E9015-B91 vs E9018-B91: Why E9015 Is the Preferred Electrode for P91 Welding
Selecting the correct SMAW electrode for welding P91 steel (Grade 91, 9Cr-1Mo-V) is one of the most critical decisions a welding engineer makes on a high-temperature power-plant or pressure-vessel project. Both the E9015-B91 and E9018-B91 electrodes satisfy the AWS A5.5 classification for 90 ksi minimum tensile strength with a matching B91 alloy deposit, yet their coating chemistries differ in ways that have measurable consequences for weld metal cleanliness, residual element control, and long-term creep performance. Getting this choice wrong can result in crater cracking on the first weld pass, unacceptable X-Factor values on the mill certificate, or, worst of all, Type IV cracking during service at 550 – 600 °C after years of operation.
This guide examines every meaningful technical difference between E9015-B91 and E9018-B91 — coating type, iron powder content, diffusible hydrogen levels, residual element control, X-Factor, PWHT sensitivity, and practical handling requirements — so you can make the right selection and specify it correctly in your P91 welding procedure. We also cover the AWS A5.5 classification logic and the metallurgical reasons why EXX15-type electrodes consistently outperform EXX18-type in critical chromium-molybdenum alloy applications.
Whether you are a welding inspector reviewing a consumable approval, an engineer writing a WPS, or a technician selecting a rod from the stores, this comparison will give you the technical grounding to justify and defend the correct choice on your project.
Understanding the AWS A5.5 Classification System
Both E9015-B91 and E9018-B91 are classified under AWS A5.5: Specification for Low-Alloy Steel Electrodes for Shielded Metal Arc Welding. Before comparing the two electrodes, it is worth decoding the designation systematically, because every character carries precise technical meaning that directly governs how you use the electrode and what you can expect from it.
The prefix E identifies the product as an arc welding electrode. The digits 90 define the minimum as-deposited tensile strength — in this case 90,000 psi (approximately 620 MPa) — matching the strength requirement for Grade 91 base metal. The third digit, 1, indicates all-position capability (flat, horizontal, vertical-up, overhead). The fourth digit encodes the coating type and current: 5 in E9015 indicates low-hydrogen sodium flux, DCEP only, no iron powder; 8 in E9018 indicates low-hydrogen potassium flux with iron powder additions, compatible with both DCEP and AC.
The suffix -B91 is the critical alloy designator. It specifies the chemical composition of the deposited weld metal: nominally 9% Cr, 1% Mo, 0.2% V, with controlled Nb, N, Ni, and Mn additions. This composition matches the modified 9Cr-1Mo-V chemistry of Grade 91 base material, ensuring compositional continuity across the fusion line — essential for long-term creep compatibility. You can find a full breakdown of electrode classification logic in our welding consumable nomenclature guide.
Chemical Composition of E9015-B91 and E9018-B91 Weld Deposits
The alloy composition requirements for both electrodes are identical under A5.5 — they both deposit a 9Cr-1Mo-V-Nb-N weld metal. The difference is not the target composition but the residual element content that results from the coating chemistry. Table 1 shows the AWS A5.5 compositional requirements for the B91 deposit alongside typical achieved values for each electrode type.
