Foseco Ferrous Foundryman''''s Handbook Part 13 pdf

Filtration and the running and gating of steel castings 293Figure 18.16 KALPUR ST unit: a for side feeder application; b for top feeder application.. The application of the theory of hea

290 Foseco Ferrous Foundryman’s Handbook

differences between conventional systems and direct pour practice Theexamples shown in Fig 18.13 illustrate the benefits obtained by the application

of a KALPUR ST unit to a valve casting

Selecting the proper size and positioning the KALPUR unit

There are two basic designs of unit, one for side feeder application and onefor top feeder application, Fig 18.16a, b KALPUR ST units are supplied

in a range of sizes (Fig 18.11) having capacity from around 30 kg to over

250 kg of carbon or low alloy steel and more for high alloy steel (Table 18.4).Metal temperature, ladle practice (bottom versus lip pour), melting practice(cleanliness), metal deoxidation, and steel composition all determine theamount of metal that can be poured before filter blockage that would preventproper pouring or adequate feeding after the mould is filled

Factors resulting in low capacities and flow rates:

Figure 18.12 The KALPUR ST unit is placed as close to the casting cavity as possible to preserve laminar flow from the filter.

Filtration and the running and gating of steel castings 291

Figure 18.13 (a) Temperature distribution within a valve casting immediately after filling through a conventional gating system Large areas are much colder than the last metal poured (b) Temperature distribution within a valve casting immediately after filling through a KALPUR unit shows higher overall temperature, illustrating reduced heat loss when direct pour units are used (This figure is reproduced in colour plate section.)

(a)

(b)

Temperature Distribution in

the Casting at the End of

Filling … Conventional Gating

Temperature Distribution in

the Casting at the End of

Filling … Direct Pour

292 Foseco Ferrous Foundryman’s Handbook

Figure 18.14 KALPUR unit used as a central feeder.

Conventional method

New KALPUR ST direct pour system

Figure 18.15 The KALPUR ST direct pour system eliminates the conventional running system and reduces feeder requirements.

Filtration and the running and gating of steel castings 293

Figure 18.16 KALPUR ST unit: (a) for side feeder application; (b) for top feeder application.

Table 18.4 Capacity and flow rates of KALPUR ST units

Capacity range (kg) Nominal flow rate (kg/s)

(at 300 mm metallostatic pressure) KALPUR High level of Low level of Carbon Stainless

Molten metal containing large quantities of inclusion material

Low metal pouring temperatures

Low metallostatic pressure on filter

A minimum pouring temperature of 1580°C for carbon steel and 1520°C forstainless steel is recommended to ensure priming

294 Foseco Ferrous Foundryman’s Handbook

The KALPUR ST system acts as an insulating feeder sleeve of similarsize The presence of a filter in a unit does not affect the feed efficiency Anefficient anti-piping compound should be applied to the KALPUR STimmediately after pouring is finished Recommended materials are:

KALPUR ST 50 and 75 FERRUX 16

The KALPUR unit is rammed into position It should be placed as close

to the casting as possible and ideally should be located no more than

150 mm above the impact area (Fig 18.12)

Cost savings through the use of

STELEX and KALPUR

An analysis of steel casting cleaning costs has been undertaken by the Institutfur Giessereitechnik, Dusseldorf covering a number of steel foundriesrepresenting 10% of German steel casting production The work showedthat the cleaning costs associated with a casting can amount to 40% of itstotal manufacturing cost Cleaning costs were further broken down as follows:

Reworking of casting defects 37%

(inclusions, penetration, shrinkage)

Removal of feeding and gating systems 27%

(fins, flash and burn-on)

Of these costs

52% were considered as avoidable31% were considered as possibly avoidable17% were defined as unavoidable

Filtration and the running and gating of steel castings 295

The use of STELEX ZR filters and KALPUR ST units enables major reductions

to be made in the avoidable cleaning costs through simplification of gatingand feeding systems and the removal of inclusions

To avoid shrinkage porosity, it is necessary to ensure that there is a sufficientsupply of additional molten metal, available as the casting is solidifying, tofill the cavities that would otherwise form This is known as ‘feeding thecasting’ and the reservoir that supplies the feed metal is known as a ‘feeder’,

‘feeder head’ or a ‘riser’ The feeder must be designed so that the feed metal

is liquid at the time that it is needed, which means that the feeder mustfreeze later than the casting itself The feeder must also contain sufficientvolume of metal, liquid at the time it is required, to satisfy the shrinkagedemands of the casting Finally, since liquid metal from the feeder cannotreach for an indefinite distance into the casting, it follows that one feedermay only be capable of feeding part of the whole casting The feedingdistance must therefore be calculated to determine the number of feedersrequired to feed any given casting

