EFFECT OF Ni ON FLUIDITY OF Cr-Mo STEELS

Transcription

1 Paper 12-3.pdf, Page 1 of 8 AFS Proceedings 212 American Foundry Society, Schaumburg, IL USA Copyright 212 American Foundry Society EFFECT OF Ni ON FLUIDITY OF Cr-Mo STEELS A. Dash, S.N. Lekakh, V.L. Richards, D.C. Van Aken Missouri University of Science and Technology, Rolla, Missouri ABSTRACT The effect of Ni on fluidity of Cr-Mo steels (.18% C,.15% Si,.7% Mn,.5% Cr,.15% Mo) was studied using several experimental techniques and CFD Fluent and Magmasoft (hereafter referenced as Software A) modeling. Computer assisted, single thermocouple thermal analysis showed that Ni decreased the liquidus, solidus and dendritic coherency (DC) temperatures, while Ni had little effect on the solid fraction at DC. Filling time was evaluated by measurement of electrical resistivity of a W-wire inserted into a spiral mold and experimental fill time data was used for modeling analysis of factors influencing spiral length fill. Measured fluidity lengths showed a monotonic increase with superheat for all steels and had a maximum value in steel alloyed with 5% Ni in comparison to un-alloyed, low alloyed by 3% Ni and high alloyed by 9% Ni steel. To understand the mechanism of fluidity increase in steel alloyed by Ni, the measured and calculated (FACTSAGE software [hereafter referenced as Software B]) physical properties of liquid and solidified Ni-alloyed Cr-Mo steel were applied in Software A simulations of spiral length and predicted results were compared to measured fluidity. Keywords: steel, nickel alloying, fluidity, modeling INTRODUCTION Fluidity is an important technological parameter in producing thin-walled steel castings. 1 Incomplete mold fill and the associated specific defect called cold shuts typically indicate poor fluidity. 1,2 Fluidity is often measured as a fill length of molten metal in a standard spiral mold at particular poured superheat above liquidus temperature. Initial work of performing fluidity tests of different iron alloys in a standard spiral mold was done by Taylor et al. 3 Subsequently, experimental work to relate fluidity to fundamental properties of the pure metals and mold materials was done by Ragone et al. 4 Measurement of fluidity of dilute alloys was conducted by Flemings et al. 5 It was concluded that fill length ceases when solidification chokes flow at or near the channel entrance due to the impingement of columnar grains with jagged solid-liquid interfaces. In contrast, flow of solute rich alloys is choked by precipitation of equiaxed grains at the leading tip of the flowing stream. Campbell 6 showed that during melt filling, the metal starts to freeze at the mold wall and a solid skin starts to form. Growing dendrites can become fragmented by the flowing stream and result in semi-liquid slurry that fills the mold. When the amount of solid phases exceeds a critical value, the dendrites bridge across or interlock and thereby stop the fluid flow. It was estimated that fluid flow would stop at.2-.5 solid fraction in the stream. The amount of solid phases to choke the flow depends on the morphology of the solid phases. Dendrites have complex shape and therefore can interlock easily as compared to columnar grains in dilute alloys or pure metals. Hence, flow would choke off at a lower volume fraction of solid phase in alloys with greater freezing ranges as compared to pure metals. Monroe 2 examined the general problems of mold filling in terms of nondimensional numbers of fluid flow and heat transfer. The first systematic study of the effect of alloying elements on steel fluidity was done by Taylor et al. 3 The effect of such important alloying elements in steels as C, Si, Mn, Al, Cu, Ni, Cr, Mo and V on the fluidity of plain carbon steels was evaluated. Fluidity of plain carbon steels increased as Ni was increased to 3.25%, and decreased on further increasing the Ni content. It was suggested that the changes in solidus temperature would govern the critical amount of solid phases formed to choke the flow. This conclusion contradicts the later published results, 5, 6 where filling of solute rich alloys in a narrow channel stops significantly before the solidus temperature when the first dendrites impinge on each other. The objective of this study was to experimentally study and model the effect of Ni on fluidity of high strength Cr-Mo steels used for critical application castings. PROCEDURES EXPERIMENTAL Five alloys with varying Ni content were produced using a 1 lb induction furnace. High purity induction iron, 75% Fe-Si, 6% Fe-Mo, electrolytic Mn and Cr and high purity carbon riser were used as charge materials. Weight percentage designated as (%) was used in this paper. An inert atmosphere cover was maintained during melting by constantly flowing Ar-gas over the melt. De-oxidation was done in the ladle by using.7-.9% Al followed by.5% Ca in the form of a CaSi-wire. The temperature readings were taken using Electro-Nite dip thermocouples. Chemical analysis (Table 1) was performed by using an arc spectrometer. The base steel

