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It is fairly amazing how much controversy arises with respect to cartridge annealing. Shooters are all over the map. Some anneal after every firing. Some never anneal. Some say annealing brings greater accuracy, others say that its impact on accuracy is negligible at best. The conflicting opinions go on and on.
This debate amazes me because, after all, is this not physics we're talking about? Physics is a science, meaning it should be provable and repeatable. I'd rather we understand the underlying science, not just hear endless opinions. All those opinions are typically based on individual experience, as if that "proves" their point. Sorry to burst any bubbles, but the lack of scientific proof ranks their opinions right up there with astrology. So it is that, despite not being a physicist, chemist, or metallurgist, I got to wondering about the actual science behind brass and annealing, and although this is lengthy I'm sharing below some of what I found.
Most cartridges are made of brass. Brass has been with us since the 5th century BC and is an alloy of copper, zinc, and trace impurities. The term "alloy" simply means a combination of two or more metals. The percentages of copper and zinc in brass can vary. Consequently, when we speak of "brass," even brass cartridges, we may be referring to different things due to variabilities in manufacture. This means the word "brass" should be considered more of a generic than exact term.
However, almost all cartridge brass has a copper content above 63% in order to maintain "ductility." More often than not, cartridges are manufactured with C26000 brass, typically referred to as C260. C260 brass generally has 70% copper and 30% zinc (thus its common name, 70/30 Brass). But the percentages of copper and zinc in C260 brass can still vary slightly. The industry standard for C260 brass allows for a copper content range between 68.5% to 71.5%, with zinc being the remainder. C260 brass must also have less than 0.05% lead content as a trace element.
Ductility of brass is important. For a metal object, ductility refers to its ability to be drawn (i.e., stretched, or more technically "plastic deformation") without fracturing. As a ductile metal like brass gets drawn/stretched, it will get thinner. The alloy used with copper makes a difference to ductility. Brass, made of copper and zinc, is both stronger and more ductile than copper alone. For comparison, brass is also more ductile than bronze (an alloy of copper and tin), thus it is easier to fracture bronze compared to brass. It is this ductility inherent to brass cartridges which allow them to expand inside the chamber and form a gas-tight seal.
People often confuse the terms ductility and malleability. One of the reasons for that confusion is because both terms refer to brass' ability to be shaped without fracturing under stress. Stress refers to a measurable amount of force applied to an object. Ductility has to do with the "tensile stress" of brass. Tensile stress is brass' ability to endure forces which stretch it, elongate it, without fracturing. Malleability has to do with the "compressive stress" of brass, its ability to be forced into a smaller size, i.e, a shape which occupies less volume, again without rupturing. With enough force applied, brass will eventually rupture from tensile and compressive stresses.
In addition to its helpful tensile and compressive stress properties, along with its aesthetic, non-magnetic, and low friction attributes, and combined with its excellent flow properties and inherent corrosion resistance, brass is an ideal alloy for cartridges. Interestingly, copper and copper alloys like brass have antimicrobial properties, and are being studied as a deterrent to Covid-19 (https://www.asminternational.org/documents/10192/1630346/20_CopperCorona_Digital_First.pdf/).
Another property of brass we hear about is its "phase" characteristic. Brass is a "single phase" alloy, although it should be noted that brass becomes a two-phase alloy once the zinc content exceeds 35%, but that won't be a factor for C260 cartridge brass. The phase of a metal refers to its crystalline structure. All metals are made of crystals and have one of three crystalline structures. I won't get into that weed patch, but suffice it to say that a given metal can change from one crystalline structure to another when it undergoes stress or temperature impacts. C260 brass is single phase, meaning its crystalline structure does not fundamentally change. This is why C260 brass is sometimes referred to as "alpha" brass, meaning it only has one, alpha phase (as opposed to a second, beta phase).
This is not to say temperature and pressure do not impact C260 brass. Brass can obviously melt and change into a liquid state under high temperature, and then cool back into a solid state, but those physical changes are not altering the fundamental crystalline structure of the alloy. What does change are the crystalline "grains."
