What is Sheet Metal Fabrication?
Sheet metal fabrication is a multifaceted process of creating metal parts or structures out of sheet metal.
Cutting: Cutting metal into pieces of precise dimensions or dividing large sheets into several parts are examples of sheet metal processing. You can cut sheet metals manually using power tools, metal saws, lasers and waterjet saws.
Blanking: Blanking involves cutting out a portion of the sheet metal based on a stencil. Excess material is discarded or recycled.
Bending: Depending on your specific design, you may need to bend portions of the sheet metal pieces you have made. You can do this using press brakes and other tools.
Galvanizing: Corrugated roofing panels, for example, are a great example of this type of use. Zinc coated materials works effectively in prolonging the life of your roof.
Milling: If the metal work piece is still too thick and further reducing its thickness through rolling or stretching is impractical, milling is the next best course of action. It requires the use of a rotary cutter.
Welding: Joining together two pieces of metal by applying heat and pressure with gas or electricity can be used to build structures and create parts.
The forging of metal sheet and plate is often just the first step in its overall processing. Sawing will give you the desired size, but depending on the intended purpose of the metal, further operations will need to be done. Material removal services enable metal producers to create the exact customer specifications for components, using hole punching, drilling, and notching. Each process results in material removal, but the choice of method can depend on the metal’s thickness, the location of where the removal will take place, and other factors.
The Material Removal Process
This shearing process involves a tool with sharp edges capable of punching through a piece of metal. Think of a cookie cutter: no matter what the shape, if the cutter is pressed quickly and firmly into the dough, you’re left with precise edges once the piece is removed. Hole punching works the same way on metal sheet and plate. Whether done using electrical, hydraulic, or pneumatic power, if sufficient force is applied, any shaped hole can be made in the metal.
Much like hole punching, notching is a shearing operation using a sharp-edged tool. The difference lies in the removal’s location. While hole punching can take place anywhere on the metal, notching occurs only on its edges. Once those notching cuts are performed on the metal’s perimeter, additional bending or forming can be done.
This type of material removal uses a drill bit rather than a shearing tool. As the drill is applied to the metal, it continually removes the material until the bit pierces through. While a hand drill can be used, more often this process involves a machine known as a drill press. The operator uses the machine handles to lower the bit onto the metal, which helps enable more precise drilling.
Advantages and Disadvantages
Hole punching is an appealing option for several reasons, but especially because of its speed. The presses used for hole punching can be incredibly quick, allowing multiple metal pieces to be processed within a short period of time.
Quick processing means a higher production volume, making this highly cost-effective.
The other advantage is flexibility, since hole punching easily creates shapes other than round. Whatever the shape needed – round, square, triangular – the hole punching press can cut that into the metal.
However, successful hole punching is limited to metal pieces with a thinner diameter. It is also dependent on the sharpness of the press, and punching through metal wears on the tool. If not kept sharp, a worn tool leads to punches that are incomplete or have sloppy, rough edges.
Since notching is a shearing process like hole punching, they share similar pros and cons. The notching tool is fast, and while the process often involves creating vertical cuts, it can be modified to create different shapes. And like hole punching, this type of material removal is best done on thinner pieces of metal. Greater ease of cutting helps ensure the notches will be clean-edged and precise.
Thicker metal can be processed, but requires greater force which is tough on the notching tool. The tool wear requires more supervision and maintenance to best help keep it sharp for notching.
Unlike hole punching and notching, which cut through the metal, drilling gradually pierces through it to remove material. While slower than a shearing method, drilling helps to prevent distortion of the metal part. The force needed for punching or notching tools can sometimes leave the metal bent around the location of the cut. Drilling is much better at keeping the surrounding area unaffected, and can be done successfully even on very thick pieces of metal. In addition, drilling is much more affordable since the necessary equipment is less expensive than the tools needed for other material removal options.
However, the use of a drill bit does have its issues. This process can only create a round hole, unlike notching and punching. There is more friction involved due to drilling being a longer process, and friction produces heat which can negatively affect the metal. The friction also wears away at the drill bit, which may need replacement or sharpening to keep the hole precise.
The word “metallic”: a term synonymous with adjectives such as bright, gleaming, and reflective. However, newly-produced metal straight from the mill isn’t very shiny; in fact, it looks and feels rather rugged. Its appearance is referred to as “mill finish”, and it has a rough and irregular surface texture. The finish is dull, and its color often patchy due to metal oxidation or contamination from mill oil. While it looks unrefined, depending on its use mill finish can be acceptable. Why spend extra time and money to make unfinished metal look better when appearance is unimportant?
