When it comes to steelmaking, production can be divided into two basic groups: cold rolled and hot rolled steel. In earlier eras, molten steel was cast into blocks known as ingots. These ingots were convenient to stack and ship, and could be melted back down when needed for use. In current times, steel manufacturing is done on a massive and continuous scale. To make this process more efficient, manufactured steel is produced to be as close to a finished product as possible.
Newly forged steel emerges as a continuous slab of red-hot metal, which is then fed into a series of mills. The process is similar to a home pasta maker: with each consecutive rolling, you will end up with a thinner and thinner result. As the steel progresses through the mills, the metal remains hot enough to stretch and thin out into a long strip. Once rolled out to the desired thickness, it can be cut into shapes such as bars, or thinned enough to be rolled into coil.
Most Popular Grades
Hot rolled steel is available in several different grades, which are the standards set for a steel type. Generally, steels in North America conform to the standards set by the American Society for Testing and Materials (ASTM), or those set by the Society of Automotive Engineers (SAE). ASTM steel grades always begin with an “A”, which is the letter assigned to ferrous metals. SAE steel grades use a four-digit number for identification.
A36 Hot Rolled Steel
One of the most popular grades of hot rolled steel, A36 is a low carbon steel alloy. The low percentage of carbon within A36 steel means it is highly versatile: it can easily be formed, machined, or welded.
The low amounts of other alloying elements such as nickel and chromium make A36 steel just average in corrosion resistance, but also keeps the price relatively low. For this reason, it is widely used in applications where cosmetic appearance is not a priority. You will often find A36 hot rolled steel used for heavy duty construction and equipment manufacturing.
Common applications include:
• Bridge and building construction
• Automotive frames and trailers
• Agriculture equipment
• Oil and gas equipment
1018 Hot Rolled Steel Bar
Another popular choice of hot rolled steel is 1018, which is a similar grade to A36. A36 is often the top choice in manufacturing, but 1018 will be selected if the application calls for bar or strip steel. It is even lower in carbon percentage than A36, and this low carbon in 1018 allows for excellent formability. The low carbon level also allows 1018 to be a fairly ductile type of steel which can be easily bent and machined.
Common applications include:
• Pins and studs
• Steel bar in square, rounded square, hexagonal and other shapes
1011 Hot Rolled Sheet and Plate
SAE 1011 is a strong, low-alloy form of hot rolled steel. Its low percentages of carbon and other alloying elements means this steel grade is very hard and strong, while remaining easy to drill, form and weld. 1011 is strong and durable, but can be vulnerable to corrosion with its low alloying percentages. Exposed to the elements, the iron content of 1011 will begin to discolor, corrode, and roughen its surface. For added corrosion protection, an outer layer is often applied to the steel’s surface, through painting or processing such as galvanization.
Common applications include:
• Building and roofing construction
• Shipping containers
• Automotive parts
• Heavy equipment
For people new to working with pipe, referring to their “schedule” may be unfamiliar. Imagine you have two pipes in front of you: one labeled schedule 40, while the other is schedule 80. Both are manufactured of the same type of metal, with the same grade, and the same 3-inch diameter. So, what is it that makes these two pipes different, other than their schedule numbers?
First, it helps to understand the functional challenges faced by pipe. Pipe is designed to move along liquid or gas under pressure – pressure which can be internal, external, or both. To sustain fluid pressure, pipe must be strong enough to withstand a great deal of stress. This means the product dimensions of pipes aren’t simply measured by length and diameter, but its wall strength as well. A high-pressure fluid will require thicker pipe walls than pipes used for low-pressure applications like drainage.
The wall thickness that determines a pipe’s strength is known as its schedule. Pipe schedules are a standard to identify wall thickness for the same size of pipe. The outer diameter of a particular pipe size will stay constant while the inner diameter varies according to the schedule type. Using the example of the 3-inch pipes, you can see the difference once you measure their inner diameter. The outer diameter for both pipes will remain approximately 3.5 inches, but their other qualities differ:
3” Schedule 40 pipe
• Wall thickness of 0.216 inches
• Inner diameter of 3.05 inches
• Weight per foot is 7.58 pounds
3” Schedule 80 pipe
• Wall thickness of 0.300 inches
• Inner diameter of 2.90 inches
• Weight per foot is 10.25 pounds
As you can tell, the schedule 80 pipe is significantly heavier due to its thicker walls. Those thicker walls mean schedule 80 pipe is better able to handle high pressure, which might cause greater wear or damage to schedule 40 pipe. Clearly, pipe schedules are quite important to ensure a project’s success and the best use of your funds. For home plumbing projects, schedule 40 pipe is sufficient, with no need to spend extra for schedule 80 pipe. For industrial uses, on the other hand, investing more money up-front for schedule 80 pipe may help prevent damage and costly repair work in the future.
It’s not uncommon for someone to assume tube and pipe are interchangeable terms. You yourself may not have ever considered what the difference might be. If asked, what might be your guess: maybe they’re called pipes when made of metal, and tubes if made of PVC? Are they simply the same thing no matter what the name, the way different people might refer to soda versus pop?
