300 Series Stainless Steels
Stainless Steel is a great choice where extreme corrosion resistance, durability, and good surface finish are required. Swissturn machines are a variety of stainless steels used in medical and dental devices, military, aerospace, photonics, laboratory instruments, and valve applications. Stainless steel is highly popular for CNC Swiss screw machine parts due to its excellent machinability.
The most commonly machined stainless steel is 303 (UNS S30300). This is an austenitic stainless with good corrosion resistance. It cannot be heat treated to make it harder, but it can be annealed to make it softer. This is often done when the finished part is going to be crimped or bent in a subsequent operation. 303 stainless has excellent machinability due to the addition of a small amount of sulfur. 303 stainless should not be exposed to moist marine environments because it is subject to rapid-pitting corrosion and can rust. For marine environments, consider passivated 316 stainless steel.
304 and 304L are often used when a part is going to be welded or cold formed. The machinability of 304 stainless steel is much lower than that of 303 stainless, so it will usually increase your cost. Swissturn only uses the 304L, even if you specify 304. This is because 304L is dual certified and meets the specifications of regular 304. The L in 304L signifies extra-low carbon (.03% maximum vs. .08% maximum in regular 304 stainless steel). 304L is the preferred alloy for welding because it does not require a post-annealing process of weld joints after welding. If 304L stainless is welded, it is recommended that the weld joints be annealed to prevent corrosion.
316L stainless steel is very commonly used for medical and dental devices that will come in contact with the body. It is highly corrosion resistant, but its machinability is not as good as 303 stainless steel, so it will increase the machining cost. 316L is also a weldable alloy.
All of the 300 series stainless steels can be passivated. Passivation rids your precision machined components of surface contaminants, including free iron, improves corrosion resistance, and leaves a thin chemical film barrier to resist corrosion. Clear passivation is the most common post-machining process we provide to our customers, but customers do not often want something known as black passivation. Black passivation is also known as “Black Oxide” for corrosion-resistant steel alloys. It is a conversion coating that produces a uniform, attractive, and non-reflective black finish.
Electropolishing is often used on stainless parts to improve surface finish. Electropolishing is an electro-chemical process in which that material to be polished is charged anodically. The parts are then submerged into a heated electrolytic bath. Think of electropolishing as a reverse plating process. Instead of adding metal to the part’s surface, metal ions are removed. You can achieve mirror-like finishes with electropolishing. The downside is that it adds significant cost and complicates tolerance limits. Before adding an electropolishing requirement to your drawing, consult with Swissturn’s engineers to be sure that it is possible with the geometry and tolerances that you have chosen.
The 300 series of stainless steels can also be fully annealed. This process leaves the parts very soft, removes residual stresses from the material, and improves corrosion resistance. Machined parts are often annealed when the finished part is bent or crimped in its application to prevent fracture.
Many military applications require stainless steel that complies with Defense Federal Acquisition Regulation Supplement (DFARS) 252.225-7009. This regulation restricts the use of certain metals to those melted in specific countries. Swissturn purchases and inventories only DFARS-compliant stainless steel.
400 Series Stainless Steels
400 series stainless steels are known for their high corrosion resistance and magnetic properties. They are typically composed of iron, carbon, and chromium as the main elements, with additional alloying elements such as nickel, manganese, or molybdenum. One of the key characteristics of 400 series stainless steels is their hardenability. Although they exhibit resistance to some corrosive environments, they are not as resistant to corrosion as the 300 series stainless steels.
Another notable feature is their magnetic properties. Unlike the austenitic stainless steels (such as the 300 series), which are non-magnetic, the 400 series stainless steels are ferromagnetic. This makes them attractive for applications where magnetic properties are desired, such as in certain electrical and electronic devices.
Precipitation Hardening Stainless Steels
17-4 PH stainless steel is a precipitation-hardening stainless steel alloy known for its excellent combination of mechanical properties, corrosion resistance, and machinability. 17-4 PH stainless steel can achieve high strength through precipitation-hardening. It can be heat-treated to enhance its mechanical properties, including hardness and toughness. This stainless steel grade exhibits good corrosion resistance in various environments, including mild atmospheric conditions, freshwater, and some chemicals. However, it may not be as corrosion-resistant as other stainless steel alloys, such as 316 or 304. 17-4 PH stainless steel offers high strength and hardness, making it suitable for applications that require excellent mechanical properties. It combines good tensile strength, yield strength, and impact resistance. The alloy’s hardness and strength contribute to its wear resistance, making it suitable for applications subject to abrasive wear, such as pump components, valve parts, and gears. It exhibits good heat resistance, allowing it to withstand moderately high temperatures without significant loss of mechanical properties. For this reason, it is often used in applications involving elevated temperatures, such as heat exchangers and jet engine components.
Swissturn is very familiar with the machining characteristics of 17-4PH. While 17-4 PH stainless steel is not as easy to machine as other stainless steel grades, it can still be machined using proper techniques and tools. This stainless steel grade has fair weldability when compared to austenitic stainless steels. Low heat input and appropriate filler materials are generally recommended to maintain the alloy’s mechanical properties.
17-4 PH is a common stainless steel used where high strength, corrosion resistance, and weldability are necessary. It consists of 17% chromium and 4% nickel, plus a variety of other alloying elements to give it its strength and corrosion resistance characteristics. 17-4 PH has relatively good machinability and can often be machined in the heat-treated state. It is commonly used in medical devices, valves, screws, and gun components.
