Inoculation of cast irons: practices and developments

Transcription

1 Inoculation of cast irons: practices and developments Dr John Pearce CEng Prof MICME BSc PhD FIM National Metal and Materials, National Science and Tech Development Agency, Ministry of Science and Technology, Thailand Introduction Continuing developments in liquid metal treatment technologies have made major contributions to the quality, performance and reliability of metal castings. Advances in melt conditioning, in magnesium treatments to produce Ductile and Compacted Graphite Irons, and in inoculation techniques have ensured that Cast Irons remain key and competitive engineering materials. The combined interactions and influences of liquid metal treatments and other factors such as alloy composition, trace elements, section size & design and cooling rate etc. are covered in standard texts (1-3). The aim of this short review is to outline the development of inoculants and inoculation practices, and to also reflect on recent improvements in our understanding and practical control of graphite nucleation. In liquid iron preparation the usual sequence of treatments involves: Initial control of composition and inherent nucleation by melt preconditioning Mg treatment (when Ductile or Compacted Graphite Irons are being produced) Inoculation. The report begins by looking at the main aspects of inoculation and the materials used as inoculants, before commenting on how inoculation can be effectively integrated with the other treatments. Inoculation: Some key points Inoculation is the term used to describe the process of increasing the numbers of nucleating sites from which eutectic graphite can grow during the solidification of flake, nodular and compacted graphite irons. The main aim of inoculation is to minimize the degree of undercooling of liquid iron during eutectic solidification, and hence to make sure that the resultant cast microstructures are completely free from eutectic carbides. Inoculation also plays a major part in the control of eutectic graphite morphology and distribution, and hence in control of the levels of pearlite and ferrite in matrix structures. In flake graphite cast irons inoculation refines the eutectic cell size Fig. 1. Effect of inoculation on eutectic cell size and chill depth in wedge samples (schematic). (a) before inoculation (b) immediately after inoculation (c) fading due to holding time after inoculation before pouring (2) of the austenite-flake graphite eutectic reducing the continuity of the weak graphite phase and thereby increasing tensile strength. Inoculation is not normally required for higher carbon equivalent flake irons which are used to produce lower strength grades but it is a must for the lower carbon equivalents used for flake iron grades requiring minimum tensile strengths in the range of N/mm2. In flake graphite (grey) irons inoculation is used to: Prevent eutectic carbides, especially in thin sections and at corners Ensure a uniform distribution of fine type A graphite flakes Avoid the presence of undercooled graphite and the associated soft free ferrite in the matrix. The formation of eutectic carbide is called chill. It increases the tendency for fracture of the casting during handling or service and gives hard spots that seriously reduce machinability. Fig. 1 shows schematic views, using wedge test type samples, of the effects of inoculation in reducing chill and refining cell size in a flake graphite iron. Inoculation also avoids the formation of large eutectic cells consisting of the very finely branched form of graphite (undercooled graphite) that can grow at high degrees of undercooling when nucleation of the melt is low. Because of its high surface area undercooled graphite promotes the formation of a ferrite rather than a pearlite matrix (fig. 2). Free ferrite lowers strength, hardness and wear resistance. It also reduces machinability since it encourages the formation of a built up edge reducing both tool life and the quality of machined surface finish. In ductile irons eutectic carbides tend to form in the intercellular regions throughout the casting but are more likely to occur in thin sections (due to high cooling rates as in fig. 3) or heavy sections (due to segregation of carbide formers). The intercellular regions, in between the austenitenodule eutectic cells are the last zones to solidify and will contain elements which segregate into the liquid: notably manganese, chromium and other carbide forming residuals such as niobium, vanadium and titanium. Inoculation must ensure that sufficient numbers of graphite nodules are nucleated to prevent the formation of intercellular carbides, and also to encourage a high degree of nodularity. At low nodule numbers, the larger the graphite nodules the greater is the tendency for reduced nodularity. Unfortunately the effects of inoculation treatments are transient and they become reduced with time when the inoculated metal is held in the ladle before castings are poured. This is called fading. As illustrated in fig.1(c), fading in flake irons leads to a greater tendency to form 28 FTJ January/February 2008

