Materials #05: What Is Nitriding (窒化処理)? Common Treatment Types in Machine Parts

In machine part design, there are positions that do not need the whole part hardened, but do need a harder surface, better wear resistance, or reduced galling when sliding.

At times like these, one of the frequently mentioned options is Nitriding (窒化処理・ちっかしょり).

In English-language documents, this method is usually called nitriding. In this article I will use the term nitriding consistently for easier reading.

Simply put, nitriding is a method of introducing nitrogen into the surface layer of steel to create a hard outer layer. The interior of the part keeps its original mechanical properties, while the surface is improved in hardness, wear resistance, anti-galling, and in some cases even better corrosion resistance.

I have written this article from a practical angle for choosing a treatment for machine parts, without going too deep into heat-treatment theory.

What is nitriding?

Nitriding is the process of making nitrogen penetrate into a metal surface, mainly steel, to create a hard surface layer.

The nice thing about nitriding is that the treatment temperature is usually much lower than other heat-treatment methods such as carburizing then quenching, or induction hardening. Because the temperature is lower, distortion after treatment is usually smaller.

This is why, for precision machine parts, if you need to increase surface hardness but still want to limit warping, nitriding is an option worth considering.

However, you should not understand nitriding as always being better than quenching or carburizing. The hard layer of nitriding is usually very hard but rather thin. So if that position bears too large a surface load, strong impact, or needs a deep hard layer, it must be checked carefully.

In short:

Nitriding suits surfaces that need wear resistance, anti-galling, reduced friction and limited distortion. But it is not an ideal choice for every heavily loaded position.

Why do machine parts often use nitriding?

In machinery, many failures do not come from a part breaking outright, but from the surface wearing down gradually.

For example:

A sliding pin wears out
A guide bush gets scored
A shaft galls with its mating part
A jig is abraded at the contact zone
A die or fixture part gets scratched after many cycles
A sliding mechanism gains friction after a long time

At these positions, just increasing the base material hardness is not necessarily the best approach. Because if the whole part is too hard, machining is harder, the price is higher, or the part becomes more brittle.

Nitriding solves it in a different direction: improving only the surface layer that needs to work.

The interior of the part still keeps its original toughness or mechanical properties, while the surface is harder and resists wear better.

General characteristics of nitriding

There are many nitriding methods, but in general they share some common points:

Create a hard layer on the steel surface
Distortion after treatment is usually small
Improve wear resistance
Reduce galling and seizing during sliding
Can improve corrosion resistance depending on the treatment type
Suit positions with sliding friction
The hard layer is usually thinner than in some other heat-treatment methods

The last point is very important.

Many people hear "very hard surface" and think it bears load very well. But in reality you must also look at the depth of the hard layer and the contact conditions.

If the part bears large surface pressure, impact load or too heavy a line contact, a thin hard layer may not be enough. In that case you must ask the heat-treatment vendor carefully or reconsider the material choice.

Advantages of nitriding

The biggest advantage is low distortion.

For parts already precisely machined, especially parts with a datum face, precision holes, sliding faces, or long slender parts, distortion after treatment is a very annoying problem.

If after heat treatment the part is bent, warped, the holes are off, or the datum face is shifted, it must be re-ground or even remade.

Nitriding is usually advantageous here because the treatment temperature is lower.

Some other advantages:

Increases surface hardness
Increases wear resistance
Reduces friction
Reduces galling between two metal surfaces
Less distortion than many other heat-treatment methods
Can treat small positions, internal holes or narrow gaps depending on the method
Suits sliding parts or parts with repeated contact

In designing jigs, fixtures, guide parts, pins, shafts, bushes, stoppers, dies or sliding mechanisms, these are quite practical points.

Drawbacks to know

Nitriding also has drawbacks.

The first drawback is that the hard layer is not deep.

If the part must bear a large surface load or has a risk of surface indentation, only a thin outer hard layer is not necessarily enough. The surface may be hard, but if the base underneath is not strong enough, there is still a problem.

The second drawback is that the treatment result depends quite a lot on the material and the treatment vendor.

Both called "nitriding", but each vendor may have different conditions, equipment, experience, treatment layer and standards. Some are strong at gas nitriding, some at plasma nitriding, some have their own trade names.

So when the drawing only writes a generic "窒化処理" without stating clear requirements, the result received sometimes does not match what was wanted.

