--- title: "Carbon Equivalent Calculations for Welding | Steel Equivalents" description: "Learn how to calculate CEIIW, Pcm, CET, CEq, and CEN from steel chemistry and use them to assess weldability." canonical: https://steelequivalents.com/wiki/calculated-values/carbon-equivalent-calculations language: en robots: index, follow og_type: article type: wiki-article language_alternates: en: https://steelequivalents.com/wiki/calculated-values/carbon-equivalent-calculations.md cs: https://steelequivalents.com/cs/wiki/vypočítané-hodnoty/výpočty-uhlíkového-ekvivalentu-pro-svařování.md de: https://steelequivalents.com/de/wiki/berechnete-kennwerte/berechnung-des-kohlenstoffäquivalents-beim-schweißen.md es: https://steelequivalents.com/es/wiki/valores-calculados/cálculos-del-equivalente-de-carbono-para-soldadura.md fr: https://steelequivalents.com/fr/wiki/valeurs-calculées/calculs-de-l-équivalent-carbone-pour-le-soudage.md hu: https://steelequivalents.com/hu/wiki/számított-értékek/hegesztéshez-szükséges-szénegyenérték-számítások.md it: https://steelequivalents.com/it/wiki/valori-calcolati/calcoli-dell-equivalente-di-carbonio-per-la-saldatura.md nl: https://steelequivalents.com/nl/wiki/berekende-waarden/berekeningen-van-het-koolstofequivalent-voor-lassen.md pl: https://steelequivalents.com/pl/wiki/wartości-obliczane/obliczenia-równoważnika-węglowego-podczas-spawania.md pt: https://steelequivalents.com/pt/wiki/valores-calculados/cálculos-do-equivalente-de-carbono-para-soldadura.md ru: https://steelequivalents.com/ru/вики/расчётные-величины/расчёт-углеродного-эквивалента-для-сварки.md tr: https://steelequivalents.com/tr/wiki/hesaplanan-değerler/kaynak-için-karbon-eşdeğeri-hesaplamaları.md zh: https://steelequivalents.com/zh/维基/计算得出的数值/焊接用碳当量计算.md ja: https://steelequivalents.com/ja/ウィキ/計算で求める値/溶接における炭素当量の計算.md ar: https://steelequivalents.com/ar/ويكي/القيم-المحسوبة/حسابات-مكافئ-الكربون-للحام.md ko: https://steelequivalents.com/ko/위키/계산으로-얻는-값/용접용-탄소-당량-계산.md pt-br: https://steelequivalents.com/pt-br/wiki/valores-calculados/cálculos-do-equivalente-de-carbono-para-soldagem.md hi: https://steelequivalents.com/hi/विकी/परिकलित-मान/वेल्डिंग-के-लिए-कार्बन-समतुल्य-की-गणनाएँ.md content_hash: sha256:729a3bcc2ea11ce933103a792ea0c410d19f517f74d30e1d4fe5cf8a3d411dac generated: 2026-09-24 --- > **Machine-readable Markdown rendering.** This is a Markdown representation of a web page, > served for AI agents and crawlers as a token-efficient alternative to the full HTML. > Full interactive HTML version: https://steelequivalents.com/wiki/calculated-values/carbon-equivalent-calculations > Canonical URL: https://steelequivalents.com/wiki/calculated-values/carbon-equivalent-calculations > Available languages: en, cs, de, es, fr, hu, it, nl, pl, pt, ru, tr, zh, ja, ar, ko, pt-br, hi > Change detection: `content_hash` is the SHA-256 of this document's Markdown body — everything > below this block, whitespace-trimmed, excluding the front-matter. Compare it to detect a real > content change; `generated` moves on every render and is not a change signal. ![Carbon Equivalent Calculations for Welding](/images/uploads/a9505256-43bd-4f1a-bccf-3273b98a70c1/wiki-hero-a-steel-plate-joint-being-welded-with-the-weld-bead-fusion-boundary-and-heat-aff-1920x823.jpg) Calculated Values # Carbon Equivalent Calculations for Welding Learn how to calculate CEIIW, Pcm, CET, CEq, and CEN from steel chemistry and use them to assess weldability. ![Portrait of Anders Bergström, steel industry reporter](/images/uploads/a7d8e5bc-7cdf-409b-913b-211e63e8c441/anders-bergstr-m-1920x1920.jpg) **[Anders Bergström](/news/author/anders-bergstrom "Anders Bergström")** Calculated Values 32 min read Updated Aug 14, 2026 Evidence-reviewed On this pageOn this page - [What Carbon Equivalent Measures—and What It Does Not](/wiki/calculated-values/carbon-equivalent-calculations#what-carbon-equivalent-measures-and-what-it-does-not "What Carbon Equivalent Measures—and What It Does Not") - [The IIW Carbon Equivalent Formula: CEIIW](/wiki/calculated-values/carbon-equivalent-calculations#the-iiw-carbon-equivalent-formula-ceiiw "The IIW Carbon Equivalent Formula: CEIIW") - [Why Several Carbon Equivalent Formulae Exist](/wiki/calculated-values/carbon-equivalent-calculations#why-several-carbon-equivalent-formulae-exist "Why Several Carbon Equivalent Formulae Exist") - [Worked Calculation Method for Steel Chemistry](/wiki/calculated-values/carbon-equivalent-calculations#worked-calculation-method-for-steel-chemistry "Worked Calculation Method for Steel Chemistry") - [Carbon Equivalent and Hydrogen Cracking in the HAZ](/wiki/calculated-values/carbon-equivalent-calculations#carbon-equivalent-and-hydrogen-cracking-in-the-haz "Carbon Equivalent and Hydrogen Cracking in the HAZ") - [Applying CE to Named Steel Grades and Specifications](/wiki/calculated-values/carbon-equivalent-calculations#applying-ce-to-named-steel-grades-and-specifications "Applying CE to Named Steel Grades and Specifications") - [Standards, Material Grouping and Welding Procedure Qualification](/wiki/calculated-values/carbon-equivalent-calculations#standards-material-grouping-and-welding-procedure-qualification "Standards, Material Grouping and Welding Procedure Qualification") - [Limits, Misuse and Practical Interpretation](/wiki/calculated-values/carbon-equivalent-calculations#limits-misuse-and-practical-interpretation "Limits, Misuse and Practical Interpretation") ## What Carbon Equivalent Measures—and What It Does Not ### From elemental composition to an equivalent carbon value Carbon equivalent (CE) is a calculated composition index. It is not a material property measured by a tensile test, hardness test, or chemical-analysis instrument. A laboratory determines the steel’s elemental composition; a selected equation then converts those weight percentages into a single number intended to represent, approximately, the combined effect of alloying elements on hardenability and welding risk. Carbon has the leading coefficient because it has a strong, direct effect on steel transformation behaviour and on the hardness that can develop when austenite cools rapidly. It also affects the carbon available to form hard transformation products, including martensite, in the weld heat-affected zone (HAZ). The other elements do not contribute equally. Manganese and silicon, chromium, molybdenum and vanadium, and nickel and copper therefore appear in grouped terms with different divisors in the International Institute of Welding equation: **CEIIW = C + (Mn + Si)/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15** AWS D8.10M:2021 identifies this equation and describes carbon equivalent as a weldability guideline, not an absolute parameter. The formula is commonly applied by substituting elemental percentages expressed as weight percentages. Its coefficients are empirical approximations, not universal physical constants. They compress complex effects—such as transformation kinetics and alloy interactions—into a convenient index. CEIIW coefficient CEq coefficient Pcm coefficient CET coefficient Selected carbon-equivalent formula coefficients show why CEIIW, CEq, Pcm and CET cannot be treated as interchangeable. That compression explains why formulas differ. For hot-rolled steel guidance published by AWS in 2007, the CE expression is **C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15**, while the corresponding **Pcm** expression is **C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B**. The **CET** expression is **C + Mn/10 + Mo/10 + Cr/20 + Cu/20 + Ni/40**. Silicon is treated differently in these equations, and boron appears explicitly in Pcm because even a small boron addition can strongly influence hardenability. The formula name, intended steel population and elemental weighting must accompany every reported value. | Formula | Key composition context | Distinctive treatment | |---|---|---| | CEIIW | Conventional