Metal Casting Materials, Processes and Selection: Gray Cast Iron

Gray Cast Iron

Picture a fractured piece of gray iron under a microscope. You'd see graphite arranged in flake-like formations — and these flakes are exactly what give the fractured surface its characteristic dark, gray appearance.

Composition: Typically 1.7 to 4.5% carbon, 1 to 3% silicon.

Key Properties:

• Excellent machinability
• High damping capacity (absorbs vibration)
• Low modulus of elasticity
• Comparatively easy to manufacture
• Easily cast into complex shapes in green-sand and other molds

Where it's used: Machine tools, automotive cylinder blocks, cast-iron pipe and fittings, agricultural implements, and anywhere vibration damping matters.

Classification (ANSI/ASTM A48-76):

The standard groups gray iron castings into classes based on minimum tensile strength. The prefix number indicates the minimum tensile strength in thousands of pounds per square inch.

High-strength gray iron castings produced by the Meehanite-controlled process can achieve various combinations of physical properties including heat resistance, wear resistance, and corrosion resistance.

White Cast Iron

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

Key Properties:

• Extremely hard
• Extremely brittle (ductility is practically zero)
• Compressive strength usually higher than 200,000 psi (compared to 65,000–160,000 psi for gray iron)
• Less resistant to impact loading than gray iron

Design Warning: White cast iron castings need particular attention. Sharp corners and thin sections will result in material failures at the foundry. These castings demand careful geometric consideration.

Primary Use: Most white iron castings are used in the production of malleable iron castings. Some are used directly where maximum wear resistance is the primary requirement.

Chilled Cast Iron

This is not a separate alloy — it's a technique applied to gray iron castings.

Many gray iron castings have wear-resisting surfaces of white cast iron. These surfaces are produced in molds having metal chills that cool the molten metal rapidly. This rapid cooling causes the formation of cementite and white cast iron at the surface, while the interior remains gray iron.

The result: A part with a tough, shock-absorbing gray iron core and a hard, wear-resistant white iron surface.

Alloy Cast Iron

When you add alloying elements — nickel, chromium, molybdenum, copper, and manganese — in sufficient amounts to appreciably change the physical properties of cast iron, you enter the domain of alloy cast iron.

These elements are added to:

• Increase strength
• Increase wear resistance
• Increase corrosion resistance
• Increase heat resistance

Applications include: Automotive cylinders, pistons, piston rings, crankcases, brake drums, machine tool castings, certain types of dies, crushing and grinding machinery parts, and components that must resist scaling at high temperatures.

Machinable alloy cast irons with tensile strengths up to 70,000 psi or higher can be produced.

Malleable Iron Castings

Here's where metallurgy becomes almost alchemical.

Malleable iron is produced by annealing (graphitization) of white iron castings. During this process, the graphite forms as "temper carbon" — compact, rounded aggregates rather than flakes.

Manufacturing Process:

1. Produce a hard, brittle white iron casting from pig iron and scrap
2. Place castings in furnaces
3. Slowly increase temperature (over ~50 hours) to 1,650–1,700°F
4. Slowly cool (over ~60 hours)

This transforms brittle white iron into a tough, ductile material.

Standard Grades (ANSI/ASTM A47-77):

Cupola Malleable Iron is another variant, produced using a cupola furnace. It exhibits good fluidity, produces sound castings, and is well-suited to galvanizing. Used primarily for pipe fittings and valves.

• Minimum tensile strength: 40,000 psi
• Minimum yield strength: 30,000 psi
• Minimum elongation: 5%

Pearlitic Malleable Iron contains some combined carbon and is produced either by stopping the heat treatment before all combined carbon transforms to graphite, or by reheating regular malleable iron above the transformation range.

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

Ductile Cast Iron (Nodular Iron / Spheroidal Graphite Iron)

This is the material that bridges the gap between cast iron and steel.

The distinguishing feature: the graphite is present in ball-like (spheroidal) form instead of flakes. This is achieved by adding small amounts of magnesium- or cerium-bearing alloys combined with special processing.

