INOCULANTS

Inoculants

The most commonly used inoculants are high silicon materials (ferro silicons and calcium silicide) and high purity highly crystallised graphite.

Silicon based inoculants require the presence of small amounts of minor elements such as aluminium, barium, calcium, cerium or strontium to obtain maximum effect.

More has been written about the effects of inoculation than practically any other metallurgical problem.

Inoculation gives the following advantages and disadvantages:

 reduces tendency to chill and promotes graphite formation

 reduces formation of fine graphite and associated ferrite

 promotes uniform structures

 increases strength

 enables high strength, low carbon equivalent irons to be cast free from chill

 increases tendency to unsoundness.

Size range

 Silicon inoculants – granules 2–12 mm

 Graphite inoculants – powder or fine particles

Control

 Specify requirements and ensure that each delivery conforms to specification, including grading.

 Store materials under cover in an area free from damp or in a waterproof container.

 Finely divided graphite absorbs moisture.

 Silicon inoculants react with moisture which reduces their efficiency and makes them dangerous in use.

 Segregate each consignment in case of variations or problems.

Using Inoculants

 Weigh each inoculant addition.

 Add it at the last possible moment before casting.

 Ensure uniform distribution throughout the metal.

 Add the inoculant to clean metal free from slag or dross.

 Ensure good mixing. Add inoculant to the metal stream or to a partly filled ladle.

 When using transfer ladles, add inoculant to the casting ladles as above.

 Do not delay pour. Inoculants are subject to fade, in most cases over a very short period of time. 

 Use late stream or mould inoculation where possible.

 Where possible use inoculant in the form of a wire as a late addition to the metal stream.

Alloy Additions

 Electric furnaces, cupolas or cupola ladles.

 The specification of the desired cast iron will determine the production route.

 In general, highly alloyed irons – Ni hard, Ni resist, high chromium and high silicon irons – are made in electric furnaces.

 Low alloy grey or ductile irons can be made in either electric furnaces or cupolas.

 Alloy additions used to provide the alloying elements and their recovery in cupola and electric furnace practice are given in Exhibit O.

Control

 All alloy additions should be accurately weighed.

 Ladle additions should not exceed a total of 1% unless the alloy addition is of a low melting point or is absorbed very easily. Metal temperatures should be as high as possible when addition is made.

 Avoid the use of alloy scraps which can give rise to obnoxious fumes or gases

Avoid the use of relatively low cost alloy scraps which contain elements deleterious to the alloy cast iron being manufactured.

 Specify requirements, type, chemical composition, size range and check each consignment.

 Segregate each consignment.

 With high alloy cast irons it is necessary to check chemical compositions of the molten metal by spectrograph and adjust before tapping.

 A summary guide to maintaining quality in foundries is given in Exhibit P. This illustrates some of the major problems which can occur if strict quality control is not carried out.

 Remember that “quality assurance” is not just two words, it is a way of life which encompasses the whole of the business. Quality is only as good as the weakest link in the chain.

 Weighbridge

Large and medium-sized foundries should have their own weighbridges to weigh accurately all incoming and outgoing materials, whereas small foundries may have to share a facility with a nearby large company or use a public weighbridge close to the plant.

The weighbridge should have a deck size suitable for road vehicles, with electronic load cells accurate to ±5 kg and have provision for checking axle loads. The computer print-out should be accessible to the production control computer system. The types of materials that the weighbridge will be used for include:

 incoming (all metallic, coke, sands, coal dust, bonding agents, ferro-alloys, etc)

 outgoing (finished castings, waste sand, slag, etc)

Foundries must keep close control on the receipt and availability of raw materials within the plant. These controls help to reduce overall costs and identify stock excesses.

 Stockyard

Foundry stockyards should be concreted areas with adequate drainage and the facility to clean out the individual raw material bays, which will be typically for:

 steel scrap

 cast iron scrap

 pig iron

 foundry returns

 ferro alloys

 coke

 limestone

Purchased scrap, pig iron and foundry returns should be stored under cover, organised in individual bays by usage frequency, density, specification, or alloy content.

Stockholding will vary according to the size of the foundry and the space available, but a minimum of three days’ supply of all raw materials should be held on site. If space is available, those items subject to price fluctuation, eg steel scrap, should be purchased and stored when prices are low. Each delivery should be from an approved supplier to avoid the need for the checking of each load. However, the quality of materials supplied must be kept under constant review to ensure quality levels do not drop.

