Process Metallurgy Effect of Nitrogen Alloying on Sulphur Behaviour during ESR of AISI M41 Steel Taha Mattar, Kamal EI-Fawakhry, Hossam Haifa, Mamdouh Eissa Steel Technology Department, Central Metallurgical R&D Institute (CMRDI), P. O. Box 87 Helwan, Egypt, tahamattar@cmrdLscLeg In this work, the effects of nitrogen alloying, physical properties and chemical composition of slag used in electro-slag refining (ESR) on phosphorus and sulphur contents of AISI M41 high speed steel have been studied. The experiments were conducted with two high speed steel grades which were melted in an induction furnace (IF). The first grade is the standard AISI M41 high-speed steel and the second one is nitrogen alloyed M41 (denoted M41N). The produced ingots were ESR remelted under three grades of calcium fluoride based slag. Results showed that the ESR process has no effect on the phosphorus content in steel but it is a good tool in removing sulphur. This study shows that a high desulphurization rate can be achieved by ESR process by optimizing slag properties where the viscosity and oxidation reactions play an important role in sulphur removal. Nitrogen alloying was found to retard sulphur removal. Keywords: High speed steel, nitrogen, induction furnace, ESR, slag, refining, phosphorus, sulphur, desulphurization, viscosity DOl: 10.2374/SRI07SP077-79-2008-691; submitted on 9 July 2007, accepted on 4 November 2007 Introduction High speed steels are produced by melting the metallic charge in an electric arc furnace (EAF) or induction furnace (IF) and refining the produced steel ingots using secondary refining processes, such as vacuum arc remleting (VAR), electron beam remelting (EBR), plasma arc remelting (PAR) or electroslag remelting (ESR). On the other hand, the presence of sulphur in high speed tool steels is likely to produce hot shortness, as revealed in forgeability tests and in hot torsion tests used to measure hot workability. In addition, fatigue strength may be reduced, and toughness may be lower compare to low sulphur grades [1]. On the other hand, nitrogen alloyed tool steels were developed to satisfy the advanced technological demands in the engineering sector. In addition to its role with aluminium as a grain refining agent, nitrogen produces marked solid solution hardening and precipitation strengthening reactions. Moreover, it is particularly beneficial to the pitting resistance of austenitic grades. In terms of mechanical properties it is concluded that nitrogen alloying up to 0.18 wt.% oflow cobalt containing steels (about 2 wt.%) leads to the rise in secondary hardness and heat resistance to the level of S 6-5-2-5 steel with 5 wt.% Co (AISI M41). The magnitude of secondary hardness and heat resistance of S-6-5-2-5 and S6-5-2-10 high cobalt steels cannot be substantially increased by nitrogen alloying. It was concluded that, by nitrogen alloying, it is possible to reach a high level of the basic properties at lower cobalt content [I]. Dephosphorization, which is an oxidation process, and oxygen removal are not compatible in ESR. In a recent study (2), the slag used in ESR showed low dephosphorization power due to the absence of oxidizers and the addition of aluminium during ESR with the consequence of iron oxide and manganese oxide being reduced and hence dephosphorization conditions are not valid [2]. On the other hand, the ESR process has been distinguished among the other refining processes by its ability to remove sulphur typically down to a few steel research int. 79 (2008) No.9 hundredths or even thousandths of percent regardless of the electrode composition. The sulphur contents in the metal pool are not different from those in a solidifying ingot [3]. It is supposed that the desulphurization is completed during the time starting with the melting of the consumable electrode surface, formation of molten droplet, dropping of the molten droplet through the molten slag pool until reaching the