{"id":342,"date":"2026-04-28T01:53:45","date_gmt":"2026-04-28T01:53:45","guid":{"rendered":"https:\/\/kurtfoundry.com\/?p=342"},"modified":"2026-07-05T09:05:07","modified_gmt":"2026-07-05T09:05:07","slug":"sand-casting-materials-grades-properties-selection","status":"publish","type":"post","link":"https:\/\/kurtfoundry.com\/fr\/blog\/sand-casting-materials-grades-properties-selection\/","title":{"rendered":"Mat\u00e9riaux de moulage au sable : nuances, propri\u00e9t\u00e9s et guide de s\u00e9lection"},"content":{"rendered":"\n

Gray iron\u2019s compressive strength reaches 3-4x its tensile strength \u2014 a property inversion that makes it the dominant choice for machine bases and engine blocks, yet disqualifies it from any application with significant tensile loading. That single fact illustrates why listing \u201ccast iron\u201d as a sand casting material, without distinguishing gray from ductile, leads engineers to specify the wrong alloy.<\/p>\n\n\n\n

Sand casting handles the widest range of metals of any casting process, but it earns its keep with ferrous metals \u2014 iron and steel \u2014 where pouring temperatures (up to 2800+ F for high-alloy steel) and casting weights exceed what die casting and permanent mold can deliver.<\/p>\n\n\n\n

Cast Iron: Gray Iron vs Ductile Iron<\/h2>\n\n\n\n

The 2% carbon threshold separates steel from iron, but within cast iron, the shape of the graphite precipitates during solidification determines everything. Gray iron contains flake graphite; ductile iron contains nodular (spheroidal) graphite. These are not interchangeable materials \u2014 the graphite morphology creates fundamentally different mechanical behavior.<\/p>\n\n\n\n

\"Diagram<\/figure>\n\n\n\n

Gray Iron (ASTM A48)<\/h3>\n\n\n\n
\"Gray<\/figure>\n\n\n\n

ASTM A48 Class 30 delivers 30,000 psi tensile strength with 174-210 BHN hardness, while Class 40 pushes to 40,000 psi tensile with 183-285 BHN. Those tensile numbers look modest until you flip the loading direction: Class 30 compressive strength hits 109 ksi, and Class 40 reaches 150 ksi. That 3-4x compressive-to-tensile ratio is why gray iron castings<\/a> dominate machine tool bases, brake drums, and pump housings \u2014 applications where compressive loads and vibration damping matter more than tensile capacity.<\/p>\n\n\n\n

Thermal conductivity of 30 Btu\/hr-ft-F and a melting range of 2050-2120 F make gray iron the easiest ferrous alloy to cast. The flake graphite structure provides natural lubricity and outstanding machinability.<\/p>\n\n\n\n

One specification trap: gray iron strength depends critically on section thickness. Thinner sections cool faster and develop higher strength, while heavy sections cool slowly and test weaker. Specifying \u201cClass 30\u201d without defining the test bar diameter or the actual casting cross-section invites surprises at incoming inspection.<\/p>\n\n\n\n

Ductile Iron (ASTM A536)<\/h3>\n\n\n\n

ASTM A536 grades encode their own properties directly in the name. Grade 65-45-12 means 65 ksi tensile, 45 ksi yield, 12% elongation. Grade 60-40-18 delivers 60,000 psi tensile with 18% elongation and compressive strength of 429 ksi \u2014 more than enough for dynamic loading, impact, and fatigue applications where gray iron fails.<\/p>\n\n\n\n

The grade range spans from 60-40-18 (maximum ductility, full ferritizing anneal) through 80-55-06 and 100-70-03 (quench and temper) up to 120-90-02 (highest strength, lowest elongation). Grade 65-45-12 and 80-55-06 can often ship as-cast, eliminating heat treatment cost entirely.<\/p>\n\n\n\n

\"Ductile<\/figure>\n\n\n\n

Ductile iron castings<\/a> meet the 60-40-18 impact requirement of 12J at -20 C per ISO 1083, making them suitable for cold-weather infrastructure \u2014 manhole covers, pipe fittings, suspension components \u2014 where gray iron\u2019s zero impact resistance is a liability. For most structural applications that don\u2019t require corrosion resistance or weldability, ductile iron delivers steel-like performance at iron prices.<\/p>\n\n\n\n

Cast Steel: Carbon and Stainless Grades<\/h2>\n\n\n\n

When ductile iron\u2019s strength or corrosion resistance falls short, cast steel fills the gap \u2014 at higher cost and greater casting difficulty. Steel\u2019s lower carbon content (below 2%) means higher pouring temperatures, greater shrinkage, and tighter process control.<\/p>\n\n\n\n

Carbon Steel (ASTM A216)<\/h3>\n\n\n\n

ASTM A216 WCB is the most commonly specified carbon steel casting grade: 70-95 ksi tensile, 36 ksi yield minimum, 22% elongation minimum. Composition limits of 0.30% C max, 1.00% Mn max, and 0.60% Si max provide good weldability \u2014 critical for carbon steel castings<\/a> used in valve bodies, pressure-containing housings, and structural components welded into larger assemblies.<\/p>\n\n\n\n

