When and How to Choose Ferroalloys for Cast Iron Foundry

When and how to choose ferroalloys for cast iron foundry

R. R. Dash S. Ghosh G. N. Rao'

ABSTRACT

Ferroalloys find extensive use in foundry for various specific purposes. The present paper discusses the use of ferroalloys in cast iron foundry as an, additive to moulding sand, as an inoculant for the production of graded cast irons, as an alloying element for the production of alloy cast irons, also as an,inoculant for the production of special cast irons such as S. G. iron, Malleable iron, compacted graphite cast irons. The paper also discusses the scope of production of complex inoculants as a present day need for foundry industry.

Introduction classified into two main classes • the conventional ferro-alloys and the special ferroalloys. Conventional ferroalloys like, Fe-Mn, Fe-Si, Ferroalloys find extensive use in foundry Fe-Cr have been in use for a long time while because they are comparatively cheaper and special alloys such as Fe-Mo, Fe-V, Fe-Ti etc. considerably easier to add to the molten iron and have been developed recently to cater to the steel. Further, their specific gravity and melting special needs of Iron and Steel production. points approach those of iron and steel which facilitate their use. Since the pertinent member In a cast iron foundry, ferroalloys are used of a ferroalloy is the alloying element, the iron for the following purposes content is relatively not very important, thus, they are identified generally by the principal 1) As an additive to moulding sand base metal present. 2) As an inoculant for the production of graded In any particular ferroalloy, gradation is cast irons. normally done according to the percentage of 3) As an alloying element, either in furnace cr the base metal present, Further gradation or sub- ladle for production of alloy cast iron. division is usually based upon the specific elements of technical importance like carbon, 4) An an inoculant for the production of spe- silicon, phosphorous, sulphur, etc. From the cial cast irons such as S. G. Iron, Malleable point of view of its use, ferroalloys can be iron, compacted graphite cast iron, etc.

• Authors are Scientists in the National Metallurgical Laboratory, Jamshedpur.

278 As an additive to moulding sand Fe-Si and Ca-Si addition to grey iron. Fig. 2 shows the influence of the inoculant on eutectic Though use of ferroalloys in mould prepa- cell count in grey iron. As silicon percentage ration is un-usual, Fe-Si is used as a hardener for is increased, the eutectic cell count in grey iron sodium silicate bonded sands to produce self set also increases. Fig. 3 depicts the influence of moulds. The process relies upon exothermic type of inoculant on decay in inoculation effect reaction between sodium silicate and pulverised (chill depth) in grey cast iron. It is seen that silicon causing dehydration of binder which Ba-Si is a more potent inoculant compared to causes the mould to harden. The strength obta- Fe-Si. ined is mainly due to the formation of a glassy silicate and silica gel. 0.3 • Fec Si However, due to some disadvantages such * a 5, as defects produced by liberated hydrogen, avai-

lability of cheaper hardeners, etc. the process % 0.2 has become less popular. Si. IN As an Inoculant for production of SE REA

graded cast iron NC 0.1 I

The purpose of inoculation of molten metal is to modify its microstructure on solidification

and thereby improve mechanical and physical 6.12 12.70 70.140 1350 1450 1550 properties which are not explainable in terms of GRAIN SIZE, MESH TEMPERATURE,.0 composition change. Ferroalloys, particularly FIG. 1: INFLUENCE OF INOCULANT MESH SIZE AND silicon based, improve the mechanical properties INOCULATING TEMPERATURE ON RECOVERY of cast iron by changing the shape, size and OF Fe-Si AND Ca-Si IN GREY IRON distribution of graphite flakes.

The usual silicon based inoculants are Fe-Si 9300 containing 75 to 80% Si, calcium silicide conta-

ining 60% Si and alloys of silicon with other

N. 8400 metals such as Mn, Fe, Zirconium or copper. I Ferro silicon acts as a good inoculant only when

R SG. 7 it contains 1-2% aluminium and upto 1.0% cal- PE

cium. There is a risk with aluminium containing S, 6 inoculants as the molten cast iron may pick up ELL C hydrogen from moisture in the mould, thus encouraging the development of sub-surface TIC 5700 pinholes in the castings. Manufacturing process TEC U of Fe-Si must aim at producing ferrosilicon with E 4800 required level of aluminium for use in cast iron foundries. Studies have shown that inoculating 3900 0 0.2 0.4 0.6 power of aluminium free Fe-Si can be increased Si , INCRFMENT.% by incorporating small percentage of strontium in ferrosilicon. FIG. 2: INFLUENCE OF AMOUNT OF ADDED INOCULANT ON EUTECTIC CELL COUNT IN Fig. 1 shows the inoculant mesh size and GREY IRON inoculating temperature on the effectiveness of

279 Since ferroalloy addition is expensive, they B. 51 must be added only when essential after making Fe 51 nl a judicious selection of the ferroalloy, based on metallurgical considerations.

