Low‐temperature annealing and graphitizing of white‐solidified low‐alloy cast irons

Kante, Stefan; Leineweber, Andreas; Holst, A.; Buchwalder, Anja

doi:10.1002/mawe.201800169

Cited by 10 publications

(7 citation statements)

References 38 publications

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“…The melt‐spun Fe–3.5C and Fe–3.5C–3Si ribbons contain similar microstructure features as their mold‐cast equivalents. The size of the microstructure features is notably refined due to rapid solidification (see Kante et al [ 21 ] for details). In Fe–3.5C–3Si, fine‐grained and coarse‐grained regions can be distinguished, which will be relevant to nitriding.…”

Section: Stability Of ε In Nitrided White‐solidified Fe–35c–15/3si Al...mentioning

confidence: 99%

“…[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. In addition, a minor volume fraction of Fe 23 Si 5 C 4 -type silicocarbide (Table 1) is contained in the alloys.…”

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

2021

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Gray-solidified cast irons are frequently used to manufacture machine and engine parts due to their low cost, good compressive strength, and high damping capacity. [1,2] Their use is, however, often limited by poor surface hardness and corrosion resistance. Both can be notably improved by a duplex surface treatment developed in recent years. [3][4][5][6] First, the surface of the cast iron is remelted, e.g., by an electron beam or plasma arc. This produces a white-solidified, ledeburitic [7,8] surface layer, which is hard and abrasion-resistant. Afterward, the material is nitrided to enhance the corrosion resistance by the formation of a dense surface layer composed of the main Fe (carbo)nitrides, γ 0 -Fe 4 (N,C) and ε-Fe 3 (N,C) 1þx (Table 1), denoted compound layer. Note that nitriding without remelting leads to less favorable microstructures because coarse graphite particles contained in gray-solidified cast irons are detrimental to the compound layer formation. [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. [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. In addition, a minor volume fraction of Fe 23 Si 5 C 4 -type silicocarbide (Table 1) is contained in the alloys. [22][23][24] The distribution of Si in the white-solidified microstructure is very heterogeneous. The Si/Fe atomic ratio in the eutectic and pearlitic θ is u Si,θ < 3 Â 10 À4 and, here, considered negligible, say u Si,θ ! 0. [12] The Si/Fe atomic ratio in Fe 23 Si 5 C 4 is u Si,Fe 23 Si 5 C 4 % 0.22; however, it might take also values slightly different from 0.22 due to a possibly mixed Fe/Si site [23,24] and extended lattice defects affecting the composition. [22] The Si content in α-Fe relates to the Si content of γ-Fe, which is affected by Si segregation during solidification. The Si content in the center of γ-Fe dendrites is typically similar to the Si content of the alloy, while the outer rim of dendrites and last-to-freeze regions are enriched in Si. [25] The Si content in eutectic γ-Fe is higher

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Section: Stability Of ε In Nitrided White‐solidified Fe–35c–15/3si Al...mentioning

confidence: 99%

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confidence: 99%

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

2021

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“…550 °C, there were only changes in the microstructure of pearlite, i. e., the pearlitic cementite became coarser. Coarsening is slower the higher the silicon content in the material [28]. This results from the accumulation of silicon at the cementite interface and is caused by the quasi insolubility of silicon in the cementite and the limited self‐diffusion of the silicon at low annealing temperatures [29].…”

Section: Resultsmentioning

confidence: 99%

Investigations on the thermal stability of microstructures generated by different thermal electron beam surface treatments

Buchwalder

Thronicke

Holst

et al. 2021

Materialwissenschaft Werkst

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Surface treatments are frequently used to improve the wear and/or corrosion resistance of metal components. In the case of cast iron, the material‐specific graphite limits both its treat‐ability and load‐bearing behaviour. A promising option for overcoming these limitations is provided by combination processes, in which near‐surface graphite is first removed in an initial liquid‐phase surface treatment – such as, e. g., remelting, alloying or cladding using electron beam (EB) – before application of thermochemical processes or hard coatings. A prerequisite for this is sufficient thermal resistance of these microstructures. This was investigated by means of annealing tests. The ranges of temperature used for annealing are based on those typically used for hard coating (250 °C–500 °C), nitriding (400 °C–600 °C) and boriding (600 °C–860 °C). The metastable microstructures produced as a result of rapid solidification during the electron beam liquid‐phase treatments differ in their alloy content and, therefore, in their microstructural components. Hardness measurements after annealing provided an initial indication of thermal stability. Based on these measurements, interesting treatment conditions were analysed in more detail using scanning electron microscopy and x‐ray diffraction. The focus of interest was on the formation of secondary graphite and the dissolution of ledeburitic carbides and other intermetallic phases.

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“…Initial investigations were carried out under conventional boriding conditions at high temperatures. Given that the remelted layers were not thermally stable in this temperature range, the reference condition was thus characterised and further development of low‐temperature technology could be derived [27, 31]. Under identical treatment conditions, the chemical composition and the microstructure of the substrate had a decisive influence on the layer structure (phase composition) and the layer thickness after boriding, Table 1, Figure 1 (i.e., surface layer).…”

Section: Resultsmentioning

confidence: 99%

Influence of different electron beam surface pre‐treatments on the results of subsequent boriding‐part I

Buchwalder

Thronicke

Jung

et al. 2021

Materialwissenschaft Werkst

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Boriding produce thick hard layers on cast iron components, which can improve their wear and corrosion behaviour. However, this potential cannot be fully exploited by a simple boriding due to the material specific presence of graphite. In that context, this paper presents results of two fundamentally different electron beam liquid surface treatments (remelting, cladding with nickel‐based additive) and their possibilities and limitations regarding subsequent boriding. The boriding behaviour under conventional high temperatures (760 °C–860 °C), and experiments on low‐temperature boriding (600 °C–700 °C) were investigated. Under identical treatment conditions, the compound layer thicknesses generated on the unalloyed surfaces (remelting) were approx. 50 %–75 % greater than those of the alloyed surfaces (cladding). A two‐layered boride layer structure were generated, though with different phase compositions. Nevertheless, the hardness of all borided layers were comparable. Surface hardness measurements revealed that the supporting effect of substrates plays a decisive role up to a boride layer thickness of approx. 57 μm. In this layer‐thickness range, the compound hardness of the alloyed substrates is higher than that of the unalloyed substrates. This knowledge should prove decisive for the selection of layer composites for corrosive and/or tribologically stressed components.

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Low‐temperature annealing and graphitizing of white‐solidified low‐alloy cast irons

Cited by 10 publications

References 38 publications

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

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

Investigations on the thermal stability of microstructures generated by different thermal electron beam surface treatments

Influence of different electron beam surface pre‐treatments on the results of subsequent boriding‐part I

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