All A216 castings require heat treatment: annealing at 890-910 C or normalizing at 870-890 C. No exceptions. WCB carries a service temperature ceiling of 1200 F (649 C); above that, you need high-alloy grades outside A216\u2019s scope.<\/p>\n\n\n\n

\"Carbon<\/figure>\n\n\n\n

A216 covers three grades \u2014 WCA (lower strength, highest ductility), WCB (workhorse), and WCC (slightly higher strength with more manganese). For 80% of carbon steel casting applications, WCB is the correct starting point.<\/p>\n\n\n\n

Stainless Steel (ASTM A351)<\/h3>\n\n\n\n

ASTM A351 CF8M \u2014 the cast equivalent of wrought 316 stainless \u2014 delivers 70 ksi minimum tensile, 30 ksi minimum yield, and 30% minimum elongation with 217 HB hardness. The key composition: 18-21% chromium, 9-12% nickel, and 2-3% molybdenum. That molybdenum is what makes CF8M different from CF8 (the 304 equivalent), providing resistance to chloride pitting and crevice corrosion in chemical processing, marine, and food-grade environments.<\/p>\n\n\n\n

Heat treatment requires solution annealing at 1900 F (1040 C) minimum followed by water quench \u2014 a rapid cool that preserves the austenitic structure and prevents carbide precipitation at grain boundaries.<\/p>\n\n\n\n

Stainless steel castings<\/a> cost two to three times more than carbon steel. Specify stainless only when the service environment demands corrosion resistance that carbon steel with coatings cannot deliver.<\/p>\n\n\n\n

What the Data Sheet Won\u2019t Tell You<\/h2>\n\n\n\n

Property tables give you minimums. Foundry reality adds variables that change material selection decisions.<\/p>\n\n\n\n

I\u2019ve seen engineers specify gray iron Class 30 for a mounting bracket, confirm the tensile strength meets their FEA model, and sign off. Six months later, brackets are cracking in service. The FEA modeled tensile loading, but the actual installation introduced impact loads during assembly \u2014 exactly the condition where gray iron\u2019s flake graphite acts as a crack propagator. Ductile iron would have handled it at nearly the same cost.<\/p>\n\n\n\n

The cost hierarchy also defies intuition. Gray iron is the cheapest casting alloy by material cost per pound. Ductile iron costs more per pound. But the finished part cost often comes out roughly equivalent, because ductile iron\u2019s superior machinability and lower scrap rates reduce post-casting operations. I\u2019ve run the numbers across hundreds of jobs \u2014 when you factor in machining time, scrap rate, and rework, ductile iron\u2019s premium shrinks to near zero on most parts under 50 pounds.<\/p>\n\n\n\n

Shrinkage behavior varies by alloy family, and each material\u2019s contraction pattern is non-uniform. Liquid contraction, solidification shrinkage, and solid-state contraction occur in three distinct phases, each affected differently by part geometry. Applying a single uniform shrinkage factor produces out-of-tolerance parts, especially on steel castings where total shrinkage is substantially higher than iron.<\/p>\n\n\n\n

How to Choose the Right Alloy<\/h2>\n\n\n\n

Before you specify the grade, understand the service conditions. The selection sequence should be: loading type, environment, then cost \u2014 not the reverse.<\/p>\n\n\n\n

Service Condition<\/th>Recommended Alloy Family<\/th>Starting Grade<\/th><\/tr><\/thead>
Compressive\/static loading, vibration damping<\/td>Gray iron<\/td>A48 Class 30<\/td><\/tr>
Dynamic\/impact\/fatigue loading<\/td>Ductile iron<\/td>A536 65-45-12<\/td><\/tr>
High tensile + weldability required<\/td>Carbon steel<\/td>A216 WCB<\/td><\/tr>
Corrosive or high-temperature environment<\/td>Stainless steel<\/td>A351 CF8M<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\"Sand<\/figure>\n\n\n\n

Start with gray iron. It is the lowest-cost, most castable option. If gray iron\u2019s tensile strength or impact resistance is insufficient, move to ductile iron \u2014 not steel. The jump from ductile iron to carbon steel adds considerable cost and introduces mandatory heat treatment, higher shrinkage, and tighter process control. Justify that jump with specific service requirements, not habit.<\/p>\n\n\n\n

The ASTM spec gives you minimums, but here\u2019s what actually matters: match the alloy to the dominant loading mode, confirm the service environment, and then let cost break the tie. Material selection is 80% of casting success \u2014 get it right, and the foundry process follows.<\/p>\n\n\n\n

Conclusion<\/h2>\n\n\n\n

Specify the alloy family based on loading and environment first, then select the ASTM grade. If you are deciding between gray iron and ductile iron, request test bars cast from the same heat as your parts \u2014 catalog minimums do not capture the section-thickness sensitivity that determines real-world performance. The most expensive casting mistake is not picking the wrong grade. It is specifying a material before defining the service conditions it must survive.<\/p>\n\n\n\n