WB36 (15NiCuMoNb5-6-4) delivers 57% higher yield strength (440 MPa vs 280 MPa) than P22 (2.25Cr-1Mo), making it the preferred choice for high-pressure boiler drums and feedwater systems operating below 400 °C. P22 is the code-correct choice for main steam piping and hot reheat systems operating above 450 °C, where its proven creep resistance up to 580 °C is essential. WB36 also has significantly better weldability — lower preheat (100–150 °C vs 200–250 °C), lower PWHT temperature (580–620 °C vs 700–750 °C), and PWHT may be waived on thin-wall sections.
Why Do Engineers Compare WB36 and P22?
In high-energy piping and pressure vessel design, material selection is a critical and often irreversible decision. Choose a grade that is too conservative for the service conditions and you pay a cost premium with no engineering benefit. Choose one that is marginal for the operating window and you accept elevated risk of creep failure, stress corrosion, or hydrogen embrittlement — consequences far more expensive than the original steel selection.
WB36 (ASTM A335 P36 / EN 15NiCuMoNb5-6-4) and P22 (ASTM A335 P22 / EN 10CrMo9-10) are among the most widely specified alloy steels in power generation and the chemical process industries. They are often evaluated as alternatives because both serve elevated-temperature, high-pressure applications. However, they are fundamentally different alloys optimised for different failure modes. This guide provides procurement engineers, piping designers, and quality managers with a structured, data-driven comparison.
What Are WB36 and P22 Alloy Steels?
WB36 — Definition and Background
WB36 (15NiCuMoNb5-6-4, EN material number 1.6368) is a bainitic nickel-copper-molybdenum alloy steel microalloyed with niobium, designed for high-pressure moderate-temperature service up to 400 °C. The designation "WB36" originates from the German term Wärmebeständiger Behälterstahl (heat-resistant pressure vessel steel). It corresponds to ASTM A335 Grade P36 for seamless pipe and ASTM A182 Grade F36 for forgings in American codes.
WB36 achieves its high room-temperature strength through copper precipitation hardening — a mechanism in which copper-rich precipitates form during normalising and tempering at 580–640 °C, creating a fine microstructure that resists dislocation movement. This is fundamentally different from P22's solid-solution strengthening mechanism and gives WB36 a significantly higher yield strength at ambient and moderate temperatures without requiring a high chromium content.
P22 — Definition and Background
P22 (2.25Cr-1Mo, EN designation 10CrMo9-10, material number 1.7380) is a chromium-molybdenum alloy steel with well-characterised creep resistance for continuous service up to 580 °C. It is specified under ASTM A335 Grade P22 for seamless pipes and ASTM A182 Grade F22 for forgings; in European standards it is covered by EN 10216-2 and EN 10222-2. P22's 50+ years of power plant service history gives designers and pressure vessel inspectors high confidence in long-term creep life predictions.
P22 achieves its elevated-temperature performance through chromium-molybdenum solid solution hardening and the formation of stable carbide phases during heat treatment, giving it excellent creep rupture strength at temperatures where WB36 is no longer serviceable.
How Do WB36 and P22 Differ in Chemical Composition?
The composition differences between WB36 and P22 are fundamental — they reflect entirely different design philosophies. The highlighted rows (★) show the critical alloying elements that drive the performance difference between the two grades.
Key difference: WB36 contains Ni (1.0–1.3%) and Cu (0.5–0.8%) with Nb grain refinement — none of which P22 contains in significant amounts. P22 contains Cr (2.0–2.5%) and high Mo (0.87–1.13%) — present only in traces in WB36. These differences drive every performance distinction described in this article.
WB36 vs P22: Mechanical Properties Comparison
At room temperature, WB36 delivers significantly higher yield and tensile strength than P22 due to copper precipitation hardening. This is the primary reason WB36 is specified for high-pressure vessel applications where design pressure — not temperature — is the governing design criterion.
