Quick Answer
CA15 and CA6NM are the two primary martensitic stainless steel casting grades specified under ASTM A217, A487, A743, and A352 for high-strength, cavitation-resistant pump impellers, hydro turbine runners, and high-pressure valve bodies. While CA15 (cast AISI 410, 12% Cr, C ≤0.15%, EN 1.4008) is an economical general-purpose hardenable grade, CA6NM (13% Cr – 4% Ni – 0.5% Mo, C ≤0.06%, EN 1.4317) drops carbon below 0.06% and adds Nickel and Molybdenum — delivering 22% higher yield strength (≥550 MPa vs ≥450 MPa), dramatic improvements in weldability without cracking, and verified Charpy V-notch impact toughness down to -46°C (-50°F).
⚙ Engineering & Foundry Verification — Qingdao Yucheng (YCUMETAL) 34,000㎡ Pingdu Facility
All casting grades on this page are poured and machined in-house at our 34,000㎡ Pingdu foundry & CNC plant (400+ staff, IATF 16949 & ISO 9001 certified). Every heat is verified by Spectro optical emission spectrometry before pouring, with internal soundness and machined datums validated via Nikon XTH 320 industrial CT, Hexagon CMM, and Nikon metallographic microscopy.
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CA15 vs CA6NM Side-by-Side Specification Matrix (ASTM A217 / A487 / A743 & EN 10213)
Engineers designing high-velocity centrifugal pump impellers, API 6A / ASME B16.34 valve bodies, and hydroelectric or oilfield fluid-end components choose martensitic cast stainless steels when austenitic grades like CF8M (cast 316) lack the yield strength or cavitation erosion resistance required:
| Property / Standard | CA15 (Cast 410 Stainless) | CA6NM (Low-Carbon 13Cr-4Ni-Mo) |
|---|---|---|
| ASTM Standards | ASTM A217 Gr CA15, A743 Gr CA15, A487 Gr CA15 | ASTM A487 Gr CA6NM (Class A / B), A352 Gr CA6NM, A743 Gr CA6NM |
| UNS Number | UNS J91150 | UNS J91540 |
| EN 10213 / Werkstoff Nr. | GX12Cr12 (1.4008) | GX4CrNi13-4 (1.4317) |
| JIS G5121 / GB/T 12230 | SCS1 / ZG12Cr13 | SCS6 / ZG04Cr13Ni4Mo |
| Carbon (C max) | ≤ 0.15% | ≤ 0.06% (eliminates brittle high-carbon martensite) |
| Chromium (Cr) | 11.5% – 14.0% | 11.5% – 14.0% |
| Nickel (Ni) | ≤ 1.00% (residual) | 3.5% – 4.5% (stabilizes tough lath martensite + reformed austenite) |
| Molybdenum (Mo) | ≤ 0.50% | 0.40% – 1.00% (suppresses temper embrittlement & boosts pitting resistance) |
| Tensile Strength (min) | 620 MPa (90 ksi) | 755 MPa (110 ksi) [Class A] / 690 MPa (100 ksi) [Class B NACE] |
| Yield Strength 0.2% (min) | 450 MPa (65 ksi) | 550 MPa (80 ksi) [Class A] / 515 MPa (75 ksi) [Class B NACE] |
| Elongation in 50 mm (min) | 18% | 15% (Class A) / 18% (Class B) |
| Typical Hardness | 185 – 241 HB (≤22 HRC when double-tempered for NACE) | 235 – 285 HB (Class A) / ≤23 HRC (243 HB max) for Class B sour service |
Compare CA15 and CA6NM with austenitic and duplex stainless steels in our ASTM A351 Stainless Casting Grades Guide and Global Casting Grade Cross-Reference Tool.
Why CA6NM Replaced CA15 in Critical Pump, Turbine & Subsea Castings
- Cavitation & Slurry Erosion Resistance: In high-head centrifugal pumps, boiler feedwater stages, and Francis/Pelton hydro runners, imploding vapor bubbles strip metal from austenitic CF8M at 3–4× the rate of martensitic steel. CA6NM’s tempered lath-martensite matrix with 5–15% finely dispersed retained/reformed austenite absorbs micro-jet shock waves without brittle spalling.