Key Properties:

• High strength with appreciable ductility
• Toughness intermediate between cast iron and steel
• Shock resistance comparable to ordinary mild carbon steel
• Good pressure tightness under high stress
• Can be welded and brazed
• Can be softened by annealing or hardened by normalizing, air cooling, oil quenching, and drawing

Standard Grades (ASTM A 536-80):

Reading the grade nomenclature: 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%.

Applications: Crankshafts, pistons, cylinder heads (automotive), forging hammer anvils, cylinders, guides, control levers (heavy machinery), wrenches, clamp frames, face-plates, chuck bodies, and metal-forming dies (tool and die field).

Ductile cast iron can also be cast in molds containing metal chills for wear-resistant surfaces, and can be surface-hardened by flame or induction methods.

Steel Castings — When Strength and Shock Resistance Are Non-Negotiable

Steel castings are the go-to choice when a part must withstand heavy shocks and loads. They are stronger than wrought iron, cast iron, or malleable iron — and they are extremely tough.

The steel used for casting may be produced by the open-hearth, electric arc, side-blow converter, or electric induction methods. The raw materials are steel scrap, pig iron, and iron ore.

Typical applications: Hydroelectric turbine wheels, forging presses, gears, railroad car frames, valve bodies, pump casings, mining machinery, marine equipment, engine casings.

Critical advantage over wrought steel: Steel castings do not exhibit the directionality effects typical of wrought steel — they have uniform properties in all directions.

Steel castings are classified into two groups: carbon steel and alloy steel.

Carbon Steel Castings

Structural and Engineering Grades — Mechanical Properties:

Engineering Grades of Low-Alloy Steel Castings:

Alloy Steel Castings

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

Two Groups:

• Low-alloy steels: Total alloy content less than 8%
• High-alloy steels: Total alloy content 8% or more

Heat-Resistant Steel Castings (ASTM A297-81)

Corrosion-Resistant Steel Castings (ASTM A743-81a)

Effect of Alloying Metals on Gear Steels (Applicable to All Steel Castings)

Understanding what each element does helps you specify the right alloy:

• Nickel: Increases hardness and strength with little sacrifice of ductility. Greater hardness penetration. Lower critical points mean less distortion during quenching. Case-hardening group carburizes more slowly, but grain growth is less.
• Chromium: Increases hardness and strength more than nickel, but with greater loss of ductility. Refines grain. Imparts higher hardness penetration. Improves wear resistance.
• Molybdenum: Improves high-temperature strength and creep resistance. Reduces temper brittleness.
• Vanadium: Strong grain refiner. Improves fatigue resistance. Increases hardness and strength at elevated temperatures.
• Manganese: Increases hardenability significantly. Improves tensile strength. Can cause brittleness if used in excess with sulfur.

Casting Processes — Choosing the Right Method

This is where the practitioner's redesign challenge gets solved. Understanding which casting process to use — and why — is arguably the most valuable knowledge in this entire guide.

Sand Casting (Green-Sand Molding)

This is the workhorse of the casting industry. Used for most sand castings, the process involves packing sand mixed with a binder around a pattern.

How it works:

1. A pattern (replica of the part) is made — usually from wood, though plastics or metal are used for production runs
2. The pattern is made in two halves, attached to opposite sides of a flat plate
3. Shaped bars and projections are fastened to the plate to form runner channels and funnels in the sand
4. Sand is packed around the pattern — by hand, with power tools, or in a vibrating/compressing machine
5. The two mold halves are assembled with cores in place (if needed)
6. Molten metal is poured
7. After solidification, the frame is removed, sand falls away, and finished castings are cut from the runners

The term "green-sand" means the binder is not cured by heating or chemical reactions.

Cores: Hollows and undercut surfaces are produced by sand cores placed in position before the mold is closed. An undercut surface is one from which the pattern cannot be withdrawn in a straight line — it must be formed by a core.

Best for: Gray iron and most general-purpose castings in complex shapes. Gray iron usually contains carbon 1.7–4.5% and silicon 1–3% by weight.