2 chill, and to increases in eutectic cell size thus decreasing tensile strength. The effects of fading of three common inoculant materials in grey and ductile irons can be seen in fig. 4. It is seen that for both types of iron the rates of fading are highest immediately after inoculation. In ductile iron the reduction in nodule number with time will tend to promote less ferrite and more pearlite in the matrix. During the holding of treated metal intended for ductile iron production the problem of fading is even more serious since both Mg loss due to volatilisation and inoculant fade occur. If metal is held too long, without any back up treatment, this will result in unacceptable castings due to the presence of sub-nodular graphite. To reduce the effects of fading castings should be poured as soon as possible after inoculation treatment but often in production situations this cannot always be achieved. The problems of inoculant fade in flake and in ductile irons, the development of reduced fade inoculants, and of late treatments via pouring stream or in the mould techniques have therefore been and continue to be major areas for R&D, and for treatment technology improvements (4-7). Technical Paper Fig. 2. Undercooled graphite formed in grey iron as a result of low melt nucleation (x1000) Inoculant materials Foundries are often confused by the extensive range of both inoculants and nodularising agents that are available from the ferroalloy producers. Examples of some typical compositions are listed in Table 1. Most inoculants are based on ferrosilicons containing about 70-75% Silicon, or on ferrosilicon - graphite mixtures. In flake irons the normal levels of inoculant ladle addition raise the silicon content by about 0.2%, whereas in ductile irons larger additions are used, raising Si level by around 0.5%. Inoculant grades containing around 45-50% Si are also used where pick up of Si must be limited. Research into understanding the effects of inoculation, and into the development of more potent ferrosilicon compositions, has been continuing since the early 1960 s. Important observations from some of this work (4-10) can be summarised as follows: The effect of silicon on eutectic graphite nucleation and chill reduction is much more marked if the silicon is added as an inoculant than if it were just added to the furnace charge. The relationship between graphite nucleation and chill reduction is not simple one in that inoculants giving the finest eutectic cells (high cell counts) do not always give the greatest chill reduction. For ferrosilicon to be an effective inoculant then it must contain small amounts of minor elements such as calcium, aluminium, zirconium, cerium, barium, manganese and strontium. Lack of control in the use of inoculants can give rise to other problems such as shrinkage defects caused by excessive mould dilation, pinholes due to Al pick up, and inclusions of undissolved inoculant and slag. The rates of fading of inoculation treatments are most rapid during the first few minutes after treatment and the effects of the treatment are halved after about five minutes of holding. Barium containing ferrosilicons tend to be more persistent and can show a reduced tendency to fade in ductile irons. Graphitic carbons with suitable crystal structures can inoculate flake irons but not ductile irons. Amorphous carbons do not act as inoculants. It is difficult to effectively inoculate grey irons with sulphur contents below 0.05%, especially below 0.03%, using conventional ferrosilicon inoculants. Fig. 3. Presence of eutectic carbides in thin section ductile iron x750 Fig. 4. Fading of inoculation effect on eutectic cell counts in grey iron and nodule number in ductile iron (5,7) 1. FeSi with Ba and Ca 2. FeSi with Sr and low AI 3. FeSi regular foundry-grade FTJ January/February

