits coarser dendrites and grains. The actual result also depends on alloy chemistry, section thickness, mould conditions and thermal gradient.

Ordinary cast structure may be equiaxed, with grains growing in many directions, or columnar where a strong thermal gradient favours directional growth. Special high-temperature components can be directionally solidified or grown as a single crystal to remove selected transverse grain boundaries. Those specialised structures are deliberately anisotropic and require representative material models for NDT.

Directional solidification and feeding should progress toward a reservoir of liquid metal. Risers, chills and gating are designed around that objective. Poor feeding geometry can concentrate shrinkage at hot spots. In welding, the molten pool is small and moves; the fusion boundary and heat flow create directional columnar growth, chemical segregation and a heat-affected region in the unmelted base metal.

4.3 Reading equilibrium phase diagrams

A phase diagram maps stable phases against composition and temperature under near-equilibrium conditions. It does not show cooling rate directly. Liquidus separates fully liquid material from liquid-plus-solid. Solidus separates fully solid from liquid-plus-solid. Solvus boundaries show solid-solubility limits.

In a two-phase region at a chosen temperature, the lever rule estimates phase fractions. If an alloy of composition C0 lies between phases of compositions C_alpha and C_beta:

fraction alpha = (C_beta - C0) / (C_beta - C_alpha)

fraction beta = (C0 - C_alpha) / (C_beta - C_alpha)

Use consistent composition units. The rule estimates equilibrium fractions, not morphology, grain size or exact service properties.

An eutectic reaction converts liquid into two solid phases at a specific composition and temperature. A eutectoid reaction converts one solid phase into two other solids. A peritectic reaction combines liquid and one solid to form another solid.

4.4 Iron-carbon foundations

The commonly taught metastable iron-iron-carbide diagram helps explain carbon steels and many cast irons. Ferrite is a low-carbon BCC iron phase. Austenite is an FCC iron phase with higher carbon solubility. Cementite is iron carbide and is hard and brittle. Pearlite is a lamellar mixture of ferrite and cementite formed by eutectoid transformation. Sufficiently rapid cooling from austenite can suppress diffusional products and form martensite, a hard, metastable, supersaturated carbon-in-iron structure discussed in the next chapter.

The exact transformation temperatures and compositions depend on chemistry and heating or cooling conditions. Values taken from a simplified classroom diagram must not replace a grade-specific heat-treatment instruction.

4.5 NDT and field connection

Solidification conditions predict discontinuity shape. Gas pores are commonly rounded; shrinkage may be irregular, interdendritic or concentrated near hot spots; hot tears and solidification cracks are planar and follow weak regions. Segregation bands can change local hardness, corrosion response and ultrasonic behaviour. An NDT response should be interpreted with location and process geometry, not shape alone.

Practical checkpoint - Trace the last liquid

Given a stepped casting or weld cross-section, mark the regions expected to cool first and last. Identify feeding paths, restraint and section changes. Predict likely porosity, shrinkage or cracking locations and choose complementary NDT methods. State what evidence would distinguish a volumetric pore cluster from a planar hot tear.

Common errors and misconceptions

Several common errors recur in this topic: treating an equilibrium diagram as a time-temperature-transformation diagram; assuming all porosity is gas porosity; using indication shape alone to name a discontinuity; ignoring the final-freezing region when planning coverage; and quoting one eutectoid temperature as exact for every steel grade.

Chapter summary

Nucleation and dendritic growth create grains while solute, gas and liquid feeding control many discontinuities. Cooling rate influences structure but works with chemistry and geometry. Phase diagrams show equilibrium phase fields; the lever rule estimates fractions. The iron-carbon system provides a foundation for steel transformations.