A supercritical steam line is a component that works entirely on creep. The metal spends its life at a temperature where the stress never approaches yield, but where time does the damage anyway. Selecting the material is therefore not a question of which alloy is strongest. It is a question of which alloy family holds its properties at the service temperature, for how long, and whether the mill’s certificate actually proves the condition you need.
This article sets out the selection logic for the ferritic alloy steel pipe grades used in supercritical and advanced supercritical power plant steam and feedwater service, using ASME BPVC 2023 Section II Part A-1. It is a framework for reading the specification and reading the certificate, not a project record.
Start from the service condition, not from the alloy name
SA-335/SA-335M clause 1.1 states the scope: seamless ferritic alloy steel pipe intended for high-temperature service, and it says plainly that selection will depend upon design, service conditions, mechanical properties and high-temperature characteristics. Note what is not in it. The specification covers room-temperature tensile requirements only. The allowable stresses and time-dependent properties that govern a supercritical design live in other volumes of the Code and in the applicable design code, not in the material specification. A mill certificate showing only room-temperature numbers is therefore incomplete evidence of fitness for the service.
What you need from the material specification is four things: the chemical composition, the heat treatment, the room-temperature mechanical properties as a baseline, and the marking and certification detail that tells you what was really done.
The grade ladder: from 1.25Cr-0.5Mo to 12Cr
SA-335 Table 1 sets the chemical composition and Table 2 sets the heat treatment. The column order in Table 1 is carbon, manganese, phosphorus, sulfur, silicon, chromium, molybdenum, then the listed alloying additions. P22, P91 and P911 are the grades most often written into a purchase specification for this service; P92 and P122 sit at the top of the ferritic ladder, where the certificate detail matters most.
| Grade | UNS | C % | Mn % | Cr % | Mo % | Key additions |
|---|---|---|---|---|---|---|
| P11 | K11597 | 0.05-0.15 | 0.30-0.60 | 1.00-1.50 | 0.44-0.65 | – |
| P22 | K21590 | 0.05-0.15 | 0.30-0.60 | 1.90-2.60 | 0.87-1.13 | – |
| P24 | K30736 | 0.05-0.10 | 0.30-0.70 | 2.20-2.60 | 0.90-1.10 | V 0.20-0.30, Ti 0.06-0.10, N 0.012 max |
| P91 Type 1 | K91560 | 0.08-0.12 | 0.30-0.60 | 8.00-9.50 | 0.85-1.05 | V 0.18-0.25, N 0.030-0.070, Cb 0.06-0.10 |
| P911 | K91061 | 0.09-0.13 | 0.30-0.60 | 8.5-9.5 | 0.90-1.10 | V 0.18-0.25, Cb 0.060-0.10, B 0.0003-0.006, N 0.04-0.09, W 0.90-1.10 |
| P92 | K92460 | 0.07-0.13 | 0.30-0.60 | 8.50-9.50 | 0.30-0.60 | V 0.15-0.25, N 0.03-0.07, Cb 0.04-0.09, W 1.5-2.00 |
| P122 | K92930 | 0.07-0.14 | 0.70 max | 10.00-11.50 | 0.25-0.60 | V 0.15-0.30, W 1.50-2.50, Cu 0.30-1.70, Cb 0.04-0.10, B 0.0005-0.005, N 0.040-0.100 |
All figures are SA-335/SA-335M Table 1. The progression across the grades is a straightforward trade: chromium and molybdenum buy oxidation resistance and creep strength, while vanadium, niobium, tungsten and boron buy grain-boundary and matrix stability.
Heat treatment is where the grade is actually secured
SA-335 Table 2 sets the heat treatment requirements, Each grade has a heat treatment type, a normalizing temperature, a cooling medium and a subcritical annealing or tempering temperature. Where ellipses appear in the table there is no requirement, so a blank cell in a certificate table may be entirely correct rather than a missing value.
| Grade | Heat treatment | Normalizing temp. | Cooling | Subcritical anneal / temper |
|---|---|---|---|---|
| P11 | Full or isothermal anneal, or normalize and temper | – | – | 1200 F [650 C] |
| P22 | Full or isothermal anneal, or normalize and temper | – | – | 1250 F [675 C] |
| P23 | Normalize and temper | 1900-1975 F [1040-1080 C] | Air or accelerated cooling | 1350-1470 F [730-800 C] |
| P24 | Normalize and temper | 1800-1870 F [980-1020 C] | Air or accelerated cooling | 1350-1420 F [730-770 C] |
| P91 Type 1 and Type 2 | Normalize and temper, or quench and temper | 1900-1975 F [1040-1080 C] | – | 1350-1470 F [730-800 C] |
| P92 | Normalize and temper | 1900-1975 F [1040-1080 C] | – | 1350-1470 F [730-800 C] |
| P122 | Normalize and temper | 1900-1975 F [1040-1080 C] | – | 1350-1470 F [730-800 C] |
| P911 | Normalize and temper | 1900-1975 F [1040-1080 C] | – | 1365-1435 F [740-780 C] |
Four footnotes change the reading of specific rows. Note A states that where ellipses appear in the table there is no requirement. Note D, on the P92 and P911 normalizing temperatures, permits accelerated cooling for section thicknesses greater than 3 in. [75 mm].