3 Table 1: Approximate compositions of some typical Ferrosilicon based inoculant materials, in each case the balance is Iron The FeSi-RE type can also contain small controlled amounts of Oxygen and Sulphur to boost nucleation where high nodule numbers are needed in producing ferritic ductile iron. Magnesium ferrosilicons used as nodularising agents normally contain around 45%Si and have a range of Mg levels from 3 10%, some grades may contain up to 3%Ca, 1%Al, and 3%RE. Nodularising agents and inoculants are supplied in controlled size ranges to suit their intended modes of application e.g. 3-25mm for MgFeSi, 2-6mm for ladle inoculant, and for late in stream inoculant. Most commercial ferrosilicon based inoculants therefore contain up to around 5% of carefully controlled levels of elements (Ca, Al, Sr, Ba, Mn, Zr, Ce, etc.) that are capable of forming microinclusions of complex oxides or oxysulphides having suitable surface and crystallographic characteristics to heterogeneously nucleate graphite (11-12). The problem of inoculation of low S grey irons has grown as increasing numbers of foundries have replaced cupolas with electric induction melting, and in turn are using less pig iron and greater proportions of steel scrap in charges. For example, many automotive iron foundries in Thailand base their charges on very low S steel scrap from their automotive pressings neighbours. These foundries do not want to add S to their grey iron melts (allowing the use of normal levels of conventional grade ferrosilicons) since they also produce ductile irons and do not want to separate foundry returns. To meet the needs of such foundries the major inoculant producers have developed special ferrosilicon inoculants containing Sr and rare earth (RE) elements such Ce and La that can be used to treat very low S grey irons at relatively low addition rates. Likewise an inoculant containing zirconium has also been developed to gather nitrogen as zirconium nitrides and so prevent N related blowhole defects. Inoculation in the production of ductile irons A number of treatment methods have been devised to introduce magnesium into liquid irons of suitable composition, with Sulphur levels preferably below 0.02%, to ensure that the graphite phase grows in a nodular form (13-17). Magnesium is volatile and extremely reactive at liquid iron treatment temperatures so it is more conveniently added in the form of a carrier alloy to avoid the dangers of explosive reaction, to ensure economic and consistent recovery of magnesium, and to minimise fume. The first alloys used were based on Nickel-15% Magnesium but foundries can now choose from a wide range of treatment alloys many of which are now based on Magnesium Ferrosilicons containing between 3-10% Mg with 50-70%Si (13). Unalloyed magnesium can be used for safe and efficient treatment if specially designed treatment equipment is employed, e.g. converter or pressurized ladle, cored wire, etc (14-16). Treatment with pure magnesium is especially suitable for base irons of higher S content where combined desulphurisation and nodularising treatment is needed. Depending upon the purity of the base iron small amounts of cerium may be included with nodularising treatments in order to inhibit the effects of subversive trace elements such as antimony, arsenic, lead, tin, etc. If cerium is not used the presence of such elements can influence nodule formation and can lead to imperfect nodular graphite structures resulting in irons with inferior mechanical properties. Most ladle treatment methods involve the use of magnesium ferrosilicon alloys in specially prepared ladles as in the sandwich and tundish (covered ladle) processes (16-18). Compared to open ladle treatments, the use of a covered tundish ladle gives better Mg recovery with much less fume and glare (18). Commercial Mg ferrosilicons typically contain around 30 FTJ January/February 2008

4 3-10%Mg, 44-50%Si, with up to 2.5%Ca, and up to 2.5% RE (Ce, La, etc.). Calcium reduces reactivity increasing Mg recovery and provides some inoculation effect. The rare earths (RE) neutralise deleterious trace elements, assist in nodularization, and like Ca, reduce reactivity and provide some inoculation. The lower Mg content ferrosilicons can be used very effectively, via flow through reaction chambers, tundish type ladles, or in the mould treatments, giving high levels of Mg recovery (due to low reactivity) together with some inoculation effect. Regardless of the form of Mg treatment employed, the treated iron must inoculated with a suitable inoculant to prevent eutectic carbides and to encourage a uniform distribution