Out of the many metals found on Earth, which is the strongest? It’s a straightforward question, but not one with a simple answer. You may find yourself becoming rather philosophical, because first you must define exactly what it is you mean by “strength”. When it comes to metal, being strong isn’t just a matter of toughness. There are several different ways of determining whether a metal is strong, and it helps to know how these assessments are made.
Types of Metal Strength
There are four basic ways to test the strength of a metal:
• Compressive strength: This measures the metal’s ability to withstand squeezing or compression. Testing involves using external force to press the metal, in order to see how much stress it can take without affecting its shape.
• Yield Strength: This tests the metal’s elastic behavior, which is the tension it can take without becoming deformed or broken. This is performed through a bend test, where the ends of a bar or beam are bent towards each other into a U-shape. The amount of force needed to bend the metal, and then past its breaking point, will identify its yield strength.
• Impact Strength: As the name suggests, this measures the impact the metal can withstand from heavy blows. Stronger metals will be able to withstand a major impact, even repeatedly, without cracking or breaking.
• Tensile Strength: A metal’s tensile strength is its ability to resist tension, specifically when the material is being stretched. Metals with high tensile strength are better able to withstand this test, while metals with low tensile strength can be easily pulled apart.
Comparing Strongest Metals
Having learned about the different types of metal strength, it becomes clear that it’s unlikely for any single metal to lead in every category. A metal can have high marks when it comes to one or more types of strength, while being more vulnerable in another area. As a general list, we can consider these to be some of the strongest metals:
Tungsten
Out of all pure metals, tungsten may have the best claim to the title of Strongest Overall. It is very tough, has the highest melting point of any unalloyed metal, and tops all others when it comes to tensile strength. However, its major pitfall is impact strength. Due to its incredible hardness, tungsten is very brittle – so much so that it’s known to shatter upon impact when dropped. But when used as an alloying element in steel, tungsten greatly enhances a metal’s strength. Due to its hardness and high melting point, tungsten is often used in military and aerospace applications.
Chromium
Chromium is not only the hardest metal on Earth, but the third hardest element overall: surpassed only by carbon (diamond) and boron in hardness. But like tungsten, that hardness makes chromium extremely brittle in its pure form. Its greatest value is as an alloy, lending itself to boosting both yield and tensile strength. It is also very resistant to corrosion and reflective when polished, so it has aesthetic appeal along with strength.
Titanium
While titanium isn’t nearly as hard as tungsten and chromium, it has extraordinary tensile strength. It is a relatively low density metal, and beats all other metals when it comes to strength-to-density ratio. What that means in actual practice is that titanium can be produced to be thin and flexible, without sacrificing strength and durability. In addition, it is highly resistant against corrosion, cracking, and other metal failure. Its lightness and strength make it a prized choice in the medical and aeronautical industries.
Steel
Unlike the other three metals on this list, steel is man-made: an alloy rather than a pure element. Alloys have the advantage of combining several different metals, resulting in a chemical composition designed to boost the metal’s most useful qualities. Carbon steel on its own is a strong material, with the carbon contributing to the steel’s hardness and toughness. With additional alloying elements such as vanadium or tungsten, steel becomes one of the strongest and most versatile metals on the market.
Category: Education
Aluminum Alloys for Anodizing
Anodizing the Aluminum Series
While aluminum is the most common metal to be anodized, not every grade of aluminum alloy receives this type of processing. As time passes, aluminum oxide naturally forms on the surface of aluminum, creating a layer of corrosion resistant protection. This layer not only halts continued oxidation and corrosion, it also helps reinforce the metal from the hardness of aluminum oxide.
However, this oxidation develops most successfully on pure aluminum – and pure aluminum is limited in its usage due to being a relatively soft and weak metal. Alloying the metal will give it greater strength and durability, but those properties come at the price of affecting aluminum’s ability to oxidize. Anodization is a convenient method of producing a thin, even layer of protective oxide on aluminum alloy.
Because anodization uses the metal’s aluminum content to form this anodic oxide layer, in theory any type of aluminum alloy can be used for this process. But some types of aluminum alloy have much greater chances of producing a successfully anodized piece. Due to the different element combinations in alloys, the anodizing of some series will produce much stronger and aesthetically appealing products than others.
