---
title: "Heat Treatment of Steel: Annealing to Tempering"
source: "https://www.laxconsteels.com/heat-treatment-of-steel/"
description: "Annealing, normalising, stress relieving, hardening, tempering, solution annealing and precipitation hardening: temperatures, cooling and what each changes."
---
# Heat Treatment of Steel: Annealing, Normalising, Hardening, Tempering and Precipitation Hardening

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Published 11 September 2026 10 min read

Heat treatment of steel is the controlled heating and cooling of the metal in the solid state to change its structure, and through the structure its hardness, strength, ductility, toughness, machinability and internal stress. The principal processes are annealing, which softens; normalising, which refines the grain; stress relieving, which removes residual stress without changing the structure; hardening, which produces martensite by quenching from the austenite range; tempering, which softens martensite to a usable toughness; solution annealing, which dissolves precipitates in austenitic stainless steel; and precipitation hardening, which strengthens a solution-treated steel by a low-temperature age. Each is defined by a temperature, a time and a cooling rate, and the condition a bar is delivered in is stated on its certificate by the name of the treatment it received.

## Why steel responds to heat

Steel is an alloy of iron and carbon, and iron changes its crystal structure with temperature. Below about 723 degrees Celsius the stable structure of plain carbon steel is body-centred cubic ferrite with the carbon held as iron carbide (cementite); above the transformation range, which for a 0.4 percent carbon steel lies between about 723 and 800 degrees Celsius, the structure is face-centred cubic austenite, which dissolves carbon freely. Heating into the austenite range and cooling slowly returns the steel to ferrite and carbide in a coarse or fine form according to the rate. Cooling fast enough traps the carbon and forms martensite, a hard, brittle, body-centred tetragonal structure. Every hardening and softening treatment of carbon and alloy steel is a way of choosing which of these structures the steel ends in, and how fine it is.

Alloying elements move the transformation temperatures and slow the transformations, which is why an alloy steel such as [EN19](https://www.laxconsteels.com/grades/en-19/) hardens fully in oil where a plain carbon steel of the same carbon content needs water and hardens only in a thin section. Austenitic stainless steels such as 304 are alloyed with enough nickel to stay austenitic at room temperature, so they cannot be hardened by quenching at all, and their heat treatment serves a different purpose.

## The processes in one table

*Heat treatment processes for steel with their temperatures, cooling and results*

| Process | Temperature | Cooling | Result |
| --- | --- | --- | --- |
| Full annealing (carbon and alloy steel) | 30 to 50 degrees Celsius above the upper transformation temperature, about 800 to 900 | Slow, in the furnace | Softest condition, coarse ferrite and pearlite, maximum ductility and machinability for low-carbon steel |
| Normalising | 850 to 950 degrees Celsius | Still air | Uniform fine grain after rolling or forging, properties slightly higher than annealed |
| Spheroidising | Just below the lower transformation temperature, about 680 to 720, for several hours | Slow | Carbide in rounded particles, the softest and most machinable state for medium and high carbon steel |
| Stress relieving | 550 to 650 degrees Celsius (carbon and alloy steel) | Slow | Residual stress from machining, welding or cold work removed; structure and hardness unchanged |
| Hardening | Austenitised at 800 to 900 (carbon and alloy), 950 to 1,050 (martensitic stainless) | Quenched in water, oil, polymer or air by grade | Martensite, maximum hardness, brittle until tempered |
| Tempering | 150 to 650 degrees Celsius | Air | Hardness reduced in exchange for toughness; the temperature sets the balance |
| Solution annealing (austenitic stainless) | 1,010 to 1,120 degrees Celsius | Rapid, in water or forced air | Carbides dissolved, full corrosion resistance and ductility restored |
| Precipitation hardening (17-4 PH) | Solution treated at about 1,040, then aged at 482 (H900) to 621 (H1150) | Air | Strength raised by fine copper-rich precipitates; higher age temperature gives lower strength and higher toughness |

## Annealing, normalising and stress relieving

Full annealing heats the steel above its upper transformation temperature, holds it long enough for the section to transform to austenite throughout, and cools it slowly in the furnace. The slow cool produces coarse pearlite and ferrite, the softest condition the steel can take, and it is used to make a hardened or cold-worked steel machinable or formable again. Normalising heats to the same range or slightly above and cools in still air. The faster cool gives a finer pearlite and a uniform grain, which is why normalising is the standard treatment after hot rolling and forging to remove the coarse and uneven structure that a large hot reduction leaves behind. A normalised steel is a little harder and stronger than the same steel annealed.