molten metal pool, and the distribution equilibrium is established at the slag/metal and gas/slag interfaces [4-8]. Among these sites of sulphur transfer and reactions, there are two reactions which govern the sulphur removal in the ESR process [9], a slag/ metal reaction and a gas/slag reaction. The transfer of sulphur from metal to slag is promoted by high slag basicity and low concentration of oxygen in the metal. Hence, the removal of sulphur from metal to slag is promoted by high lime and low silica and iron oxide contents of slag. In contrast, the transfer of sulphur from the slag to the gas phase is promoted by a high partial pressure of oxygen in the atmosphere and low slag basicity [5-9]. The results obtained by Hlineny and Buzek [10] indicated that most of the sulphur removed from the metal enters into the gas phase when using air atmosphere. Therefore, the gas/ slag reaction is more important than the slag/metal reaction. On the other hand, Kato [11] found that the interface of the tip of the electrode/ molten slag pool was the most important for the sulphur transfer. At any rate, the sulphur transfer from the surface of the slag pool to the atmosphere determines the overall desulphurization rate. Kato [11] found also that the main interface where desuplurization takes place is at the metal pool/ slag pool; where the electrochemical reaction is important in this process. Mattar et al. [5] reported that good desulphurization results can be obtained by using slag of the approximate composition 70wt.%CaF r 30wt.% Ah03. Sulphur concen-tration in CaFrCaO slag melts was found to increase linearly with N cao until this reaches a value of 0.26. Beyond this value the sulphur percentage decreases and finally reaches a negligible value for pure lime. 691 Process Metallurgy Table 1. Chemical composition of materials used in induction steel melting process. C Si Mn High carbon Fe - Mn 6.52 0.133 High carbon Fe - Cr 6.85 4.4 Extra-low carbon Fe - Cr 0.02 -- Cr V Al S N Fe 76.3 -- -- -- 0.088 -- Balance -- 61 -- -- 0.08 -- Balance -- 70 -- -- 0.1 -- Balance 1.5 -- -- Balance Fe- V -- 1.5 -- -- 80 Nitrovan 2.82 -- -- -- 78.03 -- -- 16.7 Balance Fe-Cr- N 2.95 -- -- 65 -- -- -- 10.25 Balance P S N Fe Table 2. Chemical composition of the used high speed tool steel scrap. C Si Mn Cr Mo Co V W Scrap 1 0.77 0.33 0.25 3.88 4.38 - 1.98 6.87 0.0354 0.0278 0.003 balance Scrap 2 1.01 0.344 0.25 4.02 3.62 5.06 1.72 7.49 0.0225 0.0036 0.004 balance From sulphide capacities data [8,I0] for CaF2-CaOAI203 melts, it is clear that optimum desulphurization (highest sulphide capacity for this system) is attained at the composition of 20 wt.% CaO and 80 wt.% CaF2. Desulphurization power of CaF2 slag increases with Ncao, and can be used under highly reducing as well as oxidizing conditions. The results by Kato [II] declared that even if a slag does not contain CaO, CaO is produced during the process by the decomposition of CaF2 , where about 3-5 wt.% of the CaF2 will be dissociated, and the desulphurization power of the slag is generated. The value of (S)/[S] constitutes a function of the basicity of (CaO)/ (Si0 2), and the less Ah03 is the higher will the distribution coefficient be. It was also found that the ESR process removes from 50 to 75 % of the sulphur contained in the electrode depending on both the initial sulphur content, the composition of the slag and the atmosphere above the slag [5]. Nitrogen in the metal is mostly desorbed to the gas phase. Mattar et al. [12] found that nitrogen adsorption to slag was no more than 10% of the nitrogen reduction, while the other 90% of removed nitrogen was in the gas phase. The nitrogen transfer to the gas phase is more rapid than that to the slag phase [13]. Yamanaka et al. [13] observed a reduction of nitrogen content in molten steel by about 27% with increasing remelting time. Martinez et al. [14] investigated the nitrogen solubility in its different forms for the