2 Paper 12-3.pdf, Page 2 of 8 AFS Proceedings 212 American Foundry Society, Schaumburg, IL USA Table 1. Chemistries of Experimental Steels (wt. %) Heat # C Si Mn P S Cr Mo Ni Al Ca Cu # <.1 < # <.1 < # <.1 < # <.1 < # <.1 < was from Heat 1 with no Ni addition. Heats 2, 3 and 4 had 3%, 5% and 9% Ni. Heat 5 was used to study the combined effect of 3.5% Cu and 9% Ni. Fluidity spirals had 1.1 cm 2 cross sectional area and a total length of 16 cm. Fluidity spirals were produced in silica sand using no-bake furan binder. An alcohol based zircon coating was applied to the mold surface to stabilize the quality. Prior to pouring the first spiral mold, approximately 1 lb of the melt from the ladle was poured into a pig mold to nullify the ladle lip chilling effect. After that, the spiral molds were sequentially poured with intermediate measuring of the temperature in the ladle. The filling time was evaluated by measurement of changing electrical resistivity on a.2 mm diameter W-wire placed inside fluidity spiral. When liquid steel filled the spiral, the measured electrical resistivity of the W-wire dropped proportionally. The system was calibrated before the test. Liquidus, solidus and dendrite coherency (DC) temperatures were studied using a computer assisted, single thermocouple, thermal analysis technique. 7, 8 Sokolowski 9 showed that DC could be identified as an extreme in the second derivative curve of the temperature. Briefly, thermal analysis was carried out by pouring the melt into cone-shaped, no-bake molds with an S-type thermocouple. 8 A heat balance model based on a zero line calculation was used for this analysis. The rate of latent heat liberation is proportional to the difference of the first derivatives from experimental temperature (dt/dt) and, the so called zero-line (dz/dt) during solidification as shown in Equation 1. dq = C dt dt p dz dt dt Equation 1 The zero line (Equation 2) was fit to the first and the second derivatives of the cooling curve before (coefficients A 1, A 2 ) and after solidification (coefficients A 3, A 4 ). Coefficients A 3 and A 4 accounted also for difference in specific heat capacity of the solid (C S ) and the liquid (C L ): Z = A 1 e A 2t + A 3 t t o 3 + A 4 t t o 4 Equation 2 MODELING Two computational fluid dynamic (CFD) programs were used for modeling liquid steel flow in a thin channel. The first one, Software C, was used for parametric analysis of factors influenced on length of filled straight channel. VOF (Volume of Fluid), Solidification and Transient Flow modules were coupled. The second technological software, Software A version 4.4, was used to predict fill lengths in a standard fluidity spiral having a 1.1 cm 2 semicircular cross section area (Fig.1). Fig.1. Fluidity spiral was developed for Software A modeling (pouring basin was attached to sprue for uninterrupted calculations). Initial calculations showed that the Software A algorithm correctly solves melt flow with solidification, but does not allow incomplete filling of the mold domain and artificially promotes complete filling of the mold. To avoid this computational restriction, an additional volume was attached to the down sprue. In this case, the melt solidified in the spiral before the pouring basin was completely filled and post-processing analysis was used to recognize the moment when melt stopped to flow in the spiral. Key parameters of modeling including pressure profile, heat transfer coefficient and melt viscosity were adjusted by inversed modeling to fit measured filling times and spiral lengths. Pressure at the bottom of the inlet was chosen to be 18 mbar based on actual pouring height evaluated from a video of the casting process. Values of heat transfer between the melt and the mold were varied from 3 to 1, W/m 2 K and the results will be discussed later. Steel solidification parameters, in particular, solid fraction versus temperature, were taken from thermal analysis. Viscosity of molten steel at temperatures between the liquidus and solidus were estimated from theoretical models. 1

