Preheat exists to minimise the risk of hydrogen assisted cold cracking, HACC. It needs four things at once: hydrogen, a susceptible microstructure, tensile stress, and low temperature. Take any one of them away and the risk is reduced.
AS/NZS 1554.1 works the minimum preheat out from the weldability of the steel, the heat sink from the joint, and the energy going in the joint from the welding process. That last one is heat input, and it’s the one people don’t really consider when determining preheat.
Put more heat input in and the required preheat comes down. Sometimes to nil.
Knowing when you have to preheat, and when the numbers say you don’t, changes what a job costs. Every degree of preheat is time, gas, and someone standing there with a torch instead of welding. Guessing is expensive.
How preheat stops HACC
All four have to be present at the same time. Temperature is the one you set at the joint, and raising it goes to work on the other three.
- Hydrogen. A warm joint cools slower, so hydrogen has time to diffuse out of the weld and the HAZ instead of being trapped in it.
- Susceptible microstructure. Slowing the cooling rate through the transformation temperature keeps the HAZ off the hard end. Above roughly 350 HV you are into the susceptible range.
- Tensile stress. A gentler thermal gradient means less distortion, and less residual tensile stress locked into the joint as it cools.
One control, three effects. That is why preheat works, and why applying it by habit on joints that do not need it is expensive. It is also why a heavily restrained joint can still crack at the preheat the chart gives you. Restraint drives the tensile stress leg, and the chart does not know about it.
When this applies, and when it does not
Use this for: carbon and carbon-manganese structural steels welded to AS/NZS 1554.1. The standard’s scope is steel with a specified minimum yield strength not exceeding 500 MPa.
Do not use this for:
- Pressure vessels and pressure piping. Clause 1.2 excludes them outright. AS 4041, AS 1210 and ASME B31.3 have their own requirements and they are not interchangeable.
- Quenched and tempered steels. They go to AS/NZS 1554.4.
- Boron-containing steels. CE(IIW) has no boron term, so a boron steel can be badly mis-assessed. HERA’s advisory is to add roughly 30 °C for butt welds up to 90 mm combined thickness, and 50 °C above that.
- Steels not listed in Table 5.3.4(A). Clause 5.3.4 does not assign them a group number directly. Qualify the procedure under Section 4, or have the composition assessed properly before you weld it.
- Anything where a client specification overrides the standard. Read the spec first.
One more limit worth knowing. Note 2 under Clause 5.3.4 states that the preheat prediction methods are designed to minimise heat-affected zone cold cracking. They do not address weld metal cold cracking. If you hit that, the standard’s guidance is that procedure modifications may be needed, including preheat beyond the predicted figure and lower-hydrogen consumables. It flags weld metal cold cracking as more likely on multi-pass welds in restrained plate over 20 mm with high heat input runs.
That distinction gets missed constantly. The chart is not a cracking guarantee, it is a HAZ cold cracking control.
How AS/NZS 1554.1 actually does it
Unlike the AWS D1.1 prequalified table, which is a flat lookup on grade and thickness, AS/NZS 1554.1 runs a six-step method built on continuous nomographs with energy input as an axis. That is what lets you trade energy against preheat.
Step 1: get the weldability group number
Two routes, per Clause 5.3.4(a):
- (i) For a standard steel of known specification, read the group number straight off Table 5.3.4(A), Preheat Determination. It lists grades from AS/NZS 1163, AS 1397, AS 1450, AS 1548, AS/NZS 1594, AS/NZS 1595, AS 2074, AS/NZS 3678, AS/NZS 3679.1 and 3679.2.
- (ii) For a steel type listed in Table 5.3.4(A) where you have the actual ladle or heat analysis, calculate CE, add 0.01, then read the group number from Table 5.3.4(B), Relationship Between Carbon Equivalent and Group Number.
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
All contents in per cent.
The 0.01 is mandatory and it is easy to skip. Clause 5.3.4(a)(ii) requires it. It is not the same as the ISO/TR 17844 residuals allowance of 0.03, which applies in different circumstances and is not what this standard asks for. Use 0.01 here.
Group numbers run 1 to 12, spanning CE below 0.30 up to 0.80 and above. Higher number, poorer weldability.
