Metal Casting Materials, Processes and Selection: White Cast Iron

White Cast Iron — Hard as Nails, Brittle as Glass

When nearly all the carbon in a casting exists in the combined or cementite form, you get white cast iron — named for its silvery-white fracture surface.

Properties at a Glance

• Extremely hard — compressive strength usually exceeds 200,000 psi (compared to 65,000–160,000 psi for gray iron)
• Practically zero ductility — this material shatters, it does not bend
• Less impact-resistant than gray iron
• Design-critical — sharp corners and thin sections cause failures at the foundry

Where White Iron Earns Its Keep

Most white iron castings serve as feedstock for malleable iron production — they are cast as white iron, then heat-treated into something far more useful. But some white iron castings are used directly where maximum wear resistance is the only thing that matters and nobody cares about toughness.

Chilled Cast Iron — Best of Both Worlds

Imagine you need a gray iron casting, but specific surfaces must resist severe abrasion — a roll for a steel mill, for instance, or a crusher jaw.

Chilled cast iron gives you a white iron surface on a gray iron body. Metal chills placed in the mold extract heat rapidly from specific areas, forcing the carbon in those zones to remain in the combined (cementite) form. The result: a hard, wear-resistant skin backed by the toughness and machinability of gray iron.

Alloy Cast Iron — Engineered to Specification

When standard gray iron cannot meet your performance requirements, you add alloying elements — nickel, chromium, molybdenum, copper, and manganese — in sufficient quantities to meaningfully change the physical properties.

What Alloying Buys You

Machinable alloy cast irons can achieve tensile strengths of 70,000 psi or higher. They are used extensively for automotive cylinders, pistons, piston rings, crankcases, brake drums, certain die types, and high-temperature applications.

Malleable Iron Castings — Toughness Through Transformation

Malleable iron starts life as white iron. The transformation from brittle to tough happens in the heat-treat furnace through a process called graphitization — converting combined carbon into temper carbon (graphite in the form of compact, rounded aggregates).

The Manufacturing Sequence

1. Melt and cast a hard, brittle white iron from pig iron and scrap
2. Place castings in stationary batch-type or car-bottom furnaces
3. Slowly heat (over ~50 hours) to 1,650–1,700°F
4. Slowly cool (over ~60 hours) back to room temperature
5. The result: strong, ductile, shock-resistant castings

Standard Grades (ANSI/ASTM A47-77)

Cupola Malleable Iron

An alternative production method using a cupola (or cupola with air furnace) produces malleable iron with excellent fluidity — ideal for pipe fittings, valves, and galvanized parts. Per ANSI/ASTM 197-79: minimum tensile strength 40,000 psi, yield strength 30,000 psi, elongation 5%.

Pearlitic Malleable Iron

By stopping the heat treatment before all combined carbon converts to graphite — or by reheating regular malleable iron above the transformation range — you get pearlitic malleable iron. It bridges the gap between malleable iron and steel castings.

Applications: Axle housings, differential housings, camshafts, crankshafts, machine parts, ordnance equipment, and tooling.

Ductile Cast Iron — The Spheroidal Revolution

Here is where the story gets fascinating.

the practitioner, a powertrain engineer at a heavy equipment manufacturer, faces a challenge: he needs a crankshaft material that casts like iron but performs closer to steel — without the cost of steel forgings.

The answer is ductile cast iron (also called spheroidal graphite iron or nodular iron). The distinguishing feature: graphite exists in ball-like (spheroidal) form instead of flakes. This single microstructural change transforms the material's personality.

How Ductile Iron Is Made

Small additions of magnesium- or cerium-bearing alloys combined with special processing cause the graphite to form spheroids instead of flakes. The result:

• Toughness intermediate between cast iron and steel
• Shock resistance comparable to mild carbon steel
• Melting point and fluidity similar to high-carbon cast irons
• Good pressure tightness under high stress
• Weldable and brazeable
• Can be annealed, normalized, or quenched and tempered

ASTM A 536-80 Standard Grades

Reading the grade code: Grade 60-40-18 means minimum tensile strength of 60,000 psi, minimum 0.2% yield strength of 40,000 psi, and minimum elongation in 2 inches of 18%.

