The Importance of Restricting Ni+Mn in P91 and P92 Welding

The Ni+Mn restriction in P91 and P92 welding is one of the most consequential — and most frequently misunderstood — requirements in the fabrication of creep-strength enhanced ferritic (CSEF) steels. P91 (9Cr-1Mo-V) and P92 (9Cr-1.8W-0.5Mo-V-Nb-B) are the backbone materials of modern ultra-supercritical power plants, industrial boilers, and high-temperature pressure piping. Their exceptional creep strength at 550 to 620 °C, combined with good oxidation resistance and lower thermal expansion than austenitic stainless steels, makes them indispensable in demanding thermal-cycle environments. But that exceptional high-temperature performance is conditional: it depends entirely on a stable, well-tempered martensitic microstructure that can only be produced and maintained through rigorous control of chemical composition and heat treatment.

At the centre of that control is a deceptively simple rule: the combined content of Nickel (Ni) and Manganese (Mn) in the weld filler metal must not exceed a defined limit — typically 1.0% or 1.2% depending on the applicable code and project specification. Violating this limit does not produce an immediately visible defect. The weld will look fine, pass radiographic and ultrasonic examination, and meet hardness requirements on the day of PWHT. The damage is metallurgical and cumulative: a narrowed PWHT temperature window, a risk of partial microstructure transformation during heat treatment, and a weld deposit that is more susceptible to Type IV cracking after years of high-temperature service. Understanding why this restriction exists, and how it interacts with the post-weld heat treatment (PWHT) process, is fundamental knowledge for any welding engineer, inspector, or quality professional working with P91 or P92.

This article explains the physical metallurgy behind the Ni+Mn limit in full: how each element affects the critical Ac₁ transformation temperature, what the construction codes (ASME B31.1, ASME B31.3, and ASME Section VIII) specify for PWHT temperature as a function of Ni+Mn content, what happens if the limit is exceeded, and how to verify compliance in practice. Internal links to related WeldFabWorld technical guides are included throughout so you can explore the broader P91 and P92 fabrication landscape in detail.

Post-Weld Heat Treatment in P91 and P92: Why Every Degree Matters

Post-weld heat treatment is not optional for P91 or P92 — it is mandatory for every welded joint, regardless of wall thickness or pipe diameter. As-welded P91 produces a predominantly untempered martensitic microstructure with hardness values typically in the range of 380 to 450 HV. This microstructure is brittle, susceptible to hydrogen-induced cold cracking, and completely incapable of providing the creep resistance required in service. PWHT converts this untempered martensite into a tempered martensitic structure with fine M₂₃C₆ carbides and MX carbonitrides (where M = V, Nb) distributed within the martensitic lath boundaries. These precipitates provide the dislocation-pinning mechanism that gives P91 its creep strength at elevated temperature.

The temperature at which PWHT is performed is therefore not merely a code compliance number — it directly determines the precipitate coarseness, the degree of tempering, the resultant hardness, and the long-term creep performance of the weld. Too low, and the martensite is insufficiently tempered: the weld is hard, brittle, and prone to service cracking. Too high — specifically, if the PWHT temperature exceeds the Ac₁ lower critical transformation temperature of the weld metal — the tempered martensite begins to transform back into austenite. On cooling from this excess PWHT temperature, that fresh austenite converts to new, untempered martensite. The result is a weld that has been subjected to a full PWHT cycle but still contains brittle, untempered martensitic regions, with a toughness that is far worse than an adequately tempered deposit.

This is the core reason why the Ni+Mn restriction exists: because Ni and Mn both lower the Ac₁ temperature of the weld deposit, and therefore determine how much headroom exists between the PWHT temperature and the transformation onset. For a detailed treatment of the full P91 fabrication requirements including preheat, interpass temperature, hydrogen bake-out, and PWHT procedures, refer to our comprehensive P91 welding requirements guide.

How Nickel and Manganese Individually Affect P91 and P92 Weld Metal

The Role of Nickel (Ni)

Nickel is a strong austenite-stabilising element. In the P91 alloy system, it improves the low-temperature toughness of the weld deposit, which is why some filler metal formulations intentionally include small Ni additions. However, Ni has two significant adverse effects at elevated concentrations:

• Ac₁ depression: Nickel lowers the temperature at which the tempered martensitic microstructure begins to transform back into austenite on heating. The quantitative effect is approximately 15 to 25 °C per 0.1% Ni in the context of P91 weld metal chemistry. At Ni levels approaching 0.6%, the total Ac₁ depression attributable to Ni alone can reach 30 to 45 °C.
• Reduced creep resistance: At concentrations above approximately 0.4 to 0.6% Ni, the element destabilises the MX (vanadium and niobium carbonitride) precipitate structure that provides long-term creep strengthening in P91 and P92. The net effect is a measurable reduction in creep rupture life at 550 °C and above. This is why most P91 filler metal specifications independently restrict Ni to 0.40% maximum, well below the level at which Ac₁ depression becomes severe.

