The X-Factor (Bruscato Factor): Calculator, Formula and Complete Guide
When welding chrome-moly steels for high-temperature pressure equipment, achieving a sound weld is only half the challenge. The other half is ensuring the weld retains its toughness over decades of service at elevated temperature. The X-Factor — also known as the Bruscato factor — is a compositional index specifically developed to quantify a chrome-moly weld metal’s susceptibility to temper embrittlement: the progressive loss of impact toughness that occurs when trace elements migrate to grain boundaries during extended high-temperature service.
This guide covers what the X-Factor is, why it matters for Cr-Mo steel welding, how to calculate it from certified filler metal chemistry, what acceptable limits look like, and how to select compliant filler metals for critical applications. A free, plugin-free calculator is provided near the top of the page to compute the X-Factor for any filler metal instantly.
What is Temper Embrittlement?
Temper embrittlement is a form of metallurgical degradation in which the impact toughness of certain alloy steels — particularly chrome-moly grades — is progressively reduced during exposure to a critical temperature range. The embrittlement occurs when the steel is held at, or slowly cooled through, approximately 450 to 600 degrees Celsius (840 to 1100 degrees Fahrenheit). This temperature window is frequently encountered during the normal operation of high-temperature power plant and process equipment, where startup and shutdown cycles repeatedly take the material through this range.
The mechanism is driven by grain boundary segregation. During service in the critical temperature range, trace impurity elements — phosphorus, antimony, tin, and arsenic — gradually diffuse through the steel lattice and accumulate at the prior austenite grain boundaries. These elements have very low solubility in the ferrite matrix but are strongly attracted to boundary sites, where they weaken the inter-granular cohesion. The result is a shift in fracture mode from ductile transgranular fracture to brittle intergranular fracture, measurable as a significant upward shift in the Ductile-to-Brittle Transition Temperature (DBTT) in Charpy impact testing.
Critically, temper embrittlement is reversible — if a temper-embrittled component is heated above the embrittlement range (typically above 600 degrees Celsius) and cooled rapidly, the grain boundary segregation is partly dissolved and toughness is temporarily restored. However, in service conditions where the component cannot be readily heat-treated, or where the embrittlement continues to worsen over time, the consequences can be severe: brittle fracture during cold-start events when the steel is at ambient temperature but already embrittled by years of service exposure.
What is the X-Factor (Bruscato Factor)?
The X-Factor, developed by A.T. Bruscato and first published in welding research in the 1970s, is a simple compositional index that predicts the temper embrittlement susceptibility of a Cr-Mo steel weld metal based on the concentrations of the four elements that have the strongest influence on grain boundary segregation: phosphorus (P), antimony (Sb), tin (Sn), and arsenic (As). It was designed to serve as an efficient, low-cost alternative to expensive and time-consuming step-cooling tests or creep rupture tests, both of which physically simulate embrittlement but require weeks of furnace time.
The X-Factor is computed from the certified chemical analysis of the filler metal — specifically, from the steel portion of the electrode or wire (for tubular wires, the outer strip; for solid wires, the wire itself). It is important to understand that the X-Factor applies to the weld metal composition, not to the base metal, which is separately characterised by the related J-Factor (see below).
The four elements in the X-Factor formula are each assigned a weighting coefficient that reflects their relative potency in causing grain boundary embrittlement:
Phosphorus (P) — Coefficient: 10
The highest-weighted and most potent embrittling element. Even at very low concentrations (tens of ppm), phosphorus segregates strongly to prior austenite grain boundaries. It is the primary target for control in low-residual filler metal specifications.
Antimony (Sb) — Coefficient: 5
A highly potent embrittler at low concentrations. Antimony is a tramp element introduced via steel scrap and is difficult to remove by steel-making. It has a strong thermodynamic driving force for grain boundary segregation in Cr-Mo steels.
Tin (Sn) — Coefficient: 4
Another tramp element from steel scrap. Tin segregates to grain boundaries and synergistically amplifies the embrittlement effects of phosphorus and antimony. Its coefficient reflects a moderately high potency.
Arsenic (As) — Coefficient: 1
The least potent of the four elements, with a coefficient of 1 (no multiplier). Arsenic is still included because it contributes meaningfully to overall embrittlement susceptibility, particularly at higher concentrations where the other three elements are already controlled.
The X-Factor Formula — Derivation and Worked Example
The X-Factor formula is designed to produce a single dimensionless number (expressed in ppm) that captures the combined embrittlement contribution of all four trace elements, weighted by their respective potencies. The calculation requires all element concentrations to be expressed in parts per million (ppm) by weight. If the certified test report for the filler metal expresses concentrations in weight percent, a conversion is required before applying the formula.
Where: P, Sb, Sn, As are all expressed in ppm (parts per million by weight)
Result X is expressed in ppm
Unit Conversion: wt% to ppm ppm = wt% × 10,000
Example: 0.007 wt% P = 0.007 × 10,000 = 70 ppm
Worked Example (matching original article)
Filler metal certified test report shows the following analysis (in weight percent):
- P = 0.007 wt%
- Sb = 0.004 wt%
- Sn = 0.001 wt%
- As = 0.0016 wt%
Sb = 0.004 × 10,000 = 40 ppm
Sn = 0.001 × 10,000 = 10 ppm
As = 0.0016 × 10,000 = 16 ppm
Step 2 — Apply weighted sum Numerator = (10×70) + (5×40) + (4×10) + 16
Numerator = 700 + 200 + 40 + 16 = 956
Step 3 — Divide by 100 X = 956 ÷ 100 = 9.56 ppm
Result: X = 9.56 ppm — well within the accepted limit of 15 ppm. This filler metal is acceptable for temper embrittlement-controlled applications.
