Tribological Stress Behaviour of Cast Iron without and with Surface Treatment under Concentrated Contact Load*

Abstract

The graphite inclusions typical of grey solidified cast iron materials reduce the load-bearing capacity under locally concentrated pressure and simultaneous sliding stress. Surface treatment processes such as nitriding and electron beam remelting are known to improve the local stress behaviour. In this paper, the effects of the above-mentioned individual processes and their combination on the tribological stress behaviour of ferritic and pearlitic cast irons with different graphite morphologies are discussed. The results obtained in the model wear test ball-plate show that the specific wear coefficient of the investigated cast irons with different graphite morphology can already be reduced by at least one order of magnitude by an approx. 0.5–0.9 mm thick remelted surface layer with a surface hardness of 650–750 HV1. This treatment eliminates the graphite and produces ledeburitic carbides instead. The potential of an additional nitriding treatment depends on the parameters used, i. e. the nitrided layer thickness produced as well as the phase composition and the pore fraction of the compound layer. Based on stress calculations, the experimentally determined main influences such as the coefficient of friction, the pore fraction in the compound layer and the magnitude of the Hertzian pressure on the contact stress could essentially be confirmed.

Nitriding of White‐Solidified Fe–C–Si Alloys: Diffusion Path Concept Applied to Inhomogeneous Microstructures

“…The model of the microstructure formation during nitriding of white‐solidified Fe–3.5 wt% C–1.5/3 wt% Si alloys developed in this study significantly contributes to the understanding of the nitriding behavior of the white‐solidified surface layer produced by remelting of commercially available ferritic cast irons, laying the basis for thermodynamics‐based tailoring of the nitriding treatment. In view of the heterogeneous distribution of γ′ and ε in the compound layers obtained under γ′ conditions, nitriding under process conditions already allowing for the formation of ε when nitriding pure α‐Fe may be advised to produce macroscopically homogeneous compound layers that are γ′‐free and only contain ε (and X) (cf., results in Holst et al [ 17 ] ). Also, combined nitriding treatments such as the here‐used two‐step nitriding might be expedient to tailor the Fe nitride composition and microstructure of the compound layer forming in remelted cast irons.…”

“…In addition, Fe nitride has grown on top of the original surface of Fe-3.5C-1.5/3Si (Figure 1d). The growth of such surface nitrides is known from nitriding of white-solidified cast irons [12,17] and other alloys. [62][63][64] It is likely driven by stress relief [65,66] and requires outward diffusion of Fe atoms.…”

“…Note that, in contrast to ferritic cast irons, pearlitic cast irons often contain non‐negligible additions of Mn and Cu, which may additionally affect the nitriding behavior. [ 17 ]…”

“…Note that, in contrast to ferritic cast irons, pearlitic cast irons often contain non-negligible additions of Mn and Cu, which may additionally affect the nitriding behavior. [17] The microstructure of white-solidified Fe-3.5 wt% C-1.5/3 wt% Si alloys, having C and Si contents typical of cast irons, is mainly composed of coarse eutectic cementite plates, θ-Fe 3 C 1Àz (Table 1), embedded in ferrite and pearlite. [20,21] The latter result from the decomposition of primary and eutectic γ-Fe during cooling after solidification.…”

“…[3,4,[9][10][11] Although the general advantages of the previously described duplex treatment have been demonstrated, the nitriding behavior of the white-solidified surface layers turns out to be complex and incompletely understood. [12][13][14][15][16][17][18][19] The research of the current authors [12,18,19] aims at elucidating the complex nitriding behavior of the remelted surface layers by controlled gas nitriding of white-solidified Fe-3.5 wt% C-0/1.5/3 wt% Si alloys, serving as model alloys for commercially available ferritic cast iron grades subjected to remelting and nitriding. Note that, in contrast to ferritic cast irons, pearlitic cast irons often contain non-negligible additions of Mn and Cu, which may additionally affect the nitriding behavior.…”