Si: Fe Si 50:50 Selection of ferroalloys o. a 2 Ferroalloys like Fe-Si, Fe-Mn, Fe-Cr, Fe Mo, 6.51 : Fe SI 20 : BO Fe-Ti, Fe-V, are extensively used in the process control of grey iron melts to obtain special

characteristics and produce castings to partcular 6 9 12 15 MINUTES. AFTER INOCULATION specifications. FIG 3 : INFLUENCE OF TYPE OF INOCULANT ON In case of Fe-Si, low grade as well as high DECAY IN INOCULATION EFFECT (CHILL DEPTH) IN GREY IRON grade is in use in the foundries. Apart from the silicon content, size as well as composition, As an alloying element for production of especially, the aluminium content of the ferroalloy cast iron alloy should be within specified limits. The aluminium content should not exceed 1.0%, Ferroalloy additions in melt preparation 60% grade is used for bulk furnace additions in lump form, while 75% or 85% silicon grades are The selection of a particular grade of ferro- used for ladle additions. alloy for use as alloying addition depends upon the type of melting unit employed, the alloy High carbon ferro-manganese containing composition and the specification of the castings 70% manganese is used in grey iron foundries, to be produced. In jobbing foundries where a though low carbon variety has added advantage cold blast cupola is available, ferroalloys such when used in large quantities as they have low as Fe-Si, Fe-Mn, which are higher in Si and Mn phosphorous content facilitating final adjust- are preferred due to their lower melting tempe- ment. Normally lumps are used for furnace ratures. Ferroalloys can either be added in additions while 4 to 15 mm sized ferroalloys are cupola or in the ladle. In the later case exces- used for ladle additions. Pearlite is stabilized sive loss of ferroalloys can be prevented. In using small quantities of Fe-Mn along with Tin. modern foundries where electrical melting units Ferrochrome is a carbide former and when are available, additions of ferroalloys can be used in small quantities increases tensile stren• made late in the furnace for higher recovery. gth and hardness. Usually high carbon ferro- The losses encountered in the process of chrome is used in lump form for furnace addition addition, particularly during the cupola melting and for ladle addition, 50 mm size is used. of iron are as follows : Fe-Mo is added to impart high wear resis- Elements in Ferroalloy % Losses tance. Molybdenum is specially effective in strengthening and hardening irons because of Silicon 10 to 15 its property of causing austenite to transform to Manganese 15 to 20 fine pearlite or bainite. Lumps are used for fur- Chromiurn 10 to 15 nace additions while crushed to 20 mm size is used for ladle additions. Titanium ... 20 to 30 Molybdenum No Loss The behaviour of titanium, when Fe-Ti is Vanadium ... 10 to 15 added is similar to aluminium. Titanium added

280 upto 0.05 to 0.25% promotes graphitization, have been grouped as follows for practical pur- reduces chilling tendency and refines the gra- poses : phite flake size. Above this value, however, increasing titanium percentages results in the 1) 15-17% Cr for heat and wear resistant appli- formation of titanium carbide, which is stable cation. even at high temperatures. 2) 26-28% Cr for abrasion resistant applications. Vanadium acts as a carbide former when added as Fe-V. Small additions to the extent of 3) 30-35% Cr for heat and corrosion resistant 0.05% increases the strength, hardness, and application. impact strength of grey irons. Corrosion resistance property of cast iron can not be generalised with particular composi- Ferro-alloys in alloy cast irons tion range as there are a number of corrosive Alloy cast irons are classified depending on agents with varying concentrations, temperatures their specific use as corrosion resistant, heat and conditions. It is, therefore, better to develop resistant and wear resistant cast irons. the different grades of corrosion resistant cast iron depending on their actual use in service. Silicon as Fe-Si when added to cast iron in For example, cast irons containing upto 2% nick- the range of 6 to 8% Si imparts better resistance el and 0.5% chromium resists caustic alkali to scaling compared to ordinary irons. With 13 attack. 'Ni-resist' austenitic iron containing, Ni to 18% Si, cast irons develop resistance to acid 1.2 to 16.0%, Cr. 1.5 to 4.0%, Cu 6.0 to 8.0% corrosion. has been found satisfactory to withstand sulphu- ric acid corrosion. Cast irons containing 33%Cr The property of heat resistance is improved and just over 1% of carbon retains enough chro- by alloying grey iron with chromium as Fe-Cr; or mium in the solid solution to ensure good resis- silicon as Fe-Si. Other elements which increases tance to oxidising acids. High silicon cast iron carbide stability or raise the critical temperature having 1.19% Cr, 15.0% Si, and 0.118% Ce range are also used. exhibits good resistance to acid attack and can The aim in manufacturing wear resistant also be used against other corroding media. alloys is that their microstructure should contain Ferroalloys in malleable iron production the largest possible quantity of hard constitu- ents, carbide and martensite. However, in Malleable iron castings are in fact, heat trea- applications where toughness or impact resista- ted products obtained from white iron. The ferro- nce is also important, it is essential that the size alloys which are extensively used in the produc and distribution of the hard phases should be tion of malleable iron are Fe-Mn and Fe-B. well controlled and product should be free from internal stresses, micro-cracks, and other struc- In ferritic malleable cast iron, too high Mn tural discontinuity. Chromium is the principal content is undesirable and is kept deliberately alloying element which imparts wear resistance; low, only to normalize sulphur content. In chromium addition in wear resistant cast irons pearlitic malleable irons, 0.7 to 0.9% Mn is vary from 2 to 28%. In addition to chromium, tolerated since it suppresses second stage gra- manganese upto 3% is also used alongwith Cu, phitization. The addition of Fe-Mn therefore Mo, V and Ti. While Table - 1 gives the details must be made judiciously. of wear resistant and heat resistant cast irons commercially available, Table - 2 gives the mic- The effect of Boron in malleable iron is two rostructure and commercial applications of high fold. It neutralizes small amounts of chromium chromium cast irons. High Cr alloyed cast iron but more important it acts as an inoculant. It is

