Effects of Work Hardening on 314 Stainless Steel Alloy

Abstract

Abstract314 stainless steel (314 SS) is an austenitic alloy of stainless steel which adheres to the linear-elastic model of material behavior. It is used in high-temperature environments, as well as several ballistic and aerospace applications. When 314 stainless steel is deformed past its elastic limit, its molecular structure is stressed in a way that strengthens the areas of the steel local to the deformed material. This process is called “work hardening,” or “cold working.” As materials are cold worked, properties such as ultimate tensile strength, yield stress, and hardness will change proportional to the degree of cold working. For the design of ballistics and aerospace structures, a working knowledge of these fundamental properties for 314 SS is vital. Despite this need little work has been reported on the material properties of work hardened 314 stainless steel in literature. The research conducted herein aims to characterize the behavior of 314 SS tubing which has been work hardened to various degrees.The stainless steel tubing chosen was of a 3/8 inch outer diameter and 1/8 inch outer diameter. This small diameter geometry was the best replication of the aforementioned application of this material in the aerospace industry. To work harden the tubing, the tubes were bent at 5° intervals, ranging from 5 to 15°, and then bent back to their original position. Some samples were further work hardened by repeated bending to the same degree interval. The effects of the cold work on the material properties were quantified using a tensile test, a microhardness test, and resonant ultrasound spectroscopy. The results of the experiment showed that the yield strength of the part increases with small angle bends such as 5 degrees and further increases with repeated bending. Bending beyond this 5° point appeared to produce a negative effect on the yield strength of the part. The change in yield strength between any set of trials was typically around 1–2%. The ultimate strength consistently decreased with increased work hardening, approaching 2–3% decreases with greater amounts of work hardening. The change due to repeated bending was on average less than 1%. The analysis of the failure strength was inconclusive, with the failure strength varying by as much as 19% from the control. It is likely that more trials would reveal a clear pattern, indicating what effect the work hardening has, if any, on the failure strength.KeywordsWork hardeningCold workingStrain hardening