Shell Molding

Invented by the German engineer Croning, shell molding uses a resin binder to lock sand grains in a thin (¼ to ⅜ inch) layer that adheres to a heated pattern plate.

How it works:

1. A sand/resin mixture is applied to a heated pattern plate
2. The resin hardens quickly, forming a rigid shell
3. The shell is removed from the pattern
4. Two matching half-shells are assembled (with cores if needed)
5. Molten metal is poured

Advantages over green-sand: Better dimensional accuracy, smoother surface finish, thinner walls possible.

V-Process (Vacuum Molding)

In this process, a heated plastics film is placed on a pattern. Vacuum applied through the pattern draws the film tightly over the pattern surface. Sand (without binder) is placed over the film and vacuum applied to compact it. A second mold half is made the same way.

When molten metal is poured, the plastics film melts and evaporates. After solidification, the vacuum is released and the sand falls away cleanly. The sand needs only cooling before reuse — no binder to reclaim.

Permanent Mold (Gravity Die) Casting

Mainly used for nonferrous metals and alloys.

The mold (die) is usually iron, steel, or graphite, cooled by water channels or air jets. Cavity surfaces in metal dies are coated with a thin layer of heat-resistant material.

Key considerations:

• The alloy must be sufficiently ductile to accommodate shrinkage restrictions without fracturing during cooling
• An alloy that tears or splits during cooling is said to be "hot short" and cannot be cast in rigid molds
• Rules for casting shrinkage that apply to friable (sand) molds do not hold for rigid molds
• Designers rely on temperature-based calculations and experience for shrinkage allowances

Low-Pressure Casting

Uses mold designs similar to gravity casting, but with a key difference.

How it works:

1. The crucible is sealed airtight with the mold
2. Gas or air pressure (6–10 lb/in²) is applied to the bath surface
3. Metal is forced up a hollow refractory tube (stalk) projecting from the die underside
4. The stalk extends below the bath level so metal entering the die is free from oxides and impurities floating on the surface
5. The filling rate is controlled so air can be expelled by the entering metal

Advantage: High-quality, non-porous castings with improved fine detail reproduction.

Squeeze Casting

This process uses a metal die mounted on a large hydraulic press.

How it works:

1. Molten metal is poured into the lower die
2. The upper die is brought down until the die closes
3. A slight overflow is designed to ensure complete filling
4. Pressures up to 25 tons per square inch squeeze molten metal into the tiniest recesses
5. When the press opens, ejectors push the solidified casting out

Dies are lubricated with graphite. The extreme pressure produces very dense castings with excellent mechanical properties.

Die Casting

Die casting is the high-volume, high-precision king of casting processes.

How it works:

Hardened steel molds (dies) receive molten metal injected at high speed, with pressures reaching up to 10 tons per square inch. Force is applied by a hydraulically actuated plunger moving in a cylindrical pressure chamber connected to the die cavity.

The mathematics of force:

If the plan area of the casting and runner system covers 50 in², the total clamping force required is:

Force = Pressure × Projected Area

Force = 10 tons/in² × 50 in² = 500 tons

The die-casting machine must hold the die shut against this enormous force using massive toggle mechanisms.

The Skin Effect: Metal entering the die cavity is cooled quickly, producing layers of rapidly chilled, dense material about 0.015 inches thick at the cavity surfaces. Because thin walls are possible with high injection forces, these dense layers form a large proportion of the total wall thickness, producing high casting strength. This is crucial to understand — test bars (usually thicker) may give different strength readings than the actual thin-walled castings.

Two methods:

Accuracy: Castings may be accurate within 0.001 inch or less, and tolerances of 0.002–0.003 inch per inch can be maintained routinely.

Limitations:

• High die cost — economical only for large production runs
• Cannot be used for iron or steel (too hard and high-melting)
• Porosity from trapped air limits heat treatment and welding

Porosity in Die Castings

Molten metal displaces most air in the cavity, but some is trapped. High pressure squeezes pores to very small size, but subsequent heating can cause blistering as trapped air expands. This is why die castings are seldom solution heat-treated or welded.

