Nitriding effect on fatigue resistance of grey and nodular cast irons

Abstract

The influence of nitriding on the fatigue resistance of grey and nodular (ductile) cast iron piston rings was studied for six chemical compositions. The selected materials include three grey cast irons: one with a mixed initial microstructure (martensite plus bainite), the second martensitic and another martensitic alloyed with niobium; and three nodular cast irons: one common nodular cast iron, the second alloyed with Nb and another alloyed with Al and V. These materials were gas nitrided for 3 and 6 h. The relationship between the mechanical behaviour and the treatment time (depth of nitrided layer) was established. Samples were tested under a cyclical bending load. The stresses were determined in the cross-section, where the biggest moment was experienced. Results were statistically treated by means of Weibull analysis. Nitriding showed that fatigue resistance of grey cast iron with a martensitic structure is higher than that of a mixed structure. Fatigue resistance of nodular cast irons is reduced due to nitriding.

Effect of plasma nitriding on high‐cycle fatigue properties and fracture behaviors of GJS700 nodular cast iron under cyclic bending loading

“…It could be said that there were some run‐out samples with nitriding, which had longer fatigue lifetime, since nitriding had positive effects on defects in those cases. Penha et al 27 showed that in cast irons, there were several graphite nodules and defects, either inside the material or on the sample surface. After the nitriding process, surface defects may improve, but the metallurgical defects were still present inside the material.…”

“…In addition, the nitrided layer was inter‐metallic, and thus, the changed surface material and also the change in the elastic module on the sample surface should be considered in the analysis of the fatigue behavior. It could induce a change in the crack nucleation zone, from the surface to the interface of the material 27 . Morino et al 28 claimed that the fracture always started from micro‐shrinkages or abnormal graphites, which sizes were comparatively large.…”

“…One more element of the fatigue properties depreciation would be considered as the increased surface roughness made by the plasma nitriding process, which could enhance the probability of surface crack initiation. Furthermore, in nodular cast irons, there are significant phases of graphite nodules, whose size generally depreciated the positive effects of surface treatments on the fatigue properties of the studied material 27 . Therefore, since the maximum stress under bending loading exists on the surface of the specimen, this region is a possible potential zone for the crack initiation.…”

“…The main objective of this article is to examine the effect of nitriding on HCF properties of nodular cast irons. In this field of study, Penha et al 27 investigated the nitriding effect on the fatigue resistance of gray and nodular cast irons. The material was gas‐nitrided for 3 and 6 h, and they found that the fatigue resistance decreased by increasing the time of nitriding.…”

Effects of plasma nitriding process on the corrosion behavior of GJS700 nodular cast iron

“…However, for superior properties, some treatments such as nitriding or heat treatment on the surface of cast iron are usually performed. The plasma nitriding (PN) process is one of these treatments that can enhance cast iron properties such as fatigue lifetime, wear resistance, and surface hardness [4]. In addition, other surface treatments such as pulsed electron beam irradiation and N + ion modification during the PN process were suggested in other research [5,6].…”