2.1 Ferrous Metals and Iron-Carbon Phases

ASNT NDT Level III Basic Examination Exam

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1 Introduction & Exam Overview

1.1 Current Exam Facts 1.2 Level III Role and Certification Path 1.3 Eligibility, Fees, Scheduling, and Retakes 1.4 Study Strategy and Exam-Day Approach

2 Chapter 2: Materials Fundamentals for NDT

2.1 Ferrous Metals and Iron-Carbon Phases 2.2 Non-Ferrous Metals and Alloys 2.3 Mechanical Properties and Crystal Structure

3 Chapter 3: Manufacturing Processes

3.1 Casting Processes and Cast Discontinuities 3.2 Forging, Rolling, and Forming 3.3 Extrusion, Drawing, and Machining 3.4 Heat Treatment Effects

4 Chapter 4: Welding and Joining Processes

4.1 Fusion Welding Processes 4.2 Weld Metal and HAZ Discontinuities 4.3 Brazing, Soldering, and Mechanical Joining

5 Chapter 5: Discontinuity Classification

5.1 Inherent Discontinuities 5.2 Processing Discontinuities 5.3 Service Discontinuities

6 Chapter 6: Specific Discontinuity Types

6.1 Cracks and Fracture Modes 6.2 Porosity, Inclusions, and Lack of Fusion 6.3 Laminations, Seams, Laps, and Undercut 6.4 Relevant vs Non-Relevant Indications

7 Chapter 7: NDT Method Selection and Inspection Planning

7.1 Method Selection Principles 7.2 Surface vs Volumetric Methods 7.3 Inspection Planning and Flaw Expectations

8 Chapter 8: Ultrasonic and Radiographic Testing

8.1 Ultrasonic Testing Principles and Applications 8.2 UT Limitations, Calibration, and Evaluation 8.3 Radiographic Testing Principles and Applications 8.4 RT Limitations, Image Quality, and Safety Basics

9 Chapter 9: Magnetic Particle, Liquid Penetrant, and Visual Testing

9.1 Magnetic Particle Testing 9.2 Liquid Penetrant Testing 9.3 Visual Testing

10 Chapter 10: Electromagnetic, Leak, Acoustic Emission, and Infrared Testing

10.1 Electromagnetic / Eddy Current Testing 10.2 Leak Testing 10.3 Acoustic Emission Testing 10.4 Thermal / Infrared Testing

11 Chapter 11: Specialty Methods and Method Comparison

11.1 Neutron Radiography 11.2 Magnetic Flux Leakage 11.3 Method Comparison and Supplemental Examinations

12 Chapter 12: SNT-TC-1A Recommended Practice

12.1 SNT-TC-1A Overview and Written Practice 12.2 SNT-TC-1A Training and Experience 12.3 SNT-TC-1A Examinations and Recertification

13 Chapter 13: CP-189 Standard Practice

13.1 CP-189 Overview and Scope 13.2 CP-189 Mandatory Requirements 13.3 SNT-TC-1A vs CP-189 Comparison

14 Chapter 14: CP-105 and NDT Training Programs

14.1 CP-105 Purpose and Topical Outlines 14.2 Developing and Approving NDT Training 14.3 OJT, Mentoring, and Instructor Oversight

15 Chapter 15: Building and Managing Certification Programs

15.1 Building Qualification and Certification Programs 15.2 Administering Examinations and Evaluating Competency 15.3 Program Audits and Managing Certifications

16 Chapter 16: Codes, Standards, Procedures, and Quality

16.1 Interpreting Codes, Standards, and Specifications 16.2 Developing and Approving NDT Procedures 16.3 Acceptance Criteria and Quality Control 16.4 External Audits and Inspection Documentation

2.1 Ferrous Metals and Iron-Carbon Phases

Key Takeaways Explain This Topic AI

Plain carbon steels are iron-carbon alloys with limited intentional alloy additions; alloy steels deliberately add Cr, Ni, Mo, Mn, V, or other elements to raise hardenability, strength, toughness, or corrosion resistance.

Key iron-carbon phases are ferrite (BCC, soft, ferromagnetic), austenite (FCC, ductile, non-magnetic), cementite (Fe3C, hard and brittle), pearlite (lamellar ferrite + cementite), bainite, and martensite (supersaturated BCT, hard and brittle).

The eutectoid composition is about 0.77% C at approximately 727 °C (A1); steels contain up to roughly 2.1% C, while cast irons exceed that carbon level.

Austenite is non-magnetic because of its FCC structure, so austenitic stainless steels cannot be inspected with conventional MT without special magnetic-particle adaptations that do not apply to fully non-magnetic grades.

Coarser grain and multi-phase microstructures increase ultrasonic attenuation and scatter; finer grain generally improves both mechanical toughness and UT inspectability.

Last updated: July 2026

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:

Alloy element Primary metallurgical role NDT / service implication

Chromium (Cr) Hardenability; corrosion and oxidation resistance Stainless grades; magnetic behavior depends on crystal structure, not Cr alone

Nickel (Ni) Toughness; austenite stabilizer Helps keep austenitic stainless non-magnetic and tough at low temperature

• Molybdenum (Mo) Hardenability; creep strength High-temperature piping and pressure parts
• Manganese (Mn) Deoxidation; strength; hardenability Present in almost all steels
• Vanadium, Nb, Ti Grain refinement; precipitation strengthening Finer grain aids strength and often UT

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

Phase / constituent Crystal / form Typical character Magnetic?

• Ferrite (α-iron) BCC Soft, ductile, low carbon solubility Ferromagnetic below Curie temp
• Austenite (γ-iron) FCC Soft when stable, highly ductile, dissolves more carbon Non-magnetic
• Cementite (Fe3C) Orthorhombic compound Very hard and brittle Weakly magnetic / complex

Pearlite Lamellar ferrite + cementite Moderate strength/hardness; "fingerprint" under microscope Ferromagnetic overall

Bainite Fine ferrite + carbide from intermediate cooling Stronger/tougher balance than coarse pearlite Ferromagnetic

Martensite BCT (supersaturated carbon in distorted BCC-like lattice) Hard, high strength, low toughness as-quenched Ferromagnetic

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:

Eutectoid : ~ 0.77% C , ~ 727 °C — austenite → pearlite (ferrite + cementite).

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.

Ferrite carbon solubility at room temperature is very low (~0.02% C); excess carbon must exist as cementite, pearlite, bainite, or martensite.

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.