THE EFFECTS OF NITROGEN ION IMPLANTATION ON FATIGUE PROPERTIES OF PLAIN CARBON STEELS
Plain carbon steels are widely used for simple structural applications and machinery parts because they are cheap and readily available, but they perform poorly under fatigue when compared with alloy steels. Plasma ion immersion implantation (PIII) is a surface modification technique which has shown to improve the hardness, corrosion, fatigue, and wear properties of steel. Most of the research works featuring this technique used nitrogen as the implantation species however, the materials used were mostly of alloy steels. In the present study, AISI 1018 and AISI 1045 plain carbon steels were bombarded with nitrogen ions at a bias voltage of 5 kV and a working pressure of 15 mTorr for 1 h, 2 h, 3 h and 5 h. The effect of treatment conditions on implantation depth, hardness and phase evolution were analyzed using surface roughness measurements, microhardness and nanohardness tests, X-ray photoelectron spectroscopy and X-ray diffraction. The fatigue properties of nitrogen implanted steels were evaluated using a rotating bending fatigue test at 50 Hz, while fractured surfaces of failed samples were examined using scanning electron microscopy. The surface roughness of AISI 1018 and AISI 1045 specimens remained practically unchanged after PIII treatment. It was also found that AISI 1045 steel had higher concentration of implanted nitrogen than AISI 1018 at the near surface. However, the implanted nitrogen atoms were distributed over a smaller depth from the surface in AISI 1045 compared with AISI 1018 steel. Both grades of steel showed an increase in microhardness after nitrogen implantation for all treatment times compare to their untreated counterparts. However, after 2 h of PIII treatment the hardness values measured remained practically the same. Two distinct regions characterized the fatigue behavior of both grades of steel after nitrogen ion implantation, namely: the high cycle fracture domain (HCFD) and low cycle fracture domain (LCFD). The fatigue life of both steels was increased in the HCFD, while it decreased in the LCFD. This effect was more pronounced in AISI 1045 steel than AISI 1018 steel.
Fatigue, Nitrogen ion implantation, Plasma immersion ion implantation (PIII), high cycle fracture domain (HCFD), low cycle fracture domain (LCFD)
Master of Science (M.Sc.)