Carbon Recovery and Inoculation Effect Of Carbonic Materials in Cast Iron Processing

Transcription

1 Carbon Recovery and Inoculation Effect Of Carbonic Materials in Cast Iron Processing M. Chisamera*, I. Riposan*, S. Stan*, V. Constantin*, C. Diaconu** *POLITEHNICA University of Bucharest; **ELECTROCARBON Slatina, ROMANIA Abstract Metallurgical and petroleum coke materials used as conductive packing medium to surround the carbon products or as resistor medium in electric graphitizing furnaces are compared to natural graphite, scrap electrodes and usually used calcined and graphitized petroleum coke carbon raisers, under laboratory and plant trials conditions. Carbon recovery and dissolution rate, specific sulphur increasing, slag generation, chilling tendency, shrinkage occurrence etc, are considered in cast irons production. The parameters of cooling curves are connected to graphite morphology, metal matrix make-up and chilling sensitiveness of carburized cast irons. Recovered granular carbonic materials, previously used as conductive or resistor medium were found as potential carbon raisers with visible inoculation effects in cast irons, depending on their origin (petroleum or metallurgical coke). Specific characteristics of these products and their application fields are presented (as advantages and limits), compared to conventional calcined and graphitized petroleum coke, especially in grey irons production. Key words: re-carburizers, by products, recovery parameters, inoculation 129/1

2 Introduction A large variety of carbonic materials are currently used to correct the carbon content of ferrous melt [1-9]. Generally, it is considered that amorphous phases of carbonic materials are absorbed into the melt as iron carbide. They can supply carbon to the melt, but cannot help the graphite nucleation process, while any crystalline graphite phase is particularly improving graphite nucleation [2-6,8-9]. It was found that using pet. coke the grey cast iron showed no reduction on chill tendency as compared to situations when carbon raisers with different levels of crystallinity were used [5,8]. No difference was found in ductile iron [7]. The main objective of this research is to compare different primary and recovered (by-product) carbonic materials, as far as carbon and sulphur recovery parameters and inoculation activity, under the influence of the crystallinity and purity grade, in lower sulphur (<.25%) and lower carbon equivalent (3.6-4.%) un-inoculated grey cast irons. Experimental Procedure Two thermally treated groups of carbonic materials were tested: a) new (primary) materials, as calcined (13-14 o C) or graphitized (>25 o C) petroleum coke and b) packing medium or resistor (insulator) medium materials in electric graphitizing furnaces, as granular petroleum or metallurgical coke (by-products from graphite manufacturing process) (Table 1). Metallurgical coke was usually used as packing medium to surround the carbon products in electric graphitizing furnace; if this material is covered with sand, will act as an insulator, to protect the furnace walls. Calcined petroleum coke was also tested in these positions, especially to define its behavior as recarburizer and inoculation. The purest of the spent by product material is a low ash partially graphitized coke which originates as the packing medium in close proximity to the graphite products. The least pure material is the one which originates as the material at the sand/coke interface (insulator layer) and contains free carbon, silicon carbide (up to 25% SiC) and un-reacted silicon dioxide. Carbon raisers studied in this work (Table 1) can be characterized more specifically, as follows: a) Calcined Petroleum Coke (CPC), at relatively low and high sulphur content and high ash level as representative amorphous carbonic material. b) Graphitized Petroleum Coke (GPC) as crystalline Synthetic Graphite. This list of materials includes two desulphurized petroleum coke products processed in granular form (>27 o C), at very low level of sulphur, ash and volatile matter and scrap electrodes as lower purity product. c) Recovered Partially Graphitized Packing Coke, as initially granular Petroleum Coke (GPCR) or Metallurgical Coke (GMCR) used to surround electrodes in electric graphitizing furnace. Partially graphitized carbonic materials, they will be different as sulphur, ash and volatile matter content. 129/2

