Materials Monday: Stainless Steel-The Strong and the Rustless
Stainless Steel: The Strong and the Rustless
Some of us are old enough to remember the DeLorean, a stainless steel-bodied sports car designed and manufactured by a former General Motors executive of the same name. The DeLorean Motor Company sold its shiny, every-one-looks-the-same namesake for almost two years before shutting down and is now best known for giving us the time machine in the unrivaled Back to the Future series. We won’t get into the possible rebirth of the DeLorean as a battery-electric vehicle except to say Google it, and wish “better luck next time” to the brave souls at DeLorean Motors Reimagined LLC.
Beyond time machines
Troubled business model and legal problems aside, John DeLorean’s idea of a stainless steel vehicle exterior was a good one. That’s because this family of tough, durable alloys is both rust and corrosion-resistant, putting them high on the materials list for a wide range of outdoor applications, including architecture and storage containers for chemicals and other liquids. Stainless steels are also a first choice for surgical tools like hemostats and cannulas, as well as kitchen sinks and cookware, aircraft components, and much more.
Stainless steel has been around a long time. American inventor and metallurgist Elwood Haynes applied for the first patent on “rustless steel” in 1912, although most industry experts credit Englishman Harry Brearley for the discovery during his search for a metal that would resist erosion in gun barrels. No matter, the two settled their differences and went on to form the litigious and long-defunct American Stainless Steel Corporation.
In all fairness, their work was nothing new. Even as far back as the 1700s, metallurgists recognized that adding small amounts of the element chromium to carbon steels and iron alloys made these metals stronger and more resistant to oxidation and chemical attack. As such, so-called “chromium steels” began to gain popularity over the next century, with steelmakers in Germany, Britain, and the U.S. jumping aboard the corrosion-resistant train. Some might argue that Haynes and Brearley were simply the beneficiaries of being in the right place at the right time.
Meet the family
Brief history lesson behind us, it’s important to point out that the stainless steel family is quite large. Five general classifications exist, each separated from its siblings by a unique crystalline structure and each containing a broad selection of individual alloys and material properties. However, all have at least 11% and no more than 30% chromium, and alloying elements such as nickel, molybdenum, sulfur, and manganese serve to make stainless steels stronger, more heat and corrosion resistant, less prone to intragranular attack, and to increase workability.
- Austenitic: For instance, metallurgists will tell you that austenitic stainless steels have a face-centered cubic structure. And for reasons they will be more than happy to explain but which lie beyond the scope of this article, this molecular arrangement makes these alloys both non-magnetic and non-hardenable via conventional heat-treating methods. It also puts them among the most commonly used of all stainless steels. Due to their high chromium content, they are known for their exceptional corrosion resistance and ability to withstand extreme temperatures on both ends of the spectrum.
- Martensitic: This was the original stainless steel that Haynes reportedly invented and Brearley later patented. Martensitic alloys boast a body-centered tetragonal cubic structure. Together with a smidge of carbon (around 1%, give or take), this stainless steel family can be made fairly hard, and most are also magnetic. They contain much less chromium than austenitic steels and are therefore less corrosion resistant, but are generally stronger, more brittle, and less workable.
- Ferritic: These stainless steels are similar to yet distinctly different from their martensitic counterparts. They have a body-centered cubic lattice structure like that of carbon steels, and as their name implies, contain ferrite, a ceramic-like, magnetic material. Depending on the specific alloy, the chromium content of ferritic stainless steels varies widely, but most contain very little to no nickel. They are not hardenable by heat-treatment and only minimally so with cold working methods. Small amounts of niobium and other alloying elements give some ferritic alloys good weldability.
- Duplex: In some ways, duplex stainless steels offer the best of both worlds, as their metallurgical structure straddles the fence between ferritic and austenitic. This gives them even greater strength and corrosion resistance than the latter, but because they contain less nickel (like ferritic stainless steels), they are much less costly to produce. This makes them a favorite in the oil and gas industry, which uses large amounts of duplex and super duplex steels in offshore and deep-water oil production.
- Precipitation Hardening: Readily identifiable by the suffix PH, this group of stainless steels can be thought of as the Superman of the family. When heat-treated, they offer tensile and yield strengths several times that of austenitic alloys. As with duplex steels, PH stainless may fall into multiple metallurgical categories—15-7 PH, for instance, contains both austenitic and martensitic structures, A 286 is classified as austenitic, and the ubiquitous 17-4 PH is considered martensitic. Whatever the classification, however, all are robust, and rank close to superalloys in terms of metallurgical properties and workability.