| Element | AWS A5.5 Limit (B91) | E9015-B91 Typical | E9018-B91 Typical | Comment |
|---|---|---|---|---|
| C (Carbon) | 0.08 – 0.13% | 0.09 – 0.11% | 0.09 – 0.12% | Critical for creep strength; avoid over-dilution |
| Mn (Manganese) | ≤ 1.20% | 0.50 – 0.80% | 0.60 – 1.00% | Ni+Mn sum ≤ 1.0% preferred for PWHT |
| Si (Silicon) | ≤ 0.30% | 0.10 – 0.25% | 0.15 – 0.30% | Low Si reduces oxidation; keep towards lower limit |
| Cr (Chromium) | 8.0 – 10.5% | 8.5 – 9.5% | 8.5 – 9.5% | Core creep and oxidation resistance element |
| Mo (Molybdenum) | 0.85 – 1.20% | 0.90 – 1.10% | 0.90 – 1.10% | Solid solution strengthener at elevated temperature |
| V (Vanadium) | 0.15 – 0.30% | 0.18 – 0.25% | 0.18 – 0.25% | Forms MX carbonitrides; critical for creep resistance |
| Nb (Niobium) | 0.02 – 0.10% | 0.04 – 0.08% | 0.04 – 0.08% | Stabilises grain size; acts with V in MX precipitates |
| N (Nitrogen) | 0.02 – 0.07% | 0.03 – 0.05% | 0.03 – 0.05% | Part of MX carbonitride; balance needed for stability |
| Ni (Nickel) | ≤ 0.80% (code: ≤ 0.40%) | ≤ 0.20% | ≤ 0.30% | Lowers Ac1 temperature; keep low to protect PWHT window |
| P (Phosphorus) | ≤ 0.010% | ≤ 0.008% | ≤ 0.010% | Key X-Factor element; must be tightly controlled |
| S (Sulfur) | ≤ 0.010% | ≤ 0.006% | ≤ 0.010% | Hot cracking risk; keep as low as possible |
| X-Factor (Bruscato) | < 15 (best practice) | Typically 8 – 12 | Typically 10 – 18 | Critical temper embrittlement index |
Table 1 — Comparative chemical composition and X-Factor for E9015-B91 and E9018-B91 weld deposits per AWS A5.5. Highlighted row indicates the critical differentiating parameter.
Coating Chemistry: Low-Hydrogen Sodium vs. Low-Hydrogen Potassium with Iron Powder
The coating type is the defining difference between these two electrodes, and it affects far more than which power source you can use on site. The EXX15 (low-hydrogen sodium) coating used in E9015-B91 is formulated primarily from calcium fluoride (fluorspar) and calcium carbonate (limestone), with sodium compounds to stabilise the arc on DC. It contains no iron powder. When this coating burns, it generates a CO2 and CO shielding atmosphere with very low hydrogen content, typically achieving diffusible hydrogen levels of H4 (≤4 mL/100 g of deposited metal) or better under proper storage conditions.
The EXX18 (low-hydrogen potassium with iron powder) coating used in E9018-B91 uses a similar base flux chemistry but adds potassium compounds (enabling AC arc stabilisation) and incorporates iron powder — typically 25 to 40% by weight of the total coating — into the flux mass. This iron powder addition serves a legitimate productivity purpose: it increases the deposition efficiency from roughly 85% (EXX15) to 105 to 110% (EXX18), meaning more weld metal is deposited per electrode length. The iron powder essentially melts into the weld pool, adding filler metal without consuming any of the core wire. For carbon steel and lower-alloy applications, this is an excellent feature.
For P91 specifically, the iron powder becomes a liability. Commercial iron powder used in electrode coatings is not metallurgically pure. It contains measurable quantities of phosphorus (P), antimony (Sb), tin (Sn), and arsenic (As) — the four residual elements that directly drive temper embrittlement. These elements segregate to prior austenite grain boundaries during PWHT and long-term service, weakening the boundaries and promoting intergranular cracking, particularly at temperatures where temper embrittlement is active (300 to 600 °C).
The X-Factor (Bruscato Factor): What It Is and Why It Governs P91 Electrode Selection
The X-Factor, sometimes called the Bruscato factor, is a calculated index that quantifies the susceptibility of a weld metal to temper embrittlement. It is defined as:
X = (10P + 5Sb + 4Sn + As) / 100
where P, Sb, Sn, and As are expressed in parts per million (ppm) by weight in the weld deposit. A value of X < 15 is the accepted industry threshold for P91 welds, established through creep and impact testing of service-aged specimens from actual power plant components. Exceeding this limit does not cause immediate weld failure; the consequence is a gradual reduction in low-temperature impact toughness (measured by Charpy V-Notch test) over years of service, which only becomes critical during plant shutdown, hydrostatic testing at ambient temperature, or in the event of a thermal transient that rapidly cools the component.