3 d) Resistor Coke, as by-product from graphite manufacture, where calcined petroleum coke was covered with sand to act as an insulator of furnace wall. It was selected a high purity product (MCSX), at more than 95% fixed carbon, less than.2%s and 2-3 %SiC. e) Natural Crystalline Graphite as high purity mined product (96.9% FC). Equations 1-4 describe the efficiency parameters of carburization treatment, while equations 5 the specific sulphurization parameters: m C C q Μ C ΜC m C Carbon Recovery (CR): CR = x1 [%] CR 1 Carburizing Efficiency (Ec): Ec = = [%] C Carbon Dissolution Rate (CDR): CDR = [%C / min] t C Specific Carburization (Cs): Cs = [% C /1.% ] Specific Sulphurization (Ss, S*s): S MC * q Ss = [% S /1.% ] MC q ΜC C ΜC MC * q MC S S*s = [% S / % C] C m = treated melt, kg; C = C fin - C in dissoluted carbon in iron melt, %; C MC carbon content, %; q MC, q* MC added recarburizer, kg and %; t dissolution time, min; MC recarburizer. The solubility of these commercial and experimental carbon raisers was evaluated in a series of cast iron heats prepared in coreless induction furnaces under laboratory conditions and plant trials. Two initial 8kg heats were produced from a charge of 6 kg pig iron and 2 kg Steel scrap in a medium-frequency induction furnace (1Kg, 24Hz, silica lining). The resulted base material from each initial heat was later melted in a small coreless induction furnace (1 kg, 8Hz, graphite crucible) and two experimental programs were recorded under laboratory conditions (Table 2). The established experimental algorithm consisted in holding the melt at constant temperature for about five minutes followed by deslagging. The candidate material was then added to the melt while the power was applied to the furnace to maintain the temperature at the same field ( and o C, respectively). After a fix holding time (3 or 5 minutes), a mechanical stirring was applied (2 minutes). Finally, FeSi75 was added in the furnace, before tapping. The two programs are differentiated by iron chemistry, carbonic material (type, quantity, grain size), the melt temperature and the quantity of FeSi75 addition (Table 2). (1) (2) (3) (4) (5) 129/3

4 In plant trials low frequency coreless induction furnace was used (6.t, 5Hz, silica lining). Metallic charge included molten iron heel (17-5%) and steel scrap (5-83%). Each recarburizer was added as 4 6 parts (25-3 Kg) in the steel scrap charge. Finally, the 15min holding time was applied (Table 3). Results and Discussion The results of the carbon and sulphur dissolution studies are presented graphically in Figures 1 through 3 for all of the various carbon products studied. In all of cases, the treated iron melts are characterized by low carbon (<3.%), silicon (<1.5%) and sulphur content (<.25%). The first laboratory experiment program was focused on the behavior comparison of amorphous (CPC) and crystalline (GPC, NG) carbonic materials. The carbon dissolution data presented in Figure 1 characterize the manner in which a variety of carbon raisers may be taken into solution in typical iron melt. Essentially, it appears that the carbon recovery (CR), carburizing efficiency (Ec), carbon dissolution rate (CDR) and specific carburization (Cs) depend on the structure (amorphous to crystalline ratio) and the purity of the specific carbon raiser. The laboratory scale experiments at relatively low melt temperature and carburizing time did not allow high level of the tested parameters, but at specific range depending on recarburizer type. The highest level of carbon-recovery parameters is typical for graphitized petroleum coke (crystalline synthetic graphite): higher purity grade, higher the rate of dissolution of these materials (GPC1 vs. GPC3). Despite its crystalline structure and high purity grade, mined natural graphite led to the lowest level of carbon dissolution in molten iron, in the same treatment conditions. Calcined petroleum coke products show an intermediate behavior, but closer to graphitized petroleum coke, in the experimental conditions. Partially graphitized and high purity recovered resistor petroleum coke (special selected) is characterized by a good behavior, as it is on the second place as efficiency in carburizing treatment. It could be comparable to lower purity synthetic graphite. It should be noted that if this material type contains higher amount of SiO 2 and SiC, carbon dissolution rate and recovery are visible lower. Sulphur contribution of tested carbon raisers depends on content in the added materials (see Table 1), but visible level is typical only for calcined petroleum coke products. The second experimental program developed under laboratory conditions was designed to compare the most representative amorphous and crystalline recarburizers at the one hand and partially graphitized (crystalline) recovered carbonic materials at different origin (packing Pet Coke or Met Coke), on the other hand. It was applied the same experimental procedure such as Program I was, but at higher level of iron melt temperature and carburizing time, for higher addition of recarburizer (see Table 2). The carbon and sulphur recovery parameters are summarized in Figure 2. The rate of dissolution of these materials is higher 129/4