X-Factor Acceptance Criteria — What the Numbers Mean
The primary benchmark for the X-Factor is a maximum value of 15 ppm, which has been established through decades of industrial experience and research as the threshold below which temper embrittlement risk is adequately controlled for most Cr-Mo steel applications. Some project specifications — particularly for ultra-supercritical power plant components and high-temperature P91/P92 applications — tighten this to 10 ppm.
| X-Factor (ppm) | Assessment | Action Required | Typical Specification |
|---|---|---|---|
| Below 10 | Excellent | No action required. Suitable for the most demanding high-temperature applications. | Ultra-supercritical power plant, P91/P92 applications, very stringent client specs |
| 10 to 15 | Good | Acceptable for all standard Cr-Mo applications. No action required. | Standard power plant, boilers, pressure vessels per most project specifications |
| 15 to 20 | Marginal | Marginal. Evaluate against project specification. Contact filler metal supplier for low-residual alternative. Do not use without engineering review for critical applications. | May be acceptable for low-temperature or non-critical Cr-Mo service only |
| Above 20 | Unacceptable | Do not use for temper embrittlement-controlled applications. Replace filler metal lot. Verify heat/lot traceability and re-test. | Fails all standard Cr-Mo high-temperature specifications |
Which Chrome-Moly Grades and Applications Require X-Factor Control?
The X-Factor is most relevant for filler metals used on the lower-alloy Cr-Mo steel grades that are commonly used in continuous high-temperature service. The risk is proportional to the time spent in the embrittlement temperature range and to the operating temperature — higher operating temperatures accelerate element diffusion and therefore accelerate grain boundary segregation.
| Steel Grade | ASTM / ASME Designation | Composition | X-Factor Relevance | Typical Applications |
|---|---|---|---|---|
| Grade 11 | ASTM A387 Gr.11 / SA-387 | 1.25Cr – 0.5Mo | Moderate | Low-temperature heat exchangers, pressure vessels, moderate-temperature boiler components |
| Grade 22 | ASTM A387 Gr.22 / SA-335 P22 | 2.25Cr – 1Mo | High | Boiler pressure parts, superheater headers, high-temperature process piping (up to ~595°C) |
| Grade 91 | ASTM A387 Gr.91 / SA-335 P91 | 9Cr – 1Mo – V | Critical | Main steam lines, superheater headers, high-temperature boiler components (540–610°C) |
| Grade 92 | ASTM A387 Gr.92 / SA-335 P92 | 9Cr – 0.5Mo – 1.8W – V – Nb | Critical | Ultra-supercritical (USC) power plant steam lines and headers above 600°C |
| Grade 21 / P21 | ASTM A387 Gr.21 / SA-335 P21 | 3Cr – 1Mo | Moderate | High-temperature hydrogen service in refineries and petrochemical plants |
The J-Factor — Base Metal Counterpart to the X-Factor
While the X-Factor characterises weld metal susceptibility to temper embrittlement, the related J-Factor performs the same function for the base metal plate or forging. The J-Factor includes manganese and silicon in addition to the phosphorus and tin terms, because in base metal these elements — which are more variable than in controlled filler metals — also contribute to embrittlement. Knowing both the X-Factor and J-Factor for a given fabrication ensures that neither the weld metal nor the base metal introduces a temper embrittlement weakness into the completed joint.
Where: Si, Mn, P, Sn are expressed in weight percent (not ppm)
Typical acceptance criterion: J ≤ 100 for most applications; J ≤ 150 for less critical service
Comparison Summary X-Factor: weld metal / filler metal — uses P, Sb, Sn, As (in ppm)
J-Factor: base metal / plate / forging — uses Si, Mn, P, Sn (in wt%)
Where to Find P, Sb, Sn, and As on the Certified Test Report
To calculate the X-Factor, you need the certified chemical analysis (typically provided as a Mill Test Certificate or EN 10204 3.1/3.2 certificate) for the specific heat or lot of filler metal you are purchasing. Most standard filler metal certificates report carbon, manganese, silicon, chromium, molybdenum, vanadium, and other principal alloying elements — but may not include the four X-Factor elements unless you specifically request them.
To obtain X-Factor-certified filler metals:
- Specify the X-Factor requirement on the purchase order: State explicitly that the filler metal must be certified with maximum X-Factor of 15 ppm (or 10 ppm for more demanding specifications), and that a certificate showing the actual values of P, Sb, Sn, and As (in ppm or wt%) for the specific heat/lot supplied must accompany the delivery.
- Request the supplemental analysis: Some manufacturers include trace element analysis only as a supplemental report on request. Contact the technical department directly if the standard certificate does not show these values.
- Verify heat/lot traceability: The chemistry on the certificate must correspond to the exact heat and lot number of the material delivered. Mixed-heat deliveries make traceability impossible and should not be accepted for X-Factor-controlled applications.
- Calculate and record the X-Factor: Once you have the certified P, Sb, Sn, and As values, calculate the X-Factor using the formula above (or use the calculator at the top of this page) and document it in the welding procedure record and material traceability file.
Recommended Reference Books — Cr-Mo Steels and High-Temperature Metallurgy
Deepen your understanding of temper embrittlement, chrome-moly steel metallurgy, and high-temperature weld quality with these authoritative engineering references.
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