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| Property | WB36 (15NiCuMoNb5-6-4) | P22 (2.25Cr-1Mo) |
|---|---|---|
| Min. Yield Strength Rp0.2 | 440 MPa (+57%) | 280 MPa |
| Tensile Strength Rm | 610 – 760 MPa | 480 – 630 MPa |
| Min. Elongation A | 19 % | 20 % |
| Charpy Impact at +20 °C | ≥ 100 J (average) | ≥ 27 J (average) |
| Charpy Impact at −20 °C | ≥ 60 J (average) | Not specified |
| Hardness (typical, HB) | 190 – 235 | 140 – 187 |
| Max. Continuous Service Temp. | 400 °C | 580 °C |
| Allowable Stress at 350 °C (ASME) | ~145 MPa | ~115 MPa |
| Allowable Stress at 500 °C (ASME) | Not applicable | ~82 MPa (creep-governed) |
Higher minimum yield strength vs P22 at room temperature — 440 MPa vs 280 MPa (EN 10222-2 / ASTM A182)
Higher maximum continuous service temperature — 580 °C vs 400 °C. Essential for main steam and hot reheat service.
What Is the Maximum Service Temperature for WB36 vs P22?
Selecting WB36 for service above 400 °C is not merely a non-conservative engineering decision — under EN 13480, ASME B31.1, and the European PED, it is a code violation. No allowable stress data for WB36 exists above 400 °C in these codes, and the copper precipitation strengthening mechanism degrades irreversibly above this temperature.
European Pressure Equipment Directive (2014/68/EU) — Material Assessment ReferenceThe service temperature limit for each grade is clearly defined by applicable codes and is not a matter of engineering judgment within certified pressure equipment design:
- WB36 maximum service temperature: 375–400 °C continuous. Optimised for high-pressure boiler drums, HP feedwater heaters, and deaerators where temperatures are moderate but pressures are very high (up to 25 MPa and above).
- P22 maximum service temperature: Approximately 580 °C continuous. Creep rupture data in EN 10216-2 and ASME Section II Part D supports use in main steam piping, hot reheat headers, and superheater manifolds.
Above 400 °C, WB36's copper precipitates begin to coarsen and partially dissolve back into the matrix, progressively losing the strengthening effect. For P22, 50+ years of accumulated creep rupture data across hundreds of power plants gives designers high confidence in life predictions at 500–580 °C — a data set that simply does not exist for WB36 at those temperatures.
Critical warning: Do not specify WB36 above 400 °C. Substituting WB36 for P22 in main steam or hot reheat service is non-code-compliant under ASME B31.1, EN 13480, and the PED. It also creates an unquantifiable creep failure risk because no long-term data exists to support such use.
Which Is More Weldable — WB36 or P22?
Weldability is one of the most practically significant differences between WB36 and P22 for fabricators, and WB36 has a clear, measurable advantage in this area.
WB36 Weldability — Key Facts
Despite its higher room-temperature strength, WB36 has substantially better resistance to cold cracking (hydrogen-induced cracking) than P22. Its carbon content (0.10–0.17%) and typical carbon equivalent (CE) of 0.45–0.55 result in a significantly lower risk of HAZ hardening. The nickel addition improves toughness in the heat-affected zone, and the copper addition creates no significant hardening risk during rapid cooling after welding.
P22 Weldability — Key Facts
P22's 2.25% chromium raises its carbon equivalent to typically 0.70–0.80 — well above the threshold where cold cracking becomes a significant risk. PWHT is mandatory for virtually all P22 weld joints in pressure service regardless of wall thickness, and the PWHT temperature window (700–750 °C) must be precisely controlled to achieve full stress relief and proper carbide distribution.
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| Welding Parameter | WB36 | P22 |
|---|---|---|
| Carbon Equivalent CE (typical) | 0.45 – 0.55 | 0.70 – 0.80 |
| Preheat Temp. (≥ 25 mm wall) | 100 – 150 °C | 200 – 250 °C |
| Max. Interpass Temperature | 250 °C | 300 °C |
| PWHT Temperature | 580 – 620 °C | 700 – 750 °C |
| Mandatory PWHT (thin wall) | May be waived (<30 mm, per EN 13480) | Always required |
| HAZ Hardness Risk | Low | Medium–High without preheat |
| GTAW Filler Metal | ENiCrMo-3 or matching | ER90S-B3 |
| SMAW Filler Metal | ENiCrMo / E9018-B3 | E9015-B3 |
Step-by-Step: Welding WB36 Forgings Correctly
Pre-weld hydrogen control and preheat
Use low-hydrogen filler metal (diffusible H₂ < 5 ml/100 g certified). Preheat to 100–150 °C for wall thicknesses ≥ 15 mm. Verify with a calibrated contact thermometer across 75 mm minimum each side of the joint before striking the arc.
