Foseco Ferrous Foundryman''''s Handbook Part 4 pot

5.4,the flow of inoculant can be regulated either by optical detection of the startand end of iron flow via an optical module and fibre optic system or byconnecting the system to the pou

Electric melted irons require more inoculation than cupola melted irons.Electric melting will also produce low sulphur contents.

High steel scrap charges will require more inoculation

Where inoculated iron is held for more than a few minutes after inoculation,there is a need of a higher level of treatment

It is therefore difficult to give an accurate estimate of the amount of INOCULINwhich is required for every situation In general, INOCULIN additions of0.1–0.5% by weight of metal will be satisfactory for grey cast irons, higheradditions are needed for ductile (SG) irons (see p 79) Care must be takennot to over-inoculate grey irons, otherwise problems will arise with shrinkageporosity due to too high a nucleation level Many grades of INOCULINcontain high Si content, so that by adding 0.5% of inoculant, the siliconcontent of the iron will be raised by as much as 0.3%, this must be allowedfor by adjusting the Si analysis of the furnace metal

Control methods

The wedge chill test is a simple and rapid method of assessing the degree ofchill reduction obtained by the use of INOCULIN in grey cast irons Carriedout on the foundry floor, the wedge test is frequently used as a routinecheck even when full laboratory facilities are available The most commondimensions for the wedge are illustrated in Fig 5.2

Figure 5.2 The wedge chill test.

in

4 5 5

The wedge is made in a mould prepared from silicate or resin bondedsand After pouring, it must be allowed to cool in the mould to a dull redheat (c 600°C), after which it can be quenched in water and fractured The

width at the point where clear chill ceases, t, is measured and this gives a

good indication of the need for inoculation and of the effectiveness of an

66 Foseco Ferrous Foundryman’s Handbook

inoculation process In general, casting sections should be not less thanthree times the wedge reading if chill at the edges and in thin sections is to

of an automatic pouring machine at a computer-controlled rate using theIMPREX Station (see pp 73, 78) IMPREX wire is available in a range ofdiameters from 6 mm upwards

Late stream inoculation

With the increasing number of foundries where castings are made on highlymechanised moulding and pouring lines, the requirements for inoculationare becoming more difficult to meet Particular difficulties arise with theuse of automatic pouring furnaces where conventional ladle inoculation isnot possible A method of carrying out inoculation at the casting stage isneeded and this must be consistent and automatic in operation The MSI 90Metal Stream Inoculator is intended for use in these conditions

It is designed to add controlled amounts of inoculant to the liquid castiron just before it enters the mould The use of late stream inoculationtechniques leads to the virtual elimination of fading This permits a substantialreduction in the amount of inoculant used The inoculant addition therebyproduces a smaller change in iron composition leading to improvedmetallurgical consistency The cost of inoculation is also lower

The MSI 90 Stream Inoculator consists of two units, Fig 5.3, a control unitand a dispensing unit linked together by a special cable and air line assembly.The inoculant dispensing cabinet is located in a fixed position over themould being poured A storage hopper for the inoculant is mounted abovethe dispensing cabinet In the latest version, MSI SYSTEM 90-68E, Fig 5.4,the flow of inoculant can be regulated either by optical detection of the startand end of iron flow via an optical module and fibre optic system or byconnecting the system to the pouring furnace electrical signal used to regulatethe flow of liquid iron The monitoring system checks INOCULIN 90 level,dispensing tube status, inoculant flow, gate status, compressed air anddispensing unit temperature The monitor can automatically interruptpouring in the event of malfunction The control unit is fitted with a printerport allowing records to be kept The control cabinet is positioned in asecure, easily accessible place and may be some distance from the point ofinoculation

The MSI 90 Stream Inoculator can be operated in conjunction with avariety of types of pouring equipment:

Metal stream

Dispensing unit controlling flow of inoculant

Delivery tube for INOCULIN 90

to be added to metal stream

Mould

Figure 5.3 The principles involved in the MSI System 90.

Figure 5.4 MSI System 90 Type 68E.

pouring furnaces

ladle transporters

automatic ladle pouring devices

conventional ladles with fixed or variable pouring positions (providedthe latter is within a limited radius)

68 Foseco Ferrous Foundryman’s Handbook

The inoculant used in late stream inoculators must have a number of importantfeatures:

It must be a powerful inoculant

It must be finely divided to ensure free-flowing properties and rapidsolution

It must be very accurately graded, without superfine material whichwould blow away, or large particles which jam the gate mechanism

It must dissolve rapidly and cleanly to avoid the presence of undissolvedinoculant particles in the castings

INOTAB

cast mould

Inoculant

Ratio of cross-sectional areas:

INOTAB cast mould inoculant set in pouring basin

Figure 5.5 Application of INOTAB cast mould inoculant.

