posted by Dave Arnold
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I originally wrote this piece for a print publication, but they said the tone was too dry and axed it. They said they wanted something more like the blog. Here it is on the blog.
Cast Iron Intro:
While cast-iron cookware has been available for centuries, the advent of industrialized factory production in the mid 1800’s allowed cast iron to become widely available. The cast-iron skillet quickly became an icon of American cooking. Cast iron could be cheaply produced with minimum tooling in a wide variety of shapes –waffle irons, corn-shaped muffin pans, dutch ovens (dutch meaning “fakeâ€, not “from Hollandâ€), and skillets of every size. While many of these manufacturing advantages have since been supplanted, cast iron’s characteristic properties make it an excellent cookware choice in the modern kitchen. Corn bread made the classic way, in a pre-heated cast iron skillet, highlights cast iron’s cooking advantages: its temperature delivery power generates a good crust, and its temperature-regulating power provides even, constant heat –leveling out the temperature variations of your oven. The science of cast iron shows how these advantages work.
Cast Iron as a Pan Material:
The popular wisdom that cast iron cookware provides even heat is misleading. A cast iron skillet placed on a gas burner will develop distinct hot spots where the flame touches the pan. If you heat the center of a cast iron pan you will find that the heat travels slowly towards the pan’s edge, with a significant temperature gradient between the center and the edge. The pan will heat very unevenly, because cast iron is a relatively poor heat conductor compared to materials like aluminum and copper. An aluminum pan will heat more evenly because heat travels quickly across aluminum. Because of poor heat conduction, undersized burners are incompatible with cast iron cooking. The edges of a large cast iron pan will never get hot on a tiny burner. On properly sized burners you can minimize hot spots by heating slowly, but the best way to evenly heat cast iron is in the oven.
Cast iron has a higher heat capacity than copper, so it takes more energy to heat a pound of cast iron to a given temperature than a pound of copper. More energy is stored in each pound of the cast iron. Aluminum has a higher heat capacity than iron (it stores more heat per pound) but is much less dense than iron. For a given volume, therefore, cast iron stores more heat than aluminum.  Because cast iron pans typically weigh much more and are thicker than the same size pan in another material, they tend to store more energy when heated. This combination of high heat capacity and weight means that cast iron takes a long time to get hot. Once hot, however, a cast iron pan usually contains more thermal energy than other pans at the same temperature — a significant cooking advantage. Cast iron has unparalleled searing power because it has a lot of available thermal energy – and unlike almost any other type of pan, cast iron pans won’t warp when left dry on a burner to heat up. Thick and heavy cast iron will remain flat and true.
Cast iron is slow to heat up, so it’s also slow to cool down. It is a good regulator. It retains its temperature longer than other materials and won’t produce temperature spikes. This behavior can be disconcerting to the uninitiated. Cooking with cast iron is more akin to driving a boat than a car: the pan doesn’t respond instantly to changes in the applied heat.
Cast Iron – the OG Non-Stick Material:
Cast iron is naturally non-stick when seasoned properly. New cast iron is anything but non-stick, and it can easily rust. Seasoning — rubbing oil or fat into the cast iron and subsequently heating it — fixes both problems. Unsaturated fats work best (unsaturated means that some of the carbons in the fatty acid chains contain reactive double bonds). Nineteenth century American cooks typically used lard because it was readily available and unsaturated enough to polymerize well, but almost any oil will work. When an unsaturated fat is heated to high temperatures, especially in the presence of a good catalyst like iron, it is broken down and oxidized, after which it polymerizes –joins into larger mega molecules the same way plastics do – and mixes with bits of carbon and other impurities. This tough, impermeable surface adheres to the pores and crevices in the cast iron as it is forming. The surface is non-stick because it is hydrophobic – it hates water. Water soluble proteins make foods stick to their pan; a hydrophobic surface prevents sticking. The bits of carbon in the seasoning may also act as an additional release agent.