carbon and low-alloy steels | Groups Mn + Si, Cr + Mo + V, and Ni + Cu | | CEq | AWS hot-rolled steel guidance | Does not include silicon in the stated expression | | Pcm | Modern low-carbon steels, approximately 0.11 wt% C or less | Includes 5B and gives silicon a separate divisor | | CET | AWS hot-rolled steel guidance | Uses separate coefficients for Mn, Mo, Cr, Cu and Ni | | CEN | Wider range of steels | Carbon-dependent treatment of alloying contributions | These are not interchangeable scores. TWI identifies CEIIW, Pcm, CEq and CEN as formulations used in hydrogen-cracking assessment, with each developed for particular steel populations or assessment approaches. Pcm was developed primarily for modern low-carbon steels containing approximately **0.11 wt% carbon or less**. Applying it outside that context can change the meaning of the result, even when the arithmetic is correct. Material grouping is a separate matter: **ISO 15608:2025** specifies a uniform system for grouping steels and other metallic material families for welding purposes; a group designation does not replace a carbon-equivalent calculation. ### Why CE is a weldability indicator rather than a pass/fail property A CE value answers a limited question: how strongly might the stated composition promote hardening under a relevant welding thermal cycle? It does not answer whether a particular joint will crack, whether a welding procedure is qualified, or whether a steel automatically meets a construction code. A carbon-equivalent value should guide welding decisions but cannot stand alone as a pass/fail criterion. Strong evidence AWS D8.10M:2021 is explicit on the central limitation: CE is a guideline rather than an absolute parameter. A numerical result can support procedure selection, comparison between heats, and decisions about preheat or hydrogen control, but it cannot stand alone as a pass/fail criterion. Two steels with the same CE may respond differently because their alloy balance, plate thickness, prior processing, grain structure, and actual carbon distribution differ. Conversely, steels with different CE values may be welded safely under procedures that account for their separate risks. Carbon equivalent addresses only part of the welding-risk assessment. | Factor | Why it matters to cracking assessment | |---|---| | Joint restraint | Can increase tensile stress and restrict contraction | | Heat input | Changes the weld thermal cycle and HAZ cooling behaviour | | Interpass temperature | Helps control the thermal cycle between weld passes | | Diffusible hydrogen | Provides one of the conditions required for hydrogen-assisted cracking | | Thickness | Affects heat flow and cooling rate | | Post-weld heat treatment | Can alter residual stress and temper hard regions when specified | The welding decision also depends on joint restraint, heat input, interpass temperature, hydrogen content in consumables and surfaces, cooling conditions, ambient temperature, thickness, and any post-weld heat treatment. AWS guidance on carbon and low-alloy steels identifies both chemical composition and hardenability as weldability considerations, not as a substitute for a complete welding procedure specification or qualification record. #### Do not use CE as a universal limit TWI reports recommended carbon-equivalent limits below 0.43, or 0.45 for components thicker than 1 inch, when carbon exceeds 0.18 wt%, in the context of controlling hard HAZs. These figures do not establish universal acceptance limits for every grade, thickness, service environment or welding process. A threshold can be meaningful only within its stated application. For example, TWI’s 2024 guidance on NACE hardness requirements reports recommended carbon-equivalent limits below **0.43**, or **0.45 for components thicker than 1 inch**, when carbon exceeds **0.18 wt%**, in the context of controlling hard HAZs. Those figures should not be recast as universal limits for every grade, thickness, service environment, or welding process. ![Cross-section of a welded steel joint labelled with the weld metal, fusion boundary, HAZ and base metal.](/images/uploads/0c47757a-e8db-4c8b-a3ab-338c2c04977f/wiki-inline-a-labelled-cross-section-of-a-welded-steel-joint-showing-the-weld-metal-fusion-b-1520x1920.jpg)[](/images/uploads/0c47757a-e8db-4c8b-a3ab-338c2c04977f/wiki-inline-a-labelled-cross-section-of-a-welded-steel-joint-showing-the-weld-metal-fusion-b-2027x2560.avif "Enlarge image — Cross-section of a welded steel joint labelled with the weld metal, fusion boundary, HAZ and base metal.")The HAZ is the region where the welding thermal cycle can create a harder, more crack-sensitive microstructure. ### The relationship between hardenability, heat-affected zones and cracking During welding, the HAZ is heated above transformation temperatures and then cooled. A composition with greater hardenability can form a harder microstructure as cooling becomes sufficiently rapid, especially in the coarse-grained region near the fusion boundary. The relevant danger is not hardness by itself. A hard HAZ is less tolerant of diffusible hydrogen and local stress, so it can provide the conditions for delayed hydrogen-assisted cracking. Diffusible hydrogen **Diffusible hydrogen** Hydrogen that can move through weld metal and adjacent steel during and after welding, potentially concentrating at stressed or defective regions. Hydrogen cracking generally requires a combination of diffusible hydrogen, a susceptible hard microstructure, and tensile stress or restraint. CE mainly addresses the second factor indirectly by indicating hardenability. It does not measure hydrogen, quantify restraint, predict the cooling rate at every point in the joint, or establish the final HAZ hardness. A high CE can therefore signal greater attention to preheat, interpass control, low-hydrogen practice and heat input, but it cannot predict every weld outcome. A lower value reduces one concern without removing hydrogen cracking risk. #### From chemistry to a welding decision 1. **Select the formula** Match CEIIW, Pcm, CEq, CET or CEN to the steel population and assessment method. 2. **Verify the chemistry** Use the certified heat or controlled analysis and enter all required elements in the stated units. 3. **Calculate the index** Preserve the equation structure, retain guard digits and document assumptions. 4. **Assess the joint** Consider thickness, restraint, hydrogen control, thermal conditions and post-weld treatment. 5. **Set procedure controls** Use the result to review preheat, interpass temperature, heat input and inspection requirements. The practical sequence is therefore composition calculation followed by engineering assessment. The selected formula must suit the steel and the decision being made; the resulting index must then be considered with thickness, joint design, restraint, hydrogen control, thermal conditions and post-weld treatment. CE is useful precisely because it narrows attention to a real metallurgical risk. It becomes misleading when treated as the whole weldability assessment. ## The IIW Carbon Equivalent Formula: CEIIW ### The CEIIW equation and its elemental terms \[1\] \[1\] [**AWS D8.10M:2021**](https://pubs.aws.org/Download_PDFS/D8_10M_2021_PV.pdf). American Welding Society. AWS standard, 2021. The International Institute of Welding carbon equivalent, usually written **CEIIW**, estimates the combined effect of several alloying elements on steel hardenability. AWS D8.10M:2021 gives the equation as: CEIIW=C+Mn+Si6+Cr+Mo+V5+Ni+Cu15 D-Range guidance from the British Constructional Steelwork Association presents the same equation. In this expression, **C, Mn, Si, Cr, Mo, V, Ni and Cu** mean the elemental contents of carbon, manganese, silicon, chromium, molybdenum, vanadium, nickel and copper. The values are normally entered as **weight percentages**, not fractions and not atomic percentages. Carbon