Step 2: work out combined thickness
Combined thickness is the sum of the thicknesses of all the parts meeting at the joint, t1 + t2 + t3 + t4. It measures the heat sink you are fighting, which is why it matters more than any single plate thickness.
- Butt joint, two plates: t1 + t2
- T-joint or fillet, three legs: t1 + t2 + t3
- Cruciform, four legs: t1 + t2 + t3 + t4
Figure 5.3.4(A) carries the joint diagrams and two exclusion rules worth reading properly: for welds between t1 and t2 to t3, ignore t4 unless it is already welded; for welds between t2 and t3, ignore t1 unless it is already welded. In other words you only count steel that is actually connected and able to conduct heat away.
Where a member is tapered, t1 is the average thickness over 75 mm.
The point people miss: a 12 mm fillet on a T-joint is a 36 mm combined thickness problem, not a 12 mm one. Which is exactly why fillet welds on heavy sections crack when nobody expected them to.
Step 3: get the joint weldability index
Figure 5.3.4(A), Relation of Joint Weldability Index to Joint Combined Thickness and Group Number. Group number against combined thickness gives you an index letter, A to L.
Step 4: calculate the energy for the run
Equation 5.3.4(2): Q = (60 x E x I) / (1000 x V)
Same thing in plain terms: Arc Energy (Heat Input) = Amps x Volts x 0.06 / Travel Speed mm/min
Read the symbols carefully, because this standard’s notation is a trap:
| Symbol | In AS/NZS 1554.1 | Units |
|---|---|---|
Q | arc energy | kJ/mm |
E | arc voltage, measured at the welding head | V |
I | welding current | A |
V | travel speed | mm/min |
E is voltage, not energy. V is travel speed, not volts. That is the opposite of ISO 18491:2026, where U is voltage, E is arc energy and v is travel speed. Anyone moving between the two documents in one afternoon will eventually swap them. Check your working. Note also that voltage is specified at the welding head, not at the machine.
Step 5: read the minimum preheat off the chart
Two charts, and which one you use depends on the consumable and the process:
- Figure 5.3.4(B) – Preheating determination for hydrogen-controlled manual metal-arc electrodes and semi-automatic or automatic processes.
- Figure 5.3.4(C) – Preheating determination for manual metal-arc electrodes other than hydrogen controlled.
This is not a hydrogen scale, it is a two-chart split, and the split is not purely about hydrogen. Figure B covers hydrogen-controlled MMAW and all semi-automatic and automatic processes. So GMAW, FCAW and SAW go to Figure B as a matter of course. Figure C exists only for MMAW electrodes that are not hydrogen-controlled.
A terminology warning. The clause text and Equation 5.3.4(2) call Q arc energy. The axes of Figures 5.3.4(B) and (C) are labelled heat input. Same quantity, two names, in the same clause. What matters is that Q as defined by Equation 5.3.4(2) carries no thermal efficiency factor. Do not apply k of 0.8 or 0.6 before reading the chart just because the axis says heat input. See the heat input calculator for why the distinction is worth 20 to 25 per cent.
Step 6: iterate
Nobody uses this step and everybody should. If the preheat comes out inconvenient, go back to step 4, raise the energy, and read it again. Sometimes 40 °C of preheat disappears for the cost of slowing the travel speed. Sometimes it does not. You will not know unless you run it both ways. Where you want a preheat not determined by this method, Clause 5.3.4 requires the procedure to be qualified under Section 4.
Pulsed and waveform-controlled welding: read this before you trust the number
AS/NZS 1554.1 Clause 1.1 Note 2 states plainly that GMAW includes waveform controlled welding such as synergic, programmable and microprocessor controlled processes, giving pulsed spray transfer and controlled short circuit transfer as examples.
For those, Clause 5.3.4(e) Note says to use average voltage and average current in Equation 5.3.4(2), and adds that where arc energy needs to be limited, advice should be sought from the welding machine supplier on calculating arc energy in pulsed mode.
That hedge is doing a lot of work, and ISO 18491:2026 explains why. ISO 18491 Clause 6 sets out three determination methods. Calculating from voltage, current and travel speed is method A. For waveform-controlled welding, ISO 18491 states method A can introduce errors of up to 70 per cent, and that method B or C, based on instantaneous energy or instantaneous power, should be used instead.
