Metal Casting Materials, Processes and Selection: Two Categories

Two Categories

• Low-alloy steels: Total alloy content < 8%
• High-alloy steels: Total alloy content ≥ 8%

Engineering Grades of Low-Alloy Steel Castings

Heat-Resistant Steel Castings (ASTM A297-81)

Corrosion-Resistant Steel Castings (ASTM A743-81a) — Selected Grades

Austenitic Manganese Cast Steel

One of the most remarkable engineering materials ever developed. Austenitic manganese cast steel provides an extraordinary combination of shock and wear resistance through a unique mechanism: it work-hardens under impact.

Composition:

Critical processing: In the as-cast condition, this steel is quite brittle. It must be heated to 1,830–1,940°F and quenched in cold water to develop its characteristic toughness.

Properties after quenching:

The self-hardening phenomenon: When the surface of quenched austenitic manganese steel is cold-worked (by impact in service), the hardness increases from 180–220 Brinell to 450–550 Brinell. The interior remains tough and ductile while the surface becomes extremely wear-resistant. This is why it's used for rock crushers, railway crossings, and mining equipment.

Machining reality: Heat-treated austenitic manganese steel is machined only with great difficulty since it hardens at and slightly ahead of the cutting tool contact point. Grinding is the primary material removal method, with high-speed steel or cemented carbide tools used only when grinding isn't possible.

Casting Methods: How Molten Metal Becomes a Part

Now you understand the materials. Next, you need to understand how those materials get shaped. The molding method you choose affects dimensional accuracy, surface finish, production rate, cost, and even the metallurgical properties of the final casting.

Molten metals are shaped by pouring (casting) into a mold of the required form, which they enter under gravity, centrifugal force, or various degrees of pressure. Molds are made of refractory materials like sand, plaster, graphite, or metal.

Green-Sand Molding: The Foundation of Casting

Green-sand molding is used for most sand castings. The term "green-sand" implies that the binder is not cured by heating or chemical reactions—the sand retains its moisture throughout the process.

The Process:

1. Pattern creation: A replica of the part is made in two halves, attached to opposite sides of a flat plate
2. Channel formation: Shaped bars and projections are fastened to the plate to form connecting channels and funnels for molten metal entry
3. Sand packing: Sand mixed with a binder is packed around the pattern by hand, power tools, or vibrating machines
4. Flask support: The sand is supported at the edges by a box-shaped frame (flask) with locating tabs for alignment
5. Core placement: Hollows and undercut surfaces are produced by cores (also sand) placed before mold closure
6. Core retention: Cores are held in place by tenons in grooves (called prints) formed by pattern projections
7. Pouring: Molten metal fills the cavity through the runner system
8. Extraction: After solidification, the flask is removed and sand is cleaned off

Key terminology: An undercut surface is one from which the pattern cannot be withdrawn in a straight line—it must be formed by a core in the mold.

Shell Molding: Precision in a Thin Shell

Invented by German engineer Croning, shell molding uses a resin binder to create thin, precise mold halves.

The Process:

1. A sand/resin mixture is applied to a heated pattern plate
2. The heat causes the resin to bind the sand grains into a ¼- to ⅜-inch thick shell
3. The hot resin hardens quickly, making the shell rigid enough to remove from the pattern
4. A second half mold is produced on another plate
5. Cores are inserted for internal passages
6. The shell assembly is placed in a molding box, supported with steel shot or coarse sand
7. Some shell molds are strong enough to be filled without backup — halves are simply clamped together

V-Process: Vacuum-Formed Precision

The V-Process achieves remarkable precision using dry, unbonded sand held in shape by vacuum alone.

The Process:

1. A pattern with multiple vent passages is draped with heat-softened 0.002–0.005 inch thick plastic film
2. A vacuum of 200–400 mm mercury pulls the film into intimate contact with the pattern
3. Dry unbonded sand is vibrated into a flask over the pattern
4. The top surface is sealed with another plastic film
5. The flask walls (containing hollow chambers) are connected to vacuum
6. The vacuum holds the sand in the shape defined by the pattern
7. Matching halves are assembled, molten metal is poured (plastics film melts and evaporates)
8. After solidification, vacuum is released and sand falls free

Advantages: Sand needs only cooling before reuse—no chemical binders to dispose of, no sand reclamation chemistry.