Specifying in the supplied reference automotive castings (SAE J434C), ductile iron can be specified by Brinell hardness alone — though the appropriate microstructure must also be present.

Where Ductile Iron Wins

• Automotive: Crankshafts, pistons, cylinder heads
• Heavy machinery: Forging hammer anvils, cylinders, guides, control levers
• Tooling: Wrenches, clamp frames, faceplates, chuck bodies, metal-forming dies

the practitioner selects Grade 100-70-03 for his crankshaft. It gives him 100,000 psi tensile strength with casting-friendly economics — no expensive forging dies, no extensive machining from billet. The metallurgy is complex and requires special melting stock and close process control, but the payoff is worth it.

Steel Castings — When Nothing Else Is Strong Enough

Steel castings sit at the top of the ferrous casting hierarchy. They are stronger than wrought iron, cast iron, or malleable iron, and they are extremely tough. When your part must withstand shocks, heavy loads, or both — and no other casting material can deliver — steel castings are the answer.

How Steel for Castings Is Produced

Raw materials: Steel scrap, pig iron, and iron ore — proportions vary by process and furnace type.

Typical Applications

Steel castings serve in the most demanding applications across every major industry: hydroelectric turbine wheels, forging presses, gears, railroad car frames, valve bodies, pump casings, mining machinery, marine equipment, and engine casings.

Two Critical Advantages Over Wrought Steel

1. No directionality effects — wrought steel has grain flow from rolling or forging that creates directional properties. Cast steel is isotropic.
2. Any composition — steel castings can be made from virtually any carbon or alloy steel composition produced in wrought form, and they respond similarly to heat treatment.

Carbon Steel Castings — Classified in the supplied reference castings divide into two broad groups: carbon steel and alloy steel. Carbon steel castings further divide by carbon content.

Low-Carbon Steel Castings (< 0.20% C)

Most produced in the 0.16 to 0.19% carbon range.

Tensile strength (annealed): 40,000 to 70,000 psi

Medium-Carbon Steel Castings (0.20–0.50% C)

Tensile strength: 65,000 to 105,000 psi (varies with heat treatment)

High-Carbon Steel Castings (> 0.50% C)

Tensile strength (fully annealed): 95,000 to 125,000 psi

Mechanical Properties of Structural-Grade Carbon Steel Castings

Alloy Steel Castings — Precision Performance

Alloy cast steels contain alloying elements — manganese, chromium, nickel, molybdenum, vanadium — in quantities sufficient to obtain or increase desired properties.

Two Groups

Engineering Grades of Low-Alloy Steel Castings

Heat-Resistant Steel Castings (ASTM A297-81)

These high-alloy grades serve where elevated temperatures are the primary design constraint.

Corrosion-Resistant Steel Castings (ASTM A743-81)

Governing Standards

Specifications are issued by the ASTM, SAE, Association of American Railroads (AAR), American Bureau of Shipping (ABS), and Federal authorities. The Steel Founders' Society of America publishes the definitive Steel Castings Handbook with supplements covering design rules, specifications, tolerances, drafting practices, welding procedures, and hardenability.

Standard test methods for steel castings include mechanical testing, visual inspection, liquid penetrant, magnetic particle, radiographic, and ultrasonic examination.

Austenitic Manganese Cast Steel — The Self-Hardening Warrior

This is one of the most remarkable alloys in the casting world.

Tanya, a wear-parts engineer at a mining equipment OEM, needs crusher jaw plates that survive brutal impact from rocks the size of refrigerators. Mild steel deforms. Hardened steel cracks. She needs something that gets harder the more you hit it.

Composition

The Heat Treatment Trick

In the as-cast condition, austenitic manganese steel is quite brittle. To unlock its potential, it must be heated to 1,830–1,940°F and quenched in cold water.