Spheroidising holds a medium or high carbon steel just below the lower transformation temperature for several hours, sometimes with a slight cycling about it, so that the lamellar carbide of pearlite breaks up into rounded particles. It is the treatment given to bearing and tool steels before machining and before hardening. Stress relieving is a lower-temperature treatment, 550 to 650 degrees Celsius for carbon and alloy steel, held for an hour per 25 mm of section and cooled slowly. It does not transform the structure and it does not reduce hardness materially; it allows the residual stresses of machining, welding, straightening or cold drawing to relax so that the part does not distort in later machining or in service.

## Hardening and tempering

Hardening austenitises the steel and then cools it faster than the critical rate at which pearlite would form, so that the austenite transforms to martensite instead. The quenching medium is chosen for the grade: water for plain carbon steel, oil for most alloy steels, polymer solutions between the two, and still or forced air for high-alloy and martensitic stainless grades. The hardness reached depends on the carbon content, about 55 HRC for 0.4 percent carbon and above 60 HRC for 0.6 percent and more; the depth to which the section hardens depends on the alloy content, which is the property called hardenability. An as-quenched steel is at its hardest and its most brittle, and it carries a high residual stress from the volume change of the transformation, so it is tempered as soon as it is cold.

Tempering reheats the hardened steel to a temperature below the transformation range, 150 to 650 degrees Celsius, holds it, and cools it in air. Low tempering temperatures, 150 to 250, relieve stress and retain most of the hardness, and are used for tools and bearings. Higher temperatures, 500 to 650, precipitate and coarsen the carbide, reduce the hardness to the 25 to 35 HRC range and raise the toughness sharply; this is the hardened and tempered condition in which [EN24](https://www.laxconsteels.com/grades/en-24/), EN19 and [4140](https://www.laxconsteels.com/grades/sae-4140/) are supplied for shafts, gears and bolts, with a tensile strength of 850 to 1,100 MPa. Plain carbon steel loses hardness faster with tempering temperature than an alloy steel containing chromium, molybdenum or vanadium, whose carbides resist coarsening; the alloy steel therefore keeps its strength at a tempering temperature that gives it full toughness, which is the reason the alloy is added.

## Hardening martensitic stainless steel

The martensitic stainless grades [410](https://www.laxconsteels.com/grades/aisi-410/), [420](https://www.laxconsteels.com/grades/aisi-420/) and [431](https://www.laxconsteels.com/grades/aisi-431/) carry 12 to 17 percent chromium and enough carbon to form martensite, and they are hardened by the same sequence as an alloy steel, at a higher temperature. The bar is austenitised at 950 to 1,050 degrees Celsius, quenched in oil or air (the chromium gives a hardenability high enough that air cooling hardens most sections), and tempered. The tempering range has a gap: between about 400 and 580 degrees Celsius the chromium carbides that precipitate reduce both the toughness and the corrosion resistance, so these grades are tempered either below 400, for hardness, or between 600 and 750, for toughness and machinability. 410 hardened and tempered at the lower range reaches about 40 HRC; 420, with more carbon, reaches 50 HRC and above and is the grade of cutlery and surgical instruments. The bar is normally supplied annealed for machining and hardened by the customer after machining, or supplied hardened and tempered to a stated strength.

## Solution annealing of austenitic stainless steel

Austenitic grades such as [304](https://www.laxconsteels.com/grades/aisi-304/) and [316](https://www.laxconsteels.com/grades/aisi-316/) carry enough nickel to hold the austenite structure down to room temperature, so cooling from any heat treatment forms no martensite and the grades cannot be hardened by quenching. Their heat treatment is solution annealing: heating to 1,010 to 1,120 degrees Celsius, holding until the section is at temperature, and cooling rapidly in water or forced air. The high temperature dissolves the chromium carbides that form when the steel is held or cooled slowly through 425 to 815 degrees Celsius, the sensitising range, and the rapid cool passes through that range before they can re-form. A solution-annealed bar has the full corrosion resistance and ductility of the grade, and it is the condition in which hot rolled stainless bar is supplied, usually followed by pickling. Cold drawn bar that has hardened in the draw is solution annealed again where the annealed properties are required. Post-weld solution annealing is required where a part in a standard carbon grade such as 304 or 316 has been welded or hot formed and will meet a medium that attacks sensitised material; it is not given to every welded part, and the low-carbon grades 304L and 316L and the stabilised grades [321](https://www.laxconsteels.com/grades/aisi-321/) and [347](https://www.laxconsteels.com/grades/aisi-347/) exist so that fabrications too large for a furnace can be welded and used without it. The rapid cool is the point on which the treatment turns: a slow cool through the sensitising range undoes what the high temperature did.