Charge make-up can be by a crane fitted with an electro magnet or grab or, in very small foundries, by hand. However, all materials must be weighed and an automatic print-out of the weight of each constituent material should be retained manually or, preferably, on computer for traceability purposes.

For cupola melting, coke and limestone should be stored in separate hoppers and the desired weight automatically dispensed into the charging skip. The coke and limestone should be charged into the cupola separately from the metallic to minimise the crushing and fracturing of the coke and so maintain its optimum size. The metallic skips should be charged alternately with the coke/limestone skips.

CUPOLA FURNACES

Cupola Furnaces

a) Introduction

Since the first cupola patent was taken out in England in the late eighteenth century, the furnace has remained the predominant melting unit in iron foundries on a tonnage basis. Over the two intervening centuries, the basic operations of coke fired cupola have remained relatively unchanged, although the understanding of the process involved has improved considerably.

The conventional cupola has been perceived as a relatively low cost plant which, particularly for smaller foundries, is capable of achieving the appropriate quantity of iron, especially grey iron. Thus the status of the furnace was virtually unchallenged in iron foundries until the late 1950s, early 1960s.

At that time, electric furnaces, particularly coreless induction types, began to make inroads into the market. This market penetration has accelerated in recent years with the development of more efficient and powerful solid state powered equipment. The introduction of electric furnaces was promoted by environmental concerns, by the ability to produce more easily a range of alloys, by increasing customer expectations regarding quality, and improvements of both electric furnace equipment and associated refractory systems. Many of the small to medium sized electric furnace plants installed in recent years have been justified on this basis, even though capital costs may be higher than for a simple cupola plant.

The introduction of environmental legislation requiring rigorous control of emissions to the external atmosphere has been perceived to mitigate against cupola melting in favour of electric melting, the latter being considered by most foundrymen to be cleaner. However, this is not necessarily true, since deterioration of scrap quality, the need to control workplace environment and the continued scrutiny and tightening of outside emission limits apply whatever the melting unit. Various recent waste regulations mean that the dumping of waste from furnace emission control systems will become more difficult and expensive, and the use of the cupola to reprocess such material will become a more important benefit in the future.

The largest melting cost item is charge metallic and the ability to process lower quality scrap through a cupola, due to its refining action and greater tolerance of included non-metallic material, is of considerable benefit, particularly for tonnage applications. This is why large tonnage outputs of a restricted range of unalloyed cast irons are almost invariably produced from cupolas. Such plants may include hot blast facilities and will for the most part be fitted with fume cleaning plant of sufficient efficiency to satisfy the appropriate environmental requirements. These cupola installations will employ electric holding furnaces to allow for optimum operation of the prime cupola melter.

Most iron foundries have had to consider either upgrading their emission control equipment or changing their melting practice to ensure compliance with the relevant requirements. The choice in reality has been between fitting dry bag filtration systems or changing to induction furnace operation. These options are expensive, but installing suitable emission control equipment on an existing cupola plant has no economic justification other than keeping the company out of Courts and allowing them to continue in business.

The majority of cupola plants are based on conventional cold blast units, which may or may not be fitted with improvements such as automatic blast control, oxygen enrichment, and divided blast equipment. In addition, the higher output furnaces, especially where long campaign operation is involved, have for many years been largely of the hot blast type. However, relatively recently, the impending legislation on emissions and to a lesser extent waste, the increased interest in long campaign operation and the ever present need to reduce costs – both capital and operating – have all conspired to ensure that cupola technology has not become stagnant. The following sections attempt to review the current status of the cupola furnace.

b) Conventional Cold Blast Cupola Operation

The conventional cold blast cupola is a vertical shaft furnace operating on the counter-current principle. Cold charge materials are fed into the top of the unit and are preheated by the products of combustion of coke as they descend and melt before being tapped out near the base (see Exhibit Q).

An incandescent coke bed is established in the lower part of combustion air (blast air) introduced via a series of tuyeres arranged in this area to ensure that appropriate combustion conditions are generated. Blast pressure, velocity and disposition are important and require adequate control if the correct bed conditions and temperature distribution are to be maintained.