There is no quick way to fully season a cast iron pan; the surface of cast iron becomes slicker and blacker the more it is used. Though most cast iron today is sold “pre-seasoned,†this cursory seasoning protects against rust, but not against sticking. A good non-stick surface forms over time, with use. The oil polymer on a well-used piece of cast iron is built of many thin layers deposited over time. Thick layers can flake off in large pieces. Thin layers will remain adhered to the pan and will slough off microscopically. A true seasoned surface will only form properly at temperatures well in excess of the 350-375 degree F temperature that some manufacturers recommend for seasoning cast iron. Low temperatures do not completely polymerize and break down oil and will leave a brown, somewhat sticky pan instead of a black, non-stick one. 400-500 degrees F is the effective range for seasoning.
Early cast iron was sold either polished or unpolished. Polished cast iron isn’t polished the way silver is, it merely has a surface that was sanded or machined to make it smoother. The polishing process reveals more of the internal pore structure of the iron, and these pores make the seasoning adhere better to the pan. Polished cast iron is slick like glass when properly seasoned. Most modern cast iron is unpolished, meaning its surface has a pebbly appearance from the grain of the mold in which it was cast. Eventually, through years of seasoning, unpolished cast iron can become extremely smooth, but never as smooth as polished cast iron. New, unpolished pans can be sanded with rough sandpaper to approximate polishing.
Caring For Cast Iron:
Many cooks are unnecessarily worried about maintaining their cast iron cookware. The seasoning on a good piece of cast iron is very durable. Modern soap will not harm seasoned cast iron. Old, lye based cleaners will hurt seasoned cast iron because lye dissolves the oil-polymer. Seasoned cast iron can also tolerate gentle scrubbing with non-metallic abrasives. Vigorous washing is not recommended on new, weakly seasoned pans.
Sometimes, the surface of a cast iron pan can become damaged through abuse or neglect. In this case the pan has to be stripped down to metal and re-seasoned. The best way to remove an old or bad seasoning job is to use a fireplace or the self-clean cycle of your oven to reduce the seasoning layer to ashes. This happens around 800 degrees F.
Another good maintenance technique with cast iron is to use metal cooking implements. The gentle scraping of metal along the bottom of the pan while cooking helps to even out the surface of the seasoning and make it more durable, not less.
Cast Iron Nutrition:
Studies show that cooking in cast iron can leach iron into food. Foods that are high in moisture, very acidic, or are long-cooked leach the most. For many people the extra iron is beneficial, but for a small minority of people who are sensitive to iron it can be harmful. The most quoted study on the effects of cast iron cookware on iron levels is the July 1986 study in the Journal of the American Dietetic Association. The pan used in that study had only been seasoned by daily usage for a couple of weeks prior to the study. As the study pointed out, better seasoned pans leach less iron. There are no data on iron leaching in decades-old pans.
Time to read: 8 min
Welcome to the world of post-CNC heat treatment! Now that you’ve successfully CNC machined your parts, it’s time to give them the TLC they deserve. Don’t let those raw components go out into the world without a little extra oomph. Once you’ve finished CNC machining your parts, your work isn’t done. Those raw components might have ugly finishes, may not be strong enough, or only one part of a multi-part complex product. Regardless, post-machining processes are necessary for a range of applications, and we’re here to walk you through some considerations so you can choose the right secondary operations for your project.
This article is part of a three-part series covering options and considerations for heat treatment processes, finishes, and hardware installation. Any or all of these may be necessary to take your part from its machined state to customer-ready. Here, we discuss heat treating, while Parts II and III examine finishes and hardware installation.
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Heat treatment is a processing operation option before or after machining. Why use one method over the other? Both pre-CNC-machining and post-CNC-machining heat treatment processes are common, and both offer specific benefits and considerations. The order in which you choose to heat treat and machine your metal can affect the material properties, the machining process, and the tolerances of your part.
In general, heat treatment of a metal is an exposure of metal to a specified temperature over a period of time to modify the material properties like strength, ductility, and toughness or to alleviate residual stress built up in the material. Usually, this means increasing the strength and hardness of the metal so that it can stand up to more extreme applications. However, certain heat-treating processes, like annealing, actually reduce the hardness of metals, or soften them. Let’s take a look at the different heat-treating methods.
Hardening is used to — you guessed it — make metal harder. A higher hardness means the metal is less likely to dent or be marked when impacted. Hardening of a metal actually improves its resistance to plastic deformation. Hardening also increases the tensile strength of the metal, which is the force at which the material fails and breaks. Higher strength makes the material more suitable for certain applications. Hardening typically applies to ferrous metals (like steel) although other metals like titanium or aluminum can undergo a similar process called nitriding.
