Posted December 4, 2017
Rapid Prototyping: Steel 101
Steel is an incredibly popular material, and for good reason: it comes in a huge variety of types with different specialities, a wide range of costs, and material properties. Steel is used everywhere around you: in the cars, bikes, and trains you ride along with the offices, homes and gyms you work in. Despite increased popularity of materials such as aluminum, steel is still used in a myriad of construction materials and fasteners such as nuts, bolts, nails and screws. All steels are primarily made of iron, but other elements are added to improve strength, toughness and other properties, since iron alone is fairly soft. Here are a few things to consider when working with steel.
Unfortunately, the presence of iron in materials causes rusting when they’re exposed to water and air. Due to steel’s high iron content, rust is a common problem with steel parts that are exposed to humid environments or the outdoors. The presence of salt further accelerates this process, which is why sunken ships rust quickly, even though they aren’t exposed to as much oxygen as they would be if they were above water.
An example of the corrosion of structural steel. (Source)
A great method for preventing rusting is to coat the steel in a protective material, such as oil or oil-based paint. This is common in many tools you’ll find in a machine shop and on household items such as rain gutters. Some steels also come with coatings pre-applied, such as galvanized steel, which is coated in zinc. The zinc coatings found on many structural steel members such as I-beams provide a nice uniform color to the steel and protect it from corrosion. Stainless steel often comes with a chromium coating, which also acts as a barrier to prevent oxygen from reaching the iron in the steel.
Steel is an incredible material but doesn’t come without its limitations. Due to its hardness, it’s generally more difficult and time-consuming to machine than other metals such as aluminum or brass. The additional hardness gained through additives and coatings commonly found in Stainless Steels will often require the use of special tooling in order to cut it effectively. For this reason, you should expect milled and turned stainless steel parts to always be more expensive than the same part in aluminum.
Milling stainless steel. (Source)
On the other end of the spectrum, steel is way easier to use than aluminum in some processes such as welding. Low carbon steel, for example, is one of the easiest materials to weld due to its low carbon content. Something with much higher carbon content, such as iron, is therefore much more difficult to weld.
Identifying Types of Steel
Like other metals, types of steel are differentiated with a numbering system. In the case of non-stainless or carbon steels, there are four digits, with the first indicating a modifying element of the steel. For example, a “1” indicates a carbon steel, while a “4” indicates a molybdenum steel. The second digit indicates if there are any alloying elements, which are used to modify the steel’s properties. A “0” here indicates that there are no additional alloying elements. The last two digits indicate carbon content, which is perhaps most critical in determining a steel’s properties. 1040 steel has 0.4% carbon and 1045 steel has 0.45% carbon, for example. As carbon content increases, both weldability and ductility decrease, while hardening the same steel yields great strength and toughness.
1018 Steel, typically referred to as “Ten-Eighteen Steel”, is a great low-cost steel with low carbon content. It has a good balance between toughness and strength. It also has great weldability, thanks to its low carbon content. It’s widely used for applications where the strength of high-carbon, but more expensive, steels aren’t necessary.
Machineable vise jaws made out of 1018 steel. (Source)
1040 & 1045 Steel
1045 steel has more carbon, indicated by the last two digits of its numerical code, which gives it better tensile strength and hardness than 1040 steel. Fortunately for 1040, though, its lower carbon content makes it easier to weld. If weldability is the primary driver behind the selection of steel, a lower carbon content steel such as 1018 should be used.
4130 & 4140 Steel
Both 4130 and 4140 are chromium-molybdenum steels, or “chromoly” steels. The presence of chromium in these steels, while not as dramatic as Stainless, does increase corrosion resistance. The presence of molybdenum increases the toughness of these steels, which makes them great for use in bicycle frames and car roll-cages. Like the previous comparison, the steel with more carbon, 4140, has better tensile strength and hardness. Although, also like the previous comparison, 4130 is easier to machine and weld. Both of these will be more costly than 10-series steel, but make up for it in their increased corrosion resistance and strength.
300-series stainless steels fall under the category of Austenitic steels, or those that contain a austenitic crystalline structure. These steels are better for corrosion resistance than typical steel. However, they’re generally more difficult to machine, harden, and are often more expensive than other steels.
The three most common types of stainless, all offered by Plethora, are 303, 304 and 316. 303 Stainless was made for machining, so it isn’t ideal for surviving in incredibly harsh environments, such as at sea, although the main drawback of 303 Stainless is that it isn’t weldable.
304 stainless is the most common type of stainless available in the world, and for good reason: it’s weldable, machinable (with the right tools), and has great corrosion resistance. 303 and 304 have similar resistance to corrosion, but 304 is far more weldable.
Pipe welding stainless steel. (Source)
316, on the other hand, has the addition of molybdenum, which makes it the ideal option for marine and outdoor application, thanks to its fantastic corrosion resistance. As you might expect, this brings its upfront cost past 303 and 304, but can potentially save a lot of money down the line since the parts will last far longer in a harsh environment.
Thanks to its fantastic corrosion resistance, stainless steel is ideal for food-safe and medical applications, since it can be cleaned and sterilized but remains rust-free without any additional coatings. In the end, though, there are many types of steel to choose from during the development of your product or prototype, so all should be considered with care. Certain steels lend themselves well to surviving in various environments or certain manufacturing processes, so do your research and you’ll end up with a fantastic part in the end.