CaO-CaF2-AI203 system at 1773 K. They found that at constant alumina content, the nitrogen solubility increases as CaF2 is replaced by lime. This work aims at studying the effect of nitrogen alloying and slag properties on the phosphorus and sulphur behaviour during electoslag refining of AlSI M41 high speed tool steel. Experimental Work With the objective of this study a new grade of steel comparable to AlSl M41 high speed steel and alloyed with nitrogen has been developed. Nitrogen was added to the molten metal in the form of nitrovan (vanadium-16% nitrogen alloy) and ferrochromium bearing nitrogen. The 692 conventional and nitrogen alloyed AlSl M41 high speed tool steel grade (denoted here as M41N) were electroslag refined under three grades of calcium fluoride based slag. The used steel scrap, produced steels, and slag were chemically analysed. Phosphorus and sulphur behaviour were monitored and interpreted through the analyses of their content in the used and produced materials and steels before and after different melting processes. Melting. The AISI M41 high speed tool steel scrap was melted in a medium frequency induction furnace (IF) with silica lined 100 kg capacity crucible. A material balance for production of the steel grades under investigation has been designed. The charge constituents and the chemical composition of the tool steel scraps used for producing both conventional and developed high speed steel grades are shown in Tables 1 and 2 respectively. The molten metal was tapped at I500°C in sand moulds forming round ingots with diameter 65 mm and length 1200 mm to be used as consumable electrodes in the ESR process. The conventional M41 grade samples that were produced in the induction furnace are denoted as IF while those alloyed with nitrogen produced in the induction furnace are denoted as IFN. Refining by ESR. The consumable electrodes produced in the induction furnace were electroslag refined using three grades of pre-fused CaF2 based slag in the ESR unit. Three grades of synthetic CaF2 based slag were used in solid start electroslag refining of both investigated steels, Table 2. These slag grades were produced by pre-fusion of the raw materials (fluorspar, limestone, and alumina), Table 3, in a submerged arc furnace. The working conditions of the single electrode submerged furnace were 350 A and 35 V. The pre-fused material was grounded, then kept in a sealed container to avoid water vapour capturing till being used in ESR. The electrical parameters of the used ESR unit were 2035 A and 32 V. The applied electrode descending speed was 34 mm per minute. A starting mixture in the form of a capsule was put over a steel slide. The molten metal was collected in a water cooled copper mould. After the remelting process was steel research int. 79 (2008) NO.9 Process Metallurgy completed, a steel ingot was brought out and cleaned. The bottom slide was removed due to the possibility of contamination with gangue material formed during the starting process. In addition, the produced slag was collected and weighed. The IF samples (AISI M41 steel) that were refined by ESR are denoted as FI, F2 and F3 for the Slags I, 2 and 3, respectively. Those produced from ESR refining of IFN samples (nitrogen alloyed AISI M41 steel) are denoted as FIN, F2N and F3N for the Slag 1,2 and 3, respectively. Evaluation. To evaluate the efficiency of the ESR process in dephosphorizing and desulphurizing the steels under study and the role of nitrogen alloying in these processes, samples were taken and chemically analysed using a spectrographic analyser (SPGA). Samples from the scrap, the different alloys used in the melting, ingots produced from induction furnace and elecrtoslag remelting were analysed to determine their chemical composition. Additionally, the behaviour of phosphorus and sulphur during the refining process was studied where samples from the top and the bottom across the section of the ingots were