The cupola furnace is not a “dead melting” furnace in that the melting process is accompanied by a number of compositional changes – carbon and sulphur pick-up, and oxidation losses of silicon and manganese.

Pick-up of carbon depends on a number of factors including:

 initial carbon content of the charge mixture

 the tapped silicon and phosphorus levels

 the amount and quality of the coke

 coke bed temperature

 cupola well depth

 method of tapping

 metal tapping temperature.

Clearly some of these factors are inter-related.

Sulphur pick-up in turn is governed by a number of factors, notably the sulphur content of the coke, slag basicity and charge make-up and composition.

There is invariably some oxidation loss of silicon during cupola melting, normally between 10% and 20% of the charged level of element. The actual figure will vary depending on the melting conditions, with high tapping temperatures favouring a reduction in losses and the presence of steel scrap and high silicon briquette additions having the opposite effect. Manganese is subject to oxidation losses of the order of 20–25% of the charged level, although again melting conditions will have an influence on the practical values achieved.

The cupola is refractory lined and as the melting campaign proceeds, the refractory is mechanically and chemically eroded until the remaining thickness is insufficient to allow for continued safe operation. This erosion situation limits the melting campaign and usually requires the provision of two furnaces in an installation. One will be melting while he second unit is being repaired. Eroded lining material, together with coke ash, dirt from the charge materials and appropriate fluxes (usually limestone) results in the formation of an acid slag which is tapped either with or separate to the metal.

From the above, it can be observed that there are a number of areas of possible improvements that could be explored when considering the limitations of the conventional cold blast cupola. These include:

 increasing coke bed and metal tapping temperatures

 improving carbon pick-up and reducing sulphur pick-up

 reducing silicon manganese oxidation losses

 increasing campaign length

 increasing melting rates.

In addition, the increasingly stringent emission and waste regulations have resulted in the development of re-use systems to minimise their effects on the foundry’s operations and costs.

These factors have resulted in considerable research and development of a wide variety of cupola technology in an effort to improve the performance of the furnace.

c) Divided Blast Cupola

The divided blast cupola, a development of the balanced blast cupola of earlier years, was developed in the early 1970s and became very popular as a means of improving the performance of many cold blast cupola plants installed years before and struggling to meet increasing quality standards. It also became popular in new installations at that time.

In the balanced blast system, however, the cupola is provided with two rows of tuyeres set 900 mm apart and the air blast is controlled in both areas. A distribution of 50/50 between the two rows of tuyeres appears to give the best results independent of the amount of charge coke or blast rate. Control can be obtained by using separate fans for each row of tuyeres or by installing proportioning control equipment in the ducts from a single fan (see Exhibit R).

The system enables a higher metal tapping temperature and higher carbon pick-up to be obtained for a given coke charge or a reduction in charge coke and an increase in melting rate whilst maintaining a given tapping temperature. It is necessary to recognise that all of these advantages cannot be obtained at the same time.

Exhibit S shows the relationship between coke consumption, metal temperature and melt rate. The divided blast cupola may be further enhanced by the use of oxygen applied to the lower row of tuyeres.

SUBSIDIARY RAW MATERIALS FOR MELTING

Subsidiary Raw Materials  for melting:

These materials, which must be controlled, include:

 coke

 carburisers

 fluxes

 inoculants

 alloying additions

Coke

Coke is the primary energy source in cupola operations and coke quality is the key to carbon and sulphur pick-up. Carbon pick-up is of particular importance when high steel charges are used.

Chemical and physical properties should have the following characteristics: moisture	–	4% max
ash	–	9% max
volatile matter	–	1.0% max
sulphur	–	0.9% max
50 mm shatter index	–	90% min
mean size	–	110 mm min
undersize	–	not more than 4% of pieces less than 50 mm

Serious coke breakages can occur through inefficient handling at all stages from the coke oven, to and in the foundry.

Each coke delivery should be checked visually for size to ensure that there has been no serious degradation.

Examination of the coke structure may give a clue to its behaviour in the cupola. A good close/dense grain structure normally denotes a good operating coke. Coke strength is related to its final size and weak friable coke will result in excessive breakage and fines.

Serious coke breakages can occur at the foundry, resulting in a high percentage of small coke below 50 mm in size.

