ASME SA-335M / ASTM A335M: Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service (METRIC)
1. Overview & Metric Standard Significance
ASME SA-335M is the metric version of ASME SA-335, approved for ASME Boiler and Pressure Vessel Code (BPVC) Section II, Part A construction. The "M" suffix indicates exclusive use of SI (metric) units throughout the specification - dimensions, test values, and tolerances.
Critical Distinction: SA-335M is not a direct conversion of SA-335 - it is a parallel metric specification with dimensional and testing requirements expressed solely in SI units.
2. Metric vs. Inch Standards Comparison
| Aspect | ASME SA-335M (Metric) | ASME SA-335 (Inch) | Key Differences |
|---|---|---|---|
| Unit System | Exclusively SI units | Exclusively inch-pound units | No mixing of units allowed |
| Dimensional Series | DN (Diameter Nominal) sizes | NPS (Nominal Pipe Size) sizes | Different size gradations |
| Wall Thickness | mm (millimeters) | inch (fractions/decimal) | Different schedule systems |
| Tolerances | mm based tolerances | inch based tolerances | Calculated differently |
| Test Values | MPa (megapascals) | ksi (kips per square inch) | 1 ksi = 6.895 MPa |
| Ordering | Must specify metric sizes | Must specify inch sizes | Cannot intermix in one order |
3. ASME SA-335M Grades & Metric Designations
Ferritic Alloy Steel Grades (Identical Grades, Metric Units)
| Grade | UNS No. | ASME P-No. | Equivalent to SA-335 |
|---|---|---|---|
| P1 | K11522 | P-No. 3 | P1 |
| P2 | K11547 | P-No. 3 | P2 |
| P5 | K41545 | P-No. 4 | P5 |
| P5b | K51545 | P-No. 4 | P5b |
| P9 | S50400 | P-No. 5B | P9 |
| P11 | K11597 | P-No. 4 | P11 |
| P12 | K11562 | P-No. 4 | P12 |
| P22 | K21590 | P-No. 5A | P22 |
| P91 | K90901 | P-No. 5B | P91 |
| P92 | K92460 | P-No. 15E | P92 |
| P122 | K91603 | P-No. 15E | P122 |
*Note: Chemical compositions are identical between SA-335 and SA-335M*
4. Metric Size Designations & Dimensional System
DN (Diameter Nominal) Size Equivalents
| DN Size (mm) | Approx. NPS Equivalent | Common OD (mm) | Common Wall Thickness Range (mm) |
|---|---|---|---|
| DN 15 | NPS ½ | 21.3 | 2.0 - 4.0 |
| DN 25 | NPS 1 | 33.4 | 2.5 - 6.0 |
| DN 50 | NPS 2 | 60.3 | 2.5 - 8.0 |
| DN 80 | NPS 3 | 88.9 | 3.0 - 10.0 |
| DN 100 | NPS 4 | 114.3 | 3.0 - 12.0 |
| DN 150 | NPS 6 | 168.3 | 3.5 - 14.0 |
| DN 200 | NPS 8 | 219.1 | 4.0 - 18.0 |
| DN 300 | NPS 12 | 323.9 | 5.0 - 25.0 |
| DN 400 | NPS 16 | 406.4 | 6.0 - 30.0 |
| DN 500 | NPS 20 | 508.0 | 6.0 - 35.0 |
Wall Thickness Designations (Metric)
| Designation | Description | Example |
|---|---|---|
| Schedule | Metric equivalent schedules | Sch 10S, 20, 40, 80, 160 |
| mm thickness | Direct wall thickness in mm | 6.0 mm, 8.0 mm, 12.5 mm |
| Series | Wall thickness series | Series 1, Series 2 (ISO 4200) |
5. Chemical Composition (SI Units - Identical to Inch Version)
| Grade | C (%) | Mn (%) | P Max (%) | S Max (%) | Si (%) | Cr (%) | Mo (%) | Other Elements |
|---|---|---|---|---|---|---|---|---|
| P11 | 0.05-0.15 | 0.30-0.60 | 0.025 | 0.025 | 0.50-1.00 | 1.00-1.50 | 0.44-0.65 | - |
| P22 | 0.05-0.15 | 0.30-0.60 | 0.025 | 0.025 | 0.50 | 1.90-2.60 | 0.87-1.13 | - |
