Grade 304 vs 316: The Differences between Stainless Steels
304 Stainless Steel versus 316 Stainless Steel: to the average layperson, these will appear to be very similar materials. They both fall into the 300 Series class, made up of austenitic chromium-nickel alloys. They’re the same grayish-silver color, with a bright and lustrous shine. Neither type can be hardened through heat treatments, but can be work-hardened.
But as those with more knowledge of these steels realize, it’s the small details between the two grades that make for crucial differences. Knowing more about these difference can provide consumers with better insight into the right type of steel for their use.
300 Series Steel
The steels within the austenitic class are generally alloyed with three main elements: chromium, nickel, and molybdenum. They’re known as low-carbon steels, with less than 0.8% carbon making up their chemical composition. Alloying with chromium increases the strength, hardness, and corrosion resistance of iron. Nickel also boosts the metal’s strength and hardness, while helping to prevent against loss of ductility and toughness.
However, it’s the addition of molybdenum that is one of the most significant differences between the 304 and 316 grades. Grade 304 stainless steel contains 18% nickel and 8% chromium, while 316 has 16% chromium, 10% nickel, and 2% molybdenum. Like the other alloying elements nickel and chromium, molybdenum is used to strengthen and toughen steel. But its biggest asset in 316 stainless steel is in the prevention against chloride corrosion.
Molybdenum: The Difference Maker
The boosted percentage of nickel, along with the inclusion of molybdenum, means that 316 has better chemical resistance when compared to 304 steel. The most significant corrosion protection it offers is against chlorides. In applications where the steel will be exposed to salts or seawater, 316 steel outperforms other Series 300 steels in resisting the development of pits and crevices. This is also why 316 is known as “marine grade” steel.
That’s not to say that Grade 304 isn’t strong and resistant to general corrosion on its own merits. In fact, its overall attributes and reliable performance is why 304 steel is the most widely used of all stainless steels. One other great advantage is its cost: by containing less nickel, along with no molybdenum, Grade 304 is a more affordable choice of material. So as long as the steel will be used with milder acids only, and have little to no contact to salt, it will save a bit of money to construct items out of 304. For applications that include exposure to chlorides and powerful corrosives, 316 with its added molybdenum would be the better choice.
When it comes to formability and welding, molybdenum comes into play again. Formability refers to a metal’s ability to be formed into a particular shape without incurring damage. While 316 is regularly used to form metal parts and structures, the hardness coming from molybdenum means forming requires more effort than compared to 304. The same goes for welding; both grades are commonly welded austenitic steels, but 304 is generally more easily weldable.
Advantages and Applications of Grade 304 Stainless Steel
With its good formability, resistance against heat and corrosion, and affordability, it’s understandable that 304 is the most popular of all stainless steels. It’s a reliable and versatile alloy, and readily welded by all common methods.
Some of the most common applications of Grade 304 steel include:
• Kitchen equipment and cookware
• Cutlery and flatware
• Appliances such as refrigerators and dishwashers
• Piping and fasteners
• Equipment used in dairy, brewing, and pharmaceutical production
Advantages and Applications of Grade 316 Stainless Steel
While it does come at a higher cost, when an application requires protection against severe corrosion, 316 will be the recommended choice. Its strength and durability are a necessity for usage in harsh environments, including underwater. In some cases, 316 may last many times longer than a part made of 304 – giving extended usage life to your parts and structures.
Some of the most common applications of Grade 316 include:
• Marine vessels and structures in marine environments
• Chemical processing and storage equipment
• Medical devices
• Textile, pulp, and paper manufacturing
• Jet engine parts
Annealing vs Tempering
Annealing and tempering are two of the most common heat treatment services used in metal production. But you may wonder: what is the purpose of heat treating in the first place? What differentiates these processes, and why choose one over the other?
Heat Treatment
When producing metal products, a beam or plate emerging from the casts aren’t necessarily considered to be finished. Through heat treatment, the physical and mechanical properties of the metal can be changed without affecting its shape. So the metal is heated short of the point where it would begin to melt or deform its shape, but high enough to modify its internal crystallization structure.
