Changes in the Bauschinger effect level post the autofrettage process

The favorable residual stress field generated by the autofrettage process is increasing the barrel's capacity to withstand pressure during firing - defined as the re-loading phase. There are two principal autofrettage processes: swage autofrettage and hydraulic autofrettage. Both processes include two successive stages of 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 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 level of the secondary Bauschinger effect can be reduced by introducing a low temperature heat treatment (LTHT) immediately after the autofrettage process, thus increasing the barrel's safe maximum pressure (SMP). In the utmost case, the yield stress in tension may regain its initial value prior to the autofrettage process. The aim of the present work is to quantitatively analyze the effect of different levels of the yield stress in tension post the autofrettage process, on a gun barrel's SMP, resulting from a low temperature heat treatment. The two autofrettage processes, the swage and the hydraulic, are numerically analyzed using the newly developed 3-D computer code. For the hydraulic autofrettage case five different Bauschinger effect levels were analyzed while for the swage autofrettage case, only two limiting cases were considered. The numerical results are indicating that the thermal treatment might partially or fully eliminate the influence of the secondary Bauschinger effect thus, increasing the barrel's SMP. The swage autofrettage SMP, which is higher than the hydraulic one, is less sensitive to the LTHT thus, increasing barrel's reliability.