| P91 | 0.08-0.12 | 0.30-0.60 | 0.020 | 0.010 | 0.20-0.50 | 8.00-9.50 | 0.85-1.05 | V, Nb, N |
| P92 | 0.07-0.13 | 0.30-0.60 | 0.020 | 0.010 | 0.50 | 8.50-9.50 | 0.30-0.60 | V, Nb, N, W |
6. Mechanical Properties (SI Units - MPa)
| Grade | Tensile Strength Min (MPa) | Yield Strength Min (MPa) | Elongation Min (%) | Hardness Max (HB) |
|---|---|---|---|---|
| P1, P2, P12 | 380 | 205 | 30 | 163 |
| P11, P22 | 415 | 205 | 30 | 163 |
| P5, P9 | 415 | 205 | 30 | 163 |
| P91 | 585 | 415 | 20 | 250 |
| P92 | 620 | 440 | 20 | 250 |
| P122 | 620 | 400 | 20 | 250 |
7. Dimensional Tolerances (Metric - mm)
| Dimension | Tolerance for DN ≤ 101.6 mm | Tolerance for DN > 101.6 mm |
|---|---|---|
| Outside Diameter | ±0.4 mm or ±0.75% of OD, whichever greater | ±0.75% of OD |
| Wall Thickness | +0 / -12.5% of nominal wall | +0 / -10% of nominal wall |
| Ovality | Max 1.5% of OD | Max 1.0% of OD |
| Length | +3 mm / -0 mm for random lengths | +3 mm / -0 mm for random lengths |
| Straightness | 0.5 mm per 300 mm length | 1.5 mm per meter length |
8. Hydrostatic Test Requirements (Metric)
| Pipe Size | Minimum Test Pressure Formula | Typical Test Pressure Range |
|---|---|---|
| All Sizes | P = 2St/D Where: P = Test pressure (MPa) S = Stress (MPa) - typically 70% of SMYS t = Wall thickness (mm) D = Outside diameter (mm) |
10-40 MPa (1450-5800 psi) depending on dimensions |
Example Calculation for DN 100, wall 8.0 mm:
P = (2 × 145 MPa × 8.0 mm) / 114.3 mm = 20.3 MPa (≈2940 psi)
9. Ordering Information for SA-335M
Mandatory Information for Metric Orders:
Specification: ASME SA-335M (must include "M")
Grade: P11, P22, P91, etc.
Size: DN size OR OD × WT in mm
Length: Meters (not feet)
Construction Code: Section I, III, VIII
All units must be metric
Example SA-335M Purchase Order:
text
ASME SA-335M P91 Size: DN 250, OD 273.0 mm × WT 20.0 mm Length: 8.0 meters, plain ends Quantity: 50 pieces Construction: ASME Section I Heat Treatment: Normalized & Tempered per SA-335M Additional Requirements: - 100% Ultrasonic Examination per ASME Sec V Art 4 - Hydrostatic test at 25 MPa minimum - Impact testing at -20°C - ASME MTR Form with metric units only - All dimensions in mm, pressures in MPa
10. Metric vs Inch Conversion Challenges
| Challenge | Solution | ASME Guidance |
|---|---|---|
| Mismatched sizes | Use exact metric equivalents, not conversions | ASME B36.19M for metric pipe sizes |
| Schedule differences | Use actual mm thickness, not schedule conversion | Wall thickness tables in SA-335M |
| Test pressure units | Specify MPa, not psi conversions | Hydro test formula in SA-335M |
| Material certification | MTR must show metric values only | ASME Sect II, Part A requirements |
| Design calculations | Use metric allowable stresses | ASME Sect II, Part D (metric) |
11. Global Metric Standards Harmonization
| ASME SA-335M Grade | ISO 9329-2 Grade | EN 10216-2 Grade | GOST 8731 Grade |
|---|---|---|---|
| P11 | 13CrMo4-5 | 13CrMo4-5 | 13ХМ |
| P22 | 10CrMo9-10 | 10CrMo9-10 | 10Г2 |
| P91 | X10CrMoVNb9-1 | X10CrMoVNb9-1 | - |
12. Common Applications for Metric Piping
| Industry | Typical Applications | Preferred Units |
|---|---|---|
| European Power Plants | Main steam lines, hot reheat lines | Metric (DN sizes) |
