5.3 Carbon Equivalent, Hardenability & ISO/TR 15608 Groups

5.3 Carbon Equivalent, Hardenability & ISO/TR 15608 Groups

Quick Answer: Use carbon equivalent to judge hardenability risk from certificate chemistry. The widely taught IIW CE is CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Higher CE and thickness raise cold-cracking risk and often drive preheat and low-hydrogen practice. ISO/TR 15608 groups steels so a WPQR can cover a range of materials—inspectors confirm production materials sit inside that range.

WT2.8–2.9 connect material chemistry to welding procedure controls. At IWI-S level you calculate or interpret CE, explain hardenability in plain language, and read ISO/TR 15608 group numbers on WPS/WPQR documents.

Why a Single Number for Chemistry?

Carbon is the strongest common driver of hardenability and maximum martensite hardness in steels, but manganese, chromium, molybdenum, vanadium, nickel, copper, and other elements also delay ferrite/pearlite formation and promote hard structures on cooling. Fabrication needs a compact index for:

• Comparing heats of similar grades
• Setting preheat guidance (with thickness, hydrogen, and restraint)
• Explaining why two “mild steels” weld differently

That index is a carbon equivalent (CE) or related parameter (CET, PCM/Pcm, and others appear in literature and standards). Always use the formula required by the applicable code, client specification, or training syllabus when more than one is possible.

IIW Carbon Equivalent Formula

The IIW carbon equivalent commonly taught for C–Mn and many low-alloy steels is:

CE (IIW) = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

where element symbols are mass percent from the product analysis (or heat analysis, as specified by the document you are applying).

Worked example (illustrative)

Suppose a plate heat analysis (mass %):

• C = 0.18
• Mn = 1.20
• Cr = 0.15
• Mo = 0.05
• V = 0.02
• Ni = 0.20
• Cu = 0.25

Then:

• Mn/6 = 1.20/6 = 0.200
• (Cr+Mo+V)/5 = (0.15+0.05+0.02)/5 = 0.220/5 = 0.044
• (Ni+Cu)/15 = (0.20+0.25)/15 = 0.450/15 = 0.030
• CE = 0.18 + 0.200 + 0.044 + 0.030 = 0.454

Interpretation is always relative to thickness, joint type, hydrogen level, and standard guidance—not a magic pass/fail without context. Many teaching tables treat rising CE as rising need for preheat and hydrogen control, especially above roughly 0.40–0.45 for thicker restrained joints, but project documents govern numerical limits.

Practical inspector uses

• Check that certificate chemistry is within the WPS base-metal composition range when the WPS states limits
• Understand why a WPS mandates preheat for one grade/thickness but not another
• Support NCR discussion when cracks appear in high-CE, high-restraint welds with inadequate preheat or wet electrodes
• Avoid treating “low carbon” alone as safe if Mn and Cr+Mo+V terms are high

Silicon, boron, and niobium may be critical in some formulas or steels but do not appear in the classic IIW CE expression above—do not invent terms. If the WPS or code uses PCM (Ito-Bessyo) or EN 1011-style calculations, follow that document.

Hardenability Concept

Hardenability is the capacity of a steel to transform to martensite (or other hard products) to a certain depth under a given cooling rate. It is related to, but not identical with:

Alloying elements that slow diffusion transformations increase hardenability: the HAZ of a high-CE steel can form hard martensite even at moderate cooling rates and in thicker plate, whereas a low-CE steel may form softer ferrite–pearlite structures under the same weld cooling curve.

Cooling rate after welding depends on:

• Heat input / arc energy
• Preheat and interpass temperature (raise → slower cool)
• Thickness and joint geometry (heat sinks)
• Thermal conductivity and starting temperature

Procedure designers combine CE, thickness, and hydrogen potential to choose preheat, consumable type (basic low-hydrogen vs rutile/cellulosic), and sometimes postheat or PWHT. Inspectors verify those choices are executed and recorded.

