How to Correctly Measure Preheat and Interpass Temperature

I have had welding supervisors answer the question "How do you measure interpass temperature?" without a moment's hesitation: "With the temp stick, right after preheating." Both parts of that answer were wrong – not out of carelessness, but because that is how it was learned, and because with structural steel it rarely makes a difference. With armour steel, it makes a substantial difference. Interpass temperature gets measured in almost every shop. And in most of them, it still gets measured wrong.

That costs money in two ways. Measure too early, or at the wrong spot, and you believe yourself safe while still producing a softened heat-affected zone or cold cracks – invisible until the ballistic test or the audit reveals them. And in an audit, the measurement point is one of the first questions an accredited body asks. If you cannot answer it convincingly, you have a problem that has nothing to do with actual weld quality – but still costs you the qualification.

Why temperature is a window, not a single limit

With armour steel, minimum preheat prevents hydrogen cracking, while maximum preheat or interpass temperature prevents softening of the microstructure. Both limits must be met simultaneously – that is a window, not a single threshold. The harder the grade, the narrower that window becomes. For the hardest grades, the gap between minimum and maximum temperature is often so tight that there is only a narrow band in which welding can be done safely. That is why welding armour steel simply does not work without genuine temperature monitoring – "by feel" is not an option with a window that tight.

Additions to the table value kick in automatically under certain conditions: heavily restrained joints, low heat input at the bottom end of the usual energy range, and high humidity or low ambient or joint temperature. Anyone unaware of these additions falls below the temperature actually required without noticing.

The measurement procedure that almost always gets done wrong

This is the actual core of the problem, and it breaks down into a few clear rules.

• Measure on the thicker plate of the joint, not on the thinner one and not somewhere in between.
• Before measuring, observe a wait time of at least two minutes per 25 millimetres of plate thickness – otherwise you are measuring the surface, not the part.
• The minimum temperature must be reached within a zone of 75 millimetres on either side of the weld, not just right at the weld edge.
• Measure with a contact thermometer or thermocouple, not with temperature-indicating crayon alone.

Each of these four points looks trivial on its own. Together, though, they explain why two shops using identical preheating equipment can end up with completely different results. Measure the thinner plate instead of the thicker one, and you systematically overestimate the actual temperature reached in the critical cross-section. Measure without observing the wait time, and you are reading a surface temperature that rises and falls faster than the part's core temperature – basing a decision on a number that is not representative of the actual weld. Temperature crayon only shows whether a threshold has been reached or exceeded, not the actual temperature – for a narrow window with both an upper and a lower limit, that is not enough.

Why the heating method matters just as much

How you heat affects not only how fast the target temperature is reached, but also how evenly the part is heated. Electric heating mats or induction are clearly superior to a torch, because they heat the plate evenly. Localised, uneven heating with a torch barely slows cooling across the area that matters, and additionally generates residual stress and distortion – a side effect that is doubly expensive with armour steel, since hot straightening is off the table anyway.

A shop that switches to heating mats or induction therefore does not just solve a temperature problem – it simultaneously reduces the distortion it would otherwise have to fight, later, with limited means.

This switch is not really a fundamental technology change, more a question of procurement and training. Heating mats can be cut to fit the actual part geometry and deliver a reproducible area that can also be logged – an advantage that pays off directly during an audit. Anyone who has preheated exclusively with a torch until now should not leave the switch until the day before qualification, but start early, with enough time for trial welds and for training operators on the new equipment.

What this means in an audit

As part of DIN 2303 qualification, an accredited body asks specifically about the measurement point, the wait time, and the instrument used – not as formalism, but because this is exactly where the gap between the documented welding procedure specification and actual practice on the shop floor becomes visible. A shop that only has the measurement procedure on paper, but in daily practice measures with temperature crayon right after preheating, tends to get caught out during the technical interview. That costs not only time during the audit itself, but in the worst case the qualification, or at least a condition requiring corrective action.

What to do about it

The change is manageable once it is built firmly into the welding procedure specification and the shop equipment:

• Anchor measurement point and wait time in the welding procedure specification itself, not left to the operator's judgement.
• Make contact thermometers or thermocouples the standard instrument, with temperature crayon at most as a supplementary check.
• Switch to electric heating mats or induction wherever torches have been the default.
• Document additions for restraint, humidity, and low ambient temperature as a fixed part of the work instruction.

What this means for your shop

If you are not certain whether your welding supervision is capturing interpass temperature at the right spot, at the right time, and with the right instrument, that is exactly the point I examine on site – the welding procedure specifications and the practice actually in use on the shop floor. The Defense-Readiness-Check examines this systematically, over two days on site, for a fixed fee, and ends with a written roadmap. If you would like to clarify beforehand, with no obligation, whether that is the right starting point for your shop: initial call, 45 minutes, remote, free of charge – book a first conversation.