So: AS/NZS 1554.1 tells you to use averages and check with your machine supplier. ISO 18491 tells you what happens if you do not. Both are current, both are right, and the practical answer is that if you are on a synergic or pulsed set, the Q you calculate off average volts and amps may not be the Q in the joint, and your preheat determination inherits that error.
If you are working to 1554.1 on a modern power source, get the energy off a meter that reports true energy or true power.
Worked example
Joint: butt weld, two 20 mm plates. Process: GMAW, 280 A, 29 V at the head, 350 mm/min travel. Ladle analysis: C 0.18, Mn 1.20, no other listed elements.
Carbon equivalent: CE = 0.18 + 1.20/6 = 0.38
Add 0.01 per Clause 5.3.4(a)(ii): CE = 0.39
Combined thickness: 20 + 20 = 40 mm
Arc energy: Q = (60 x 29 x 280) / (1000 x 350) = 1.39 kJ/mm
Then read the standard. CE 0.39 gives a group number from Table 5.3.4(B). That group against 40 mm on Figure 5.3.4(A) gives an index letter. That letter against 1.39 kJ/mm on Figure 5.3.4(B), because GMAW is a semi-automatic process, gives the minimum preheat.
I am not reproducing the tables or nomographs here. They are copyright Standards Australia and you need a licensed copy to work to the standard anyway. The calculator runs the method; the standard is what you cite.
The lever: if that comes back at a preheat you do not want, drop the travel speed to 250 mm/min. Q goes to 1.95 kJ/mm. Read it again.
Putting the range on the WPS
Note 3 under Clause 5.3.4 gives the convention for the permitted energy range on a WPS, and it is worth following because it is what an auditor will expect:
- Minimum arc energy from low amps, low volts, high travel speed
- Maximum arc energy from high amps, high volts, low travel speed
Qmin = (60 x Emin x Imin) / (1000 x Vmax) and Qmax = (60 x Emax x Imax) / (1000 x Vmin)
Straightforward once you see it, and routinely done backwards.
Related requirements in the same clause
Extent of preheat, 5.3.3. Applied so the full thickness reaches temperature, and maintained while welding is in progress. Measurement to AS ISO 13916.
Maximum inter-run temperature, 5.3.5. For structures subject to seismic loading using seismic resisting steels, groups 2S and 5S in Table 4.6.1(B), maximum inter-run is 300 °C unless the procedure is qualified higher.
Cooling. Uniform and as slow as practicable. Delayed cooling with insulation or heat may be worth it on extreme or complex joints.
Severe restraint, 5.7.1. Where welding under severe external shrinkage restraint, the joint is to be completed, or taken to a point that ensures freedom from cracking, before it is allowed to cool below the minimum preheat and inter-run temperature. Restraint is not an input to the chart, but the standard has not ignored it.
How this differs from the other methods
| AS/NZS 1554.1 Cl. 5.3.4 | EN 1011-2 Method A | EN 1011-2 Method B | AWS D1.1 Annex B | |
|---|---|---|---|---|
| Carbon equivalent | CE(IIW), plus 0.01 | CE(IIW) | CET | Pcm |
| Thickness input | Combined thickness | Combined thickness | Single plate thickness | Single thickness |
| Consumable split | Two charts by consumable and process | Five-point hydrogen scale A to E | Continuous, in the equation | H1 / H2 / H3 |
| Energy input | Yes, chart axis | Yes, nomogram axis | Yes, Q term | Not in the prequalified table |
| Restraint | Not a chart input | Implicit | Implicit | Explicit |
Two differences worth carrying in your head. AS/NZS 1554.1 and EN 1011-2 Method A both use combined thickness. Method B and AWS use single plate thickness. That makes 1554.1 more conservative on cruciform and T-joints, and it is the right call, because those joints genuinely do cool faster.
AWS D1.1’s prequalified table will not let you trade energy against preheat. You have to go to Annex B for that. If you are working to D1.1 and the preheat looks punishing, that is usually why.
On overseas steel: SCNZ’s guidance for EN-grade sections is to add 0.02 to the calculated CE before selecting the group number. That is a compensation for CE variability in imported product, not part of the standard. HERA has measured the practical divergence between the 1554.1 result and the EN methods at around 50 °C depending on settings and thickness.