Properties After Quenching

That last line is the magic. When you impact the surface, it work-hardens to nearly three times its original hardness — while the interior remains tough and ductile. Tanya's crusher jaws start at 200 BHN and climb to 500+ BHN in service, right where the rocks are pounding.

Machining Reality Check

Heat-treated austenitic manganese steel is machined only with great difficulty — it hardens at and slightly ahead of the cutting tool's point of contact. Machining options:

• Grinding on specially adapted machines for boring, planing, keyway cutting
• High-speed tool steel or cemented carbide tools with heavy, rigid equipment and slow, steady operation
• Both approaches are tedious and expensive

Welding

Arc welding is possible with manganese-nickel steel welding rods containing 3–5% nickel, 10–15% manganese, and typically 0.60–0.80% carbon.

Casting of Metals — Molding Methods Explained

Now that you know what to cast, you need to understand how.

Molten metals are shaped by pouring into a mold of the required form. The metal enters the mold under gravity, centrifugal force, or various degrees of pressure. Molds are made from refractory materials: sand, plaster, graphite, or metal.

Green-Sand Molding — The Industry Standard

This is the process used for most sand castings worldwide.

How it works:

1. Sand mixed with a binder is packed around a pattern (replica of the part) — by hand, power tools, or vibrating/compressing machines
2. The term "green-sand" means the binder is not cured by heating or chemical reactions
3. The pattern is made in two halves, attached to opposite sides of a flat plate
4. Shaped bars and projections form runner channels and funnels for the molten metal
5. A box-shaped frame (flask) with locating tabs holds the sand and aligns the two mold halves
6. Cores (also sand) create hollows and undercut surfaces, held in place by tenons in grooves called prints
7. After pouring and solidification, the frame is removed and the sand cleaned off

Key term — Undercut: A surface from which the pattern cannot be withdrawn in a straight line. Undercuts must be formed by cores.

Shell Molding — Precision in Thin Shells

Invented by the German engineer Croning, shell molding produces higher-precision castings than green-sand methods.

The process:

1. A resin binder locks sand grains into a thin shell (¼ to ⅜ inch thick) of sand/resin mixture
2. The mixture adheres to a heated pattern plate — excess mixture is dumped back
3. Hot resin hardens, making the shell rigid enough to remove from the pattern
4. Two half-molds are assembled with cores inserted
5. Shell assemblies may be filled with or without backup material (steel shot, coarse sand)

Advantages: Higher dimensional accuracy, better surface finish, thinner walls possible.

V-Process — Sand Without Binders

This elegant method uses dry, unbonded sand held in shape by vacuum.

How it works:

1. A heat-softened plastics film (0.002–0.005 in. thick) is draped over a vented pattern
2. Vacuum (200–400 mm Hg) draws the film tight against the pattern
3. Dry, unbonded sand fills a flask placed over the pattern
4. Vibration compacts the sand; a second plastics film seals the top
5. Vacuum holds the sand firmly in the pattern's shape
6. The flask lifts off, carrying the rigid sand half-mold
7. Two halves are assembled; molten metal is poured
8. Metal melts and evaporates the plastic film between mold surfaces
9. After solidification, vacuum is released — sand falls free, and castings emerge clean

The beauty: Sand needs only cooling before reuse. No binders to buy, no binder fumes, no binder disposal.

Permanent Mold (Gravity Die) Casting

Mainly used for nonferrous metals and alloys.

• Mold material: iron, steel, or graphite, cooled by water channels or air jets
• Cavity surfaces coated with heat-resistant material
• Metal poured into a funnel at the top (or tilted in via mechanisms)
• Ejectors push castings out when temperature drops enough for sufficient strength

Critical concept — Hot Short: An alloy that tears or splits during cooling in the die is "hot short" and cannot be cast in rigid molds. The casting must be sufficiently ductile to accommodate restricted shrinkage without fracturing.

Important: Shrinkage rules for sand molds do not apply to rigid molds. Metal mold designers rely on temperature-based calculations and experience.

