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?

  1. 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.

Metal Aging through Precipitation Hardening

When it comes to metal aging, the simplest way to understand the process is in terms of heat. In general, if a metal has the ability to withstand high temperatures during heat treatment, then it can be aged. For alloys containing aluminum, copper, magnesium or nickel, aging is the principal method of strengthening the finished product.

When metal is exposed to heat, any impurities (precipitates) contained within it begin to form on the surface. These precipitates help to prevent dislocations, which are defects in the metal’s crystal structure. Because dislocations are a primary cause of metal weakness, this means the precipitates are acting as reinforcements to strengthen the metal. Aging it has made the material stronger, more stable, and more resistant. So it’s clear why accelerating these changes through artificial aging is a popular choice!

Precipitation Hardening

The basic process of precipitation hardening, or age-hardening, consists of three steps:

  1. Solution treatment: Also known as “solutionizing”, this involves heating a metal alloy to extremely high temperatures. This mix creates a solution, where the alloying material is suspended within the liquid base metal. More importantly, it dissolves the precipitates and helps disperse these particles evenly throughout the solution.
  • Quenching: Once an alloy solution has been created, the liquid metal is then cooled as quickly as possible. This quenching can be done using compressed air, oil, water, or brine. Whatever the method used, the aim is to “flash freeze” the metal so that the solid is as evenly-mixed as the solution. The faster it can be cooled, the less time the precipitates have to form on its surface.
  • Aging: The metal is heated again, although to a lower temperature to avoid any dissolving. Applying heat a second time ensures the precipitates within the metal are evenly dispersed. Afterwards the heated metal item is quenched a final time.

However, there are risks involved with heat treatment. Over-aging occurs when the metal is held too long at too high a temperature. This can result in uneven disbursement of precipitates in solution, which leads to cracking and distortion in the cooled product. When monitored carefully throughout the age-hardening process, metal alloys that have completed these steps will be a harder, stronger material.

The Aluminum Extrusion Process

What is extrusion?

Extrusion is the process of shaping material during manufacture. Generally, this is done by forcing a block of metal, called a billet, through a shaped die. Think of it like a frosting tip: whatever is squeezed out appears with the specific design you selected.

How is aluminum extruded?

For aluminum, the two main methods of extrusion are direct and indirect.

Direct extrusion is the most commonly used method, using a stationary die. The billet is heated to 800 – 925 degrees F, then laid on a loader and pushed through the die using a hydraulic press. The steady pressure squeezes the softened metal through the die opening. Using direct extrusion, this process produces a wide variety of solid bars, rods, and hollow tubing.

With indirect extrusion, the process is reversed – the billet remains stationary while the die is forced onto the metal itself. This creates far less friction on the billet than using direct extrusion. The result is a product with more consistent dimensions, grain structure, and mechanical properties. However, the method also has its disadvantages, mainly related to the lack of friction. Billets must be carefully cleaned, since little to no friction means any substances on the metal will affect the extrusion’s surface.

Why use the extrusion process?

Extrusion is favored for many metals since it is easier to manufacture, with aluminum being particularly suited for the process:

  • Quick fabrication and assembly: compared to other tooling processes such as stamping, casting or injection molding, extrusion has a shorter lead time and done at a lower cost. This means items will be much more quick-to-market, from prototype development to product launch.
  • Easy tailoring: there are already a number of standard aluminum extrusion designs already available. This speeds production and assembly, by improving performance and cutting down on secondary operations.
  • Strength: with the extrusion process, the metal’s strength can be concentrated in specific areas by varying the wall thickness and internal reinforcement of the design. This is even more of an advantage with aluminum extrusions intended for use in cold environments. Unlike other metals which can become brittle with cold, aluminum strengthens with lower temperatures. The combination of the extra reinforcement through extrusion, coupled with the property of the aluminum itself, makes for a dependably strong metal.
  • Excellent thermal and electrical conductors: aluminum is nearly twice as conductive as copper, and much less expensive a material. It also conducts both heat and cold better than many other common metals. Because of this, extruded aluminum products are an attractive choice for home builders. Its lower price, heat dissipation properties, and resistance to fire are all advantages for house framing.
  • Sustainability: aluminum can be recycled infinite times, with no degradation of the metal’s properties. This means extruded aluminum products often contain a high percentage of recycled content. The addition of recycled material to the primary aluminum has no effect on the finished product’s overall aesthetics or functionality.

What is the Difference between Annealing and Tempering?

