The corrosion resistance of stainless steel, thanks to its greater chromium content, makes it an obvious choice for parts that require protection against oxidation. However, there are many different types of stainless steel, each offering different benefits. In this article, we’ll discuss the different grades, their limitations, and heat treating processes used to enhance material properties for varying applications.
Grades of Stainless Steel
There are four primary grades of stainless steel. Austenitic stainless steels, such as 304 and 316, are high-chromium steel grades with the greatest corrosion resistance of all stainless steels. Heat treatment does not increase the hardness of austenitic stainless steel, but it can enhance corrosion resistance and reduce hardness.
Cold working manufacturing processes can sometimes introduce ferromagnetism to austenitic stainless steels, which is undesirable for applications such as surgical instruments, medical devices, electronics, and some types of fasteners. Annealing is a common process for these types of parts because it can eliminate the ferromagnetism caused by cold working.
Ferritic stainless steels, such as 430, have less nickel content than austenitics and comparable amounts of carbon. However, the effect of heat treatment is generally the same. Ferritic stainless components can become more corrosion-resistant as a result of annealing, but thermal processing will not increase its hardness.
Compared to austenitic and martensitic stainless steel, ferritic stainless is best suited for cost-conscious applications where components don’t require high durability or extreme temperature resistance. For example, car exhausts and bathroom fixtures are commonly made from ferritic stainless steel.
Martensitic stainless steels like 410, 416, 420, and 440C are generally the strongest grade of stainless steel available due to higher carbon content and less nickel than their austenitic counterparts.
Mold-grade stainless steels are a subgroup of 420. These special grades have higher corrosion resistance and polishability, ideal for machined parts, plastic molding dies, and firearm components.
Grade 440C is the ultimate tool steel stainless steel weighing in at 1% carbon and with a higher wear resistance than most steels. It’s mostly used in cases that require good corrosion and wear resistance, such as food processing. Industrial cutting and grinding tools are often made from 440C.
Precipitation Hardening-Grade Stainless Steel
Stainless steels are commonly “aged,” a process also known as precipitation hardening. Precipitation hardened stainless steels such as 17-4 and 13-8Mo provide increased corrosion resistance and are ideal for service in high-heat environments. During the aging process, particles “precipitate” out of the intermetallic structure, thus allowing the material to harden—hence, why this process is called “precipitation hardening.”
Key Specs for Precipitation Hardening
- A286 stainless steel is commonly used to make fasteners for applications that require high corrosion and heat resistance, namely aerospace and specialty automotive. For example, cars with turbochargers will use exhaust fasteners made from A286.
- 15-5, 13-8Mo, and 17-4 stainless steels are used in aerospace tooling and engine parts. These materials are also used widely in tool and die applications and firearms components because they tend to be extremely dimensionally stable and amenable to tight tolerances and provide better corrosion resistance than martensitics.
- 17-7 stainless steel is primarily used in aerospace, typically requiring an austenite condition followed by age hardening in a vacuum furnace.
Unlike most materials, stainless steel shrinks during the precipitation hardening process. The exact shrinkage rate depends on the steel grade, but it’s predictable. We typically observe a shrinkage rate from 0.0004–0.0008 inches per inch, depending on the length of the part.
Other Key Processes for Stainless Steel
Annealing is a controlled heating and cooling process that maximizes corrosion resistance and reduces hardness. It’s the first step in heat treating any stainless steel.
When cold forming, annealing is especially important. Stainless steel work hardens quickly when bent, and the material often needs to be annealed between forming operations to keep it workable. As we previously mentioned, annealing also wipes away undesired ferromagnetism introduced by cold working in austenitic stainless steel.
The annealing process for stainless steel is typically completed in the mill, and the material is purchased in its annealed form. Austenitics can also be purchased in cold-worked conditions and are commonly referred to as quarter-hard, half-hard, full-hard stainless steels, to name a few.
Martensitic and precipitation hardening-grade stainless steels can both be through hardened. Through hardening, or rapid quenching, is a process that involves heating an alloy to enhance its physical properties and quickly quenching to preserve those properties achieved during the heat treating process.
Through hardening is commonly used on stainless steel springs, seating components, hand tools, and other parts that are required to sustain heavy loads.
Case Hardening for Special Applications
Case hardening, nitriding, and FNC are used to increase wear resistance in special applications where carburization is required. These processes work to break down the oxide layer that forms on the surface of stainless steel to let oxygen in to carburize the material effectively. All case hardening processes are used for the same reasons, but FNC lends the greatest degree of corrosion resistance in the finished parts. Martensitic and austenitic stainless steels can both be carburized; the only downside is that they can reduce the corrosion resistance of the stainless material.
Case hardening is typically used for applications requiring greater wear resistance, most commonly for industrial food processing (grinders, pelletizers, etc.).
Reducing Risk with Stainless Steel
In a previous article, we discussed many factors that make stainless steel procurement risky, and there are also risks that come with heat treatment processes. To achieve the most predictable outcomes for your stainless steel parts, it’s best to consult the metallurgy experts on your heat treatment team. At Paulo, we often work with our customers to ensure part designs are optimized so that heat treatment yields finished parts that meet all required specifications for service in the field.At Paulo, we have a long history of working with stainless steel. We work closely with engineering teams and materials suppliers to help our customers deliver precise, reliable parts for automotive, aerospace, and beyond. Have questions about an upcoming stainless steel project? Request a quote or connect with a Paulo expert today.