The goal is that after completing its heat treatment, the item will have been transformed to produce a more useful type of metal. With heat treatment, there will be improvements in the metal’s toughness, strength, and durability. But it can also boost assets in different areas: one process can produce a more ductile product, while another creates a tougher one. This is where the choice of heat treatment is key: once you identify what qualities are most important for the finished product, it will help determine which method is best to use.
Tempering
Tempering is usually performed on steel and other iron alloys, because those freshly-cast metals are extremely hard. This might sound like a nice benefit – but that high degree of hardness causes the steel to be brittle and susceptible to cracking. This means the untempered metals are considered unsuitable for most applications due to their structural vulnerability. But with tempering, some of this excess hardness is reduced. What remains afterwards is still a very hard metal, but enough internal stress has been alleviated to make it stronger overall.
When casting steel, once the metal has been formed into its desired shape, it’s immediately quenched to cool it as fast as possible. Think of it like immersing blanched vegetables in an ice bath: the goal is to lock in the iron’s structure at its peak. However as mentioned previously, while this quenched metal is incredibly hard, it is quite brittle. It is critical to follow the quenching process immediately with tempering, to help refine the steel before it can cool with existing flaws or weaknesses.
To begin tempering, the metal is placed in an oven. The oven’s heat is then carefully and gradually raised to avoid any metal cracking. Once it hits the desired temperature, the steel is held there for a length of time to allow the heat to permeate it completely. It is then removed and allowed to cool in open air.
When the tempering process is completed, it will have changed the metal’s ductility, strength, toughness, and hardness. What initially was an item too hard and brittle for use is now ready for sale or additional processing. The use of tempered steels include applications where toughness and strength are critical: construction, industrial machinery, and cutting implements.
Annealing
At first, annealing seems virtually the same as tempering: slowly heating metal to a specific temperature, and then letting it cool. But the big difference comes down to cooling. With tempering, the metal is removed from the oven and allowed to cool on its own to room temperature. Annealing, on the other hand, involves great effort to extend that cooling period as long as possible.
Why is this slow cooling significant? It’s because as the metal cools, its crystal structure continues to change with the temperature. Tempering is a simpler process because the main objective is to reduce brittleness. In annealing, the extended cooling allows for even more transformation and refinement of the metal’s structure. One method of cooling is simply turning down the oven until the temperature reaches zero, and letting the metal cool with its surroundings. Another approach is removing the metal from the oven and insulating it in a low-conductivity material such as sand or ashes. Either way, by the time it reaches room temperature, annealed metals will be a softer material compared to tempered ones.
This softness makes annealed metal well-suited for applications where it won’t experience major stresses, but ductility is important. You will find annealed copper in electrical wiring, annealed brass in pipes and fixtures, and annealed aluminum in a variety of sheet and shapes.
What is a Non-Ferrous Metal?
If you were asked the definition of a non-ferrous metal, the answer may seem obvious: it’s a metal which contains no iron. And while that’s true, it might surprise you that the answer is not entirely correct! Non-ferrous metal is a sprawling category, which covers iron-free metals such as aluminum or copper. But a metal is also defined as “non-ferrous” when its chemical composition does not include a significant amount of iron. This means even an alloy with trace amounts of iron can be correctly identified as being made of non-ferrous metal. A ferrous metal will have iron as the first or second most-abundant element in its makeup. But if iron is present in a non-ferrous metal, it will typically be less than 1% of the metal’s overall composition.
So because the non-ferrous category covers so many different varieties of metal, it can be very difficult to identify common properties shared by them all. Some non-ferrous metals are very soft and ductile, while others are hard and brittle. One non-ferrous metal may be durable enough to weather freezing temperatures, but another is well-suited to withstand extremely high heat. However, there is one common denominator to be found amongst non-ferrous metals: they don’t rust. Since they contain very minimal to no iron, there’s little opportunity for the development of a significant amount of iron oxide. And that means the metal doesn’t show signs of rust.