However, the majority of consumers prefer their metal products to be smooth and shiny. Beyond the look of mill finish, other processes such as shearing can leave metal with rough or sharp edges. To improve the overall quality of the metal, polishing is often done to create a smooth and shiny surface. But when a finer degree of finishing is needed for certain applications, additional processing such as superfinishing is done.
Simply put, superfinishing is a level beyond the typical processing done to metal products. The metal has already gone through some type of finishing such as polishing or another surface modification process. To the naked eye, polishing leaves a gleaming surface and metallic shine, but the process can actually result in microscopic structural irregularities on its surface. These irregularities are referred to as the residual amorphous layer. Unlike the rest of the metal which retains its crystalline structure, the amorphous layer has scattered atoms creating a disordered surface. Superfinishing is necessary for situations where removal of that amorphous layer is critical to the metal component’s use.
The Superfinishing Process
During a primary finishing process like polishing, a great deal of friction is needed to smooth away any pits or bumps marring the metal’s surface. On the other hand, the secondary process of superfinishing is a much more precise operation using lower friction. The best way to create lower friction is by the use of a very fine abrasive agent. The height of a superfinishing abrasive agent may only measure from 5 to 8 micrometers: too small for successful polishing, but effective in wearing away the amorphous layer.
After the selection of stone or sandpaper with an extremely fine grit, the abrasive is applied on the metal’s surface while the item rotates in the opposite direction. This is done one small section at a time, both for precision and to keep friction to a minimum. Additional thermal protection is provided by using a lubricant, which helps reduce any heat produced from friction.
Advantages of Superfinishing
Superfinishing is useful for both functional and aesthetic purposes:
• Appearance: the metal is left with tiny crosshatch marks, creating an attractive brushed-metal finish.
• Low Tolerances: the amorphous layer removal performed during superfinishing means the metal’s measurements are much more precise. Polishing may leave metal looking shiny, but on a microscopic level, the surface is still uneven. Superfinishing ensures that the metal’s surface is uniform, because the crosshatching is the same height on every part of the component. The metal’s dimensional accuracy is improved, creating better seals and more precise assembly when the part is used in applications.
• Prolonged Use: precise assembly allows for less friction created during use, because pieces are less likely to press and slide against each other. This reduces wear on the metal, which then requires less maintenance and enables a longer period of use. Automotive parts like rods and shafts are typically superfinished to help extend the lifetime of the part.
Disadvantages of Superfinishing
• Finish: depending on the consumer’s taste, a polished finish may be considered more appealing than a superfinished one.
• Cost: additional equipment is needed to perform superfinishing, adding to the metal’s overall cost.
• Time: while superfinishing is not a particularly lengthy process, the operation does add to the overall processing period for that part. This prolongs the time before a finished product can be shipped or sold.
Carbon steel is just as described: a steel alloy containing added carbon, with the carbon making up 0.05% to 2.0% of its weight. This percentage range means carbon steel is a sprawling category, covering many different types of steels with varying levels of carbon. Steel with any other combination of alloying elements can be included in this category, so long as they contain carbon.
Clearly the many possible chemical compositions of carbon steel make it difficult to make blanket statements about this metal. Carbon increases the hardness and strength of steel, but the percentage of carbon added creates different results with the steel’s mechanical properties. Those properties then affect how the metal reacts to further processing such as welding. It is very important to understand the exact qualities of a selected carbon steel to produce a successful weld; doing otherwise risks metal damage or a weld that may fail later during use.
The most important decision before beginning a weld will be the type of steel to use. Carbon steels are organized as three basic types:
• Low carbon steels: containing less than 0.30% carbon by weight
• Medium carbon steels: containing 0.30% to 0.60% carbon by weight
• High carbon steels: containing 0.61% to 2% carbon by weight
Each of these groups will weld differently due to their carbon content. The higher the steel’s carbon level, the more prone it will be to weld cracking. Low carbon steels can be much more easily welded, which is why they are the typical choice for this process.. However, high carbon steels can be welded, provided extra care is taken with the more brittle material. Heating high carbon steel prior to the weld, as well as afterwards, will help prevent cracking. Additional filler metals may also be used with high carbon steel to create a better weld.
Once the type of carbon steel is selected, further determination of its weldability is done using the carbon equivalency formula. This formula examines what effect the other alloying elements besides carbon may have on a weld. The percentages of alloying elements are calculated as an additional degree of carbon – and that boosted carbon level translates into a steel considered more difficult to weld. As an example: using the formula means a medium carbon steel with no added manganese will be judged as easier to weld than the same carbon metal including manganese alloy.