Low-Pressure Casting

Similar die designs to gravity casting, but with a pressure assist:

• An airtight seal between crucible and mold
• 6–10 psi of gas or air pressure forces metal up a hollow refractory tube (stalk) from below the bath surface
• Metal entering the die is free from surface oxides and impurities
• Controlled fill rate allows air to escape

With good design, both gravity and low-pressure methods produce high-quality, non-porous castings.

Squeeze Casting

The heavyweight process — literally.

• One die half on the press bed, the other on the vertically moving ram
• Molten metal poured into the lower die
• Upper die brought down under pressures up to 25 tons per square inch
• Slight metal overflow ensures complete cavity filling
• Dies lubricated with graphite; heated dies
• Ejectors push out the solidified casting

Squeeze casting produces some of the densest, highest-integrity castings possible.

Die Casting — High-Speed, High-Volume Production

Wen, a product design engineer at a consumer electronics company, needs 500,000 aluminum housings per year. Each housing has thin walls, fine details, and tight tolerances. Sand casting cannot deliver the surface finish. Machining from billet is absurdly expensive. Die casting is his only viable option.

How Die Casting Works

Hardened steel molds (dies) receive molten metal injected at high speed under pressures up to 10 tons/in².

The math of clamping force:

$$F_{clamp} = P_{injection} \times A_{projected}$$

Where:

• $F_{clamp}$ = Force the machine must exert to hold the die shut
• $P_{injection}$ = Injection pressure (up to 10 tons/in²)
• $A_{projected}$ = Plan area of the casting + runner system

Example: 10 tons/in² × 50 in² projected area = 500 tons of clamping force. Massive toggle mechanisms stretch heavy steel tie bars (~6 in. diameter) through about 0.045 in. to generate this force.

The Skin Effect

Metal entering the die cavity is cooled quickly, producing dense, rapidly chilled layers approximately 0.015 in. thick where metal contacts die surfaces. Because thin walls mean these dense layers form a large proportion of total wall thickness, die castings achieve surprisingly high strength.

Engineering note: Test bars are typically thicker than actual casting walls. The skin effect means castings can have higher strength than test bar data suggests.

Hot-Chamber vs. Cold-Chamber Methods

Why the split: Aluminum would dissolve the ferrous pressure chamber in a hot-chamber setup. Brass requires higher pressures than the hot-chamber can deliver.

Porosity — The Persistent Challenge

Molten metal injected into a die cavity displaces most of the air, but some is trapped and mixed with the metal. High injection pressure squeezes pores small, but subsequent heating can cause blistering as trapped air expands. Consequences:

• Die castings are seldom solution heat treated or welded
• Machining depths limited to 0.020–0.035 in. to avoid exposing pores
• Special pore-sealing techniques needed for pressure-tight applications

Solutions to porosity:

• Vacuum die casting — evacuate cavity atmosphere before injection
• Oxygen displacement — fill cavity with oxygen before injection; hot metal burns the oxygen, eliminating porosity

Designing Die Castings — Rules That Prevent Failure

Die Casting Alloys — Complete Reference

Aluminum-Base Alloys

The most widely used die-casting alloys. Casting temperatures around 1,200°F. Linear shrinkage: 12.9–15.5 × 10⁻⁶ in./in.-°F.

Zinc-Base Alloys

Extremely fluid when molten — cast into very intricate shapes. Closer dimensional limits and thinner walls than aluminum. Linear shrinkage: 9–13 × 10⁻⁶ in./in.-°F. Casting temperatures: 750–800°F.

Established alloys (3, 5, 7): Each contains 3.5–4.3% aluminum for strength and hardness. Produced by the fast, easily automated hot-chamber process. Extremely smooth surfaces — excellent for plating.

New high-aluminum zinc alloys:

Copper-Base Alloys

For plumbing, electrical, and marine applications requiring corrosion resistance + strength + wear resistance.

Magnesium-Base Alloys

Light weight + good mechanical properties + excellent damping. Magnesium does not dissolve iron, so die life is much longer than for aluminum. Production is about 50% faster than aluminum due to lower specific heat and faster solidification.