However, it doesn’t mean non-ferrous metals are corrosion-free. In fact, some non-ferrous metals such as zinc are highly corrosive – much more so than iron itself! But because the term ‘rust’ only applies to the formation of iron oxide, non-ferrous metals technically do corrode but they don’t rust.
Common Non-Ferrous Metals
Aluminum
One of the most widely used non-ferrous metals, aluminum in its pure form is soft and not particularly strong. Once alloyed, it gains strength and durability while remaining relatively lightweight. These assets, along with its machinability, makes it very popular in manufacturing. Common applications for aluminum range from aircraft fuselage and cars, to drink cans and kitchen utensils.
Copper
Like aluminum, unalloyed copper is softer and less strong in comparison to carbon steel. One of its most desirable qualities is its high thermal and electrical conductivity, which is why pure copper is commonly found in wiring and high-end cookware. When alloyed with zinc, it forms another non-ferrous metal, brass. Brass is stronger than copper, while retaining a high degree of malleability. This makes it popular for fittings and castings in a variety of shapes. Copper can also be alloyed with tin to create bronze – again creating a stronger and harder metal than the original copper, with better durability. Given the toughness of bronze parts, it’s a popular choice to manufacture bearings, electrical connectors, and springs.
Zinc
Zinc is a non-ferrous metal with a low melting point. As mentioned before, it is more likely to corrode than iron. However, the type of corrosion produced by zinc is beneficial: the zinc oxide which forms on the layer of the metal stops any further corrosion from reaching inside. For this reason, of the most common uses for zinc is in galvanizing other metals. The outer layer of zinc forms a protective coat on steel or iron to prevent rust.
A36 Steel: A Grade Guide
The global market for structural steel is enormous, valued at 100 billion dollars USD yearly. It’s easy to see why steel is such an appealing choice for construction due to its qualities: high strength, toughness, formability and ease in welding. And among the many types of structural steels, grade A36 stands above the crowd.
Key Attributes of A36 Steel
A36 is a low carbon steel, which means its composition contains less than 0.3% carbon by weight. At its most basic, steel is formed when alloying iron with carbon. Carbon grants more strength and hardness to the alloyed metal in comparison to pure iron. However, as the percentage of carbon in the steel grows larger, the hardness can become brittle. That same hardness also produces steel with lower ductility and less workability.
In contrast, low carbon steels like A36 are relatively soft and not quite as strong compared to other steels. But their greatest attribute is ductility: that same softness means low carbon steel is much more formable and malleable. This makes low carbon steel an excellent choice for machining and welding. So it’s not surprising to learn that this category is the most widely used type of carbon steel – and A36 is the most utilized type of low carbon steel.
A36 steel is unique in its grading: unlike many other steels whose number is determined by their chemical composition, A36 is designated by its mechanical properties. What this means is that most grades have alloys that must be added in specific percentages in order to qualify as that steel type; meanwhile, A36 steel must meet the set mechanical standards for its grade. So while it does need to conform to certain chemical composition standards, the most important characteristic of A36 steel as established by ASTM International is its yield strength. In order to be graded as A36, the steel must meet a minimum yield strength requirement of 36,000 pounds per square inch. The metal must be able to withstand that amount of pressure with only 0.2% or less deviation to its shape under stress.
A36 is also a relatively ductile steel, able to elongate to around 20% of its original length during tensile testing. Along with its yield strength, this ductility gives A36 steel a high degree of impact strength at room temperature. This toughness helps prevent the metal from fracture and other damage.