| Asian Petrochemical | High-temperature process piping | Metric (mm sizes) |
| International EPC Projects | Turnkey plant construction | Metric (SI system) |
| Nuclear (outside US) | Reactor coolant lines, steam lines | Metric (ISO standards) |
| Marine (non-US) | Boiler piping, exhaust systems | Metric (DN/PN system) |
13. Design Considerations for Metric Piping
| Design Aspect | Metric Practice | Conversion Factor |
|---|---|---|
| Allowable Stress | Table 1A (MPa values) | 1 ksi = 6.895 MPa |
| Pipe Weight | kg/m (kilograms per meter) | 1 lb/ft = 1.488 kg/m |
| Flow Rates | m³/h (cubic meters per hour) | 1 gpm = 0.227 m³/h |
| Pressure Rating | PN (Pressure Nominal) classes | PN 40 = Class 300 approx |
| Flange Standards | EN 1092-1, ASME B16.47 Series B | Different bolt patterns |
14. Certification Requirements for SA-335M
| Document | Metric Requirements | Notes |
|---|---|---|
| ASME MTR | All values in SI units | Must state "SA-335M" |
| Test Reports | MPa, mm, °C only | No dual units allowed |
| Heat Treat Records | Temperatures in °C | No °F conversions |
| Dimensional Reports | Measurements in mm | With metric tolerances |
| Marking | Include "M" in stamp | SA-335M P91 |
15. Procurement Best Practices
Specify Clearly: Always include "M" in specification
Avoid Conversions: Order exact metric sizes, don't convert from inch
Verify Units: Check all documents for consistent SI units
Global Suppliers: Source from mills familiar with metric production
Design Integration: Ensure all components are metric (flanges, valves, fittings)
16. Common Errors to Avoid
| Error | Consequence | Correction |
|---|---|---|
| Ordering SA-335 for metric project | Inch sizes supplied, project delays | Always specify SA-335M |
| Mixing units in PO | Confusion, wrong material supplied | Use only metric units |
| Assuming schedule equivalence | Wrong wall thickness delivered | Specify actual mm thickness |
| Using converted test pressures | Over/under testing of pipes | Use MPa values from standard |
| Not specifying "M" on MTR | Documentation rejected by inspector | Require "SA-335M" on all docs |
17. Industry Adoption Trends
| Region | Primary Standard | Transition Status |
|---|---|---|
| Europe | EN/ISO metric | Fully metric |
| Asia | Mixed - Japan (JIS metric), China (GB metric) | Mostly metric |
| North America | ASME inch with metric option | Dual system, inch dominant |
| Middle East | Project specific | Often metric for new projects |
| South America | Mixed | Increasing metric adoption |
18. Future Outlook
ISO harmonization: Increasing alignment between ASME SA-335M and ISO 9329-2
Digital twins: Metric data integration for smart plant design
Global projects: Metric becoming default for international EPC work
Code unification: ASME embracing more ISO metric standards
ASME SA-335M provides the Code-approved metric pathway for high-temperature alloy steel piping in global projects. Its exclusive use of SI units eliminates conversion errors and ensures compatibility with international engineering standards. For projects outside North America or for multinational corporations, SA-335M is often the preferred specification for seamless alloy piping.
