Preheat and Interpass Temperature: How to Measure and Control Heat in Welding
Preheat is one of the few welding variables that costs almost nothing to control and almost everything to ignore. On thin mild steel it rarely matters. On a 2-inch A572 base plate in February, skipping it is how a fabricator ends up grinding out a crack that appeared three days after the weld passed visual inspection. This guide covers what weld preheat and interpass temperature are, why procedure specifications call for them, how to measure and hold temperature correctly, and how the two requirements differ in practice.
What Is Weld Preheat?
Preheat is the controlled heating of a base metal to a minimum temperature before welding begins. It slows the cooling rate of the weld and the heat-affected zone, which reduces hydrogen cracking, hardness spikes, and residual stress in thick or high-carbon steel sections.
The mechanism is a timing problem. A weld deposited on cold steel loses heat into the surrounding mass almost instantly. That rapid quench transforms the heat-affected zone into hard, brittle martensite and traps diffusible hydrogen before it can escape. Hard microstructure plus trapped hydrogen plus residual stress is the three-ingredient recipe for hydrogen-induced cracking — the delayed kind that appears hours or days after the arc is out.
Preheat attacks all three at once: it stretches cooling time through the critical transformation range, raises the diffusion rate so hydrogen can migrate out of the joint, and narrows the temperature gradient between puddle and base plate to lower residual stress.
How much you need depends on material thickness, chemical composition expressed as carbon equivalent, and the hydrogen level of the filler metal. North American structural fabricators work to the prequalified preheat and interpass tables in AWS D1.1, the Structural Welding Code for Steel. As rough orientation: many carbon steels need no preheat below 3/4 inch, roughly 150°F from 3/4 to 1-1/2 inch, and 225°F or more above 2-1/2 inch — but the governing WPS always wins over a rule of thumb.
What Is Weld Preheat Used For?
Preheat and interpass control show up anywhere failure is expensive. Typical applications include:
- Structural steel fabrication — column splices, moment connections, and plate girders where code minimums are enforced by third-party inspection.
- Pressure vessels and piping — thick-wall joints where a crack is a containment failure, not a rework ticket.
- Heavy equipment repair — buildup and hardfacing on high-carbon wear parts that would crack on contact if welded cold.
- Cast iron repair — where preheat and slow cooling are the entire difference between a repair and a broken casting.
- Cold-weather field welding — most codes require warming the joint whenever base metal is below 32°F, regardless of thickness.
- High-strength low-alloy steels — quenched and tempered grades where both a minimum and a maximum interpass temperature apply.
There's a compliance dimension too. Preheat is a documented WPS variable, so it gets verified — inspectors trained under the practices maintained by the American Society for Nondestructive Testing will ask how and where temperature was measured. And because preheating means open flame or resistance heating near combustible material, the hot work requirements in NFPA 51B, the standard for fire prevention during welding and cutting, apply to the preheating operation itself.
How to Measure and Control Preheat
Most preheat failures are not failures to heat. They are failures to measure — heating the torch-side surface, reading it immediately, and calling it good while the far side of the plate sits 200 degrees colder.
1. Determine the required temperature from the WPS
Pull the minimum preheat and maximum interpass temperature from the governing welding procedure specification. If no WPS applies, work from the prequalified code tables using base metal group, thickness, and filler metal hydrogen designation. Low-hydrogen consumables reduce required preheat, which is a large part of why they exist.
2. Keep the consumables dry
Preheat controls hydrogen already in the joint; dry electrodes keep it from getting there. Low-hydrogen stick electrodes must be held in a rod oven at manufacturer-specified temperature once the hermetic can is opened, and exposure limits are measured in hours, not days. Midland stocks welding rod ovens alongside TIG rods and MIG and MAG wires.
3. Heat evenly, through the thickness
Apply heat over a zone extending at least 3 inches in every direction from the joint, and heat through the section rather than scorching the surface. Resistance heating pads give the most uniform result on heavy plate; oxyfuel rosebuds work but demand constant movement to avoid local overheating.
4. Measure correctly — and wait
Check temperature on the face opposite the heat source, or at least 3 inches from the point of application, and allow soak time so the reading reflects through-thickness temperature rather than a hot skin. Crayons, contact pyrometers, digital surface thermometers, and stick-on indicators all work; the tools live in our temperature testing category. Record the reading — on code work, an unrecorded measurement is the same as no measurement.
