The Process For Powder Coating Stainless Steel
What Is The Difference Between Steel And Stainless Steel?
As the backbone of modern industry, steel is the world's most important engineering and construction material, which is used in nearly every aspect of our lives. It's so versatile that it's virtually everywhere, from the cars we drive and homes we live in, to washing machines, cutlery, and surgical instruments.
However, the term "steel" actually covers a wide variety of metals with different properties, which can be separated into different groups. In today's article, we'll discuss the difference between steel and stainless steel — both reliable types of metal found in nearly every conceivable industry.
What is Steel?
Steel is the most common metal alloy in the world, and it consists of iron (base metal) and varying amounts of carbon (alloying element). Since both elements have been known to humanity since ancient times, the alloy has been produced, in various forms, for the better part of the last 2,000 years. Some of the earliest steel discovered has been dated to 1400 BCE.
Iron is the base metal for the alloy, with carbon being the second most important and also cost-effective element used to alloy iron. Iron, in its solid metal state, has horrible mechanical properties; it's relatively soft, rusts easily, and readily dissolves in acids.
Carbon, and other alloying elements, are used to fill in microscopic weak spots within the crystalline structure of the iron, which gives steel alloys their flexibility and tensile strength. In most cases, steel alloys contain anywhere between 0.002% and 2.14% carbon. If the carbon content is higher than 2.14%, the alloy is then defined as cast iron.
In addition to carbon, many other elements can be added to enhance the alloy's physical, thermal, and electromagnetic properties. These mostly include chromium and tungsten, though other elements can also be used.
Types of Steel
Steel, as an alloy of iron, is divided into four main types. We'll mention all four types, but for this discussion, our main focus will be carbon steel vs stainless steel.
Carbon Steel
Carbon steel got its name because of the fact that it contains a very small amount of other alloying elements. It's a dull-looking, matte-like metal that's exceptionally strong but vulnerable to corrosion. They contain up to 2% carbon and lack any other alloying element that would classify as alloy steel.
Carbon steel is used to make knives, car parts, and high-tension wires. It's further classified into three groups:
- Low Carbon Steel — Low carbon steel is also known as mild steel and refers to steel that has less than 0.30% carbon content. It's the most widespread type of steel on the metal markets since it's relatively inexpensive, easy to form, and well-suited for applications in which tensile strength requirements aren't high or precise, like industrial hardware.
- Medium Carbon Steel — These contain anywhere between 0.30% and 0.60% carbon and between 0.60% and 1.60% manganese as an additional alloying element. This enhances the strength of the steel, but the tradeoff is low toughness and ductility, which negatively affects machinability and weldability.
- High Carbon Steel — High carbon steel has between 0.60% and 1.70% carbon by weight and manganese content up to 1.65%. It's incredibly tough and hard, but it's also quite difficult to cut, weld, or form. It's mostly used for cutting and milling tools.
Alloy Steel
Alloy steel is manufactured by blending carbon steel with alloying elements to give steel distinct qualities. These commonly include Chromium, Cobalt, Molybdenum, Nickel, Tungsten, Vanadium, and traces of other alloying elements.
Due to incredible variety, you can make steel with particularly any property, though some might end up being pricey, and their strength depends on the alloying element concentration.
Tool Steel
Tool steel alloys are known for being exceptionally hard and resistant to high temperatures. Their name is derived from the fact that they're mostly used to make metal tools. They're commonly blended with molybdenum, vanadium, tungsten, and cobalt.
Stainless Steel
Last but not least, stainless steel is the best-known variety of steel on the market, and besides the usual iron and carbon, it also contains between 10% and 20% chromium. This combination makes steel corrosion-resistant and easily shaped. Stainless steel is used in aerospace, medical equipment, residential applications, cookware, and commercial/industrial building cladding due to its easy manipulation, flexibility, and quality.
What is Stainless Steel?
All stainless steel is steel, but not all steel is stainless — in other words, stainless steel is one of four types of steel. All stainless steel has one key property, and that's corrosion resistance. This is attributed to the high chromium content, which ranges from 10.5% to 20% by weight. Stainless steels have a relatively low carbon content that's lower than 1.2%.
The chromium within the material reacts with oxygen from the environment and creates a passive surface layer of chromium oxide, which prevents the oxidization of iron within the alloy. The protective layer further prevents the formation of iron oxide, so the steel can't corrode. Besides that, it encourages self-healing in case of abrasions.
Beyond corrosion resistance, the mechanical and other physical properties of stainless steel greatly depend on additional alloying elements such as molybdenum, nickel, copper, titanium, etc.
