The beneficial influence of bauschinger effect mitigation on the barrel's safe maximum pressure

The favorable residual stress field generated by the autofrettage process is increasing the barrel's capacity to withstand pressure during firing-defined as the reloading phase. There are two principal autofrettage processes: swage autofrettage and hydraulic autofrettage. While the theoretical solution for hydraulic autofrettage has been available and accessible for a long time, the available models for swage autofrettage have been quite limited. Both processes include two successive stages of pressure loading and unloading followed by an additional reloading during firing. Reyielding during the firing phase of an autofrettaged barrel is strongly affected by the secondary Bauschinger effect (BE) that involves a reduction of the yield stress in tension due to previous plastic deformation in compression, occurring in the unloading phase of the autofrettage process. The secondary Bauschinger effect can be completely mitigated by introducing a low temperature heat treatment (LTHT) immediately after the autofrettage process, thus increasing the barrel's safe maximum pressure (SMP). The aim of the present work is to quantitatively analyze the effect of low temperature heat treatment on the safe maximum pressure of a gun barrel. Two extreme cases are considered: In the first case, it is assumed that low temperature heat treatment was applied to the barrel, and that it completely mitigated the secondary Bauschinger effect, while in the second case it is assumed that no low temperature heat treatment was applied to the barrel. Both the swage and the hydraulic autofrettage processes are numerically analyzed using a newly developed 3D computer code. The numerical results confirm that a low temperature heat treatment, which fully eliminates the influence of the secondary Bauschinger effect, increases the barrel's safe maximum pressure especially in the case of hydraulic autofrettage.