- Weldability & Foundry Upgrading: Because CA15 contains up to 0.15% carbon and almost no nickel, it is highly prone to cold hydrogen cracking in thick sections and requires strict 200–300°C preheating before any weld repair. CA6NM’s ultra-low carbon (≤0.06%) and 4% Ni produce low-carbon lath martensite that welds cleanly with matching E410NiMo filler metal followed by post-weld tempering.
- Sub-Zero Impact Toughness (-46°C / -50°F): Standard CA15 suffers a steep ductile-to-brittle transition near ambient temperature. Under ASTM A352 Gr CA6NM and EN 10213 1.4317, CA6NM routinely achieves 40–70 J Charpy V-notch impact energy at -46°C, making it the standard martensitic choice for Arctic pipelines and cold-climate pressure valves.
Heat Treatment Discipline: Why Ac1 Temperature Control Makes or Breaks CA6NM
Martensitic stainless castings must never be shipped in the as-cast condition. At YCUMETAL’s heat treatment workshop, CA15 and CA6NM undergo strictly instrumented thermal cycles:
| Grade & Class | Austenitizing (Normalize / Quench) | Tempering Cycle | Critical Metallurgical Precaution |
|---|---|---|---|
| CA15 (ASTM A217 / A743) | 955–1,010°C (1,750–1,850°F), air cool or oil quench | Temper at 595–760°C (never temper between 400°C and 565°C to avoid 475°C embrittlement) | Cool casting below 150°C before tempering so martensite transformation completes. |
| CA6NM Class A (ASTM A487) | 1,010–1,065°C (1,850–1,950°F), air or polymer quench to <50°C | Single temper at 595–620°C (1,100–1,150°F) | Because 4% Ni lowers the $ re-austenitizing temperature to ~610–630°C, tempering above 625°C forms fresh untempered martensite on cooling! |
| CA6NM Class B (NACE MR0175 / ISO 15156) | 1,010–1,065°C air quench to <50°C ($ is near 60–90°C) | Mandatory Double Temper: 1st temper at 650–680°C (cool to <50°C) + 2nd temper at 595–620°C | Required to guarantee hardness ≤23 HRC (243 HB) throughout thick valve walls for sour H2S oilfield service. |
Inspection & Non-Destructive Testing (NDT) Checklist for OEM Buyers
- Chemistry & Carbon/Nickel Ratio: Verified on every melt via Spectro OES at our Pingdu metallurgy lab (confirming C ≤0.06% and Cr/Ni balance to prevent excessive delta-ferrite stringers).
- Internal Soundness & Surface NDT: Because martensitic grades have narrower solidification ranges than austenitics, directional feeding is verified using our Nikon XTH 320 industrial CT (see Casting Porosity Acceptance Criteria) plus Magnetic Particle Inspection (MPI per ASTM E709) on machined sealing surfaces (see NDT Methods for Castings).
Frequently Asked Questions (FAQ)
What is the wrought equivalent of cast CA15 and CA6NM?
CA15 is the cast counterpart of wrought AISI 410 (UNS S41000 / EN 1.4006) 12% Cr martensitic stainless steel. CA6NM corresponds to wrought F6NM (ASTM A182 Gr F6NM / UNS S41500 / EN 1.4313 X3CrNiMo13-4).
Why did my CA6NM casting fail the 23 HRC max hardness test after tempering at 660°C?
In CA6NM, the 3.5–4.5% Nickel addition lowers the lower critical transformation temperature ($) to approximately 610–630°C. If a single temper is performed at 650–680°C, part of the matrix reverts to austenite during holding and transforms into fresh, hard untempered martensite upon cooling. The solution is a double temper: first at 660–680°C (cooled below 50°C), followed by a second temper at 595–615°C.
When should I specify 17-4PH (CB7Cu-1) or CD4MCuN instead of CA6NM?
Specify 17-4PH (ASTM A747 CB7Cu-1) when you need yield strength above 690–1,000 MPa in precision investment castings. Specify CD4MCuN duplex stainless steel when the fluid contains high chlorides or seawater where 13% Cr martensitic steel would suffer pitting corrosion.