Category: Processes

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.

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.

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.

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.

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.

2. 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.

3. 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.