You should check in advance:

Is the material suitable?
About what hardness is needed?
What depth of hard layer is needed?
Is corrosion resistance needed?
Is anti-galling needed?
Is re-grinding needed after treatment?
Is there any zone that must not be treated?
Does it affect the mating/fit dimensions?

Especially for precision parts, you must talk with the treatment vendor before finalizing the drawing.

Common types of nitriding

The names in practice are quite many. Some are method names, some are trade names, and some are called slightly differently by each company.

Below are the types commonly encountered when reading documents or discussing with a treatment vendor.

Gas nitriding – ガス窒化

Gas nitriding is a nitriding method using a gas environment.

This is a fairly traditional method that can create a relatively deep penetration layer compared to some other nitriding methods. With materials such as SACM645, the treatment effect is usually very good.

Advantages:

Relatively deep penetration layer
Good surface hardness
Suits some steels specifically for nitriding
Less distortion than many other quenching methods

Points to note:

Long treatment time
Cost can be high
Not every material gives the same result
Stainless/SUS usually needs careful checking; do not just assume it can be treated

Gas nitriding suits when a relatively good penetration layer is needed and the part has high wear-resistance requirements, but there is no great rush on time.

Gas soft nitriding – ガス軟窒化

Gas soft nitriding is a faster method that usually costs less than ordinary gas nitriding.

In Japanese you may encounter names such as ガス軟窒化, SN処理, エスナイト処理 or SNプロセス depending on the vendor.

Common characteristics:

Faster than gas nitriding
Usually lower cost
Usually a shallower penetration layer
Hardness can be lower than gas nitriding
The surface can have a milky white or characteristic color depending on the treatment

This method is often used for parts that need surface improvement but do not require too deep a hard layer.

For ordinary machine parts, if you need wear resistance, reduced galling, and do not want too long a delivery time, gas soft nitriding is a fairly practical option.

Salt bath soft nitriding – 塩浴軟窒化 / Tufftride

Salt bath soft nitriding is a method treated in a molten salt bath. A commonly encountered trade name is Tufftride.

Characteristics:

Relatively short treatment time
Cost is usually more affordable
Can be applied to many materials
The surface is usually gray or dark depending on the treatment conditions
Can improve wear resistance and anti-galling

In some cases, gas soft nitriding is used instead of salt bath soft nitriding, depending on environmental requirements, cost and the vendor.

The point to note is that this method involves a salt solution, so each vendor will have its own standards and management conditions. When appearance control or environmental requirements matter, confirm in advance.

Plasma nitriding / Ion nitriding – プラズマ窒化 / イオン窒化

Plasma nitriding, or ion nitriding, is a method that uses plasma to introduce nitrogen into the surface.

This method is usually regarded as clean, well-controlled and suitable for many types of steel. The surface after treatment can have a matte silver or bright gray color depending on conditions.

Advantages:

Can control the treatment zone relatively well
A better-looking surface in many cases
Low distortion
Suits many types of steel
Less contamination than some other methods

Points to note:

Cost is usually higher
For deep holes, narrow gaps or complex internal faces, treatment can be harder
Careful confirmation with the vendor is needed if the part has a complex shape

Plasma nitriding suits parts that need a good appearance, well-controlled treatment, or reduced distortion on a precision part.

Radical nitriding – ラジカル窒化

Radical nitriding is a nitriding method characterized by limiting the formation of a brittle compound layer on the surface.

In practice, this method is mentioned when you want to avoid problems such as cracking, edge chipping, or need a more stable surface after treatment.

Characteristics often mentioned:

Little brittle compound layer formed
Limits cracking or chipping at sharp edges
May not need re-polishing depending on requirements
A diffusion-type treatment layer, so less worry about coating peeling
In some cases the part can still be welded or plated after treatment, depending on conditions and the vendor

This part should not be decided by feeling. If the part has sharp edges, small grooves, small holes, or requires further operations after treatment such as plating, discuss with the vendor in advance.

Sulfur nitriding / Gas sulfonitriding – ガス浸硫窒化

This is a method that adds a sulfur element to improve slidability and reduce friction.

In sliding mechanisms, anti-galling is a very important issue. If two metal surfaces contact and slide against each other, かじり can occur — that is, galling or surface seizing.

Gas sulfonitriding can be used when you want to improve anti-galling and slidability.

