The reduction of residual stresses in cold expanded thick-walled cylinders by plastic compression

Thick-walled cylinders (D/d≥2) constitute a large group of parts with precise small diameter holes (d=1…10 mm). Improvement of surface finish and accuracy of small diameter holes is an actual task and requires the development of new methods of processing and cold expansion is one of the most effective methods of finishing and hardening of holes in such parts. Along with high productivity, cold expansion helps to increase accuracy, improve surface roughness, considerably work-harden surface layer and generate favorable compressive residual stresses. However, residual stresses generated during cold expansion of parts such as thick-walled cylinders can be undesirably high in some cases. We suppose that in order to maintain high accuracy of holes and to lower residual stresses after cold expansion of thick-walled cylinders, which undergo throughout plastic deformations, it is needed to perform axial plastic compression with subsequent cold expansion with small interferences. To test the hypothesis, we studied accuracy of holes as well as hoop, radial and axial residual stresses in cylinders made of steel grade 50 (0.5% C, НВ 2170…2290 MPa) with hole diameter d=5 mm, outer diameter D=15 and length L=30 mm by Sachs method. It is found that double-cycle cold expansion with total interference аΣ/d=5.1% generates hoop residual stresses with largest absolute value equal to 284 MPa, which, after plastic compression with strain ∆L/L equal to 0.5% and 1% and single-cycle cold expansion with interference a/d=0.9%, changed to 177 MPa. It is shown that high hole accuracy (IT7) achieved through double-cycle expansion remained at the same high level after plastic compression and single-cycle expansion.

thick-walled cylinders, cold expansion, plastic compression, residual stresses