Foseco Ferrous Foundryman''''s Handbook Part 5 ppsx

but since the developmentof spheroidal graphite ductile iron its use has declined, due to the high cost of the annealing treatment which requires expensive furnace equipment.Malleable ir

There are two types of malleable iron, blackheart and whiteheart Malleableiron has a long history, whiteheart iron having been developed in 1722 bythe French metallurgist, Réaumur, while blackheart iron was developed inthe USA in 1820 Malleable iron was widely used for automotive andagricultural components, pipe fittings, valves etc but since the development

of spheroidal graphite ductile iron its use has declined, due to the high cost

of the annealing treatment which requires expensive furnace equipment.Malleable iron is still widely used for small pipe fittings, electrical fittingsand builders hardware, particularly for thin section castings and castingswhich are subsequently galvanised

Whiteheart malleable

In the whiteheart process, the white, as-cast iron is decarburised duringannealing leaving a structure of iron carbide in a metallic matrix Whenfractured, the appearance is whitish, giving rise to the name, ‘whiteheart’.Decarburisation is only possible in thin sections; in heavier sections, someconversion of carbide to graphite nodules occurs so that the annealed castinghas a white rim with a core having different structure and mechanicalproperties This limits the applications to which it can be put Whiteheartcan be melted in a cupola and is a low cost material which still findsapplications in small, thin section castings

Composition of whiteheart malleable

Typical compositions are

Malleable cast iron 91

Annealing is a combined decarburisation and graphitisation processperformed in an oxidising atmosphere Originally it was done by packingcastings into iron ore mixtures but now it is carried out in continuous,

fully decarburised and are referred to as weldable malleable irons Table 7.1lists the European specifications for whiteheart malleable iron The USA has

no equivalent standard

Table 7.1 Specifications for whiteheart malleable cast irons

France: NF A32-701(1982); Germany: DIN 1692 (1982); UK: BS6681:1986.

Notes: It is advisable to consult the original standards for details of the mandatory values, methods of testing etc.

USA has no standard for whiteheart malleable iron.

92 Foseco Ferrous Foundryman’s Handbook

Blackheart malleable iron

The iron is typically melted in a cupola and duplexed into an electric furnacewhere temperature and composition are adjusted The cupola metal has thecomposition:

The final composition in the electric furnace is:

0.01%) of bismuth are added in the ladle to ensure fully white as-caststructures White irons contract on solidification so to ensure freedom fromshrinkage, the castings must be fed, see Chapter 19 As-cast malleable iron

is extremely brittle, allowing feeders and running systems to be brokeneasily from the castings Castings which have abrupt changes of sectiondevelop internal stresses on cooling which may be enough to cause cracking

of the castings after shakeout Slow cooling in the mould may be needed toavoid this happening

The castings are annealed to develop the required graphite clusters (Fig.2.3) A typical cycle is about 48 hours long (Fig 7.1) it may be carried out inbatch-type or continuous furnaces in a controlled atmosphere to avoidoxidation of the castings The rate of cooling in the final section of the heat

500

12 24 36 48 60 Time (h)

Figure 7.1 Typical heat treatment cycle for a short cycle blackheart malleable iron (From Elliott, R (1988) Cast Iron Technology, Butterworth-Heinemann, reproduced

by permission of the publisher.)

Malleable cast iron 93

treatment determines the matrix structure of the castings which can beferritic or pearlitic according to the physical properties required

Specifications for malleable cast irons

Table 7.2 lists the European and International specifications for blackheartmalleable cast irons The European Standard CEN 1562:1997 has supersededthe former national standards Table 7.3 lists the US ASTM specifications

Table 7.2 European and international specifications for blackheart malleable cast irons

France: NF A32-702(1982); Germany: DIN 1692 (1982); UK: BS6681:1986.

Notes: It is advisable to consult the original standards for details of the mandatory

values, methods of testing etc.

94 Foseco Ferrous Foundryman’s Handbook

Table 7.3 US specifications for blackheart malleable irons

ASTM A602-94 covers automotive malleable castings.