These requirements are met by INOCULIN 90, specially developed for thispurpose INOCULIN 90 is an inoculating grade of ferroalloy containingbalanced proportions of Si, Mn, Al, Ca and Zr, and is an excellent inoculantfor grey and ductile irons INOCULIN 90 should not be used for normalladle inoculation because of its very fine size grading.

Stream inoculation is very efficient since fading is eliminated The normaladdition rate for grey iron is from 0.03–0.20%, typically 0.1%, much lessthan would be used for ladle inoculation For ductile iron, addition ratesrange from 0.06–0.3%, typically 0.2%

INOPAK sachets are sealed paper packets containing 5, 10 or 20 g ofgraded, fast-dissolving inoculant which can be placed in the runner bush, atthe top of the sprue or in some other situation where there is a reasonabledegree of movement in the metal stream For most purposes, the additionrate should be 0.1%, i.e 5 g of INOPAK for each 5 kg of iron poured.INOTAB cast mould inoculant tablets are designed to be placed in therunner where they gradually dissolve in the metal stream as the casting ispoured, giving uniform dissolution This ensures that inoculation takes placejust before solidification of the iron Application is simple using core prints

to locate the INOTAB tablet

INOTAB tablets are normally applied at 0.07–0.15% of the poured weight

of iron The metal temperature and pouring time of the casting must beconsidered when selecting the tablet weight A minimum pouring temperature

of 1370°C (2500°F) is recommended It is important that the INOTAB tablet

is located where there is continual metal flow during pouring to ensureuniform dissolution and the typical application methods are shown inFig 5.5

Chapter 6

Ductile iron

Production of ductile iron

Ductile iron, also known as spheroidal graphite (s.g.) iron or nodular iron,

is made by treating liquid iron of suitable composition with magnesiumbefore casting This promotes the precipitation of graphite in the form ofdiscrete nodules instead of interconnected flakes (Fig 2.4) The nodular iron

so formed has high ductility, allowing castings to be used in criticalapplications such as:

Crankshafts, steering knuckles, differential carriers, brake callipers, hubs,brackets, valves, water pipes, pipe fittings and many others

Ductile iron production now accounts for about 40% of all iron castings and

is still growing

While a number of elements, such as cerium, calcium and lithium areknown to develop nodular graphite structures in cast iron; magnesiumtreatment is always used in practice The base iron is typically:

having high carbon equivalent value (CEV) and very low sulphur Sufficientmagnesium is added to the liquid iron to give a residual magnesium content

of about 0.04%, the iron is inoculated and cast The graphite then precipitates

in the form of spheroids It is not easy to add magnesium to liquid iron.Magnesium boils at a low temperature (1090°C), so there is a violent reactiondue to the high vapour pressure of Mg at the treatment temperature causingviolent agitation of the liquid iron and considerable loss of Mg in vapourform This gives rise to the familiar brilliant ‘magnesium flare’ duringtreatment accompanied by clouds of white magnesium oxide fume During

Mg treatment, oxides and sulphides are formed in the iron, resulting indross formation on the metal surface, this dross must be removed ascompletely as possible before casting It is important to remember that theresidual magnesium in the liquid iron after treatment oxidises continuously

at the metal surface, causing loss of magnesium which may affect the structure

of the graphite spheroids, moreover the dross formed may result in harmfulinclusions in the castings

Several different methods of adding magnesium have been developed,with the aim of giving predictable, high yields Magnesium reacts withsulphur present in the liquid iron until the residual sulphur is about 0.01%.Until the sulphur is reduced to near this figure, the magnesium has littleeffect on the graphite formation In the formation of MgS, 0.1%S requires0.076%Mg A measure of the true Mg recovery of the treatment process can

Magnesium content of treatment materials

Typical analysis of magnesium ferrosilicon nodulariser

RE (rare earths) contain approximately 50%Ce

Treatment methods include:

Sandwich ladle: the treatment alloy is contained in a recess in the bottom

of a rather tall ladle and covered with steel scrap The method is suitablefor use only with treatment alloys containing less than 10% Mg (Fig 6.1a)

Tundish cover: this is a development of the treatment ladle in which a

specially designed cover for the ladle improves Mg recovery and almosteliminates glare and fume (Fig 6.1c)

72 Foseco Ferrous Foundryman’s Handbook

Plunger: the alloy is plunged into the ladle using a refractory plunger bell

usually combined with a ladle cover and fume extraction (Fig 6.1d)

Porous plug: a porous-plug ladle is used to desulphurise the metal with

calcium carbide and the treatment alloy is added later while still agitatingthe metal with the porous plug

Converter: a special converter-ladle is used, containing Mg metal in a

Cover Alloy

Treatment alloy

Figure 6.1 Treatment methods for making ductile iron (a) Sandwich treatment (b) Pour-over treatment (c) Tundish cover ladle (d) Plunging treatment (e) GF Fischer converter (f) IMPREX cored-wire treatment station (g) In-mould system.

