Mechanism of Layer Formation during Gas Nitriding of Remelted Ledeburitic Surface Layers on Unalloyed Cast Irons

Abstract

Unalloyed cast iron materials exhibit low tribological and corrosive resistance. In this respect, nitriding has a wide range of applications for steels. In the case of cast iron, the advantageous properties of nitrided layers are impaired by the presence of graphite. Electron beam remelting of cast iron surfaces prior to nitriding removes graphite. The homogeneous ledeburitic microstructure within the approx. 1 mm-thick remelted layer enables the formation of a dense compound layer during subsequent nitriding. The main objective of this study is to investigate the nitriding mechanism of unalloyed ledeburitic microstructures. Due to the complex relationships, investigations were carried out on both conventional ferritic and pearlitic cast irons and Fe-based model alloys containing one to four additional alloying elements, i.e., C, Si, Mn and Cu. The iron (carbo-)nitride composition (γ’, ε) of this compound layer depends on the gas nitriding conditions, the chemical composition of the substrates and the microstructural constituents. As a result, a schematic model of the nitriding mechanism is developed that includes the effects of the nitriding parameters and alloy composition on the phase composition of the nitriding layer. These findings enable targeted parameter selection and a further optimization of both the process and the properties.

Effect of a surface treatment consisting of electron beam remelting and gas nitriding on the corrosion resistance of cast iron

“…Depending on the nitriding potential, γ' grew exclusively on the material surface during nitriding with K N = 1 atm −1/2 (KN1) and ε grew exclusively during nitriding with K N = 4 atm −1/2 (KN4). [21] This observation was consistent with the Lehrer diagram [34] valid for pure iron, which specified the formation of the observed iron nitrides from pure iron at the selected nitriding temperature of 540°C and the nitriding potentials mentioned.…”

“…[28,29] The SC formed along the eutectic cementite plates and also occurred in a duplex structure consisting of alternating cementite and SC lamellae. [21,30] The alloying elements contained in the cast iron (see Table 1) were not homogeneously distributed in the white-solidified microstructure. In accordance with the trends implied by the literature, [31][32][33] the EDS examination revealed that the eutectic cementite could be considered to be essentially Si-and Cu-free.…”

“…These elements were enriched in the ICRs, namely in the ferrite and the ferrite fraction of the pearlite, respectively. [21,22] The eutectoid cementite in the pearlite, like the eutectic cementite, is not expected to contain Si and Cu. Due to the fine lamellar structure of the eutectoid cementite of the pearlite, however, the elemental composition could not be determined by the EDS measurements.…”

“…The precipitation limit of silicon nitrides visible in the SEM using BSE contrast was used as a criterion to determine the diffusion-layer thickness of the remelted and nitrided surface layers. [21,27] The standard deviations calculated from the individual values were indicated with ± in the tables containing the mean value.…”

“…[18][19][20] Previous studies have shown that it is not only the high silicon (Si) content of the cast iron that has an influence on the phase composition of the compound layer, but also the alloying elements-such as manganese (Mn) and Cu-that are added in varying amounts to achieve the desired matrix microstructure. [21,22] In this article, the effect of Cu on the phase composition of the compound layer and the corrosion resistance of combination-treated surface layers was investigated on a ferritic, low-Cu cast iron and a pearlitic cast iron containing 0.73 wt% Cu. The comparative potentiodynamic polarization measurements were performed in 5% NaCl solution on the untreated, remelted, gas-nitrided only, and combination-treated cast irons, respectively.…”