Link to Preheat and Hydrogen Control

Preheat reduces cooling rate in the critical temperature range, lowers residual hydrogen concentration gradients by aiding diffusion out of the joint (when held appropriately), and reduces residual stress peak severity somewhat by lowering thermal gradients. Combined with:

• Low-hydrogen consumables and correct baking/handling of basic electrodes
• Controlled interpass temperature (minimum and maximum)
• Suitable heat-input window
• Good joint cleanliness (moisture, oil, rust scale)

…preheat is a primary engineering control against hydrogen cold cracking in hardenable HAZs.

Inspector checkpoints:

1. WPS states preheat method, measurement location, and minimum temperature for thickness/CE range.
2. Production shows measured preheat before welding (and after interruptions as required).
3. Consumable hydrogen classification and storage match WPS.
4. Certificate CE or chemistry is not outside the qualified envelope when limits apply.

If production material CE is higher than the WPQR coupon chemistry in a way that exceeds the allowed range of the qualification standard, the procedure may not cover the job—even if the “grade name” looks familiar.

ISO/TR 15608 Material Groups for Steels (Overview)

ISO/TR 15608 Welding — Guidelines for a metallic materials grouping system assigns materials to groups and subgroups for welding procedure qualification systems (used with standards such as ISO 15614 series). For steels, groups commonly referenced in training and WPS forms include:

Exact chemical and mechanical boundaries are those of the current ISO/TR 15608 text and the application standard’s use of it. IWI-S candidates must know the purpose and logic, not memorise every ppm limit. Subgroup numbers (1.1 vs 1.3, 8.1 vs 8.2, etc.) refine ranges of approval.

Other metals

ISO/TR 15608 also groups aluminium, copper, nickel, titanium, and other alloys (higher group numbers). This section focuses on steels for WT2 foundations; later materials chapters reuse the same grouping idea.

Why Grouping Matters for WPS Qualification Ranges

Welding procedure qualification is expensive. Standards therefore allow a WPQR on a test piece of material in group X (and thickness t, process, etc.) to cover production welding of materials in a defined range of groups/subgroups, subject to the rules of ISO 15614 (or ASME IX, AWS D1.1, etc., which use their own P-Number/Group-Number systems).

Consequences for inspectors:

• A WPS that says “ISO/TR 15608 group 1.2” does not automatically cover group 3 Q&T steel or group 8 austenitic stainless.
• Moving to a higher-strength or different metallurgy subgroup may require a new WPQR or a narrower essential-variable check.
• Thickness, pipe diameter, joint type, process, filler, and PWHT ranges are separate essential/non-essential variables—grouping is only one axis.
• Material delivery condition (normalised, TMCP, QT) can change subgroup and weldability even when carbon content looks similar.

On the shop floor, the inspector’s verification sequence is typically:

1. Read drawing/material list grade and standard.
2. Read mill certificate grade, heat, chemistry, mechanicals.
3. Confirm WPS parent metal: standard/grade and/or ISO/TR 15608 group/subgroup and thickness range.
4. Confirm consumable and preheat fit that material’s hardenability risk.
5. Hold production if certificate material is outside the WPS range—do not “stretch” groups informally.

Comparison Sketch: CE vs Group Number

Smart inspection uses all three: grade for purchase compliance, group for procedure coverage, CE/chemistry for cracking-risk awareness.

Exam Focus for IWI-S

Typical questions:

• Write or select the IIW CE formula and compute a simple CE
• Define hardenability vs hardness
• Explain why high CE + thick plate + high hydrogen → preheat and low-hydrogen practice
• State the role of ISO/TR 15608 groups in range of approval of a WPS/WPQR
• Recognise that group 1 C–Mn steels and group 8 austenitics are not interchangeable under one steel WPS

Exam tip: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Groups organise materials for qualification ranges; CE organises chemistry for hardenability and preheat thinking.