The combination of these two effects means that even a modest Ni addition — entirely within the individual element limit — consumes a portion of the available Ac₁ margin that Mn cannot also consume without risk. This is precisely why the restriction is expressed as a combined Ni+Mn limit rather than two separate independent limits.

The Role of Manganese (Mn)

Manganese serves as a deoxidiser and toughness improver in the weld deposit. In carbon steel welding, Mn contents of 1.0 to 2.0% are common and entirely unproblematic. In P91 and P92, however, Mn behaves like Ni in its effect on Ac₁: it depresses the transformation temperature by approximately 10 to 20 °C per 0.1% Mn in typical P91 weld metal chemistry. The effect is roughly half as potent as Ni per unit weight, which is why the combined index weights them equally (Ni+Mn) as a reasonable engineering approximation.

AWS A5.5 permits Mn up to 1.20% in B91 weld deposits, but achieving the full AWS limit alongside any Ni content makes compliance with the Ni+Mn ≤ 1.0% project requirement essentially impossible. In practice, well-controlled P91 filler metal producers target Mn in the range of 0.50 to 0.85% and Ni below 0.20%, giving a combined Ni+Mn that comfortably meets the 1.0% limit with margin to spare. For an understanding of how filler metal designations relate to chemical composition requirements, see our welding consumable nomenclature guide.

The Ac₁ Temperature: What It Is and Why Exceeding It Is Catastrophic

The Ac₁ temperature — the lower critical transformation temperature on heating — marks the onset of reverse transformation from ferrite/martensite to austenite. For P91 base metal under controlled equilibrium heating conditions, Ac₁ is typically cited in the range of 820 to 830 °C. However, for P91 weld metal, the Ac₁ is typically lower — measured values range from 780 to 830 °C depending on Ni+Mn content and heating rate. Research using Gleeble thermo-mechanical simulation has shown that at a practical slow heating rate of 28 °C/hour (typical of field PWHT), the Ac₁ of P91 weld metal can be as low as 792 °C when Ni+Mn is at the borderline level of approximately 1.2%.

With a maximum PWHT temperature of 790 °C (as permitted by ASME B31.1 when Ni+Mn ≤ 1.0%), the margin above Ac₁ in that scenario is only 2 °C — effectively zero. Thermocouple calibration tolerances, furnace temperature non-uniformity, and the location of the hottest junction relative to the weld can easily place local areas of the weld at or above Ac₁. This is not a theoretical risk: power plant weld failures attributed to inadvertent intercritical heating during PWHT have been documented in the literature and in regulatory investigation reports.

What Happens Physically When Ac₁ Is Exceeded

When a P91 weld metal at a carefully produced tempered martensitic condition is heated above its Ac₁, the following sequence occurs:

1. Tempered carbides begin to dissolve back into the austenite forming at the lath boundaries.
2. The austenite is enriched in C and other austenite-stabilising elements from the dissolving carbides.
3. On cooling from the PWHT temperature, this newly formed austenite transforms to fresh, untempered martensite (M_s for P91 weld metal is approximately 380 to 410 °C; M_f is approximately 80 to 100 °C).
4. The fresh martensite is not subjected to any further tempering because the PWHT cycle is complete.
5. The resulting microstructure contains a mixture of well-tempered martensite (from the correctly heat-treated regions) and brittle, untempered martensite (from regions that were intercritically heated).

The mechanical consequences of this mixed microstructure are severe. Charpy V-Notch impact values in the weld fusion zone, which should be in the range of 80 to 130 J after correct PWHT, can fall to 15 to 30 J after inadvertent intercritical heating. Hardness may be locally elevated above the code maximum of 265 HV, but this is not guaranteed — the mixed microstructure may produce a hardness reading that appears acceptable even though the microstructure is severely damaged. Standard NDE methods (radiography, ultrasonics, liquid penetrant) cannot detect this damage. Only metallographic examination or hardness mapping of the specific cross-section can identify it.

Code Requirements: ASME B31.1, B31.3, and Section VIII

The ASME construction codes were substantially updated in the 2010s and 2020s to formalise the PWHT temperature restrictions for Grade 91, following documented service failures and a broader industry recognition that previous code provisions were insufficient for the metallurgical complexity of CSEF steels. The current requirements, as expressed in ASME B31.1 Table 132.1.1-1 and ASME B31.3 Table 331.1.1, establish the following tiered maximum PWHT temperatures for welds made with matching Grade 91 filler metal:

Table 1 — PWHT temperature tiers for Grade 91 matching filler metal per ASME B31.1 / B31.3. Practical best-practice PWHT temperature for all tiers is 740 to 760 °C, providing margin above the minimum and below the maximum.

The code minimum of 730 °C applies universally: below this temperature, the tempering effect on the untempered martensite is insufficient and the weld will have unacceptably high hardness and poor toughness. The recommended practical PWHT temperature for most P91 applications, balancing adequate tempering against the risk of Ac₁ exceedance, is 740 to 760 °C. This range provides at least 20 to 30 °C above the minimum and at least 30 °C of margin below the maximum for a Ni+Mn ≤ 1.0% deposit, which in turn places the PWHT temperature approximately 50 to 70 °C below the Ac₁ of a well-controlled weld metal deposit.