Basically, it comes down to use: tube is a structural element, while pipe is used to transport fluid or gas. While both types are lengthy hollow cylinders, their use means they are different in their measurements, desired qualities, and even shapes.
10 Differences Between Tube and Pipe
1. Size Measurements
Pipe is more approximate in its measurements, with sizes being referred to as nominal pipe size (NPS). The pipe’s inner diameter and wall thickness are the most important dimensions, and vary according to strength. A very strong pipe will have thick walls with a smaller inner diameter than an equivalent NPS with thinner walls.
When measuring tube, its outer diameter and wall thickness are also important dimensions. Unlike pipe, these will be the exact measurements of the tube’s cross section.
2. Wall Thickness
Pipe wall thickness is known as its schedule, and determines the liquid capacity of a pipe. Two pipes with the same NPS number but different schedule numbers will have a difference in inner wall thickness.
Tube wall thickness is defined by gauge, with the wall’s width increasing along with a higher gauge number.
3. Production Sizes
Pipe comes in a wide variety of lengths, often 6 feet or more. Tubes, on the other hand, are most often smaller parts. Many are 5 inches or less, although larger tubes can be used for some applications.
Many different materials can be used for pipe: iron, copper, brass, and PVC. Tube is most often made of some type of steel, whether that’s carbon, low-alloy, or stainless steel.
A pipe will always be round. Tube can be made in round, square, and other shapes.
6. End Connections
The ends of pipes can be straight, beveled, or screwed. Tube ends are commonly threaded or grooved for quicker connections.
An item’s tolerance is the acceptable range in its measurements: any deviations in its straightness, shape, or strength. Pipe tends to have a wider tolerance, so long as it meets its designated schedule requirements. Tube is the opposite, being made with strict tolerance guidelines. Because tube is intended for structural and often mechanical use, strength is a high priority. Repeated quality checks are made on a tube’s straightness, gauge, and outer surface.
8. Production Process
Pipe manufacturing is quite efficient and speedy, with many items being made to stock. Production of tube is more exacting due to its strict tolerances and quality checks, which means a lengthier, more labor-intensive process.
Due to the difference in production, availability also varies between pipe and tube. Pipe, especially if ordered in made to stock sizes, is likely to have a quick delivery. Tube generally takes longer to produce and ship
As you may have already guessed, the time and effort used to produce tube also translates to a higher price tag. While pipe can be pricey, especially if made of metals like copper, the average price of pipe will be lower than tube.
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.
At first glance, aluminum and stainless steel may appear similar: both a silvery gray, softly shiny, and used to make many of the same products. Many food service and kitchenware items, for instance, are made available from a manufacturer in both types of metals. What separates these two, other than price?
1. Thermal Conductivity
Aluminum is a metal with a high degree of thermal conductivity. What that means in real terms is that a water will boil more quickly in a stockpot than one made of stainless steel. However, aluminum also cools more quickly than stainless, so stainless will help keep a pot of soup warm longer.
2. Thermal Properties
Its lower degree of thermal conductivity means stainless steel is much more resilient to use in high temperatures. Aluminum will begin to soften around 400 degrees Fahrenheit; meanwhile, stainless steel can function well at temperatures up to 800 degrees. Some stainless steel grades can withstand temperatures nearly double that for short periods of work, up to 1500 degrees.
Both metals are quite strong and durable, but stainless steel more so than aluminum.
4. Strength to Weight Ratio
Stainless steel is very strong, but at the cost of a heavier weight. While aluminum is not as strong, manufacturing with this metal will result in an item nearly one third the weight of a steel part. This incredible strength to weight ratio makes aluminum very attractive to manufacturers, especially in the aerospace industry.
Most grades of stainless steel are relatively simple to weld, while aluminum’s high thermal conductivity makes it a more difficult task. However, both are among the most popular types of metals used in welding.
6. Electrical Conductivity
Aluminum has the distinct advantage over stainless steel here, with it being an excellent conductor of electricity. Stainless steel does not conduct electricity well in comparison to other metals.
Pure aluminum is a very soft and malleable metal, and even after alloying, it is easy to cut and form. Due to its strength and hardness, stainless steel takes effort to form into shape.
8. Corrosion Resistance
Both metals are known for excellent corrosion resistance. Chromium is one of the principal alloying agents in stainless steel, and boosts the corrosion resistance of the steel alloy. Aluminum is naturally resistant to corrosion on its own, although it is more vulnerable to damage from highly basic or acidic exposure.
In general, a part made of aluminum will be more affordable than the same item made of stainless steel.
10. Reaction to Foods
Consumers have the choice between aluminum and stainless steel cookware, and often make the decision to purchase aluminum based on price. However, aluminum is a more reactive compound than stainless steel. Acidic foods such as tomato sauces can leave aluminum damaged or with unsightly marks, while other foods like eggs can discolor if cooked or stored in aluminum. While both metals produce high-quality items, choosing stainless steel may be worth some additional money for non-reactive cookware.