Root pass and interpass temperature control
Use GTAW for the root pass with matching or nickel-bearing filler per approved WPS. Maintain interpass temperature between 100 °C (minimum to prevent cold cracking) and 250 °C (maximum to prevent HAZ coarsening) throughout the weld operation.
Post-weld hydrogen bake (if PWHT is waived)
Where PWHT is waived on sections under 30 mm, perform a hydrogen bake at 200–250 °C for 2–4 hours immediately after welding — before the joint cools to ambient. Minimum 1 hour per 25 mm of wall thickness. Do not interrupt this bake cycle.
Post-weld heat treatment (PWHT where required)
Heat uniformly to 580–620 °C at a rate not exceeding 80 °C/hr above 400 °C. Hold for minimum 1 hour per 25 mm of maximum wall thickness (2-hour minimum total). Cool at ≤ 80 °C/hr to 400 °C, then unrestricted air cooling. Document with continuous furnace chart.
Post-PWHT non-destructive examination
Perform visual and MT/PT inspection of weld caps and the full HAZ. For forgings over 50 mm wall thickness in pressure service, supplement with TOFD or phased-array UT for volumetric examination. Document and retain all NDE records with the MTR package.
Heat Treatment Requirements: WB36 vs P22 Forgings
For forged components, both grades require specific heat treatment to achieve their specified mechanical properties. The regimes differ significantly, and the WB36 tempering temperature in particular is a critical manufacturing parameter.
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| Heat Treatment Stage | WB36 Forgings | P22 Forgings |
|---|---|---|
| Normalising Temperature | 900 – 940 °C | 930 – 980 °C |
| Tempering Temperature | 580 – 640 °C ⚠ critical window | 680 – 730 °C |
| Tempering Purpose | Forms Cu precipitates + stress relief | Carbide distribution + stress relief |
| Min. Tempering Hold | ≥ 2 hours (+ 1 hr per 25 mm) | ≥ 2 hours |
| Q+T Option | Yes — quench from 900 °C, temper 580–640 °C | Not standard |
| Target Grain Size | ASTM No. 6 or finer | ASTM No. 5 or finer |
| Heavy Section Risk (>100 mm) | Moderate — Cu precipitate uniformity | Lower — more uniform through-thickness |
WB36 tempering window is critical: If the tempering temperature is too high (above 640 °C), copper precipitates dissolve back into solution and the strengthening effect is lost — the forging will fail to meet the 440 MPa yield requirement. If too low (below 580 °C), precipitates do not form fully. A furnace chart and post-heat treatment tensile test are essential quality records for every WB36 forging batch.
ASTM, EN, and International Code Recognition: WB36 vs P22
Both grades have full recognition across all major pressure equipment codes, but their listing in specific code sections and nuclear standards differs. Always verify the grade is listed in the applicable code edition for your project jurisdiction.
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| Standard / Code | WB36 | P22 |
|---|---|---|
| ASTM — seamless pipe | A335 Grade P36 | A335 Grade P22 |
| ASTM — forgings | A182 Grade F36 | A182 Grade F22 |
| ASME — boiler & PV code | SA335 P36 / SA182 F36 | SA335 P22 / SA182 F22 |
| EN — seamless pipe | EN 10216-2 (15NiCuMoNb5-6-4) | EN 10216-2 (10CrMo9-10) |
| EN — forgings | EN 10222-2 | EN 10222-2 |
| AD-Merkblatt W2 (Germany) | Listed — full recognition | Listed — full recognition |
| KTA 3201.1 (Germany nuclear) | Listed | Listed |
| RCC-M (France nuclear) | Limited | Full recognition |
| NORSOK (Norway offshore) | Limited — verify edition | Recognised |
| JIS (Japan) | Limited | STPA24 (equivalent) |
| MTR — standard | EN 10204 Type 3.1 | EN 10204 Type 3.1 |
| MTR — third-party (on request) | EN 10204 Type 3.2 | EN 10204 Type 3.2 |
For European projects under the Pressure Equipment Directive (PED 2014/68/EU), Jiangsu Liangyi supplies both WB36 and P22 forgings with EN 10204 Type 3.1 material test reports as standard. EN 10204 Type 3.2 inspection (countersigned by an independent third party) is available on request through the customer's nominated inspection body.