appears as a separate term, so each unit increase in carbon contributes directly to CEIIW. Manganese and silicon are divided by 6; chromium, molybdenum and vanadium are divided by 5; nickel and copper are divided by 15. The denominators are part of the empirical formulation. They are not conversion factors that can be changed to suit a particular grade. Hardenability **Hardenability** The tendency of steel to form harder transformation products to a given depth or under a given cooling condition; it is different from hardness itself. The equation reflects a metallurgical concern: increasing carbon and many alloying additions generally increases hardenability. During welding, a more hardenable steel can form a harder heat-affected zone under suitable cooling conditions. If diffusible hydrogen and restraint are also present, that harder zone may have greater susceptibility to hydrogen-assisted cracking. AWS guidance on carbon and low-alloy steels therefore treats chemical composition and hardenability as weldability considerations, alongside procedure and fabrication conditions. CEIIW is not the only carbon-equivalent formulation. TWI distinguishes CEIIW, Pcm, CEq and CEN as separate methods used in hydrogen-cracking assessment. Pcm, for example, was developed mainly for modern low-carbon steels containing approximately **0.11 wt% carbon or less**, so selecting it merely because it produces a convenient number can be misleading. AWS welding guidance also gives related expressions such as CE, Pcm and CET, each with different elemental weightings. ### How to substitute steel chemistry into the formula Use the certified heat analysis, product specification, or another controlled chemical analysis for the steel being evaluated. The chemistry must correspond to the material and product condition under consideration. Do not insert a grade name, nominal maximum, or decimal fraction unless the formula and source explicitly require that form. **Symbolic calculation method** Suppose a material certificate reports the following elemental percentages: - carbon: C=c - manganese: Mn=m - silicon: Si=s - chromium: Cr=r - molybdenum: Mo=o - vanadium: V=v - nickel: Ni=n - copper: Cu=u The calculation is then written: CEIIW=c+m+s6+r+o+v5+n+u15 First add the manganese and silicon percentages, then divide that sum by 6. Separately add chromium, molybdenum and vanadium, dividing by 5. Finally add nickel and copper and divide by 15. Add the three alloy-group contributions to the carbon value. This preserves the structure of the formula and makes transcription errors easier to identify. For a real calculation, the symbolic variables would be replaced by certificate values for the actual heat. A value reported as 0.20 wt% carbon is entered as **0.20**, not 20 and not 0.0020. If an element is not listed, its treatment should follow the governing material specification or assessment procedure; silently assuming zero can be unsafe where trace alloying affects the result. Product standards may also report ranges, while a certificate gives a heat-specific analysis. Those are different inputs and should not be mixed without stating the choice. Material grouping is a separate task. ISO 15608:2025 specifies a uniform system for grouping metallic materials for welding purposes, including steels and other alloy families. A steel may therefore have both an ISO 15608 group designation and a calculated CEIIW value, but the group number does not replace the calculation. ### Interpreting the result without false precision A CEIIW result is a composition-based indicator, not a complete welding decision. AWS D8.10M:2021 describes carbon equivalent as a **weldability guideline rather than an absolute parameter**. Reporting a result to several decimal places does not make the underlying prediction more exact. Certificate rounding, segregation, sampling, formula selection and actual welding conditions all limit what the number can show. A higher CE generally indicates greater hardenability and a need for closer welding-process review, but it does not by itself predict cracking. Limited evidence A higher CEIIW generally signals greater hardenability and a greater need to examine hydrogen control, preheat, interpass temperature and cooling behavior. It does not, by itself, specify a preheat temperature or prove that cracking will occur. The equation does not independently encode plate thickness, joint restraint, diffusible hydrogen, cooling rate, welding heat input, ambient conditions, post-weld heat treatment or the thermal cycle produced by a particular process. Thickness and restraint can change the cracking risk even when two steels have the same CEIIW. Conversely, different heat inputs can produce different heat-affected-zone cooling rates in the same grade. A procedure assessment must therefore consider the actual joint, consumable hydrogen level, welding sequence, preheat, interpass control and any post-weld treatment. Published limits also belong to their stated context. TWI reports recommended carbon-equivalent limits below **0.43**, or **0.45 for components thicker than 1 inch**, when carbon exceeds **0.18 wt%**, in guidance concerned with controlling hard heat-affected zones under NACE requirements. Those figures are not universal acceptance limits for every steel, thickness or welding process. CEIIW is useful for comparing compositions and identifying when closer procedure control is warranted; it should not be mistaken for a standalone weldability score. ## Why Several Carbon Equivalent Formulae Exist Carbon equivalent is not a universal weldability score. It is a composition-based estimate of how alloying elements affect hardenability and the risk of hydrogen-assisted cracking, but each equation was developed for a particular steel population, data set and cracking model. The numerical result therefore depends on more than the heat analysis: it depends on which equation is selected. AWS D8.10M:2021 describes carbon equivalent as a weldability guideline, not an absolute parameter. That qualification matters. A carbon equivalent does not by itself determine preheat, interpass temperature, heat input or post-weld treatment. Hydrogen level, restraint, thickness, cooling rate, joint detail and the selected welding process also affect the result. ### CEIIW, CEq, Pcm and CEN The International Institute of Welding equation, commonly identified as CEIIW, is widely associated with conventional carbon and low-alloy steels. AWS D8.10M:2021 gives it as: CEIIW=C+Mn+Si6+Cr+Mo+V5+Ni+Cu15 The elemental values are inserted as weight percentages. Thus, the carbon, manganese, silicon, chromium, molybdenum, vanadium, nickel and copper values must all use the same compositional basis. A closely related expression is often labelled CEq, or simply CE, in welding specifications and guidance. The AWS guidance for hot-rolled steel gives: CE=C+Mn6+Cr+Mo+V5+Ni+Cu15 This differs from the CEIIW expression by the treatment of silicon: silicon appears in the numerator with manganese in CEIIW, but is not included in this stated CE expression. Some documents use CEq as a general label for a conventional carbon-equivalent calculation, while others reserve it for a particular equation. The designation must therefore be read with its source, not treated as a fixed international definition. Pcm and CEN arose from attempts to represent different steel compositions and cracking behaviour more closely than one conventional expression can do. TWI’s discussion of hydrogen-cracking formulae distinguishes CEIIW, Pcm, CEq and CEN rather than presenting them as interchangeable names. That distinction reflects historical development. The equations assign different weights to carbon and alloying elements because the steels and experimental relationships behind them were not identical. CEN is a carbon-equivalent formulation with carbon-dependent treatment of alloying contributions. It was developed to cover a wider range of steels than the low-carbon population associated with Pcm, while retaining a composition-based link to hardenability and hydrogen-cracking susceptibility. CEIIW, CEq, Pcm and CEN can consequently produce different values for one certified heat. That is not a calculation error; it is the expected consequence of using different models. The formula name must accompany every reported value. “CE = 0.42” is incomplete unless the document identifies whether the value is CEIIW, CEq, Pcm, CEN or another specified index. Comparing an IIW value with a Pcm limit can lead to an invalid welding decision. Material grouping is a separate matter. ISO 15608:2025 specifies a uniform grouping system for metallic materials for welding, including steels and other alloy families. A steel’s ISO grouping designation does not replace carbon-equivalent calculation, and a carbon-equivalent value does not establish the ISO 15608 group. ### The low-carbon-steel context of Pcm \[2\] \[2\] [**What is the difference between the various carbon equivalent formulae used in relation to hydrogen cracking?