Diffusion chromium plating of steel – diffusion enrichment of surfaces with chromium, process, properties and applications

Diffusion chrome plating is a process thermochemical treatment consisting in saturating the surface layer of the material chrome Cr at elevated temperatures. Unlike electroplating, it does not involve depositing a finished chromium coating on the surface. Chromium atoms diffuse into the material and form a bond with the substrate. diffusion layer with gradually changing chemical composition and microstructure.
The purpose of chrome diffusion plating is primarily to increase resistance to oxidation, high-temperature corrosion, aggressive environments and – in appropriately selected materials and conditions – wear resistance.
The process is mainly used for steel and other alloys on a matrix iron, but the technologies of diffusion surface enrichment with chromium can also be used for other metallic materials.

What is diffusion chrome plating?

During the process, the surface of the workpiece is in an environment that provides active chrome.
At a sufficiently high temperature the following occurs:

1. production of active forms of chromium in the process environment,
2. chromium transport to the surface workpiece,
3. chromium adsorption on the surface,
4. diffusion of chromium atoms into the material,
5. creation of a chromium-enriched layer and – depending on the composition of the material – the appropriate phases and compounds.

Since the basic mechanism of layer growth is diffusion, its properties depend primarily on temperature, time, chromium activity in the environment and the chemical composition of the substrate material.

Diffusion chrome plating and electroplating chrome plating

Diffusion chrome plating should not be confused with classic electroplating.

The most important difference is that after diffusion chromium plating, chromium does not only form a separate coating lying on the surface, but becomes a chemical component of the material layer.

Why does chrome increase surface resistance?

Chromium is one of the most important elements used to increase steel's resistance to oxidation and corrosion.
With a sufficiently high chromium content, a dense layer of chromium oxide Cr may form on the surface.₂O₃, limiting further transport of oxygen and environmental components into the material.
This mechanism is one of the main reasons for the high oxidation resistance of many stainless steel, heat-resistant steel and high-chromium alloys.
Diffusion chromium plating makes it possible to obtain a high chromium content, primarily in the surface layer, without having to manufacture the entire element from a high-alloy material.

Structure of the diffusion chrome layer

The structure of the layer depends strongly on the content coal in the substrate material.
In steels with a low carbon content, a layer of solid solution rich in carbon may be formed. chrome.
In steels containing higher amounts of carbon, chromium can react with carbon and form chromium carbides.
The phases occurring in the Fe–Cr–C systems include, among others, carbides of the following types:

• Cr₂₃C₆,
• Cr₇C₃,
• complex carbides also containing iron.

Therefore, the properties of the layer after diffusion chromizing can vary greatly depending on the material composition.

Diffusion chromium plating of low-carbon and high-carbon steels

For this reason, the term "diffusion chromium-plated layer" does not mean one identical microstructure for all steel grades.

Diffusion chrome plating temperature

Diffusion chrome plating requires a relatively high temperature because the diffusion rate chrome in the material must be sufficient to produce a useful layer.
Depending on the technology, material and required layer, the process can be carried out in a wide temperature range, often around 850-1100 ° C.
The exact temperature must be selected according to:

• type of material,
• required layer depth,
• chrome plating methods,
• process time,
• composition of the chromium-plating environment,
• required core microstructure,
• further heat treatment.

High temperature accelerates diffusion, but at the same time it can cause changes in the core structure, grain growth and element deformation.

The effect of time on layer growth

Diffusion chrome plating is a process largely controlled by diffusion.
As time increases, the depth of material enrichment increases chrome, however, layer growth is usually not linear over time.
For many diffusion processes, the relationship can be approximately described by the parabolic law:

where:

• x – characteristic thickness of the diffusion layer,
• k – a factor depending on, among other things, the temperature and composition of the material,
• t – trial time.

In practice, the kinetics can be more complex, especially when distinct carbide phases are formed during the process.

Diffusion chrome plating methods

Chromium can be delivered to the surface by various methods.

One of the classic methods is pack cementation technology , in which the workpiece is placed in a powder mixture containing a chromium source and an activator enabling its transport.

Chrome plating using the pack cementation method

In the pack cementation process , the element is surrounded by a powder mixture containing:

• source chrome,
• an activator enabling the transport of the ingredient,
• inert filler that limits sintering of the mixture.

