Numerical investigation on melting characteristics of scrap with heat and mass transfers in molten steel

Herein,a numerical simulation with simultaneous heat and mass transfers is carried out to investigate the scrap melting characteristics in molten steel after model verification by published experimental data.The numerical results show that the scrap melting stages consist of the frozen shell formation stage,the frozen shell remelting stage and the parent scrap melting stage.The heat transfer coefficient and the carbon mass transfer coefficient between the scrap and the molten steel are,respectively,in the range of 4209-6249 W m^(-2) K^(-1) and 6.4×10^(-5) m s^(-1).Meanwhile,the effects of process parameters on scrap melting time were studied.An increase in the scrap preheating temperature(T_(scrap)),the molten steel temperature(T_(steel))and the carbon content of molten steel(C_(steel)),and a decrease in the scrap thickness dscrap,can reduce the frozen shell existence time,as well as the scrap melting time.On this basis,a quantitative relationship between the aforementioned process parameters and the scrap melting time is obtained to predict the formation of frozen shell(W),which provides process guidance for shortening the scrap melting time.The quantitative relationship is expressed as:lnΨ=311.32-2.34ln(T_(scrap))-39.99ln(T_(steel))-0.08ln(d_(scrap))-0.57ln(C_(steel)).

Effect of partial replacement of carbon by nitrogen on intergranular corrosion behavior of high nitrogen martensitic stainless steels

The microstructure evolution and intergranular corrosion(IGC) behavior of high nitrogen martensitic stainless steels(MSSs) by partial replacing C by N were investigated by using microscopy, X-ray diffraction, nitric acid tests and double-loop electrochemical potentiokinetic reactivation(DL-EPR) tests. The results show that the partial replacement of C by N first reduces and then increases the size and content of precipitates in high nitrogen MSSs, and converts the dominant precipitates from M23C6 to M2N,furthermore first improves and then deteriorates the IGC resistance. The high nitrogen MSS containing medium C and N contents provides good combination of mechanical properties and IGC resistance.

Effects of nitrogen content on pitting corrosion resistance of non-magnetic drill collar steel

High-nitrogen (N) austenitic stainless steel (Cr-Mn-N series) is commonly used for non-magnetic drill collars, which exhibits excellent mechanical properties and corrosion resistance. The effects of N content (0.63 to 0.86 wt.%) on the pitting corrosion behavior of the experimental non-magnetic drill collar steel were investigated using the electrochemical tests and immersion tests. Besides, X-ray photoelectron spectroscopy was used to analyze the constitution of the passive film. The results show that with the enhancement of N content from 0.63 to 0.86 wt.%, the metastable pitting corrosion sensitivity of the tested materials in 3.5 wt.% NaCl solution decreased and the pitting corrosion resistance increased. Meanwhile, the corrosion rate in 6 wt.% FeCl3 solution at 30 ℃ decreased from 10.40 to 4.93 mm/a. On the other hand, nitrogen was concentrated in the form of ammonia (NH4+ and NH3)on the outermost surface of the passive films. The contents of Cr2O3 and Fe2O3 raised in the passive films, together with the content of CrN, at metal/film interface increased as N content increased from 0.63 to 0.86 wt.%, which facilitated protective ability of the passive films, thus contributing to higher pitting corrosion resistance.

Precipitation behavior of P550 steel for non-magnetic drill collars during isothermal aging at 650–900℃

Precipitation behavior of P550 steel for non-magnetic drill collars was investigated by microstructure characterization as well as thermodynamic calculation.The results demonstrate that the main precipitate formed at 650–900℃was cellular Cr_(2)N,and its precipitation depended heavily on the aging temperature.The most sensitive precipitation temperature of cellular Cr_(2)N was 750℃.At 750℃,the cellular Cr_(2)N exhibited fast-slow precipitation kinetics with the aging time prolonging.The initial precipitation of cellular Cr_(2)N was governed by the short-range intergranular diffusion of Cr.During long-term aging,its growth was controlled by the long-range bulk diffusion of Cr.In addition,cellular Cr_(2)N induced the precipitation of r phase ahead of the cell after long period of aging.Increasing the nitrogen content resulted in the increment of both the nucleation site and the driving force for the cellular Cr_(2)N,which jointly promoted its precipitation.