What the certificate must demonstrate
SA-335 Table 3 sets the tensile requirements, For P24 and for P91 Type 1 and Type 2 the minimum tensile strength is 85 ksi [585 MPa] with minimum yield 60 ksi [415 MPa]. For P92 and P911 it is 90 ksi [620 MPa] and 64 ksi [440 MPa]. For P122 it is 95.5 ksi [660 MPa] and 66.5 ksi [460 MPa].
Note 9.5 to that table is frequently missed: for Grade P91 Type 1 and Type 2, when quenching and tempering has been performed, the tensile and hardness properties shall be met and verified on material taken from the half-thickness location. A P91 certificate quoting a surface tensile result is not what the specification asks for in the quenched and tempered condition.
SA-335 also controls the one repair tightly enough to name processes. Clause 7.6 requires weld repair to be permitted only subject to the approval of the purchaser, and clause 7.6.1 states that all repair welds in P91 shall be made with one of the listed welding processes and consumables, including SMAW with A5.5/A5.5M E90XX-B9 and SAW with A5.23/A5.23M EB9 plus neutral flux.
The tube side and the austenitic alternative
SA-213/SA-213M is the tube specification and carries the same grade families in T form. SA-213 Table 3 sets heat treatment and grain size. T91 Types 1 and 2, UNS K90901, is normalize and temper, austenitizing at 1900-1975 F [1040-1080 C] with subcritical tempering at 1350-1470 F [730-800 C]. T92, UNS K92460, has the same temperatures.
SA-213 Table 4 sets tensile and hardness for the same families. T91 Types 1 and 2: minimum tensile 85 ksi [585 MPa], minimum yield 60 ksi [415 MPa], minimum elongation 20 %, hardness 190 to 250 HBW or 196 to 265 HV, or 90 HRB to 25 HRC. T92: 90 ksi [620 MPa] and 64 ksi [440 MPa], 250 HBW or 265 HV, 25 HRC. T122: 90 ksi [620 MPa] and 58 ksi [400 MPa], 250 HBW or 265 HV, 25 HRC.
One verification point is worth flagging: the UNS number for T122 is printed as K91271 in SA-213 Table 1 and Table 4, but as K91261 in Table 3. The chemistry table and the mechanical properties table agree on K91271, so that is the number to use on a purchase order.
Where the operating temperature and corrosion environment allow it, austenitic stainless is the alternative. SA-312 Table 2 sets the annealing requirements: TP321H, TP347H and TP348H are cold finished at 2000 F [1100 C] or hot finished at 1925 F [1050 C]; TP304H and TP316H are cold finished or hot finished at 1900 F [1040 C]. It is the point, because grain boundaries are where creep damage initiates and a coarse grain buys the same creep resistance with less carbide precipitation at the boundary.
The selection logic, in order
Step 1, establish the metal temperature.
Step 2, choose the family.
Step 3, specify the treatment, not just the grade. The grade designation does not carry the heat treatment with it. Write the SA-335 grade, the UNS number and the type into the purchase order, and state the normalizing and subcritical temperatures and cooling media required by Table 2, the Table 3 tensile and hardness limits, and for P91 in the quenched and tempered condition the half-thickness verification required by note 9.5.
Step 4, check the marking and the certificate agree. The heat number, the heat-treatment lot identification where the grade requires it, the WR mark for a weld repaired length and the RT mark for radiographically examined double welded pipe all have to reconcile with the certificate. For P91, ask for the repair-weld documentation and the half-thickness results before accepting the heat.
Step 5, match the fittings and flanges. SA-182 carries the F 91, F 92 and F 122 forging grades, and clause 19.1.6 requires Grade F 91 to be additionally marked with the appropriate Type, which tells you which forging route the mill used.
For a supercritical pipe enquiry, the information that lets us quote correctly is the maximum metal temperature, the pipe specification and grade, the nominal size and schedule, the minimum wall if that is what you want, the quantity, the heat treatment and supplementary requirements, and the dimensional and pressure-temperature standard the pipe has to match. Send that and we will confirm the grade, the certification scope and the marking before pricing. Our Incoloy 825 seamless pipe and the wider seamless pipe and tube catalogue are supplied with full material certification to the applicable specification, and the ANSI B36.19 dimension and tolerance reference covers the dimensional side of the same purchase. Request a quotation through the contact page or on WhatsApp at +86 186 2137 7760.
