Ferrous Metals for Manufacturing: Steel, Cast Iron, Machinability and Selection

Ferrous metals are iron-based materials, but that label is only the first step in a manufacturing decision. You still need to identify whether the part is steel or cast iron, then lock the grade, product form, condition, critical properties, machining route, and inspection basis. ISO 4948-1:1982 classifies steel by chemical composition and uses 2.0% carbon as the usual dividing line between steel and cast iron, with exceptions. ISO 185:2020 classifies grey cast iron by tensile strength or Brinell hardness under defined sample and wall-thickness conditions. This guide turns those classification rules into material-selection, machining, and RFQ decisions for engineers and buyers.

1. What Are Ferrous Metals?

Ferrous metals are metals and alloys in which iron is the principal metallic constituent. Common ferrous alloys used in manufacturing fall mainly into steel and cast-iron families, and those families lead to different grade systems, product routes, mechanical behavior, and machining assumptions. A drawing that says only “steel” or “cast iron” is incomplete for any part whose strength, hardness, heat treatment, surface condition, or inspection matters.

1.1 Examples and Types of Ferrous Metals

Material family	Common engineering names	What the name tells you	What still needs to be specified
Steel	carbon steel; alloy steel; stainless steel; tool steel; bearing steel; structural steel; free-machining steel	Broad chemistry, property, or application family	Exact grade, product standard, product form, condition, heat treatment, and acceptance criteria
Cast iron	grey/gray cast iron; ductile iron; white cast iron; malleable cast iron; compacted graphite iron	Cast-iron family and, in some cases, graphite morphology	Exact grade, section-related property basis, heat treatment, test location, and purchase requirements
Other iron-based materials	wrought iron and specialized iron-based alloys	General material identity	Whether the actual product form and specification fit the design and process route

Decision use: family names help you navigate material options. They do not replace a grade or purchase specification.

Common examples of ferrous metals include carbon steels, alloy steels, stainless steels, tool steels, and several types of cast iron. ISO terminology is narrower than this shop language. ISO 4948-1 classifies steel into unalloyed steel and alloy steel based on specified ladle-analysis values. It does not create a parallel top-level list of carbon steel, alloy steel, and stainless steel. Those common names remain useful, but the exact grade and applicable product specification still control purchasing and acceptance.

2. Ferrous vs Non-Ferrous Metals

The basic difference between ferrous and non-ferrous metals is the base material: ferrous metals are iron-based, while non-ferrous metals are based primarily on other elements. The properties of ferrous metals still vary widely by grade, condition, microstructure, and product form. That distinction helps with first-pass material selection, but it does not tell you enough to specify a part. Magnetism, corrosion resistance, density, machinability, and cost all have important exceptions.

Decision factor	Ferrous metals	Non-ferrous metals	What to do with the comparison
Base material	Iron is the principal metallic constituent	Iron is not the principal metallic constituent	Use this as the classification starting point, not as a full property prediction
Magnetic response	Many steels and irons are magnetic; some ferrous grades are weakly magnetic or effectively non-magnetic in service	Many are non-magnetic, but the family label alone is not a test specification	Verify the actual grade when magnetic response matters
Corrosion	Plain carbon and low-alloy steels often need protection; stainless steels are a major exception	Many alloys offer useful corrosion resistance, but behavior is alloy- and environment-specific	Specify grade, environment, and required finish
Weight	Often relatively dense	Some are lighter, but copper and nickel alloys are not lightweight substitutes	Compare actual density only after the candidate grades are fixed
Machining	Ranges from free-machining steel to hardened steel and abrasive cast structures	Also varies widely by alloy, temper, and microstructure	Do not use “ferrous” or “non-ferrous” as a machinability rating

Scientific boundary: these are family-level tendencies with explicit exceptions. Final selection requires the exact alloy or cast-iron grade and condition.

For a sourcing or design decision, the useful next question is narrower: which exact material family, grade, condition, and product form fit the load case and manufacturing route? That is why this article returns quickly to steel and cast iron instead of expanding into a general non-ferrous materials guide.