**](https://www.twi-global.com/technical-knowledge/faqs/faq-what-is-the-difference-between-the-various-carbon-equivalent-formulae-used-in-relation-to-hydrogen-cracking). TWI. TWI Technical Knowledge, 2024. Pcm was developed primarily for modern low-carbon steels containing approximately 0.11 wt% carbon or less, according to TWI (2024). That applicability note should stay attached to the formula. Pcm is not automatically the correct substitute for CEIIW whenever a steel has a low numerical equivalent, nor should it be applied outside its intended composition range without guidance from the relevant standard or procedure qualification. AWS welding guidance gives the following Pcm expression: Pcm=C+Si30+Mn+Cu+Cr20+Ni60+Mo15+V10+5⁢B Here, B is boron in weight percent, and the boron term is essential rather than optional. Even a small boron concentration can make a measurable contribution because it is multiplied by 5. Omitting that term changes the defined index. Pcm places greater emphasis on carbon and treats several alloying additions differently from CEIIW. That construction suits the low-carbon, [microalloyed steels](/categories/microalloyed-steels "microalloyed steels") for which Pcm was developed, but it does not make Pcm a universal replacement for the IIW equation. For steels above approximately 0.11 wt% carbon, the engineer should confirm that the chosen Pcm method and its limit apply to the grade, product form and cracking assessment. ### CET as an alternative composition index CET is another composition index used in welding guidance: CET=C+Mn10+Mo10+Cr20+Cu20+Ni40 The AWS hot-rolled-steel guidance published in 2007 presents CET alongside CE and Pcm. Its coefficients differ from both, so the same heat will normally receive three different numerical results when all three equations are calculated. CET should therefore be paired with the limits, diagrams or procedure rules developed for CET, rather than converted informally into a CEIIW or Pcm value. The practical question is not which formula produces the smallest number. It is which index matches the steel population, governing standard and hydrogen-cracking model. AWS guidance identifies chemical composition and hardenability as weldability considerations for carbon and low-alloy steels; the resulting welding procedure must also control diffusible hydrogen, restraint, thickness, preheat, interpass temperature and heat input. In some applications, hardness requirements add another constraint. TWI reports recommended carbon-equivalent limits below 0.43, or 0.45 for components thicker than 1 inch, when carbon exceeds 0.18 wt%, in guidance aimed at controlling hard heat-affected zones. Such limits are conditional requirements, not evidence that one carbon-equivalent formula governs every steel or weld. ## Worked Calculation Method for Steel Chemistry A carbon-equivalent result is only meaningful when the formula, chemistry basis and reporting convention are known. AWS D8.10M:2021 identifies the International Institute of Welding equation as: CEIIW=C+Mn+Si6+Cr+Mo+V5+Ni+Cu15 AWS also describes carbon equivalent as a weldability guideline, not an absolute parameter. The result does not replace a welding procedure qualification or determine preheat by itself. Preheat, interpass temperature, heat input, diffusible hydrogen, restraint, thickness and post-weld treatment remain relevant. ### Reading carbon and alloying values from a chemical analysis Begin with the product specification and the actual chemical analysis. Confirm whether the reported values are heat analysis, product analysis or another contractual analysis, then identify the elements required by the selected equation. A mill test certificate may list carbon as C, manganese as Mn, silicon as Si, chromium as Cr, molybdenum as Mo, vanadium as V, nickel as Ni, copper as Cu and boron as B. The abbreviations should be checked against the certificate legend rather than assumed from column position. For the IIW equation, record carbon, manganese, silicon, chromium, molybdenum, vanadium, nickel and copper. None should be silently omitted. An element with a small concentration still contributes to the equation, and an omitted term can make the reported result irreproducible. Boron is not part of the IIW expression above, but it must be captured when a formula includes it. The AWS expression for Pcm is: Pcm=C+Si30+Mn+Cu+Cr20+Ni60+Mo15+V10+5⁢B The coefficient 5⁢B makes even a small boron value relevant. Pcm was developed primarily for modern low-carbon steels containing approximately 0.11 wt% carbon or less, according to TWI’s 2024 comparison of carbon-equivalent formulas. It should not be selected merely because it produces a convenient number. The values must be elemental percentages on the basis required by the formula, normally weight percent. A certificate value of 0.18 wt% carbon is entered as 0.18, not as 18 and not as 0.0018. If the analysis is given in mass fraction, parts per million or another unit, convert every required element before beginning. Do not mix a carbon value in wt% with boron in ppm. A certificate calculation is an arithmetic result obtained from reported chemistry. It is not a laboratory measurement of weldability, hardness or cracking susceptibility. The laboratory measured the composition, subject to its sampling and analytical uncertainty; the carbon equivalent is calculated afterward from those reported values. If the certificate gives a detection limit, range or “less than” result, the calculation should identify that limitation instead of presenting an unjustified exact value. ### Unit discipline and arithmetic order Write the selected formula before inserting numbers. This prevents a CEIIW calculation from being confused with Pcm, CEq or CET. AWS welding guidance for hot-rolled steel, published in 2007, gives CEIIW in the same form and also lists: CET=C+Mn10+Mo10+Cr20+Cu20+Ni40 These equations are not interchangeable. TWI identifies CEIIW, Pcm, CEq and CEN as different formulations used in hydrogen-cracking assessment. For an explicitly illustrative example, assume a hypothetical chemistry—not a named grade and not a sourced statistic—reported in wt% as C 0.18, Mn 1.20, Si 0.25, Cr 0.30, Mo 0.10, V 0.05, Ni 0.20 and Cu 0.25. Apply the IIW equation in grouped terms: CEIIW=0.18+1.20+0.256+0.30+0.10+0.055+0.20+0.2515 Illustrative CEIIW contribution The worked chemistry example yields contributions of 0.1800, 0.2416667, 0.0900000 and 0.0300000 before summation. Calculate each contribution first: C=0.1800 (Mn+Si)/6=1.45/6=0.2416667 (Cr+Mo+V)/5=0.45/5=0.0900000 (Ni+Cu)/15=0.45/15=0.0300000 Then sum the unrounded contributions: CEIIW=0.5416667 Reported to three decimal places, the result is **CEIIW = 0.542**, with the formula and chemistry basis stated beside it. The