Expected Results of Anodizing Aluminum Series
1xxx Series
This series covers pure aluminum, or aluminum with such tiny amounts of other elements that it can be considered virtually pure. 1xxx series can be anodized, but the pure metal remains weak and can be easily damaged. With or without anodizing, 1xxx aluminum is not strong enough for most structural applications.
2xxx Series
The primary alloying element for 2xxx is copper, which produces a very hard and strong type of aluminum. Anodization does not offer much additional protection, because the copper impedes the development of an anodic layer. The processing also gives the metal a yellow tint which consumers generally find unappealing.
3xxx Series
Manganese is the main alloying element in this series, and results in a layer of good-quality anodization. Unfortunately, the anodic layer is likely to be an unattractive brown tint that can vary from piece to piece, making it difficult to match when using multiple sheets in a project.
4xxx Series
Like the 3xxx series, the main alloying element in 4xxx causes the metal to turn an unappealing color after anodizing. 4xxx is alloyed with silicon, and this results in a dark gray anodized aluminum with sooty black patches. These blotches are very difficult to remove, so when 4xxx is anodized, it is generally used in architectural applications.
5xxx Series
This series is alloyed with magnesium, and is well-suited to anodizing. Once complete, the anodic layer is transparent, strong, and offers long-lasting protection. However, the chemical composition in some grades of 5xxx aluminum should be examined carefully, because some elements within may make anodizing a bit tricky. If the magnesium content is very high, or it contains over 0.1% silicon, the oxide layer may appear streaky.
6xxx Series
Both magnesium and silicon are the alloying agents in the 6xxx series, and these aluminum grades are considered to be excellent candidates for anodizing. The anodic oxide layer is clear and strong, as long as the alloy’s magnesium content is kept below a certain percentage. The strength of anodized 6xxx aluminum makes it a good choice for structural and mechanical applications, but its attractive finish means it can function well for aesthetic purposes too.
7xxx Series
Zinc is the primary alloying element in 7xxx series aluminum, and it takes well to the anodizing process. This series is already known for being some of the strongest types of aluminum, and anodizing increases that quality even further. The only risk comes if the chemical composition of the alloy is high in zinc. For 7xxx grades with heavy zinc content, the otherwise clear oxide layer can turn brown.
What is Anodizing?
One of the greatest advantages of aluminum is its natural resistance to corrosion. Ferrous metals such as steel are highly vulnerable to corrosion due to their iron content; iron atoms react with oxygen to produce rust, which wears away and damages the metal. Aluminum atoms react even more quickly to oxygen, but its oxide doesn’t flake off as iron oxide does. Aluminum oxide is quite hard and difficult to remove from the metal’s surface, so it become a natural layer of protection from oxygen permeating the metal underneath.
However, this natural oxidation takes some time to occur – and manufacturers understandably want faster results to produce corrosion-resistant aluminum for sale. This is where anodizing comes into play: this electrochemical process enables oxide to form much more quickly than it would on its own. Unlike paint or plating, which can chip or wear away, the anodized surface layer is fully integrated with the metal itself to provide long-lasting protection.
The Anodizing Process
Anodizing is a multi-step process, but at its most basic it can be described as highly controlled oxidation. By treating the metal and applying an electric current, the aluminum atoms are prompted into an accelerated interaction with oxygen. The end result is a metal surface that is evenly and completely anodized, with a high degree of corrosion resistance.
These steps include:
1. Cleaning: A thorough cleaning must be done to prepare the aluminum for the processing. Either an alkaline or acid-based cleaner is used to remove any surface grease or dirt from the metal.
2. Pre-Treatment: The benefits of anodizing can be both functional and aesthetic. During processing, dyes and other treatments can be applied to create a more attractive appearance in the final product. To modify the metal’s finish, two procedures can be done as pre-treatment:
a. Etching: To achieve a matte finish, heated sodium hydroxide solution is applied to the aluminum. Sodium hydroxide, also known as lye, is an extremely caustic substance capable of corroding metal. The solution strips away any minor surface imperfections, leaving it smooth and matte.
b. Brightening: Mirror-finish aluminum is created by treating the aluminum with a concentrated mixture of phosphoric and nitric acids. Similar to the lye solution, the acids corrode away the metal’s top layer along with minor imperfections. Once complete, the result is smooth, shiny metal.