of well formed graphite nodules throughout the structure, the amount of inoculant used depending on the nature of the prior Mg treatment. For example, treatment with pure Mg or Ni-Mg type alloys requires the use of larger inoculant additions for effective nucleation. As mentioned earlier, treated iron must be poured into moulds as soon as possible after treatment so that the effects of both Mg loss and inoculant fade are minimised. There are a number of post-ladle or late inoculation treatment systems that have been developed to avoid these fading problems. These include metering measured amounts of controlled size inoculant from a dispensing unit to the pouring stream as it enters the mould, and the use of inoculant containing cored wire in a fed-wire arrangement, the latter being used particularly in automatic pouring systems. Cored steel wires from 5-13mm in diameter are used to transport powdered treatment alloy or inoculant into the iron at the pouring station. Experience in the use of such systems has shown that they can replace all or part of the ladle inoculation resulting in savings in the amount of inoculant used and reduced Si increments in the iron. These treatments are consistent and reproducible but it must be remembered that they can only back up or replace ladle inoculation if there is sufficient residual Mg left in the iron. If Mg treatment has been ineffective (e.g. too high a treatment temperature giving low recovery) or if too much Mg has been lost because of delays in pouring then good nodularity cannot be obtained even if the degree of nucleation is high. An alternative approach is to use in the mould inoculation where the inoculant is placed in the pouring bush or in a cavity in the runner system, this latter technique also being used for Mg treatment. In this case a specially designed treatment chamber is incorporated into the filling system to make sure that the treatment alloy (Mg alloy +inoculant) dissolves uniformly, treating consistently all the metal that enters the mould. Ceramic filters are also used in the system such that only clean, correctly treated metal enters the casting cavity. Each mould is a separate treatment and this can present greater inspection problems than ladle treatment. The in the mould type treatment is most suitable for long runs of simple shaped castings where automatic assessment of nodularity can be easily made. Pre-conditioning of cast iron melts Pre-conditioning involves the treatment of liquid iron before nodularising and/or inoculation treatments. The aim is to promote the formation of type A graphite in grey irons and the formation of nodular graphite in ductile irons by ensuring that melts have consistent and sufficiently high states of nucleation. Improvements in our understanding of the roles of Oxygen and Sulphur levels and associated oxy-sulphide micro-inclusions in graphite nucleation (11,19), and of the influences of recarburiser characteristics on nucleation potential (20-22), coupled with developments in interpreting the data from cooling curves during thermal analysis (20,21,23) have encouraged foundries to pay much more attention to melt preparation. In particular work on the use of recarburisers has shown that foundries can improve the metallurgical condition of melts in terms of control of Carbon Equivalent and nucleation potential if graphitic carburisers with high degrees of crystallinity are used in place of amorphous carbons (22). Pre-conditioning has proved its effectiveness in the production of ductile irons (20,21,24). For example it is reported (24) that with pre-conditioning the spread of hardness and % elongation values were Hb and 18-24% respectively compared to values of Hb and 10-24% without pre-conditioning. Also in producing ferritic ductile irons pre-conditioning with very small additions of Cerium (20-60ppm) and Bismuth (10-20ppm) has been recommended to ensure sufficiently high nodule numbers. With the increasing interest in reducing wall thicknesses in iron castings and in producing Compacted Graphite Irons (CGI) without the use of Ti, pre-conditioning of melts, computer aided thermal analysis, and improved Mg and inoculation treatments will remain the key areas for future developments. For example, attention has been drawn to the care needed in inoculating CGI (25) where inoculation must prevent eutectic carbides without promoting the formation of graphite nodules. This work has also shown that with correct pre-conditioning, if a suitable inoculant material is used in place of steel