Which carbon equivalent formula applies
CE numbers are meaningless without the formula that produced them.
CE(IIW) = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. Plain C-Mn and low-alloy structural steel, carbon above about 0.18 per cent, yield up to roughly 500 MPa, which is also 1554.1’s own scope limit. Valid range CE 0.30 to 0.70.
Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B. Modern low-carbon microalloyed, HSLA, TMCP and linepipe steels, carbon below about 0.12 per cent. Used in AWS D1.1’s hydrogen control method. CE(IIW) understates risk on these because it under-weights carbon.
CET = C + (Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40. High-strength, quenched and tempered and fine-grain steels, yield up to 1000 MPa, thickness 10 to 90 mm. EN 1011-2 Method B, and it returns a temperature directly. Watch the grouping: molybdenum pairs with manganese over 10, nickel stands alone over 40. The TWI FAQ page renders this one incorrectly, so do not use it as a source.
Standard and edition
AS/NZS 1554.1:2014, Structural steel welding, Part 1: Welding of steel structures. Ninth edition, 2014. Reissued incorporating Amendment No. 1 (September 2015) and Amendment No. 2 (September 2017). Prepared by Joint Standards Australia / Standards New Zealand Committee WD-003, Welding of Structures, superseding AS/NZS 1554.1:2011.
Amendments 1 and 2 both touched this clause. The Preface lists Clause 5.3.4 and the title of 5.3.5 among revised clauses, and Table 5.3.4 among revised tables. Clause 5.3.1 carries an A1 text-deleted marker. If you are working from a pre-2015 print, you are working from a superseded method.
Preheat sits at Clause 5.3.4, inside Section 5 Workmanship, at page 56. There is no preheat appendix. The 2014 appendices are A to H, with Appendix H, Selection of materials for the avoidance of lamellar tearing, new in this edition.
What is verified, what is derived, what is a modelling choice
Verified against the standard. Technoweld holds a licensed copy of AS/NZS 1554.1:2014 incorporating Amendments 1 and 2, read 16 August 2026. Confirmed directly: the clause and subclause structure of 5.3; the six-step method at 5.3.4; the CE equation and the mandatory 0.01 addition; the titles and function of Tables 5.3.4(A) and (B); group numbers 1 to 12; index letters A to L; the titles of Figures 5.3.4(A), (B) and (C) and which serves which case; combined thickness and the 75 mm averaging rule for tapered members; Equation 5.3.4(2) and its symbol definitions; the heat input axis labelling; the pulsed-mode note; the Qmin and Qmax convention; the HAZ versus weld metal cold cracking limitation; AS ISO 13916 for measurement; the 300 °C seismic inter-run limit; the 500 MPa scope limit; the pressure equipment exclusion; the edition and amendment record; and the appendix list.
Verified against ISO 18491:2026, licensed copy: the three determination methods and the up-to-70-per-cent method A error on waveform-controlled welding.
Derived: the worked example arithmetic. The comparison table, assembled from four source standards rather than lifted from one.
Modelling choice: the calculator implements the published method. Where the standard uses a continuous nomograph, the tool interpolates. Chart read-off by eye and software interpolation will not always agree to the degree, and the standard governs.
From secondary sources, flagged as such: the HERA boron and divergence figures, and the SCNZ 0.02 addition for EN-grade steel. Both are guidance, not clauses.
Limitations
A teaching and checking tool. Not a welding procedure, and it qualifies nothing.
Preheat is a minimum, not a target. Restraint is not an input to the AS/NZS 1554.1 chart and heavy restraint can crack a joint the chart says is fine. Consumable condition matters: a hydrogen-controlled electrode left out of the oven overnight is not hydrogen-controlled any more. The method controls HAZ cold cracking, not weld metal cold cracking.
Work to a licensed copy of the standard. Confirm against your WPS, your PQR and the acceptance criteria in the applicable code.
Related tools
- Heat Input Calculator – arc energy and heat input, and why they are not the same number
- Pipe Schedule and Wall Thickness
- Shielding Gas Waste Calculator
- Volts, Amps and the GMAW Arc
- GTAW Electrode Angle and Penetration
If a preheat decision is contested, or you have cracking a procedure says should not be happening, that is independent technical review rather than a calculator.