3. Anodizing: The cleaned and pre-treated aluminum is then submerged in an electrolyte solution. Anodizing requires electricity, and an acid solution provides a boost in conductivity. As the electric current passes through the tank containing the aluminum and solution, oxygen ions are released from the electrolytes and interact with the aluminum to form a layer of aluminum oxide. The process is carefully monitored to achieve the degree of oxide thickness for the desired product.
4. Coloring: To produce colored anodized aluminum, four different methods can be used. Since this is done during the anodizing process, the color will be highly resistant against fading, scratches, and other wear. Generally this is done through these four methods:
a. Dye: The anodized metal is immersed in a dye and lye solution.
b. Electrolytic coloring: An electric current sent through an electrolyte solution the dye to the aluminum’s surface.
c. Integral coloring: This process combines anodizing and coloring into one step to anodize, color, and seal the aluminum.
d. Interference coloring: The aluminum is treated with sulfuric acid and then soaked in dye.
5. Sealing: Although freshly anodized aluminum has the corrosion resistance of aluminum oxide, sealing offers an additional layer of protection. During the processing, the various chemicals used will leave microscopic “pores” open on the metal’s surface. Left unsealed, contaminants can be absorbed more easily onto the surface during storage, risking corrosion or discoloring. Once the pores in the anodic film are closed by applying a final seal, anodized aluminum will be highly resistant to scratches, abrasions, and color fading.
What are Pipe Schedules?
For people new to working with pipe, referring to their “schedule” may be unfamiliar. Imagine you have two pipes in front of you: one labeled schedule 40, while the other is schedule 80. Both are manufactured of the same type of metal, with the same grade, and the same 3-inch diameter. So, what is it that makes these two pipes different, other than their schedule numbers?
First, it helps to understand the functional challenges faced by pipe. Pipe is designed to move along liquid or gas under pressure – pressure which can be internal, external, or both. To sustain fluid pressure, pipe must be strong enough to withstand a great deal of stress. This means the product dimensions of pipes aren’t simply measured by length and diameter, but its wall strength as well. A high-pressure fluid will require thicker pipe walls than pipes used for low-pressure applications like drainage.
Pipe Schedules
The wall thickness that determines a pipe’s strength is known as its schedule. Pipe schedules are a standard to identify wall thickness for the same size of pipe. The outer diameter of a particular pipe size will stay constant while the inner diameter varies according to the schedule type. Using the example of the 3-inch pipes, you can see the difference once you measure their inner diameter. The outer diameter for both pipes will remain approximately 3.5 inches, but their other qualities differ:
3” Schedule 40 pipe
• Wall thickness of 0.216 inches
• Inner diameter of 3.05 inches
• Weight per foot is 7.58 pounds
3” Schedule 80 pipe
• Wall thickness of 0.300 inches
• Inner diameter of 2.90 inches
• Weight per foot is 10.25 pounds
As you can tell, the schedule 80 pipe is significantly heavier due to its thicker walls. Those thicker walls mean schedule 80 pipe is better able to handle high pressure, which might cause greater wear or damage to schedule 40 pipe. Clearly, pipe schedules are quite important to ensure a project’s success and the best use of your funds. For home plumbing projects, schedule 40 pipe is sufficient, with no need to spend extra for schedule 80 pipe. For industrial uses, on the other hand, investing more money up-front for schedule 80 pipe may help prevent damage and costly repair work in the future.
What are the Differences between Tube and Pipe?
It’s not uncommon for someone to assume tube and pipe are interchangeable terms. You yourself may not have ever considered what the difference might be. If asked, what might be your guess: maybe they’re called pipes when made of metal, and tubes if made of PVC? Are they simply the same thing no matter what the name, the way different people might refer to soda versus pop?
Basically, it comes down to use: tube is a structural element, while pipe is used to transport fluid or gas. While both types are lengthy hollow cylinders, their use means they are different in their measurements, desired qualities, and even shapes.
