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.