Make a mental picture of a window that has a few cracks running across it. Then picture that same window pane with dozens of cracks spread across it. The window is still intact, it still has the same rectangular shape, the only difference is the greater number of cracks coursing through it. We might say that the window with the greater number of cracks has a more grainy appearance than the window with fewer cracks.
All metals are made up of grains. Grains are individual crystalline structures and form when a metal cools from a molten state to a solid state. As the metal cools, those crystal grains butt up against neighboring crystal grains and metal hardness develops. When those crystalline structures are large, akin to the pane of glass with fewer cracks, the brass is softer. When those crystalline structures are finer akin to the glass pane with many more cracks, then the brass is harder.
The act of firing subjects the brass cartridge to an almost instantaneous change in both temperature and pressure. Those changes are tremendous. For example, the SAAMI pressure specification for a 6.5 PRC is 65,000 pounds per square inch at the same time the brass is subjected to an internal chamber temperature of approximately 5600 degrees Fahrenheit. Depending on the percentage of zinc in the alloy, the typical melting point for brass is between 1652 and 1724 degrees Fahrenheit. As you can see, for the briefest instant the brass cartridge inside the chamber will be in a molten state, thus resulting in increased hardening of the grain structure after each firing.
Annealing at a certain temperature for a set time more or less recovers the ductility (softness) of the brass after firing made it harder. In other words, annealing with thermal energy recovers the larger crystalline grain structure in the brass after firing made it harder with smaller grains; in effect, annealing is a recrystallization process. When brass is annealed at 570° F, microscopic changes begin to occur to the brass' grain structure, and at 660° F its hardness will become softer and larger crystalline grains will start to reform. Most reloaders consider 750° F to be the optimal temperature for annealing brass, but that needs further analysis. What we do know is that the annealing temperature for C260 brass should never exceed 1370° F; keep going above this temperature and you will begin to destroy the brass, at which point it must be thrown away as too dangerous to use.
Guess I'll leave it there, with perhaps more to come regarding the physics of annealing and practical impacts on accuracy. Hopefully the above information is correct, but since I'm not a scientist there may be inadvertent mistakes. If so, I hope NWFA members who better understand the science behind brass will correct any errors.
This debate amazes me because, after all, is this not physics we're talking about? Physics is a science, meaning it should be provable and repeatable. I'd rather we understand the underlying science, not just hear endless opinions. All those opinions are typically based on individual experience, as if that "proves" their point. Sorry to burst any bubbles, but the lack of scientific proof ranks their opinions right up there with astrology. So it is that, despite not being a physicist, chemist, or metallurgist, I got to wondering about the actual science behind brass and annealing, and although this is lengthy I'm sharing below some of what I found.
Most cartridges are made of brass. Brass has been with us since the 5th century BC and is an alloy of copper, zinc, and trace impurities. The term "alloy" simply means a combination of two or more metals. The percentages of copper and zinc in brass can vary. Consequently, when we speak of "brass," even brass cartridges, we may be referring to different things due to variabilities in manufacture. This means the word "brass" should be considered more of a generic than exact term.
However, almost all cartridge brass has a copper content above 63% in order to maintain "ductility." More often than not, cartridges are manufactured with C26000 brass, typically referred to as C260. C260 brass generally has 70% copper and 30% zinc (thus its common name, 70/30 Brass). But the percentages of copper and zinc in C260 brass can still vary slightly. The industry standard for C260 brass allows for a copper content range between 68.5% to 71.5%, with zinc being the remainder. C260 brass must also have less than 0.05% lead content as a trace element.
Ductility of brass is important. For a metal object, ductility refers to its ability to be drawn (i.e., stretched, or more technically "plastic deformation") without fracturing. As a ductile metal like brass gets drawn/stretched, it will get thinner. The alloy used with copper makes a difference to ductility. Brass, made of copper and zinc, is both stronger and more ductile than copper alone. For comparison, brass is also more ductile than bronze (an alloy of copper and tin), thus it is easier to fracture bronze compared to brass. It is this ductility inherent to brass cartridges which allow them to expand inside the chamber and form a gas-tight seal.