(f)

Magnesium chamber

Joint

Ingate to casting

or riser (cope or drag)

74 Foseco Ferrous Foundryman’s Handbook

pocket The ladle is filled with liquid iron, sealed and rotated so that the

Mg metal is submerged under the iron (Fig 6.1e)

Cored wire treatment: wire containing Mg, FeSi, Ca is fed mechanically

into liquid metal in a covered treatment ladle at a special station (Fig.6.1f)

Treatment in the mould (Inmold): MgFeSi alloy is placed in a chamber moulded

into the running system, the iron is continuously treated as it flows overthe alloy (Fig 6.1g)

All the methods have advantages and disadvantages; simple treatmentmethods can only be used with the more costly low-Mg alloys, generallycontaining high silicon levels which can be a restriction since a low Si baseiron must be used In order to use high Mg alloys and pure Mg, expensivespecial purpose equipment is needed so the method tends to be used only

by large foundries

A survey on ductile iron practice in nearly 80 US foundries in 1988 (AFS

Trans 97, 1989, p 79), showed that the biggest change in the previous 10

years was the increase in the use of the tundish ladle, used by over half ofthe foundries in the survey The growth had come at the expense of open-ladle, plunging, porous plug and sandwich processes More recently, cored-wire treatment has been developed and its use is growing

Melting ductile iron base

While the cupola can be used for the production of ductile iron, the need forhigh liquid iron temperatures and close composition control has encouragedthe use either of duplexing with an induction furnace, or using a corelessinduction furnace as prime melter

In the US survey referred to above, coreless induction furnaces wereused by 84% of the smaller foundries (producing less than 200 t/week).Almost all larger foundries duplexed iron from an acid cupola to an inductionfurnace, with channel furnaces being favourite

Cupola melting and duplexing

If magnesium treatment with MgFeSi alloy is used, a low Si base iron isneeded The process may be summarised as follows:

Melt in acid cupola, charge foundry returns and steel scrap plus lowsulphur pig iron if necessary

Tap at around 2.8–3.2%C

0.6–1.0%Si0.08–0.12%S

Desulphurise, using porous plug treatment with calcium carbide, to about0.10%S, carburise to 3.6–3.8%C.

Transfer to induction furnace, adjust C and Si and temperature to requiredlevels

Treat with MgFeSi and inoculate

Cast

Induction furnace melting

Charge foundry returns, steel scrap, ferrosilicon and carburiser to achievethe desired composition

If sulphur is below 0.025%, desulphurisation is not necessary, but thehigher the sulphur content, the more magnesium must be used so thecost of treatment increases

Treat with Mg and inoculate

Cast

If a converter or cored-wire Mg treatment is used, high silicon base irons aresatisfactory Separate desulphurisation is not necessary since, with theseprocesses it is economical to use pure magnesium as a desulphuriser

Use of the tundish cover ladle

The most commonly used treatment method, particularly in smaller foundries

is the tundish covered treatment ladle The principle is shown in Fig 6.1c.The use of a refractory dividing wall to form an alloy pocket in the bottom

of the ladle gives improved Mg recovery compared to a pocket recessed inthe bottom of the ladle Treatment batches are usually in the range, 450–

1000 kg Figure 6.2 shows the design of a ladle suitable for the treatment ofabout 450–500 kg of iron The diameter of the filling hole is chosen to minimisethe generation of fume while allowing the ladle to be filled quickly withoutexcessive temperature loss It is essential that the MgFeSi alloy is not exposed

to the liquid iron until quite late in the filling procedure, so the filling hole

is positioned to introduce liquid iron away from the alloy pocket in theladle bottom The Mg alloy in the alloy pocket is covered with steel turnings

or FeSi pieces of size 25 × 6 mm, then when the level in the ladle reaches thedividing wall, iron flows over and forms a semi-solid mass with the covermaterial allowing the ladle to be almost filled before the reaction starts, thusensuring good recovery of Mg

In order to minimise temperature losses during treatment, the ladle andcover should be separately heated with gas burners before assembly.Immediately before use, the ladle should be filled with base iron from themelting furnace and allowed to soak for a few minutes before returning theiron to the furnace The prescribed weight of MgFeSi alloy is charged through

76 Foseco Ferrous Foundryman’s Handbook

the alloy charging tube which is plugged after removal of the chargingfunnel The treatment alloy may be any of the MgFeSi alloys with Mg in therange 3–6%, additions of 1.5–3.0% are made giving Mg additions of 0.08–0.15% Tapping time is usually around 40 seconds

The temperature loss during treatment is around 50°C, so the tappingtemperature must be adjusted accordingly, treatment temperatures of around1530°C are commonly used After treatment, the tundish cover is removed,the metal transferred to a pouring ladle where inoculation may take place,then it is cast

Figure 6.2 Plan and cross-section of tundish/cover Iadle (From Anderson, J.V.

and Benn, D (1982) AFS Trans, 90, 159–162.)