as the cover in sandwich ladle treatment, then it is possible to produce satisfactory CGI structures without any post-inoculation. It has also been found that CGI can be successfully produced without Ti by using fed wire treatment with Ce bearing MgFeSi (26). In spite of many advances in the understanding and technology of inoculation treatments, some foundries still do not pay sufficient attention to the correct use of inoculants. For example, putting the inoculant at the bottom of a slag covered ladle (instead of adding it to the pouring stream during filling of a clean properly maintained ladle) and then wondering why there are problems with carbides and inclusions. The do s and don ts of inoculation practice are well documented (e.g. 27). Foundries with inoculation problems should revisit these and also seek the detailed technical advice readily available from the major ferroalloy and inoculant suppliers to make sure that they are using the best inoculant for the job, and using it in the correct way. References 1. Angus H T, Cast iron: physical and engineering properties. 2nd edition, Butterworths, London, UK. (1976). 2. Pearce J T H and Dudley J J. Foundry Metallurgy. Chapter 3 in Fundamentals of Foundry Technology. Edited by P.D. Webster. Portcullis Press, Redhill, UK. (1980). 3. Elliot R. Cast Iron Technology. Butterworths, London, UK. (1988). 4. Lownie H W. Barium inoculants resist fading. Foundry 1963, vol. 91, Moore A. Some factors influencing inoculation and inoculant fade in flake and nodular graphite irons. Transactions of the American Foundrymen s Society, 1973, vol. 81, Patterson V H and Lalich M J. Fifty years of progress in cast iron inoculation. 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5 7. Fuller A G. Fading of inoculants. Proceedings of Modern inoculating practices for gray and ductile iron, Rosemount, Illinois, USA. February 1979, AFS/CMI, Session 8, Dawson J V. The stimulating effect of strontium on ferrosilicon and other silicon containing inoculants. Modern Castings 1966, vol. 49, Dawson J V and Maitra S. Recent research on the inoculation of cast iron. British Foundryman 1967, vol. 60, Clark R A and McCluhan T K. American experience with a new strontium inoculant in gray iron. Modern Casting 1966, vol. 49, Skaland T. Inoculation of ductile iron and the mechanisms of graphite nucleation. Proceedings of best practices in the production, processing and thermal treatment of castings, October, 1995, Raffles City Convention Centre, Singapore. 23/1-23/ Loper C R Jr. Inoculation of cast iron: a summary of current understanding. Proceedings of the 6th Asian Foundry Congress, January, 1999, Calcutta, India, Morrison J C. What s in a name nickel and ductile iron. Foundryman 1998, vol.91, Hauke W. Production of SG Iron with the pure magnesium converter. Foundryman 2000, vol.93, Hasse S and Hauke W. Procedure for the production of ductile iron. Giesserei-Praxis Special Supplement, October 2002, Berlin, Germany, 23pp. 16. Lerner Y S. Overview of ductile iron treatment methods. Foundry Trade Journal 2003, vol.177, Part 1 September, p19-22; Part 2 October, p Deelokcharoen K. Magnesium treatment of ductile iron. Proceedings of the Third Thai Foundry Conference, 23 November 2006, Thai Foundrymen s Society, Bangkok, Thailand, pp.4/ Forrest R D. The progress and industrial acceptance of the covered ladle for ductile iron production. British Foundryman 1982, vol.75, Best K J. Oxygen and Sulphur metallurgical partners in cast iron melts. Foundry Trade Journal 2004, vol.178, Loper C R Jr. The pre-conditioning of cast iron melts and its use in safety components production. Foundry (India) 2006, vol.18, Loper C R Jr. Preconditioning effect from crystalline recarburisers and their use in safety cast components production. Foundry Trade Journal 2006, vol.180, Jentsch A. Influence of recarburisers in the microstructure and properties of cast components: cost analysis. Foundry Trade Journal 2006, vol.180, Sillen R. Adjusting cast iron melts using Active Carbon Equivalent. Foundry (India) 2002, vol.14, Forrest R D and Mullins J D. Achieving and maintaining optimum ductile iron metal quality. Foundry (India) 2003, vol.15, Ecob C M and Hartung C. An alternative route for the production of compacted graphite irons. Proceedings of the Eighth Asian Foundry Congress October 2003, Bangkok, Thailand, pp Fallon M J. Experiments on the treatment of compacted graphite iron. Foundry Trade Journal 2004, vol.178, Ladle-inoculation practice. BCIRA Broadsheet FTJ January/February 2008