Stainless steel, a material synonymous with strength and versatility, has a subset that plays an even more ubiquitous role – the common stainless steel grades. These grades apply from household essentials to vital medical instruments in various aspects of our lives. In this exploration, we uncover the essence of common stainless steel, its defining attributes, and the diverse industries it influences. Join us as we delve into common stainless steel’s unassuming yet indispensable world.

What is Stainless Steel?

Stainless steel is an alloy primarily composed of iron, carbon, and chromium, with some grades having slightly higher carbon content to enhance strength and hardness. It is common to add other metals, such as nickel, to stabilize the microscopic structure of the iron, molybdenum, or titanium to improve the whole steel alloy’s heat and corrosion resistance. Stainless steel exhibits relative ductility and malleability, depending on the heat treatment.

Stainless steel alloys’ good corrosion resistance and rust prevention come from their behavior when exposed to oxygen. Once the bare metal is exposed to the air, the chromium in the alloy forms a thin oxide layer over carbon steel. This oxide layer inhibits further metal oxidation, preventing corrosion and rust caused by moisture and oxygen. This corrosion resistance and rust prevention make many stainless steel grades ideal for producing parts exposed to the natural environment for extended periods.

There are five main categories of stainless steel:

1. Austenitic

2. Ferritic

3. Martensitic

4. Duplex

5. Precipitation-Hardened

While different types of stainless steel and alloys can be challenging to machine, the machinability of all these categories can be improved by following various best practices, including using sharp cutting tools with appropriate cutting geometries and selecting the right feed rates and cutting depths for each specific alloy.

Adding elements like sulfur, copper, lead, and other alloying elements can alter the material’s machinability. Sulfur, in particular, reduces the ductility of chips, making them easier to break off.

Austenitic Stainless Steels

Austenitic, stainless steel grade, named after the austenite crystal microstructure in iron, is the most common form of stainless steel. These grades are highly corrosion-resistant and strong, exhibit impressive machinability and good weldability, and typically have higher nickel content than other stainless steel alloy types. Austenitic steels are designated by numbers within the 300 range.

For instance：

Grade 304 steel (often called standard 18/8 stainless steel) contains at least 18% chromium, 8% nickel, and a maximum of 0.07% carbon. They are used in various domestic and industrial components, including cookware, screws, and machinery.

Grade 316 steel is very similar but contains higher amounts of nickel and molybdenum, enhancing acid and chloride resistance. As a result of better corrosion resistance, Grade 316 is well-suited for parts used in marine and chemical processing environments. However, the additional metals increase material costs.

Both 304 and 316 steels have low-carbon versions (304L and 316L) to reduce the risk of chromium carbide precipitation (which significantly decreases the corrosion resistance of welds). These are preferred in highly corrosive marine environments only.

Austenitic stainless steel can be the most challenging machine, mainly due to its gummy behavior and rapid work hardening. Cooling and lubricants are crucial during machining to prevent heat concentration.

Ferritic Stainless Steel

Ferritic stainless steel is a magnetic, high-chromium, low-carbon alloy known for its resistance to stress corrosion cracking and oxidation at high temperatures.

These grades are commonly used in automotive components, kitchenware, petrochemical equipment, and washing machine components, among other applications requiring resistance to potentially corrosive materials. It also exhibits excellent thermal conductivity, making it suitable for boiler heat exchangers and furnaces.

While not as strong as austenitic stainless steel, its low carbon content provides excellent ductility, allowing for extensive forming without compromising strength. It cannot be hardened through heat treatment. Ferritic stainless steel is often more cost-effective than austenitic stainless steel due to lower corrosion resistance and the absence of nickel. The grades are designated by numbers within the 400 range, with Grade 434 being common. Molybdenum enhances corrosion resistance, providing good mechanical properties, high heat, and oxidation resistance.

