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Kyrie · 05-22-2019, 07:31 AM

Quick recap:

The above description of surface carbonization pertains to the heat treatment of low carbon steel parts that require a very hard surface to resist abrasion, but also require a fairly soft core that allows the part to take repeated shocks without failing by cracking, splitting, shattering, etc. Examples of such parts in Lugers include the frame, barrel extension, and the toggle train parts. In Mauser C96s, example parts are the frame, barrel/barrel extension, bolt, and hammer. In Mauser Model 98 rifles, example parts include the receiver and bolt.

In terms of those parts that show straw or fire blue color, these are (relatively) small parts that also require a hardened surface to resist abrasion. But these parts have characteristics that make surface carbonization a poor choice to attain that hard surface. Examples of such parts include safety levers, locking blocks (in Lugers), bolt stops (in C96s), tangent rear sight adjustment bars, triggers, extractors, and so on.

The major material characteristic that make surface carbonization a poor hardening choice is the relatively small size of the parts, which means the part isn’t big enough to retain a soft core after surface carbonization (meaning the entire part become high-carbon steel). Additionally, these parts don’t usually experience the repeated mechanical shocks that make a soft core desirable.

The net result of all this is these parts are not made hard by making them, in part or whole, high carbon steel. Rather hardness is imparted to them by changing the internal crystalline structure of the steel of which they are composed.

Caveat: this is a subject that can get fairly complex fairly quickly, so I’m going to try to keep it simple at the expense of detail.

All steel, including low carbon steel, takes the form of steel crystals called "grains" or crystallites. Generally speaking, the smaller these crystals are the shorter the boundaries between crystals, and the harder (and more brittle) the steel will be. Conversely, the larger these crystals are the longer the boundaries between crystals, and the softer (and more ductal) the steel will be. The size of these grains in a piece of steel can be changed by heating the steel; the hotter the steel (top end around 1500F – 1600F) the smaller the grains.

If a piece of steel is heated sufficiently to change the size of the grains from large to small and then allowed to cool slowly the grains will revert to their original large size as the object cools. If the steel object is cooled suddenly (“quenched”) by dipping it in water (or other heat sink material) the small size of the grains is generally retained.

Our small gun parts composed of low carbon steel are heated to the 1500F – 1600F temp and then quenched. This leaves them extremely hard, but also too brittle to be useful. We want to moderate (i.e. “temper’) the brittleness while retaining most of the hardness, and this is done by re-heating the part to some specific temperature range. Our target temperature re-heat range is determined by how much hardness we want to trade off for how much reduction in brittleness.

The surface of piece of hardened steel will, when being tempered in an environment where oxygen is present, oxidize (essentially, rust). The color of this surface oxidation is largely determined by the temperature to which the piece of steel has been heated.

Light straw color appears at about 350F, moderate straw color at about 400F, and dark straw at about 440F. These are the tempering temperatures commonly used on pre-1938 Luger triggers, safeties, locking blocks, etc.

Dark blue color appears at about 500F. This is the tempering temperature for the leaf spring used to provide pressure on the Luger’s magazine release button, and on the extractor and safety of a Mauser C96.

These are the heat treatment colors (really, the heat treatment temper colors) found on Luger small parts, and the origin of those colors.

A final note, concerning the corrosion resistance properties of heat treatment colors. Heat colors do provide some small, and unintentional, protection from additional surface rust. This protection occurs because the colors themselves are a form of rust, and that rust locks up the surface iron atoms in a form of iron oxide, making those atoms unavailable for further oxidation. But, again, this is an incidental effect and not an intentional attempt to produce rust resistance. Ultimately, there are other, more effective and less labor intensive ways than heating and tempering to prevent surface rust (among those ways, and the most commonly used way, are the surface oxidation techniques we know as “bluing”).

05-22-2019, 07:31 AM	#15
Kyrie User Join Date: Jun 2002 Posts: 757 Thanks: 0 Thanked 212 Times in 101 Posts	Quick recap: The above description of surface carbonization pertains to the heat treatment of low carbon steel parts that require a very hard surface to resist abrasion, but also require a fairly soft core that allows the part to take repeated shocks without failing by cracking, splitting, shattering, etc. Examples of such parts in Lugers include the frame, barrel extension, and the toggle train parts. In Mauser C96s, example parts are the frame, barrel/barrel extension, bolt, and hammer. In Mauser Model 98 rifles, example parts include the receiver and bolt. In terms of those parts that show straw or fire blue color, these are (relatively) small parts that also require a hardened surface to resist abrasion. But these parts have characteristics that make surface carbonization a poor choice to attain that hard surface. Examples of such parts include safety levers, locking blocks (in Lugers), bolt stops (in C96s), tangent rear sight adjustment bars, triggers, extractors, and so on. The major material characteristic that make surface carbonization a poor hardening choice is the relatively small size of the parts, which means the part isn’t big enough to retain a soft core after surface carbonization (meaning the entire part become high-carbon steel). Additionally, these parts don’t usually experience the repeated mechanical shocks that make a soft core desirable. The net result of all this is these parts are not made hard by making them, in part or whole, high carbon steel. Rather hardness is imparted to them by changing the internal crystalline structure of the steel of which they are composed. Caveat: this is a subject that can get fairly complex fairly quickly, so I’m going to try to keep it simple at the expense of detail. All steel, including low carbon steel, takes the form of steel crystals called "grains" or crystallites. Generally speaking, the smaller these crystals are the shorter the boundaries between crystals, and the harder (and more brittle) the steel will be. Conversely, the larger these crystals are the longer the boundaries between crystals, and the softer (and more ductal) the steel will be. The size of these grains in a piece of steel can be changed by heating the steel; the hotter the steel (top end around 1500F – 1600F) the smaller the grains. If a piece of steel is heated sufficiently to change the size of the grains from large to small and then allowed to cool slowly the grains will revert to their original large size as the object cools. If the steel object is cooled suddenly (“quenched”) by dipping it in water (or other heat sink material) the small size of the grains is generally retained. Our small gun parts composed of low carbon steel are heated to the 1500F – 1600F temp and then quenched. This leaves them extremely hard, but also too brittle to be useful. We want to moderate (i.e. “temper’) the brittleness while retaining most of the hardness, and this is done by re-heating the part to some specific temperature range. Our target temperature re-heat range is determined by how much hardness we want to trade off for how much reduction in brittleness. The surface of piece of hardened steel will, when being tempered in an environment where oxygen is present, oxidize (essentially, rust). The color of this surface oxidation is largely determined by the temperature to which the piece of steel has been heated. Light straw color appears at about 350F, moderate straw color at about 400F, and dark straw at about 440F. These are the tempering temperatures commonly used on pre-1938 Luger triggers, safeties, locking blocks, etc. Dark blue color appears at about 500F. This is the tempering temperature for the leaf spring used to provide pressure on the Luger’s magazine release button, and on the extractor and safety of a Mauser C96. These are the heat treatment colors (really, the heat treatment temper colors) found on Luger small parts, and the origin of those colors. A final note, concerning the corrosion resistance properties of heat treatment colors. Heat colors do provide some small, and unintentional, protection from additional surface rust. This protection occurs because the colors themselves are a form of rust, and that rust locks up the surface iron atoms in a form of iron oxide, making those atoms unavailable for further oxidation. But, again, this is an incidental effect and not an intentional attempt to produce rust resistance. Ultimately, there are other, more effective and less labor intensive ways than heating and tempering to prevent surface rust (among those ways, and the most commonly used way, are the surface oxidation techniques we know as “bluing”).