---
title: Preheat and Interpass Temperature: Meaning | Wolf Machine Reno
url: https://wolfmachinenv.com/glossary/preheat-and-interpass/
business: Wolf Machine & Fabrication, Inc.
address: 4812 Longley Ln, Reno, NV 89502
phone: (775) 359-5570
hours: Shop Monday-Friday 7:00 am-3:30 pm; closed Saturday and Sunday
type: Metal and plastic fabrication and machining (custom and CNC machining, welding, metal and plastic fabrication, gearbox and part repair, align boring, metal spray)
---

[Photo: Stock photo: a welder in leather gloves running a MIG bead]

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- Preheat and interpass temperature

# Preheat and interpass temperature

Preheat is the temperature a part is warmed to before welding, and interpass temperature is the range it is kept in between weld passes. Both slow the cooling of the weld and the metal beside it so the joint is less likely to crack.

Stock photo: a welder in leather gloves running a MIG bead

A cold, thick part pulls heat out of a weld very quickly. Warming it first lets the weld cool more slowly and gently, which matters for metals that turn hard and brittle when they cool fast.

## Why parts are warmed

Thick sections, cast iron and steels with more carbon or alloy are the usual candidates. A fast-cooling weld in those metals can make a hard, brittle structure in the [heat-affected zone](https://wolfmachinenv.com/glossary/heat-affected-zone/), and shrinkage stress can pull it apart. Warming the whole area evenly lowers the temperature difference and gives trapped hydrogen time to escape.

The amount of preheat depends on the metal, the thickness and the job. It is set by the welding plan or the base metal rather than by guess. Cast iron is warmed all over, not just at the crack, because a hot spot in a cold casting sets up the same stress the preheat is meant to prevent. Heavy steel sections are warmed for a different reason: they are so large that they would otherwise carry the heat away from the weld before it could spread.

## Interpass and slow cooling

Interpass temperature is the temperature of the weld area right before the next pass. A minimum keeps the part from cooling too far between passes, while a maximum prevents too much heat from softening steel or damaging stainless. After the last pass, a casting is often wrapped in insulation so it cools slowly and evenly, sometimes for hours. Opening it too early is a common way to crack a good weld.

On a cracked casting the whole procedure matters. See the page on [cast iron welding repair](https://wolfmachinenv.com/services/cast-iron-welding/) for how it is used.

### Sources

- [Heat-affected zone (overview)](https://en.wikipedia.org/wiki/Heat-affected_zone) (2026-10-18)
- [Cast iron (welding section)](https://en.wikipedia.org/wiki/Cast_iron) (2026-10-18)

### Talk to the shop

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|---|---|
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### Keep reading

- [Cast iron welding repair](https://wolfmachinenv.com/services/cast-iron-welding/)
- [Welding, all types](https://wolfmachinenv.com/services/welding/)
- [Steels and steel alloys](https://wolfmachinenv.com/materials/steel-and-steel-alloys/)

## See also

[Heat-affected zone (HAZ)](https://wolfmachinenv.com/glossary/heat-affected-zone/) [Rockwell hardness](https://wolfmachinenv.com/glossary/rockwell-hardness/) [Tolerance](https://wolfmachinenv.com/glossary/tolerance/)

## Where this comes up

[Metal spray, sleeve or weld: rebuilding a worn surface](https://wolfmachinenv.com/guides/metal-spray-sleeve-or-weld/) [Cast iron welding repair](https://wolfmachinenv.com/services/cast-iron-welding/) [Steels and steel alloys](https://wolfmachinenv.com/materials/steel-and-steel-alloys/)

[Photo: Stock photo: a lathe turning a bar, hot chips flying]

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Stock photo: a lathe turning a bar, hot chips flying