Characteristics:

Improves slidability
Reduces friction
Supports anti-galling
Can be applied to some high-Cr alloy steels depending on conditions
Cost is usually higher than gas soft nitriding

This method should be seen as a more specialized option, used when the sliding and anti-galling requirements are important.

Some trade-name treatments

Besides general method names, in practice you also encounter many trade names or each vendor's own treatments.

For example:

MIC処理
カナック処理
エスナイト処理
SN処理
Tufftride

These names should not be simply understood as "all the same". Each has different treatment conditions, surface color, treatment layer, hardness, wear resistance and cost.

When you meet an unfamiliar treatment name, the best approach is to ask the vendor:

Which materials can be treated?
About what hardness after treatment?
What depth of treatment layer?
Is there a compound layer on the surface?
Is grinding or polishing needed after treatment?
What color is the surface after treatment?
Can internal holes or narrow gaps be treated?
Is there any zone that needs masking?
Does it affect the dimensions?

Do not just look at the treatment name and write it on the drawing if you do not understand it clearly.

Where in the machine should you consider nitriding?

In automatic machines, nitriding can be considered at positions such as:

Guide pins
Sliding shafts
Bushes or sliding parts
Stoppers bearing repeated contact
Jig parts that get heavily rubbed
Die parts or fixture parts
Mechanisms with a risk of galling
Contact faces that need wear resistance but should not distort much

However, if it is a position bearing strong impact, too large a surface load, or needing a deep hard layer, you should re-check. You may need a different material, a different treatment, or a structural change to reduce the surface load.

For example, if a small pin bears a large lateral force and is repeatedly impacted, nitriding alone will not necessarily solve the root of the problem. You may have to increase the pin diameter, change the support, add a bush, change the material or change the mechanism to reduce the force.

Surface treatment should not be used to "firefight" a weak mechanical design.

Can you mask off zones you don't want treated?

You can, but you must ask the vendor.

In many cases, if there is a zone where you do not want nitrogen to penetrate, the vendor can use an anti-penetration method, shielding, masking or a separate treatment depending on the technology.

For example:

A precision mating zone
A hole that must keep its dimension
A surface to be welded later
A zone needing further machining
A position where you do not want the hardness changed

But you should not casually write it on the drawing without confirming the method. Each treatment method has its own masking approach and limits.

Can welded parts be treated?

In principle, there are cases where even parts with a welded zone can be treated. But the result depends on the material, the weld metal, the heat-affected zone and the treatment method.

If the part has welding, you should clearly notify the vendor from the start.

For parts requiring precision, there is usually already residual stress after welding. If you send it to heat treatment or surface treatment without controlling the stress, distortion can still arise.

So for large welded parts or parts needing precision, you should consider:

Is stress relief needed first?
Does the welded zone affect the treatment layer?
Is a dimensional re-check needed after treatment?
Is there any position prone to distortion?

How should you note it on the drawing?

This point is quite important.

If the drawing only writes a generic:

窒化処理

Nitriding

that is not necessarily enough.

Depending on the requirements, you should clarify further:

Type of nitriding treatment
Base material
Required hardness
Depth of hard layer
Zone to be treated
Zone not to be treated
Appearance requirement
Whether grinding is needed after treatment
Inspection standard

In practice, for parts that are not too strict, you can note it in a simpler way, but for precision parts or important parts you should discuss with the vendor before releasing the drawing.

In production, one unclear surface-treatment note can generate a lot of work: wrong quotes, wrong treatment, a surface color different from what was wanted, affected mating dimensions, or a life that falls short.

Conclusion

Nitriding (窒化処理) is a very useful method when you want to increase surface hardness, resist wear, reduce friction and limit distortion after treatment.

It especially suits parts with sliding friction, guide parts, pins, shafts, jigs, stoppers, or repeatedly contacted surfaces.

But you should not regard nitriding as a treatment that is "always good just by doing it".

You need to check the material, the depth of the hard layer, the required hardness, the load conditions, the surface pressure, the part shape and the supplier's treatment capability.

In machine design, surface treatment is only one part of the design. If the initial structure is wrong, the load too large, the contact too small, or the mechanism eccentric, then nitriding cannot save everything.

The safest approach is to choose the material, design the structure and choose the surface treatment at the same time, then discuss early with the vendor before finalizing the drawing.

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