## Precipitation hardening of 17-4 PH

Precipitation hardening strengthens a steel by precipitating a fine dispersion of a second phase from a supersaturated solid solution at a temperature far below the transformation range. [17-4 PH](https://www.laxconsteels.com/grades/17-4-ph/), the most used grade of the family, is a martensitic stainless steel containing about 3 to 5 percent copper. It is solution treated at about 1,040 degrees Celsius and cooled to room temperature, which leaves it martensitic but comparatively soft (Condition A, about 35 HRC or below) and machinable. A single ageing treatment then hardens it. At 482 degrees Celsius for one hour, the H900 condition, copper-rich precipitates form throughout the martensite and the steel reaches a minimum tensile strength of 1,310 MPa, a 0.2 percent proof stress of 1,170 MPa and a hardness of about 44 HRC. At 621 degrees Celsius for four hours, the H1150 condition, the precipitates coarsen and the strength falls to a minimum of 930 MPa tensile and 725 MPa proof, with a hardness near 33 HRC and a toughness and stress corrosion resistance much improved. The intermediate conditions H925, H1025 and H1075 lie between. Because the age involves no quench, the dimensional change it produces is small and consistent, a contraction of roughly 0.05 percent on ageing to H900 and about 0.1 percent to H1150, so a part can be machined in Condition A to close to finished size and that change allowed for on the drawing. Freedom from the distortion a quench produces is the reason the family is specified for pump shafts, valve stems and aerospace fittings. The [precipitation hardening stainless steel](https://www.laxconsteels.com/understanding-precipitation-hardening-stainless-steel-process-types-and-benefits/) article describes the family, and [15-5 PH](https://www.laxconsteels.com/grades/15-5-ph/) is the higher-toughness variant.

## How the treatment appears on the certificate

The delivery condition is a required entry on a mill test certificate to EN 10204. It names the treatment in the words of the product standard: annealed, normalised, hardened and tempered, solution annealed, or the ageing condition for a precipitation hardening grade. EN 10088-3 codes the condition on stainless bar (+AT for solution annealed, +QT for quenched and tempered, +P for precipitation hardened), and ASTM A276 and A564 name it as Condition A, T, H or by the ageing designation. The mechanical test results reported beside the condition were obtained on a sample in that condition, so a certificate for an annealed bar says nothing about the strength the same bar will show after the customer hardens it; the hardened values come from the product standard or from a test on the hardened part.

## Heat treatment at a bar mill

Laxcon Steels supplies [hot rolled round bar](https://www.laxconsteels.com/products/hot-rolled-round-bars/) in the annealed and pickled condition, [precipitation hardening steel bar](https://www.laxconsteels.com/products/precipitation-hardening-steels/), and the martensitic and alloy grades listed in the [grade reference](https://www.laxconsteels.com/grades/), with the delivery condition named on the certificate. The heat treatment furnaces and the testing that follows them are described on the [technology](https://www.laxconsteels.com/technology/) page.

## Frequently asked questions

### What is the difference between annealing and normalising?

Both heat the steel above its transformation temperature. Annealing cools it slowly in the furnace and gives the softest, most ductile condition; normalising cools it in air and gives a finer grain with slightly higher strength and hardness. Normalising is the usual treatment after rolling or forging, and annealing is the usual treatment before heavy machining or forming.

### Can 304 stainless steel be hardened by heat treatment?

No. The nickel in 304 and the other austenitic grades holds the austenite structure down to room temperature, so quenching from any temperature forms no martensite. They can be hardened only by cold work, and their one heat treatment, solution annealing, softens them and restores their corrosion resistance. The stainless grades that harden by heat treatment are the martensitic grades such as 410 and 420 and the precipitation hardening grades such as 17-4 PH.

### Why is steel tempered after hardening?

As-quenched martensite is at its hardest but brittle and carries high internal stress, so a part left untempered can crack in the quench or in service. Tempering at 150 to 650 degrees Celsius trades some of the hardness for toughness, and the temperature chosen sets where on that trade the part sits: low for tools and bearings, high for shafts and bolts.

### What does H900 mean on a 17-4 PH certificate?

H900 is the ageing condition: the solution-treated bar was held at 900 degrees Fahrenheit, 482 degrees Celsius, for one hour and air cooled. It is the highest-strength condition of the grade, with a minimum tensile strength of 1,310 MPa. H1025, H1075 and H1150 are aged at higher temperatures for longer and give progressively lower strength with higher toughness.

## Sources

- ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and Processes.
- ASTM A564/A564M, Standard Specification for Hot-Rolled and Cold-Finished Age-Hardening Stainless Steel Bars and Shapes.
- ASTM A276/A276M, Standard Specification for Stainless Steel Bars and Shapes.
- EN 10088-3, Stainless steels: technical delivery conditions for semi-finished products, bars, rods, wire, sections and bright products.
- EN 10083-1 and EN 10083-3, Steels for quenching and tempering.
- EN 10204, Metallic products: types of inspection documents.

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