P-Number and F-Number Implications

Grade 91 base metal is classified as P-No. 5B, Group 2 in ASME Section IX. Filler metals used for P91 welding are assigned F-Number 4 (for SMAW stick electrodes) or F-Number 6 (for solid wire GTAW/GMAW). The A-Number for the B91 deposit (9Cr-1Mo-V) is A-9. These groupings affect which procedure qualification records (PQRs) are valid for production welds. When a WPS is qualified specifically for P91 using an E9015-B91 or E9018-B91 electrode, any subsequent change to a filler metal with higher Ni+Mn content constitutes a change to an essential variable (chemical composition suffix in A5.5), requiring requalification. See our P-Number, F-Number, and A-Number guide for the full classification reference. The ASME Section IX quiz is also a useful self-assessment tool for welding engineers.

P91 vs P92: How the Ni+Mn Restriction Differs Between the Two Grades

P92 (ASTM A335 Grade P92, UNS K92460; Alloy designation 9Cr-2W) replaces a portion of the molybdenum in P91 with tungsten and adds a small boron addition to further improve creep strength — by approximately 25 to 30% compared to P91 at the same temperature. Because P92 incorporates different alloying additions, its Ac₁ temperature profile under the effect of Ni+Mn is somewhat different from P91, and its welding metallurgy has additional complications.

Table 2 — Comparison of Ac₁ temperature range and Ni+Mn restriction considerations for P91 and P92 weld metals.

For P92, the situation is further complicated by the risk of delta ferrite formation in the weld deposit. P92 weld metal contains elevated levels of ferrite-stabilising elements (W, Nb, and the reduced Mo content compared to P91), and the balance between ferrite and austenite stabilisers must be carefully managed. If Ni+Mn is kept very low to maintain Ac₁, the austenite-stabilising contribution of these elements is reduced, potentially allowing delta ferrite to form during solidification. Delta ferrite in P91 and P92 weld deposits is highly detrimental to toughness. For a full explanation of the delta ferrite issue and its measurement, see our guide to delta ferrite in alloy steel welds.

Practical Compliance: How to Verify and Control Ni+Mn in Production Welding

Step 1: Filler Metal Procurement and Certificate Review

The Ni+Mn control process begins at the procurement stage, before a single weld is made. When purchasing E9015-B91, E9018-B91, ER90S-B9, or equivalent consumables, specify in the purchase order that the supplier must provide:

• Full all-weld-metal chemical analysis for each production heat or lot, including Ni and Mn as separate values.
• Confirmation that the calculated Ni+Mn does not exceed the project limit (typically 1.0%).
• For SMAW electrodes, confirmation that the analysis is of the deposited weld metal, not of the electrode core wire alone.
• Residual element data including P, S, Sb, Sn, and As for X-Factor calculation (see our article on E9015-B91 vs E9018-B91 for the X-Factor explanation).

Step 2: Incoming Inspection and Traceability

Upon receipt of each batch of P91 filler metal, verify the certificate against the purchase order requirements. Calculate Ni+Mn from the certificate values. Assign each batch a unique heat/lot identification and record it in the material traceability system. Do not mix batches — if a weld uses more than one batch, the highest Ni+Mn value of any used batch governs the PWHT temperature for that joint.

Step 3: WPS Documentation

The Welding Procedure Specification must state the Ni+Mn content of the approved filler metal and the resulting maximum PWHT temperature. When qualifying a new WPS, the PQR test coupon must be welded using a filler metal with Ni+Mn content representative of production intent. A PQR qualified with a low Ni+Mn filler metal does not automatically permit production welding with a higher Ni+Mn batch at a higher PWHT temperature — the PWHT temperature on the WPS is tied to the filler metal chemistry used in qualification.

Step 4: PWHT Temperature Control and Verification

PWHT must be performed with calibrated thermocouples placed on or immediately adjacent to the weld. The number and placement of thermocouples depends on the component geometry and the requirements of the applicable code. Temperature recording charts must be retained and reviewed to confirm that the maximum temperature recorded by any thermocouple on the weld did not exceed the Ni+Mn-driven limit. For components with high thermal gradients (thick-wall transitions, nozzle connections), additional thermocouple placement may be needed to confirm that local hot spots did not breach the limit.

Summary: Consequences of Ni+Mn Non-Compliance

Table 3 — Summary of the metallurgical and service consequences of various Ni+Mn non-compliance scenarios in P91 welding.

Recommended Books on P91, P92, and CSEF Steel Welding

These references are recommended reading for engineers and inspectors responsible for P91 and P92 welding programmes, PWHT specification, and quality control.

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Related Technical Guides on WeldFabWorld

Continue exploring P91 and P92 welding metallurgy, consumable selection, and ASME code compliance with these related articles.