Quenched and tempered steels

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Technical perfection for the ideal material

As a regular reader of our WE-Know-How articles, you already know that not all steel is the same. There are approximately 3,500 standardized types of steel worldwide, each with its unique applications.

Particular attention should be given to the group of tempered steels, which, as you might have guessed, derive their name from the complex process of tempering. This process allows the precise adjustment of essential physical properties such as hardness, toughness, tensile strength, and fatigue strength.

In this article, we will explain in detail how tempering works, why the different material properties are so crucial, and the practical applications of tempered steel.

The path to quenched and tempered steel:

quenching and tempering


The quenching and tempering process fundamentally consists of two steps that may seem quite simple at first glance. However, upon closer inspection, a multitude of details reveal themselves, each having a significant impact on the resulting material. Specifically, the process involves:

#1 Quenching

The name already indicates it: hardening a steel alters the metal's structure in such a way that its mechanical resistance increases. Metallurgy knows various hardening processes, but in the case of tempering, the so-called transformation hardening is almost always used.

Simply put, in transformation hardening, steel is first heated to 900 to 1,000 °Celsius, then kept constant at this temperature for a certain period (soaking), and then quenched.

The purpose of heating is to transform the ferrite (α-iron) into austenite (γ-iron), which means dissolving the carbon contained in the metal from its cubic structures. Upon subsequent rapid cooling, the carbon atoms recombine. However, this new structure is no longer cubic but, figuratively speaking, interlocked and thus significantly more stable than in the original material. Austenite transforms into martensite; the steel gains hardness.

Transformation hardening is precision work: the amount of martensite obtained depends primarily on the temperature difference before and after quenching and the time required for the process. Therefore, critical factors include the heating temperature and the nature of the cooling medium. Oil, water, or gases lead to entirely different results.

Also important is the chemical composition of the starting material. Alloying elements can significantly influence the material properties, and the initial carbon content of the steel, typically between 0.2 percent and 0.65 percent, is also crucial.

#2 Tempering

Tempering refers to reheating the steel after hardening. This time, however, without subsequent quenching and at lower temperatures of 80 to 550 °Celsius. The parameters selected are also decisive for the end result: how long and at what temperature the steel is tempered and which elements, such as chromium, manganese, nickel or molybdenum, are added to the original steel have a major influence on the physical properties of the end product.

The aim of tempering is always a further precise adjustment of the material properties: it allows the hardness of a steel to be subsequently reduced again and its toughness to be increased at the same time.

Only with the help of this two-stage process is it possible to produce lexibly and individually a tempered steel from a starting material that is 100% tailored to its intended use.

Hardness vs. toughness

What is it all about?


So much effort in hardening just to make the steel softer again afterwards? Indeed, because the hardness and toughness of a material are almost diametrically opposed to each other:

Hardness is the mechanical resistance that a material offers to the penetration of another body. A hard wood is insensitive to scratches; a hard drill head penetrates a concrete wall easily.

Toughness, on the other hand, is the resistance to breakage or cracking and consists of strength (ability to withstand mechanical stress before deformation) and ductility (ability to deform under shear stress before breaking). A metal wire should be able to bend without breaking; the shock absorbers of a car deform with every bump in the road.

If the hardness of a material increases, its toughness decreases; if it gains toughness, it loses hardness. In order to tailor a steel perfectly to its intended use, it is important to find the right balance between hardness and toughness:

A blade, for example, should be as hard as possible because its intended use is to penetrate other materials. However, it will break under a strong hammer blow. The axle of a car, on the other hand, toughness should not be lack, otherwise you would have to go to the car workshop with a broken axle after the first pothole.


Quenched and tempered steels in practice

Due to this versatility and their flexible physical properties, tempered steels are regularly used in numerous industries. For example, unalloyed quenched and tempered steels are used for components with lower loads, while alloyed quenched and tempered steels are indispensable in dynamic and statically highly stressed parts such as aircraft turbines or gear wheels in mechanical and plant engineering.

To give you an exemplary overview:

Automotive industry

In the automotive industry, alloyed heat-treatable steels such as AISI 4140 or AISI 4340 are used for block cylinders, industrial fittings, transmission gears and axles to ensure the required hardness for the drive train.

In aerospace

Heat-treatable steels such as AISI 4340 are used in engine components and structural parts of aircraft and spacecraft. These materials offer high load-bearing capacity while at the same time reducing weight.


Mechanical engineering

In the manufacture of machinery and equipment, materials such as AISI SAE 6150 or 4340 are used for block cylinders, industrial valves, gears, pinions and gear wheels to ensure durability and performance.


Medical technology

In medical technology, stainless tempered steels such as AISI 316LVM or AISI 631 are used for surgical instruments and implants. These materials are characterized by precision and biocompatible properties.


Without the versatility of quenched and tempered steels, technological leadership in key industries would be unthinkable, as selecting the right material for each application plays a key role in ensuring safety, performance and reliability in numerous industries.

Material testing

The final, all-important step:

The consistently high quality of our materials is the standard we set ourselves. Comprehensive and thorough inspection is essential, especially for quenched and tempered steels that are used in places where material failure can not only be costly, but can even endanger human life.

Our quality assurance department therefore monitors our entire production process from the first to the last step. Our quenched and tempered steels are also subjected to intensive material tests in our test laboratory. These include

  • The hardness test
  • The tensile test
  • The notched bar impact test

The hardness test

Hardness testing is defined as "a test to determine the resistance of a material to permanent deformation by the penetration of a harder indenter". Classic methods such as Brinell, Vickers and Rockwell are used here. The choice of the right method depends on factors such as the material, the hardness and the type of test material.

The tensile test

In the tensile test, a material sample is stretched until it breaks. Important characteristic values such as yield strength, tensile strength and elongation at break are determined. The tensile strength is the maximum mechanical tensile stress that a sample can bear before it fails. It is determined using a tensile test and measured in megapascals (MPa) or newtons per square millimeter (N/mm²).

The notched bar impact test

The notched bar impact test is used to determine the toughness of a material sample. A test pin of the material to be tested is notched on one side and tested in a tempered state. A pendulum hammer strikes the unnotched back of the sample and shatters it. The impact energy W determined in joules provides information about the deformation capacity of the material under different stress conditions.

Tempered steels does not leave our mill untested. Ordering from us therefore means absolute reliability for you.

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