10 Differences Between Tube and Pipe
1. Size Measurements
Pipe is more approximate in its measurements, with sizes being referred to as nominal pipe size (NPS). The pipe’s inner diameter and wall thickness are the most important dimensions, and vary according to strength. A very strong pipe will have thick walls with a smaller inner diameter than an equivalent NPS with thinner walls.
When measuring tube, its outer diameter and wall thickness are also important dimensions. Unlike pipe, these will be the exact measurements of the tube’s cross section.
2. Wall Thickness
Pipe wall thickness is known as its schedule, and determines the liquid capacity of a pipe. Two pipes with the same NPS number but different schedule numbers will have a difference in inner wall thickness.
Tube wall thickness is defined by gauge, with the wall’s width increasing along with a higher gauge number.
3. Production Sizes
Pipe comes in a wide variety of lengths, often 6 feet or more. Tubes, on the other hand, are most often smaller parts. Many are 5 inches or less, although larger tubes can be used for some applications.
4. Materials
Many different materials can be used for pipe: iron, copper, brass, and PVC. Tube is most often made of some type of steel, whether that’s carbon, low-alloy, or stainless steel.
5. Shapes
A pipe will always be round. Tube can be made in round, square, and other shapes.
6. End Connections
The ends of pipes can be straight, beveled, or screwed. Tube ends are commonly threaded or grooved for quicker connections.
7. Tolerances
An item’s tolerance is the acceptable range in its measurements: any deviations in its straightness, shape, or strength. Pipe tends to have a wider tolerance, so long as it meets its designated schedule requirements. Tube is the opposite, being made with strict tolerance guidelines. Because tube is intended for structural and often mechanical use, strength is a high priority. Repeated quality checks are made on a tube’s straightness, gauge, and outer surface.
8. Production Process
Pipe manufacturing is quite efficient and speedy, with many items being made to stock. Production of tube is more exacting due to its strict tolerances and quality checks, which means a lengthier, more labor-intensive process.
9. Delivery
Due to the difference in production, availability also varies between pipe and tube. Pipe, especially if ordered in made to stock sizes, is likely to have a quick delivery. Tube generally takes longer to produce and ship
10. Price
As you may have already guessed, the time and effort used to produce tube also translates to a higher price tag. While pipe can be pricey, especially if made of metals like copper, the average price of pipe will be lower than tube.
Popular Metals for Welding
Welding is a complicated process, and there can be several pitfalls on the way to completing a successful weld. The choice of welding method, how carefully all materials are cleaned before proceeding, the experience of the welding operator: just a few of the many factors that can make or break a welding job. But one of the most important is your choice of metal. Working with metals that are more compatible to welding will make this process much easier to complete, and boost your chances of a good outcome.
During its exposure to the intense heat used during welding, metal begins to expand and soften. Welding involves connecting those softened edges together, with the use of a filler metal to create a join. Doing this quickly helps reduce the risk of the metal heating to the point of melting, which can thin the material and create a weaker join. So from the start, choosing a metal more compatible to welding helps reduce the risk of a bad weld.
The Four Most Popular Welding Metals
1. Low Carbon (Mild) Steel
Mild steel is one of the most weldable metals available, and this is due to the very quality featured in its name: its low percentage of carbon. In other circumstances, carbon is a great benefit when it comes to alloying steel; it reinforces the strength and hardness of the material, making it less ductile. However, that strength can become a downfall when it comes to welding. The higher the carbon content in the steel, the greater the risk that the heat of welding will create microstructures such as martensite within the material. These microstructures are more brittle than the surrounding metal crystal structures, and end up weakening the metal’s overall strength.
The more ductile a metal, the less likely it is to develop these microstructure flaws within the material. Mild steel is the most ductile of the steel family, so it takes easily to the welding process. Choosing low carbon steel means eliminating much of the risk of a disappointing weld before the process even starts.
2. Cast Iron
On the other side of the spectrum from mild steel, cast iron has an extremely high carbon content — about 2% of its overall formulation is carbon. Just as in steel, the high amount of carbon makes the metal quite strong but also very brittle. However, cast iron is still a good candidate for welding because of its relatively low melting point. This quality means a shorter period of sustained heat is used during welding, which prevents heat damage or burn-through.