Uses for A36 Steel
As you might tell from its attributes, A36 is quite a versatile material. It comes in a variety of shapes suited for riveted, bolted, and welded construction. Many of these shapes are designed for structural applications such as bridges and buildings. Wide flange beams (shaped like an “H”) are used as columns or beams. Tee bars (formed in a “T” shape) can be found used as beams or bracing. Angles are generally used as bracing or finishing elements. However, its uses are not limited to large construction projects. The versatility of A36 steel lends itself to other uses such as ship building, heavy equipment manufacturing, and automotive parts. Whatever its purpose, it’s clear why grade A36 steel is so popular: tough, strong, easily welded…and all this at a relatively low cost!
The Stages of Metal Fatigue
It’s well known that the long term consequences of living under chronic stress are harsh, taking a toll on our health and overall well-being. When it comes to the harmful effects of stress, metal is no different: too much repeated wear and tension will lead to fatigue, and eventually to a breaking point. And much like our bodies, a metal’s breakdown won’t necessarily occur from one major incident. Often the cracking and weakening that leads to metal fatigue will be gradual, going unnoticed until it’s too late.
In the development of metal fatigue, there are three stages:
Stage 1: Crack Initiation
Fatigue cracks will almost always begin around a stress riser point, an area of the metal where the stress becomes concentrated. The stress riser’s susceptibility to cracking may stem from irregularities in the material itself, flaws in the metal part’s construction, or from vulnerabilities caused by damage such as scuffs and scratches. But whatever the reason for a stress riser, its weakness means that micro-cracks in the metal are likely to begin there. The edges of a metal part can be stress risers, since they’re most likely to be receive the brunt of an impact and be damaged. Weld joints are often the culprit as well, since the join of the weld may be weaker than the surrounding material.
Stage 2: Crack Propagation
Repeated stresses will cause micro-cracks, and if they continue, those faults within the metal will begin to grow. Typically crack propagation will be the longest-running stage of metal fatigue, worsening slowly over time with steady repetition. However, it can be accelerated by other factors. Increased moisture collecting on the surface from weather, vibration from moving parts, and greater stress placed on the metal will all increase the likelihood of crack growth. The danger is that these accelerants are obvious pitfalls leading to metal failure, but metal fatigue is often much more subtle and easy to miss. Because of this, a lack of maintenance which allows the unchecked growth of micro-cracks can lead to a sudden and unpredictable failure.
Stage 3: Final Catastrophic Failure
If metal fatigue is allowed to progress to this stage, it will result in a final failure event: the metal will fracture. Depending on the material’s thickness, metal composition, and applied stress, the fracture will be either ductile or brittle. In a ductile fracture, the metal is deformed by being bent out of shape. The edges of the fracture may crumple and bend, but not necessarily break all the way through. When it comes to a brittle fracture, it’s exactly as it sounds: the metal shatters or snaps. In a typical case of brittle fracture metal fatigue, the pieces will snap completely apart. There will be little to no plastic deformation (a change in shape) in the metal as compared to a ductile fracture. Instead, you’ll see smooth snapped edges where the break occurs. In either case, the final stage of metal fatigue is often sudden and without obvious warning signs.
What is Normalizing?
After going through a hardening process, the end results is a much harder steel. Yet this does come at a cost, since the gain in hardness is accompanied by decreased ductility. Ductility is a measure of a metal’s ability to withstand tensile stress, which is any force that pulls the two ends of an object away from each other. If left as is without any further processing, the hardened steel will become weaker and increasingly brittle the more it is stretched and worked.
To avoid this issue, some type of heat treatment is commonly performed after hardening to boost the ductile properties of the steel. The aim is to make the metal more formable, more machinable, and reduce residual stresses in the material. Normalizing is one common method of producing a more resilient form of steel.
There are three stages in the normalizing process:
- Recovery: This involves heating the steel to a high enough point to relieve its internal stresses. After being placed in a furnace, the steel is heated at a slow rising rate until is reaches the desired temperature.
- Recrystallization: In the second heating phase, the temperature is raised to a range above the steel’s recrystallization point while remaining below its melting point. It will glow red-hot, but still retain its shape. During this stage, new metal grains will begin to form without preexisting stresses in the material.
