2.1 Ferrous Metals and Iron-Carbon Phases

Why Ferrous Metallurgy Matters for Level III Basic

The ASNT NDT Level III Basic exam expects more than name recognition of steel grades. Domain 4 (materials science and process technology) tests whether you can predict discontinuity families and choose or approve inspection approaches from the material and its thermal history. Ferrous metals dominate pressure equipment, structural steel, pipelines, cast machinery, and welded fabrications—so iron-carbon phase behavior is a recurring exam and field foundation.

Ferrous metals are alloys in which iron is the primary base metal. They range from nearly pure iron through plain carbon steels and highly alloyed stainless grades to high-carbon cast irons. What unifies them for NDT is that allotropic iron, carbon content, and cooling rate control the phases present—and those phases control hardness, toughness, magnetism, and acoustic behavior.

Plain Carbon Steels vs Alloy Steels

Plain carbon steels

Plain carbon steels are iron-carbon alloys with only residual or minor amounts of other elements (typically manganese for deoxidation and strength, silicon, sulfur, and phosphorus kept within limits). Carbon content is the main property lever:

• Low-carbon (mild) steel (~0.05–0.25% C): good ductility and weldability; relatively soft; common structural plate and pipe.
• Medium-carbon steel (~0.25–0.55% C): higher strength; can be heat treated; shafts, gears, fasteners.
• High-carbon steel (~0.55–~1.0%+ C): high hardness after heat treatment; springs, tools, wear parts; less weldable.

As carbon rises, strength and hardenability increase while ductility and toughness generally decrease, and the risk of hard, crack-sensitive heat-affected zones in welding grows.

Alloy steels

Alloy steels intentionally add elements beyond the plain-carbon package. Common additions and their NDT-relevant effects include:

For Level III work, the practical distinction is: plain carbon responds mainly to carbon and cooling rate; alloy steels can form hard microstructures in thicker sections (higher hardenability) and may be magnetic or non-magnetic depending on whether the structure is ferritic/martensitic or austenitic.

Cast Iron Types

When carbon exceeds roughly 2.1%, the alloy is classed as cast iron rather than steel. Freezing and carbon form dominate properties:

• Gray cast iron: carbon largely as graphite flakes; excellent damping and machinability; weak and brittle in tension; graphite scatters ultrasound heavily.
• White cast iron: carbon mostly as cementite; very hard and brittle; wear surfaces.
• Ductile (nodular) iron: graphite as spheres (nodules); much better ductility and toughness than gray iron.
• Malleable iron: heat-treated white iron producing temper carbon nodules; improved ductility versus gray iron.

Cast irons are often inspected with RT and VT/PT for surface and volumetric casting flaws; UT is limited by coarse structure and graphite scatter. Many cast irons are ferromagnetic, so MT may be usable on ferromagnetic grades when surface conditions allow.

Iron-Carbon Phases You Must Own

Equilibrium and near-equilibrium phases

Cementite is iron carbide (Fe3C). Pearlite is not a phase in the single-phase sense but a eutectoid product: alternating plates of ferrite and cementite that form from austenite of eutectoid composition on slow cooling. Bainite forms with faster cooling than pearlite but without a full quench to martensite. Martensite is a diffusionless shear transformation from austenite on rapid quench—carbon is trapped, the lattice is strained, and hardness soars.

Critical temperatures (exam-relevant)

On heating a hypoeutectoid steel (below ~0.77% C):

• A1 (~727 °C): pearlite → austenite (eutectoid reaction temperature on heating/cooling under equilibrium).
• A3: upper critical temperature where ferrite finishes dissolving into austenite (composition-dependent; higher for lower carbon).
• Acm: for hypereutectoid steels, temperature where cementite finishes dissolving into austenite.

On cooling, the reverse transformations control whether the product is coarse pearlite, fine pearlite, bainite, or martensite. Ms (martensite start) and Mf (martensite finish) mark the temperature range of the martensitic transformation for a given steel.

Fe-C diagram anchors for the exam

Memorize the landmarks, not every curve:

1. Eutectoid: ~0.77% C, ~727 °C — austenite → pearlite (ferrite + cementite).
2. Steel vs cast iron boundary: roughly ~2.1% C maximum solubility of carbon in austenite; above this, carbon-rich liquid and cementite/graphite paths define cast irons.
3. Ferrite carbon solubility at room temperature is very low (~0.02% C); excess carbon must exist as cementite, pearlite, bainite, or martensite.
4. Pure iron is BCC (α) at room temperature, becomes FCC (γ austenite) above about 912 °C, then BCC (δ) near melting—this allotropy is why steel can be heat treated at all.

How Structure Affects Properties and NDT Response

Grain size

Fine grain generally increases yield strength (Hall–Petch) and often toughness. For UT, fine equiaxed grain reduces scatter and attenuation, improving signal-to-noise. Coarse grain (castings, overheated welds, some stainless weld metal) scatters sound, can create grass/noise, and may force method changes (e.g., RT instead of UT, or specialized UT techniques).

Attenuation and multi-phase structure

Pearlite colonies, carbide networks, graphite, and mixed hard/soft phases all raise ultrasonic attenuation relative to clean, fine-grained ferrite. Level III procedure approval should anticipate higher gain, lower frequency, or alternate methods when the microstructure is known to be coarse or multiphase.

Magnetic properties for MT

Magnetic particle testing requires materials that can be strongly magnetized—typically ferromagnetic ferritic, pearlitic, bainitic, or martensitic steels and many cast irons.

Scenario — Why austenite is non-magnetic: Austenite’s FCC structure does not support the ferromagnetic domain behavior of BCC ferrite. Fully austenitic stainless steels (e.g., 304, 316 in the annealed condition) are effectively non-magnetic and are not candidates for conventional MT. Use PT, ET, UT, RT, or VT as appropriate. Partial magnetism can appear after cold work (strain-induced martensite) in some grades—a Level III must not assume "stainless equals non-magnetic" without knowing grade and condition, but pure austenite remains the classic non-magnetic case on the exam.

Scenario — Why martensite is hard and brittle: Rapid quench traps carbon in a highly strained body-centered tetragonal (BCT) lattice. Dislocation motion is severely hindered, so hardness and strength are high, but ductility and toughness collapse. As-quenched martensite is crack-sensitive (including quench cracks). Tempering after quench reduces hardness slightly and restores toughness—an essential process-property link when evaluating heat-treated parts and their expected flaw types (quench cracks vs tempered service cracks).

Connecting heat treatment language to inspection

• Anneal / normalize: softer, more equiaxed structures; generally better UT; magnetic if ferritic/pearlitic.
• Quench: risk of quench cracks, residual stress, hard martensite; MT/PT for surface cracks; UT for internal cracks depending on geometry.
• Temper: toughness recovery; residual hardness still high in many tool and alloy steels.

A Level III writing or approving procedures should ask: What phases are present? Is the material ferromagnetic? How coarse is the grain? What cracks are most likely from the last thermal cycle? Those four answers drive method selection more reliably than alloy trade name alone.