Types of Stainless Steel
Just like typical steel, stainless steel comes in five major types of stainless steel, which differ in their chemical composition and their crystalline structure.
Austenitic Stainless Steel
Steel grade SAE 304 is the most popular austenitic stainless steel due to its desirable properties, like high chromium content and relatively low nickel content, which makes it great for producing surgical equipment and implants. Its austenitic structure allows for easier drawing without any previous annealing while the steel retains its hardness.
Austenitic stainless steels, like SAE 304 and SAE 316 (and their variants), often have non-magnetic properties, despite relatively high iron content. The secret to austenitic steel's magnetic impermeability lies in the austenitic structure itself, which arranges electrons in the alloy in a way so that they're not affected by the magnetic fields.
It's worth noting that cold working may disrupt non-magnetic properties, turning the steel magnetic, but this is usually resolved with low-temperature heat treatment.
Ferritic Stainless Steel
Ferritic stainless steels have less than 0.2% of carbon, anywhere between 10.5% to 27% chromium, and no nickel. This means that they can be cold-formed to a reasonable degree of hardness since the hardness can't be increased using heat treatment. SAE 409, SAE 430, SAE 439, and SAE 441 are the most commonly used ferritic grades of stainless steel.
Martensitic Stainless Steels
Martensitic stainless steels, which typically contain up to 1.2% carbon and anywhere between 11.5–18% chromium, and occasionally a percentage of nickel as well, can be hardened with heat treatment. These steels feature great ductility and strength, good magnetic properties, and relatively good corrosion resistance. They usually show up in 4xx-level steel grades and are typically used to make cutlery, turbines, tools, and surgical instruments.
Duplex Stainless Steel
Duplex, or austenitic-ferritic stainless steel, combines the austenitic and ferritic steels equally, often resulting in chromium content of 21-27%, 1.35-8% nickel, 1.05-3% copper, and 0.05-5% molybdenum. Duplex steels are generally tougher than either ferritic or austenitic on their own, thanks to the mix of different chemical elements. The resulting duplex steel is frequently used for manufacturing storage tanks and containers for the transport of harsh chemicals.
Precipitation Hardening Stainless Steel
Precipitation-hardening stainless steel is exceptionally strong. Besides nickel and chromium, which make up for 15–17.5% of the weight combined, copper, niobium, and aluminum make up the remaining 3-5% of the steel's overall mass. The corrosion resistance of precipitation-hardening steel is also quite good, as it's comparable to that of austenitic stainless steel.
What Is the Difference Between Stainless and Ordinary Steel?
Besides their obvious differences in chemical composition, stainless steel and ordinary carbon steel differ in several ways. Carbon steels, especially high-carbon varieties, are used to make cutting tools and machine components, as it's less susceptible to wear and tear. Stainless steel is used in applications that demand high levels of hygiene, such as the medical and food industry, but also in industries that generally require the use of corrosion-resistant steel alloys.
Additionally, stainless steel has higher heat resistance than typical steel and can be fabricated in a way that makes the material unsusceptible to magnetism. It's also much more resistant to impact due to higher chrome contents and isn't easy to fabricate compared to mild steel.
Mixing carbon steel with additional alloying elements to create stainless steel greatly affects the cost of steel production, making stainless steel a pricier option. However, stainless steel is an incredibly hygienic material, and some variants are even body-friendly, making them great for implant production.
The Difference Between Stainless Steel and Galvanized Steel
Just like stainless steel, galvanized steel also offers good corrosion resistance, but at a fraction of the price — usually marginally pricier than untreated carbon steel. However, the two materials differ significantly.
Stainless steel contains chromium, nickel, and other alloying elements, which are chemically bonded with the iron-carbon mixture. Galvanized steel is any type of carbon or alloy steel that has been coated with zinc — usually through electroplating.
This creates a barrier between carbon steel and the environment, preventing oxidation. While suited for a myriad of uses, this type of steel can't be used for food processing and isn't recommended for food storage. It reacts with heat and releases toxic fumes (while cooking food), and it also reacts with acidic foods.
Summary
In summary, there's no clear-cut winner when it steels vs. stainless steel discussion since each type is best suited for different applications. Using carbon steel in medicine or food processing could lead to many problems, and using stainless steel as bearing beams in construction would be too pricey, so the choice of materials actually greatly depends on the application.
If you want to learn more about Stainless Steel and Surgical Steel, please visit Reid Supply, and refer to their Engineer Resource Guides or some of their guides on Surgical and Stainless steel.