3. Steel and Cast Iron: The Main Manufacturing Families

Steel and cast iron are both ferrous materials, but they are not interchangeable versions of one material. ISO 4948-1 defines steel as an iron-based material whose carbon content is usually below 2.0%; the standard also notes exceptions, including some chromium steels above that level. Cast iron is a separate family with different solidification behavior and graphite or carbide structures.

3.1 Steel Types Used in Manufacturing

The types of steel most often named on engineering drawings include carbon steel, alloy steel, stainless steel, structural steel, tool steel, bearing steel, free cutting steel, and free machining steel. These steel grades and families do not all describe the same classification level. Their properties of steel also depend on condition and product form. On a drawing or RFQ, “alloy steel” still leaves the exact grade, condition, heat treatment, hardness, and certificate requirement unresolved.

For a grade-level example, the published EPOC CRAFTER comparison of 4140 alloy steel vs 1018 and 1045 carbon steel shows why chemistry, condition, and hardenability must be fixed before machining or procurement decisions are made.

3.2 Cast Iron Types Used in Manufacturing

The main types of cast iron used in manufacturing include grey cast iron (gray cast iron), ductile iron, white cast iron, malleable cast iron, and compacted graphite iron. Their cast iron properties differ because their graphite or carbide structures differ, so they should not share one generic machining rule. ISO 185:2020 applies to unalloyed and low-alloyed grey cast irons for castings made in sand moulds or moulds with comparable thermal behavior. It excludes grey-iron pipes and fittings, continuous-cast products, and general technical delivery conditions.

For CNC process planning, the published guide on machining cast iron grades and chip behavior goes deeper into casting-blank checks, chip formation, dust, and machining-specific risks.

4. How Steel Is Classified Under ISO 4948

For steel, classification should be read in two layers. ISO 4948-1 handles the chemical-composition split between unalloyed and alloy steel. ISO 4948-2 then adds quality classes and main property or application characteristics. Neither document is a complete purchase specification for a specific bar, plate, tube, forging, or casting.

4.1 Unalloyed vs Alloy Steel Under ISO 4948-1

ISO 4948-1:1982 classifies steel from specified ladle-analysis values. A steel is unalloyed when each listed element remains below its classification limit. It is alloy steel when at least one listed element reaches or exceeds its limit. The values are not added across several elements, and the standard contains specific rules for how minima, ranges, and maximum-only composition limits are interpreted.

Element	ISO 4948-1 classification limit, mass %	Decision meaning
Al	0.10	Classification threshold only
B	0.0008	Classification threshold only
Bi	0.10	Classification threshold only
Cr	0.30	Classification threshold only
Co	0.10	Classification threshold only
Cu	0.40	Classification threshold only
Mn	1.65*	Use 1.80% when only a maximum Mn value is specified
Mo	0.08	Classification threshold only
Ni	0.30	Classification threshold only
Nb	0.06	Classification threshold only
Pb	0.40	Classification threshold only
Se	0.10	Classification threshold only
Si	0.50	Classification threshold only
Te	0.10	Classification threshold only
Ti	0.05	Classification threshold only
W	0.10	Classification threshold only
V	0.10	Classification threshold only
Zr	0.05	Classification threshold only
Each lanthanide	0.05	Classification threshold only
Other specified elements excluding S, P, C, N	0.05	Classification threshold only

Source: ISO 4948-1:1982, as extracted in the supplied technical summary. These are classification limits, not guaranteed grade chemistry or incoming-inspection acceptance limits.

That last point prevents a common error. A 0.30% chromium classification limit does not mean an alloy steel grade must contain exactly 0.30% Cr, and it does not make 0.30% the purchase acceptance limit. The actual grade chemistry and product-analysis tolerances come from the applicable material and product specification.

