Originally published on fastradius.com on December 7, 2020
Stainless steels are among the most popular material options for metal manufacturing, and are used in applications ranging from cookware to automotive components to chemical processing equipment. Many stainless steel items are produced via CNC machining, a versatile manufacturing method that leverages the precision of computer-guided mills, drills, lathes, and other cutting tools to efficiently and cost-effectively create precise, repeatable parts.
However, the term “stainless steel” actually refers to more than just a single material. Stainless steel is a category of metals, each of which exhibits different characteristics. Selecting the right stainless steel for a particular application can improve or hinder the part’s performance in significant ways. Here’s what product teams need to know.
What are stainless steels?
Stainless steel is an alloy composed primarily of iron, carbon, and chromium, though many grades include a slightly higher percentage of carbon for greater strength and hardness. Incorporating other metals — such as nickel to stabilize the iron’s crystalline microstructure, or molybdenum or titanium to increase the alloy’s heat and corrosion resistance — is also common practice. Stainless steels can be relatively malleable and ductile, depending on the heat treatment.
Stainless steels’ corrosion and rust resistance stem from the alloys’ behavior when exposed to oxygen. As soon as the bare metal is exposed to open air, the chromium in the alloy produces a thin oxide layer. This oxide layer inhibits further oxidation of the metal, preventing corrosion and rust otherwise caused by moisture and oxygen. This resistance to corrosion and rust makes many stainless steels well-suited for parts that will be exposed to the elements for extended periods.
There are five primary categories of stainless steels:
- Austenitic
- Ferritic
- Martensitic
- Duplex
- Precipitation hardened
Though stainless steels are generally difficult to machine, the machinability of all each of these categories can be improved by following a number of best practices, including: Using sharp tooling with proper cutting geometry and choosing the appropriate feed rates and depth-of-cut for each specific alloy.
Furthermore, the inclusion of elements like sulfur, copper, lead, and other alloys can alter the material’s machinability. Sulfur in particular reduces the ductility of chips, enabling them to break away with greater ease.
1. Austenitic stainless steels
Austenitic stainless steels — named for the austenitic crystalline microstructures in the iron — are the most common form of stainless steel. These steel grades provide high corrosion resistance and strength, impressive post-machining formability, good weldability, and typically have much higher nickel content compared to other alloy types. Austenitic steels are denoted by numbers in the 300 range.
Grade 304 steels — commonly known as standard 18/8 stainless — have a minimum of 18% chromium and 8% nickel, and a maximum of 0.07% carbon. They are used to create a wide range of household and industrial components, including cookware, screws, and machinery.
Grade 316 steels are fairly similar, but contain higher levels of nickel and molybdenum, which offer greater resistance to acids and chlorides. Grade 316 is therefore well-suited for parts used in marine and chemical processing contexts. The additional metals increase the material cost, however.
Both 304 and 316 steels are available in low-carbon grades (304L and 316L), which provide reduced risk of chrome carbide precipitation (a phenomenon that drastically reduces corrosion resistance along weld seams). These are preferred for highly corrosive environments.
Austenitic stainless steels can be the most difficult to machine, in large part because they exhibit gumminess and work harden rapidly. Coolants and lubricants are especially important during machining to prevent heat from concentrating.
2. Ferritic stainless steels
Ferritic stainless steels are magnetic, high-chromium, low-carbon alloys with high resistance to stress corrosion cracking (a common type of steel degradation) and oxidation at high temperatures.
These grades are often used for automotive components, kitchenware, petrochemical equipment, and other applications that will need to interact with potentially corrosive materials. Ferritic stainless steels also exhibit excellent thermal conductivity properties, making them ideal for applications like boiler heat exchangers and furnaces.
While ferritics do not possess the same strength and corrosion-resistance as austenitic stainless steels, their low carbon content provides superior ductility, allowing these grades to undergo extensive shaping without weakening the material. They are also non-hardenable by heat treating. Due to their lack of nickel, ferritic stainless steels are often less costly than austenitic grades. These grades are designated by numbers in the 400 range.
Grade 434 is one of the most commonly used ferritic stainless steels. Molybdenum increases the alloy’s corrosion resistance, resulting in a material that provides good mechanical properties as well as strong resistance to heath and oxidation.