- Grain Growth: The newly-formed grains are allowed to fully develop during cooling in the final stage of normalization. Unlike quenching or other methods designed for rapid cooling after heat treatment, the normalizing process calls for metal to be cooled naturally at room temperature.
Once the process is complete, the metal is left with an altered microstructure. Grains which were previously more coarse are now transformed to finer, more uniform grains which improves the overall toughness of the material.
One of the biggest advantages of normalizing is its cost-effectiveness, being less expensive than other forms of heat treatment. For example, aannealing follows the same basic process as normalizing, but at a much slower and controlled pace. The lengthy use of a furnace in annealing, including during the cooling process, makes it more costly to produce finished steel as compared to normalizing. Normalization also creates a harder and stronger metal than annealing. So depending on the material’s intended use, the cost-savings and strengthening of normalization makes it a popular choice to treat hardened steel.
Common Metals That Don’t Rust
When it comes to protecting and maintaining metal, the most constant battle encountered will be against rust. Rust compromises a metal’s chemical characteristics, eventually leading to its disintegration. And even if it doesn’t progress to a destructive point, it’s just not very appealing on an aesthetic level. The distinctive orange-brown of rust forming on metal can make it look old and shabby long before its time. So one of the best solutions to this problem is to eliminate it from the start: choosing a metal that won’t rust.
Common rust-free metals include:
- Aluminum
- Stainless steel
- Red metals (copper, brass, and bronze)
- Galvanized steel
Aluminum
Aluminum and aluminum alloys cannot rust because they contain no to very little iron, and ‘true’ rust is made up of iron oxide. That doesn’t make aluminum indestructible, since it can still oxidize when exposed to water. But unlike iron oxide which will wear away at the underlying metal, the forming of aluminum oxide actually becomes a protective barrier. Once it develops on the aluminum’s surface, the oxide layer will be quite resistant to any additional corrosion.
Stainless steel
Most grades of stainless steel include at least some amount of iron, the element which leads to rust. However, the other alloying elements – particularly chromium – lends it protection to the material. Chromium tends to oxidize very quickly and like aluminum, the resulting oxide then forms a barrier against rust. With this chromium oxide layer in place, oxygen is no longer able to reach and react to the metal underneath. Other alloying elements in stainless steel such as nickel and molybdenum provide resistant to rust development.
Copper, brass, and bronze
Collectively known to as “red metals”, these metals can oxidize without rusting since they contain virtually no iron. Copper is very slow to react to oxygen and other environmental factors, but once corrosion does occur it will gradually turn the bright reddish metal to a verdigris green patina. Brass and bronze are copper alloys, with the dual benefits of copper’s own corrosion resistance and the rust-free properties of alloying elements. Both brass and bronze are even more resistant to corrosion than pure copper.
Galvanized steel
Unlike the other three types mentioned, galvanized steel relies upon the application of a physical barrier to prevent rust. Carbon steel is galvanized after being coated with a thin layer of zinc. After bonding with the surface, any oxidation which does occur becomes zinc oxide. A zinc layer exposed to water will become zinc carbonate, which is water-insoluble and puts a stop to any further chemical reactions. More importantly, whatever oxidation that takes place will affect the wear of the zinc before reaching the steel underneath.
What is Metal Pickling and Oiling?
A spool of steel coil is an impressive sight: thousands of pounds of material, having been forced through immense pressure to form smooth, thin, gleaming metal. But this is the end product ready for sale, after the finishing process. Upon being freshly rolled, you may be surprised to find the steel’s appearance to be much more rough! This can be due to various causes from staining to rust, but most often the metal’s unpolished look will be due to mill scale. Mill scale, also referred to as scale, is a mixture of iron oxide residues which cling to the metal’s surface after rolling. It is typically a dark grayish color, with a rough and flaky texture.