5 than in Program I was, due to better conditions as temperature and time: more than 8% C-recovery, compared to 45-64% for the same materials type, in the first program. The crystallinity of these carbonic materials appears to be more important than their purity level to influence the carbon recovery parameters. It is clear that graphitized petroleum coke is better than only calcined petroleum coke. Very good results were also obtained for packing petroleum coke, despite higher ash and lower fixed carbon content (comparable with commercial synthetic graphite). Recovered graphitized metallurgical coke as packing material from high temperature treatment furnace is also better than calcined petroleum coke as C- recovery for the same dissolution rate despite high level of ash content. As specific sulphurization, the highest contribution is typically for petroleum coke, while the lowest level for graphitized petroleum coke (a little bit higher for recovered packing product). Graphitized packing metallurgical coke has an intermediate position, but closer to graphitized petroleum coke than only calcined material. The visible difference as ash content will differentiate these carbonic materials also as specific slag generation. Good relationship was established between ash content in the recarburizer and slag quantity generated in the melting furnace (CPC3=.8, GPC2=.13, GPCR1=.28 and GMCR1=1.68 kg/%c x ton, respectively). Prevalent crystalline structure and sulphur removal, as result of high temperature treatment of packing carbonic materials into the electric graphitizing furnaces sustain these materials as good recarburizers, especially for petroleum coke as origin. Also metallurgical coke could be considered in this position, but its specific high ash level will decrease dissolution rate at more slag volume production and higher sulphur contribution. Graphitized packing coke by products could be attractive for cast iron foundries, as possibility to obtain high performance in iron treatment, at lower cost. Table 3 and Figure 3 summarize the foundry results as carbon and sulphur dissolution and slag volume. The high stirring capacity of the 5Hz coreless induction furnace led to high rate of dissolution of these materials which exhibited recoveries of carbon near 1%, at relatively lower melt temperature ( o C). Despite its high ash content, graphitized packing metallurgical coke had a good behavior, especially for lower carbon content in the initial melt (see Table 3). No visible difference between calcined petroleum coke and recovered graphitized packing petroleum coke as carbon recovery. Clear difference appears to be as sulphur contribution and slag, but in opposite way: the highest sulphurization and the lowest slag volume are typically for calcined petroleum coke, while the packing by-products are characterized by lower sulphur and higher slag contribution (petroleum coke has intermediate position). The specific production of this foundry by the use of these byproducts includes grey and ductile iron castings: GPCR is usually used in 129/5

6 both grey and ductile iron field, while GMCR especially for grey iron production (also ductile iron, but desulphurization treatment is usually applied). Plant trials confirmed the laboratory experiment and sustain the possibility to use selected graphitized packing coke by-products as carbon raisers in cast iron foundries (inclusively cheaper metallurgical coke). LABVIEW6 thermo-analysis system was used, and chill tendency, concentrated shrinkage sensitiveness, mechanical properties and structure characteristics were evaluated during the first laboratory program. Eutectic undercooling degree ( Tm), recalescence degree ( Tr), maximum rate of recalescence (TEM) and the minimum value of the first derivative at the end of solidification (FDES) were found to be the representative parameters of the cooling curves capable to distinguish the solidification features of these cast irons. The highest undercooling level (21-22 C) was typically seen on treatments using natural graphite, while synthetic graphite products appear to determine a lower level of undercooling (1-13 C) and higher level of recalescence ( C). Calcined petroleum coke presents an intermediate position. When used, natural graphite was not effective as carbide avoidance. No obvious difference was found as far as chill tendency between calcined and graphitized petroleum coke materials, but the last group led to lower shrinkage incidence. Resistor petroleum coke as the least partially graphitized material which included silicon carbide as active component appeared to have the highest graphitizing influence, according to the lowest undercooling degree (1.4 C) and FDES level (-3.61 C/s) (Fig.4). Cast irons microstructure was more influenced by the crystallinity of carbonic materials rather than chill tendency: higher homogeneous grade of both graphite phase and metal matrix make-up for crystalline recarburizers addition in the furnace. Prevalent A-type graphite and pearlitic matrix are typical for synthetic graphite or graphitized resistor coke application. Mechanical properties, as tensile strength (Rm= N/mm 2 ) and Brinell Hardness ( HB) are in good relationship with microstructure characteristics and recarburizer type, respectively. At the same range of initial sulphur and manganese content (.16-.2%S, %Mn), lower level of equivalent carbon ( %) was tested during the second laboratory experiments program (II-Table 2). Better dissolution conditions (higher temperature and time) and higher recarburizer addition rate led not only to a lower chill tendency level, but also to more evident differentiation between carbonic materials efficiency (Fig. 5). Two types of samples were used, lower cooling rate (W 3 ½ Wedge Test) and higher cooling rate (4C Chill Test) samples (ASTM A367). In the experimented conditions, it is clear that the both graphitized petroleum coke products (granular synthetic graphite-gpc and packing coke as byproduct-gpcr) are more effective than calcined petroleum coke for the both cooling rate levels. Graphitized Packing Metallurgical Coke appears to be also more efficient than amorphous calcined petroleum coke. 129/6