Permanent Mold (Gravity Die) Casting

Permanent mold casting uses reusable metal molds — mainly for nonferrous metals and alloys.

• Mold material: Iron, steel, or graphite
• Cooling: Water channels or air jets
• Cavity surfaces: Coated with thin heat-resistant material
• Design: Usually two halves, with loose sand or metal cores for undercuts

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

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

Low-Pressure Casting

Low-pressure casting uses designs similar to gravity casting but adds a controlled pressure mechanism:

• Pressure: 6–10 psi applied to the molten metal surface in a sealed crucible
• Delivery: Metal is forced up through a hollow refractory tube (stalk) projecting from the die underside
• Purity advantage: The stalk extends below the bath level, so metal entering the die is free from oxides and surface impurities
• Fill control: Rate is controlled to allow air expulsion from the die

Result: Higher density and improved reproduction of fine detail compared to gravity methods.

Squeeze Casting

Squeeze casting represents the maximum metal density achievable in a casting process:

• Equipment: Large hydraulic press with die halves on bed and ram
• Process: Molten metal poured into lower die; upper die brought down to close
• Metal control: Slight overflow ensures complete cavity filling
• Die lubrication: Graphite
• Pressure: Up to 25 tons per square inch
• Extraction: Ejectors push solidified casting out when press opens

Die Casting: High-Speed Precision for Production Quantities

Die casting is a method of producing finished castings by forcing molten metal into hardened steel dies arranged to open after solidification for casting removal.

Why Die Casting Exists

• Accuracy: Parts may be accurate within 0.001 inch or less
• Repeatability: Limits of 0.002–0.003 inch per inch maintained routinely
• Finish: Parts are often completely finished when removed from the die
• Machining: Either eliminated entirely or greatly reduced

How Die Casting Works

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

Force calculation example:

$$F = P \times A$$

Where:

• $F$ = clamping force required
• $P$ = injection pressure (up to 10 tons/in²)
• $A$ = plan area of casting + runner system

Example: A casting with 50 in² plan area at 10 tons/in² pressure requires 500 tons of clamping force. Massive toggle mechanisms stretch heavy (6-inch diameter) steel tie bars approximately 0.045 inches to generate this force.

The Skin Effect

This is critical for design engineers to understand. Metal entering the die cavity is cooled rapidly, producing layers of dense, rapidly chilled material approximately 0.015 inches thick on all surfaces having direct die contact. Because thin walls mean these dense layers constitute a large proportion of total wall thickness, die castings achieve high strength relative to their weight.

Warning: Tensile strengths measured in thicker test bars may not reflect the actual strength of thin-walled die castings, which benefit from the skin effect.

Two Injection Methods

Hot-Chamber Method:

• Pressure chamber is immersed permanently in molten metal
• Automatically refills as the vertical plunger retracts
• Best for: Zinc, lead, tin, and magnesium (low melting point, high fluidity)

Cold-Chamber Method:

• Water-cooled horizontal pressure chamber is outside the molten metal
• Slower than hot-chamber
• Required for: Brass (needs higher pressure) and aluminum (attacks ferrous chamber material)

Porosity: The Die-Casting Challenge

Some trapped air is inevitable in die castings. High pressure squeezes the pores to very small size, but they're still present. This creates specific limitations:

• No solution heat treating — heating softens the casting, allowing pore expansion and blistering
• No welding — same blistering problem
• Limited machining depth: 0.020–0.035 inches to avoid exposing pores
• Pore-sealing techniques required for pressure-tight applications

Solutions to porosity:

• Vacuum die casting — cavity atmosphere evacuated before injection
• Oxygen displacement — cavity filled with oxygen, which is burned by hot metal

Designing Die Castings — Essential Rules

• Uniform wall thickness to reduce cooling stresses
• Simple core shapes for easy die extraction
• Avoid heavy sections — core them out to reduce porosity concentrations
• Route metal through thick sections to reach thin ones
• Metal cores only — sand cores can't withstand injection pressures
• Avoid small/slender cores — easily bent or broken; pierce or drill after casting
• Use ribs to strengthen thin sections
• Fillets on all inside corners to reduce stress concentrations
• Avoid sharp outside corners
• Draft allowance: 0.5 to 1.5 degrees per side

Die-Casting Alloy Families

Aluminum-Base Alloys

Most widely used die-casting alloys due to superior strength combined with castability.