Not only is scale unattractive-looking, it becomes a nuisance when left on the metal. Any coating applied over scale will be rough and uneven, and vulnerable to wear. Once water seeps under the scale, it will flake and fall off. So the effort to paint the metal is wasted; not only will the bare patch need repainting, but other scaly areas will eventually flake off and require repainting as well. For these reasons, scale is usually removed by the manufacturer before being sold.
One of the most common methods of scale removal for steel is pickling and oiling. This involves a lengthy multi-step procedure, but at its most basic, the metal is immersed in “pickle liquor” and oiled it as the final step. So what exactly happens during the pickling process?
- Loading: the material is carefully arranged on racks. Crowding the material, or allowing pieces to overlap, means the solution will not be able to reach all surfaces evenly.
- Cleaning: the rack of steel is immersed in a highly alkaline cleaning solution, which will remove dirt and oil. While this step will clear the metal of surface debris, scale still remains on the metal.
- Rinsing: the steel is carefully and thoroughly rinsed with water to remove the cleansing solution. This also helps to raise its pH level prior to the pickling.
- Pickling: the rack is then lowered into a bath of hydrochloric acid, referred to as “pickle liquor”. The immersion in the potent acid effectively eats away at the bits of scale, as well as improving discoloration on the metal’s surface.
- Second Rinsing: immediately after the pickling, the steel is rinsed to cleanse it of the acid.
- Second Cleaning: the metal is again placed in an alkaline cleansing solution. This will neutralize any remaining acid residue from pickling.
- Final Rinsing: the rack is removed from the cleaner and given one last thorough rinse.
- Oiling: after pickling and rinsing, the metal will now have the smooth and shiny appearance we associate with steel. However, if we were to stop at this step, the steel would again be vulnerable to the accumulation of surface debris. The freshly cleaned surfaces are also now more fully exposed to air, which makes it more likely to rust. So the final step of the process is oiling, which protects the metal and provides a barrier to air and contamination. This involves placing the rack of steel in an oil bath to give it an overall coating. Whether using mineral oil or a water-based oil, the cover will help preserve the steel from developing flash rust while in storage. Once the metal is selected for additional fabrication, the oil will be removed via cleansing.
As you can tell, the pickling process is lengthy and work-intensive. However, it’s also a necessary step to prevent oxidation and to prepare the metal for later processing. Without pickling, leveling a scale-covered coil to sheet would result in a product of subpar quality. And just as pickling a cucumber helps to preserve the vegetables’ shelf life, metal pickling and oiling does the same for the material. The manufacturer may not need to process their coil immediately, but it would be unfortunate to discover their stored stock has experienced “spoilage”: rust development and other damage. Properly pickled and oiled, the corrosion of your steel will be prevented for a much longer period of time.
How to Prevent Rust
We’ve all seen rust, whether it’s the dirty-orange flaking off a fence or swing set and marking up your children’s clothes, to the brownish water flowing in houses with old plumbing. But rust isn’t just an annoyance with its unsightly appearance and tendency to stain. For iron equipment and structures, rust can become a real danger when allowed to progress unchecked. In cases such as the Genoa bridge collapse in August 2018, a lack of maintenance to prevent rust and corrosion can lead to deadly consequences.
When it comes to iron corrosion, the equation is simple: oxygen from air or water + iron = rust. The longer iron is left exposed to oxygen, the more quickly and completely it will rust. So when it comes to rust prevention, the best solution is to somehow attempt to keep these elements apart. These methods include:
Materials
A common choice is to attempt to avoid the issue from the start by using steel alloys, weathering steels, or other alloys which contain virtually no iron. These materials are either naturally resistant to rust, or manufactured to be as rust-free as possible. Stainless steel contains at least 11% chromium, which forms a protective film of chromium oxide preventing any further corrosion. Weathering steels may include up to 21% alloying elements like chromium, phosphorus, nickel and copper. In comparison to stainless, weathering steel will form a patina and begin to look to look orange and rusty. However, appearances can be deceiving: unlike the damaging rust formed on iron structures, the rust formed on weathering steel is actually beneficial. The alloying elements stop any internal corrosion with the rust as an outer layer.