7 Conclusions The results of this research illustrate that the various carbon raisers respond differently when they are applied to ferrous melt, not only from the perspective of carbon recovery parameters, but also as sulphur contribution and inoculation efficiency: Medium sulphur and high carbon content amorphous Calcined Petroleum Coke (CPC) is the reference carbon raiser for grey cast iron. It was confirmed that Graphitized Petroleum Coke (GPC) products are characterized by a more rapid solubility and higher C-recovery at an evident lower S-contribution and higher inoculation efficiency than CPC in grey cast iron. Graphitized Conductive Packing Petroleum Coke (GPCR) is superior to CPC as far as higher C-recovery, lower S-contribution and higher inoculation activity being close to GPC. Graphitized Conductive Packing Metallurgical Coke (GMCR) is generally superior to CPC as far as C-recovery performance but at the same level of dissolution rate. CMCR has an intermediary position as S-contribution and inoculation efficiency, usually closer to GPC. Graphitized Resistor Petroleum Coke (MCSX) at a high C-level and presence of a low amount of SiC is very efficient from the perspective of C-recovery, inoculation efficiency, and very low S-contribution. Table 4 illustrates the typical recarburizers, resulted from the present research program and validated by foundry applications. References 1. Coates R.B., Types, Selection and Applications of Recarburisers in the U.K. Foundry Industry, The British Foundrymen, August Loper C.R.Jr., Liu S.L.,Shrivani S. and Witter, T.H., The Dissolution of Carbon in Cast Iron Melts as Studied Using Commercial Raisers and Experimental Materials, AFS Transactions, 1984, pp Terrel P.B., Carbon Raisers and Cast Irons, Foundry Management and Technology, Aug and Sept Riposan I., Recarburisers for Cast Iron Foundries, Romanian Foundry Journal (RO), No.1, 23, pp Jentsch A., The Influence of Carburisers on the Microstructure, Quality and Overall Production Cost of Cast Iron Parts, 66 th World Foundry Congress, Istanbul, Turkey, 6-9 Sept.24, pp pub. Toksad: The Foundrymen s Association of Turkey, Salmon E and Jentsch A., Analyse Comparative de Produits Recarburants, Effect Inoculants et Trempe des Fontes Spheroidales, Hommes and Fonderie, April 23, pp Henning W.A, Comparing Crystalline and Noncrystalline Recarburizers in Ductile Iron Production, AFS Transactions, 1999, pp Suzuki S, AFS Transactions, 1982, pp Riposan I., High Quality Romanian Recarburisers for Cast Irons, Romanian Foundry Journal (RO), No. 5-6, 24, pp /7

