Heat input, carbon equivalent and preheat: the formulas and how they connect
Last reviewed
Short answer
Arc energy is volts times amps divided by travel speed. Heat input is arc energy multiplied by a process thermal efficiency factor k, which is 0.8 for MMAW, GMAW and FCAW, 0.6 for GTAW and PAW, and 1.0 for SAW and electroslag. In kJ/mm with travel speed in mm/min the formula is Q = (V x A x 60) / (1000 x S) x k. Carbon equivalent measures how hard the steel is to weld, with the IIW formula CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. AS/NZS 1554.1 Clause 5.3.4 combines the carbon equivalent band, the combined thickness of the joint and the arc energy to read a preheat temperature off a chart.
Key numbers
- Heat input
- Q (kJ/mm) = (V x A x 60) / (1000 x S) x k, with S in mm/min
- k for MMAW, GMAW, FCAW
- 0.80
- k for GTAW and PAW
- 0.60
- k for SAW, ESW and EGW
- 1.00
- Carbon equivalent (IIW)
- CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
- Pcm
- Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B
Arc energy and heat input are different numbers
Arc energy is the electrical energy delivered per unit length of weld. Heat input is the portion of that energy that actually enters the workpiece, after losses to radiation, spatter and the arc column. The ratio between them is the thermal efficiency factor k, and ignoring it overstates heat input by up to 40% on GTAW.
This matters because heat input limits on a WPS are what control cooling rate, and through it the hardness of the heat affected zone and the toughness of the weld metal. Reporting arc energy where the standard asks for heat input is a genuine conformance issue, not a rounding difference.
- Arc energy
- E = (V x A x 60) / (1000 x S) kJ/mm, with S in mm/min
- Heat input
- Q = E x k
- AS/NZS 1554.1 Equation 5.3.4(2)
- Q = (60 x E x I) / (1000 x v)
- AS/NZS 1554.3 arc energy
- Q = 60 x E x I / (1000 x V) kJ/mm
- Efficiency source
- EN 1011-1 thermal efficiency factors (typical values)
Carbon equivalent tells you how much trouble the steel will give you
Carbon equivalent condenses an alloy composition into a single number expressing hardenability, and therefore susceptibility to hydrogen assisted cold cracking. Higher carbon equivalent means a harder heat affected zone at a given cooling rate, which means more preheat is needed to slow that cooling.
Two formulas are in common use and they are not interchangeable. The IIW formula is the general purpose one and is what AS/NZS 1554.1 Equation 5.3.4(1) uses. Pcm is weighted for modern low carbon microalloyed steels, where the IIW formula tends to overstate the risk.
- IIW carbon equivalent
- CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15, elements in weight percent
- Pcm critical metal parameter
- Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B
- Where to get the numbers
- The material test certificate (MTC) for the actual heat, not the grade nominal
The AS/NZS 1554.1 preheat method, step by step
Clause 5.3.4 does not give a formula for preheat temperature. It gives a procedure that ends in a chart lookup, and each step needs a real input rather than a default.
- Resolve the steel group, 1 to 5, from the grade table or from the carbon equivalent
- Add 0.01 to the carbon equivalent and read the carbon equivalent group from Table 5.3.4(B)
- Work out the combined thickness of the joint and read the weldability index letter from Figure 5.3.4(A)
- Calculate arc energy Q from Equation 5.3.4(2)
- Read the preheat temperature from Figure 5.3.4(B) for low hydrogen consumables, or Figure 5.3.4(C) for non low hydrogen
- Combined thickness
- The sum of the thicknesses of the parts meeting at the joint, which is why a fillet on a thick flange needs more preheat than the plate alone suggests
- Chart arc energy range
- The 1554.1 charts cover roughly 1.0 to 6.0 kJ/mm; outside that range the lookup is extrapolation and should be treated as a warning
- Flat section effective thickness
- tE = min(W, 75) x t / 75
- Round bar effective thickness
- tE is approximately 0.01 x d squared
- Consumable hydrogen
- Low hydrogen and non low hydrogen use different charts, so the consumable choice changes the preheat before any calculation
Which method applies to which code
Preheat is one of the least portable requirements between codes. A preheat derived from AS/NZS 1554.1 is not a defensible answer on an ASME B31.3 line, and the reverse is equally true.