taken and chemically analysed. The chemical composition of the fluxing materials and the final slag was determined to evaluate the behaviour and distribution of phosphorus and sulphur during ESR process. Table 3. Chemical composition and physical properties of the used pre-fused slags. ESR Flux Chemical com osition ,wt% CaF, CaO Aba) Physical properties CaO/ Ah 0 3 Density [g/crrr'] Viscosity [Poise] Interfacial tension [mN/m] FI 65 15 20 0.75 2.555 0.433 1375 F2 75 IS 10 1.5 2.55 0.225 1405 F3 55 30 15 2.0 2.53 0.3 1395 M41 and M41N steel grades were produced by IF in the form of consumable electrodes that were refined by ESR technique using three slag grades. These slags had approximately the same density and different viscosity and basicities (Table 3). Specimens from steels produced by IF and ESR were analysed to determine their compositions. Table 4 and 5 show the chemical compositions of these steels. Tables 4 shows that there is no pronounced deviation in the chemical composition between the top and bottom or between the edge and the centre of the produced ingots, which could be attributed to the rapid solidification rate in the ESR process and the homogeneous melt produced in the induction furnace. Pre-fused initial slag and final slag were examined to monitor the behaviour of phosphorus and sulphur. Results and Discussion The effects of nitrogen alloying and ESR process on the behaviour of phosphorus and sulphur in AISI M41 high speed tool steel were studied. Steel Grade ESR Flux Position Top FI Bottom Edge Center Edge Center Average Top M41 F2 Bottom Edge Center Edge Center Average Top F3 Bottom Edge Center Edge Center Average Top FI Bottom Edge Center Edge Center Average Top M41N F2 Bottom Edge Center Edge Center Average Top F3 Bottom Edge Center Edge Center Average steel research int. 79 (2008) No.9 C 0.91 0.92 0.99 0.97 0.95 0.8\ 0.82 0.89 0.90 0.86 0.91 0.87 0.95 0.95 0.92 0.95 0.9\ 0.92 0.94 0.93 0.94 0.92 0.94 0.95 0.94 0.97 1.02 1.13 1.09 1.05 Mn 0.25 0.25 0.25 0.26 0.25 0.27 0.27 0.30 0.31 0.29 0.28 0.27 0.31 0.29 0.29 0.16 0.17 0.19 0.19 0.18 0.16 0.16 0.20 0.19 0.18 0.17 0.17 0.23 0.23 0.20 Si 0.42 0.43 0.48 0.48 0.45 0.39 0.39 0.30 0.30 0.35 0.4\ 0.40 0.45 0.43 0.42 0.52 0.51 0.47 0.50 0.50 0.43 0.45 0.55 0.53 0.49 0.52 0.50 0.46 0.49 0.49 Behaviour of Phosphorus. Tables 4 and 5 clarify that neither nitrogen alloying nor the ESR process have any effect on the phosphorus behaviour and there is no dephosphorization observed during the ESR of both P 0.020 0.021 0.021 0.021 0.021 0.020 0.022 0.021 0.020 0.021 0.023 0.022 0.022 0.022 0.022 0.021 0.023 0.025 0.023 0.023 0.023 0.023 0.023 0.023 0.023 0.025 0.024 0.023 0.025 0.024 Chemical composition ,wt% S Cr Mo 0.0096 4.\1 4.\7 0.009 4.12 4.20 0.0086 3.98 4.01 0.0086 4.13 3.99 0.0098 4.05 4.13 0.0073 4.15 4.06 0.0076 3.93 4.09 0.0075 3.81 3.33 0.0077 3.79 3.36 0.0075 3.73 3.90 0.0050 4.23 4.06 0.0050 4.04 4.13 0.0055 3.86 3.99 0.0057 3.94 4.17 0.0053 3.98 4.13 0.0067 4.17 3.74 0.0060 4.23 3.73 0.0059 3.49 3.80 0.0058 3.95 3.67 0.0061 3.66 4.04 0.0077 4.24 3.62 0.0078 3.61 4.\9 0.0079 3.8\ 3.39 0.0088 3.46 3.93 0.0080 4.04 3.52 0.0084 4.22 3.80 0.0085 3.77 4.18 0.0083 3.44 4.\0 0.0082 3.45 4.15 0.0083 4.16 3.62 Co 5.02 4.98 4.97 4.99 4.99 5.\0 5.08 4.81 4.8\ 4.95 5.04 5.08 5.00 4.88 5.00 4.97 5.03 5.06 4.90 4.99 5.01 5.03 4.96 5.02 5.0\ 5.02 5.01 4.99 4.96 4.99 V 2.00 2.01 1.95 2.02 1.99 1.91 1.84 1.80 1.80 1.84 2.08 2.03 2.04 2.08 2.06 2.14 2.23 1.89 1.97 2.06 2.11 2.08 1.89 1.93 2.00 2.\8 2.17 2.12 2.\6 2.16 w 6.22 6.4 6.2 6.33 6.29 6.47 6.54 5.73 5.69 6.1\ 6.46 6.5\ 6.42 6.46 6.46 6.94 7.19 6.27 6.58 6.75 6.41 6.45 6.04 6.06 6.24 6.98 6.87 6.96 6.85 6.92 693 Process Metallurgy 0.014 ,g, ~ 80 M411 rO IIIM411't 0.012 ~ ~ 70 ~ 60 0 cr 0.010 iii c: ~ 0.008 50 N 40 ss: 0.004 '5 en Q) Q. Cl 0.002 iii (5 f- 0.000 IF F1 F2 F3 IFN F1N F2N ~ 30 M411'!! r-r-r- 20 10 L......