grouping matters. Dividing manganese and silicon separately can produce the same arithmetic result, but writing the prescribed groups makes the calculation easier to audit and less likely to acquire an accidental denominator. ### Rounding, reporting and traceability Retain more digits during intermediate steps than will appear in the final report. Rounding 1.45/6 to 0.24 before adding it changes the displayed result, and repeated early rounding becomes more significant when several terms are present. A practical report should preserve the certificate’s stated precision, retain guard digits through the calculation and round only the final value according to the governing specification or project convention. The record should include the heat or sample identification, source document, chemistry values, units, selected formula, intermediate contributions, unrounded result and final rounded result. If a value was converted from ppm or mass fraction, record that conversion. If an element was absent from the certificate, state whether it was treated as zero, bounded by a detection limit or referred back for clarification; never hide the assumption. Finally, keep carbon-equivalent calculation separate from material grouping. ISO 15608:2025 provides a uniform system for grouping metallic materials for welding purposes, including steels and other alloy families. A group designation does not replace a chemistry calculation, and a CE result does not establish the ISO 15608 group. Both may appear in a welding record, but they answer different questions. ## Carbon Equivalent and Hydrogen Cracking in the HAZ Carbon equivalent connects steel chemistry with hardenability, but it does not predict hydrogen cracking by itself. The International Institute of Welding equation identified by AWS D8.10M:2021 is: CEIIW=C+Mn+Si6+Cr+Mo+V5+Ni+Cu15 The elemental values in this expression are composition percentages by weight. A higher result generally indicates greater hardenability and a greater possibility of forming hard regions in the heat-affected zone (HAZ), particularly when welding conditions produce rapid cooling. AWS describes carbon equivalent as a weldability guideline, not an absolute parameter. That distinction matters because cracking depends on the weld thermal cycle, hydrogen level, and mechanical restraint as well as on chemistry. TWI distinguishes CEIIW, Pcm, CEq, and CEN as formulations used for hydrogen-cracking assessment. They do not apply with equal meaning to every steel population. For example, TWI states that Pcm was developed primarily for modern low-carbon steels containing approximately 0.11 wt% carbon or less. AWS guidance gives the related expressions: CE=C+Mn6+Cr+Mo+V5+Ni+Cu15 Pcm=C+Si30+Mn+Cu+Cr20+Ni60+Mo15+V10+5⁢B CET=C+Mn10+Mo10+Cr20+Cu20+Ni40 Selecting one equation should therefore follow the steel type, the governing standard, and the welding assessment being performed. ISO 15608:2025 groups materials for welding purposes; that grouping system is separate from calculating a carbon equivalent. ![Schematic of hydrogen-assisted cracking caused by hydrogen, a hard microstructure and tensile stress.](/images/uploads/44071dec-3c0d-4093-be64-32d6626cf4e5/wiki-inline-a-schematic-showing-the-three-conditions-required-for-hydrogen-assisted-cracking-1920x1920.jpg)[](/images/uploads/44071dec-3c0d-4093-be64-32d6626cf4e5/wiki-inline-a-schematic-showing-the-three-conditions-required-for-hydrogen-assisted-cracking-1920x1920.avif "Enlarge image — Schematic of hydrogen-assisted cracking caused by hydrogen, a hard microstructure and tensile stress.")Hydrogen-assisted cracking requires hydrogen, a susceptible microstructure and tensile stress or restraint. ### The three conditions behind hydrogen-assisted cracking Hydrogen-assisted cracking requires the coincidence of three conditions: diffusible hydrogen, a susceptible hard or brittle microstructure, and tensile stress or restraint. Remove one of these conditions and the probability of cracking falls, although no single control measure guarantees immunity. Diffusible hydrogen enters through moisture, contamination, damp flux, poorly stored electrodes, surface coatings, and other hydrogen-bearing sources. During welding, some hydrogen dissolves in molten metal and then migrates into the HAZ as the joint cools. It can collect at defects, inclusions, prior-austenite grain boundaries, or regions under high local stress. The relevant quantity is not simply the hydrogen in the consumable before welding, but the hydrogen that becomes available to diffuse through the weld and adjacent steel. The second condition is a susceptible microstructure. A hard HAZ can have reduced tolerance for hydrogen and local stress, especially when its transformation products are brittle. The third is tensile stress. Joint restraint, shrinkage, residual stress, applied load, and stress concentration at an unfused region or notch can provide the driving force for a crack. A high CE value raises concern mainly because it can increase hardenability; it does not establish that hydrogen is present or that restraint is sufficient to cause cracking. ### Hard microstructures and cooling rate The HAZ is heated above transformation temperatures and then cooled at a rate controlled by plate thickness, joint geometry, heat input, preheat, interpass temperature, and surrounding material. A high-hardenability steel may transform into hard martensitic or otherwise crack-sensitive regions during rapid cooling. The same chemistry can produce a less susceptible HAZ under a slower thermal cycle. Thickness affects this cycle because a thick plate or a highly conducting joint can draw heat away rapidly. Joint restraint also tends to rise with joint size, stiffness, and the number of components preventing free contraction. Heat input changes the width and cooling behavior of the HAZ, but increasing heat input is not a universal remedy: excessive heat can impair toughness, enlarge the HAZ, or affect distortion and residual stress. The procedure must balance these effects rather than treat CE as a pass/fail score. \[3\] \[3\] [**Complying with NACE hardness requirements**](https://www.twi-global.com/technical-knowledge/job-knowledge/complying-with-nace-hardness-requirements-119). TWI. TWI Job Knowledge, 2024. TWI’s NACE-related guidance illustrates the limited context of numerical CE advice. When carbon exceeds 0.18 wt% and the objective is control of hard HAZs, it reports recommended carbon-equivalent limits below 0.43, or below 0.45 for components thicker than 1 inch. Those figures are not universal acceptance rules for every steel, thickness, service condition, or welding process. They belong to that hardness-control context. ![Technician checking preheat on a clean steel joint beside stored low-hydrogen electrodes.](/images/uploads/925ccb15-9626-4982-8d3a-ceb168d5fbe7/wiki-inline-a-welder-measuring-preheat-on-a-restrained-steel-joint-before-welding-with-dry-l-1920x1094.jpg)[](/images/uploads/925ccb15-9626-4982-8d3a-ceb168d5fbe7/wiki-inline-a-welder-measuring-preheat-on-a-restrained-steel-joint-before-welding-with-dry-l-1920x1094.avif "Enlarge image — Technician checking preheat on a clean steel joint beside stored low-hydrogen electrodes.")Preheat, dry consumables and clean surfaces work together to reduce cracking risk. ### Preheat and hydrogen-control logic Preheat slows cooling, giving a hardenable HAZ more time to transform into less crack-sensitive products and allowing hydrogen to diffuse away from highly stressed regions before the joint reaches low temperature. Interpass control maintains the intended thermal cycle between weld passes. Both values must be specified and monitored as part of the qualified welding procedure, with attention to actual thickness, joint restraint, ambient conditions, and the steel’s chemistry. Hydrogen control begins before the arc is struck. Consumables must be