Organic coatings
A simple and cost-effective method of preventing rust is paint. Covering a metal item in an overall coat of paint creates a physical barrier between the metal and oxygen. Oil-based paints are usually the preferred option since they contain no water. It’s also appealing because the oil paint adheres better, is durable, and will dry to a more even finish.
Powder coatings
Like paint, a powder coating creates a protective layer to prevent rust. Powders are commonly applied to the steel by using a compressed air sprayer. Once the powder particles are clinging to the object’s surface in an even layer, it’s ready to be heat-cured. This involved placing the object in a hot oven, which will melt and fuse the powder particles into a continuous coat. So while this method involves more time and expense than painting, the biggest advantage of powder coating is its durability. Not only is it rust-resistant, it’s more resistant to chipping, scratching and other wear due to the thermal bonding of the curing process.
Galvanization
Steel is galvanized by applying a layer of zinc, which provides two benefits: it forms a strong physical barrier, and if corrosion does occur, it will affect the outer zinc rather the metal underneath. For items such as car exteriors that will later be painted, electroplating galvanization is used to bond the zinc to the steel. The process will leave the metal with a soft, even shine. For hot-dipping galvanization, the steel is immersed in a bath of molten zinc and dries to a flat finish. Hot-dipped galvanized steel is often preferred for construction projects because the resulting zinc layer may be up to 5 to 10 times thicker with this process. With each layer of zinc comes more protection against rust.
Maintenance
No matter what material is chosen or coating applied, the best protection against rust will always involve continued routine maintenance. Any deposits and dirt on the metal should be cleaned on a regular basis. If any rust does form on the surface, it should be removed it as quickly as possible, with a protective coating applied or re-applied to the item. Neglect means even a structure as strong as a bridge can be destroyed from the corrosive effects of air, water and salt rusting the steel.
What is a Ferrous Metal?
When classifying metals, focusing on a particular property is most often used as a way to divide them into two groups. Is this metal ductile or non-ductile? Is it magnetic or not?
When it comes to ferrous metals, one basic quality determines the groups: whether the metal contains iron. If iron makes up a large percentage of its composition, the metal is considered to be ferrous. If it contains no iron, or just trace amounts of it, it will be labeled a non-ferrous metal.
Beyond that, it becomes more difficult to apply general labels on the groups and the metals’ properties. While ferrous metals can range from iron itself to stainless steel, the alloying elements greatly affect the metal’s characteristics. For example, most ferrous metals are magnetic. But austenitic stainless steel is not, due to the high levels of nickel added to the steel for alloying. The nickel allows the steel to form in a crystal structure that is mostly austenite – and austenite is not magnetic.
So although it can be difficult to generalize about all ferrous metals as a group, there are some general characteristics that can be made about them. Ferrous metals are very hard and strong, especially in comparison to non-ferrous ones such as tin or copper. They’re vulnerable to rust due to their high percentage of iron, unless given corrosion resistance through alloying elements or protective coatings. And they’re usually (but not always) magnetic, which makes them very useful for motor and electrical applications.
The most common categories of ferrous metals include:
- Carbon steel: there’s certainly no question of this being a ferrous metal, with over 90% of its composition being made up of iron. It is very hard and can keep a sharp edge, making it well-suited for mechanical uses such as drill bits and blades.
- Cast iron: this metal is exceptionally hard due to its high levels of carbon, but the carbon also makes it quite brittle. For this reason, cast iron is now primarily used for smaller machine components or cookware.
- Stainless steel: the most commonly used type of ferrous metals, especially for consumer goods. The addition of chromium is what makes a steel stainless, and gives it good corrosion resistance. And it’s magnetic, which is why you can stick magnetics on your refrigerator.
- Alloy steel: the properties of this group of ferrous metals can vary much more widely than the others, since the alloy is specifically formulated for a particular purpose. So while alloy steels are ferrous, the added elements allows the metal to be tailored for more strength, ductility, hardness or other property.