As you might guess, tool steel earns its name by being primarily used for tools. If you then picture a steel screwdriver, pliers, or a wrench, you’d be right – but the items created of tool steel group consist of much more. Within the steel industry, tool steel also refers to machine tools, including those used to manufacture other metal products. To meet the standards of this demanding usage, tool steel is known for being extremely hard, resistant to abrasion and deformation, and its ability to hold a cutting edge. While the steel’s chemical composition is important, the method of production must be carefully controlled to achieve the proper qualities expected in tool steel.
Tool Steel Groups
Tool steel is classed into six groups, all made of carbon steel alloyed with one or more of these main elements: molybdenum, tungsten, chromium, and vanadium. Most tool steel originates from recycled steel scrap, but not all steel scrap is suitable for tool steel. Alloy scrap containing elements which resist oxidation (such as nickel, cobalt or copper) will impede the development of carbides in the metal’s structure, and carbides are crucial to creating tool steel. To ensure the metal will be an optimal mix, tool steel will typically consist of 75% mill scrap supplemented by purchased steel scrap.
The first step is primary melting, where the scrap is heated until molten. Most often this is done in an electric arc furnace (EAF) due to it being a lower-cost production method. However, it is key to avoid contamination during the melting process to create the highest-quality steel. While the EAF is widely used for tool steel, there is some risk of contamination from dust or traces of oxidation-resistant metals from previous batches. For top notch tool steel, an alternate method known as electroslag refining (ESR) is used to melt the steel. Instead of a furnace, ESR uses electric currents to superheat and slowly melt the metal. While a more expensive process, ESR will produce a more refined type of steel without apparent surface imperfections.
After melting and alloying, the steel is poured into ingot molds and then forged into the desired shapes. This is where the six groups of tool steel are determined by their processing:
• Water hardening: Also known as W-group tool steel, this group’s defining property is being water quenched. Quenching in water does risk warping or cracking steel with its rapid cooling, and results in a relatively brittle metal. For this reason, water-hardened tool steel is not considered appropriate for high-temperature industrial uses; once the temperature nears 300 degrees Fahrenheit, there is noticeable softening of the steel parts. Typical items made of W-group steel include scissors, smaller hand tools, and springs.
• Shock-resisting: S-group tool steel is notable for its toughness and ability to withstand repeated impact. This is achieved by using low carbon steel during its production, since higher carbon content results in a harder but more brittle product. Alloying with silicon, tungsten, and chromium gives additional wear resistance and tensile strength. It’s then quenched in oil, which allows for a gentler cooling than water and helps produce a metal with a high degree of durability. Items made of S-group tool steels include tableware dies, impact hammers for nails guns, and shear blades used in cutting heavy steel plate.
• High speed: This group is mainly used for cutting tools, which requires a very hard and abrasion-resistant type of steel. Tungsten, chromium, and vanadium allow HS steel to withstand very high temperatures, such as the heat friction created while cutting other metals. It also manages to hold a cutting edge with repeated use, allowing for higher speeds while cutting without having to pause to resharpen. HS steel can be found in various cutting tools such as drill bits, saw blades, and milling cutters.
• Hot work: Hot-working (H-group) steels are used to cut and shape materials at very high temperatures. To achieve a steel that can withstand prolonged exposure to intense heat, H-group steels are made of a low carbon steel with a higher percentage of alloying content. This produces a tool steel with a high amount of carbides, giving it good overall toughness and wear resistance. The most common use for hot work steel is to produce other metal items because it can withstand the heat needed to forge and cast other metals. It’s used in machinery such as pressure dies, extrusion, and forging equipment.
• Cold work: This tool steel type is narrowed down further into three groups known as the A series (air-hardened), O series (oil-hardened), and D series (high carbon/chromium). All three are intended to be used to cut or form materials at low temperatures.
1. A-series: As the name suggests, steel in this series is hardened via air. Because of the quenching method and its high chromium content, A-series cold work steel is known for low distortion during heat treatment. This makes it a good fit for machining purposes, and so can be found in dies, forming tools, and gauges.
2. O-series: Oil-quenching avoids the cracking and warping risks brought on by water-quenching, and is used for larger parts which require minimal distortion. It offers good wear resistance and toughness, lending itself to a wide range of applications as a general-use cold work steel. Typical uses of O-series tool steel include stamping dies, bushings (metal parts used to absorb vibration and friction), shear blades, and other cutting tools.
3. D-series: Made of a high-carbon steel with a high alloy percentage of chromium, this can also be categorized as a type of stainless steel. However, unlike typical stainless steel, its corrosion resistance is limited and thus is not used to make consumer products. Instead, D-series tool steel applications include cutters, dies, plastic injection molds, and machining rollers.
• Special purpose: this group includes tool steels with alloying elements such as nickel, which typically resist the formation of carbides. The extra time and effort needed to produce these tool steels makes them too expensive for general purposes, so they are manufactured for specific use. Special purpose tool steel can be found in plastic molding dies, zinc die casting, and drills.
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.
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:
- Electrical Resistance
- Thermal Expansion
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.
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.
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.
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.
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.
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.