What Makes Stainless Steel Stainless?

Stainless steel is a fundamental part of the prototyping and manufacturing industry. It is vital for making machine parts and some tools for plastic injection molding, and of course it`s found in a million-and-one common everyday items. Yet, even with stainless steel playing a pivotal role in our lives, many people don`t know what exactly makes stainless steel [stainless" or that there are many kinds of stainless steel. Knowing the chemistry that goes into making this useful metal will help you to choose which material is best for your next tooling, low-volume manufacturing or prototyping project.

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 highly magnetic and conducts electricity so it`s still useful in some applications. But pure iron is soft and not suited for mechanical work.

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 (up to .2% carbon) through to high carbon (2%). 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 plain steel has limitations.

Why Is Stainless Steel 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 types of steel 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.