3. Stainless Steel
Welding is already a complicated process, and the complex chemical composition of stainless steel can make it even more tricky. The chromium in stainless is what produces the metal’s distinctive shine and excellent corrosion resistance, but the same element can be a stumbling block during welding. The intense heat of welding leads to chromium bonding with the carbon in stainless steel, leading to warping and making it more vulnerable to rust.
It’s important to properly identify the grade of stainless before processing begins, because the formulation of stainless steel will help best determine what type of welding method should be used. Ferritic and austenitic stainless steels can be more easily welded. Martensitic stainless steel grades, which are primarily alloyed with chromium, are more prone to cracking. However, with careful attention paid throughout the process, especially with monitoring welding torch temperature, stainless steel can be welded successfully.
4. Aluminum
Aluminum welding is a balancing act, because the metal possesses qualities that are both beneficial and risky to welding. Its ductility makes it easy to weld, but its high degree of thermal conductivity is an obstacle. The heat during welding can easily spread throughout the metal, making it expand more significantly than heated steel. Then during its cooling period, the aluminum must be monitored to prevent the development of craters and cracking as it shrinks back to its original size.
Yet despite the difficulties, aluminum is one of the most popular welding metals. Its appealing properties such as corrosion resistance, durability, and relative low cost make it worthwhile to take the extra care during welding.
Ten Differences between Aluminum and Steel
At first glance, aluminum and stainless steel may appear similar: both a silvery gray, softly shiny, and used to make many of the same products. Many food service and kitchenware items, for instance, are made available from a manufacturer in both types of metals. What separates these two, other than price?
1. Thermal Conductivity
Aluminum is a metal with a high degree of thermal conductivity. What that means in real terms is that a water will boil more quickly in a stockpot than one made of stainless steel. However, aluminum also cools more quickly than stainless, so stainless will help keep a pot of soup warm longer.
2. Thermal Properties
Its lower degree of thermal conductivity means stainless steel is much more resilient to use in high temperatures. Aluminum will begin to soften around 400 degrees Fahrenheit; meanwhile, stainless steel can function well at temperatures up to 800 degrees. Some stainless steel grades can withstand temperatures nearly double that for short periods of work, up to 1500 degrees.
3. Strength
Both metals are quite strong and durable, but stainless steel more so than aluminum.
4. Strength to Weight Ratio
Stainless steel is very strong, but at the cost of a heavier weight. While aluminum is not as strong, manufacturing with this metal will result in an item nearly one third the weight of a steel part. This incredible strength to weight ratio makes aluminum very attractive to manufacturers, especially in the aerospace industry.
5. Welding
Most grades of stainless steel are relatively simple to weld, while aluminum’s high thermal conductivity makes it a more difficult task. However, both are among the most popular types of metals used in welding.
6. Electrical Conductivity
Aluminum has the distinct advantage over stainless steel here, with it being an excellent conductor of electricity. Stainless steel does not conduct electricity well in comparison to other metals.
7. Workability
Pure aluminum is a very soft and malleable metal, and even after alloying, it is easy to cut and form. Due to its strength and hardness, stainless steel takes effort to form into shape.
8. Corrosion Resistance
Both metals are known for excellent corrosion resistance. Chromium is one of the principal alloying agents in stainless steel, and boosts the corrosion resistance of the steel alloy. Aluminum is naturally resistant to corrosion on its own, although it is more vulnerable to damage from highly basic or acidic exposure.
9. Cost
In general, a part made of aluminum will be more affordable than the same item made of stainless steel.
10. Reaction to Foods
Consumers have the choice between aluminum and stainless steel cookware, and often make the decision to purchase aluminum based on price. However, aluminum is a more reactive compound than stainless steel. Acidic foods such as tomato sauces can leave aluminum damaged or with unsightly marks, while other foods like eggs can discolor if cooked or stored in aluminum. While both metals produce high-quality items, choosing stainless steel may be worth some additional money for non-reactive cookware.
What is Tool Steel?
