Precipitation hardening is the main strengthening mechanism in Al-Cu alloys, which is largely driven by the interaction between dislocations and disc-shaped precipitates. The metastable theta' phase, a representative precipitate in these alloys, generates stress-free transformation strain (SFTS) arising from its lattice mismatch and shear transformation within the matrix, which affects dislocations motions. Dislocations are usually bypassed around precipitates via the Orowan looping mechanism instead of penetrating precipitates, leaving residual loops around the precipitates. These accumulated loops exert a back-stress on subsequent dislocations, contributing to hardening through a mechanism analogous to dislocation pile-up. While recent simulations indicate that SFTS significantly enhances the resistance to dislocation motion, the detailed quantitative relationship between the number of accumulated loops and the resulting time-dependent hardening behavior has not yet been fully clarified. Therefore, in this study, discrete dislocation dynamics (DD) simulations incorporating STFS were performed to investigate the influence of accumulated loop numbers on hardening behavior. Our DD model can reproduce the accumulation of Orowan loops around theta' precipitates. The forces acting on dislocations were calculated by incorporating the SFTS of the precipitates. We calculated the critical resolved shear stress (CRSS) increment due to precipitates using our DD model. We found that the CRSS increment is lowered by including the SFTS. This is attributed to the mechanism of the accumulation of Orowan loops; Orowan loops are accumulated far from the precipitates because of large stress around them derived from SFTS in materials with stress-free transformation effects. We also found that precipitate orientations and channel width between precipitates affect the CRSS increment, which can vary by up to a factor of about three.