Hardening steel is not a single operation. Annealing, stress relieving, hardening and tempering form a chain; skip one link and the part either cracks, distorts, or fails to reach the hardness you expected. This article walks through the four basic heat treatments in order, in shop-floor language.
What is heat treatment, and what changes inside the steel?
Heat treatment means heating steel to a given temperature, holding it there, then cooling it at a controlled rate in order to change its internal structure. The shape stays the same and the composition stays the same, but because the grain structure changes, so do hardness, toughness and machinability.
The core of it is this: above roughly 723 °C steel turns into a structure called austenite, which holds carbon in solution. Cool slowly from there and the carbon comes back out, forming a soft structure. Cool fast and the carbon has no time to escape, so the structure is forced into martensite. Martensite is the hardest form of steel – and also the most brittle. The whole logic of hardening sits in those two sentences.
1. Annealing (softening)
Annealing means holding the steel between 600 and 800 °C for a long soak, then cooling it slowly. After this the steel is as soft as it gets, easy to machine, and in the best condition for later hardening.
Tool steels are supplied to you already annealed. The bar you buy is soft inside, so it turns and mills comfortably. There are two typical reasons you would need to anneal something yourself:
- You have to re-machine a part that was hardened previously.
- Welding, flame cutting or heavy cold forming has hardened the material locally.
Annealing is the foundation of heat treatment: it resets the structure and makes everything that follows predictable.
2. Stress relieving
Turning, milling, drilling and welding all build up invisible stresses inside the material. Those stresses release themselves the moment the part is free to move – which is exactly when it heats up in the hardening furnace – and the part distorts.
Stress relieving is done by holding the material between 600 and 700 °C and usually cooling it slowly. Before hardening this step is mandatory, especially on:
- parts where a lot of material has been removed,
- die components with thick and thin sections in the same part,
- work with deep pockets, holes or slots.
3. Hardening (quenching)
Hardening means soaking the steel somewhere between 780 and 1250 °C depending on the grade, then cooling it rapidly. The hardening temperature is the temperature at which that particular steel transforms to austenite, and it is different for every grade – it is never guessed, it is read from the material data sheet.
Why is preheating done in stages?
Throw a part straight into a 1000 °C furnace and the outer surface heats fast while the core stays cold. The difference creates stress and produces what are known as heating cracks. That is why heating is done in several stages: the part is held at an intermediate temperature until surface and core even out, then taken up to the next step.
The risk is greater still on high-alloy steels and on tools with complicated geometry; skipping preheat there means scrap.
How long should the part be soaked?
The accepted guide is half a minute per millimeter of thickness at lower temperatures and one minute per millimeter at high temperatures. So a 40 mm thick part is soaked for roughly 40 minutes at high temperature. The point is to let the core reach hardening temperature too.
How is the quenching medium chosen?
Cooling speed is chosen according to the alloy content. Alloying elements slow the transformation down, so the more alloyed the steel, the gentler the quench it needs:
| Steel | Quenching medium | Why |
|---|---|---|
| Unalloyed | Water | The transformation is fast, so the harshest quench is needed. This carries the highest cracking risk. |
| Alloyed | Oil or salt bath | A gentler quench is enough, and distortion and cracking risk drop. |
| High alloy | Air | The alloy delays transformation so much that even air cooling forms martensite. |
When quenching in water or oil, agitation matters. A part left motionless builds a vapor blanket around itself; that blanket blocks heat transfer and the area under it does not harden properly. Moving the part or the fluid breaks the blanket up.
In a salt bath, once the part reaches bath temperature it is taken out and cooling continues in air.
What happens immediately after the quench?
This is the most commonly skipped step on the shop floor. As soon as the part drops to 80 °C, before it has finished cooling, it goes into a furnace at 100-150 °C and is held there for a while. The purpose is to reduce the temperature difference between surface and core. On bulky parts especially, this equalizing step prevents cracks and keeps distortion to a minimum.
4. Tempering
Tempering means reheating the hardened part to a given temperature and cooling it again. On most tool steels this is repeated several times – a single temper is usually not enough.