During heating, volatile compounds are formed that transport chromium towards the surface of the material.
Reactions take place on the surface, leading to the release of active chromium, which then diffuses into the substrate.
This process is an example chemical transport in the gas phase combined with diffusion in the solid state.

Diffusion chrome plating and oxidation resistance

One of the most important applications of chromium diffusion plating is to increase the material's resistance to high-temperature oxidation.
The area is rich in chrome can form a protective scale rich in Cr₂O₃.
This layer limits the speed:

• transport of oxygen to the material,
• transport of metal ions towards the surface,
• further growth of scale.

This allows the oxidation rate to be significantly lower than that of unprotected steel with a low chromium content.

Diffusion chrome plating and corrosion resistance

Surface enrichment in chrome can also improve resistance to many corrosive environments.
However, effectiveness depends on:

• chromium content in the layer,
• continuity and microstructure of the layer,
• presence of carbides,
• type of environment,
• operating temperatures,
• surface condition.

Chromium plating should not be considered a universal protection against every type of corrosion. The coating must be designed for a specific operating environment.

Diffusion chrome plating and wear resistance

In steels with appropriate content coal chromium can form hard carbides, so chromium diffusion plating can significantly increase surface hardness and wear resistance.
Hard carbide phases of the Cr type are of particular importance₇C₃ and Cr₂₃C₆.
However, the tribological properties depend on the entire structure of the layer, its thickness, the hardness of the substrate and the type of wear mechanism occurring.

The effect of carbon on chromium diffusion plating

Coal is one of the most important components influencing the structure of the chrome layer.
Chromium is a strong carbide-forming element.
If sufficient carbon is available in the material, chromium carbides may be formed during the process.
This can increase hardness and abrasion resistance, but at the same time limit the amount of chromium remaining in solid solution.
Therefore, a layer designed primarily for corrosion and oxidation resistance may require a different structure than a layer designed primarily for wear resistance.

Influence of alloying elements

Diffusion chromium plating and stainless and heat-resistant steels

The protective mechanism of the chromium-rich layer is similar to one of the basic mechanisms providing resistance stainless and heat-resistant steels.
In high chromium steels the appropriate content chrome occurs throughout the entire volume of the material.
In diffusion chrome plating, the high chrome content is created primarily at the surface.
This allows the core properties of the manufactured part to be combined with the chemical and high-temperature properties of the modified surface layer.
This does not mean, however, that chromium-plated steel becomes equivalent to stainless or heat-resistant steel in every respect. The composition and core properties remain essentially those of the starting material.

Diffusion chrome plating and boriding

Boronizing and diffusion chromizing are processes of diffusion surface saturation, but they lead to the formation of different phases and are used for different purposes.

Diffusion chrome plating and nitriding

W nitriding the surface is enriched primarily in nitrogen, while during diffusion chrome plating – in chrome.
Nitriding is usually carried out at a much lower temperature and is particularly effective in increasing surface hardness, wear resistance and fatigue resistance.
Diffusion chromium plating requires a higher temperature, but allows for a fundamental change in the chemical composition of the surface in the direction characteristic of high-chromium materials.

Diffusion chrome plating and carburizing

During carburization is introduced into the surface layer carbon. After later hardening This allows for a hard martensitic layer to be obtained while maintaining a more ductile core.
Diffusion chrome plating, on the other hand, uses chrome and may lead to the formation of a layer resistant to oxidation, corrosion or wear.
In carbon steels, chromium and carbon can work together to form very hard carbide phases.

Diffusion chromium plating and nitrocarburizing

During nitrocarburization the surface is enriched mainly in nitrogen and additionally carbonThe process is usually carried out at a much lower temperature than diffusion chrome plating.
Nitrocarburizing is used primarily to improve tribological properties, resistance to seizure and fatigue and – in appropriate variants – corrosion resistance.
Diffusion chromium plating is particularly interesting when high chromium content at the surface and resistance to oxidation and high-temperature corrosion are crucial.