As you might guess, tool steel earns its name by being primarily used for tools. If you then picture a steel screwdriver, pliers, or a wrench, you’d be right – but the items created of tool steel group consist of much more. Within the steel industry, tool steel also refers to machine tools, including those used to manufacture other metal products. To meet the standards of this demanding usage, tool steel is known for being extremely hard, resistant to abrasion and deformation, and its ability to hold a cutting edge. While the steel’s chemical composition is important, the method of production must be carefully controlled to achieve the proper qualities expected in tool steel.
Tool Steel Groups
Tool steel is classed into six groups, all made of carbon steel alloyed with one or more of these main elements: molybdenum, tungsten, chromium, and vanadium. Most tool steel originates from recycled steel scrap, but not all steel scrap is suitable for tool steel. Alloy scrap containing elements which resist oxidation (such as nickel, cobalt or copper) will impede the development of carbides in the metal’s structure, and carbides are crucial to creating tool steel. To ensure the metal will be an optimal mix, tool steel will typically consist of 75% mill scrap supplemented by purchased steel scrap.
The first step is primary melting, where the scrap is heated until molten. Most often this is done in an electric arc furnace (EAF) due to it being a lower-cost production method. However, it is key to avoid contamination during the melting process to create the highest-quality steel. While the EAF is widely used for tool steel, there is some risk of contamination from dust or traces of oxidation-resistant metals from previous batches. For top notch tool steel, an alternate method known as electroslag refining (ESR) is used to melt the steel. Instead of a furnace, ESR uses electric currents to superheat and slowly melt the metal. While a more expensive process, ESR will produce a more refined type of steel without apparent surface imperfections.
After melting and alloying, the steel is poured into ingot molds and then forged into the desired shapes. This is where the six groups of tool steel are determined by their processing:
• Water hardening: Also known as W-group tool steel, this group’s defining property is being water quenched. Quenching in water does risk warping or cracking steel with its rapid cooling, and results in a relatively brittle metal. For this reason, water-hardened tool steel is not considered appropriate for high-temperature industrial uses; once the temperature nears 300 degrees Fahrenheit, there is noticeable softening of the steel parts. Typical items made of W-group steel include scissors, smaller hand tools, and springs.
• Shock-resisting: S-group tool steel is notable for its toughness and ability to withstand repeated impact. This is achieved by using low carbon steel during its production, since higher carbon content results in a harder but more brittle product. Alloying with silicon, tungsten, and chromium gives additional wear resistance and tensile strength. It’s then quenched in oil, which allows for a gentler cooling than water and helps produce a metal with a high degree of durability. Items made of S-group tool steels include tableware dies, impact hammers for nails guns, and shear blades used in cutting heavy steel plate.
• High speed: This group is mainly used for cutting tools, which requires a very hard and abrasion-resistant type of steel. Tungsten, chromium, and vanadium allow HS steel to withstand very high temperatures, such as the heat friction created while cutting other metals. It also manages to hold a cutting edge with repeated use, allowing for higher speeds while cutting without having to pause to resharpen. HS steel can be found in various cutting tools such as drill bits, saw blades, and milling cutters.
• Hot work: Hot-working (H-group) steels are used to cut and shape materials at very high temperatures. To achieve a steel that can withstand prolonged exposure to intense heat, H-group steels are made of a low carbon steel with a higher percentage of alloying content. This produces a tool steel with a high amount of carbides, giving it good overall toughness and wear resistance. The most common use for hot work steel is to produce other metal items because it can withstand the heat needed to forge and cast other metals. It’s used in machinery such as pressure dies, extrusion, and forging equipment.
• Cold work: This tool steel type is narrowed down further into three groups known as the A series (air-hardened), O series (oil-hardened), and D series (high carbon/chromium). All three are intended to be used to cut or form materials at low temperatures.
1. A-series: As the name suggests, steel in this series is hardened via air. Because of the quenching method and its high chromium content, A-series cold work steel is known for low distortion during heat treatment. This makes it a good fit for machining purposes, and so can be found in dies, forming tools, and gauges.
2. O-series: Oil-quenching avoids the cracking and warping risks brought on by water-quenching, and is used for larger parts which require minimal distortion. It offers good wear resistance and toughness, lending itself to a wide range of applications as a general-use cold work steel. Typical uses of O-series tool steel include stamping dies, bushings (metal parts used to absorb vibration and friction), shear blades, and other cutting tools.