-- 0 F3N F1 F2 F3 F1N ESR Slag ESR Slag Figure 1. Effect of ESR slag on sulphur content in the produced steel ingots. , - OM41 ~ 0 0.006 c: - 0'< ~ '- F2N F3N Figure 2. Effect of ESR slag on total desulphurizaion ratio. Table 5. Average chemical composition of steels before and after the ESR process. Steel grade Process M41 ESR Flux IF ESRI ESR2 ESR3 IFN ESRNI ESRN2 ESRN3 M41N C 0.92 0.95 0.86 0.92 0.90 0.93 0.94 1.05 FI F2 F3 FI F2 F3 Si 0.43 0.45 0.35 0.42 0.51 0.50 0.49 0.49 Chemical composition Mo Co V 4.21 5.02 2.12 4.13 4.99 1.99 3.89 4.95 1.84 4.13 5.0 2.06 4.20 4.78 2.13 4.04 4.99 2.06 4.04 5.01 2.00 4.16 4.99 2.16 Cr 4.08 4.05 3.73 3.98 3.79 3.66 3.52 3.62 Mn 0.29 0.25 0.29 0.29 0.18 0.18 0.18 0.20 conventional and nitrogen alloyed AISI M41 steels. The obtained data show that the maximum dephosphorization power that could be attained is 2.5%. The obtained data are in good agreement with previous results [2]. ,wt% W 6.53 6.28 6.11 6.46 6.82 6.75 6.24 6.92 P 0.020 0.020 0.020 0.022 0.022 0.022 0.022 0.024 S 0.012 0.009 0.007 0.005 0.008 0.006 0.008 0.008 N 0.045 0.0082 0.036 0.031 0.135 0.093 0.1139 0.1167 M41N is only 25.6 % of the initial sulphur contained in the consumable electrode. To understand the effect of slag composition on the mechanism of the desulphurization process, the distribution of sulphur between metal, slag and gas phases are determined based on the analysis of the charge materials (metal electrode and flux) and the produced metal ingot and slag (Table 6). The effect of slag composition on the ratio of sulphur in the ingot, evaporated sulphur and sulphur captured by slag to the total sulphur content and total desulphurization ratio are shown in Figures I and 2 and Table 6. Sulphur behaviour during ESR has not been thoroughly clarified. The results obtained by Hlineny and Kato et al. Behaviour of Sulphur. Figure I illustrates the variation of sulphur content in the produced steels remelted under the different used slag compositions compared with the initial sulphur content in the consumable electrode. It is clear from Figures 1 and 2 and Table 6 that 25.6 to 56.2 % of the initial sulphur content of steel M41 is removed, depending on the slag composition. On the other hand, the maximum desulphurization degree of steel Table 6. Sulphur distribution between different phases and constituents. S Input S Output S Distribution <lJ e OIJ <lJ OIJ .". OIJ .". "0 >< :::s u 0 - Z 0) E '" f/J UJ I oi ~ '0 g <lJ W3 '0 en en ~ os OIJ .". '0 OIJ .5 '" 1.80 21.0 Cii '- en en en en ~ '" ~ oil os '0 ~ ~ OIJ .". ~ 0 '" ~ >< :::s 0:;::: >< :::s .5 o: ;:;: 0 ~ OIJ ;:;: 0 ~ en en '" oi 0 <lJ W3 .5 W3 .5 tr: '0 en tr: ~ ~ l::: o 2 t><lJ OIJ "0 <lJ "0 .5 o: '0 . 0 c: OIJ ;:;: 0 ~ ~ ~ OIJ ct OIJ .5 ;; iOIJ .5 .5 .5 .5 '0 ] [Jl ~ r/i' Q. 0 E- ~ 20.8 1.95 0.0017 0.0306 0.012l 2.541 2.572 0.009 ~ ~ '" ~ ~ . OIJ os Cii .5 OIJ '" Cii .5 os o .5 §: - '- '0 0 §: ;:;: o OIJ oil os ~ '" ~ '" ~ Ci~ c: • c: .- 0 .- 0 f/J _ ~o 0 .~ ~ ~o ~* f/J_ ~* ~ ~ ~rv os .... oc: ~. .0 _0 f/J ~'L -f/J o ~ 1.872 0.0018 0.035 0.665 72.8 1.3 25.9 ;:;: 0 ~ 0 ig ~- is :Sr/J :Sr/J :s~ O~ O?:- 0 ;:;:f/J en en ~ ~ ~ . r;:; ;:::: ~ ]~ <lJ Of!' 0 ~~ ~ ~"' ~ ~ ;2= ~ 19.9 0.36 27.2 25.6 2 1.80 19.64 19.4 1.84 0.0467 0.8406 0.0121 2.376 3.217 0.0075 1.455 0.0031 0.057 1.705 45.2 1.8 53.0 29.4 1.17 54.8 38.0 3 1.80 19.85 19.6 1.95 0.0892 1.6056 0.0121 2.402 4.008 0.0053 1.039 0.02 64.4 6.6 2.49 74.1 56.2 IN 1.95 20.0 1.208 0.0187 0.379 0.086 72.2 22.7 5.1 0.2 0.07 27.8 25.6 2N 1.80 18.23 18.0 1.93 0.0467 0.8406 0.0082 1.495 2.336 0.008 1.440 0.0387 0.747 0.149 61.6 32.0 6.4 0.2 0.1 3N 1.85 20.5 1.660 0.0411 0.801 0.870 49.8 24.1 26.1 1.1 0.52 50.2 0 694 19.8 2.05 0.0017 0.0331 0.0082 1.64 1.673 0.0061 20.0 1.95 0.0892 1.6502 0.0082 1.681 3.331 0.0083 0.390 2.579 25.9 9.7 38.4 2.4 steel research int. 79 (2008) No.9 Process Metallurgy 70 80 . Cl Cl o 70 0 60 II:: 50 c . . 0 ;: ~ o 60 ° M41N ~ ~ M41 50 -- 40 .c Gl 30 :; . 20 _-0 --- _0 ~ Ui 40 !! 