Chapter 8

Special purpose cast irons

Heat resisting alloys

Unalloyed cast iron shows only slight scaling and no growth at temperatures

occur in less than one year in most grey, ductile and malleable cast irons Toachieve greater resistance to heat, special purpose irons must be used

Temperatures up to 600 °C

phosphorus up to 1.0% improves the scaling resistance and can be used inapplications such as fire bars where mechanical strength is not a majorconsideration

Temperatures up to 700 °C

The addition of about 0.5% Cr improves the oxidation resistance of greyirons and is also used for fire bars Automotive exhaust manifolds andturbo-charger casings are subjected to increasingly high temperatures inmodern engines, unalloyed cast iron can be used at temperatures up to

iron with Cr and Mo, e.g

Low alloy grey iron

96 Foseco Ferrous Foundryman’s Handbook

temperature creep resistance They are used for exhaust manifolds and charger casings and are better able to resist thermal stress than grey ironcastings Typical compositions are:

turbo-High silicon ductile iron

trade name of the International Nickel Company) are used Nickel incombination with Mn and Cu produces a stable austenitic matrix and Cr incombination with Ni forms an effective oxidation resistant scale Mostcountries have similar standard specifications for austenitic cast irons, based

on the original Ni-Resist specifications Each country designates the alloysdifferently Table 8.1 lists the designations of the flake graphite austeniticirons Table 8.2 lists the specifications of austenitic flake graphite irons.Spheroidal graphite Ni-resist can also be made Table 8.3 lists theinternational designations Table 8.4 lists the British specifications

Temperatures up to 1000 °C

High Cr white irons have excellent resistance to oxidation at temperatures

to impact loading They are used for furnace parts, sinter pallets, recuperatortubes etc Typical analyses are:

T

T

Special purpose cast irons 101

The iron is of eutectic composition or slightly hyper-eutectic, since this isfound to give the maximum mould life Compacted graphite iron is alsoused for this application (see p 84)

Corrosion resistant cast irons

Unalloyed cast irons exhibit reasonable corrosion resistance, particularly toalkali environments, but special alloys have been developed for use incorrosive conditions These include, high nickel (Ni-Resist), high siliconand high chromium alloys Table 8.5 summarises the corrosion resistance ofvarious alloy irons in different media

Table 8.5 Corrosion resistance of cast irons

acid quite well

acid

hydrochloric acid

Ni-Resist alloy irons are used for the manufacture of pumps for the handling

of sulphuric acid and alkalis, for sea water pumps and for handling sourcrude oils in the petroleum industry

High silicon cast irons, 10–16% Si, are suitable for use in chemical plantand for cathodic protection anodes The grades shown in Table 8.6 are used.High silicon irons are low strength, brittle materials which are susceptible

to cracking due to stresses induced by cooling Moulds and cores used fortheir manufacture should have good breakdown properties Electric furnacemelting is necessary because of their low carbon content The alloys are

102 Foseco Ferrous Foundryman’s Handbook

prone to unsoundness due to hydrogen gas and care must be taken duringmelting to avoid contact with hydrogen, by the avoidance of moisture oncharge materials and refractories and maintaining low levels of Al in the iron.Heat treatment is advisable to eliminate internal stresses, the castingsshould be stripped from the moulds as hot as possible and charged into afurnace at about 600°C then heated to 750–850°C for 2–8 hours, depending

on metal section They should then be slowly cooled to below 300°C beforeremoving from the furnace

High silicon irons have high hardness and are difficult to machine; grinding

is preferred

Wear resistant cast irons

Wear resistant alloy irons are an important class of cast irons, they find wideapplication for the manufacture of mineral crushing plant, e.g hammers forrock crushers, grinding balls, liners for crushing mills etc They are alsoused in shot blasting machines for impellers, liners etc Three types of castiron are used:

Unalloyed and low alloy grades of white iron having a structure of massivecarbides in a pearlitic matrix These alloys are extremely brittle and havebeen largely superseded by tougher, alloyed white irons

Ni-hard irons containing Ni to increase hardenability by ensuring thataustenite transforms to martensite after heat treatment They also contain

Cr to increase the hardness of the carbide

High Cr-Mo white irons combining abrasion resistance with toughness.The Mo increases hardenability allowing heavy section castings to bemade with a martensitic structure, either as cast or heat treated

Low alloy cast irons

These include irons having the compositions:

Table 8.6 High silicon cast irons

Grade Si 14 is used for general applications

Si 10 has higher tensile strength but lower corrosion resistance

Si 16 has high corrosion resistance but lower strength

Si Cr 14 4 is used for the manufacture of cathodic protection anodes

For further details, refer to BS 1591.