Small cokes can reduce tapping temperatures and carbon pick-up and may require an increased coke percentage to maintain good cupola operation. Increasing coke percentage will reduce melting rates unless air volumes are increased.

Care should be taken if coke is stocked for any length of time as this will result in degradation, and possibly a reduction in carbon pick-up and tapping temperatures.

Control

Inspect each delivery for size and structure. Record the name of the supplier, haulier and time and delivery of each consignment.

Retain a sample from each delivery for chemical analysis. If laboratory facilities are available, check ash, sulphur and volatile matter. If laboratory facilities are not available and coke quality is suspect, arrange for an outside analysis.

If the load contains excessive small coke and fines, do not use unless absolutely necessary. Under these circumstances retain a proportion of the load for supplier’s examination.

Do not store coke except on an emergency basis. Use the stored coke at regular intervals and replace with fresh supplies.

Carburisers

Carburisers are mainly used in electric furnaces to carburise high steel charges to the appropriate carbon level.

They are also used in external treatment processes such as porous plug ladles, and shaking ladles, again to raise carbons to required levels.

Carburising materials available in the UK are mainly based on the synthetic graphites, petroleum cokes and to a lesser degree metallurgical cokes.

Approximate analyses are given in Exhibit N.

Graphites

These materials are used for carburising base irons for ductile irons, malleable irons and also grey irons when it is desirable to restrict nitrogen pick-up, or when sulphur content is in excess of 0.05% and a high degree of nucleation is required.

Non graphites (petroleum or metallurgical cokes)

These materials are used for ductile and malleable iron production. In the case of ductile irons, petroleum and metallurgical cokes are only used when the metal is to be desulphurised.

They are used extensively in the production of grey irons for the following reasons:

 To obtain an increase in tensile strength due to increased nitrogen content

 Care must be taken that the increased nitrogen content does not give rise to nitrogen fissure defects in the finished casting.

 To avoid over-nucleation which could give rise to shrinkage defects.

 To increase sulphur contents of some irons to above 0.05% making them more responsive to inoculation.

Control

 Select the correct carburising material and grade for the application. When purchasing specify these two criteria.

 Grading is of prime importance and can affect solution rate, carburising potential and carbon losses. If the material is too fine, the carbon will float away on thermal air currents. If too coarse, low temperatures and low turbulence will result in slower carbon solution and low recovery.

 Check every consignment for specific type and grading, and store the material under cover.

Segregate carburising agents if more than one type is used.

 Check carbon recoveries on a statistical basis to ensure quality continuity.

Fluxes

 Fluxes are added to the cupola to give a fluid slag with the impurities in the charge, sand, rust, ash, etc, plus refractory lining attacked and removed during the melting process.

 Standard fluxes are limestone and dolomite but certain other fluxes such as fluorspar may be used in special circumstances.

Limestone

 Calcium carbonate which reverts to CaO (lime) and CO2 in the cupola.

 The CaO (lime) combines with the impurities to give a fluid slag.

Dolomite

 This is the double carbonate of calcium and magnesium which calcines to CaO and MgO to form a fluid slag with the impurities. The melting point of the double carbonate is somewhat lower than CaO and gives more flexibility with “sticky slags” of high acidity or basicity.

 Both limestone and dolomite should contain a minimum of 96% CaCO3 and MgCO3.

 Size range should be 2.5 cm to 7.5 cm.

Amount

 Normal cupola practice requires the flux to be 3–4% of the metallic charge.

 This should provide a fluid slag with a basicity, in this case an acidity of 0.6–0.8, using the standard formula CaO + MgO/SiO2.

 It is extremely important to obtain a fluid slag in acid cupola operations.

 Too little flux will give a highly acid viscous slag which in extreme cases can cause bridging.

 Too much flux will tend to give a basic slag which will quickly neutralise itself by attacking the acid refractory lining resulting in a large slag volume and misuse of energy in the form of coke.

 As there are very few basic lined cupolas in operation in Europe, or the rest of the World, there is little or no point in discussing the operating parameters for basic slags.

Fluorspar (calcium fluoride)

 Fluorspar may be used in small amounts as a slag fluidiser or to reduce sulphur pick-up.

It is a very vicious slagging material and will severely attack acid linings.

