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.

Heat’s Effect on Metal Properties

Often when we speak of heat, it concerns scenarios we’d rather avoid: getting sweaty and sunburnt from the summer sun, overheating laptops or car engines, maybe singeing your hair after leaning too close to a candle. And rightly so! In all these cases heat can be damaging, whether to your possessions or your own self.

Yet despite some risks, heat is greatly beneficial. The sun’s beams feel warm and pleasant after a cold winter. The flames of a stove burner or grill cook your food. And when it comes to metal, heat is no different. Too much heat can damage metal to the point of weakening or destroying it. But heat is also a transformative force, from forging the metal to hardening it and modifying its mechanical properties.

There are four ways the application of heat affects metal:

  • Structural
  • Electrical Resistance
  • Magnetism
  • Thermal Expansion

Structural

Heat is a crucial part of the metal making process, with furnaces heated to such high temperatures that metal ore turns molten. However, it’s not always necessary to go to such extremes to affect the properties of the metal. Any time metal is heated, the atoms making up the metal or alloy begin to move. Given enough heat and then carefully cooled, the crystal structure of the metal can be reshaped into a lattice that is much stronger than prior to heat treatment. In the case of high carbon steels, heat application allows the iron to absorb additional carbon, which produces an exceptionally hard and strong steel.

Electrical Resistance

As metal is heated, the electrons within it will absorb energy and move faster. The faster the electrons move, the more likely they are to collide with the metal’s atoms and scatter them. Electrical resistance measures how strongly the metal works against this potential scattering as the temperature rises. That means the higher the electrical resistance of a metal, the lower its conductivity.

Magnetism

There are three elements with naturally magnetic properties, known as ferromagnetic metals: iron, cobalt, and nickel. But once heat is applied – and especially as the temperature goes up – the metal’s natural magnetism is reduced. If heated to a certain point known as the metal’s Curie temperature, its magnetic property will be reduced completely.

Thermal Expansion

As metal is heated, it begins to expand; its volume, length and overall surface area will grow as it continues to absorb more heat. This is due to the atoms making up the metal, which increase their movement as the temperature rises. This movement creates vibrations within the metal’s structure to the point that it swells, an occurrence which is known as thermal expansion. For safety reasons, any metal structure such as bridges or buildings must be designed to accommodate a certain degree of thermal expansion and contraction. Otherwise, you run the risk of damaging the metal as it warms and cools.

Quenching Explained

What is Quenching?

If you’ve watched medieval-themed movies and shows, this is a familiar scene: a blacksmith hammering away at an anvil, before plunging a sword into water amid huge clouds of steam. What you may not realize is that you’ve just witnessed quenching in action. While water might be the favored choice for its dramatic effects onscreen, professionals in metal working use a variety of methods ranging from air to oil to brine. But whatever the medium used, the basic concept is the same: forcing the rapid cooling of heat-treated metal.

Purpose of Quenching
During the heat treatment process, the metal is heated to a point above its recrystallization temperature but just below melting. It’s then left for a set period of time to allow the heat to completely permeate the metal. Once removed, it’s considered to be annealed metal if left to cool naturally at room temperature. However, this method can result in a drastically altered metal microstructure, which may not be the desired effect. With quenching, the rapid cooling effectively locks in the microstructure existing in the heated metal. What this means is that a harder, stronger metal is produced once it cools.

Quenching Media
The substance used in quenching is referred to as the medium, and the medium determines the quench’s severity. The more quickly you are able to draw heat out of the metal, the more severe the quench. However, faster isn’t always ¬-better. The rolling clouds of steam produced by plunging that red-hot sword into water? In reality, while water is a very effective medium for hardening metal, it can also be a risky one. If the metal is too hot and the water too cool, you may be left with cracked or distorted pieces.
The chemical composition of the metal, as well as the desired degree of hardness, also factors into the choice of quenching medium. Carbon steel and low-carbon steel alloys generally require a more severe quench in order to produce the right degree of hardness. In contrast, high-carbon steel alloys are already a harder type of metal. So lower-impact, less severe quenching media can be used to produce the hardened steel.

Here are the most commonly used types of quenching media, ranked from most to least severe:
Caustics: This involves some combination of water, salt, and caustic sodas to create a brine. However, heat dissipates so quickly in water that it can create an overload of tension between the metal’s surface and its core. When this occurs, the metal part may be left with damage such as warping. So special care is taken to properly balance the temperatures, while workers protect their eyes and skin against any exposure to the caustic solution.

