The Effects of Nickel/Tin Ratio on Cu Induced Surface Hot Shortness in Fe

Abstract

Surface hot shortness is of concern in scrap-based electric arc furnace (EAF) steelmaking. The excessive amount of residual copper (Cu) in the steel scrap enriches during the oxidation of the solid steel, and as a result a Cu-rich layer could form which causes inter-granular grain boundary cracking. Other residuals can also influence hot-shortness. Ni is known to improve the resistance to hot shortness whereas Sn is known to worsen it. In this paper, the mechanism through which nickel (Ni) counters the detrimental effects of tin (Sn) on surface hot shortness are investigated in detail. A series of Fe-0.3 wt%Cu-x wt%Ni-0.03 wt%Sn alloys with x content ranging from 0.03 to 0.45 wt% was oxidized in air at 1,423 K (1,150°C) for 60, 300 and 600 seconds using thermogravimety (TG). The microstructure investigation under scanning electron microscopy (SEM) showed that, significant grain boundary cracking was suppressed by eliminating the highly embrittling Sn containing Cu-rich liquid, through increasing Cu solubility and/or solidifying the liquid phase by the presence of sufficient amount of Ni. Occlusion and decreased oxidation kinetics caused by Ni were shown not to play a significant role. A previous proposed diffusion numerical model was applied to the Fe-Cu-Ni-Sn system to quantitatively support experimental results and provide insight to the enrichment kinetics at the oxide/metal interface.

Strain aging behaviour of Cu-containing microalloyed low carbon seamless pipeline steel

“…It implies that copper is completely dissolved in iron at tempered temperature 700°C. It was reported 13,14 that the solubility limit of copper in steel increases with the addition of Ni to some extent which could contribute to this phenomenon. In addition, Cu precipitation will not occur during the tempering process due to the rapid cooling rate in air.…”

Synergistic effect of residual elements on oxidation rates and oxide/metal interface characteristics in a low-carbon steel oxidized at 1180°C for 3 hours

“…The Cu and Ni maps of the oxidized Steel 2 sample, Figure 7, show that a continuous layer of Ni and Cu-rich Fe phase (referred as Fe(CuNi) phase) was formed along the scale/metal interface, and some Fe(CuNi) phase was occluded into the scale. This continuous layer of Fe(CuNi) phase was not seen in the steel containing Cu and Ni after short-time oxidations, suggesting a higher degree of residual element enrichment after long-time oxidation [18,19]. Meanwhile, large Fe oxides appeared close to the scale interface.

Figure 7. BSE-SEM image and elemental maps of the oxidized Steel 2 (0.15Cu-0.15Ni-0.03Sn).

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“…The major problem the Cu-rich liquid phase can cause is hot shortness when the liquid phase penetrates the Fe grain boundaries and leads to surface cracking during the subsequent hot rolling [11]. It has been found that Sn lowers the solubility of Cu in the steel and lowers the solidus temperature of the Cu-Fe alloy, promoting the formation of Cu liquid phase; meanwhile, the presence of Ni can counteract the effect of Sn by expanding the Cu solubility range in Fe [12][13][14][15][16][17][18][19]. In Imai et al's work [12,13], a mild steel containing 0.3 wt-% Cu (in wt pct) was subjected to oxidation for 2 h at 1000-1300°C followed by tensile testing with a strain of 40% at the oxidation temperature.…”

“…The phase constituents and their fractions at the oxide/ metal interface depend on the degree of enrichment of the different residual elements, which is related to the bulk composition and the oxidation temperature and time [16][17][18][19]21,22]. A numerical model coupled with Thermo-Calc software was developed by Webler et al [17] to predict the concentrations of different elements in the Fe phase and Cu liquid phase at the interface as a result of selective oxidation of Fe.…”

Hot shortness mechanism and heat treatment of Cu containing carbon steel

“…Authors indicated that removing chlorined copper from steel is technically feasible [14]. Nickel alloying to steel increases both copper containing phases melting temperature and copper solution in austenite [15]. Increasing copper solubility in austenite prevents intergranular copper segregation and hot shortness.…”