Materials #06: Choosing Stainless Steel for Machined Parts – Don't Default to SUS304

When designing machine parts from stainless steel, a great many drawings default to writing SUS304.

The reason is easy to understand.

SUS304 is easy to buy, common, corrosion-resistant, and usable in many environments. For the designer, choosing SUS304 usually creates a feeling of "safety".

But if that part is a machined part — for example turning, milling, drilling, tapping, reaming, making many small holes or mass machining — then the story is not so simple.

There are parts that only need moderate corrosion resistance, are not welded, are not used outdoors, and do not contact water or strong chemicals. But because the drawing says SUS304, the quote comes back higher than necessary.

The problem is not that SUS304 is "no good".

The problem is that SUS304 is good, but it is not always easy to machine or cost-optimal.

Why is stainless harder to machine than ordinary steel?

Stainless steel, especially the Austenitic group like SUS304 and SUS316, is usually regarded as a hard-to-machine material.

Not just because it is "hard".

In reality, the difficulty of stainless lies in a few points:

Prone to work hardening during machining
Poor thermal conductivity
A ductile and tough material
Chips that are hard to break and easily wrap around the tool
Easily wears the tool if the cutting conditions are unsuitable

On the drawing, these points do not show up.

But at the machining shop they show up very clearly through machine run time, tool life, surface quality and the number of stops needed to clear chips.

Machining stainless easily causes surface work hardening

A rather annoying feature of SUS304 is 加工硬化 – work hardening from machining.

As the cutting tool passes over the material surface, the surface layer can harden. If the next cut then bites right into that hardened layer, the tool bears a greater load, wears faster, and may even chip the edge.

That is why, with stainless, you sometimes cannot cut as "gently" as you think.

If you leave too little machining allowance, the depth of cut is too shallow, or you split into too many small passes unwisely, the tool may keep cutting into the already-hardened surface layer. The result is slow machining, fast tool wear and higher cost.

The designer should be aware of this, especially when ordering parts with tolerance, surface or precision-hole requirements.

Stainless conducts heat poorly, so the tool overheats easily

Another issue is that stainless conducts heat less well than ordinary carbon steel.

During cutting, heat is generated at the contact zone between tool and material. With a material that conducts heat well, part of the heat escapes with the chip and through the body of the material.

But with stainless, heat easily concentrates at the tool tip.

The tool heats up faster, the tool coating bears a greater thermal load, and material sticking to the tool is also more likely. When that happens, the machined surface can turn out poor, dimensions become unstable, and the tool wears fast.

For parts requiring high precision, heat causes one more problem: the part can expand during machining. When it cools back to normal temperature, the dimensions may change slightly.

So machining precision stainless is not just a matter of choosing a good machine. It relates to the material, the tool, the coolant, the cutting conditions and even the part shape.

Stainless chips are tough and easily wrap

Anyone who has stood at a lathe or watched stainless being machined will understand how troublesome chips are.

Stainless is ductile and tough, so the chips do not break into fine pieces as easily as some free-cutting steels. The chips can stretch into a string, wrap around the tool, wrap around the part, or scratch the surface.

For automatic machining or high-volume runs, this is a big problem.

The machine is running but the chips keep wrapping continuously, so it cannot run stably. The operator has to watch, stop the machine, remove chips, adjust the cutting conditions. All these tasks ultimately turn into cost.

So when designing a stainless part, don't just look at the material price. You must also think about whether the machining shop can run it easily.

SUS304: common but not always cheap

SUS304 is a very common type of stainless.

It has good corrosion resistance, is easy to buy, and is usable in many mechanical applications, equipment, covers, brackets, machine parts, jigs, and light structures.

If the part needs:

Good corrosion resistance
Weldability
Use in a humid environment
A common material that is easy to replace
No supply risk

then SUS304 is a reasonable choice.

But if the part is just a small shaft, spacer, collar, pin, small block, an auto-turned part, or a part with many holes to machine, then you should think further.

Because SUS304 is harder to machine than many people think.

There are cases where the material is not too expensive, but the machining cost is high.

SUS316: better corrosion resistance, but even harder to machine

SUS316 is usually used when higher corrosion resistance than SUS304 is needed, especially in environments with salt, sea spray, chemicals or harsher conditions.