The application of the theory of heat transfer and solidification allowsthe calculation of minimum feeder dimensions for castings which ensuressound castings and maximum metal utilisation

Natural feeders

Feeders moulded in the same material that forms the mould for the casting,usually sand, are known as natural feeders As soon as the mould andfeeder have been filled with molten metal, heat is lost through the feedertop and side surfaces and solidification of the feeder commences A correctlydimensioned feeder in a sand mould has a characteristic solidification pattern:that for steel is shown in Fig 19.1 the shrinkage cavity is in the form of acone, the volume of which represents only about 14% of the original volume

Feeding of castings 297

of the feeder, and some of this volume has been used to feed the feederitself, so that in practice only about 10% of the original feeder volume isavailable to feed the casting The remainder has to be removed from thecasting as residual feeder metal and can only be used for re-melting

Figure 19.1 Solidification pattern of a feeder for a steel casting (schematic).

Aided feeders – feeding systems

If by the use of ‘feeding systems’ the rate of heat loss from the feeder can beslowed down relative to the casting, then the solidification of the feeder will

be delayed and the volume of feed metal available will be increased Thetime by which solidification is delayed is a measure of the efficiency of thefeeding aid The shape of the characteristic, conical, feeder shrinkage cavitywill also change and in the ideal case, where all the heat from the feeder islost only to the casting, a flat feeder solidification pattern will be obtained(Fig 19.2) As much as 76% of an aided feeder is available for feeding thecasting compared with only 10% for a natural sand feeder This increasedefficiency means greatly reduced feeder dimensions with the followingadvantages for the foundry:

A greater number of castings can be produced from the given weight ofliquid metal

Smaller moulds can be used, saving on moulding sand costs

A reduction in the time needed to remove the feeder from the casting ispossible

More castings can be mounted on the pattern plate and thus into themould

Less metal is melted to produce a given volume of castings

Maximum casting weight potential is increased

Smaller feeders mean less fettling time and cost

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Feeding systems

Side wall feeding aids are used to line the walls of the feeder cavity and soreduce the heat loss into the moulding material For optimum feedingperformance, it is also necessary to use top surface feeding aids These arenormally supplied in powder form and are referred to as anti-piping or hot-topping compounds Figure 19.3 illustrates how the use of side wall and topsurface feeding aids extends the solidification times Now, however, suitableinsulating discs (lids) are increasingly being used in place of hot-toppingcompounds for environmental reasons For mass production castings 90%

of feeders are closed or ‘blind’

Figure 19.2 Ideal feeder solidification pattern where all the heat from the feeder has been lost to the casting (schematic).

Solidification time Sand wall

open top

KALMINEX sleeve open top 29.2

minutes

39.8 minutes

Sand wall FERRUX cover

KALMINEX sleeve FERRUX cover

44.9 minutes

73.1 minutes

Figure 19.3 Extension of solidification times with side wall and top surface feeding products for a steel cylinder 250 mm dia and 200 mm high.

Feeding of castings 299Calculating the number of feeders – feeding distance

A compact casting can usually be fed by a single feeder In many castings ofcomplex design the shape is easily divided into obvious natural zones forfeeding, each centred on a heavy casting section separated from the remainder

of the casting by more restricted members Each individual casting sectioncan then be fed by a separately calculated feeder and the casting shapebecomes the main factor which determines the number of feeders required

In the case of many extended castings however, for example the rim of agear wheel blank, the feeding range is the factor which limits the function

of each feeder and the distance that a feeder can feed, the ‘feeding distance’,must be calculated

The feeding distance from the outer edge of a feeder into a casting sectionconsists of two components:

The end effect (E), produced by the rapid cooling caused by the presence

of edges and corners

An effect (A), produced because the proximity of the feeder retards freezing

of the adjacent part of the casting (Fig 19.4)

Where a casting requires more than one feeder the distance between feeders

is measured from the edge of the feeder, not from its centre; and when thefeeder is surrounded by a feeder sleeve the distance between feeders ismeasured from the outside diameter of the sleeve

The effective distance between feeders can be increased by locating achill against the casting mid-way between the two feeders (X1) and thenatural end effect can be increased by locating a chill against the naturalend (X) Chills should be of square or rectangular section with the thicknessapproximately half the thickness of the section being chilled

There are therefore four possible situations:

Sections with natural end effect only (A+E)

Sections with natural end effect plus end chill (A+E+X)

Sections with no end effect (A)

Sections with no end effect plus chill (A+X1)