selected and handled according to the applicable procedure and manufacturer’s storage requirements; low-hydrogen electrodes and fluxes can absorb moisture if drying, holding, or exposure controls are neglected. Joint surfaces should be free from water, oil, rust scale, paint, and other contaminants that can add hydrogen or create defects. Inspection timing is also part of the logic. Hydrogen cracks may form after welding rather than during the visible welding operation, so delayed inspection can be required where the procedure or governing code identifies a risk. The delay is not a substitute for preheat, sound consumable control, or restraint assessment. It gives a later examination a better chance of detecting delayed cracking. A carbon-equivalent result is therefore a starting point for procedure decisions. The final assessment must combine chemistry with diffusible-hydrogen control, cooling rate, thickness, restraint, heat input, preheat, interpass temperature, and any specified post-weld treatment. ## Applying CE to Named Steel Grades and Specifications ### Carbon and low-alloy structural steels Grade designation, heat chemistry, material grouping and carbon equivalent answer different questions. | Information type | What it tells the engineer | What it does not replace | |---|---|---| | Product grade | Required product properties and specification class | Heat-specific chemistry calculation | | Mill heat analysis | Reported elemental composition for the material being joined | Complete weldability assessment | | ISO 15608 group | Relationship to materials for welding-procedure qualification | Carbon-equivalent value | | CE calculation | Composition-based indication of hardenability and welding risk | WPS qualification or joint-specific controls | Carbon equivalent calculations can be applied to carbon steels and low-alloy structural steels when the welding assessment needs a composition-based indication of hardenability and hydrogen-cracking risk. Typical product specifications include ASTM A36/A36M, ASTM A572/A572M and EN 10025-2 grades such as [S275](/materials/gost/s275 " — composition, equivalents and standards") and S355. The designation identifies a class of material and its required product properties; it does not itself provide the composition of the particular heat being welded. For the International Institute of Welding method, AWS D8.10M:2021 gives: **CEIIW = C + (Mn + Si)/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15** The elemental values are inserted as weight percentages. Some welding guidance for hot-rolled steel writes the same expression without the subscript: **CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15**, because silicon is not included in that presentation. The calculation is an estimate of the combined effect of alloying elements on hardenability, not a direct measurement of weldability. That distinction matters even for familiar structural grades. A heat certified to ASTM A36/A36M or EN 10025-2 S355 must satisfy the chemistry limits and mechanical requirements of its specification, but the reported carbon, manganese, chromium, molybdenum, nickel, copper and vanadium contents may differ from another compliant heat. The resulting CE values can therefore differ while both materials carry the same nominal grade designation. AWS guidance on carbon and low-alloy steels identifies chemical composition and hardenability as weldability considerations. The practical consequence is that CE may help determine whether a procedure needs tighter control of preheat, interpass temperature, heat input, consumable hydrogen and cooling conditions. Thickness, joint restraint, ambient conditions and the method of hydrogen control also affect the assessment. A CE result cannot, by itself, select a welding procedure or establish that a joint is safe from delayed hydrogen cracking. Other expressions may be more suitable for a particular steel population or decision. AWS welding guidance published in 2007 gives **Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B** and **CET = C + Mn/10 + Mo/10 + Cr/20 + Cu/20 + Ni/40**, alongside the IIW equation. TWI reported in 2024 that Pcm was developed primarily for modern low-carbon steels containing approximately 0.11 wt% carbon or less. Selecting Pcm simply because it produces a convenient number would ignore the composition range and purpose for which the formula was developed. ### High-strength low-alloy and quenched-and-tempered steels HSLA steels require the same separation between grade identification and heat-specific calculation. Examples include ASTM A572/A572M and ASTM A709/A709M HSLA products, while quenched-and-tempered construction steels may be specified under ASTM [A514](/materials/material-no/1.8974 " — composition, equivalents and standards")/A514M, EN 10025-6 [S690Q](/materials/material-no/1.8931 " — composition, equivalents and standards") or another applicable product standard. These designations carry requirements for strength, toughness, delivery condition and chemistry, but they do not replace examination of the mill test report. Microalloying additions such as vanadium, niobium and titanium can affect precipitation, grain refinement and strength; chromium, molybdenum and manganese can affect hardenability and transformation behaviour. A formula weights these elements according to its construction. It does not reproduce the full thermal history of a weld or the resulting heat-affected-zone microstructure. In quenched-and-tempered steel, the welding thermal cycle can also alter the original tempering condition, so hardness, toughness and post-weld requirements may be as important as the CE value. For these steels, the applicable specification and welding standard govern the final assessment. A welding procedure qualification may need to demonstrate mechanical properties, hardness, toughness or delayed-cracking controls under specified essential variables. Preheat and interpass limits, heat input, bead sequence, consumable storage, restraint and post-weld heat treatment must be considered together. Where hardness control is central, TWI’s 2024 discussion of NACE requirements reports recommended carbon-equivalent limits below 0.43, or 0.45 for components thicker than 1 inch, when carbon exceeds 0.18 wt%; those values belong to that stated context and should not be transferred to every HSLA or quenched-and-tempered grade. AWS D8.10M:2021 describes carbon equivalent as a weldability guideline rather than an absolute parameter. That wording is especially important for [high-strength steels](/categories/high-strength-steels "high-strength steels"), where strength level, toughness requirements, plate thickness, restraint and delivery condition can change the welding risk without changing the nominal grade. ### Why grade designation alone cannot replace heat chemistry A grade designation defines a specification range or product requirement. A heat-specific CE calculation uses the actual reported elemental values for the material being joined. Those are different kinds of information. The first establishes what the product is required to meet; the second estimates how its measured chemistry may influence hardenability and welding controls. The correct workflow is therefore to identify the product standard, obtain the applicable heat chemistry, select a formula suited to the steel and cracking assessment, and then compare the result with the requirements of the welding procedure and governing code. The calculation should state whether the values are from a certified heat analysis, a product maximum, or another permitted source. Substituting specification maxima for measured values may produce a conservative screening result, but it is not the same as reporting the heat’s actual CE. Material grouping adds another layer rather than replacing the calculation. ISO 15608:2025 provides a uniform system for grouping metallic