What Makes Stainless Steel Stainless?

Stainless steel is ubiquitous in the manufacturing of machine parts and some mold tools for plastic injection molding, and of course it’s found in a million-and-one common everyday items. Not all stainless steels are the same, and many people don’t realize what exactly makes steel “stainless”, or rust-proof. Knowing the chemistry that goes into making this most useful metal will help you to choose which type may be best for your next project.

Hot steel on conveyor

Hot steel on conveyor

First Of All, What Is Steel?

You won’t find steel on the periodic chart because it’s not an element but an alloy, or chemical combination of many elements. The foundation of all steel is iron. Iron in its pure form is useful for some industrial applications but it’s soft and brittle and not suited for mechanical work. But iron is by far the most common element in the Earth’s crust and so it’s very abundant.


Some genius in ancient history discovered that by adding up to 2% of carbon to iron you get steel, a relatively simple mixture we now call “mild steel.” Steel is harder, stronger and more durable than iron and forms the backbone of the industrialized world.

It’s important to remember that steel has excellent mechanical properties for which there is still no substitute – so steel will be with us for a long time. It’s stronger than aluminum, and it doesn’t shatter or crack like carbon fiber. But mild steel has limitations.

What Is Rust?

One of the most chemically reactive of all elements is oxygen. It loves to join with other elements to create new chemical compounds. When it joins with iron, it makes iron oxide, or rust. This happens to mild steel all the time, which is one reason why we use so many surface coatings to protect mild steel: painting, galvanizing, powder coating, electroplating, etc. Eventually water will get in, and since water contains some free oxygen radicals, rust will form.


What Is Stainless Steel?

The secret to stopping rust is to prevent oxygen from forming bonds with iron. The best way to do this is to get something else to bind with the iron first. After many experiments, it was discovered that by adding at least 10% of chromium to mild steel you get a new alloy that doesn’t rust – stainless. This innovation, by the way, took place in Sheffield, England in the 1930’s, giving that city its reputation for making some of the finest cutlery in the world.

Stainless is not as tough as mild steel in many applications and it’s more expensive, but it also resists the corrosive effects of water and many other chemicals. Now, scientists play with different trace additives like manganese, copper and nickel to get stronger steels that are still rust-free.


Why Is This Good For Tool Making?

Modern alloys can be very hard and durable, and this is true for tools and dies routinely used every day. But for plastic injection mold tools, it’s possible to inject plastic compounds that have a corrosive effect even on hard metals. This is especially true with polyvinyl chloride (PVC). Steel with high chromium content, like P20 or NAK80, will resist this corrosion and will therefore last much longer, which helps to offset their relatively higher cost. These types of steel are not completely stainless but are highly corrosion-resistant.

Another interesting point is that chromium in the alloy helps to produce a high polish, which is good for the surface finish of the parts. At the same time, these steels exhibit less thermal heat transfer during the extreme heating/cooling cycles of pressure die casting. This insulating effect may help to avoid tool damage over long production runs.


stainless steel heat sink

3D printed stainless steel part from 316L

What Kind Of Stainless Do We Use?

We can purchase and work with any commercially available grade of stainless steel. For most of our common work we use four main types:

  Type 303

Because of its relatively high sulfur content, this stainless steel is softer and therefore is used for most CNC machining work. It is non-magnetic and does not accept heat treatment, so the hardness is not affected by the heat induced via the machining process (work hardening). However, the additional sulfur makes it a little less corrosion-resistant than type 304.

  Type 304

Higher corrosion resistance than type 303, it is also harder and tougher. It is a bit more difficult to work (machine) than type 303, but it also makes a stronger finished part. This is the most common type of stainless steel used in manufacturing.

  Type 316L

A specialty stainless steel with a low carbon content but more molybdenum. This makes it even more corrosion-resistant than type 304. And because it is low in carbon, it does not carburize or give off heavy fumes when welded. This is often used for marine applications in corrosive environments, and it’s the choice for 3D printing such as in direct metal laser melting (DMLM).

  Type S136

An especially high purity of stainless steel. It is used mostly for plastic injection mold tools when the molded part needs to be an optically clear plastic lens, because the steel takes a very high polish.

Knowing how to work with a variety of raw materials is only part of the secret to making great parts. When you’re ready for your next project, contact us for a free quotation and let our experts help you to decide which tool steel is best for your schedule, budget and order volume.

Knowing how to work with a variety of raw materials is only part of the secret to making great tools and parts. When you’re ready for your next project, contact us for a free quotation and let our experts help you to decide which tool steel is best for your schedule, budget and order volume.


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