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Probably going to bury us with data he is digging up!!!
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Maybe not bury, but there is a lot of info out there that tends to support bart's position... when applied to smaller capacity cartridges. The highpower community has published reams of material on the topics at hand. The guys at Hart have some definate opinions as well. My personal belief tends to lean more towards Kirby's experences, especially in low expansion ratio cartridges, but definately counter to Kirby's in terms of the abrasive nature of extruded propellants. Lots of published material on this as well.
Anyway, for those interested...
HEAT CHECKING IN RIFLE BORES
By Fred Barker
copyright 2005 Precision Shooting Magazine
The surface cracking that we rifle shooters have in the throat areas of our barrels—a major part of "barrel erosion"—also is important in the metal manufacturing industry. The casting of aluminum and magnesium objects from the molten metal, forging, extruding and cutting red-hot metal, and similar operations has led metallurgists to study the behavior of steels at high temperatures and to develop the "hot-work" tool steels for such work. The qualities needed for hot-work tool steels include resistance to deformation and wear at high temperatures and to thermal shock. Another quality, especially desired for the dies used to cast aluminum, magnesium and some other metals at temperatures in the 1,000 o to1,300 o -plus F range, is resistance to surface cracking or "heat checking". Heat checking is the term metals experts use to describe the networks of fine, shallow cracks that develop in steels subject to repeated cycles of heating from room temperature to very hot and back to room temperature. The depth of such cracks is mostly proportional to their width. In such a thermal cycle heating causes the surface layer to expand (relative to the cooler, substrate metal) and be in compression; and in cooling—when the surface metal becomes cooler than the substrate—the surface layer is in tension. Such thermal stresses initiate cracking of that very thin surface layer. Heat checking in casting dies may progress to a degree where the surface finish of the castings becomes rough and a costly die has to be scrapped. For a technical discussion of hot-work tool steels see Roberts, Krauss and Kennedy's Tool Steels , 5 th Ed., ASM Int., 1998.
A semi-quantitative scale has been developed to rate the damage done to a tool or die surface by heat checking. This is the Uddeholm scale (see Figure 1), in which two aspects of a crack system—the width of the largest or leading cracks and the relative density or network of cracking—are each visually compared to the scale, assigned numerical values and then given a summary score of the two. Rifle shooters who have looked closely at the throats of moderately to much-shot .22-250, .25-06, .300 magnum or other high-intensity cartridges will look at the Uddeholm scale and see identical cracking patterns. Having established this similarity in heat checking between industrial hot-work tools and our rifle bores, can we learn anything from the metallurgists—both as to their hot-work steels and how they are used—that may help us rifle shooters get longer life from our barrels?
The hot-work steels that have been developed over many decades for industrial use for casting dies, and which resist heat checking better than any others, are the half-dozen or so H-series steels. These contain 0.3-0.4% carbon, 3-5.5% chromium, about 1% silicon (to prevent surface oxidation, for these steels when very hot are exposed to the air), and a fraction to a few percent each of the carbide-forming metals molybdenum, tungsten and vanadium. The H-series steels thus are chromium steels with relatively low carbon contents ( in comparison with other tool steels) and low alloy-metal contents. The physical qualities desired in a hot-work steel are an optimal combination of toughness (the ability to deform rather than fracture under stress) and hardness at high temperatures—which involves the common trade-off in steels that very tough ones are not hard and very hard ones are not tough; resistance to thermal shock and heat checking; and minimal distortion and scaling during heat treating. Resistance to heat checking involves two factors: 1) the absence of inclusions of sulfides or other grains where cracking may be initiated, and 2) relatively high heat conductivity so that high surface temperatures are quickly lowered. Production of H-series steels having very low sulfur contents and other "clean" chemistry thus involves vacuum degassing, vacuum-arc remelting or other special procedures. The heat treating of these steels also is extensive, ending with double or triple tempering at more than 1.000 o F to give both optimal toughness and the small, very hard grains of vanadium carbide that only form at such high temperatures. The working hardness of the H-series steels is in the range 38-55 Rockwell C.
Could rifle barrels be made of an H-series steel, precisely and at reasonable cost? Probably not. These steels are at less than 20 Rockwell C in the as-rolled condition and could be worked then, but scaling and dimensional changes during heat treating would preclude making precision barrels. Working H-series steel after heat treating at the minimal hardness of about 38 R C might be an option for some barrel makers. The costs of the steel and its heat treating also would be high. We probably won't see barrels of this steel.