materials for welding purposes, including steels and other alloy families. A group designation supports procedure qualification and material control; it is not a carbon-equivalent formula. Final decisions must follow the applicable product and welding standards, supported by the actual chemistry and the complete set of welding conditions. ## Standards, Material Grouping and Welding Procedure Qualification ### ISO 15608:2025 and material grouping \[4\] \[4\] [**ISO 15608:2025**](https://www.iso.org/standard/88796.html). International Organization for Standardization. ISO standard, 2025. ISO 15608:2025 specifies a uniform system for grouping metallic materials for welding purposes. Its scope is broader than carbon and low-alloy steel: the system also covers [stainless steels](/categories/stainless-steels "stainless steels"), nickel and nickel alloys, aluminium and aluminium alloys, copper and copper alloys, titanium and titanium alloys, and other material families addressed by the standard. A material is assigned a group and, where applicable, a subgroup according to its specified chemical composition, product form, mechanical characteristics and metallurgical classification. The designation is used to organize welding procedure qualification and to determine when qualification data may apply to another material in the same permitted group or subgroup. For example, a structural steel identified by its product standard and assigned to an ISO 15608 group is not being given a weldability score. The group records its relationship to other materials for qualification purposes. That distinction matters because material grouping is a classification system, whereas carbon equivalent is a composition-based calculation. ISO 15608:2025 does not turn a group number into a CE value. Conversely, a CE result does not establish an ISO 15608 material group. The material certificate, product designation, applicable edition of ISO 15608 and governing construction code must still be checked. A group designation cannot replace the actual heat analysis. Two steels can fall within a related material grouping while differing in carbon, manganese, chromium, molybdenum, nickel, copper, vanadium, boron or other elements that affect hardenability and hydrogen-cracking risk. Thickness and delivery condition also affect the thermal cycle and the resulting heat-affected zone. Grouping simplifies the qualification framework; it does not erase those differences. ### Separating grouping from carbon-equivalent calculation Carbon equivalent should be selected for the welding question being asked, not treated as a universal number attached to a grade. AWS D8.10M:2021 identifies the International Institute of Welding equation as CEIIW=C+Mn+Si6+Cr+Mo+V5+Ni+Cu15. The elemental values in this and the other equations are normally entered as weight percentages. A 0.20 wt% carbon value therefore enters the equation as 0.20, not as 20. The result is useful for judging hardenability and the likelihood that a weld heat-affected zone will become susceptible to cracking under particular hydrogen, restraint and cooling conditions. Other formulations weight the elements differently. AWS guidance for hot-rolled steel published in 2007 gives: CE=C+Mn6+Cr+Mo+V5+Ni+Cu15, Pcm=C+Si30+Mn+Cu+Cr20+Ni60+Mo15+V10+5⁢B, and CET=C+Mn10+Mo10+Cr20+Cu20+Ni40. TWI’s 2024 guidance distinguishes CEIIW, Pcm, CEq and CEN rather than presenting them as interchangeable labels. Pcm was developed primarily for modern low-carbon steels containing approximately 0.11 wt% carbon or less. Applying it outside the material population and cracking model for which it was developed can give a misleading impression of precision. AWS D8.10M:2021 describes carbon equivalent as a weldability guideline, not an absolute parameter. That wording should control how the number is used. A CE calculation can support a decision to review preheat, hydrogen control, heat input or hardness risk, but it cannot approve a WPS by itself. The governing fabrication code may specify a particular equation, limit or procedure qualification route. Its requirements take precedence over a generic formula. The same point applies to published limits. TWI’s 2024 discussion of NACE hardness requirements reports recommended CE limits below 0.43, or below 0.45 for components thicker than 1 inch, when carbon exceeds 0.18 wt%, in the context of controlling hard heat-affected zones. Those figures are not a universal acceptance rule for every steel, thickness, service environment or welding process. ### Procedure qualification variables that CE cannot replace A qualified welding procedure connects the material to an actual joint and welding operation. It specifies the joint design and preparation, base-metal thickness range, welding position, process, polarity or current type, consumable classification, shielding gas where relevant, preheat, interpass temperature, heat input and travel conditions. It may also control hydrogen levels, bead sequence, restraint, cooling, post-weld heat treatment and hardness or cracking requirements. #### CE is not a welding-procedure approval Do not approve a WPS, waive preheat, or assume immunity from delayed hydrogen cracking from CE alone. Confirm the governing code, qualified procedure, joint restraint, thickness, consumable hydrogen control, thermal cycle and inspection requirements. CE informs that control system. A high CE can indicate greater hardenability and a need for tighter thermal and hydrogen controls, while a lower CE does not guarantee freedom from cracking. Thick sections cool differently from thin sections; highly restrained joints behave differently from lightly restrained joints; and a low-hydrogen consumable does not remove the effects of an unsuitable preheat or excessive restraint. The welding process itself changes the thermal cycle. Shielded metal arc welding, gas metal arc welding, submerged arc welding and flux-cored arc welding can deliver different heat input, hydrogen exposure and cooling rates even on the same grade. Consumable classification matters as well, since strength, diffusible hydrogen designation and deposited-metal chemistry are qualification variables, not terms contained in a CE value. Post-weld heat treatment can alter residual stress and temper hard regions, but its required temperature, holding time and heating or cooling rate must come from the applicable code and qualified procedure. Hardness testing, hydrogen-cracking inspection and delayed inspection may be mandatory where service conditions or specification requirements demand them. AWS guidance on carbon and low-alloy steels identifies chemical composition and hardenability as weldability considerations, but neither AWS nor ISO 15608:2025 treats CE as a substitute for procedure qualification. The practical sequence is therefore clear: identify the material and its ISO 15608 group, obtain the certified chemistry, calculate the CE formulation required by the governing code, then qualify and apply the WPS with the specified joint, thickness, process, consumable and thermal controls. CE is one input to that decision. It is not the qualification itself. ## Limits, Misuse and Practical Interpretation ### The error of treating CE as a universal threshold Carbon equivalent is a composition-based estimate of hardenability and hydrogen-cracking risk. It is not a complete weldability score, and it does not produce a pass-or-fail answer independent of the welding conditions. AWS D8.10M:2021 states this directly: carbon equivalent is a weldability guideline, not an absolute parameter. A common error is calling every carbon-equivalent calculation “CEIIW.” In AWS D8.10M:2021, CEIIW is specifically: CEIIW=C+Mn+Si6+Cr+Mo+V5+Ni+Cu15 The elemental values are normally entered as weight percentages. That designation should not be applied automatically to Pcm, CET, CEq, or CEN. The equations reflect different