30 Q. "" 20 Gl C 10 10 0 0 0 1.5 0.5 2.5 2 0.2 0.25 0.3 (a) Figure 3. Effect of CaO/AI203 ratio in the ESR slag on the total desulphurization ratio. (b) 70 r - - - - - - - - - - - - - - - - - - - - , o o 0 70 M41 :; ° M41N II:: 50 ~ 0 N .;: Gl 30 \ 20 10 50 40 Q. 30 c Gl 20 ~ 10 .. :; \ \ \ r-------- \ ~ 0.45 M41 60 e- .c . .. ° M41N c 40 0.4 , 80 ~ 60 " -, , " , -, 0.35 CaOI AbO, Ui ;:.:. !eo M41 ° M41N - ----... -------- I- 0 0 0 0.01 0.02 0.03 0.04 0.05 0.2 0.25 0.3 0.35 Slag viscosity, Poise 0.4 0.45 - Ll [N), wt.% Figure 5. Effect of nitrogen reduction during ESR on sulphur removal into the gas phase. [10,11] pointed out that most of the sulphur in the consumed electrode is evaporated during the ESR process and Klyuev [15] reported that the degree of desulphurization depends mainly on the reducibility of the atmosphere. Mattar et al. [5] found that more than 80 % of sulphur in the charge could be removed by ESR under flux with the composition 60% CaF2 - 20% CaO - 20% Ah03. Although the role of flux composition in the desulphurization process during ESR is clearly concluded by Paton [16], the effect of the physical properties of the flux on the desulphurization process has not attracted much attention. Both sulphur content in slag and sulphur distribution between slag and metal were found to be increased by increasing CaO IAh03 ratio of the ESR slag (Table 6). Thus, desulphurization by slag Imetal reaction is favoured by using basic slag of high CaO IAl 203 ratio. The highest sulphur distribution between slag and metal was found in steel M41N with the high sulphur content and consequently lower desulphurization degree. Therefore, it can be concluded that the desulphurization process is not only dependent on slag composition but also on initial nitrogen content and the slag Imetal reactions do not represent the rate controlling step overall the desulphurization process. The removal of sulphur during the ESR process takes place in the following steps: (1) Removal of sulphur from the molten metal to the slag layer and its capture by CaO in the slag. This reaction occurs at the slag I metal interface. steel research int. 79 (2008) NO.9 Figure 4. Effect of slag viscosity on (a) percent of sulphur removed into the gas phase; {S}/St %; and (b) total desulpherization ratio during the ESR process. (2) Oxidation of sulphur on the slagl air interface as a result of its diffusion from the molten metal through the slag layer to the slag I air interface and the exposition of the slag surface to atmospheric oxygen. Slag Imetal reactions occur at the electrode tip, during passage of the metal droplet through the slag pool and at the slag pooll metal pool interface. Therefore, desulphurization power of the used flux depends mainly on CaO activity in this slag. Figure 3 clarifies the increase of total desulphurization degree with increasing CaOI Ah03 ratio of the ESR slag. Furthermore, a higher CaOI Al203 ratio leads to a lower slag viscosity, which in tum increases the desulphurization rate by a gasl slag reaction. Figure 4 shows the increase of the sulphur evaporation ratio and total desulphurization degree with decreasing slag viscosity. A low viscosity slag encourages a strong stirring action mainly resulting from electromagnetic forces, and this enhances the diffusion of sulphur from molten metal through the slag layer to the slagl air interface. The obtained results show that 27.2 to 74.1 % and 27.8 to 50.2 % of initial sulphur content of charge materials are removed by remelting steel M41 and steel M41N, respectively, depending on the slag composition. In steel M41 the greatest portion of sulphur was removed as gas. On the other hand, the greatest portion of removed sulphur was transferred into slag when remelting steel M41N 695 Process Metallurgy 90 0.16 oft i ~ C Gl C 0 - I~M41N 0.14 0.12 DM41 ;!!. iii 0.10 <: Gl 0 a) 0.08 ..J e Gl Cl 0.06 g 0.04 Z CI Z 60 III 0 ,g o M41 • M41N 80 70 n 0.00 IF F1 F2 F3 30 .... "", ' a) 20 nn 0.02 50 40 ._-----------_ - .. .. 