3. D-series: Made of a high-carbon steel with a high alloy percentage of chromium, this can also be categorized as a type of stainless steel. However, unlike typical stainless steel, its corrosion resistance is limited and thus is not used to make consumer products. Instead, D-series tool steel applications include cutters, dies, plastic injection molds, and machining rollers.
• Special purpose: this group includes tool steels with alloying elements such as nickel, which typically resist the formation of carbides. The extra time and effort needed to produce these tool steels makes them too expensive for general purposes, so they are manufactured for specific use. Special purpose tool steel can be found in plastic molding dies, zinc die casting, and drills.
Top Metals for Knifemaking
If there is one constant amongst professional and home chefs, it’s the belief that the single most important tool to possess is a good knife. A sharp knife is easier to control, requiring less force that might lead to slippage and hand injuries. It slices through foods more cleanly, requiring less effort to cut and chop. Professional chefs prize their knives so highly, they carry them to and from work to ensure they always have their preferred tools on hand.
But what differentiates one knife from another? What is the reason for one knife being priced at $20, and another at $200? Most often, it comes down to its material: when it comes to knife construction, the type of steel will have the greatest impact on its performance.
First, we need to determine the qualities that make up a good quality knife. Some of the most important characteristics include:
• Corrosion resistance: prevention against rust and pitting
• Edge-holding capability: the length of time a knife can keep its sharp edge
• Strength and ductility: the knife’s ability to absorb force without damaging its shape
• Hardness and wear resistance: the toughness of the knife, along with how frequently it can be sharpened
• Workability: the steel’s ability to be formed into the desired knife shape
The difficulty lies in producing a knife that fulfills as many of these criteria as possible, because they can conflict with each other. A stainless steel knife with great edge retention will require a lot more effort when it comes to sharpening. Ductile knives have good tensile strength, which allows them to flex during cutting and therefore avoid breaking – but they also need regular sharpening since they lose their edge more quickly. Added chromium or vanadium in the steel will boost a knife’s corrosion resistance but make it easier to snap with too much lateral pressure.
So the construction of knives depends on balancing these factors, while producing the best type of knife for a consumer’s needs. With these criteria in mind, we can examine the best choice of metal for knifemaking.
Carbon Steels
Carbon steels are just as the name describes, steels where carbon is the main alloying element. A higher level of carbon produces a very hard and wear-resistant steel. This is highly desirable for knifemaking, because that strength and hardness means the knife can be ground to an acute degree of sharpness. The knife will also retain the razor edge for an extended time during its use.
However, that sharpness often comes at a high sticker price, plus the added fuss of regular maintenance. High carbon steels are more prone to rusting than other types of steels, which means they will begin to develop rust if left wet. In humid environments (either due to climate or a small, steamy kitchen), it may be recommended to keep the blade oiled between uses to guard against rust. The high carbon content also reacts to acidic foods such as tomatoes and onions, darkening the blade and leaving a patina. While this patina does protect the steel from corrosion, some users may find it unsightly. If so, washing the knife and patting it dry immediately after each use will help preserve the blade’s shine. With careful maintenance, a carbon steel knife can be a lifetime investment; its hardness and durability will allow for many years of use.
Tool Steels
This class of steel is typically alloyed with four main elements: chromium, tungsten, vanadium, and molybdenum. This alloy mixture produces a steel that is extremely hard and resistant to abrasion and deformation. Because of this toughness, it is most often used in machinery for extruding, pressing, and cutting other metals, which is why it earned the name “tool steel”.
When used for knifemaking, tool steel generally goes to the production of utility and combat knives. Like high carbon steels, tool steels are quite hard which means they can be ground to an extremely sharp edge. But it also has pitfalls that make it less suitable for the regular wear of cooking knives: a propensity to rust, increased risk of snapping with lateral pressure, and the edge can develop nicks and chips. The alloying elements help protect against corrosion, but depending on the knife’s use, it may not always be convenient to clean it immediately which will contribute to its wear. But it will stay very sharp for a long time, making these knives valued assets to soldiers and hunters in the field.