ForgeQM implements the AS/NZS 1554.1 Clause 5.3.4 chart method as the primary path, with AS/NZS 1554.5 sharing those charts on an advisory basis. AWS D1.1 and ASME B31.3 are provided as advisory transcriptions rather than mandate gates, and AS 4041 has no preheat temperature table at all, so the app says so rather than inventing a number.
- AS/NZS 1554.1
- Clause 5.3.4 chart method. Primary implemented path
- AS/NZS 1554.5
- Shares the 1554.1 charts, advisory
- AWS D1.1
- Simplified Table 3.2 and Annex I encoding, advisory only, not the licensed table
- ASME B31.3
- Table 330.1.1 transcription, advisory
- AS 4041
- No preheat temperature table in the standard. Refer to project WPS or B31.3
Frequently asked questions
Do I use arc energy or heat input on a WPS?
Use whichever the governing standard specifies, and label it explicitly. AS/NZS 1554 works in arc energy for its preheat charts. Codes that impose heat input limits for toughness usually mean heat input including the thermal efficiency factor. Because the two differ by up to 40%, an unlabelled number in kJ/mm is ambiguous and will be queried in audit.
What is a typical carbon equivalent limit for welding without preheat?
There is no single limit, because the risk depends on carbon equivalent, combined thickness, hydrogen level and arc energy together. As rough orientation, carbon equivalent below about 0.35 is comfortable and above about 0.45 needs careful preheat control, but those bands are guidance rather than a standard limit. AS/NZS 1554.1 deliberately requires the full Clause 5.3.4 procedure instead of a single threshold.
Why does combined thickness matter more than plate thickness?
Preheat is about cooling rate, and heat drains into every part meeting at the joint. A 12 mm plate welded to a 40 mm section cools far faster than two 12 mm plates, so it needs more preheat despite the thinner member being identical. Figure 5.3.4(A) uses combined thickness for exactly this reason.
Does ForgeQM calculate preheat automatically?
Yes for AS/NZS 1554.1. The preheat calculator resolves the steel group, applies the carbon equivalent grouping from Table 5.3.4(B), derives the weldability index from combined thickness, computes arc energy, and interpolates the Figure 5.3.4(B) or 5.3.4(C) chart. It warns when arc energy falls outside the charted range rather than silently extrapolating. Other codes are provided as advisory references.
Sources
This guide is written against the published standards below. Standards are revised, and project specifications override general guidance, so confirm clause numbers against the edition your contract names before relying on them.
- AS/NZS 1554.1 Structural steel welding, Part 1: Welding of steel structuresClause 5.3.4 preheat procedure, Equation 5.3.4(1) carbon equivalent, Equation 5.3.4(2) arc energy, Table 5.3.4(B) and Figures 5.3.4(A) to 5.3.4(C).
- EN 1011-1 Welding, Recommendations for welding of metallic materials, Part 1Typical thermal efficiency factors used to convert arc energy to heat input.
- AWS D1.1 Structural Welding Code, Steel and ASME B31.3 Process PipingAlternative preheat regimes, implemented in ForgeQM as advisory references rather than mandate gates.
- AS/NZS 3678, AS/NZS 3679.1, AS/NZS 1163 and related product standardsSteel grade designations used to resolve the steel group for the preheat lookup.
Related guides
- Welding Qualification Ranges ExplainedA qualification test on one plate qualifies a band of production work. How welder thickness, position, joint type, process and material group ranges are derived, where they differ from procedure ranges, and what triggers reapproval.
- WPS vs PQR vs WQR: What Each Record ProvesA PQR records a test weld and its results, a WPS turns that evidence into welding instructions with qualified ranges, and a WQR proves an individual welder can work to it. Written order, what carries between them, and the AS/NZS 1554.1 ranges.
- AS/NZS 1554 Weld Categories: SP vs GP vs FPSP and GP are weld categories in AS/NZS 1554.1, and FP belongs to AS/NZS 1554.5. What each category changes about qualification, workmanship, inspection extent and acceptance, and which part of the 1554 series applies to your material.
Stop checking this by hand
ForgeQM applies the AS/NZS 1554 series as checks while you write the record, not after. Qualification ranges are generated from the PQR, NDT extents come back with the clause they came from, and job data carries across every document through to the MDR.
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