10 0 IFN F1N F2N F3N 0 5 10 15 20 25 A120a, wt.% 90 90 ;;e. iii III 0 ..J <: 80 80 70 70 ~ z M41 ;!!. b) 60 ui 60 III 50 ... 50 40 Gl Cl 40 0 <: Gl CI o • M41N g 30 Z 30 20 20 10 10 0 b) 0 F1 F2 F3 F1N F2N F3N 0 0.5 1.5 2 2.5 ESRSlag CaOI A120a Figure 6. Effect of the ESR process on (a) nitrogen content of the ingot produced and (b) nitrogen loss. Figure 7. Effect of (a) AI20aand (b) CaO/AI20a ratio in the ESR slag on denitrogenization of steels during the ESR process. (Table 6). Table 6 illustrates that the amount of sulphur evaporated by oxidation into gas is 6.6 to 29.4 times that picked by slag for steel M41. On the other hand, for steel M41N the amount of sulphur evaporated by oxidation into gas is only 0.2 to 1.1 times that picked by slag. It is evident from Figure 5 that nitrogen removal has a great effect on sulphur evaporation. As the denitrogenization during ESR increases, the sulphur evaporation decreases. This result can be explained by transfer of nitrogen as gas phase with the result of reducing the oxygen partial pressure in the atmosphere above the molten slag layer and consequently retarding the sulphur removal by slag lair reaction. The obtained results are in good agreement with that obtained by Klyuev and Shpitsberg [15]. They found that the desulphurization by evaporation is restrained when the oxygen partial pressure in the atmosphere is reduced by argon or by the formation of a high partial pressure of nitrogen atmosphere because of other metallurgical phenomena. It is clear from Figure 6 that the ESR process lowers the nitrogen content of steel. This figure shows also that the initial N content has a great effect on both the final nitrogen content and nitrogen removal where a higher initial N content results in a higher final nitrogen content in the steel ingot and lower nitrogen loss in the ESR process. These results can be attributed to the formation of an N rich atmosphere and slag during the ESR process which in tum hinders the further N removal both to slag and atmosphere. Furthermore, from Table 5 and Figure 6, it is clear that the amount of nitrogen removed by ESR in case of M41 ranges between 90 to 368 ppm, whereas in M41N it ranges between 183 and 420 ppm, i.e. in case of M41N the amount of N removal is larger than that in case of M41. It is also clear that amongst the used slag grades, slag No. I with highest alumina content as well as highest viscosity is the most powerful slag in removing N. This could be attributed to the higher capacity of Al to N as well as the fact that higher slag viscosity results in longer residence time of N rich inclusions in the slag and allowing the dissolution reaction of N inclusions via chemical reaction, which seems to need longer time. From this conclusion, it could be assumed that the rate determining step for N removal is not the N gas nor inclusion particles diffusion but the chemical reaction. Figure 7 shows the effect of slag chemical composition on the nitrogen behaviour during ESR of M41 and M41N steel. This figure confirms the previous conclusions that increasing alumina content and consequently decreasing CaOI Aha} ratio enhance the nitrogen inclusion decomposition reaction and hence increases the nitrogen removal process. On the other hand, the viscosity of slag plays an important role in the behaviour of nitrogen during ESR of both grades (AISI M41 and AISI M41N). It is clear from Figure 8a that a higher viscosity enlarges the nitrogen loss slightly for high nitrogen grades whereas the nitrogen loss rises markedly at higher viscosity in low nitrogen grade. Regarding the mechanism of nitrogen removal, Figure 8b shows that most of the removed nitrogen is captured by the slag where it gives the same trend as the total nitrogen removal. This figure clears that more than 55% of removed nitrogen is captured by slag. This behaviour depends on the slag viscosity but also on the initial nitrogen content in metal. As it is clear from Figure 8c, in 696 steel research into 79 (2008) NO.9 Process Metallurgy case of the low nitrogen grade most of the removed nitrogen is captured by slag while in case of the high nitrogen grade the removed nitrogen in slag and gas phase (to be removed by evaporation) is in a close range. This result could be attributed to the saturation of slag with nitrogen in case of high nitrogen grades, which makes the nitrogen evaporation is easier and possible. 