Application of diffusion chrome plating

The process can be used for parts operating in environments where increased surface resistance is required.
Typical applications include:

• elements of industrial furnaces,
• parts of devices operating at high temperatures,
• elements of chemical installations,
• valve and fitting parts,
• elements exposed to high-temperature oxidation,
• parts operating in chemically aggressive atmospheres,
• elements exposed to wear if the generated layer contains hard carbide phases,
• selected tools and machine parts.

Advantages of diffusion chrome plating

• metallurgical bonding of the layer to the substrate,
• lack of the classic sharp boundary characteristic of many deposited coatings,
• increased resistance to oxidation at high temperatures,
• possibility of improving corrosion resistance,
• the possibility of obtaining high wear resistance in layers containing chromium carbides,
• the possibility of combining a cheaper or more durable core material with a chrome-rich surface.

Limitations of diffusion chrome plating

• high process temperature,
• relatively long time required to obtain the appropriate diffusion depth,
• possibility of changing the core microstructure,
• grain growth risk,
• possibility of deformation of elements,
• strong dependence of the layer structure on the content coal,
• the need to precisely control the temperature, time and activity of the chroming environment.

Quality control of the chrome diffusion layer

Evaluation of process effectiveness may include:

• measurement of the diffusion layer thickness,
• metallographic tests,
• analysis of content distribution chrome as a function of distance from the surface,
• identification of phases and carbides,
• measurement microhardness,
• oxidation resistance tests,
• corrosion resistance tests,
• wear resistance testing, if this is the primary objective of the process.

The mere presence of chromium on the surface is not a sufficient criterion for proper process performance. The chromium concentration profile, layer depth, its microstructure, and properties corresponding to the actual operating conditions of the component are also important.

Chromium plating – comparison with other thermochemical treatment processes

Diffusion chromium plating is a thermochemical treatment process consisting in the diffusion enrichment of the surface layer of the material in chrome.
Unlike electroplating, chrome plating doesn't just create a surface coating. The chromium diffuses into the material, creating a layer that is metallurgically bonded to the substrate.
In low-carbon steels, the layer may be primarily a chromium-rich Fe–Cr solution, but with increasing availability coal the importance of hard chromium carbides increases.
Thanks to this, chrome diffusion plating can be designed both to increase resistance to oxidation and high-temperature corrosion, as well as – in appropriate material arrangements – hardness and wear resistance.
The process requires a relatively high temperature and a correspondingly long time, so when designing the technology, it is necessary to take into account not only the properties of the layer, but also possible changes in the microstructure and properties of the core.

Methods of heat and thermochemical treatment of steel

Chromium plating is just one of the processes used to modify the microstructure and properties of steel. Depending on the desired effect, normalizing, stress relieving, hardening, tempering, etc., as well as thermochemical processes such as carburizing, nitriding, and others, are also used.

Steel heat treatment methods

• Normalizing steel – grain refinement and microstructure unification.
• Stress relief annealing – reduction of residual stresses.
• Softening of steel – reducing hardness and improving susceptibility to further processing.
• Full annealing – austenitization and slow cooling.
• Isothermal annealing – transformation carried out while holding at a properly selected temperature.
• Ball annealing – spheroidizing – change in the morphology of cementite or other carbides.
• Recrystallization annealing – removal of the effects of crushing and restoration of plasticity.
• Homogenizing annealing – reducing segregation and homogenizing the material composition.
• Hardening and heat treatment – shaping the microstructure and properties by hardening and tempering.
• Tempering steel – controlled modification of steel properties after hardening.
• Induction hardening – surface induction heating and rapid cooling.
• Flame hardening – surface hardening after local heating with a flame.
• Oversaturation – obtaining a supersaturated solid solution by an appropriate thermal cycle.
• Aging – controlled phase separation leading to material strengthening.

Methods of thermochemical treatment of steel

• Nitriding of steel – diffusional enrichment of the surface layer with nitrogen.
• Carburizing steel – diffusional enrichment of the surface with carbon.
• Carbonitriding – simultaneous enrichment of the surface layer with carbon and nitrogen.
• Nitrocarburization / nitrocarburization – enrichment of the layer primarily with nitrogen and carbon.
• Drilling – diffusion saturation of the surface with boron.
• Diffusion chrome plating – diffusion enrichment of the layer with chromium.
• Diffusion aluminizing – diffusion enrichment of the layer in aluminum.