Materials science: what does chromium do as an alloying element in steel?

Chromium is a true all-rounder in steel metallurgy. Alloying with chromium (Cr) influences a wide range of engineering properties. Increasing the chromium content can achieve, among other things:

• higher tensile strength,
• better wear resistance,
• greater hardenability,
• an improved response to quenching and tempering,
• higher scaling resistance,
• greater high-temperature strength and tempering resistance,
• improved corrosion resistance,
• greater surface-layer hardness during nitriding,
• greater resistance to hydrogen attack under pressure,
• lower thermal and electrical conductivity.

This list raises several questions: which metallurgical mechanisms account for this wide range of effects? Can these effects be traced back to a few fundamental properties of chromium? To answer these questions, it is useful to begin by looking at unalloyed steel.

The starting point: unalloyed steel

Unalloyed steel consists of iron (Fe) and carbon (C). The vast majority of C atoms combine with Fe atoms to form iron carbides. Most of the carbon combines with iron to form iron carbides. Their shape, size and distribution within the comparatively soft iron crystal lattice largely determine the mechanical properties.

The hard iron carbides hinder the movement of dislocations, thereby increasing strength. They also improve wear resistance. At higher temperatures, however, the morphology and stability of the iron carbides change; above approximately 720 °C, they begin to dissolve completely. This limits the use of unalloyed steels at elevated temperatures.

In an oxidising atmosphere, iron also reacts with oxygen. At low temperatures and in the presence of moisture, rust forms. At high temperatures, such as in a forging furnace, scale forms.

Chromium and carbon: forming stable carbides

One of chromium’s key properties is its strong affinity for carbon. Chromium combines with carbon to form chromium carbides, which are harder and more resistant to high temperatures than iron carbides. This explains several effects:

• increased strength and wear resistance,
• greater high-temperature strength and tempering resistance,
• improved resistance to hydrogen attack under pressure.

The last of these is particularly important in engineering: at high temperatures and pressures, hydrogen can diffuse into steel and react with carbon to form methane. When carbon is removed in this way, the steel loses structural integrity. Chromium binds carbon in stable carbides, reducing this risk.

Chromium and oxygen: passivation and corrosion protection

Chromium also has a stronger affinity for oxygen than iron. At a dissolved chromium content of around 12% or more, a thin, dense chromium oxide layer forms on the surface. This acts as a passive layer and prevents further oxidation.

Stainless steels therefore contain at least around 12% chromium. It is essential that sufficient chromium is present in the metallic state. Since chromium also forms carbides, the chromium content used in practice is usually higher, ensuring that enough “free” chromium remains available for passivation despite carbide formation. These mechanisms explain both the corrosion resistance and the scaling resistance of chromium-alloyed steels.

Chromium and nitrogen: nitrides and surface-layer hardness

Chromium also has a strong affinity for nitrogen. During nitriding, nitrogen atoms diffuse into the surface layer of the component and form, among other compounds, very hard chromium nitrides. These are very hard and significantly increase surface-layer hardness. For this reason, nitriding steels are often alloyed with chromium.

Chromium influences not only chemical bonding but also diffusion processes in the iron crystal lattice. Chromium atoms hinder the diffusion of iron and carbon atoms, regardless of whether the chromium is freely dissolved or bound in carbides.

Since microstructural transformations during heat treatment are largely controlled by diffusion, inhibiting diffusion increases hardenability. Microstructures that form without diffusion, such as martensite, can develop. This explains improvements in strength, hardenability and the response to quenching and tempering.

Electron movement and conductivity

Chromium hinders not only atomic diffusion but also the movement of free electrons. Since electrical and thermal conductivity in metals are largely determined by free electrons, both properties decrease as the alloy content increases.

This effect is not unique to chromium but occurs with alloying elements in general. Highly alloyed austenitic steels, for example those containing around 18% chromium and 10% nickel, therefore have comparatively low thermal conductivity. This is one reason why they are suitable for applications where reduced heat transfer is desirable, such as coffee mugs.

Summary: chromium in steel

The wide-ranging effects of chromium can largely be traced back to three fundamental properties:

• a strong affinity for carbon (carbides),
• a strong affinity for oxygen (passive layer),
• a strong affinity for nitrogen (nitrides),
• as well as the inhibition of diffusion and electron movement.

To increase wear resistance or high-temperature strength beyond the level of chromium-alloyed steels, elements are used whose carbides are even harder and more resistant to high temperatures. Examples include tungsten, vanadium and molybdenum, as used in high-speed steels. Chromium nevertheless remains one of the key alloying elements in steel engineering, owing not to a single property but to its wide range of metallurgical effects.

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