• Linear shrinkage: 12.9–15.5 × 10⁻⁶ in./in.-°F
• Casting temperature: ~1,200°F
• Silicon increases fluidity but reduces machinability
• Copper adds hardness but reduces ductility

Zinc-Base Alloys

Extremely fluid when molten, enabling intricate shapes and tighter tolerances than aluminum.

• Closer dimensional limits and thinner walls than aluminum
• Linear shrinkage: 9–13 × 10⁻⁶ in./in.-°F
• Casting temperature: 750–800°F (low temperatures enable hot-chamber process)
• Extremely smooth surfaces ideal for plating and finishing
• High production rates with simple automation

Established alloys (ASTM B86): Alloys 3, 5, and 7 each contain 3.5–4.3% aluminum for strength and hardness.

New high-performance zinc alloys with 8%, 12%, and 27% aluminum deliver:

• Tensile strength: 50,000–62,000 psi
• Hardness approaching cast iron: 105–125 Brinell
• Suitable for gears, racks, and shaft housings without bearing bushes

Copper-Base Alloys

Used where corrosion resistance + strength + wear resistance are required simultaneously.

Magnesium-Base Alloys

• Lightest structural die-casting material
• Excellent damping characteristics
• Low specific heat → does not dissolve iron → longer die life than aluminum
• Production ~50% faster than aluminum
• Requires CO₂/air atmosphere with ~0.5% SF₆ to prevent oxidation
• Most widely used: AZ91D (ASTM B94) — Al 9%, Zn 0.7%, yield strength 23,000 psi

Tin-Base Alloys

Used primarily for bearings and components requiring acid/alkali/moisture resistance:

• SAE No. 10: Sn 90%, Cu 4–5%, Sb 4–5% — automotive main-shaft and connecting-rod bearings
• SAE No. 110: Sn 87.75%, Sb 7–8.5%, Cu 2.25–3.75% — similar applications
• Also: Milking machines, soda fountains, syrup pumps

Lead-Base Alloys

Used where cheap, noncorrosive metal is needed and strength is not critical:

• SAE No. 13: Pb 86%, Sb 9.25–10.75%, Sn 4.5–5.5% — batteries, wheel weights
• SAE No. 14: Pb 76%, Sb 14–16%, Sn 9.25–10.75% — higher antimony for hardness
• Applications: Lead-acid batteries, X-ray apparatus, wheel balancing weights

Dies for Die-Casting Machines

• Material: Generally steel (low-carbon); chromium-vanadium and tungsten steels for aluminum, magnesium, and brass
• Design requirements: Metal must flow rapidly to all parts; air must escape through vent channels 0.003–0.005 inch deep
• Shrinkage: 0.002–0.007 inch per inch (exact values require experimentation)
• Refractory dies: Used specifically for bronze or brass castings that would damage steel dies

Die-Casting Bearing Metals in Place

Practically all bearing-suitable metals can be die cast in place — automobile connecting rods being a prime example.

Recommended bearing composition: 85% tin, 10% antimony, 5% copper (copper should not exceed 9%)

The die-cast bearing metal becomes harder after seasoning a few days.

Metal Injection Molding: The Hybrid Process

Die casting meets injection molding by mixing powdered metal (5–10 µm particle size) with thermoplastic binders.

Process Summary

1. Powdered metal mixed with thermoplastic binder
2. Mixture injection-molded at moderate pressures and temperatures
3. Parts harden on cooling and eject from mold
4. Binder removal: Several days (slow to avoid distortion)
5. Sintering: High-temperature controlled-atmosphere furnace consolidates metal
6. Density achievement: ~95% of conventionally produced material

Key Characteristics

• Shrinkage: 10–35% after mold removal (larger than die casting)
• Tolerances: Similar to die casting
• Post-processing: Some parts sized by coining
• Size limitation: Parts restricted to approximately 1.5-inch cube

Precision Investment Casting: The Lost-Wax Art

Investment casting is one of humanity's oldest manufacturing processes — and still one of its most capable. When metals are too hard to machine, when contours are too intricate for other methods, or when conventional casting can't deliver the required accuracy, investment casting provides the solution.