Oil: The most popular medium of choice is oil, since it combines a relatively severe quenching with a lower risk of warps and cracks. It also provides more flexibility during the process, depending on the composition and temperature of the oil. If a higher degree of quenching severity is required, a “fast” oil is used – one formulated to cool the metal very quickly. Low-carbon steels do particularly well in fast oils, because it provides the rapid cooling needed to give the metal more hardness. Meanwhile, high-carbon steels and more intricately-designed metal parts benefit from a less drastic approach. An immersion in heated oil will cool the metal, but at a more controlled and gradual pace.
However, oil brings its own risks during the quenching process. Unlike water and other caustic solutions, oil is flammable. Workers must pay attention to each oil’s flashpoint to avoid any risk of fire during the process.

Salts: Also known as salt baths, this method provides a less severe quenching because the molten salt is hotter than heated oils. This level of heat allows for a slower and more uniform cooling, producing metal with fewer distortions and other defects. So when quenching precision-machined parts and tools, salt baths might be your best choice. The salt baths can also be a long-lasting and cost effective method, so long as they’re properly maintained. The removal of any remaining oxides or other contaminants from the salt bath after quenching will allow for its extended use.

Air/Gas: Air is another popular option for quenching, in large part due to its affordability. If the material is left to cool on its own down to room temperature, it is considered air-quenched (although this is generally done for annealing). For quenching purposes, forced air is used to speed the rate of cooling. However, while this is more efficient from still air, it does require an extended period of cooling – and more time ups the risk of allowing the metal’s microstructure to change. A quicker quenching can be done with gases such as nitrogen, although the trade-off is the increased cost in comparison to air.

MTR: Mill Test Reports

Each day, we take for granted that the metals we encounter and use in our ordinary lives are safe. The steel framing in apartments and high-rise buildings, the aluminum fuselage of a passenger plane, the metal making up our cars and appliances: very rarely do we question the composition of the metal itself. Our lives go smoothly because we can trust these metals are safe and sturdy. We’re able to make that assumption because of the quality controls practiced by the metal industry, and that quality assurance rests largely on the mill test report (MTR).
At every step of the manufacturing process, the metal is accompanied by its MTR. From the mill forging the material, to the service center performing heat processing and finishing, to the company purchasing the items, the MTR passes from hand to hand. But what exactly is included in a mill report that makes it so important?


Mill Test Report
¬As might be expected, most mills will have their own style and layout for an MTR. They may not even call it a Mill Test Report at all: MTR is industry jargon that can also refer to a Material Test Report, a Mill Certification, or Mill Inspection Certificate. But whatever the name, the information the MTR includes is standard across the industry. When reviewing the document, you can expect it to list the following:

• Production information: the metal’s country of origin, melt location, and manufacturer’s name

• Product description: the item’s alloy, temper, grade, finish, width, thickness, and weight

• Material heat number: also referred to as the heat lot, this is an identifying code that is stamped directly onto the metal itself. As each lot of metal is produced, it is assigned a unique code to identify it as part of that batch. This provides a high degree of traceability, no matter where the metal might end up or its purpose. If there is any type of recall or issue with the metal, this number allows it to be traced back to its origin.

• Mechanical and physical properties: this states the metal is compliant with the criteria set by an international standards organization, such as ASME or ANSI. It is especially important in the case of alloys, because it certifies the chemical makeup of the metal. This is where the “test” portion of the MTR applies. While the metal is molten, a sample is drawn from each batch for metallurgical chemical analysis. Once the item is forged, mechanical testing is then performed on the finished item. Putting items such as steel plate through hardness, tensile and impact tests establish that the material will perform to the necessary standards under specific conditions.

• Inspection: in addition to the mill’s test information, the metal requires certification from an independent inspector. This means the MTR typically includes two signatures verifying the material’s makeup: one from the mill itself while producing the metal, and a second from the inspection agency. This third party inspection provides additional authentication of the metal’s physical and mechanical properties, along with confirmation that the heat number matches the actual item. If all information is correct, the inspector signs off on the report as a final step.



Do I Need the MTR?
For companies involved in the industrial side of manufacturing, distribution, and use of metals, MTRs are a necessity. These standards ensure that the metals will be appropriate for their intended use, and able to measure up successfully to further processing such as welding with compatible metals. If the finished goods or structures later experience some type of metal failure, the information provided by the MTR enables every item can be traced back to its source.
However, MTRs aren’t a necessity for the average consumer. Buying from reputable mills and distributors means that they have done the groundwork for you by collecting the MTRs for their products. Companies are required to keep their MTR documentation for a minimum of three years. However, many companies keep their records for much longer, priding themselves on being able to trace back their materials whenever the occasion may arise.

Grade 304 vs 316

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.