As suggested earlier, much of this metal-speak is only of interest to metallurgists and materials engineers. For those of you designing tractor widgets or the next greatest medical device, what’s important is how the metal will perform in the field, and how much it will cost to machine, cut, form, stamp, and weld into its finished shape.
The last of these is easy enough to find out—just upload a file to the Prismier Rapid Quote page and we’ll be happy to quote it for you. As for which stainless steel is right for your application, the answer is more complex. Many dozens of different alloys are available, each with its own strengths and weaknesses, price range, availability, and especially manufacturability. However, here are some of the more common stainless steels in use today, along with a few application examples for each:
- 304 stainless steel is probably the most popular of all stainless steels. Like all 300-series stainlesses, it is austenitic. It’s also slightly magnetic, fairly machinable and formable, and given its high amounts of chromium and nickel, provides excellent corrosion resistance. It’s also known as food grade stainless steel. Refrigerators and dishwashers are often made of 304 SS, as are commercial food processing equipment, heat exchangers, and industrial fasteners.
- 301 stainless has a higher carbon content than 304 (0.15% compared to 304’s 0.08% by weight) which makes it better able to withstand mechanical force. Since it has less chromium and nickel than 301, it is a less expensive option. However, it also tends to be a little less corrosion resistant, a drawback that becomes more pronounced at higher temperatures.
- 316 stainless kicks it up a notch. Its 2% to 3% molybdenum content increases both strength and corrosion resistance, but sadly, reduces its manufacturability somewhat. Despite this, it is commonly used in marine applications, for chemical processing equipment, and for food and surgical components. It is slightly more expensive than 304 SS. And as with 304, there is also an “L” version. This signifies low carbon, which makes the alloy more weldable.
- 303 stainless steel contains a very small amount of sulfur, making it the most free-machining of all stainless alloys. Unfortunately, the sulfur also makes the material less corrosion resistant and a bit less tough, although it remains a favorite for aircraft fittings and gears, nuts, bolts, and other hardware items, as well as bushings and bearing housings.
- 430 stainless is a ferritic class stainless steel grade which is probably the second-most-used grade after 304. It’s also a food grade stainless steel, though is better suited for applications where the need for corrosion resistance is only moderate since it contains only 16 to 18 percent chromium. It’s ideal for situations where chemical exposure is possible due to its high resistance to nitric acid. It can become brittle at extreme temperatures and should not be welded for heavy load or high impact situations. In spite of these limitations, its lower cost makes it a fine option for the right applications such as commercial kitchen and professional catering equipment.
- 440C is a martensitic grade of stainless steel. Its high carbon content allows 440C to be heat-treated to very high hardness levels. Many knife blades are made of 440C, as are chisels, valve seats, ball and roller bearings, and other parts requiring excellent wear characteristics and moderate corrosion resistance.
- 17-4 PH stainless steel contains 17% chromium and 4% nickel. As noted, it can be made quite hard, and boasts corrosion resistance on par with austenitic stainless steels (which is damned good). Like 15-5, 13-8, and the other grades of PH steel, 17-4 is often supplied in a solution treated version (making it relatively workable) and then age-hardened after fabrication.
- 17-7 PH stainless steel contains 16 to 18% chromium, 6.5 to 7.75% nickel, 0.75 to 1.50% aluminum. The addition of manganese increases its hardenability. It’s the most formable of all PH grades and has the highest strength and hardness. It’s well suited for aerospace and other applications demanding high strength and corrosion resistance. ‘
This is just the tip of the stainless steel iceberg. We didn’t delve too deeply into the duplex steel family, the ferritic stainless steels (which overlap with the others in the 400-series, most of which are martensitic), 200-series stainless (the stuff of sinks and spoons), and more. Nor did we mention that all of the superalloys—among them titanium, cobalt chrome, Inconel, Hastelloy, etc.—can be considered “stainless steels,” even though their exceptional toughness and heat-resistance puts them into another class entirely. Regardless, we’re happy to discuss all of these excellent materials and have years of experience turning each into beautiful and highly accurate custom parts. Just give us a call or get uploading.
If you'd like to know more, pick up the phone and call us at (630) 592-4515 or email us at email@example.com. Or if you're ready for a quote, email firstname.lastname@example.org. We'll be happy to discuss your options.