90 70 ~ 60 '" .2 50 c ~ 40 g Z 30 20 Conclusions The ESR process has no pronounced effect on the dephosphorization of steel due to the absence of oxidizers in the used fluxes. The dephosphorization degree is low at about 1 to 2.5% due to slag/ metal reaction. The ESR process is effective in desulphurizing the steel. Up to 56% and 26% of the sulphur contained in metal electrode and up to 74% and 50% of the total sulphur contained in the charge for steels M4l and M41N, respectively, can be removed by remelting the consumable electrodes under the suitable flux composition. In ESR of steel M41, the greatest portion of sulphur is removed in the gas phase where up to 64% of the initial sulphur content of the charge is removed as gas by slag/ air reaction. In ESR of steel M4l N, the greatest portion of sulphur is removed in the slag where up to 32% of the initial sulphur content of the charge is removed in slag by slag/air reaction. The removal of sulphur by slag/metal reactions is enhanced by increasing the CaO activity in slag, i.e. increasing the CaO/ Ab03 ratio in the ESR slag. The removal of sulphur by slag/ air reaction is enhanced by decreasing the slag viscosity. Increasing the initial nitrogen content in the consumable electrode retards the desulphurization process. o M41 • M41N 80 ~-. 10 0 0.25 0.2 0.45 60 50 ~ .Z -~ o M41 • M41N 40 0 0 ~ 30 20 10 0 0.20 --- --~-. 0.25 0.30 0.35 0.40 0.45 (b) 12 o M41 • M41N 10 g ~ 8 6 4 --- 2 steel research int. 79 (2008) NO.9 0.4 (a) References [I] Hoyle, c.: "High Speed Steels"; Butterworth Co. Ltd., (1988), pp. 222. [2] Mattar, M. T.: Ph. D. Thesis, Faculty of Science, Helwan University, Cairo, Egypt (1996) [3] Trenkler, H. and Krieger, W.: "Metallurgy of Iron", Springer Verlag, Berlin, (1983), pp.239-251. [4] Eissa, M.M. and Alaa, M.: Steel Research, 69 (1998), No. I, II. [5] Mattar, T.; EI-Faramawy, H.; Fathy, A.; Eissa, M. and EI- Fawakhry, K.: Steel Grips, 2 (2004), No.4, 287-290. [6] Scholz, U.B.H.: "Electroslag remelting technologies in the past and in the future", MPT International, (1998), No.3, 36-43 [7] Hines, A.L. and Chung, T.W.: Metallurgical and Materials Transactions B; 27B (1996), No.2, 29-34. [8] Mills, K.C. and Keeme, B.1.: "Physicochemical Properties of Molten CaF 2-Based Stags", International Metals Reviews, 9 (1981), No. I, 21-69. [9] Miska, W.H. and Wahsten, H.M.: Archiv Eisenhiittenwesen, 44 (1973),19. [10] Hlineny, J. and Buzek, Z.: "Desulphurization in Electro-slag Melting", Hutnicke Iisty, 8 (1966), 524. [II] Kato, M.: "Electro-slag Remelting", Nagoya International Training Centre, Naggoya, Japan, (1985), pp. 238. 0.35 0.3 0 0.20 0.25 - ._ - - - - ~ 0.30 0.35 0.40 0.45 Slag viscosity, Poise (c) Figure 8. Effect of slag viscosity on (a) nitrogen loss, %; (b) nitrogen captured in the final slag (N)/Nt*100, %; (c) nitrogen distribution between slag and gas phase (N)/{N). [12]Mattar, T. M., Essia M. M.; El-Fawakhry, K.: "Nitrogen behavior during Electro-slag remelting (ESR) of tool steels" 5th Int. Tooling Conf., Sep. 29 - Oct. 1,1999, Leoben, Austria. [13]Yamenaka, R.et al.: ISIJ Int.,32 (1992), No. I, 136-141. [14] Martinez, E.; Espejo, O.V. and Majarrez, F.: ISH Int., 33 (1993), No. 1,48-52. [15] Klyuev, M.M. and Shpitsberg, V.M.: "Removal and Formation of Non-metallic Inclusions in Metal Produced by Electro-slag Remelting", Stahl (in English); (1969), Feb., 168-171. [16] Paton, A.: "Practical Aspects of ESR Technology", International Metals Reviews, 7 (1979), No. 5- 6, 240. 697