4.2 Quality Classes and Main Characteristics Under ISO 4948-2

ISO 4948-2:1981 starts after the unalloyed/alloy split. For unalloyed steel it defines base steel, unalloyed quality steel, and unalloyed special steel. For alloy steel it defines alloyed quality steel and alloyed special steel; there is no alloyed base steel category. It then uses a separate dimension for principal properties or applications, including structural, free-machining, tool, bearing, corrosion-resistant, heat-resistant, and other groups under defined conditions.

A name such as tool steel or bearing steel therefore does not, by itself, prove a quality class, a finished-part property, or suitability for a specific service. The classification route must still end at the exact grade, product form, delivery condition, and product specification.

5. How Grey Cast Iron Grades Work Under ISO 185

Grey cast iron needs a different reading method because the grade is tied to a defined test basis. ISO 185:2020 allows classification by tensile strength of cast samples, tensile strength of samples cut from a casting by agreement, Brinell hardness on the casting or cast-on knob by agreement, or a specified combination. The standard defines eight tensile grades and six hardness grades.

5.1 Tensile Grades and Sample Basis

ISO 185 tensile grade	Separately cast or side-by-side cast sample range, MPa	How to use the value
ISO 185/JL/100	100–200	Material-grade classification for the stated sample basis; not a guaranteed value at every casting location
ISO 185/JL/150	150–250	Same limitation
ISO 185/JL/200	200–300	Same limitation
ISO 185/JL/225	225–325	Same limitation
ISO 185/JL/250	250–350	Check sample type and relevant wall thickness before applying the number
ISO 185/JL/275	275–375	Same limitation
ISO 185/JL/300	300–400	Same limitation
ISO 185/JL/350	350–450	Same limitation

Source: ISO 185:2020 Table 1, as extracted in the supplied technical summary. Values shown are for separately cast or side-by-side cast samples, not universal local properties of the finished casting.

The sample route changes the interpretation. A cast-on sample is read against relevant wall thickness, and a specimen cut from the casting requires an agreed location and minimum value or permitted range. For example, the supplied ISO 185 summary records that ISO 185/JL/250 has a 210 MPa minimum for a cast-on sample representing a relevant wall thickness of 20–40 mm. Reading the grade number “250” as the minimum at that location would be wrong.

5.2 Hardness Grades and Relevant Wall Thickness

ISO 185 also defines six Brinell-hardness grades: HBW155, 175, 195, 215, 235, and 255. The designation number is not a fixed hardness for every section. The applicable range changes with relevant wall thickness and agreed test location. ISO 185 notes that hardness classification can be useful when machinability or wear resistance matters, but it does not provide a machinability rating, tool grade, cutting speed, feed, or wear-life guarantee.

Tensile and hardness grades should not be converted as if one uniquely determines the other. ISO 185 treats the strength-hardness relationship as informative and approximate. Use the property actually specified on the order, with the correct sample and location basis.

6. Machinability: Steel and Cast Iron Need Different Inputs

Neither steel nor cast iron has one machinability value. The machinability of steel changes with grade, hardness, inclusions, delivery condition, and heat treatment; the machinability of cast iron changes with iron family, matrix, hardness, casting skin, and local hard regions. Before quoting or programming, the shop also needs geometry, feature type, surface requirement, and inspection plan. ISO 4948 and ISO 185 classify materials; they do not prescribe a universal CNC process window.

6.1 Machining Steel

For steel machining, supplied condition is often as important as the grade name. Hot-rolled, cold-finished, annealed, normalized, quenched-and-tempered, and hardened stock can behave differently under the tool and during stress release. If heat treatment comes after rough or finish machining, distortion and finishing allowance become part of the process plan.

The EPOC CRAFTER guide on heat treatment before or after CNC machining explains how hardness targets, atmosphere control, stock allowance, and final tolerance change the sequence.

Free-machining steel is also a useful reminder that a family name is not a complete process specification. ISO 4948-2 recognizes non-alloy free-machining steel as a principal-characteristic group, but the label does not define one cutting-speed number or guarantee downstream welding, heat-treatment, or finishing behavior.

6.2 Machining Cast Iron

Cast-iron machining starts with the iron family and the casting blank. Grey iron, ductile iron, white iron, and compacted graphite iron can differ in chip form, abrasive wear, hard spots, and surface behavior. For grey iron under ISO 185, relevant wall thickness and local hardness can matter to both purchasing and machining, especially where the tool enters a thick boss, casting skin, or another section that may not match the coupon condition.

Casting skin, residual moulding material, porosity exposed by machining, local chilled regions, and stock allowance are process-planning concerns rather than ISO 185 acceptance criteria. They still matter because the first cut sees the casting that arrived, not an idealized material table.

For a direct material and process comparison, cast iron vs steel for CNC machining covers grade effects, machining behavior, and what changes after casting.

7. Steel vs Cast Iron: How to Choose

There is no general winner between steel and cast iron. The choice becomes meaningful only after you fix the load case, geometry, blank route, required properties, production volume, and inspection basis. A steel product-standard value and an ISO 185 grey-iron coupon value also need comparable test context before you put them side by side.

Decision factor	When steel moves up the list	When cast iron moves up the list	What must be fixed first
Load and deformation	A suitable steel grade is often preferred where tensile loading, impact resistance, welding, or plastic deformation before fracture matters	A specific cast-iron family may fit compressive or rigid structures when its property basis is adequate	Exact grade, condition, load case, and failure mode
Blank route and geometry	Bar, plate, tube, forging, fabrication, or steel casting can support the geometry	A cast-iron route can suit integrated near-net shapes, housings, bases, and large sections	Geometry, casting feasibility, machining allowance, and quantity
Damping	Use grade-specific data if damping matters	Grey cast iron is often considered where damping is a design driver	Required damping metric and validated material data
Wear and hardness	Steel grade and heat treatment can tailor surface or through hardness	Selected cast-iron grades and hardness states can be useful where wear matters	Contact mode, hardness, lubrication, counterface, and service temperature
Machinability	Depends on grade, hardness, inclusions, condition, and heat-treatment sequence	Depends on iron family, matrix, hardness, casting skin, and local hard regions	Actual material state, features, tool/process, and finish requirement
Inspection	Product standard, condition, and drawing define acceptance	Material grade plus sample basis, relevant wall thickness, and agreed test location may all matter	Test method, sample/location, MTR, and drawing requirements

Decision use: compare specific grades and conditions. Do not rank “steel” and “cast iron” as universal winners.

When the load case is driving the choice, use the yield strength vs tensile strength guide to separate permanent-deformation limits from ultimate tensile strength before selecting a steel grade.

When several metal families are still in play, the CNC machining materials selection guide is the better cross-material decision page; this article stays focused on ferrous materials.

8. Material Grades, MTRs and RFQ Requirements

A classification standard answers what class a material belongs to. A purchase specification must answer what exact material is being supplied, in what form and condition, and what evidence will prove conformity. ISO 4948 and ISO 185 do not remove that procurement work.

8.1 Steel RFQ

Steel material grades, steel standards, and steel specifications answer different parts of the purchase question. Specify the exact grade, applicable product standard, product form, delivery condition, heat treatment, critical mechanical properties, hardness when required, material certificate or MTR, traceability, inspection requirements, and substitution rules. A steel grades chart is useful only when it states the relevant condition and product basis. Even “4140 steel” can be incomplete when the part depends on quenched-and-tempered condition, hardness, section size, or a particular product specification.

8.2 Grey Cast Iron RFQ

Cast iron material grades also have to be read through the applicable cast iron standards and purchase requirements. For grey cast iron, state the full ISO 185 designation and whether tensile strength, Brinell hardness, or both are the characterization basis. When local properties matter, identify relevant wall thickness, sample type where required, agreed test location, and the required minimum or permitted range. These cast iron specifications are meaningful only when the test basis is clear. Representative samples must follow the casting heat treatment when the casting is heat treated and must remain traceable to the production they represent.

8.3 RFQ Checklist

RFQ item	Why it matters
Exact material grade	Removes family-level ambiguity and prevents the supplier from choosing a different material interpretation
Applicable material/product standard	Defines chemistry, product form, delivery condition, test method, or acceptance requirements as applicable
Product form or casting route	Bar, plate, tube, forging, and casting can carry different requirements and machining allowances
Delivery condition and heat treatment	Changes hardness, strength, machinability, and possible distortion
Critical property and test basis	Prevents a generic material reputation from replacing the property that actually controls the design
Grey-iron relevant wall thickness and test location	Connects ISO 185 property interpretation to the section that matters
MTR/certificate and traceability	Links the supplied material to documented chemistry, properties, heat/lot, or representative test evidence
Tolerance, GD&T, and surface finish	Connects the material callout to machining and inspection rather than treating material selection as a standalone decision
Substitution approval rule	Prevents an “equivalent” grade from being accepted without checking condition, product form, properties, and test basis

Decision use: every line should either define the material, define the required condition, or define how conformity will be checked. Delete requirements that do not affect the part.

For drawing-level inspection controls, use the published CNC tolerances and GD&T standards to separate general tolerances, feature-specific tolerances, datum controls, and measurement methods.

Where only selected features need tighter control, the tight tolerance machining guide shows how to tighten bores, mating features, and datum-related geometry without over-tolerancing the full drawing.

9. Applications That Change the Material Decision

Applications matter only when they change the required property or process route. Steel shafts, pins, gears, brackets, fixtures, welded structures, and wear parts may need specific strength, toughness, hardness, heat treatment, or stock-form availability. Grey cast iron often appears in housings, machine bases, bearing supports, and large integrated cast shapes where the casting route and material behavior fit the design.

Once the material callout is fixed, steel CNC machining capabilities become relevant for setups, feature access, tolerances, and inspection rather than for redefining the material grade.

Corrosion protection is a separate decision. Plain carbon and low-alloy steels may need plating, coating, black oxide, paint, or another specified finish depending on the environment. Stainless steels remain ferrous but follow their own corrosion and passivation requirements.

Use the published steel and stainless steel surface finishing options only after the exact grade, service environment, coating thickness, masking, and dimensional consequences are understood.

10. FAQ

10.1 What is the difference between ferrous and non-ferrous metals?

Ferrous metals are iron-based; non-ferrous metals are based primarily on other elements. That classification does not by itself guarantee magnetism, corrosion resistance, low weight, or machinability. For manufacturing, continue to the exact alloy or cast-iron grade and condition.

10.2 What is the difference between iron and steel?

Iron is the base element. Steel is an iron-based material whose composition and classification fall within the applicable steel definition and grade system. In ISO 4948-1, 2.0% carbon is the usual steel/cast-iron dividing line, but the standard explicitly treats it as a usual boundary rather than an exception-free rule.

10.3 Why is cast iron not simply another type of steel?

Because steel and cast iron occupy different composition and material-behavior regimes and use different classification systems. Grey cast iron also contains flake graphite, which changes its mechanical and machining behavior. Treating cast iron as “high-carbon steel” would hide the casting route, graphite structure, and test-basis rules that matter to design and procurement.

10.4 Can stainless steel be magnetic and still be a ferrous metal?

Yes. Ferrous means iron-based, not “always magnetic.” Magnetic response depends on stainless-steel family, microstructure, and processing condition. A magnet check alone is not sufficient evidence that a supplied stainless part is actually carbon steel; verify the grade and material documentation.

10.5 What should a buyer check before machining cast iron?

Start with the cast-iron family and grade, then check the casting blank, relevant hardness or strength basis, wall-thickness effects where applicable, machining allowance, local hard regions or chilled areas, critical features, finish requirement, and inspection plan. For grey iron under ISO 185, keep sample type and test location separate from machining assumptions.

Use the Material Callout to Control the Decision

For a real RFQ, the material decision should end with an exact grade, product form or casting route, delivery condition, critical property, machining sequence, and inspection basis. That is the point where “ferrous metal” becomes a manufacturable and verifiable requirement rather than a broad material label.