Do our stainless #416 and chrome-moly #4130 or #4140 steels compare with the H-series hot-work steels? At their hardness range of about 20 to 32 R C they are tough, but, of course, softer. The sulfur content of #416 is a minimum of 0.15%, which gives about 1% by volume of inclusions of manganese sulfide (to give the free-machining behavior); and the chrome-moly steels have an upper specification of 0.04% sulfur, so they, too, have an appreciable content of crack-initiating inclusions. The chrome-moly steels have the advantage of heat conductivity about 70% greater than that of #416 steel, but this is partly offset by the latter having thermal expansion coefficients about 25% less. To my knowledge we don't have any quantitative results on the barrel life (and hence resistance to heat checking) of #416 steel versus that of chrome-moly steel. When #416 was first used in quantity in the 1960's and 1970's many shooters hoped for longer barrel life, but Parker Ackley and some other observers saw little difference. Heat checking in either steel starts relatively early. Recent #416 barrels of mine in 6.5-284 and .270 Win. show fine heat checking through a Hawkeye bore scope after only 400 rounds (but it does not affect accuracy). See Figure 2 for some other examples.
Making barrels of lower sulfur content should result in less heat checking. The low-sulfur equivalent of #416 stainless, #410, contains a maximum of 0.03% sulfur and thus only 20% or so as many sulfide inclusions as #416. Difficult machining of #410 in past years has prohibited some barrel makers from using it. However, the grade containing the 0.03% maximum sulfur is termed "enhanced-machining" by the steelmakers, and may be a possibility. Krieger Barrels (262-628-8558) now offers sport-weight barrels #410 stainless, and I'm interested in knowing if they will give less heat cracking and longer life than #416 or chrome-moly barrels. (Note: #416 barrel steel was discussed in PS , December 2001.)
Can any of the experience of industrial users of hot-work tool steels in casting and forming red-hot metals be applied to the use of our #416 and chrome-moly rifle barrels? Bore coatings may help. Some military rifles have been issued with chromium-plated bores. This prevents corrosion, but the plating in the throat does tend to flake off during prolonged firing; and applying chromium of an optimum thickness in a precision barrel is a serious problem. Other coatings may work. Small-scale testing of hot-work steels coated with titanium nitride/titanium carbide has shown a delayed onset of heat checking and 30-40% less checking after 500-5,000 cool-to-hot-to-cool thermal cycles. Ceramic coatings are now being tried for rifle barrels, but I can't cite any results.
Industrial practice commonly involves both pre-heating hot-work tools to reduce initial thermal shock, and keeping molds and other tools warm during down times. On the other hand, some casting dies are water-cooled to prevent softening (H-series steels start to soften at about 800 o F). It's a given, of course, that high-intensity (i.e., both high-temperature and high-pressure) rifle cartridges heat their barrels differently than the molten metal in a casting die: the powder burns at about 5,000 o -5,500 o F for about a millisecond, and the metal at the bore surface in the throat region is heated about that hot only to a very shallow depth—perhaps a ten-thousandth at most. There also is a complication that the surface of the bore's throat is thinly nitrided by the nitrogen in the powder gas, giving a brittle coating that behaves differently than the underlying steel (see PS , October 1999, for a discussion of bore erosion and nitriding). The depth of nitride coating is a function of total number of rounds fired, duration of burn of the powder gas, temperature of the throat surface just prior to firing, the content of nitrogen in the powder gas (higher in double-base powders?), and perhaps other factors. The most important factor in heat checking, though, remains the temperature difference between the very thin skin of hot steel and iron nitride at the surface and the underlying steel substrate—i.e., the large temperature gradient. As mentioned above, this gradient causes the surface to expand (on firing) or contract (on cooling) relative to the substrate and to give the thermal stresses that lead to cracking.
So the following recommendations may be made to reduce heat checking in a bore, and thus extend barrel life:
1) in extended firing (e.g., highpower and long range competition and prairie dog shooting) keep the barrel warm; keep the number of cool-to-hot-to-cool cycles as low as possible; rack the rifle in the hot sun between relays;
2) if the outer surface of the barrel becomes too hot to touch, cool it down to where it can be handled—this to reduce the nitridation reaction on firing;
3) do not run any coolant down a hot bore: that would give thermal shock and induce cracking;
4) preheat the bore before firing: set the rifle in the sun, run boiling water down the bore, put a heated rod in the rear part of the barrel—anything to raise the temperature of the surface of the throat prior to firing and reduce thermal shock; and:
5) use a rifle configuration that maximizes heat flow from the barrel to the surrounding air: enlarge the barrel channel in a conventionally bedded rifle; use a bedding block set-up that exposes most of the barrel; set a M700 Remington barreled action in one of Sinclair International's ( PS advertizer) new aluminum F-Class stocks; perhaps flute and then stress-relieve (at 1,000 o -1,100 o F) thick match barrels; or other method.
Or a shooter may wish to use a smaller case: a .223 Rem. rather than a .22-250; the 6BR rather than a .243 Win., a .300 short Rem. or Win. rather than a big magnum; and so on. And by all means get a bore scope and see what's going on in your barrels.
