Evaluation of Tensile Strength of Heat Treated AISI 304 Stainless Steel with Automated

Ball Indentation (ABI) Technique
A. U. Iwuoha
Department of Mechanical Engineering
Imo State University, Owerri. Imo State. Nigeria.
Email: nmatoha@yahoo.com.
ABSTRACT: This paper evaluated the tensile strength and hardness of AISI 304 stainless
steel heat treated to temperatures of 400oC, 600oC and 800oC and soaking durations of 30
minutes, 2 hours, 12 hours, 24 hours and 72 hours in each temperature level. The tensile
strength was measured using Automated Ball Indentation (ABI) technique while the hardness
was on the Brinell scale. To establish confidence in the ABI technique, values obtained were
compared with similar studies conducted with the conventional tensile measurement tool. The
tensile and hardness properties of type 304 SS increased with treatment temperatures of
400oC and 600oC for all the holding durations up to 72 hours. The long stay was akin to work
hardening which increased the tensile strength. The cooling in hot sand bed may have
equally contributed to increased surface hardness measured. At temperature of 600oC,
precipitation of chromium carbides along grain boundaries coupled with a dense formation
of sigma phase and other phases like the chi combined to influence the tensile strength.
Temperature of 800oC and soaking duration beyond 30 minutes produced decreased tensile
and hardness properties in the steel. At this temperature, transformation of the steel grains by
re-crystallisation and relief of internal stresses occurred. The extended soaking duration of
72 hours led to increase of Cr diffusion to the surface.