• Most used alloy: AZ91D (ASTM B94; UNS 11916) — 9% Al, 0.7% Zn, yield strength 23,000 psi
• Oxidation prevention: atmosphere of CO₂ and air with ~0.5% SF₆ gas

Tin-Base Alloys

Used for bearings and applications requiring acid/alkali/moisture resistance.

Also used for milking machines, soda fountains, syrup pumps, and equipment resisting acids, alkalies, and moisture.

Lead-Base Alloys

Used where a cheap, noncorrosive metal is needed and strength is relatively unimportant.

Dies for Die-Casting Machines

• Material: Generally steel; cast iron or refractory materials for brass/bronze castings (high melting temps damage ordinary steel dies)
• Most common: Low-carbon steel
• For aluminum, magnesium, brass: Chromium-vanadium and tungsten steels
• Vent channels: 0.003 to 0.005 in. deep, cut into the die parting line
• Shrinkage allowance: 0.002 to 0.007 in./in. (exact values require experimentation for multi-element alloys)

Die-Casting Bearings in Place

Virtually all bearing metals can be die cast directly into the housing.

• Best metals: Babbitts with ~85% tin, remainder copper and antimony (max 9% Cu)
• Recommended high-class composition: 85% Sn, 10% Sb, 5% Cu
• Die-cast metal hardens after seasoning a few days
• Work located from bolt holes drilled prior to die casting — accuracy of bolt holes relative to machined surfaces is critical

Injection Molding of Metal — Casting Meets Powder Metallurgy

Die casting and injection molding combined: powdered metal (5–10 µm particle size) mixed with thermoplastic binders is injection-molded at moderate pressures and temperatures.

The Process

1. Metal/binder mixture injection-molded into die cavities
2. Parts harden as they cool and are removed as solids
3. Binder removal — takes several days to avoid distortion
4. Sintering in controlled atmosphere furnace at high temperatures
5. Density reaches ~95% of conventional processes
6. Optional coining for tighter accuracy

Limitations

• Shrinkage: 10–35% after removal (much greater than die casting)
• Size: Parts restricted to approximately a 1.5 in. cube

Advantages

Tolerances similar to die casting, with the ability to process materials that cannot be die cast (certain high-melting-point alloys, hard metals).

Precision Investment Casting — The Lost-Wax Mastery

the technical practitioner, a turbine design engineer at an aerospace propulsion company, needs blades cast from a nickel superalloy — a material with a melting point so high and hardness so extreme that machining from billet would be absurdly slow and wasteful. The blade geometry includes internal cooling passages that cannot be machined at all.

Investment casting is the only answer.

What Makes Investment Casting Special

• Capable of extreme accuracy and extremely intricate contours
• Can cast metals too hard to machine
• Can produce contours that could not be machined at all
• Eliminates or minimizes machining — surfaces used as-cast
• Economical from a few pieces to thousands of duplicates

The Process Step by Step

1. Create expendable patterns — wax or injection-molded plastics
2. Join patterns to wax runner bars and pouring funnels to form a "tree"
3. Invest the tree — dip into thick refractory slurry, dry, repeat until shell is thick enough
4. Bake the shell — hardens the mold and melts/vaporizes the wax (lost-wax process)
5. Back up lighter molds with solid refractory material (heavier castings)
6. Preheat mold to 700–1,000°C before pouring
7. Pour under gravity, vacuum, inert gas, pressure, or centrifuge
8. Remove investment — water dissolution, pneumatic tools, shot blasting, tumbling
9. Cut gates and runners — abrasive wheel or band saw

Materials That Can Be Cast

The precision investment process applies to a wide range of ferrous and nonferrous alloys:

• Aluminum and bronze alloys
• Stellite, Hastelloys
• Stainless and other alloy steels
• Iron castings (especially thick/thin section combinations)

The process can control porosity/density, achieve hardness variations between sections, and vary corrosion resistance and strength through special alloying.