When you think of the materials you come across in the course of your day, you think of wood, concrete, glass, etc. For most of us, stainless steel doesn’t even cause a blip on the radar, which means that it’s doing its job. Created to satisfy a need for a material that was resistant to rust and oxidation but also had strength comparable to steel itself, stainless steel is right under your nose and in your face every day.
Sounds a little melodramatic, right? The next time you’re at a stop sign, remember that stainless steel clamps are what keep the sign on its post. It’s in your car, the restaurant you had dinner at last week, even your bathroom. Open up the drawer where you keep your kitchen knives and, voila! – stainless steel. Now, imagine the impact of stainless steel in the production of vital commodities from electrical power to oil to chemicals and you begin to understand its true importance in personal and industrial use.
Stainless is more…
To be honest, stainless steel is somewhat of a misnomer. It’s not so much that the material doesn’t stain but that it is designed to resist staining and corrosion. The “stainless” tag comes from the fact that it doesn’t change color within a normal range of temperatures, an essential property. Corrosion in normal steel can be controlled in a typically dry and uncontaminated environment, but there are a number of applications in environments that are far from typical. What steel needed in order to do its job properly was protection.
This is where chromium comes in. A minimum of roughly 11% of this element in the steel mix is what gives us stainless steel. The chromium naturally reacts with oxygen, creating an oxidic film that covers the surface of the steel. This transparent layer protects the metal by virtually stopping the oxidation process, preserving the materials underneath. All stainless steels have an oxidic film (also known as a “passive” film/layer), which is unreceptive to water and air. This layer also possesses the ability to “heal” itself if it’s breached or damaged. Once the chromium is exposed due to the breach, it reacts with the oxygen in the air and repairs the integrity of the protective film.
Types of Stainless Steel
Stainless steel is always in high demand, with over 200 mixes available. There are new environment-specific compositions under development as you read this, with new applications on the horizon. The common stainless steels are divided into three major groups: Ferritic, Martensitic, and the most popular variety, Austenitic. Each of these types have unique chemical (corrosion resistance), physical (gravity, electrical, etc.) and mechanical (strength and toughness) characteristics that offer a seemingly endless array of properties to choose from, depending on the specific application requirement. Let’s take a look at these three types.
Ferritic
Ferritic stainless steels are commonly known as a binary steel alloy because of the two dominant elements being chromium and iron. The chromium is what makes them stainless, but it’s carbon that makes them steel.
The Ferritic group features a minimum of 11.5% Chromium. This type of stainless steel is magnetic at room temperature (up to its curie point) and the more commonly used Ferritic type AISI 409 is also referred to as utility stainless steel. Ferritic stainless steels are typically less expensive than Austenitic types because of the absence of nickel, which means it cannot be heat treated or easily strengthened by cold working. It is most commonly used in high volume production of components such as exhaust systems.
Martensitic
Martensitic stainless steels contain a higher carbon content than the other two types, allowing them to be hardened to specifications. While they can be significantly hardened, the resulting compromise is that Martensitic stainless steels are less resistant to corrosion than Ferritic or Austenitic types. If both resistance and hardening properties are of crucial importance to the application, alternative stainless steels such as Precipitated Hardened (PH) and Duplex-type materials can be considered.
The more common Martensitic grade is AISI 410, which has a 12% Chromium and 0.15% carbon content. For specific applications such as cutlery, the compositions of this grade could be as high as 18% Chromium and 0.75% Carbon in order to maintain that sharp knife edge.
Austenitic
The most commonly used group stainless steels is the Austenitic type. Austenitic stainless steels account for roughly 75% of all stainless steel production worldwide these days. In the U.S., the most popular type of Austenitic stainless steel is AISI type 304 which, in some cases, exceeds the production of Ferretic and Martensitic materials combined.
At temperatures between 1670 and 2550 F, iron becomes Austenitite which has a very small carbon presence and is non-magnetic. Both of these are highly sought-after properties, but you can imagine how difficult it would be to maintain that fridge door at these temperatures, not to mention manually screwing on the handle during manufacturing. The element that saves the day in this case is nickel, and in certain cases a combination of manganese and nitrogen, which are also known as Austenite stabilizers.
Nickel has suffered from shortages in the past and high prices in the present, opening the door for a mix of manganese and nitrogen to step in as a substitution in the austenitic structure. This material is not as resistant to corrosion as the nickel-infused austenitite, but it still offers substantial resistance properties in addition to mechanical advantages that are superior.
Austenitic stainless steels are non-magnetic, high-chromium alloys. They cannot be heat-treated, but can be hardened to some extent by cold working. The widely used type 18-8 (typically in sheet form) stainless steels with 18% Chromium and 8% Nickel offers significant corrosion-resistance properties due to the high Chromium level. This mix is even further enhanced by the addition of nickel that allows for its application in harshly corrosive environments.
Other classifications
Stainless steel classifications such as Precipitation Hardened (PH), Duplex Stainless Steels (a roughly equal mix of Ferritic and Austenitic), Trip Steels, Stainless Steel Clad and Cast Stainless Steels are viable options that are best served in a separate article.
Stainless Steel Designations
The most visible organization for assigning classification tags to stainless steel compositions is the American Iron and Steel Institute (AISI) with the main categories being 300-series for Nickel Austenitic, 200-series for Manganese-Nitrogen Austenitic and 400-series for Ferretic and Matensitic stainless steels. Precipitation Hardened and other specialty nickel alloys are grouped under the 600-series designation. With the exception of AISI 329, Duplex stainless steels and PH stainless steels are not covered by the AISI codes.
The Society of Automotive Engineers conforms to the original AISI specification, but the American Society for Testing Materials (ASTM) implemented a set of Unified Numbering System (UNS) tags for identifying stainless steel compositions (although, in some cases the AISI codes are retained in the first set of numbers for the ASTM codes). The UNS code consists of a number (most cases “S”), followed by 5 numbers, that may or may not contain the AISI designation.
The Last Hurrah
Stainless steel has made its way into so many aspects of our today’s world, making it hard to imagine our lives today without it. Some of you may be able to afford true silverware, but most of us would be faced with the unappetizing reality of rusty cutlery.