Thermo-chemical erosion in gun barrels

The temperature and heat transfer per round was measured at the bore surface of a 155 mm AS90 extended range ordnance. The ammunition was fired with and without wear-reducing additive and the measurements were made using an eroding-type surface thermocouple having response time of about a microsecond. The heat transfer was computed from the measured temperature-time curves. It was found that the wear-reducing additive gradually reduced the surface temperature fluctuation from about 950°C to about 600°C, and reduced heat transfer per round from about 950 kJ/m 2 to about 600 kJ/m 2 , over a period of 50 rounds. From these measurements an assessment was made of the wear rate, the number of rounds to cook-off and the increase in barrel fatigue life. Similar experiments on 30 mm RARDEN and 40 mm Bofors guns, using different additives, resulted in comparable reductions in bore temperature and heat transfer, but the mechanism for these reductions was different and it is still not clear exactly how these additives reduce barrel temperature and heat transfer.

The Influence of Additives on the Temperature, Heat Transfer, Wear, Fatigue Life, and Self Ignition Characteristics of a 155 mm Gun

Heat transfer—A review of 1978 literature

A more realistic thermal shock analysis of a radially multicracked thick-walled cylinder

: A review is made of existing empirical models for predicting erosion rates of large caliber guns. All the empirical models are based on a correlation between calculated bore surface temperature and the measured wear rate. This prompted an investigation into correlating measured total heat input with the erosion rate. A correlation was obtained even for rounds with different wear- reducing additives. Such rounds have nearly identical interior ballistics but differ in the amount of heat transferred to the barrel and the erosion rate.

Prediction of Erosion from Heat Transfer Measurements

Reduction of Heat Transfer to High Velocity Gun Barrels by Wear-Reducing Additives

: The erosivity of a series of LOVA propellants was evaluated with the BRL 37-mm blowout gun and with heat input measured in a 105-mm M68 tank cannon. The results in the 37-mm blowout gun suggested that three of the eight propellants were as erosive as M30. Subsequent review of the closed bomb data used to calculate charge weights indicated that it is likely the extraordinary erosivity of these three propellants was caused by overestimating the propellant needed to match peak pressure of M30.

Erosivity of LOVA Propellants

Reduction of Heat Transfer to Gun Barrels by Wear-Reducing Additives

: Earlier analysis of gun tubes and steel nozzles exposed to propellant combustion gases showed that a tenacious oxide layer is left on the surface after firing. The thickness of the oxide layer was inversely proportional to the flame temperature of the propellant. Experiments were performed in a 37-mm blowout gun to determine whether the oxide layer influences wear. The experiments were performed with a nozzle that had been conditioned with M1, M30,M5, or M8 propellant. The results showed the oxide layer insulates the steel and reduces erosion. The wear with M8 propellant decreased as the flame temperature of the propellant used for the previous shot decreased. Conversely, the wear with M1 propellant increased as the flame temperature of the propellant from the previous shot increased. These results show the Effective Full Charge (EFC) factors determined from standard wear tests may not predict total wear when rounds with different propellants are fired in combination.

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Role of Surface Oxide on Gun Barrel Wear.

: Gun barrel wear tests have uncovered the need for a small scale device to test barrel treatments like 'thick' chromium. Small caliber barrels have traditionally served this role, but results with high-firing rates used to produce measurable wear are ambiguous. An ideal erosion test device would be a small caliber gun with standard projectiles, cartridge cases and propellants which would produce wear comparable to wear in large-caliber guns. Interior ballistic tests were done with M2 or M9 propellant in place of the standard ball powder in the M55A2TP-T round to determine charge masses which would not exceed the peak chamber pressure of the reference M55A2TP-T round. The wear produced with the high-flame temperature propellants was estimated with the Frankle-Kruse empirical formula. The results showed an M9 propellant could be loaded to the same charge mass as the reference WC870 ball powder with a lower peak chamber pressure but 100 m/s higher velocity. The wear estimated for such a round is 5 micrometer/shot, comparable to wear -limited Army guns, but less than the desired 25 micrometer/shot (1 mil/shot) for an erosion test device. Hence the 20-mm gun with M9 propellant could be useful for evaluating chromium-plating for existing guns, but it has too little wear for conveniently assessing future platings and coatings.

J. R. Ward

Papers

Reduction of Heat Transfer to High Velocity Gun Barrels by Wear-Reducing Additives

Erosivity of LOVA Propellants

Reduction of Heat Transfer to Gun Barrels by Wear-Reducing Additives

Role of Surface Oxide on Gun Barrel Wear.

Interior Ballistic Evaluation of High-Flame Temperature Propellants in 20mm Ammunition and Assessment as an Erosion Test Device