Control

Limestone and Dolomite

 Specify high purity limestone and dolomite.

 Specify and check the size range.

 Weigh or check by volume the amount per charge.

 Remember too little limestone can mean cupola problems.

 Too much is a waste of coke and refractories.

Fluorspar

Briquettes are recommended for acid lined cupola operation. Specify size of briquettes and ensure the correct number per charge is made.

PIG IRON

Pig Iron

Pig irons are produced by blast furnaces from iron ore and from ferrous scrap in cupolas or electric furnaces (refined pig irons), or as by-products from other processes such as vanadium or titanium oxide production. These latter pig irons are usually high purity irons most suitable for ductile iron production.

The composition of refined pig irons can be adjusted to comply with customer requirements, including a known residual content or added alloys with a specified range of alloy content.

Typical pig iron compositions, with the exception of high purity irons, are shown in Exhibit M.

Control

Pig iron is ordered to a given specification range and should be accompanied by a supplier’s certificate of composition which should include the major residual contents.

No two batches are the same and unless the operation can withstand a reasonable tolerance in casting compositional range, each batch (which in the case of blast furnace irons may have variations in the batch) should be kept separate. This ideal may not be possible in large melting plants.

Foundries should insist on a certificate of analysis and also carry out analytical checks to ensure a quality material.

Pig iron is an expensive metallic and should be used as a general quality control in the metallic charge, i.e.:

 provide and even out carbon “pick-up” variations

 provide and even out silicon levels

 control phosphorus levels

 ensure a better mixing in cupola operations which have inadequate wells and fore hearth facilities

 as a control for excessive residual element levels and possible gas levels when high steel charges are used.

Return Scrap

The return scrap of each individual foundry should be of a known and generally consistent composition. It should be used to the maximum consistent with obtaining the desired composition of the castings to be manufactured.

Control

Each grade of scrap should be kept separate. Large pieces should be broken to a size suitable for individual furnace requirements.

c) Ferro-Alloys

Ferro alloys are used as additions to ensure control of elements such as silicon and manganese in the standard non or low alloy irons.

The high alloy irons such as Ni hard, Ni resist and high chromium iron, represent a small percentage of the iron foundry industry and require special attention to their choice of alloys and ferro alloys. Ferro silicon	Most common grades in lump form contain 75–80% and 45–50% silicon levels.
Silicon carbide	In lump form containing 55–60% Si and 25–30% C
Ferro manganese	Usually obtained in lump form and contains 75–80% manganese.

The use of larger amounts of ferro silicon in cupolas can result in excessive silicon variations unless suitable mixing volumes are provided in wells, receivers or ladles.

Do not rely too much on the cupola for mixing purposes as it is very possible to obtain a wide variation in silicon content from the well, even with a tap and bott system.

Control

Ferro alloys should be checked that they comply with the foundry’s specification for size, grading and composition.

In particular, ferro silicon should be checked for fines as this is a source of high silicon losses and hence variation in silicon content in melting operations.

Briquettes should be checked for consistency of weight and friability. Instability of briquette can also lead to high silicon losses in the cupola.

In both cases alloy content should be occasionally checked by the foundry or by an independent laboratory.

Accurate weighing of ferro alloy additions on suitable scales is an absolute essential to control silicon and manganese contents. When briquettes are used, ensure the chargers can count!

MELTING PROCESS IN FOUNDRY

Raw Materials

a) Controlling Raw Materials

Raw materials for the manufacture of castings must be selected and controlled to ensure that castings have the mechanical properties and chemical composition desired by the customer and are free from defects.

Amongst the various raw materials input into the foundry, metallic take the lion’s share both in terms of technology and economics.

Metallic Raw Materials

I) Steel scrap

ii) Bought cast iron scrap

iii) Pig iron

iv) Return scrap

Ferro Alloys

Each of these materials should be purchased to:

 range of compositions

 range of quality

 range of physical form.

The selections and quantity requirements of these metallic will depend on the melting method and techniques, the types and grades of irons to be produced, the skill of the operators and possibly local factors such as scrap availability.

b) Ferrous Scrap

Steel scrap has quite deliberately been placed as the number one raw metallic.

Over the last decade technical advances in cupola techniques, electric furnace melting and metallurgical skills have resulted in an increase in the use of steel scrap in foundry melting operations.