3 Solid fraction Solid fraction T', ⁰C/s Paper 12-3.pdf, Page 3 of 8 AFS Proceedings 212 American Foundry Society, Schaumburg, IL USA f cr k μ = μ f 1 f f cr Equation 3 where μ is the apparent viscosity of the slurry, μ f is the viscosity of base liquid at the liquidus temperature, f is the solid fraction, f cr is the critical solid fraction and k is a crowding factor. The value of (f cr k) depends on the shape of solid particles and was chosen to be 2.5 in. these simulations. Software B version 6.2 was used to calculate the liquidus and solidus temperatures of Ni-alloyed steels. The FStel database was used for Fe-BCC, Fe-FCC and Liquid iron phases. RESULTS EXPERIMENTAL RESULTS An example of the thermal analysis, i.e. the cooling curve, first and second derivatives and calculated solid fraction versus solidification time and temperature, are shown in Fig. 2 for the 5% Ni (Heat #3). The changes in the slope of the first derivative (T ) were used to determine the critical points (solidus and liquidus temperatures) and a minimum on second derivative was used for estimating the dendrite coherency point. In this case, the solid fraction at the dendrite coherency point was approximately equal to.3 and the dendrite coherency temperature was close to 1492C (2718F). The variation of solid fraction formed during the solidification of steels with different Ni contents is shown in Fig. 3. It was seen that alloying with Ni significantly affected steel solidification kinetics. The spiral filling time was experimentally evaluated using the electrical resistivity method for the 5% Ni steel (Heat 3) at high (15C [32F]) superheat above liquidus temperature. The measured voltage drop and recalculated spiral length as a function of pouring time is shown in Fig. 4. The flow process could be divided into two phases. During the first phase (approximately.9 sec), the fill length was proportional to time with a melt velocity of.6 m/sec. In phase two, due to fluctuations at the tip of the melt, the melt velocity dropped to.26 m/sec. The total filling time was 1.36 sec at an average melt velocity of.48 m/sec. The results (fill length and time) from this experiment were used for parametric modeling of the fill length and time in a spiral mold using CFD software. The experimentally measured fill lengths in the spiral molds were plotted for all studied steels with respect to the pouring temperatures and super heat above the liquidus (Fig. 5). The measured fill length of all steels (Heat 1 to Heat 5) varied linearly with both pouring temperature and superheat. At the same time, it was seen that the data trend is nearly the same for each steel at superheats greater than 8C (176F). These results showed that alloying has more significant effect of steel fluidity at low superheat. Higher fill distances were obtained for the 5% Ni steel (Heat 3). This steel showed 35% longer spirals length than other steels at a superheat of 6C (14F). It was also seen that addition of 3.5% Cu to the 9% Ni steel (Heat 5) did not show any significant difference from the 9% Ni steel (Heat 4) T T' Time, s Time, s Solid fraction T" Solid fraction T" c) Fig. 2. Graphs show (the cooling curve and first derivative of temperature, ( solid fraction versus time and (c)solid fraction versus temperature for Heat 3 alloyed by 5% Ni T", ⁰C/s T", ⁰C/s 2

4 Voltage, V Spiral length, cm Spiral length, cm Solid fraction Spiral length, cm Paper 12-3.pdf, Page 4 of 8 AFS Proceedings 212 American Foundry Society, Schaumburg, IL USA Heat 1 (base) Heat 2 (3% Ni) Heat 3 (5% Ni) Heat 4 (9% Ni) Heat 5 (9% Ni, 3.5% Cu) Heat 1 (base) Heat 2 (3% Ni) Heat 3 (5% Ni) Heat 4 (9% Ni) Heat 5 (9% Ni, 3.5% Cu) Fig. 3. Graph shows the solid fraction versus temperature for all studied steels Voltage Length Time, s Fig. 4. Graph shows the experimentally measured changing voltage drop and spiral length during mold filling Heat 1 (base) Heat 2 (3% Ni) Heat 3 (5% Ni) Heat 4 (9% Ni) Heat 5 (9% Ni, 3.5% Cu) Super Heat, ⁰C Fig. 5. Graph shows the effect of ( pouring temperature and ( super heat on fluidity. MODELING RESULTS To understand the effect of rheological properties of liquid and solidified steel and heat extraction from a metal stream moving into a thin channel mold, a parametric study was done using Fluent CFD software. In this modeling, coupled boundary conditions were used assuming no additional heat resistance between melt stream and mold surface. Calculations were done for the 5% Ni steel (Heat 3). In an effort to avoid the possible effect of meshing on calculated result, the melt flow in a straight channel was modeled. This approximation is reasonable, considering the relatively large ratio between spiral curative radius and section dimensions. Figure 6 shows an example of calculation, which shows that flow blockage could start behind melt tip. It is difficult to evaluate at what moment melt stopped to flow in the thin channel because theoretically a central vein could supply a tip with hot melt, while the blocking region was formed behind a tip.