But in return, SUS316 is usually harder to machine than SUS304.

The tool wears faster, heat is harder to manage, and machining cost is usually higher.

So you should not choose SUS316 just because you think "more premium is better".

If the part is not used in an environment needing high corrosion resistance, choosing SUS316 can be an unnecessary cost increase.

In machine design, the best material is not the most expensive material.

The best material is the one that is sufficient in function, easy to machine, stable in supply and reasonable in cost.

SUS303: easier to machine, but you must understand its limits

If the main goal is machining, SUS303 is a very worthwhile option to consider.

SUS303 is stainless with improved machinability. Compared to SUS304, it is easier to turn, mill and drill. The chips are easier to handle, the tool lasts longer, and the machining time can be shorter.

The interesting point is that the material price of SUS303 can be a little higher than SUS304, but the total post-machining cost can be lower.

Why?

Because for a machined part, the cost is not just the material money.

The cost also includes:

Machine run time
Tool life
The number of tool changes
The ability to run automatically
The defect rate
Chip-handling time
The risk of wrong dimensions or poor surface

For mass-turned parts, spacers, shafts, collars, pins, small shafts, small blocks with many holes, SUS303 is often more advantageous than SUS304.

But SUS303 is not a material that "replaces SUS304 in every case".

This is a very important point.

When should you not use SUS303?

SUS303 has better machinability because it contains an added component that improves cutting.

But that very thing gives it some drawbacks:

Poorer corrosion resistance than SUS304
Not suitable for welded parts
Should not be used in humid, wet or chemical environments without careful checking
Should not be used if cleanliness, corrosion durability or high surface reliability is required

In particular, if the part is welded, you almost never should choose SUS303.

When welding is needed, SUS304 or SUS316 is usually the more reasonable choice.

Simply put:

Machined part, not welded, mild environment → you can consider SUS303.

Part that needs welding, needs better corrosion resistance → prefer SUS304 or SUS316.

SUS430: cheaper, easier to machine, but poorer corrosion resistance than SUS304

Another option is SUS430.

SUS430 belongs to the Ferritic group. This material is usually cheaper, easier to machine than SUS304, and is magnetic.

If the part is used in an indoor environment, does not require very high corrosion resistance, and does not need the special features of SUS304, then SUS430 can be an option worth considering.

However, SUS430 does not resist corrosion as well as SUS304.

So you should not switch from SUS304 to SUS430 just to lower the price without checking the use conditions.

For example:

A part inside a machine, dry environment, no water contact → can be considered
A part outdoors, near water vapor, near chemicals, requiring long-term appearance → needs care

Quick comparison of common stainless types

Material	Machinability	Corrosion resistance	Welding	Note
SUS303	Good	Medium	Not suitable	Suits turned/milled parts, no welding
SUS304	Harder	Good	Good	Common, balanced, widely used
SUS316	Hard	Very good	Good	Used when high corrosion resistance is needed
SUS430	Fairly good	Medium	Needs care	Better price, magnetic, suits mild environments
SUS410	Depends on heat treatment	Lower	Needs care	Can be heat-treated to increase hardness

This table is not for choosing a material mechanically.

It just helps you ask the right question before writing a material onto the drawing.

Don't choose a material just by the feeling of "safety"

In design, we often choose SUS304 because we think choosing it will get fewer questions back.

But for a machined part, this way of choosing can raise the price.

For example, a part used only as a spacer in a machine, not welded, no water contact, only needing normal corrosion resistance. If it says SUS304, the shop can still machine it, but the price can be higher.

If you switch to SUS303, the part can be easier to machine, the machine run time shorter, the tool worn less. The total cost can drop while the function is still sufficient.

Conversely, if the part is used in a humid environment, has the possibility of water contact, needs welding or needs good corrosion resistance, then you should not switch to SUS303 just because it is cheaper.

Material design is like that.

No material is always right.

There is only the material that suits the use conditions and the fabrication method.

Tolerance also raises stainless machining cost very fast

A point often overlooked is tolerance.

With stainless, if the drawing writes too tight a tolerance in unnecessary places, the machining cost rises noticeably.