The scrap industry has made considerable efforts to ensure that it supplies the qualities of scrap required by the foundry industry. Many scrap merchants have geared their business to local foundries. However, it is up to the individual foundry to:

specify the quality and quantity requirement

 carry out the necessary visual check on every individual load of scrap – remember that visual inspection means checking before and after tipping

 visit the scrap merchant’s yard on a regular basis to ensure continuity of quality and supply.

The scrap merchant’s quality is only as good as his scrap source, plus his processing equipment. The former can change, especially when prime steel is at a premium, resulting in scrap of unknown qualities being introduced into the system.

Bought Cast Iron Scrap

Similar remarks concerning quality control apply to cast iron scrap as to steel scrap.

Categories and approximate ranges of composition for the main types of cast iron scrap are shown in Exhibit L.

The controlling element in bought cast iron scrap is considered to be phosphorus and even relatively medium phosphorus contents of 0.3% or less are detrimental to the production of the higher strength grey irons.

However, with certain specific exceptions, the phosphorus levels of cast iron scraps have reduced considerably over the last 20 to 30 years. In fact, the only commercial way to produce a high phosphorus iron is to add ferro phosphorus.

Control

As with steel scrap loads, a visual inspection of each individual load is essential, and again check before and after tipping, including the bottom of the transport vehicle.

Attention must be paid to possible sources of contamination such as: gas works scrap		high sulphur
enamelled scrap	–	lead, boron and antimony
petrol engine scrap	–	lead, aluminium, etc, plus the possibility of small quantities of chromium

The vast majority of cast iron borings originates from low phosphorus sources and as such are a useful, relatively low cost raw material for many iron foundries.

It can be briquetted, packed in canisters and used in cupola operations. Commercial success depends on the quality of briquette or canning operations as there can be serious losses in the melting operation.

The coreless induction furnace is an ideal melting unit providing the borings are dry and relatively free from rust and foreign matter.  Briquetted cast iron borings should be free from corroded lumps and excessive cutting fluids.

A RETREAT A HEAD

Here’s to Blaine and Jean Harker, those lovable two,

with joy so contagious and counseling so true.

A mourner in grief is a magnet to Jean,

since few are the pains she’s not suffered or seen.

At the parties they give there is greatness of table,

and every last diner eats more than he’s able.

Jean’s food pantry likewise, for the hungry and poor,

was much like her heart—a wide open door.

Their lives are committed to lifting the fallen,

through talkin’ and workin’ and sweatin’ and bawlin’.

An unspoken concern here is needful of saying—

for Jean’s own self-healing we are fervently praying.

While Blaine may have yet to get milk from a cow,

in spite of the Amish folks showing him how,

he’s mastered the art of infectious laughter

that shatters the silence from floor-joist to rafter.

They’ve moved to the country near Old Shipshewana,

but they can’t quite move in yet, as much as they wanna—

while waiting for lodgers to kindly dislodge

they have set up their home in a large upper garage.

We honor the Harkers today, Blaine and Jean,

and the Power behind them, so strong yet unseen.

May God bless their home, the retreat of their dreams,

granting laughter which heals, and the grace which redeems

FLOWER IN VASE

This budding daffodil contains

A universe in birth:

Each molecule a galaxy,

Each quark a tiny earth.

And what we call our universe,

All matter, time, and space,

May be a single atom of

A macrocosmic vase.

Thus up and down the scale of size

Throughout Infinity,

Both “small” and “large” are limitless

And join Eternity.

Great men have puzzled over God

To place Him in their plan,

As Primal Cause, or Sourceless Source,

Or vast Omniscient Man.

But God can never be confined

Within a man-made phrase;

He hides behind unnumbered veils

Impossible to raise.

And yet we see His evidence

In every time and place—

Behind each seed and universe,

Within each flower and vase.

Inside our inmost soul of souls,

If we can meditate,

We find a spark of light divine

And feel it radiate.

While nowhere, and yet everywhere,

Our God resides within;

Though still and small, His guiding voice

Transcends life’s noisy din.

To hear His voice and understand,

Then fearlessly obey,

Is that which mystics, martyrs, saints,

And wise men call “The Way.”

Consider every universe

And every point in space

As God in God in God in God,

As vase in flower in vase.