Heat Treatment · Heat Treat

Magnet (Heat-Treat Reference)

A magnet is used to estimate critical temperature by watching for the moment steel stops attracting it. It is a rough, lower-bound indicator that suits simple carbon steels and should never be the only reference for a quench.

StageHeat TreatRank#20

Magnet (Heat-Treat Reference) — illustration

Specifications

At a glance
Type	Small permanent magnet
Use	Detect loss of magnetism
Applies to	Simple carbon steels
Limit	Rough indicator only
Method	Touch to heated steel
Not a soak	Cannot hold temperature

Technical parameters

Typical values for this class of tool
Curie temperature	About 770 °C for iron
Typical reading	Near lower austenitize
Steel relevance	1084, 1095, W2
Alloy steels	True temp often higher
Reading window	Narrow, watch closely
Accuracy	Rough, not precise
Best use	Beginner cross-check
Backup to	Thermocouple or Tempilstik

Heat TreatmentBeginner$ typical cost

Overview

A magnet is used in heat treatment as a rough indicator of the critical temperature, the point where steel changes crystal structure as it is heated. As carbon steel is heated, it loses its magnetism at the Curie temperature, which for simple carbon steels sits close to the lower end of the austenitizing range. A smith touches a magnet to the blade and watches for the moment it stops attracting; that point is taken as a sign the steel is approaching the temperature where it will harden. It is one of the oldest field tricks in blacksmithing.

When choosing, any small magnet works, but understand what it can and cannot tell you. The Curie temperature is a property of the iron, not of the hardening range, and for many alloy and stainless steels the true austenitizing temperature is well above the point where the steel goes non-magnetic. So the magnet is a lower-bound indicator: it tells you the steel is hot enough to be near critical, not that it has reached the correct soak temperature. Reading it also takes practice, because the transition happens over a range and the magnet can stick, drag, or drop suddenly.

In the workflow the magnet is a beginner aid and a backup, used mainly with simple carbon steels such as 1084 and 1095. It is checked as the blade heats, and when the steel goes non-magnetic the smith knows the transformation is underway and can judge the remaining heat by color. It cannot replace a thermocouple, a Tempilstik, or a controlled oven for alloy steels, and it is no substitute for a soak. Used honestly, it teaches what critical temperature feels and looks like, and it is a genuinely useful cross-check when the lights are dim or your eye is unsure.

Where it fits

This tool belongs to the Heat Treat stage of the build.

ForgeShapeGrindHeat TreatFinish

Safety

Before you use thisRead the manufacturer’s instructions. The points below are the hazards that come up most often with this class of tool.

Use tongs and a magnet on a handle or rod; touching a magnet to red-hot steel with bare hands causes serious burns.
Do not trust the magnet alone to decide when to quench, since steel that has gone non-magnetic may still be too cool.
Wear eye protection and keep the magnet clear of the quench tank, since a dropped magnet can splash hot oil.

Using it well

Treat the magnet as a lower bound: it tells you the steel is near critical, not that it has soaked at the right temperature.
Use it mainly on simple carbon steels; for alloy and stainless steels the true austenitizing temperature is often much higher.
Watch for the exact moment the pull drops off, then keep heating to color; the transition happens over a short, easy-to-miss range.
Pair the magnet with a Tempilstik or thermocouple, and never let it become your only temperature reference for a critical blade.

Compatible steels

Steels this tool is commonly used with, and which suit its working range.

1084 1095 W2 5160 80CrV2

Related processes

NormalizingHardeningAnnealing

Educational reference. Forge & Anvil is an independent information resource and does not sell tools, steel or knives. Specifications and temperature ranges quoted here are typical values for the category of tool described and vary between manufacturers — always work from the documentation supplied with your own equipment. Nothing here is professional safety, legal or engineering advice.