5 Spiral length, cm Paper 12-3.pdf, Page 5 of 8 AFS Proceedings 212 American Foundry Society, Schaumburg, IL USA Solid fraction.68 Solid fraction.36 Solid fraction.18 Solid fraction Time, sec Fig. 6. This is the example of mold filling (red is steel and blue is air) and solid phase development at the final moment (red is liquid and blue in solid). Central "vein" could supply tip by hot melt while blocking region was formed behind tip. Virtual experiments, with variation in solid fraction at the moment that melt flow in thin channel stops, were performed and both parameters including predicted spiral length and filling time were compared to experimental results. It is important to note, that the unique combination of spiral length and filling time at constant inlet pressure depends on the heat balance between the molten metal and the mold surface. At the same time, an assumption of the condition in mushy zone when the melt ceased to flow (critical value of solid fraction) also affects the calculated spiral length and filling time (Fig.7). The observed combination of spiral length and filling time were fitted to the model when the critical value of solid fraction was in the range.18 to.2, which is below the solid fraction determined by thermal analysis, i.e. the dendrite coherency. Fig. 7. The graph shows the effect of solid fraction at the moment of blocking melt flow on combination of filling time and spiral length. These predictions were used in modeling melt fluidity in the spiral mold using technological Software A. In the first step, the experimentally measured combination of filling time and spiral length was used for the optimization where the coefficient of interfacial heat transfer and viscosity of steel were systematically changed to produce the best fit to the experimental results. It might be expected that significantly more heat transfer takes place initially when the pouring melt comes in contact with the cold wall of the sand mold as compared to when the melt solidifies. Also, the viscosity is expected to increase with some amount of solid phase. Adjusted parameters were used to model fill length and superheat with respect to Ni content of the steel. Analysis of spiral filling kinetics showed that partial freezing was detected in the channel, but calculated filling process was continued as a result of a remelting effect. However, it is difficult to believe that this effect has taken place during the short time of filling the thin wall channel. To correctly estimate the fill length and fill time, a plot of the spiral fill length and temperature of the spiral tip were drawn for each case (Fig. 8). It was seen that the temperature of the tip of spiral dropped quickly. After approximately one second, the temperature at the spiral tip was in the mushy zone and the model predicted the continuation of filling. The experimentally measured combination of filling time and spiral length for this case (5% Ni, 15C [32F]superheat) was achieved in modeling just before DCP (1492C [2718F]) at.2 critical value of precipitated solid phase. This critical value is significantly less as compared to that reported in literature. 5, 6 The small critical value may indicate that the morphology of the solid phases, and possible fragments from the peretectic reaction, can interlock more easily to form a choke as compared to columnar austenite dendrites in steel alloyed by Ni.

6 Fill length, cm Freezing range, ⁰C Temperature ( C) Paper 12-3.pdf, Page 6 of 8 AFS Proceedings 212 American Foundry Society, Schaumburg, IL USA Fe(L) Fe(L) + BCC Fe(L) + BCC + FCC Fe(L) + FCC FCC % 2% 4% 6% 8% 1% Ni content (wt%) Fig. 9. Graph shows the calculated (Software B) phase diagram for Ni alloyed Cr-Mo steel Length Experimental T tip Calculated (FACTSAGE) Time, s Fig. 8. These illustrate ( temperature field in spiral at the end of filling) and ( variation of fill length and tip temperature with fill time. DISCUSSION Software B was used to obtain a phase diagram for the steels being investigated. The effect of Ni on the formation of different phases in Cr-Mo steels is shown in Fig. 9. The formation of δ-ferrite phase during peritectic solidification takes place in steels with less than 3% Ni. The steels containing higher amounts of Ni would not form any δ-ferrite phase, since austenite would be the primary solidified phase. Comparison of the calculated and experimentally measured freezing ranges is shown in Fig. 1. The 3% Ni steel exhibited the minimum freezing range. From computer assisted single thermocouple thermal analysis, the dendritic coherency temperature linearly decreased with the increase in Ni content at a rate of 2.5 C/%Ni (Fig. 3). At the same time, the increase in the Ni content affected the shape of solid fraction curves versus temperature in mushy zone. In the base Cr-Mo steel (Heat 1) the rate of increase in solid fraction was delayed beyond a value of.4 to.5, which can be associated with the peretectic reaction mechanism Ni content, wt. % Fig. 1. Graph shows the effect of Ni on experimental and predicted freezing range in Cr-Mo steel. A comparison of the fluidity, predicted by using Software A and experimentally measured, was done with respect of two factors: superheat and Ni content. A close agreement between the predicted and the experimental fill lengths for the 5% Ni steel at various super heats was obtained (Fig. 11. However, this would be expected, since the model was fit to these experiments. At the same time, the trend of experimentally observed effect of Ni on fluidity of Cr-Mo steel was different when compared to that predicted by our model (Fig. 11. Experimental data showed that the steel alloyed by 5% Ni had higher fluidity as compared to other steels. This effect was more prominent at lower and medium superheats. At the same time, the predicted lenths from the fluidity model did not show this concentration optimum and did not decrease in the 9% Ni steel.