How Iron Powder Drives the X-Factor Up
When iron powder is added to the E9018-B91 coating, the phosphorus, antimony, tin, and arsenic within that powder partition into the molten weld pool during welding. Because the pool volume is relatively small and the cooling time is short, these elements do not have sufficient time to be rejected by the solidifying austenite — they become locked into the deposit microstructure. The net effect is that E9018-B91 electrodes, sourced from reputable manufacturers, can still achieve X < 15, but the margin is much tighter and the batch-to-batch variability is higher. Less-controlled sources may produce heats with X values of 16 to 25, well above the acceptable limit.
E9015-B91, with no iron powder contribution, consistently achieves X values of 8 to 12 in practice, providing a comfortable safety margin. This is why leading power-generation engineering specifications (EPRI TR-115566, the Babcock and Wilcox P91 code of practice, and the German VDTV guidelines) all mandate EXX15 electrodes for creep-critical P91 SMAW applications.
Crater Cracking: Why Iron Powder Electrodes Present Greater Risk on P91
Crater cracking (also called solidification cracking or "solidification anomalies" in some P91 inspection specifications) occurs at the termination point of a weld bead, where the last portion of weld metal to solidify is enriched in low-melting-point segregates. On P91 welds, crater cracks are treated as a zero-tolerance defect; their presence on the mill certificate is grounds to reject the entire heat of consumable.
The mechanism is as follows: as the weld pool solidifies from the outside inward, solute elements — including residual P, S, Sb, and Sn — are rejected by the solidifying dendrites and concentrate in the last liquid remaining at the bead centre. If the concentration is high enough, this liquid solidifies well below the nominal solidus, forming a brittle intergranular film that cracks under the tensile shrinkage stresses of cooling. E9018-B91's higher residual element input directly increases the probability of this segregation mechanism reaching a critical level.
E9015-B91, with lower P and S levels and the absence of the iron powder residual element source, deposits metal that is less susceptible to this centreline enrichment. In practice, fabricators monitoring Grade 91 weld quality often use observation of crater cracking as a consumable quality gate: if crater cracks appear in test welds, the electrode batch is returned regardless of what the test certificate shows, because it indicates that the residual element content is higher than the certified value.
Mechanical Properties: As-Deposited and PWHT Condition
Both E9015-B91 and E9018-B91 produce weld deposits with virtually identical mechanical properties after correct PWHT, provided the residual element content is within the acceptable range. The AWS A5.5 minimum requirements for the B91 deposit are:
| Property | AWS A5.5 Minimum | Typical E9015-B91 (PWHT) | Typical E9018-B91 (PWHT) |
|---|---|---|---|
| Tensile Strength | 620 MPa (90 ksi) | 650 – 720 MPa | 650 – 720 MPa |
| 0.2% Proof Stress | 530 MPa (77 ksi) | 560 – 650 MPa | 560 – 650 MPa |
| Elongation | 17% | 19 – 22% | 18 – 21% |
| CVN Impact (20°C) | 47 J | 80 – 130 J | 60 – 120 J |
| Hardness (HV10) | 200 – 265 max | 210 – 255 | 215 – 260 |
| CVN Impact (after ageing at 550°C) | No standard minimum | > 60 J (typical) | Variable: 20 – 80 J |
Table 2 — Comparative mechanical properties for E9015-B91 and E9018-B91 deposits after normalise + temper PWHT. The critical difference emerges in the aged impact toughness row, which reflects long-term service performance.
The critical difference is visible only in the service-aged condition. After 50,000 to 100,000 hours at 550 – 600 °C, E9018-B91 deposits with X-Factor values approaching or exceeding 15 can show a significant ductile-to-brittle transition temperature (DBTT) shift, reducing room-temperature CVN impact values to below 27 J — the minimum typically required by ASME Section VIII and ASME B31.1. E9015-B91 deposits with X < 12 show a much smaller DBTT shift, maintaining adequate toughness through the full design life of the component.