WB36 vs P22: Total Cost of Ownership
Material cost per kilogram is rarely the correct basis for alloy steel comparison in pressure equipment. The relevant metric is total installed cost, which integrates material weight, fabrication, heat treatment, NDE, and long-term inspection costs.
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| Cost Factor | WB36 | P22 |
|---|---|---|
| Raw material $/kg (relative) | Medium–High (Ni, Cu premium) | Medium |
| Wall thickness (same design pressure) | Thinner — typically 15–25% less steel weight | Thicker — more material required |
| Preheat energy cost | Lower (100–150 °C only) | Higher (200–250 °C) |
| PWHT cost (thin wall ≤30 mm) | May be eliminated entirely | Full PWHT cycle always required |
| PWHT cost (thick wall) | Required (580–620 °C) | Required (700–750 °C — higher energy) |
| Fabrication schedule | Faster — fewer mandatory cycles | Slower — mandatory PWHT all sections |
| Long-term in-service inspection | Standard — lower temperature regime | Creep monitoring required above 500 °C |
| Lead time from China (forgings) | 4 – 8 weeks | 4 – 6 weeks |
In practice, WB36's higher material cost per kilogram is frequently offset by two factors: first, the thinner wall sections its higher strength enables (reducing total steel consumption by 15–25% in high-pressure vessel designs); second, the fabrication savings from reduced preheat requirements and conditional PWHT waiver on thin sections. For large boiler drum forgings and HP feedwater heater shells, WB36's total installed cost can be equal to or lower than P22, despite the material premium.
WB36 vs P22 Decision Framework: How to Choose
The following decision matrix covers the most common engineering and procurement scenarios. Most applications will clearly favour one grade once these parameters are applied systematically.
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| Design or Procurement Criterion | Condition | Recommended Grade |
|---|---|---|
| Service temperature | Below 375 °C continuous service | WB36 |
| Service temperature | 375 – 450 °C — code calculation required | Case by case |
| Service temperature | Above 450 °C continuous service | P22 |
| Design pressure class | PN 320 or above — thin wall required | WB36 |
| Equipment type | Boiler drums, HP feedwater heaters, deaerators | WB36 |
| Equipment type | Main steam headers, hot reheat piping, superheaters | P22 |
| Cold service / startup | Sub-zero ambient or low-temperature startup cycles | WB36 |
| Fabrication constraint | Limited PWHT capacity / tight project schedule | WB36 |
| Nuclear compliance | French RCC-M compliance required | P22 |
| Creep life | 30+ year life above 450 °C, creep life prediction required | P22 |
| Hydrogen service | Hydrocracker / reformer — Nelson curve compliance needed | P22 |
| Inspector familiarity | Third-party inspection body with no WB36 experience | P22 |
Real-World Applications: Where Is Each Grade Used?
Typical WB36 Forging Applications
- High-pressure boiler steam drums in subcritical and supercritical units — typically up to 25 MPa, below 375 °C
- HP feedwater heater shells, heads, and tubesheet forgings — pressure-governed, moderate temperature
- Deaerator storage vessels and feed pump suction headers
- Forged valve bodies for high-pressure feedwater isolation and control valves (PN 250 – PN 400)
- Nuclear auxiliary feedwater system components — KTA 3201.1 code listed
- Chemical plant pressure vessels where high pressure (above 200 bar) and moderate temperature (below 375 °C) coincide
Typical P22 Forging Applications
- Main steam piping in subcritical power plants — typically 540–570 °C, 16–25 MPa
- Hot reheat headers and crossover piping — typically 540–565 °C
- Superheater and reheater outlet manifolds
- High-temperature bypass valve bodies and steam conditioning valve components
- Hydroprocessing reactor nozzles and head forgings — ASME VIII Div. 2, Nelson curve compliance
- Chemical reactor internals in environments with hydrogen at elevated temperature (above 230 °C at high H₂ partial pressure)
How to Source WB36 and P22 Forgings: A Procurement Checklist
Both grades are routinely produced by certified Chinese forging manufacturers and exported to power generation, petrochemical, and industrial projects globally. For WB36 in particular, the tempering temperature control is a critical manufacturing parameter — buyers should always request complete heat treatment records, not just the final mechanical test certificate. A WB36 forging tempered outside the 580–640 °C window may pass a hardness check while having below-specification yield strength, which only appears on a full tensile test.