Grade 444 has lower carbon and nitrogen content, offering excellent corrosion resistance to pitting, crevice corrosion, and chloride stress corrosion cracking. It is suitable for water heaters, breweries, and chemical storage tanks.

Ferritic stainless steel is easily machinable, though higher chromium alloys like Grade 446 may pose machining challenges.

Martensitic Stainless Steel

Martensitic stainless steel is structurally similar to ferritic steel but has higher carbon content (usually above 0.10% and can go up to 1% or more). This creates a martensitic microstructure within the material, giving these alloys excellent strength and wear resistance after processing, but it also increases material brittleness compared to other stainless steels.

Moreover, the increased carbon content allows martensitic steel to undergo heat treatment and tempering processes to harden further and strengthen the metal. However, Martensitic stainless steel increases the material’s susceptibility to rust and higher corrosion resistance. As a result, this group is well-suited for applications requiring high strength and durability but only general to moderate corrosion resistance. Examples include turbine components, high-end cutlery, and mechanical valves and pumps.

Martensitic stainless steel is also designated by numbers within the 400 range, and like ferritic stainless and other steel alloys, it is relatively easy to machine, but the increased carbon content reduces its machinability. Grade 440C is known for providing the highest strength, hardness, and wear resistance that martensitic stainless steels steel alloys can offer, but achieving these properties requires heat treatment. In an annealed state, 440C steel is in its most machinable form, but using sturdy tools is recommended.

Duplex Stainless Steels

Duplex stainless steel is a highly corrosion-resistant alloy containing austenite and ferrite microstructures. As a result, it combines the characteristics of both categories, offering excellent ductility and weldability.

Duplex alloys can provide twice the strength of austenitic steel or ferritic stainless steels, and their resistance to stress corrosion cracking is much higher than standard austenitic alloys like Grade 304 and 316 but lower than standard ferritic steel. This combination of high strength and poor corrosion resistance also makes duplex alloys an ideal choice for underwater applications, as components must endure long-term exposure to corrosive saltwater.

Resistance to chloride pitting and crevice corrosion depends on the specific alloy’s chromium, molybdenum, and nitrogen content. However, duplex steels contain less nickel and molybdenum than austenitic alloys, making them more cost-effective. Additionally, their high strength allows designers to reduce the cross-sectional thickness of certain components, further contributing to cost savings and overall weight reduction.

Perhaps the most common duplex stainless steel is 2205 (named for its 22% chromium and 5% nickel content), frequently used in chemical and food processing equipment, storage equipment, and ship cargo holds. Due to their high annealed strength, duplex steels are often more challenging to machine.

Precipitation-hardened stainless steels

Precipitation-hardening stainless steel combines the advantages of austenitic and martensitic alloys, allowing it to withstand corrosion and achieve high strength and mechanical properties through various heat treatment processes while maintaining excellent corrosion resistance.

Apart from providing good oxidation resistance, these precipitation-hardened stainless steels generally perform comparably to austenitic 304 stainless steel in most cases. The most common precipitation-hardened stainless steel is 17-4 PH, or Grade 630, for its 17% chromium and 4% nickel content.

While the machinability magnetic properties of precipitation-hardened stainless steel vary with specific alloys, one notable advantage of these metals is that they can be easily machined under solution-annealed conditions and then undergo aging to improve the steel’s mechanical properties and strength.

Conclusion

Stainless steel finds extensive use in various industries, including construction, transportation, healthcare, food processing, the oil and gas industry, and more. Its properties make it ideal for kitchen utensils, medical equipment, automotive parts, building structures, pipelines, and even art installations. The wide range of grades and types available allows customization to meet specific requirements across different applications.

STCNC offers efficient on-demand manufacturing services. Our experienced team of engineers and technicians works closely with customers at every stage of design and production, ensuring the manufacturability of parts is optimized, and the most suitable manufacturing method (or combination of methods) is employed. We assist customers of different sizes in different industries in producing high-quality parts at competitive prices and with rapid turnaround times.