8 Table 1 Carbonic Materials Characteristics Type (origin) Source Symbol Characteristics* Structure CPC1 98.7%FC, 2.5%S,.5%Ash Calcined Petroleum Coke Calcining Furnace CPC2 95.6%FC,.64%S, 1.5%Ash CPC3 98.3%FC,.87%S,.6%Ash CPC4 97.1%FC,.96%S, 1.6%Ash Graphitized Petroleum Coke [Synthetic Electric Graphitizing FurnaceProduct GPC1 99.3%FC,.7%S,.1%Ash Graphite] GPC2 99.7%FC,.6%S,.1%Ash Turnings GPC3 99.7%FC,.27%S,.68%Ash Electric Electrodes Graphitizing Furnace Packing Coke (to surround electrodes) [PC Petroleum Coke; MC Metallurgical Coke] GPCR1 97.4%FC,.1%S, 2.4%Ash GPCR %FC,.5%S, 2.%Ash GMCR1 87%FC,.34%S, 11.5%Ash GMCR2 83%FC,.19%S, 16.7%Ash Amorphous Prevalent Crystalline Partially Graphitized Resistor Coke (furnace insulator) Electric Electrodes Graphitizing Furnace MCSX 96%FC, 3.%SiC,.19%S Partially Graphitized Natural Graphite Mined Graphite NG 96.9%FC Crystalline * Fixed Carbon (FC) = 1 - (%Ash + %Volatile Matter + %H2O) Progr. Table 2 Laboratory Experiment Conditions I II Chemical Comp. of Iron Melt, wt.% (in/fin) Equiv. C, CE, Iron Temp. Recarburizer addition Recarb. holding in the furnace, min C Si S (in/fin, %) ( o C) Type (Tab.1) % mm Without Stirring Stirring CPC1, CPC2, GPC1 GPC3, MCSX, NG CPC3, GPC2, GPCR1, GMCR Fin. FeSi75 add. in furnace, (%) Table 3 Plant Trials Results Heat Metallic Charge, (%) Recarburizer C, (%) S, (%) C-Recov. Param. S-Recovery Parameters Heel Steel Scrap Type (Tab.1) Addition Rate, (%) Final iron Holding, ( o C / min) Metallic Charge Final Metallic Charge Final CR, (%) Ec, (%) Ss, (%S/1.%MC) CPC / GPCR / GMCR / S*s, (%S/%C) Specific Slag Quantity, (kg/%cxtiron) Table 4 Typical Designed Recarburizers Recarb. Type Thermal Treat., ( o C) Origin Fixed Carbon*, FC, (%) Sulphur S, (%) Ash (%) Volatile Matter (%) Moisture Spec. Contribution(1.wt.% add) (%) C (%) S (%) CPC Selected Calcined Petroleum Coke GPC >25 Graphitized Petroleum Coke <.5 GPCR >25 Petroleum Coke Conductive Packing Medium, in Electric Graphitizing Furnace GMCR >22 Metallurg. Coke, as Conductive Packing Medium, in Electric Graphitizing Furnace MCSX >15 Petroleum Coke as Insulator in Electric Graphitizing Furnace * Fixed Carbon (FC) = 1 - (%Ash + %Volatile Matter + %H2O) min.96 max..2 max.3. max..3 max <.15 min.82 max..5 max.17 min.96 FC 2-3 SiC max.1. max max..2 max.1. max..2 max < /8

9 C - Recovery Parameters, CR, Cs, CDR Calcined Pet.Coke CR (%) Cs (%C/1.%MCx1) CDR (%C/minx1) Graphitized Pet.Coke Resistor Pet.Coke Natural Graphite CPC1 CPC2 GPC1 GPC3 MCSX NG. Fig. 1 C-Recovery (CR), Specific Carburization (Cs) and C-Dissolution Rate (CDR) in Iron Melt (Program I) C - Recovery (CR), Specific Carburization (Cs) CR (%) Cs (%C/1.%MCx1) CDR (%C/min).15 CPC3 GPC2 GPCR1 GMCR C-Dissolution Rate (CDR) Specific Sulphurization, Ss; S*s Ss (%S/1.%MC) S*s (%S/%C) CPC3 GPC2 GPCR1 GMCR1. Fig. 2 C-Recovery and S-Recovery Parameters (Program II) C-Recovery (CR), Carburizing Efficiency (Ec) 1 % CPC4 GPCR2 GMCR2. CR Ec Specific Sulphurization, Ss; S*s Ss (%S/1.%MC) S*s (%S/%C) CPC4 GPCR2 GMCR2. Fig. 3 C-Recovery and S-Recovery parameters in 6.t/5Hz Coreless Induction Furnace (Plant Trial) 129/9

10 Chill Tendency (CC, TC); Shrinkage (SV) Clear Chill (CC, mm) Total Chill (TC, mm) Shrinkage Volume (SV, cm 3 ) Calcined Pet.Coke Graphitized Pet.Coke Resistor Pet.Coke Natural Graphite CPC1 CPC2 GPC1 GPC3 MCSX NG. Fig. 4 Clear Chill (CC), Total Chill (TC) and Shrinkage Volume (SV) of Un-Inoculated Grey Irons (Program I) 14 Chill Tendency, CC, TC Clear Chill (CC, mm) Total Chill (TC, mm) Wedge Test Chill Test W edge Test Chill Test W edge Test Chill Test W edge Test. Chill Test CPC3 GPC2 GPCR1 GMCR1 Fig. 5 Chill Tendency of Un-Inoculated Grey Irons (Program II) 129/1