Why Investment Casting

• Extreme accuracy: Comparable to many machined parts
• Intricate contours: Both interior and exterior
• Difficult materials: High-melting-point alloys, materials too hard to machine
• Minimal machining: Often eliminates finishing entirely
• Production flexibility: Economical from a few pieces to thousands

The Process (Lost-Wax Method)

1. Expendable patterns created from wax or injection-molded plastics
2. Pattern assembly: Multiple patterns joined to wax runner bars ("tree")
3. Investing: Tree dipped repeatedly into thick refractory slurry
4. Shell building: Coatings dried and repeated until sufficient thickness
5. Dewaxing: Shell baked to melt and evaporate pattern material
6. Pre-heating: Mold heated to 700–1,000°C for pouring
7. Pouring: Under gravity, vacuum, pressure, or centrifugal force
8. Investment removal: Shell destroyed (water, pneumatic tools, blasting)
9. Finishing: Gates and runners removed by abrasive wheel or band saw

Master Mold Construction

The permanent mold/die for making expendable patterns is made from:

• Carbon steel or soft metal alloy
• Rubber, alloy steels, or other materials
• the practitioner designed for clusters of patterns with shrinkage compensation
• Pattern material injected by pressure, gravity, or centrifugal method

Dimensional Tolerances

• General: ±0.005 to ±0.006 in./in.
• Specified dimensions: ±0.0015 to ±0.002 in./in.
• Very small dimensions: Even tighter tolerances achievable
• Surface finish: 30–300 microinches depending on refractory and process control

Weight and Size Ranges

• Weight: Fractional ounce to 75+ pounds (practical limit: 10–15 pounds)
• Length: Typically under 12–15 inches, but longer parts are castable
• Minimum thickness: 0.020 in. (high castability alloys) to 0.040 in. (low castability alloys)

Design Rules for Investment Casting

• Uniform wall thickness: 0.040–0.375 inches preferred
• Gradual transitions from thick to thin sections
• Metal flow path: Never force metal through thin sections to fill thick areas
• Avoid thin edges (difficult to produce in wax patterns)
• Fillets on all internal corners to avoid stress concentrations
• Allow for thermal contraction distortion
• Machining allowances: 0.010 in. (small parts) to 0.040 in. (large parts)

Casting Milling Cutters in the supplied reference

A compelling proof-of-concept: High-speed steel milling cutters of various forms and sizes have been successfully produced by investment casting. Only riser removal, sand blasting, and grinding of cutting edges are required. The bore is used as cast. Testing has shown these cutters perform comparably to conventionally manufactured cutters.

Pattern Materials and Weight Estimation

Woods for Patterns

Critical requirement: Wood must be well-seasoned — either kiln-dried or kept 1–2 years before use. Pattern shrinkage occurs almost entirely across the grain, not lengthwise.

Pattern Varnish

• Material: Yellow shellac varnish (gum shellac dissolved in grain alcohol)
• Application: At least three coats, with sandpaper rubbing between preliminary coats
• Core prints: Use black shellac varnish to distinguish prints from pattern body
• Purpose: Protect against moisture absorption from damp molding sand

Shrinkage Allowances for Patterns

Critical insight from the Steel Castings Handbook: A straight round steel bar required ~9/32 inch per foot shrinkage. The same bar with large knobs on each end needed only 3/16 inch per foot. With large flanges on each end, only 7/64 inch per foot. Casting geometry dramatically affects shrinkage allowance. Always get values from the foundry that will produce the casting.

Metal Patterns

Finishing Operations: From Raw Casting to Finished Part

Removal of Gates and Risers

After solidification and cooling, castings are removed from molds (manually or by vibratory shake-out machines). Gates and risers are then removed by:

Residual material (fins, riser pads, chaplets, parting-line flash) is removed by:

• Pneumatic chipping hammers
• Floor or bench-stand grinders
• Portable grinders
• Swing-frame grinders

Blast Cleaning of Castings

Blast cleaning serves multiple purposes:

• Remove adhering sand
• Remove cores
• Improve casting appearance
• Prepare for final finishing (painting, machining, assembly)
• Remove scale from heat treatment

Methods:

Heat Treatment of Steel Castings

Steel castings undergo heat treatment to achieve:

• Diffusion of carbon or alloying elements
• Softening for machinability
• Hardening for wear resistance
• Stress relief for dimensional stability
• Toughening for impact applications
• Improved machinability
• Hydrogen removal from casting surfaces

Key principle: Heat treatment of steel castings of a given composition follows closely that of wrought steel of similar composition.