steel populations and different attempts to represent hardenability or hydrogen-cracking susceptibility. Pcm illustrates the distinction: Pcm=C+Si30+Mn+Cu+Cr20+Ni60+Mo15+V10+5⁢B TWI reported in 2024 that Pcm was developed primarily for modern low-carbon steels containing approximately 0.11 wt% carbon or less. It gives silicon and several alloying elements different weighting from CEIIW, and it explicitly includes boron. Omitting boron is not a harmless simplification when the steel contains a deliberate boron addition. Conversely, comparing a Pcm value directly with a CEIIW limit as though both numbers had the same meaning is technically unsound. Thresholds also belong to their source and application. TWI’s 2024 discussion of NACE hardness requirements reports recommended carbon-equivalent limits below 0.43, or 0.45 for components thicker than 1 inch, when carbon exceeds 0.18 wt%, in the context of controlling hard heat-affected zones. That statement does not establish a universal limit for every grade, joint, thickness, hydrogen level, restraint condition, or service environment. A limit from a sour-service requirement cannot simply be transferred to a bridge fabrication procedure, nor can a limit from one material standard replace the governing welding code. The calculation itself may be precise while the conclusion is wrong. Carbon content, cooling rate, diffusible hydrogen, restraint, heat input, preheat, interpass temperature, and post-weld treatment all influence the actual risk. ### When formula selection changes the engineering conclusion Formula selection matters most near a procedural boundary. Consider a low-carbon microalloyed steel with a small boron addition. CEIIW may produce a moderate result because boron is absent from that equation, while Pcm increases through the 5⁢B term. If the engineer uses Pcm for a hydrogen-cracking assessment but calculates it without boron, the result can understate the intended effect of the composition. If the same value is then compared with a CEIIW threshold, two separate errors compound. The reverse problem occurs when CEIIW is applied to a steel outside the population for which another equation was developed. TWI distinguishes CEIIW, Pcm, CEq, and CEN as commonly used formulations for hydrogen-cracking assessment. They are not interchangeable labels. AWS guidance for hot-rolled steel published in 2007 lists CEIIW-type CE, Pcm, and CET separately: CET=C+Mn10+Mo10+Cr20+Cu20+Ni40 CET may therefore give a different ranking from CEIIW for the same heat, particularly when manganese, molybdenum, chromium, copper, or nickel contents are substantial. The difference is not a mathematical defect; it reflects a different empirical weighting and intended use. Chemistry quality is another source of changed conclusions. A mill certificate may provide a heat analysis, while a specification may show only nominal maximum or range values. Substituting nominal values without saying so can conceal whether the result is conservative, non-conservative, or merely illustrative. Actual heat chemistry should be used when the decision depends on a narrow margin. The report should state whether each value is an actual analysis, a specified maximum, a nominal composition, or an assumed value. Material grouping must also remain separate from carbon-equivalent calculation. ISO 15608:2025 specifies a uniform system for grouping metallic materials for welding purposes, covering steels and other alloy families. A group number helps determine whether an existing welding procedure may apply; it does not replace a CE calculation or prove that two steels have equivalent cracking behavior. ### A decision checklist for engineers and inspectors #### Carbon-equivalent review checklist - **Formula and source** Identify CEIIW, Pcm, CET, CEq or CEN and the document that governs its use. - **Chemistry basis** Record the heat analysis, units and treatment of missing or bounded values. - **Steel and thickness** Record the grade, product standard, ISO 15608 grouping and controlling thickness. - **Cracking conditions** Assess diffusible hydrogen, restraint, cooling rate and hard HAZ risk. - **Welding controls** Document preheat, interpass temperature, heat input, delayed inspection and post-weld treatment. - **Decision record** State the calculated value, applicable limit, assumptions, deviations and resulting controls. Before accepting a carbon-equivalent result, document: - **The formula and source:** identify CEIIW, Pcm, CET, CEq, or CEN, and cite the applicable standard, AWS document, project specification, or technical guidance. - **The chemistry basis and units:** verify that C, Mn, Si, Cr, Mo, V, Ni, Cu, and B are entered as weight percentages where required; distinguish actual heat analysis from nominal or maximum values. - **The steel family and designation:** record the grade and material standard, such as the relevant ASTM, EN, API, or other designation, and keep ISO 15608 material grouping conceptually separate from CE. - **The thickness and carbon content:** identify the controlling thickness and note whether carbon exceeds 0.18 wt% where a cited NACE-related limit is being considered. - **The formula’s intended population:** confirm whether the steel is a modern low-carbon material suited to Pcm or whether CEIIW, CET, or another expression is required by the governing document. - **The cracking conditions:** assess diffusible hydrogen, consumable control, joint restraint, fit-up, thermal history, cooling rate, and the risk of hard heat-affected zones. - **The welding controls:** determine preheat, interpass temperature, heat input, hydrogen precautions, delayed inspection, and any post-weld heat treatment from the governing standard or qualified procedure. - **The decision record:** state the calculated value, applicable limit if one exists, assumptions, deviations, and the resulting welding controls. CE is most useful as one transparent input to a broader weldability assessment—not as a universal threshold or a substitute for an approved welding procedure. ## References 1. \[1\] American Welding Society. [AWS D8.10M:2021](https://pubs.aws.org/Download_PDFS/D8_10M_2021_PV.pdf). AWS standard, 2021. [](/wiki/calculated-values/carbon-equivalent-calculations#wiki-cite-ref-1) https://pubs.aws.org/Download\_PDFS/D8\_10M\_2021\_PV.pdf 2. \[2\] TWI. [What is the difference between the various carbon equivalent formulae used in relation to hydrogen cracking?](https://www.twi-global.com/technical-knowledge/faqs/faq-what-is-the-difference-between-the-various-carbon-equivalent-formulae-used-in-relation-to-hydrogen-cracking). TWI Technical Knowledge, 2024. [](/wiki/calculated-values/carbon-equivalent-calculations#wiki-cite-ref-2) https://www.twi-global.com/technical-knowledge/faqs/faq-what-is-the-difference-between-the-various-carbon-equivalent-formulae-used-in-relation-to-hydrogen-cracking 3. \[3\] TWI. [Complying with NACE hardness requirements](https://www.twi-global.com/technical-knowledge/job-knowledge/complying-with-nace-hardness-requirements-119). TWI Job Knowledge, 2024. [](/wiki/calculated-values/carbon-equivalent-calculations#wiki-cite-ref-3) https://www.twi-global.com/technical-knowledge/job-knowledge/complying-with-nace-hardness-requirements-119 4. \[4\] International Organization for Standardization. [ISO 15608:2025](https://www.iso.org/standard/88796.html). ISO standard, 2025. [](/wiki/calculated-values/carbon-equivalent-calculations#wiki-cite-ref-4) https://www.iso.org/standard/88796.html **At a glance** Primary purpose Estimate hardenability and welding risk from steel chemistry CEIIW equation C + (Mn + Si)/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 Pcm context Primarily modern steels with approximately 0.11 wt% carbon or less Status Weldability guideline, not an absolute parameter [Back to Calculated Values](/wiki/calculated-values "Calculated Values — Steel Wiki")