Essential documentation to request for both grades:
- EN 10204 Type 3.1 Mill Test Certificate — covering heat analysis (chemical composition), product analysis, and mechanical test results (tensile, impact, and hardness) per the applicable product standard
- Complete heat treatment records — continuous furnace chart, soak time log, and cooling rate record — not just the final certificate
- UT inspection report per EN 10228-3 or ASTM A388, with agreed acceptance levels stated in your purchase order
- Forging ratio certification — minimum 3:1 for continuously cast stock, 5:1 for ingot stock
- Charpy impact test results for WB36 — at minimum at +20 °C; at −20 °C if required by your design code or project specification
- Third-party inspection report (EN 10204 Type 3.2) if required by your EPC or end-user specification
Jiangsu Liangyi Co., Limited manufactures WB36 open-die forgings and seamless rolled rings (ASTM A335 P36 / EN 15NiCuMoNb5-6-4) and P22 / F22 forgings with ISO 9001:2015 certification and EN 10204 3.1 MTRs as standard. EN 10204 3.2 inspection is available on request through the customer's nominated third-party inspection body. Typical lead times are 4–8 weeks for forgings depending on section size and heat treatment complexity.
Conclusion: WB36 or P22 — Which Grade Should You Choose?
WB36 and P22 are not competing grades in the strict sense — they occupy adjacent but distinct engineering spaces. WB36 is the stronger, tougher, and more weld-friendly alloy for high-pressure moderate-temperature service below 400 °C. P22 is the proven, code-recognised creep-resistant grade for the high-temperature side of the power plant above 450 °C.
When service temperature is below 375 °C, WB36 will almost always deliver better engineering value — higher allowable stress, thinner walls, faster fabrication, and often lower total installed cost despite its higher raw material price. When service temperature is above 450 °C, P22 is not just preferable — in most codes it is the only permissible option, with no credible alternative at that temperature level.
For the transition zone between 375 and 450 °C, a detailed code calculation comparing allowable stresses, wall thickness outcomes, weld qualification costs, and total installed cost is the only defensible method of selection. In many borderline cases, WB36's superior room-temperature strength and lower fabrication burden still win.
If you are specifying forged components for a high-pressure boiler or pressure vessel project and need material selection support, drawing review, or a quotation for WB36 or P22 forgings with full EN/ASME documentation, contact the Jiangsu Liangyi Co., Limited technical team.
Need WB36 or P22 Forged Parts? Request a Quote
Jiangsu Liangyi Co., Limited manufactures open-die forgings and seamless rolled rings in both grades. ISO 9001:2015 certified. EN 10204 3.1 standard. 3.2 on request.
Frequently Asked Questions: WB36 vs P22
What is the difference between WB36 and P22 steel?
WB36 (15NiCuMoNb5-6-4, EN 1.6368) and P22 (2.25Cr-1Mo, EN 1.7380) are both alloy steels for elevated-temperature, high-pressure service — but they use entirely different strengthening mechanisms and have different service temperature limits.
WB36 achieves its high strength (minimum yield 440 MPa) through copper precipitation hardening and niobium microalloying, with a maximum service temperature of approximately 400 °C. P22 relies on chromium-molybdenum solid solution hardening for long-term creep resistance up to 580 °C, but has a lower room-temperature yield strength of 280 MPa. WB36 has significantly better weldability; P22 is the correct code choice for main steam and hot reheat service above 450 °C.
Can WB36 replace P22 in main steam piping?
No. WB36 cannot replace P22 in main steam piping systems operating above 400 °C. Main steam piping in subcritical power plants typically operates at 540–570 °C — well outside WB36's allowable temperature range.
Using WB36 in main steam service would violate EN 13480, ASME B31.1, and the European Pressure Equipment Directive (PED 2014/68/EU), as no allowable stress data for WB36 exists above 400 °C in these codes. It would also create an unquantifiable creep failure risk because copper precipitation strengthening degrades irreversibly above this temperature.