PWHT Requirements and the Ni+Mn Constraint
Post-weld heat treatment for P91 is not optional — it is mandatory for every welded joint in service. The standard thermal cycle is:
- Normalise: Heat to 1040 – 1090 °C, hold for sufficient time to dissolve carbides and homogenise the microstructure, then air or forced-air cool to <80 °C (below the martensite finish temperature Mf).
- Temper: Heat to 760 – 790 °C (depending on code and filler metal chemistry), hold at temperature for a minimum of 1 hour per 25 mm of thickness, then slow cool to ambient.
The maximum tempering temperature is constrained by the lower critical transformation temperature (Ac1) of the weld deposit. Exceeding Ac1 produces fresh untempered martensite on cooling, causing catastrophic loss of toughness. The key variable that determines Ac1 is the Ni+Mn sum in the weld metal — both elements depress Ac1, narrowing the safe PWHT temperature window.
Most construction codes and leading engineering specifications require that the total Ni+Mn content of the B91 filler metal does not exceed 1.0% (some specifications set a limit of 1.5%, but the tighter 1.0% limit is best practice for maximum PWHT window). E9015-B91 electrodes, with their lower iron powder input, more consistently achieve Ni+Mn ≤ 1.0%, providing a maximum tempering temperature of 790 °C per ASME B31.1 and similar codes. For a detailed treatment of P91 PWHT, including code-by-code temperature comparisons and soaking time calculations, see our dedicated guide on P91 material welding requirements.
When to Use E9015-B91 vs E9018-B91: Decision Summary
The table below provides a direct decision framework based on the technical factors discussed in this article.
| Factor | E9015-B91 | E9018-B91 |
|---|---|---|
| Recommended for P91 critical service | Yes — first choice | Only with verified X < 15 and Ni+Mn ≤ 1.0% |
| AC power source required on site | No (DCEP only) | Yes — will operate on AC |
| Residual element control | Superior; no iron powder source | Adequate if manufacturer certifies residuals |
| X-Factor risk | Low; typically 8 – 12 | Moderate; can reach 15+ with lower-grade iron powder |
| Deposition efficiency | ~85% | ~105 – 110% |
| Diffusible hydrogen | H4 or better | H4 or better (properly stored) |
| Crater cracking risk | Lower | Higher; monitor during qualification |
| Power plant / boiler / pressure vessel use | Strongly preferred | Acceptable with additional QC verification |
| Cost per kg deposited | Slightly higher per electrode; lower deposition efficiency | Lower per kg deposited due to iron powder |
Table 3 — Decision summary comparing E9015-B91 and E9018-B91 across key application factors for P91 welding.
Key Takeaway
For any P91 weld in pressure-containing or creep-critical service, specify E9015-B91 as the standard. The deposition rate advantage of E9018-B91 is real but marginal for most SMAW applications, and it does not justify the increased risk of elevated residual element content, potential X-Factor non-compliance, and greater crater cracking susceptibility. If AC-only power sources are the only site option, use E9018-B91 but mandate a full residual element certificate and independently calculate the X-Factor before approving the heat.
Storage, Baking, and Handling Requirements for B91 Electrodes
Low-hydrogen electrodes — both EXX15 and EXX18 types — are highly susceptible to moisture absorption. The calcium fluoride-based flux coating acts as a desiccant in reverse: in a humid environment it absorbs water vapour from the atmosphere, and this absorbed moisture becomes the source of diffusible hydrogen in the weld. For P91, where the material is susceptible to hydrogen-induced cold cracking in the martensite-finish temperature range (typically 80 – 250 °C), diffusible hydrogen must be kept to an absolute minimum.
Recommended storage and reconditioning practice for E9015-B91 and E9018-B91:
- Store sealed canisters or boxes in a dry room or storage oven at 50 – 80 °C indefinitely until use.
- Bake opened electrodes at 300 – 350 °C for 1 to 2 hours before use to restore H4 hydrogen designation.
- After baking, transfer to a portable heated rod box maintained at 100 – 150 °C at the weld station.
- Discard any electrodes left outside the heated box for more than 4 hours in ambient conditions (or 2 hours in high-humidity environments).
- Rebaking is permitted once only for most manufacturers; consult the electrode data sheet for the manufacturer-specific limit.
