Guide
Machining cast iron parts: what should a buyer specify for gray and ductile iron?
Machining cast iron is the finishing of gray or ductile iron castings, or continuous-cast iron bar, into precision parts such as housings, manifolds, pulleys and machine bases. The iron itself is usually easy to cut; what decides whether the parts come out right is the casting: the grade on the drawing, where the blank comes from, how porosity is handled, and how bare machined iron is protected from rust.
By Saola Manufacturing engineering team · Published · Updated
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Gray iron or ductile iron: which grade goes on the drawing?
Call out gray iron to ASTM A48 by class when stiffness, damping and wear on a flat surface matter, and ductile iron to ASTM A536 by grade when the part takes tension, impact or pressure. The specification and the grade both belong in the material block, not just “cast iron”.
Per the abstract of ASTM A48/A48M, gray iron castings “are classified on the basis of the tensile strength of the iron in separately cast test bars,” and the specification subordinates chemical composition to tensile strength. That makes the class a strength requirement on a test bar, not a guarantee of hardness or machinability in a particular section of your casting.
ASTM A536 covers ductile iron, defined as cast iron with substantially spheroidal graphite. Its grades, such as 60-40-18, 65-45-12, 80-55-06, 100-70-03 and 120-90-02, are named by tensile strength, yield strength and elongation. The abstract notes that 60-40-18 receives a full ferritizing anneal, 65-45-12 and 80-55-06 are as-cast grades, and the two strongest grades are heat treated. Hardness, chemistry, microstructure, pressure tightness and radiographic soundness are requirements only when the purchase order calls for them, which is the key point for a machined part: if you need them, write them in.
| Question | Gray iron (ASTM A48) | Ductile iron (ASTM A536) |
|---|---|---|
| Graphite form | Flakes | Spheroids (nodules) |
| How it is graded | Class, by tensile strength of separately cast test bars | Grade, by tensile, yield and elongation |
| Optional requirements to add on the PO | Hardness range, test bar size, soundness | Hardness, chemistry, microstructure, pressure tightness, radiographic soundness |
| Chips | Short, broken, dusty | Longer chips, closer to steel |
| Common parts | Machine bases, brake and pump housings, cylinder liners | Hydraulic manifolds, gears, pressure housings, brackets under load |
| Porosity treatment | Impregnation possible (iron is in MIL-STD-276A scope) | Impregnation possible (iron is in MIL-STD-276A scope) |
| Rust after machining | Fast on bare surfaces | Fast on bare surfaces |
Where do the iron blanks for a machined part come from?
Iron parts start as sand castings from a foundry, as continuous-cast iron bar for round or rectangular parts, or as castings the customer already owns and ships to the machine shop. Which route applies decides who owns casting quality and what the machining drawing has to say.
A sand-cast blank is the norm for housings and complex shapes. It brings a casting drawing with machining stock, draft, a parting line and cored holes, and the machining drawing then works from datums set on the casting. Our guide to datums on cast parts covers how to choose them, and the casting vs machining drawing guide covers what goes on each print.
Continuous-cast bar is the route for smaller cylindrical or block parts where a pattern is not worth making; it machines like a casting but has a denser, more uniform structure than a typical sand casting. When the customer supplies castings, write down who is responsible for scrap found during machining, such as porosity that opens up on a sealing face, before the first lot arrives. That single clause prevents most disputes on supplied-casting work.
How do you handle porosity in machined iron castings?
Decide where porosity is acceptable before machining starts: mark sealing faces and pressure-holding passages, state an acceptance standard for visible porosity on machined faces, and specify a leak test or impregnation if the part holds fluid.
Iron castings can carry gas porosity and shrinkage cavities beneath the skin, and machining is what exposes them. A pore that is harmless on a mounting face is a leak path on an O-ring groove. On the drawing, identify the critical faces and passages, and in the purchase order give an acceptance rule for pores revealed by machining, by size, count and location. Our casting shrinkage defects guide covers what those cavities look like and where they form.
For fluid-holding parts, vacuum impregnation seals interconnected porosity after machining. MIL-STD-276A covers impregnation of structurally sound castings and powder metal components in aluminum, magnesium, copper, iron (excluding steels) and zinc alloys. Impregnation seals micro-leaks; it does not repair a cracked or structurally unsound casting. If you specify it, also specify the leak test that follows, because impregnation is only proven by a test.
What makes cast iron easy or difficult to machine?