Solutions to porosity:

• Vacuum die casting — cavity atmosphere evacuated before injection
• Oxygen displacement — filling cavity with oxygen before injection; the hot metal burns the oxygen, preventing porosity
• Machining limits — depths must be kept to 0.020–0.035 inch to avoid exposing pores
• Pore-sealing techniques — required when pressure tightness is needed

Designing Die Castings

• Use uniform wall thicknesses to reduce cooling stresses
• Use simple core shapes to facilitate extraction
• Avoid heavy sections (or core them out) to prevent porosity concentrations
• Arrange for metal to travel through thick sections to reach thin ones
• Small, slender cores are easily bent or broken — prefer piercing or drilling on finished castings
• Ribbing adds strength to thin sections
• Fillets on all inside corners to avoid stress concentrations
• Draft allowances: Typically 0.5 to 1.5 degrees per side

Die Casting Alloys — A Complete Reference

Aluminum-Base Alloys

The most extensively used die-casting alloys because of superior strength combined with castability.

Linear shrinkage: 12.9 to 15.5 × 10⁻⁶ in./in.-°F. Casting temperatures: ~1,200°F.

Zinc-Base Alloys

Extremely fluid in the molten state — can cast very intricate shapes with closer dimensional limits and thinner walls than aluminum.

Key advantages:

• Low casting temperatures (750–800°F) allow high production rates
• Hot-chamber process enables simple automation
• Extremely smooth surfaces ideal for plating and finishing

Copper-Base Alloys

Used for plumbing, electrical, and marine components requiring corrosion resistance combined with strength and wear resistance.

Magnesium-Base Alloys

Primary advantages: Lightweight, good mechanical properties, excellent damping characteristics.

• Lower specific heat and rapid solidification make production ~50% faster than aluminum
• Die life is usually much longer than for aluminum
• Can be die cast by cold- or hot-chamber methods
• Most widely used alloy: AZ91D (Al 9%, Zn 0.7%), yield strength 23,000 lb/in²
• Oxidation prevention: An atmosphere of CO₂ and air containing ~0.5% SF₆ gas is used

Tin-Base Alloys

Also used for milking machines, soda fountains, syrup pumps, and apparatus requiring acid/alkali/moisture resistance.

Lead-Base Alloys

Used where an inexpensive, noncorrosive metal is needed and strength is relatively unimportant.

Dies for Die-Casting Machines

Dies are generally made of steel, though cast iron and refractory materials have been used for bronze or brass castings that would damage ordinary steel dies.

Critical design requirements:

• Metal must rapidly flow to all parts of the impression
• Air must escape through shallow vent channels (0.003 to 0.005 inch deep) cut into the die parting
• Gates and vents must be located with reference to the particular shape
• Shrinkage: Typically 0.002 to 0.007 inch per inch, but exact allowance depends on the alloy composition and must often be determined experimentally

Die-Casting Bearing Metals in Place

Practically all bearing metals can be die cast directly into their housings — automobile connecting rods are a classic example.

Recommended bearing metal composition: 85% tin, 10% antimony, 5% copper. The antimony may vary from 7–10% and the copper from 5–8%. The die-cast metal becomes harder upon seasoning a few days.

Critical manufacturing note: The bolt holes must be drilled accurately in relation to the machined surfaces before die casting the bearing metal.

Injection Molding of Metal

A hybrid of die casting and injection molding, this process uses powdered metal (5–10 µm particle size) mixed with thermoplastic binders.

Process:

1. Mixture is injection-molded at moderate pressures and temperatures
2. Binder removal takes several days (to avoid distortion)
3. Sintering in a controlled atmosphere furnace at high temperatures
4. Density increases to ~95% of conventionally produced material

Shrinkage: 10 to 35% (much higher than die casting due to binder evaporation and consolidation)

Limitation: Parts restricted to about a 1.5-inch cube

Tolerances: Similar to die casting, with some parts sized by a coining process for greater accuracy.

Precision Investment Casting — The Art of the Lost Wax

Let's return to the practitioner. By now, he's deep into his redesign. But the housing his client needs has internal passages, thin walls, and surfaces that can't be machined. None of the processes above quite fit.