Why is tempering essential?
- It removes stresses. The internal stresses created during quenching are released by tempering, and the risk of cracking falls.
- It brings toughness back. You give up a few points of hardness and in return the part survives impact. An untempered cutting edge breaks on the first knock.
- It transforms retained austenite. In high-alloy tool steels the martensite transformation does not finish at room temperature, so some austenite is left in the structure. This is called retained austenite. Tempering converts that remainder as well – which is exactly why one temper is not enough.
Secondary hardening: the point where hardness goes up
“Raise the tempering temperature and hardness drops” is the rule everybody knows – but it does not hold for every steel.
- In cold-work tool steels the rule really does apply: hardness falls steadily as the temperature rises. That is why these steels must be kept below 200 °C in continuous service. If the part exceeds its tempering temperature while working, it loses hardness permanently.
- In hot-work tool steels and high-speed steels (HSS) hardness starts climbing above 350 °C and peaks a second time around 550 °C. The cause is the precipitation of alloy carbides that were dispersed in the structure. This is called secondary hardening.
As a general rule, tool steels should be used below their own tempering temperature. If a die was tempered at 550 °C, its working temperature must stay below that.
The order of operations – summary
| Step | Operation | Temperature | Purpose |
|---|---|---|---|
| 1 | Annealing | 600-800 °C, slow cool | Reset the structure, make it machinable |
| 2 | Rough machining | – | Remove the bulk of the material |
| 3 | Stress relieving | 600-700 °C, slow cool | Release machining stresses, prevent distortion |
| 4 | Finish machining | – | Approach size, leaving a hardening allowance |
| 5 | Hardening | 780-1250 °C + quench | Form martensite |
| 6 | Equalizing | 100-150 °C once the part drops to 80 °C | Prevent cracking and distortion |
| 7 | Tempering | Per grade, usually several times | Relieve stress, restore toughness, convert retained austenite |
| 8 | Grinding / final size | – | Finish to size after hardening |
The six most common mistakes
- Skipping stress relief. This is the single biggest cause of distortion. When the part comes out of the furnace bent, the furnace gets the blame – but the stress was put there on the machine.
- Preheating in one step. On thick and complex parts this invites heating cracks.
- Leaving the part still during the quench. A vapor blanket forms and that area stays soft. Later it gets reported as “patchy hardness”.
- Delaying the temper. Leaving a quenched part overnight seriously increases the risk of cracking.
- Settling for a single temper. High-alloy steels retain austenite; a second or even third temper is needed.
- Measuring hardness on a hot part. A part fresh out of the furnace does not give a true reading, and scale and the decarburized layer must be cleaned off first.
Frequently asked questions
My part came out of the furnace distorted – why?
The most likely cause is skipped stress relief. The second is section difference: the thin part of the component cools faster than the thick part, and areas shrinking at different rates bend the piece. Softening the section transitions in the design and radiusing sharp corners reduces the risk.
Does the part change size after hardening?
Yes. Martensite occupies more volume, so the part grows by a few tenths of a millimeter. That is why precision surfaces are not finished to size before hardening; a grinding allowance is left and the final size is given by grinding afterwards.
Why is the core not as hard as the surface?
Because the core does not cool as fast. How deep a steel can harden is called its hardenability, and it increases with alloy content. Use an unalloyed steel on a thick part and only the outer skin hardens. The answer is to choose a more heavily alloyed grade. That subject is covered in detail in Quenching and Hardenability.
How much does hardness drop after tempering?
It depends on the grade and the temperature. On a low-alloy steel, tempering at 400 °C drops the hardness noticeably; on a hot-work tool steel the same temperature barely moves it, and around 500 °C it may even raise it. For an exact figure, look at the tempering curve for that material.
Which hardness test is used after heat treatment?
On solid parts Rockwell C (HRC) is the most practical. If a thin hardened layer or case depth is to be measured, the part is sectioned and microhardness in Vickers is used. On large castings, Brinell is preferred. If you need to convert a reading to another scale, the Hardness Conversion Calculator does that.