What is the maximum service temperature of WB36?
The maximum recommended continuous service temperature for WB36 (15NiCuMoNb5-6-4, ASTM A335 P36) is 375–400 °C. This limit is defined by the degradation of copper precipitation strengthening above this temperature as precipitates coarsen and dissolve.
WB36 is specifically optimised for high-pressure, moderate-temperature service such as boiler drums, HP feedwater heaters, deaerators, and feedwater valve bodies — applications where temperatures stay below 400 °C but pressures can exceed 20 MPa.
Does WB36 require PWHT after welding?
PWHT requirements for WB36 depend on wall thickness and the applicable code. Under EN 13480 and ASME B31.1:
- Walls under 30 mm: PWHT may be waived subject to preheat controls (typically 100–150 °C)
- Walls 30–60 mm: PWHT typically required at 580–620 °C
- Walls over 60 mm: PWHT mandatory
This is a significant fabrication advantage over P22, which requires a full PWHT cycle (700–750 °C) for all wall thicknesses regardless of section size. WB36 also requires lower preheat (100–150 °C vs 200–250 °C for P22 at ≥ 25 mm wall).
What is the yield strength of WB36 vs P22?
WB36 has a minimum yield strength (Rp0.2) of 440 MPa — 57% higher than P22's minimum of 280 MPa at room temperature (per EN 10222-2 and ASTM A182 respectively).
WB36 also has a higher tensile strength range (610–760 MPa vs P22's 480–630 MPa) and higher allowable stress at 350 °C (~145 MPa vs ~115 MPa per ASME). This strength advantage is the primary reason WB36 is preferred for high-pressure vessels and boiler drums where wall thickness — not creep — is the governing design limit.
Is WB36 the same as ASTM A335 P36?
Yes. WB36 is the trade designation for the alloy steel grade standardised as ASTM A335 Grade P36 for seamless pipe and ASTM A182 Grade F36 for forgings in American codes.
In European standards it is designated 15NiCuMoNb5-6-4 under EN 10216-2 (pipe) and EN 10222-2 (forgings), with material number 1.6368. The designation WB36 comes from the German term Wärmebeständiger Behälterstahl (heat-resistant pressure vessel steel).
Which is more weldable — WB36 or P22?
WB36 is significantly more weldable than P22. Key metrics:
- Carbon equivalent (CE): WB36 = 0.45–0.55; P22 = 0.70–0.80
- Preheat (≥ 25 mm wall): WB36 = 100–150 °C; P22 = 200–250 °C
- PWHT temperature: WB36 = 580–620 °C; P22 = 700–750 °C
- Mandatory PWHT on thin wall: WB36 = may be waived; P22 = always required
WB36's lower carbon equivalent means the heat-affected zone (HAZ) is less susceptible to hydrogen-induced cold cracking. For fabricators working with tight schedules, WB36's relaxed PWHT requirements can meaningfully reduce production time and cost.
What certifications are available for WB36 forgings from China?
WB36 forgings from Jiangsu Liangyi are supplied with:
- EN 10204 Type 3.1 Material Test Report (standard on all orders) — covering heat analysis, product analysis, tensile properties, Charpy impact, and hardness per EN 10222-2
- EN 10204 Type 3.2 (countersigned by an independent third party) — available on request through the customer's nominated inspection body
- ISO 9001:2015 quality management system certification
- Full heat treatment records and UT inspection report
Jiangsu Liangyi Co., Limited holds ISO 9001:2015 certification. EN 10204 3.2 inspection is available on request through the customer's nominated inspection body. Typical lead time is 4–8 weeks depending on section size and heat treatment complexity.
Continue Reading: Related Technical Guides
- WB36 Forging vs. WB36 Seamless Pipe: Key Differences for Engineers
- How to Weld WB36 Steel Forgings: Preheat, PWHT, and Filler Selection Guide
- WB36 vs P91: When Does the Lower-Alloy Option Win?
- EN 10204 3.1 vs 3.2 Material Certificates for WB36 Forgings Explained
- Heat Treatment of WB36 Forgings: Normalising, Q+T, and Tempering Parameters
- WB36 / ASTM A335 P36 Forging Parts — Product Specifications and Quote Request