Overall Chemical Composition
Even after selecting a lower carbon steel, and calculating an acceptable carbon equivalency, the overall chemical composition should still be reviewed. Some elements will just not take well to welding, even with a great deal of effort. Any amount of lead in carbon steel, for instance, will significantly raise the probability of weld cracks. Other common alloying elements such as sulfur or phosphorus can have the same effect, resulting in a weak or failed weld. While carbon steel with trace amounts of either sulfur or phosphorus can still be welded, the percentages must remain extremely low (approximately 0.05% or less) to avoid weld cracking.
The job is still not complete after creating a weld, because as it cools, cracking may develop. Carbon is again the main culprit here, due to it making the steel more brittle. With higher levels of carbon and equivalent alloys, the metal must be cooled at a slower rate to prevent cracks from occurring.
In addition to the steel’s carbon percentage, other factors must be accounted for during weld cooling. The thickness of the steel being welded will affect the cooling rate, and thinner material has a higher risk of distortion. The temperature where the weld is being performed is also important. In cold temperatures, preheating may be necessary even for lower carbon steels, while it can be performed without prior operations in warmer environments.
Cracking, spatter, incomplete fusion: just a few of the many pitfalls you may face when attempting a weld. While no approach is completely foolproof, your chances of a successful weld are much better once you have a good understanding of the basics. It is also important to tailor your methods to your choice of material; what works best for one type of stainless steel may not be as helpful with another. In this case, we’ll examine a few of the most popular methods of stainless steel welding.
There are many different grades of stainless steel, but they all have at least one alloying element in common: chromium. The addition of chromium is what makes steel ‘stainless’, with its characteristic shine and boosted corrosion resistance. However, adding chromium to the steel’s chemical composition – along with other elements such as molybdenum, silicon, or manganese – affect how the metal will react to heat. These elements lower the thermal conductivity of stainless steel, which means heat will not spread as easily through the material. This lack of efficient conductivity requires a stronger heat input, but higher temperatures or an extended welding time increases the chances of a burn-through or other heat-related faults. Whatever process you use for welding stainless steel, it will be a matter of balancing necessary heat input vs the potential for heat damage.
There are several different methods of welding stainless steel, but most welders use one of these three types. There are other factors that can be considered in more detailed, but generally the selection depends upon the material’s thickness:
1. Tungsten Inert Gas (TIG) welding
This process is one of the most commonly used for welding stainless steel. TIG welding, often referred to as tungsten gas arc welding, creates a low heat input which makes it the best choice for thinner materials. The heat is generated using electricity, which powers an electric arc between the tungsten electrode and the base metal. An inert gas such as argon creates a sheathing environment around the electrode, protecting and cooling the tungsten. This dampening effect helps to keep a relatively low heat input, as well as creating less smoke and oxidation.
Low heat input means TIG welding is ideal for thin stainless steels or precision welds on critical joints. Its concentrated heat allows for a narrow fusion zone, which calls for a high degree of control over the welding process. After completion, a TIG-produced join requires little to no post-weld finishing.
2. Metal Inert Gas (MIG) welding
Also known as gas metal arc welding, MIG uses electricity to melt and join metal during a weld. However, unlike TIG welding, MIG requires the use of both an inert shielding gas and a solid wire electrode. This filler metal wire is continuously fed during the welding to create the arc between the electrode and base metal.
Due to the higher heat during the weld, MIG is best selected when welding thicker widths of stainless steel or steel plate. It can be adapted for semi-automatic welding processes, since the continuous wire feed allows for long joins. It also produces little spatter and easy cleanup after processing.
3. Resistance welding (RW)
TIG relies upon a welding operator and MIG can be done manually or through automation, but for manufacturers who require a highly automated process, resistance welding is key. The method involves an electrical current run through weld electrodes, which apply force to the pieces being joined (the “resistance” part of the process). This force is converted to heat, and the resulting heat melts the metal enough to join it together. When completed successfully, the molten metal at the point of contact will form a rounded weld, known as a “nugget”.
Unlike the other two processes, RW has a steep initial cost of equipment setup and requires a high amount of power. This means it has little portability and cannot be used in the field like TIG and MIG welding. However, for manufacturing purposes, resistance welding is ideal. It is fast, reliable, requires no filler metals or other materials, and is excellent for sheet metal applications. Stainless steel can get up to weld temperature very quickly using the RW method, so welding can be completed within a few milliseconds.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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?
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 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.
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.