Effect of carbon on the grain boundary segregation of phosphorus in α-iron

Friction stir welding (FSW) of a high-P (0.3 wt%) weathering steel was performed at temperatures above A₃ and below A₁. Three-dimensional atom probe tomography analysis revealed that the grain boundaries in the stir zone formed below A₁ (which had a considerably finer grain size) had higher P and lower C concentrations than those of the grain boundaries in the stir zone formed above A₃. The effect of grain size on non-equilibrium grain boundary segregation during cooling after FSW was analyzed using the diffusion equation, assuming grain boundaries acted as sink sites for solutes. The fast diffusion of C is postulated to result in an almost equilibrium segregated state, such that its segregation decreases with decreasing grain size. In contrast, P diffuses only an extremely small distance, presumably leading to the grain-size-independent segregation of P. Therefore, the experimental result that the grain boundary concentration of P in the stir zone formed below A₁ was larger than that in the stir zone formed above A₃ is assumed to stem from the site competition effect between P and C and/or the enhanced P diffusion to grain boundaries due to dynamic recrystallization. Moreover, the segregation heat treatment was performed after the FSW to investigate the effect of the grain boundary segregation of P on the toughness. An investigation into the correlation between grain boundary segregation and toughness revealed that when the grain boundary concentration is less than approximately 7 at%, the effect of P segregation on intergranular fracture is relatively small and the toughness can be effectively improved by grain refinement.

First-principles calculations of the Σ5(310) grain boundary in Fe with B, C and P were performed to reveal the mechanism of P-caused embrittlement and de-embrittling effect of B and C. Independent and/or joint effect of B, C and P on the grain boundary energetics and cohesion were determined as a function of concentration. It is found that interstitial segregation sites are more favorable than substitutional sites for all the three elements, and only substitutional P aggravates the grain boundary cohesion, which explains the experimental observation that P only embrittles the grain boundary beyond a critical content. The energetic preference of interstitial B and C makes interstitial P at a disadvantage during the site competition, whereas the de-embrittling cannot be simply explained by the intrinsic strengthening effect of B and C. The influence of these elements on the grain boundary cohesion is further interpreted as a net result of mechanical contribution and chemical contribution, which proved to play the dominant role in the embrittling/strengthening effect of substitutional P and interstitial segregants, respectively. It turns out that replacing part of P atoms by B and C can mitigate the strong mechanical distortion, and thus alleviate the P-caused embrittlement. In the spirit of the Rice-Wang model, we propose a possible method to quantify the variation in chemical bonding upon fracture based on the concept of integral of crystal orbital Hamilton populations (ICOHP). A close relationship was found between the change in total ICOHP of bonds across the grain boundary and the calculated chemical contribution.

Boron segregation behavior during tempering at 600 and 790 °C was examined in a boron-doped 9Cr-steel by using high-resolution SIMS imaging (nano-SIMS). Boron segregation that developed at prior austenite grain boundaries (PAGBs) during austenitizing was found to decrease reduced during tempering because the equilibrium concentration of B segregation for austenite is larger than that for ferrite so that the segregated boron atoms diffused into block and packet boundaries. The size and the volume fraction of the carbide precipitates at PAGBs were smaller than those at packet boundaries, suggesting that boron segregation at PAGBs suppresses the carbide precipitation upon tempering.

This work aimed to investigate the segregation sequence of impurity elements in the weld metal of carbon steel with various solidification sequences, such as δ single-phase, hypo-peritectic, hyper-peritectic, and γ-single-phase solidification. Moreover, the relationship between the solidification sequence and toughness of the weld metal of carbon steel was studied. The solidification sequence was controlled by the nickel content of the base metals. Various amounts of sulfur and phosphorous were added to the base metals as impurity elements, and the segregation amount of the impurity elements in the weld metals increased in the order of δ-single-phase, hypo-peritectic, hyper-peritectic, and γ-single-phase. The amount of sulfur was higher than that of phosphorous in each solidification sequence. In δ-single-phase solidification, sulfur segregated during solidification and relaxed during cooling after solidification. In contrast, in γ-single-phase solidification, sulfur continued to segregate during and after solidification. The segregated sulfur was in the grain boundaries of the prior austenite at low temperatures. Charpy impact test results revealed that the ductile-brittle transition temperature (DBTT) increased, and the upper shelf energy decreased with an increase in the nickel content, specifically, only for the specimen of γ-single-phase solidification with S addition. In particular, the tendency in the specimens containing sulfur was more significant compared to the specimens containing phosphorous. Moreover, the intergranular fracture surface was only observed in the γ-single-phase solidification specimen, and sulfur was concentrated on the surface. Therefore, the superposition of segregation during and after solidification induced intergranular embrittlement and deterioration of toughness.

The present study aims at unveiling the influence of trace amounts of phosphorus on the macro-scale delamination in a ferritic steel. Two different steels with trace amounts of phosphorus were examined to reveal the cause: One was made by long-time high temperature holding to maximize the phosphorus segregation, and the other one was made by short-time high temperature holding to minimize the phosphorus segregation. The atom probe results at the grain boundaries ahead of the delamination-related cracks provide a strong evidence that phosphorus-enrichment and carbon-depletion for the long-time high temperature holding induces a larger degree of delamination as compared to the short-time high temperature holding. Reasons for the different segregation tendency were discussed by correlating the isothermal holding time at high temperature and the competitive segregation between phosphorus and carbon.

Segregation of boron at random austenite grain boundaries with known misorientation angle in a boron-added low carbon steel quenched from austenite state at different temperatures was investigated quantitatively using three-dimensional atom probe. Boron segregation is reduced at low angle boundaries or low quenching temperatures. The latter result indicates that non-equilibrium boron segregation plays an important role and is enhanced at higher quenching temperature due to more excess vacancies and longer diffusion distance during quenching from higher temperature.