People often confuse the terms ductility and malleability. One of the reasons for that confusion is because both terms refer to brass' ability to be shaped without fracturing under stress. Stress refers to a measurable amount of force applied to an object. Ductility has to do with the "tensile stress" of brass. Tensile stress is brass' ability to endure forces which stretch it, elongate it, without fracturing. Malleability has to do with the "compressive stress" of brass, its ability to be forced into a smaller size, i.e, a shape which occupies less volume, again without rupturing. With enough force applied, brass will eventually rupture from tensile and compressive stresses.
In addition to its helpful tensile and compressive stress properties, along with its aesthetic, non-magnetic, and low friction attributes, and combined with its excellent flow properties and inherent corrosion resistance, brass is an ideal alloy for cartridges. Interestingly, copper and copper alloys like brass have antimicrobial properties, and are being studied as a deterrent to Covid-19 (https://www.asminternational.org/documents/10192/1630346/20_CopperCorona_Digital_First.pdf/).
Another property of brass we hear about is its "phase" characteristic. Brass is a "single phase" alloy, although it should be noted that brass becomes a two-phase alloy once the zinc content exceeds 35%, but that won't be a factor for C260 cartridge brass. The phase of a metal refers to its crystalline structure. All metals are made of crystals and have one of three crystalline structures. I won't get into that weed patch, but suffice it to say that a given metal can change from one crystalline structure to another when it undergoes stress or temperature impacts. C260 brass is single phase, meaning its crystalline structure does not fundamentally change. This is why C260 brass is sometimes referred to as "alpha" brass, meaning it only has one, alpha phase (as opposed to a second, beta phase).
This is not to say temperature and pressure do not impact C260 brass. Brass can obviously melt and change into a liquid state under high temperature, and then cool back into a solid state, but those physical changes are not altering the fundamental crystalline structure of the alloy. What does change are the crystalline "grains."
Make a mental picture of a window that has a few cracks running across it. Then picture that same window pane with dozens of cracks spread across it. The window is still intact, it still has the same rectangular shape, the only difference is the greater number of cracks coursing through it. We might say that the window with the greater number of cracks has a more grainy appearance than the window with fewer cracks.
All metals are made up of grains. Grains are individual crystalline structures and form when a metal cools from a molten state to a solid state. As the metal cools, those crystal grains butt up against neighboring crystal grains and metal hardness develops. When those crystalline structures are large, akin to the pane of glass with fewer cracks, the brass is softer. When those crystalline structures are finer akin to the glass pane with many more cracks, then the brass is harder.
The act of firing subjects the brass cartridge to an almost instantaneous change in both temperature and pressure. Those changes are tremendous. For example, the SAAMI pressure specification for a 6.5 PRC is 65,000 pounds per square inch at the same time the brass is subjected to an internal chamber temperature of approximately 5600 degrees Fahrenheit. Depending on the percentage of zinc in the alloy, the typical melting point for brass is between 1652 and 1724 degrees Fahrenheit. As you can see, for the briefest instant the brass cartridge inside the chamber will be in a molten state, thus resulting in increased hardening of the grain structure after each firing.
Annealing at a certain temperature for a set time more or less recovers the ductility (softness) of the brass after firing made it harder. In other words, annealing with thermal energy recovers the larger crystalline grain structure in the brass after firing made it harder with smaller grains; in effect, annealing is a recrystallization process. When brass is annealed at 570° F, microscopic changes begin to occur to the brass' grain structure, and at 660° F its hardness will become softer and larger crystalline grains will start to reform. Most reloaders consider 750° F to be the optimal temperature for annealing brass, but that needs further analysis. What we do know is that the annealing temperature for C260 brass should never exceed 1370° F; keep going above this temperature and you will begin to destroy the brass, at which point it must be thrown away as too dangerous to use.
Guess I'll leave it there, with perhaps more to come regarding the physics of annealing and practical impacts on accuracy. Hopefully the above information is correct, but since I'm not a scientist there may be inadvertent mistakes. If so, I hope NWFA members who better understand the science behind brass will correct any errors.
) and it's hard to stay current if it's not your occupation, avocation, or hobby. So I thank you for your attempt to make sense out of this subject. For my use, right or wrong, I anneal shouldered cases but not straight walled cases.