- Never use electrodes with damaged, cracked, or flaking coating — the low-hydrogen integrity is compromised regardless of prior storage.
For a more complete treatment of electrode baking procedures and the science behind moisture-induced hydrogen cracking, see our article on SMAW welding fundamentals, which covers electrode conditioning in detail. You will also find the F-Number and A-Number guide useful when qualifying a new WPS for B91 electrodes under ASME Section IX.
Industry-Accepted E9015-B91 Electrode Brands
A number of major welding consumable manufacturers produce E9015-B91 electrodes with consistently low residual element content. The following brands appear regularly in P91 welding procedure qualifications on power-generation and petrochemical projects:
- ESAB OK 76.98 (E9015-B9 / E CrMo91)
- Metrode Chromet 9MV-N (E9015-B9)
- Lincoln Electric Lincolnweld P91 (for SAW; Jetweld LH-9015 for SMAW)
- Bohler Fox P91 (E CrMo9 1 B 4 2 H5 per EN ISO 3580)
- Kobelco LB-116 (E9015-B9)
- Ador Cromoten 9M-15 (E9015-B91)
Always verify the brand's test certificate for the specific batch/heat, not just the brand reputation. Demand full compositional analysis including P, S, Sb, Sn, and As. If those elements are not listed, request a supplementary certificate or select a different supplier. Cross-reference also with your applicable construction code (ASME B31.1, B31.3, Section VIII, or EN 13480) to confirm the electrode classification is an approved filler metal group for the specific P-Number base material combination. See our P-Number and F-Number guide for a full reference table.
P91 Steel in Context: Why Electrode Selection Is Non-Negotiable
Grade 91 steel (ASTM A335 P91 for pipe, A182 F91 for forgings, A213 T91 for tubing) is the pre-eminent material for high-temperature, high-pressure steam service in modern power plants. Its nominal composition of 9Cr-1Mo-V-Nb-N delivers exceptional creep strength at 550 – 620 °C — roughly three times that of the older 2.25Cr-1Mo (P22) steel it replaced — along with good oxidation resistance and relatively low thermal expansion.
However, P91 is metallurgically complex and unforgiving of improper fabrication. Unlike carbon steel, it does not tolerate welding shortcuts. The martensite start temperature (Ms) is approximately 410 °C and the finish temperature (Mf) is approximately 200 °C, meaning the weld must be cooled below 80 °C before PWHT to ensure complete martensite transformation. Failure to do so leaves retained austenite in the structure that transforms to fresh untempered martensite during PWHT, creating brittle islands with hardness exceeding 450 HV. Calculating the carbon equivalent of the base material and filler metal is also recommended to understand preheat requirements fully.
The consequences of using the wrong electrode — or a correctly classified electrode from a batch with elevated residual element content — are not immediately visible. The weld will pass all radiographic, ultrasonic, and hardness tests immediately after PWHT. The failure mode is long-term: Type IV cracking after 30,000 to 80,000 hours of service, presenting as circumferential cracking in the outer HAZ of a pipe weld, often discovered only during a planned inspection or as an unplanned leak event. This makes P91 electrode selection a matter of lifecycle engineering, not just meeting the next inspection milestone.
Recommended Books for P91 Welding and High-Alloy SMAW
The following books are recommended reading for engineers and inspectors working with P91 and other chromium-molybdenum alloy steels in SMAW applications.
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Frequently Asked Questions
Why is E9015-B91 preferred over E9018-B91 for welding P91 steel?
E9015-B91 uses a low-hydrogen sodium (EXX15) coating that contains no iron powder, resulting in lower residual element content in the weld deposit. This keeps the X-Factor below 15 and reduces the risk of temper embrittlement and grain boundary precipitation. E9018-B91 contains iron powder in its coating, which can introduce additional contaminants and increase the risk of crater cracking and Type IV cracking in the P91 HAZ. For most creep-critical power plant applications, E9015-B91 is the specified first choice.
What is the X-Factor and why does it matter for P91 welding?