Gray iron cuts easily and breaks into short chips because its graphite flakes act as chip breakers and lubricant; ductile iron cuts more like steel. The real trouble usually comes from the casting: hard skin, sand inclusions, chilled edges and hard spots.
The first cut through a sand-cast surface meets scale and embedded sand, which are abrasive on tools, so cast iron drawings should leave enough machining stock for the cut to get under the skin. Thin sections and sharp corners in a casting can cool fast enough to form hard carbides, which shows up as a sudden hard spot under the tool. A hardness range on the purchase order, measured on the casting, is the simplest way to catch a batch that will machine badly before it reaches the machine.
Gray iron is commonly machined dry or with little coolant, and its fine graphite dust needs extraction and cleaning; ductile iron is usually cut with coolant. Neither iron needs a special drawing callout for machining itself. What helps the shop most is the grade, the hardness range and generous stock on rough faces.
How do you keep machined cast iron from rusting?
Bare machined iron can start to rust within a short time in humid air, so the purchase order should state the protection: a rust-preventive oil, a coating, or VCI packaging, and how long it must hold. Leaving it unstated is the most common cause of rusty iron parts on arrival.
Machined iron surfaces have no protective film, and the porous structure holds coolant and moisture. The usual answers are a rust-preventive oil dip right after cleaning, a painted or powder-coated finish on non-functional faces with machined faces oiled, or vacuum-sealed or VCI packaging for export shipments. For sea freight in particular, state the protection and the expected storage time, and say whether the oil must be removed before assembly. Our phosphate vs black oxide guide covers conversion finishes on ferrous parts if a permanent dark finish is wanted.
Also write down what is not allowed. Some assemblies cannot tolerate oil residue on a sealing or bonding face, and some paint systems need a clean, oil-free surface. A one-line note such as “MACHINED SURFACES: RUST PREVENTIVE OIL, REMOVABLE WITH SOLVENT” saves a lot of rework.
What should the RFQ for a machined iron part include?
The material specification and grade, the blank route, porosity acceptance and leak test, hardness range, rust protection and packaging. With those six items, a machine shop can quote the part without guessing at the casting.
- Material. “GRAY IRON ASTM A48 CLASS …” or “DUCTILE IRON ASTM A536 GRADE …”, plus any optional requirements.
- Blank. Casting drawing and pattern ownership, continuous-cast bar, or customer-supplied castings with the scrap rule.
- Porosity. Critical faces marked, acceptance rule for pores revealed by machining, leak test and impregnation if needed.
- Hardness. A range and where it is measured.
- Protection. Rust preventive, coating or packaging, and how long it must hold.
- Certificates. Material certificate with test bar results if you need them.
How does Saola handle cast iron parts?
Cast iron is not on the material lists of the Saola capability pages, so the iron grade, the source of the blanks and the casting quality responsibility are confirmed at drawing review before anything is quoted. Oil-dip rust protection is on the finish menu as a managed process.
The CNC milling page describes second-operation machining of castings at the lead plant, with the castings listed there being ADC12 aluminum, Zamak 3 zinc and sand-cast brass and bronze. The surface finishing page lists the finishes applied through partner plants, which are never named. Material certification is available on request, as the quality page explains. Send the machining drawing, the casting drawing if there is one, and the material grade through the quote form.
Frequently asked
Is ductile iron harder to machine than gray iron?
Generally, ductile iron behaves more like steel: longer chips and more tool load, especially in the higher-strength grades. Ferritic grades are typically the easiest of the ductile family to machine.
Can cast iron be welded to repair a machining defect?
It is possible but difficult, and many customers forbid it. If weld repair is acceptable, say so and state the conditions on the purchase order; otherwise assume it is not allowed.
Does gray iron need coolant?
It is often machined dry. Ductile iron is usually machined with coolant. Either way, the machined part needs rust protection afterward.
What is the difference between ASTM A48 class and A536 grade?
An A48 class is defined by the tensile strength of separately cast test bars; an A536 grade is named by tensile strength, yield strength and elongation. They are different materials with different specifications.
Should I ask for impregnation on every iron casting?
No. Impregnation adds a process and a leak test. Ask for it on parts that hold fluid or pressure, where machining may open porosity into a passage.
Sources
- ASTM A48/A48M-22, Gray Iron Castings, ASTM International, 2022
- ASTM A536-84(2019)e1, Ductile Iron Castings, ASTM International, 2019
- MIL-STD-276A, Impregnation of Porous Nonferrous Metal Castings (scope), U.S. Department of Defense, 1992