In the hardening process, the workpiece is heated to a specific temperature above the metal’s critical temperature or the point at which its crystal structure and physical properties change. The critical temperature is where the metal undergoes a phase change (such as when steel becomes austenite) or when solute-rich precipitates dissolve. The metal is held at that temperature and then cooled by quenching it in water, brine, or oil.
The quenching process allows the formation of a very fine solution-rich precipitate which inevitably stabilizes the crystalline structure of the metal and leaves less room for deformation at the grain boundaries. This increases the hardness and yield strength of the metal. The quenching liquid depends on the specific alloy of the metal. Each quenching liquid has a unique cooling rate, so it’s chosen based on how quickly you need to cool the metal.
Case hardening, or surface hardening, is a type of hardening that only affects the outer surface of the material. This process is often done after machining to create a durable outer layer.
The case hardening process is for steel alloys and works by diffusing carbon, nitrogen, or boron (known respectively as carburizing, nitriding, and boriding) into the outer layer of the steel at a high temperature and then heat treating to improve the hardness. The depth of the penetration of carbon, nitrogen or boron is dependent on the time of exposure and the temperature of exposure.
The outer layer of a part is often case hardened to be pressure and impact resistant, while the inside of the part can remain softened and ductile. This combination of a soft core and hard shell allows for improved fatigue and high stress resistance and is especially useful for gears and bearings.
The depth of hardening can vary by changing process parametersPrecipitation hardening is a heat treatment process that takes place at low temperature and is used on specific metals that have certain alloying elements. These elements include copper, aluminum, phosphorus, and titanium. There are three phases of precipitation hardening:
Don’t roll your eyes, but annealing is like a wellness retreat for metals. Annealing is used to soften the metal, as well as to relieve stress and increase the ductility of the material. This process makes the metal significantly more machinable. On a microstructure level, the metal is more refined and therefore softened. You can anneal steel, copper, aluminum, and brass alloys.
To anneal a metal, the metal is heated slowly to a specific temperature (above the material’s critical temperature), held at that temperature, and finally cooled at a slow, set rate. This slow cooling process is accomplished by burying the metal in an insulating material or keeping it in the furnace while both the furnace and the metal cool.
There are three metallurgical phases or stages in the annealing process:
From a thermodynamics perspective, annealing actually speeds up the spontaneous process of internal stress relief (acts as a catalyst by adding heat) which is technically a reduction in the Gibbs free energy of metal. The cooling actually promotes the elimination of lattice vacancies within the metal to reduce stress by creating a more homogenous, robust crystal structure.
Tempering is a special process that is performed on hardened and quenched metals to relieve stress and soften them—similar to the annealing process. The tempering process is basically re-heating already hardened metals. Tempering typically occurs at lower temperatures than hardening and results in a phase transformation (i.e., bainite, martensite, and ferrite in steel alloys) via precipitation of intermetallic particles that result in strengthening and ductility. Aging is a specific type of tempering for aluminum alloys.
Precipitation hardened alloys must undergo a solutionizing process whereby the metal is heated to allow uniform distribution and dissolution of alloying elements and quenching at a fast rate to prevent these elements from falling out of solution. These precipitation hardened alloys are tempered at lower temperatures than the solutionizing temperature. The mechanism is similar to how tempering of steels works, but the specific mechanism is the precipitation of intermetallic compounds, which add strength to the metal by preventing the formation of dislocations within the crystalline structure. The amount and general size of these intermetallic compound precipitates is controlled by time and temperature during the tempering process.
Heat treating CNC machined parts is often necessary to achieve the physical properties needed for a given application. And while heat treating material before CNC machining can save overall production time, it adds cost to the machining process. Meanwhile, heat-treating parts after CNC machining makes it easier to machine the material, but adds an extra step to the production process.
Depending on your application, you’ll have to weigh the benefits of added hardness and tighter tolerances against the disadvantages of longer machining time associated with pre-heat-treated material. The good news is that Fictiv now offers hardening, case hardening, and tempering as finishing options for select CNC machined materials. Plus our experts can help you choose the right material for your next CNC project.
Create an account and upload a part to see what our CNC machining services can do for you today!
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