Grade 444, on the other hand, includes low levels of carbon and nitrogen, offering superior resistance to pitting, crevice corrosion, and chloride stress corrosion cracking — rendering it ideal for applications such as hot water tanks and brewery or vineyard equipment.
Ferritics are among the easiest stainless steels to machine, though alloys with higher chromium contents — such as grade 446 — often present machining difficulties.
3. Martensitic stainless steels
Martensitic stainless steels are structurally similar to ferritic steels, but include higher percentages of carbon (the carbon content of ferritic steels is typically below 0.10%, while martensitic grades can contain up to 1% or more). This creates martensite microstructures within the material, which give these alloys superior strength and wear resistance after machining compared to other stainless steels, but can also increase the brittleness of the material.
Further, their increased carbon content allows martensitic steels to be heat- and age-treated to further harden and strengthen the metal. However, it also increases the materials’ susceptibility to rust and corrosion. This family is therefore ideal for applications that require high strength and durability but only average corrosion resistance, such as turbine components, high-end cutlery, and mechanical valves and pumps.
Martensitic stainless steels are also denoted by a 400 number and, like ferritics, are relatively easy to machine, though increased carbon content reduces machinability. Grade 440C is notable for providing some of the greatest strength, hardness, and wear resistance possible for stainless steel alloys — but require heat treatment first to fully realize these characteristics. When in the annealed condition, 440C steels in the state that is most-easily machined, though robust tooling is advised.
4. Duplex stainless steels
Duplex stainless steels are extremely corrosion-resistant alloys that contain both austenite and ferritic microstructures. The result is a fairly malleable and weldable set of steels with a mix of characteristics from both categories.
Duplex alloys can provide twice the strength of austenitic or ferritic stainless steels, and demonstrate stress corrosion cracking resistance far greater than common austenitic alloys like grades 304 and 316, if less than standard ferritic steels. However, this combination of high strength and corrosion resistance make duplex alloys ideal for underwater applications where parts must withstand corrosive saltwater over extended periods.
Chloride pitting and crevice corrosion resistance are determined by the chromium, molybdenum and nitrogen content of a particular alloy, but duplex steels contain less nickel and molybdenum than austenitic alloys, and are therefore generally less expensive. Additionally, their high strength also enables designers to reduce section thickness for some components, further contributing to reduced cost and overall part weight.
Perhaps the most common form of duplex stainless steel is 2205 (named for its 22% chromium and 5% nickel composition), which is commonly used in chemical processing and storage equipment and cargo tanks. Duplex steels tend to be more difficult to machine due to their high annealed strength.
5. Precipitation-hardened stainless steels
Precipitation-hardened stainless steels combine the benefits of both austenitic and martensitic alloys, making them capable of achieving high strength and mechanical properties through heat treatments while retaining good corrosion resistance.
In addition to providing good oxidation, these precipitation-hardened stainless steels perform comparably to austenitic grade 304 in most conditions. The most common form of precipitation-hardened stainless steel is 17-4 PH, or grade 630, which gets its name from its composition of 17% chromium and 4% nickel.
While the machinability of precipitation-hardened stainless steel varies based on the particular alloy, one significant advantage of these metals is that they can be easily machined when in solution-treated condition — which can be followed with an age-hardening process to improve the steel’s strength.
Find the ideal stainless steel alloy for every project
Stainless steels come in a number of different forms. If product teams are looking for super strength with average corrosion resistance, then martensitic alloys would be a good fit. Likewise, teams that require a steel offering high strength, high pitting, and corrosion resistance may find a duplex alloy ideal.
Determining the appropriate material for a particular component is a crucial part of the product design and production processes, and it behooves product teams to be thorough in vetting material options. That said, finding the right material to meet a project’s unique requirements can be simplified with the help of an experienced manufacturing partner like SyBridge .
SyBridge provides efficient on-demand manufacturing services. Working side-by-side with customers during each stage of design and production, our team of seasoned engineers, designers, advisors, and technologists ensure that parts are optimized for manufacturability and that the best-fit manufacturing method — or combination of methods — is always employed. Fast Radius helps teams of all shapes and sizes create superior parts at a competitive price point and fast turnaround time. Contact us today to learn more.