For example, a face that only needs an ordinary assembly datum but is marked ±0.01 mm. Or a hole that is only for a bolt but is required to be too precise. Places like these make the shop reduce the cutting speed, change tools more carefully, measure more, and sometimes add a finishing operation.

Writing a tight tolerance is not automatically good design.

Good design knows which places need to be tight and which should be left normal.

With stainless, this matters even more because the material is already harder to machine than ordinary steel.

A few positions to examine carefully:

Bearing-fit diameters
Locating holes for pins
Assembly datum faces
Positions needing concentricity
Positions directly affecting motion or precision

For faces that do not affect function, use a more reasonable tolerance.

Part shape also greatly affects the price

With stainless, the harder a shape is to hold rigidly, the harder it is to machine.

Shapes that easily raise cost include:

Thin walls
Deep grooves
Small deep holes
Many small tapped holes
Long parts with small cross-sections
Wide flat faces requiring high flatness
Internal corners requiring too small a radius

These shapes easily cause vibration, deformation, local overheating or tool breakage.

If the design can be adjusted a little to be easier to machine, the cost can drop quite a lot.

For example:

Increase the internal corner radius if it does not affect function
Avoid overly deep holes if not necessary
Split the part into two pieces if machining one block is too hard
Only require precision at the zone that truly needs it
Leave a reasonable clamping or datum face for machining

This is the part designers and machining shops should discuss early.

Waiting until the drawing is finalized before asking for a price reduction is usually much harder.

Do you need to specify passivation?

Stainless resists corrosion thanks to a very thin passive film on the surface.

But after machining, the surface can be affected by the cutting tool, cutting oil, iron dust or contaminants from the machining process. In some cases, this can reduce the part's corrosion resistance.

So for stainless parts that require better corrosion resistance, especially parts used in humid environments, clean environments, medical devices, food, chemicals or appearance parts, you should consider specifying:

Passivation / 不動態化処理 / passive treatment

This treatment helps remove free iron on the surface and supports the re-forming of a more stable passive film.

With SUS303, if you still want to use it to reduce machining cost but also need relative corrosion resistance, passivation is a point to discuss with the fabricator.

However, you should not mechanically write passivation for every stainless part. If the part is inside a machine, dry environment, no appearance requirement, then it may not be needed.

It is still the old principle: the technical requirement must follow the real use conditions.

Quick selection suggestions in practice

For a machined stainless part, you can think along the following lines.

Cases where SUS303 is worth considering

Turning/milling is the main operation
No welding
The use environment is not too humid
No contact with water or chemicals
You want to reduce machining time
There are many holes, many cutting operations
Mass production

Cases where SUS304 should be chosen

Needs more stable corrosion resistance
Needs weldability
Used in an ordinary environment but needs reliability
You want a common, easy-to-buy material
You do not want the risk of changing material

Cases where SUS316 should be chosen

A more strongly corrosive environment
Near salt, sea spray, chemicals
High corrosion resistance required
You accept higher material and machining cost

Cases where SUS430 can be considered

Indoor, dry environment
Corrosion resistance not required to be too high
You want to reduce cost
You accept a magnetic material
Not used in positions requiring SUS304-like features

One sentence to remember when ordering stainless machining

If the part is not welded and is mainly machined, don't rush to default to SUS304.

Ask further:

Can this part use SUS303?

Just such a question sometimes helps reduce quite a lot of cost.

But if the part needs welding, needs good corrosion resistance, or is used in a humid, wet or chemical environment, then don't change the material just because you see that SUS303 is easier to machine.

Machining is cheaper, but if it later rusts, cracks at the weld or fails in the field, the repair cost is far greater.

Conclusion

When choosing stainless for a machine part, you need to look at all three factors:

Function – Machining – Cost

SUS304 is a good material, but it is not always the optimal choice for a machined part.

SUS303 can help reduce machining cost, but must be avoided for welded parts or environments needing high corrosion resistance.

SUS316 resists corrosion better, but is harder to machine and costs more.

SUS430 can be an economical option in a mild environment, but cannot casually replace SUS304.

In machine design, choosing the right material is not just choosing the "good type".

It is choosing the right material for the right function, the right environment and the right fabrication method.

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