Figure 19.4 shows the basis for calculating feeding distance in steel castingsand all other ferrous alloys which freeze white, e.g malleable and highalloy irons

Ductile and grey iron castings

Alloy composition, casting section, mould materials and mould hardnessall play a part in determining the actual feeding distance The followingtables are guidelines for green sand moulds having mould hardness 90° Bscale, variations from these conditions will result in other feeding distances

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Table 19.1 Feeding distance factor (FD) for ductile iron castings

Carbon equivalent (%) Feeding distance factor (FD)

The calculation of feeder dimensions

The majority of foundrymen, even today, decide on feeder dimensions onthe basis of experience However, the application of calculation based onestablished theory and experimental data ensures the most efficient design

of natural and aided feeders In this section some guidance is given forcalculating feeder dimensions from first principles whereas on page 334there is a description of the various aids such as tables, nomograms, andcomputer programs developed by Foseco to make the determination offeeder dimensions much easier

The modulus concept

Although this concept has some shortcomings it is, with the exception of

Table 19.2 Feeding distance factor (FD) for grey iron castings

Carbon equivalent (%) Feeding distance factor (FD)

Table 19.3 Feeding distance factor for non-ferrous alloys

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computer programs, the most widely used acceptable and accurate methodfor calculating feeder dimensions

The solidification time of a casting section is given by Chvorinov’s rule:

where

tc is the solidification time of the casting section;

Vc is the volume of the casting section;

Ac is the surface area of the casting section actually in direct contactwith the material of the mould;

k is a constant which is governed by the units of measurement

being used, the thermal characteristics of the mould material andthe properties of the alloy being cast

Mc is the ratio of the volume of the casting section to its cooling surface areaand is known as the casting’s Geometric Modulus It is expressed in units oflength:

of the casting section therefore, the modulus of the feeder is calculated as

where MF is the modulus of the feeder required to feed a casting section

having a modulus of Mc This equation applies to natural feeders for mostalloys For grey and ductile iron castings because there is a graphite expansionphase during solidification the safety factor of 1.2 is considerably reduced

The modulus extension factor (MEF)

The object of using feeding aids is to slow down the rate of heat loss fromthe surface of the feeder It is possible to calculate how the improved thermalproperties of the feeding aid compared with sand can reduce the feedersize

From equation (1) the solidification time for a feeder is expressed as

tF= kMF2 (4)

The constant k is composed of two parts:

the thermal characteristics of the mould material surrounding the whole

of the feeder the properties of the metal within the feeder

Feeding of castings 303

Figure 19.5 Modulus formulae for some common shapes.

D H

Diameter = D Thickness = T

Diameter = D

Thickness = T

Thickness = T Width = W

OD = D1Dia core = D2Wall thickness = T

OD = D1Dia core = D2

a flat surface, calculated moduli for endless cylinders may be reduced

TH

T H

Length = L Width = W Thickness = T

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k can therefore be reduced to its two constituent components so that

k = cf2

where

c is a constant depending on the properties of the metal being cast;

f2 is a constant depending on the properties of the mould material.There is no significance in the fact that this constant is expressed as a squareother than mathematical convenience Equation (4) can now be rewritten

The expression (fMF) is known as the Apparent Modulus and f as the Modulus

Extension Factor of the mould material surrounding all of the feeder’s surfaces.This approach provides a quantitative means of evaluating and comparingdifferent feeding aids which should be designed to have the maximumpossible Modulus Extension Factor compatible with the other propertiesnecessary for a satisfactory product In practice it is not customary to determinethe absolute values of the constants relating solidification time to the modulus

as these are seldom of interest

Of greater concern is the improvement which can be expected from avariety of feeder lining materials when compared with the results obtainedfrom the same size of feeder lined with the conventional moulding material

– sand For this purpose the Modulus Extension Factor (f) for sand is equated

to unity and this serves as a basis for comparing other materials

Example

Using this information it is possible to consider as an example a sound steel

casting fed by one cylindrical feeder moulded in sand with a radius (r) of

16 cm and a height (h) of 32 cm Because the feeder is attached to the steel

casting, the bottom circular face of the feeder is a non-cooling face and the

Geometric Modulus (MF) of the feeder moulded in sand therefore is

If in place of sand, a feeding aid system with a Modulus Extension Factor (f)

of, for example 1.6 were to be used to line the feeder cavity and to cover thetop surface of the molten steel in the feeder, then the aided feeder wouldremain liquid for the same time as the sand lined feeder if the ApparentModulus of the aided feeder were to be equal to the Geometric Modulus ofthe sand lined feeder, i.e for the equal solidification times: