Cast iron in oil and gas service is specified by grade first, geometry second: ASTM A536 60-40-18 ductile iron is the default for valve bodies, pump casings and ANSI 150/300 fittings handling produced water and crude, while grey ASTM A48 Class 30/40 covers non-pressure housings and compressor housings where damping matters more than ductility [S6].
For sour (H2S-containing) service, only specific ductile and austempered ductile iron (ADI) grades with documented hardness limits are accepted; this narrows the field considerably compared to general industrial cast iron use [S6].
Why Cast Iron, and Why Now
Carbon steel still dominates line pipe, but cast iron remains the economical material for static, thick-walled pressure-containing components (valves, fittings, pump bodies, wellhead blocks) where casting geometry avoids welded fabrication. Cast iron's damping and wear behaviour also keeps it specified for compressor housings and gearbox cases; the broader family is covered in the cast iron encyclopedia entry, and downstream pressure-control components sit inside the oil seal and sealing components entry when the discussion moves to sealing hardware. [S6]
Steel service centers stock cast iron as a parallel product line to carbon and alloy steel, with the same dimensional and traceability expectations [S6]. The industrial channel therefore treats cast iron as a steel-mill-style stocked commodity, with material test reports (MTRs) per ASTM A536 / A48 / A532, rather than as a foundry-only special.
The Four Cast Iron Families You Will See on a Spec Sheet
Grey iron (ASTM A48, graphite flakes) is the cheapest and best damping. Class 30 and Class 40 cover the bulk of oilfield non-pressure housings; tensile strength sits roughly 30,000 to 40,000 psi (207 to 276 MPa) depending on class. Ductile iron (ASTM A536, graphite nodules) is the workhorse for pressure-containing castings: the common grades 60-40-18, 65-45-12 and 80-55-06 cover most valve and fitting bodies, with elongations of 18%, 12% and 6% respectively [S6].
White iron (ASTM A532, chromium-molybdenum martensitic matrix) is a wear material, not a pressure material. It carries 550 to 800 BHN hardness and is used for slurry-handling pump liners, pipe bends and rod-mill applications. Malleable iron (ASTM A47/A220) sits in the gap between grey and ductile for small threaded fittings. A practical side-by-side for the procurement desk:
Family comparison by decision criterion: (1) Pressure rating: ductile A536 60-40-18 and above for ANSI 150/300; grey A48 only for non-pressure or low-pressure. (2) Corrosion/abrasion resistance: white A532 Cr-Mo grades for slurry and rod wear; austempered ductile (ASTM A897) as the tough upgrade. (3) Machinability: grey A48 Class 30 is the easiest to machine; white A532 is unmachinable and ground-only. (4) Damping/noise: grey iron is the reference material for compressor housings and gear cases; ductile and steel transmit vibration. Stocking reality mirrors the engineering hierarchy: A48 and A536 are shelf items, A532 and high-alloy ADI are made-to-order [S6].
Sour Service: NACE MR0175 and the Hardness Ceiling
Sour service (NACE MR0175 / ISO 15156) is the single biggest constraint on cast iron in oil and gas. The standard imposes a 22 HRC maximum hardness on most carbon and low-alloy steels, and historically restricted cast iron itself to limited non-pressure service. Modern revisions permit ductile and austempered ductile iron under tightly defined metallurgical conditions: nodularity, ferrite/pearlite ratio, hardness limit, and post-casting verification [S6].
For procurement, the rule of thumb is: never accept any cast iron component into sour service without a written NACE MR0175 compliance statement naming the specific part and material lot. The same discipline applies to elastomer selection in the same valves; the comparison between industrial rubber selection and chemical compatibility and the underlying metal spec drives the actual service envelope. NACE MR0175 references the casting standards but adds a hardness and microstructure gate on top, so a "ductile iron body" in isolation is not NACE-compliant; only a documented NACE-certified heat is.
Where Each Grade Lands in the Field
Valve bodies and bonnets for ANSI 150/300 gate, globe, check and ball valves handling produced water, crude and natural gas are almost universally A536 60-40-18 or 65-45-12 in NACE-qualified heats. Pump casings for water-handling, amine and glycol service follow the same pattern. Compressor housings and gear reducers lean on A48 Class 40 grey iron for its damping and machinability; pressure is not the design driver, vibration and NVH are [S6].
Slurry pump liners, pipe elbows in desanding service, and rod-mill wear parts move into A532 Cr-Mo white iron or austempered ductile (ASTM A897) grades 1 through 4 depending on impact and wear balance. Downhole tooling uses high-alloy and ADI grades in a narrower band, and only when NACE compliance is documented. The same engineering logic (spec-first, family-first, then grade) used for polymeric components in PEEK grade selection applies cleanly to cast iron: family sets the envelope, grade sets the limit.
What Cast Iron is NOT For
Thin-wall, high-pressure line pipe is the carbon steel domain (API 5L, ASTM A106); cast iron is not specified for sour line pipe replacement. Cryogenic service (below -29 degrees C / -20 degrees F) rules out standard ferritic cast iron; austenitic ductile or austenitic steel is the only option. Boiler and fired-heater tube applications are steel and high-alloy, not cast iron. Direct chlorine, strong oxidising acid or HF service takes the discussion to high-silicon cast iron (Duriron / Durichlor) or to alloy 20 / Hastelloy territory, which is outside the standard A48/A536/A532 procurement basket [S6].
Threaded high-pressure fittings for hydrogen service at 6000+ psi are forged steel (A105, A182), not cast iron. The blanket "cast iron is brittle" is technically wrong in the ductile family, but operationally right for old grey-iron pipelines that should be replaced rather than re-rated. Any cast iron component in cyclic, fatigue-loaded service (downhole, reciprocating compressor frames) needs a documented fatigue curve from the foundry, not a generic bar stock figure.
Selection Criteria and Sourcing Discipline
Spec by family and grade, then by standard, then by NACE requirement. A useful checklist for any oil and gas cast iron buy: (a) ASTM designation and class (A48 Class 30, A536 65-45-12, A532 Class II Type D, A897 ADI 900/650/09). (b) Required MTR with heat analysis, mechanical properties, hardness, and nodularity for ductile grades. (c) NACE MR0175 / ISO 15156 compliance statement for any sour or potentially sour service, including the production environment definition. (d) Dimensional and NDT requirements: visual, dimensional, magnetic particle or UT for critical bodies, radiographic for high-pressure classes. (e) Traceability: heat code, foundry name, country of melt, and any customer-specific qualification (API 6D for valves, API 610 for pumps). (f) Pressure rating backing: hydrotest pressure and acceptance criteria on the print [S6].
The stocking channel for these materials is the same steel service center network that stocks bar, plate and tube; lead times for shelf A48 and A536 are measured in days, while A532 and qualified NACE ADI are typically weeks. The buyer should treat the foundry qualification as a one-time project and the heat-by-heat MTR as a per-shipment check, not a substitution.
Watch points for the next buying cycle: foundry consolidation in the U.S. Midwest, which tightens qualified NACE ductile supply; and the gradual migration of new sweet-service valve bodies to higher ductile classes (65-45-12, 80-55-06) as designs thin out for weight reduction. Both signals are trackable through ASTM committee A04 ballots and service-center stock reports.
For component-level specifications, see construction machinery and equipment.