5. Hold interpass temperature between passes
Preheat is not a one-time event. Check before every pass. Falling below the minimum means reheating; climbing above the maximum means letting the joint cool before continuing. On quenched and tempered steels, exceeding the maximum degrades the properties the heat treatment paid for.
6. Control the cooldown
On crack-sensitive material, cover the joint with an insulating blanket so it cools slowly, or move into post-weld heat treatment or a hydrogen bakeout if the procedure calls for one. Preheat and cooling rate are one continuous problem. The general welding safety requirements in OSHA's welding, cutting and brazing standards apply throughout the cooldown, while the work is still hot enough to ignite surrounding material.
Building procedures from scratch? Our overviews of how MIG welding works and choosing tungsten electrodes for TIG cover the process variables that sit alongside preheat in a complete WPS.
Preheat vs Interpass Temperature
The two terms describe the same measured quantity at different moments, which is why they get used interchangeably — and why that causes problems. Preheat is a minimum you must reach before the first arc strike. Interpass temperature is a window, usually with both a floor and a ceiling, maintained across every subsequent pass of a multi-pass weld.
| Preheat Temperature | Interpass Temperature | |
|---|---|---|
| When measured | Before the first pass | Immediately before each subsequent pass |
| Limit type | Minimum only | Minimum and, on many alloys, a maximum |
| Heat source | External — pads, rosebud, or oven | Usually residual heat from the previous pass |
| Failure mode if too low | Hydrogen cracking, brittle heat-affected zone | Same, plus lack of fusion between passes |
| Failure mode if too high | Rarely an issue at preheat levels | Reduced toughness and strength in Q&T steels |
| Typical documentation | Recorded once per joint | Recorded per pass on code work |
The practical takeaway: hitting preheat and then walking away is only half the job. On a heavy multi-pass joint, interpass control is where most of the actual risk lives.
Weld Preheat FAQs
Preheat temperature is set by thickness, carbon equivalent, and filler metal hydrogen level, not by a single universal number. For common carbon steels many prequalified tables require no preheat below 3/4 inch, about 150°F from 3/4 to 1-1/2 inch, and 225°F or higher above 2-1/2 inch. Always follow the governing welding procedure specification.
Preheat is the minimum temperature the base metal must reach before the first pass, while interpass temperature is the range maintained between every subsequent pass of a multi-pass weld. Preheat is a floor only. Interpass usually has both a floor and a ceiling, because exceeding the maximum degrades toughness in quenched and tempered steels.
Yes. Most structural welding codes require warming base metal to at least 70°F whenever it sits below 32°F, even on thin sections that would otherwise need no preheat. Cold steel pulls heat from the weld faster and can carry surface moisture, which introduces hydrogen. In Michigan field work this requirement applies for much of the year.
Measure on the face opposite the heat source, or at least 3 inches from where heat was applied, after allowing soak time. Temperature-indicating crayons, contact pyrometers, digital surface thermometers, and stick-on indicators are all accepted methods. Reading the torch-side surface immediately after heating is the most common error and consistently overstates the through-thickness temperature.
A propane torch works for thin sections and small joints but struggles to heat heavy plate uniformly through its thickness. Oxyfuel rosebud tips handle moderate sections with constant movement. For plate above roughly 1 inch, resistance or induction heating pads give far more uniform results and hold temperature between passes without an operator standing there.
Skipping required preheat on thick or high-carbon steel typically produces hydrogen-induced cracking in the heat-affected zone. The dangerous part is the delay — cracks often appear 24 to 72 hours after welding, long after the joint passed visual inspection. The weld also cools into a harder, more brittle microstructure with elevated residual stress.
Ready to Get Started with Preheat Control?
Midland Tool has supplied Michigan's industrial and fabrication shops since 1962, and we stock the welding side of the house from Lincoln Electric, ESAB, Miller, and Hobart. Browse temperature testing equipment for crayons, pyrometers, and stick-on indicators, keep consumables dry with welding rod ovens, and stock up on TIG rods and MIG and MAG wires in the low-hydrogen classifications your procedures require. If you need procedure support or on-site weld testing, our team can help — that's what the last six decades have been about.