Stainless Steels
Produced by adding chromium and other elements to steel, stainless steel offers a high degree of corrosion resistance. While it can begin to rust if treated carelessly, a stainless steel knife is easier to maintain compared to a carbon steel knife. Keeping the blade clean and dry will generally be enough to prevent against corrosion and darkening. The chromium in the alloy also gives the distinctive silvery sheen of stainless steel, and helps resist tarnishing.
The main disadvantage of stainless steel is that it is a softer and more malleable material than carbon steel. Carbon steel’s extreme hardness gives it the ability to be honed to a razor edge. In contrast, a stainless steel knife is more difficult to sharpen, is more prone to deformation while sharpening, and is unlikely to reach the same peak sharpness of a carbon blade. However, that softness may not necessarily be a drawback. Most home cooks don’t require a super-sharp knife, and a stainless steel knife does keep its edge for quite some time. Its ductility also means less risk of chipping the blade, since the softer steel is better able to absorb force during chopping. The affordable price of most stainless steel knives, along with their low maintenance, is why they are the best-selling option on the market.
Classes of Stainless Steel
When it comes to steel types, stainless is a broad category. Any ferrous alloy containing at least 10.5% chromium can call itself stainless steel. While additional alloying elements may be included, chromium is the significant ingredient in transforming a steel into “stainless”. Chromium boosts the corrosion resistance, durability, and strength of steel. It also results in a distinctive shine and boosts the metal’s stain-resistant properties – which is where we get its name.
So we see that all steels within the stainless family share one common alloy in chromium. What then separates them into classes are the other alloying elements, such as molybdenum or nickel. The different chemical compositions affect the steel’s crystal structure, which is how they are divided into 5 basic classes:
- Austenitic stainless steel
- Ferritic stainless steel
- Martensitic stainless steel
- Duplex (ferritic-austenitic) stainless steel
- Precipitation-hardening (PH) stainless steel
Austenitic stainless steel
The most popular grades of stainless steel come from this group, which are alloyed with high levels of chromium, molybdenum, and nickel. This gives the metal a superior degree of corrosion resistance and malleability, along with excellent weldability. While this class of stainless steels cannot be hardened through heat processing, and are prone to cracking under heat, they can be successfully hardened through cold working. Another asset is its strength against corrosive elements, which means austenitic steel is used in varied applications from marine and aerospace environments to chemical and food processing.
Ferritic stainless steel
These steels are best known for having low levels of carbon, making them a more iron-centric (ferritic) type of steel. It contains high levels of chromium and nickel, and like austenitic stainless, is strengthened through work hardening or cold working. It may not be as strong or hard as austenitic steel, but ferritic stainless is very resistant to stress-induced corrosion cracking. This makes it a popular choice for any components encountering corrosive substances or environments, such as industrial or automotive parts.
Martensitic stainless steel
Martensitic steels make up the hardest class of stainless steel, with a high level of carbon content alloyed mainly with chromium. However, that high degree of hardness also limits its use, since it can be brittle and less tough than the other classes of stainless steel. This means martensitic steel use is reserved for parts requiring great tensile strength and impact resistance, without being exposed to corrosive elements. Surgical instruments, valves, and pumps are often made of these steels.
Duplex (ferritic-austenitic) steel
Also known as ferritic-austenitic stainless steels, duplex is engineered to combine the best properties of both classes. They are alloyed with over 20% chromium and 5% nickel, boosting their yield strength and corrosion resistance. It also has approximately double the overall strength compared to ordinary austenitic stainless steel, while still being less expensive than austenitic due to the lower amount of nickel. With these qualities, duplex is heavily used in the oil industry, especially for underwater oil and chemical processing. It has the toughness to stand up to the corrosion caused by chloride and other elements.
PH (precipitation hardening) stainless steel
Steels in this class are notable for including aluminum, copper, and titanium among their alloying elements. When these steels are heat treated, the aluminum and other alloys form precipitates within the metal, reinforcing its crystal microstructure. This natural precipitation-hardening means that after its forging, it requires just a single low-temperature hardening to be ready for use. PH steel ranks highest in tensile strength among all stainless steel classes, which makes it well suited for the stresses of industrial use. Turbines, nuclear power plants, and aerospace components often make use of this class of steel.