The X-Factor (Bruscato factor) is calculated as X = (10P + 5Sb + 4Sn + As) / 100 using residual element concentrations in ppm. A value below 15 is required for P91 weld metal to resist temper embrittlement — a form of intergranular cracking that occurs during slow cooling or long-term service in the 300 to 600 °C range. E9015-B91 electrodes are formulated to achieve X < 15 consistently, while E9018-B91 may produce higher values due to the iron powder in its coating. Always verify the X-Factor on the electrode batch test certificate before approving a heat for P91 welding.
What is the difference in coating chemistry between E9015 and E9018 electrodes?
The EXX15 designation indicates a low-hydrogen sodium coating with DCEP current only and no iron powder. The EXX18 designation indicates a low-hydrogen potassium coating with iron powder additions, which allows both DCEP and AC current. The iron powder in EXX18 increases deposition efficiency to approximately 105 to 110%, compared to roughly 85% for EXX15, but also introduces phosphorus, antimony, tin, and arsenic from the powder into the weld deposit. For a full explanation of electrode designation logic, see our consumable nomenclature guide.
What is Type IV cracking in P91 welds and how is it related to electrode selection?
Type IV cracking occurs in the fine-grained or intercritical heat-affected zone of P91 welds during high-temperature service. It is accelerated by grain boundary precipitates — carbides, nitrides, and phosphide and antimony segregations — promoted by elevated residual element content. Selecting E9015-B91 with its lower residual content reduces the formation of these precipitates, helping maintain HAZ ductility and creep resistance over the component design life. This is particularly important for seam welds in steam headers and main steam pipework operating continuously at 550 °C and above.
What preheat and interpass temperature is required for P91 SMAW welding?
P91 requires a minimum preheat of 200 °C (392 °F) and a maximum interpass temperature of 300 °C (572 °F). The preheat prevents hydrogen-induced cold cracking during the pass, while the interpass limit prevents excessive retained austenite and loss of toughness from high heat input. These requirements apply to both E9015-B91 and E9018-B91. Monitoring interpass temperature with a contact pyrometer or thermal crayon is mandatory, not optional, on any Code-qualified P91 weld. See our mechanical testing guide for the hardness and CVN test requirements after PWHT.
What PWHT is required after welding P91 with E9015-B91 or E9018-B91?
P91 welds require a normalise at 1040 to 1090 °C followed by tempering at 760 to 790 °C. The exact maximum tempering temperature depends on the Ni+Mn content: if Ni+Mn is at or below 1.0%, the maximum is 790 °C per most construction codes. Exceeding the upper critical transformation temperature (Ac1) during PWHT produces fresh untempered martensite and causes catastrophic loss of toughness. Holding time is typically 1 hour per 25 mm of thickness. Full PWHT is mandatory before the weld is placed in service — a hydrogen bake alone is not a substitute.
Can E9018-B91 ever be used for P91 welding?
Yes, E9018-B91 can be used for P91 welding where AC power sources are the only site option, or for lower-criticality applications. When specifying E9018-B91, the engineer must verify the test certificate to confirm X-Factor below 15 and Ni+Mn at or below 1.0%. Crater cracking should be monitored during procedure qualification testing. If either criterion cannot be confirmed, the heat should be rejected in favour of a verified E9015-B91 batch. Some project specifications explicitly prohibit E9018-type electrodes for P91 — always check your governing engineering specification before making the selection.
How should E9015-B91 and E9018-B91 electrodes be stored and conditioned?
Both electrode types use low-hydrogen coatings that are hygroscopic and must be stored in sealed, moisture-proof containers or ovens at 50 to 80 °C until use. Before welding, bake at 300 to 350 °C for 1 to 2 hours to drive off absorbed moisture. After baking, hold in a portable heated rod box at 100 to 150 °C at the welding station. Electrodes exposed to atmosphere for more than 4 hours should be rebaked. Rebaking is normally permitted once only — confirm the limit with the electrode manufacturer's data sheet. Proper storage is a mandatory QC hold point on any P91 welding programme and should be documented in the SMAW welding procedure.
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