Question 1 of 7: Steel vs. Cast Iron, Property-Governing Factors, and the Effect of Alloying and Heat Treatment

Question 1: Steel vs. Cast Iron, Property-Governing Factors, and the Effect of Alloying and Heat Treatment (20 marks: 6/7/7)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) Steel vs. Cast Iron

Steel and cast iron are both iron-carbon alloys and sit on either side of the same phase diagram, but the carbon content divides them into fundamentally different families of engineering materials:

• Carbon content. Steel is iron with carbon typically between about 0.02% and 2.11% (by weight), the maximum solubility of carbon in austenite; cast iron carries carbon above roughly 2.11%, usually in the 2%–4% range, well beyond what iron can hold in solid solution.
• Microstructure and free graphite. Because cast iron's carbon content exceeds the solubility limit, the excess carbon precipitates out — usually as graphite flakes, nodules, or in combined form as cementite/carbide, depending on cooling rate and silicon content — whereas steel's lower carbon stays largely in solid solution or as fine carbide, with no free graphite phase.
• Melting point and castability. The eutectic composition near 4.3% C gives cast iron a much lower melting point (roughly 1150–1200 °C) than steel (roughly 1450–1540 °C), and its lower shrinkage and better fluidity make it far easier to cast into complex shapes; steel is comparatively difficult and costly to cast.
• Mechanical behaviour. Steel is ductile and tough and can be forged, rolled, and cold- or hot-worked extensively; cast iron is brittle (the graphite acts as an internal stress-concentrator and crack-initiation site) and is essentially unforgeable — it is used almost exclusively in the as-cast condition, machined to final shape rather than plastically deformed.
• Other properties. Cast iron's graphite gives it good compressive strength, excellent damping capacity (vibration absorption), good wear resistance, and self-lubricating machinability, at the cost of poor tensile strength and near-zero ductility; steel is strong in both tension and compression, weldable, and heat-treatable across a wide hardness range.

(ii) Factors Governing the Properties and Behaviour of Metals and Alloys — During Manufacture and in Service

The properties an engineer sees on a data sheet, and the behaviour the same material shows on the shop floor and in the field, both trace back to the same underlying set of factors:

• Chemical composition. The base metal and the type, amount, and distribution of alloying elements set the possible phases, solid-solution and precipitation strengthening, and corrosion behaviour available to the material.
• Microstructure. Grain size, phase constitution (e.g. ferrite/pearlite/martensite in steels), and the presence of inclusions, porosity, or segregation — largely set by solidification and thermal history — control strength, ductility, toughness, and fatigue life.
• Prior processing/thermomechanical history. Casting, hot or cold working, and heat treatment each leave the material in a different structural state (grain flow, residual stress, work-hardening, dislocation density) that persists into service.
• Temperature. Both during manufacturing (hot vs. cold forming behaves very differently) and in service (creep, temper embrittlement, loss of strength at elevated temperature, or brittle fracture at low temperature).
• Strain rate and loading history. High-rate forming or impact loading, and cyclic service loading (fatigue), can change effective strength/ductility and eventually initiate cracking even below the static yield strength.
• Environment. Corrosive media, moisture, and reactive atmospheres during both processing (e.g. oxidation during hot working) and service (corrosion, stress-corrosion cracking, hydrogen embrittlement) degrade properties over time.
• Geometry and defects. Section size (through governing cooling rate and hence hardenability), surface finish, and stress concentrators (notches, inclusions, porosity) all influence the effective strength and life of the actual part, not just the material.

In short: composition sets what is possible, microstructure (itself a product of processing and thermal history) sets what is actually achieved, and the in-service environment/loading determines how long those properties are retained.

(iii) Properties Most Strongly Influenced by Alloying Elements and Heat Treatment

Alloying and heat treatment are the two principal levers an engineer has to tailor a metal's properties after the base composition is fixed, and they act primarily on:

• Strength and hardness. Alloying (solid-solution and precipitation strengthening, carbide formation) and heat treatment (quench-and-temper martensitic hardening, age/precipitation hardening, strain hardening via cold work) together produce the widest achievable range of strength/hardness in a given alloy system.
• Ductility and toughness. These generally trade off against strength; tempering after quenching, or annealing/normalizing, restores ductility and impact toughness at the cost of some strength, and alloying elements such as nickel improve low-temperature toughness.
• Hardenability — the depth to which a section can be hardened by quenching — is controlled almost entirely by alloy content (Cr, Mo, Mn, Ni retard the pearlite/bainite transformation, allowing martensite to form even at slower cooling rates deeper in a thick section).
• Corrosion and oxidation resistance. Alloying elements such as chromium (passive oxide film in stainless steels), nickel, and molybdenum are the dominant control, largely independent of heat treatment.
• Wear resistance. Carbide-forming alloying elements combined with hardening heat treatments (case hardening, through-hardening) raise surface hardness and abrasion resistance.
• Residual stress and dimensional stability. Stress-relief and annealing heat treatments reduce internal stresses left by prior casting, welding, or cold working, improving dimensional stability in service and during subsequent machining.