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What is P1 Alloy Steel (Carbon-½ molybdenum)?
Quick answer
- Definition
- P1 / T1 is a carbon-½% molybdenum steel, the simplest of the ASTM A335/A213 creep-resistant grades, used for moderate-temperature pressure piping and tubes.
- Family
- Chromium-molybdenum alloy steel
- Key strength
- About 0.5% molybdenum
- Main forms
- Pipes, Tubes, Fittings, Flanges
- Typical uses
- Moderate-temperature steam piping; Refinery process lines; Boiler tubes (T1)
Adding about 0.5% molybdenum to carbon steel raises its strength at elevated temperature and its resistance to creep compared with plain carbon steel such as A106. P1 was historically used in steam and refinery piping.
Long service above about 425 °C can cause graphitisation in C-Mo steels, so many operators now prefer 1¼Cr-½Mo (P11) or 2¼Cr-1Mo (P22) for new high-temperature designs. P1 remains in use for replacement and moderate-temperature lines.
Key characteristics
- About 0.5% molybdenum
- Better creep strength than carbon steel
- Graphitisation risk in long service above about 425 °C
- Mostly used for replacement and moderate-temperature lines
What are the specifications and equivalents of P1?
| Product form | Specification / designation | Product page |
|---|---|---|
| Pipes | ASTM A335 P1 | P1 pipes → |
| Tubes | ASTM A213 T1 | P1 tubes → |
| Fittings | ASTM A234 WP1 | P1 fittings → |
| Flanges | ASTM A182 F1 | P1 flanges → |
What is the chemical composition of P1?
| Element | Limit (%) |
|---|---|
| C | 0.10 – 0.20 |
| Mn | 0.30 – 0.80 |
| P | 0.025 max |
| S | 0.025 max |
| Si | 0.10 – 0.50 |
| Mo | 0.44 – 0.65 |
What are the mechanical properties of P1?
| Property | Value |
|---|---|
| Tensile strength | 380 MPa (55 ksi) |
| Yield strength (0.2% offset) | 205 MPa (30 ksi) |
| Elongation | 30% (longitudinal) |
Physical properties and service temperature
| Property | Value |
|---|---|
| Density (typical) | 7.85 g/cm³ (7850 kg/m³) |
| Magnetism | Magnetic (ferritic/martensitic). |
| Heat treatment | Supplied annealed, or normalised and tempered, per specification. |
| Service temperature | Designed for elevated-temperature pressure service. |
How is P1 welded and fabricated?
Requires preheat, interpass control and post-weld heat treatment.
Always follow a qualified welding procedure (for example ASME Section IX) and the filler-metal recommendations for P1. Verify incoming material by PMI and check the mill test certificate against the specification.
Which P1 products do we supply?
P1 Alloy Steel (Carbon-½ molybdenum) is available in 4 product forms, from ready stock or mill-sourced to your specification, with EN 10204 3.1 mill test certificates (3.2 with third-party inspection on request).
P1 PipesASTM A335 P1Specification & sizes
How it's madeSeamless pipe is pierced from a solid billet and hot-finished or cold-drawn. Welded pipe is formed from strip or plate and welded longitudinally (ERW/HFW, TIG/plasma, EFW, LSAW) or spirally (SAWH). Stainless and nickel-alloy pipe is then solution annealed and pickled. Carbon and alloy steel pipe is supplied as-rolled, normalised, or normalised and tempered according to the specification.
- Specification
- ASTM A335 P1
- Size range
- NPS ⅛" to 48" and larger (DN 6 – DN 1200+), schedules 5S to XXS, single random, double random and cut lengths
- Finish
- Annealed & pickled, bright annealed, mill finish, black, varnished, galvanised or 3LPE / FBE coated (by material)
- Ends
- Plain end (PE), bevelled end (BE, ASME B16.25), threaded (NPT / BSP), grooved
Testing & documents
- Chemical and mechanical (tension, flattening, flare/bend) tests
- Hydrostatic test or non-destructive electric test (UT/ET) on each length
- Dimensional and visual inspection
- Intergranular corrosion (stainless), impact (low-temperature), hardness (sour service) when specified
- PMI and third-party inspection on request
- EN 10204 3.1 certificate; 3.2 with third-party inspection
P1 TubesASTM A213 T1Specification & sizes
How it's madeSeamless tubes are hot-extruded or pierced and then cold-drawn or pilgered to size. Welded tubes are formed from strip, welded (TIG/laser/HF), bead-worked and drawn or sized. Heat-exchanger and boiler tubes are annealed after final forming, and U-tubes are bent and stress-relieved as required.