The Complete Iron and Steel Casting Selection Guide

Back to the practitioner. After the crankshaft incident, he created a decision framework that he now shares with every junior engineer who walks through his shop door. Here's the expanded version:

Quick-Reference Selection Matrix

Your Next Step: From Knowledge to Execution

the practitioner now runs the casting division of a major industrial equipment manufacturer. The broken crankshaft that started his education cost his first employer dearly—but it built a career founded on understanding that every casting specification exists for a reason.

Here's what separates an engineer who specifies castings from one who truly understands them:

If you're a designer or engineer:

• Start every casting specification with the loading conditions — static, dynamic, impact, cyclic fatigue
• Map those conditions to the graphite form that best serves them — flakes for damping, spheroids for toughness, temper carbon for shock
• Verify the grade delivers the mechanical properties you need with appropriate safety factors
• Confirm the casting method can achieve the geometry, tolerances, and surface finish your design requires
• Communicate with your foundry about shrinkage allowances — there are no universal values

If you're in procurement or quality:

• Learn to read fracture surfaces — the color and texture tell you immediately what type of iron you're looking at
• Verify certifications against ASTM/ANSI specifications for every critical casting
• Understand that the cheapest casting grade isn't always the cheapest solution — the practitioner's gray iron crankshaft proved that

If you're a student or early-career professional:

• Master the relationship between carbon form (graphite shape) and mechanical properties — this is the single most important concept in ferrous casting metallurgy
• Build a physical sample collection if you can — seeing the actual fracture surfaces of gray, white, malleable, and ductile iron makes the theory real

The casting you specify today may be in service for decades. Get it right from the start.

What's the most critical casting specification challenge you've faced? What application pushed you to move from one casting type to another? The answers to those questions define your growth as an engineer.

Every Process, Material, and Method You Need to Master

What Castings Actually Are (And Why They Matter More Than You Think)

Before diving into processes and alloys, you need to understand the foundational principle that makes casting one of the most enduring manufacturing methods in human history.

Casting is the process of shaping metal by pouring it — in a molten state — into a mold of the required form.

The metal enters the mold under gravity, centrifugal force, or various degrees of pressure. The mold itself can be made from refractory materials like sand, plaster, graphite, or hardened steel. Once the metal solidifies, you have a near-net-shape part — often requiring little or no machining.

That's the principle. But the magic — and the complexity — lies in the details.

Here's why castings matter to you, regardless of your role:

• For designers: Castings allow geometries that are impossible or prohibitively expensive to machine — internal passages, complex curves, hollow forms, thin walls, and intricate contours.
• For production engineers: Castings scale from single prototypes to millions of identical parts with consistent quality.
• For procurement and project managers: Choosing the right casting process can reduce your manufacturing budget by 30–70% compared to machining from solid stock.
• For machinists: Understanding castings means understanding the material you're cutting — its grain structure, porosity zones, hardness variations, and stress patterns.

Iron and Steel Castings — The Foundation of Heavy Industry

Cast irons and cast steels form a massive family of ferrous alloys. As the name implies, they are cast to shape rather than formed by working in the solid state.

In general, cast irons contain more than 2% carbon and from 1 to 3% silicon. By varying the balance between carbon and silicon, alloying with different elements, and changing melting, casting, and heat-treating practices, you can produce an extraordinarily broad range of properties.

In most cases, the carbon exists in two forms:

• Free carbon — in the form of graphite
• Combined carbon — in the form of iron carbide (cementite)

The mechanical and physical properties of any casting depend strongly on the shape and distribution of the free graphite and the type of matrix surrounding the graphite particles.

The four basic types of cast iron are:

1. Gray iron
2. White iron
3. Malleable iron
4. Ductile iron

In addition to these, there are specific forms such as chilled iron, alloy iron, and compacted graphite cast iron.

Let's break each one down.