7 Fill length, cm Spiral Length, cm Paper 12-3.pdf, Page 7 of 8 AFS Proceedings 212 American Foundry Society, Schaumburg, IL USA Experimental Modeled Super Heat, ⁰C 2C(exp) 2C(model) 6C(exp) 6C(model) 8C(exp) 8C(model) Ni content, wt% Fig. 11. The graphs show the comparison fill lengths, predicted by Software A and experimentally measured, for the 5% Ni steel with respect to ( superheat and ( effect of Ni content at different superheats. This indicates that others factors, which were not considered in the model, affected steel fluidity. One of the possible mechanisms to explain maximum fluidity of 5% Ni steel could be related to the differences in primary solidification and the absenceof the peritectic reaction. In particular, primary austenite directly forms from the melt at Ni contents greater than 3% (Fig. 9). The effect of the peritectic reaction on melt viscosity was not considered in the computational model. Steel with 9% Ni also exhibited primary solidification with austenite, but produced a lower fluidity than the 5% Ni steel. Unlike the experimental curves, the curves from modeling showed increase fluidity at 5% Ni, after which the curves were mostly flat (Fig. 11. The experimentally confirmed lower fluidity of high nickel steels is consistent with previous work by Taylor. 3 It could be the results of combination of different factors which were also not considered in the model. This deviation between predicted and experimental fill lengths for 9% Ni steel could be due to the effect of large concentrations of Ni on the melt viscosity and surface tension of liquid steel at temperatures close to the liquidus temperature. Unfortunately, literature shows contrasting results on the effect of Ni on surface tension in Fe-Ni alloys. Kingery et al. 12 showed that addition of nickel linearly increased surface tension of Fe-Ni system, whereas Sharan and Cramb 13 reported that Ni additions to liquid steel resulted in gradual decrease in surface tension at low oxygen potentials. An additional factor that could diminish fluidity in high alloyed by Ni steel are a more intensive oxide surface film formation in the melt and its effect on melt flow in thin channels. This phenomenon was intensively studied in different alloys by Campbell 6 and could also be applicable for high alloyed by Ni steel. CONCLUSIONS Experimental and computational methods were applied to study the effect of Ni alloying on fluidity of high strength Cr-Mo steels. The combination of measured filling time and final length of spiral were used for CFD model parameter adjustment. The estimated critical volume of solid fraction when melt stopped to flow in thin channel was approximately.2. This predicted solid fraction is less than that predicted by thermal analysis and the experimentally determined dendrite coherency, which was near.3 for all studied steels. This indicates that initial solidified phases could block the melt flow in thin channels before development of the equilibrium dendrite coherency. Measured fluidity showed a monotonic increase with superheat above liquidus for all steels and had extreme higher value in steel alloyed by 5% Ni in comparison to un-alloyed, low alloyed by 3% Ni and high alloyed by 9% Ni steel. It was seen that the model correctly predicts the effect of superheat on fill length and time for 5% Ni steel, while we still have a way to go with correct modeling fluidity results for whole Ni range studied. The formation of primary δ-ferrite and the peritectic reaction during filling might be a possible explanation for lower fluidity of the alloys with less than 3% Ni. However, the experimental results contradict the model predictions for Cr-Mo steel with 9% Ni. This difference could be attributed to the effect of Ni on viscosity and surface tension of the melt at lower temperatures. ACKNOWLEDGMENTS The authors would like to thank U.S. Army Benet Labs for funding this research and SFSA for discussion and managing.

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