- Specification
- ASTM A213 T1
- Size range
- OD 3 mm to about 127 mm (¼" to 5") typical, wall 0.5 mm to 12 mm; BWG or mm; straight lengths up to about 30 m, U-bent to drawing
- Finish
- Annealed & pickled, bright annealed (BA), polished (sanitary), cold-drawn
- Ends
- Plain square-cut, deburred; U-bends with stress-relieved bends on request
Testing & documents
- Eddy-current and/or ultrasonic testing on each tube
- Hydrostatic or pneumatic test
- Flattening, flaring and flange tests
- Grain size (H grades), IGC and hardness when specified
- Surface roughness (Ra) for sanitary and instrumentation tube
- EN 10204 3.1 certificate; 3.2 with third-party inspection
P1 FittingsASTM A234 WP1Specification & sizes
How it's madeButt-weld fittings are formed from seamless or welded pipe or plate by hot or cold forming (mandrel bending, hydraulic bulging, pressing), then heat treated, bevelled and finished. Socket-weld and threaded fittings are machined from forgings or bar.
- Specification
- ASTM A234 WP1
- Size range
- Butt-weld ½" – 48" (larger fabricated to order) in schedules matching the pipe; socket-weld and threaded ⅛" – 4" in Class 2000, 3000, 6000 and 9000
- Finish
- Annealed & pickled, shot-blasted, black painted or galvanised (by material)
- Ends
- Bevelled (butt-weld), socket-weld, threaded NPT/BSP
Testing & documents
- Chemical and mechanical tests of the starting material
- Dimensional inspection to B16.9 / B16.11
- Radiography of welded fittings (seamed WPW/WX)
- Hardness, IGC and impact tests as specified
- EN 10204 3.1 certificate; 3.2 with third-party inspection
P1 FlangesASTM A182 F1Specification & sizes
How it's madeFlanges are forged from billets (or cut from plate for some blind and plate flanges), heat treated to the material specification, and machined: bore, face, raised face or ring groove, bolt holes and hub bevel.
- Specification
- ASTM A182 F1
- Size range
- NPS ½" – 24" (B16.5), 26" – 60" (B16.47 Series A/B); Class 150, 300, 600, 900, 1500, 2500; EN 1092-1 PN 6 – PN 100 on request
- Finish
- Raised face (RF), flat face (FF) or ring-type joint (RTJ); serrated spiral or concentric face finish (typically 3.2 – 6.3 µm Ra)
- Ends
- Weld-neck bevel matched to pipe schedule; socket bore; threaded
Testing & documents
- Chemical and mechanical tests of the forging
- Dimensional inspection to ASME B16.5 / B16.47
- Hardness, impact and IGC tests as specified
- Ultrasonic testing of forgings on request
- EN 10204 3.1 certificate; 3.2 with third-party inspection
Where is P1 used?
Typical applications
- Moderate-temperature steam piping
- Refinery process lines
- Boiler tubes (T1)
- Replacement of existing C-Mo systems
What should you specify when buying P1?
- Grade and specification, e.g. ASTM A335 P1
- Product form, size and tolerance
- Heat treatment / condition and surface finish
- Quantity, delivery location and required date
- Tests: PMI, hydro/NDT, IGC, impact or hardness as required
- Certificate type (EN 10204 3.1 / 3.2) and third-party inspection
See our RFQ checklist guide and what drives metal prices.
Explore more on P1
P1 products
Comparisons & articles
Frequently asked questions
What does P1 mean in ASTM A335?
P1 is the carbon-½% molybdenum grade of ASTM A335 seamless ferritic alloy pipe. The "P" denotes pipe; the tube equivalent in ASTM A213 is T1.
What is P1 used for?
Moderate-temperature steam piping, Refinery process lines, Boiler tubes (T1), Replacement of existing C-Mo systems. It's used across Power Generation, Petrochemical & Refining, Boiler Manufacturing.
What is the UNS number of P1 / T1 (C-½Mo)?
P1 / T1 (C-½Mo) has no UNS designation. See the specification notes above.
Which product forms is P1 available in?
Pipes, Tubes, Fittings, Flanges, to ASTM A335 P1, ASTM A213 T1, ASTM A234 WP1 and related specifications.
What is the density of P1 / T1 (C-½Mo)?
About 7.85 g/cm³ (7850 kg/m³), used for weight calculations.
Is P1 / T1 (C-½Mo) magnetic?
Magnetic (ferritic/martensitic).
What temperature can P1 withstand?
Designed for elevated-temperature pressure service.