At first glance, it might be difficult to distinguish annealing from tempering. Both are heat treatments designed to alter the physical and mechanical properties of a metal, and both involve heating that metal and gradually cooling it. So what makes the annealing steel different, and what are the advantages of this process?

When it comes to annealing, it’s important to remember how dependent the process is on precision and control. Like tempering, annealing involves reheating quenched steel and then allowing it to cool. However, at each stage of the annealing process, careful oversight is crucial to producing the most high-quality result possible.

Annealing involves three separate stages:

  1.  Recovery: simply put, this is applying heat to soften the metal. To ensure the most even heat distribution, air should be allowed to circulate freely around the items being annealed. For this reason, the heating is most often done in large ovens which can be tightly sealed, raised to the desired temperature, and closely monitored. Recovery then occurs when the heat breaks down dislocations and other irregularities within the metal’s structure.
  1. Recrystallization: during this stage, the heat is raised to above the metal’s recrystallization temperature while still remaining just below its melting point. This means that new smaller grains are formed within the steel, replacing older grains with pre-existing stresses. So while the finished product will be less hard then it was before, the uniform structure of the new grains will give the steel more strength and resiliency.
  1.  Grain growth: this is the cooling stage of the annealing process. In contrast to tempering, which allows the steel to cool naturally at room temperature, the cooling of annealed metals must be highly controlled. To do this, cooling is often done by immersing the hot steel into a low-conductivity environment such as burying it in sand or ashes. It can also be done by switching off the oven and allowing the metal to slowly cool within the machinery’s fading heat. Whatever the method used, the aim is to have as slow and gradual a cooling process as possible. When fully cooled, the steel will now possess a more refined micro-structure. In real terms, this means it has more elasticity, so that it can take the stress of machining or grinding with far less risk of cracking.

While all heat treatments result in a strengthened alloy, annealing is crucial for items that have previously been cold worked. Cold working produces stresses within the metal, which annealing then helps to reverse by bringing it closer to the metal’s original properties. That means the benefits of annealing are twofold: eliminating as much residual stresses as possible while restoring its strength and ductility. So while tempering is used for products such as structural beams, the more ductile steel produced through annealing is found in items like mattress springs, wiring, and tools.

What is the Tempering Process?

Steel is a term we consider synonymous with strength. Having a steely glare, being tough as steel: both expressions used to describe someone who is hard, strong, and determined.

So then, it may surprise you to learn that untempered steel can be nearly as fragile as glass! Without the tempering process, any steel produced will be extremely hard but also quite brittle. If it remains this way, it would be too prone to breakage to use in most applications.

Tempering is a method of heat treatment used to increase the resilience of iron-based alloys such as steel. After an initial heat treatment has been done to boost the steel’s hardness, tempering then reduces some of that hardness to help improve its strength. The end result is a steel which is less brittle, with increased ductility and abrasion resistance.

The tempering process begins after the steel has gone through an initial hardening treatment. With hardening, steel is brought to very high temperatures just short of melting, and quenched to cool it as quickly as possible. This quenching essentially locks the steel’s crystal structure and creates a very hard material. Tempering is then done immediately afterwards; if left to sit after quenching, the risk of cracking within the material will increase. To temper the steel, it involves reheating it once again to a high temperature and cooling, but in a less extreme or abrupt way than with hardening. Once exposed to the desired temperature, the steady heat application helps to relieve any internal stresses within the steel. Finally, the metal is removed and allowed to cool naturally in still air. So perhaps the best way to imagine tempering is as though you were baking a cake: you’ll have the best result if you are careful to set the right temperature, length of time, and gradual cooling.

Adjusting the peak temperature for tempering allows you the opportunity to create a product for a specific need. There are 3 temperature range categories when tempering steel:

  1. Low temperature: this tempering will somewhat reduce the metal’s brittleness, while retaining its hardness. Steel produced this way is often used for case hardening components and cold work tools.
  1. Medium temperature: the heat used in this range will produce a more elastic product. This means it’s now a more machinable and formable metal, which can be shaped and worked without losing the original shape. This steel is often used to manufacture knives.
  1. High temperature: the higher the temperature used during the tempering process, the greater toughness that is given to the steel. This combination of resiliency and toughness makes it a good choice when producing structural steel and machine components.

Overall, it’s a rule that any hardened steel must be tempered. And without tempering, your stainless steel and other alloys won’t have the high level of ductility and weldability that we’ve come to expect from high quality materials.