His senior colleague walks over, looks at the drawing, and says two words: "Investment casting."

What Is Investment Casting?

Investment casting is a highly developed process capable of great casting accuracy that can form extremely intricate contours. It is sometimes the only practical method of producing a part.

When to use it:

• Metals too hard to machine
• The only practical method of producing the part
• More economical than any other method for the quality required
• Interior or exterior contours of intricate form that couldn't be machined
• Parts like turbine blades made from alloys with high melting points

The accuracy and finish may eliminate machining entirely or reduce it to a minimum. Production can range from a few parts to thousands of duplicates.

The Process: Step by Step

1. Create the Expendable Pattern

The pattern is made from wax, plastics, or a mixture of both, injected into a master mold (usually carbon steel or soft metal alloy). The master mold cavity is designed to allow for shrinkage and compensate for distortion.

Pattern material is injected by pressure, gravity, or centrifugal method. The mold is kept at correct temperature by electrical, steam, or water jacket means.

2. Assemble the "Tree"

Several wax replicas are joined together to bars of wax that form runner channels. Wax shapes for pouring funnels are attached to the runner bars.

3. Invest the Assembly

The wax assembly is dipped into a thick slurry containing refractory particles — this is the investing process. After drying, it's repeated until sufficient thickness builds up to form a one-piece mold shell.

Because the mold is one piece: undercuts, apertures, and hollows can be produced easily — no parting lines.

4. Remove the Pattern

The invested shell is baked to increase strength. The wax melts and runs out or evaporates — hence the name "lost-wax casting."

5. Pour the Metal

The mold is pre-heated to 700–1,000°C to remove any remaining wax, harden the binder, and prepare for pouring. Hot molds help ensure complete filling of intricate details.

Pouring methods: Under gravity, under vacuum, under pressure, or with a centrifuge.

6. Remove the Investment

After solidification, the investment material is destroyed. Methods include water dissolution (for some investments), pneumatic tools, hammers, shot blasting, abrasive blasting, and tumbling.

7. Finish the Casting

Gates, sprues, and runners are removed by abrasive cutting wheels or band saws.

Materials That May Be Cast

The investment process can be applied to a wide range of both ferrous and nonferrous alloys:

• Aluminum and bronze alloys
• Stellite
• Hastelloys
• Stainless and other alloy steels
• Iron castings (especially with thick and thin sections)

By controlling the process, you can change the porosity or density, obtain hardness variations in different sections, and vary corrosion resistance and strength through special alloying.

Shrinkage Allowances for Investment Patterns

Important: Experimental preliminary casting operations may be necessary to determine the exact shrinkage allowance and possible effects of distortion for accurate parts.

Tolerances and Surface Finish

• General dimensions: ±0.005 inch
• Specified dimensions: As low as ±0.002 inch
• Under optimal conditions: ±0.005 or ±0.006 inch per inch, down to ±0.0015 to ±0.002 inch per inch
• Very small dimensions: Even smaller tolerances possible
• Surface discontinuities: 30 to 300 microinches in height

Weights and Sizes

• Weight range: Fractional ounces to 75 pounds or more (practical limit: 10–15 pounds)
• Length: Typically ≤12–15 inches, but much longer parts can be cast
• Minimum wall thickness: ~0.020 inch for high-castability alloys, ~0.040 inch for low-castability alloys
• Preferred wall thickness range: 0.040 to 0.375 inches

Design Rules for Investment Casting

These rules will save you from costly mistakes:

• Uniform wall thicknesses (0.040–0.375 inches) for both cast components and runner channels
• Gradual transitions from thick to thin sections
• Never route molten metal through a thin section to fill a thick section
• Avoid thin edges — difficult to produce in the wax pattern
• Use fillets in all internal corners to avoid stress concentrations from sharp angles
• Allow for thermal contraction (causes distortion)
• Machining allowances: 0.010 inch on small parts, 0.040 inch on large parts
• With proper arrangement in the mold, grain size and orientation can be controlled and directional solidification used to ensure desired physical properties