281

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0 0 0 0 0 0 0 0 MI C) CO 0 0 0 0 co 6 •ct 0 r- CV CY) 0 I 6 6 6 6 N N N I I 0 CO N a strong nitride former. Boron nitrides or carbo- sium and Fe-Si Mg master alloys for the produc- nitrides formed during solidification act as cen- tion of S. G. Iron. tres of crystallization and reduce the annealing time. Too high a Boron content reduces the Compacted graphite iron is produced by the

mechanical properties . the simultaneous action of a nodulariser and a denodulariser. For this purpose master alloy is Ferro-alloy in S. G. and C. G. cast iron prepared using 75% Fe-Si grade, Magnesium production and Ferro-titanium. Composition of the master alloy is same as that of alloy used for the pro- Ferre-alloys are being extensively used for duction of SG iron except that it contains 4 to making master alloys for the production of 4.5% Ti which acts as denodulariser. spheroidal graphite cast iron. S. G. cast iron is essentially a cast iron in which graphite is pre- Production of complex inoculants sent in the form of tiny balls or spherulites rather than flakes as in normal grey iron. Metallurgical considerations lead to the development of complex inoculants which pro- This shape of graphite can be obtained by duce irons of higher strength and uniform micro- inoculating grey iron with Mg. Since Mg is a structure. Multi component alloys such as very reactive metal and its vapour pressure is Si-Ca-Mg-Fe have advantages over Si-Mn and very high at the temperature of the molten cast other types of alloys, since they make it possible iron, it is added in the form of a master alloy. to produce high strength irons with no black Chemical composition of the master alloys is spots or chill crystals and with better casting shown in Table - 3. properties. However, it is almost impossible to melt inoculants with increased Mg contents by TABLE-3 carbo thermal process at atmospheric pressure Chemical composition of master alloys owing to high vapour pressure of Mg at the pro- for S. G. Iron cessing temperature. Complex inoculants with a Si-Ca-Mg-Fe composition is reported to have 1. Magnesium-Nickel Alloys : % Composition been melted on an industrial scale by silico Mg Ni Si thermal method following a method based on a) 15 85 the technology of producing Ferrosilicon calcium. b) 15 55 30 2. Magnesium-Ferro Silicon Alloys : Composition Barium or strontium containing inoculants are also reported to have been produced like- Mg Si Ca Ce Fe wise, where-in the charge materials consisted of a) 9 45 1.5 — Balance 200 Kg. of lime, 190 Kg. Fe-Si, 50 Kg. fluorspar b) 9 45 1.5 0.50 Balance and 100-122 Kg. of barium sulphate. The resul- 3. Magnesium-Silicon Alloys :% Composition ting alloy analysed as :

Mg Si Ca Ce Fe % Ba %, Ca % Fe % Mg. a) 18 65 2.0 0.60 Balance 5.8 15.0 27.0 1.2

After melting with Mg, the composition of Fe-Si of 75% grade is made use of for the complex inoculant included : preparation of the master alloys. National Metal- lurgical Laboratory has developed technical % Ba %, Ca % Fe Mg. knowhow for the production of nickel magne- 5.1 -5.7 12 - 14 24 - 26 5 - 6.

283 The utilization of Mg was 82%. The cost suggestions and permission to present this paper of the alloy containing 8-12% Mg produced by in the seminar. melting was lower than the cost of the alloy made by the silico-thermal method using dolo- The authors also thank Dr. R. Kumar, Scien- mite. tist-Director for his continued encouragement.

With the use of complex inoculants high References strength iron could be produced without using autoclaves for introducing Mg. Cast iron foun- 1) Summer School on Advanced Course on dry will be largely benefitted if such alloys are Metal Melting, May-June '77, organised by produced. R. I. T., Jamshedpur. Acknowledgement 2) Principle of Metal castings - By Heine, The authors wish to thank Prof. V. A. Loper Et Rosenthal, Mcgraw Hill Publication Altekar, Director, National Metallurgical Labora- tory, Jamshedpur for his keen interest, valuable 3) Stal in English No. 12, December, 1970.