Special purpose cast irons 103

Cr content increases the hardness of the eutectic carbide The choice of alloycomposition depends on casting thickness and intended use For maximumabrasion resistance, the C content should be at the top of the range, 3.2–3.6%, but for impact resistance it is usual to use lower carbon contents, 2.7–3.2% Table 8.7 lists a number of national specifications for alloy irons of theNi-hard type Note that the correspondence between specifications is notexact and the standards themselves must be referred to for details

improve strength and impact resistance

High Cr-Mo irons

The addition of Cr in the range 12–28%, together with Ni and Mo forms arange of alloys which combines abrasion resistance with toughness suitablefor large size equipment in the mining, coal and mineral processing industries

martensitic or pearlitic according to the application Some components may

be cast pearlitic to aid machining, then heat-treated to give an abrasionresistant martensitic structure Tables 8.8 and 8.9 list specifications of thistype

Production of high chromium irons

An induction furnace is normally used to achieve the required temperature

Special purpose cast irons 107

The Cr–Fe oxides formed during melting attack silica lining materials, soalumina linings should be used Charges are based on steel scrap and foundryreturns with additions of high and low carbon ferrochromium to obtainiron of the required carbon content Ferromolybdenum is added as needed

A loss of about 5% of the Cr content will occur during melting

carbon alloys

A viscous oxide film forms on the surface of the molten metal in thefurnace and the ladle The iron must be carefully skimmed before pouring,and running systems must be designed to trap oxide dross Metal filtersshould be used if possible To avoid oxide defects on the casting surface,gating should provide rapid filling of the mould with minimum turbulence

(6.0–10.0%) that occurs on freezing Steel castings range in size from a fewgrams to hundreds of tonnes Steel castings are often used in critical situationswhere optimum mechanical properties are essential so that freedom fromcasting defects is particularly important The advantages of cast steel overother cast metals, such as ductile iron, are:

High modulus – minimising deflection under heavy static loading Shock and impact resistance – to cope with severe dynamic loading Heat-treatable – to give the required toughness, hardness and strength Weldable – so that castings can be joined to wrought steel products High temperature strength – special alloys are available

Steel castings compete in many areas with steel weldments In welding, it isdifficult to avoid stress concentrations at junctions while castings have well-radiused joints so that fatigue resistance of cast steel junctions is superior tothat of welded junctions Also welds, especially in large structures, are notalways amenable to stress-relieving after welding so that internal stressesare likely to be greater in weldments than in steel castings This may lead todistortion of weldments or even failure in highly stressed components.Castings allow freedom of design which is not possible with weldments.The main applications of steel castings, roughly in order of tonnage used

in the UK, are:

Valves, pumps and compressors

Construction and earth moving equipment

Crushing, grinding, quarrying and dredging

Oil and gas rigs

Mining equipment

Motor vehicles

Defence equipment

Railways

Types of steel castings 109

Shipbuilding

Iron and steelworks

Nuclear power plant

General engineering equipment

Electrical plant

Agricultural equipment

While the overall tonnage of steel castings has been on a downward trendfor a number of years, the overall value has increased due to a growingtrend towards increased quality standards, reflecting the high standardsneeded for nuclear plant, gas and oil rigs etc

Processes used for steel castings are:

surface finishLost wax investment casting for smaller castings made in large

numbers where good surface finish anddimensional accuracy is neededThe Lost Foam process is not normally used for steel casting because of thedanger of carbon pick-up from the foam pattern

Specifications for steel castings

Steel casting specifications in Europe are undergoing major changes as newEuropean specifications are being drawn up At the time of writing, theonly European standard to be issued in the UK is BS EN 10213:1996 coveringSteel Castings for Pressure Purposes During the next few years, EN standardscovering steels for General Engineering, Corrosion Resisting, Heat Resisting,Wear Resisting, Structural and Centrifugal applications will replace theexisting national standards in European countries

Until the new standards are issued, individual national standards apply.However, attention should be paid to the International Standard ISO 4990-

1986 Steel Castings, General Technical Delivery Requirements which specifiesthe methods of testing steel castings and other matters which need to beagreed between purchaser and supplier The contents of this standard arebeing incorporated into the forthcoming European standard

ISO 4990 requires the purchaser to supply details with the enquiry covering:

A description of the casting(s) by pattern number or drawing

Dimensional tolerances, machining allowances and datum points formachining

The material standard and grade of steel

110 Foseco Ferrous Foundryman’s Handbook

The type of document required for inspection and testing

Additional information such as agreed manufacturing method

Size of test lot

Procedures for marking, machining, protection, packaging, loading,dispatching and the destination

Submission of sample castings

Methods of statistical control

Special tests such as intergranular corrosion and pressure tests

Test reports must provide the required traceability of the castings theyrepresent

It is normally required that the manufacturer shall maintain records of results

of all the chemical and mechanical tests performed by the foundry for aminimum of 5 years

Other ISO Standards detail testing methods:

Determination of lower yield stress and proof stress and provingtest

ISO 2605/1 Steel products for pressure purposes – Derivation and

veri-fication of elevated temperature properties – Part 1: Yield orproof stress of carbon and low alloy steel products

ISO 2605/2 Steel products for pressure purposes – Derivation and

veri-fication of elevated temperature properties – Part 2: Proof stress

of austenitic steel products

principles

ISO 3651/2 Austenitic stainless steels – Determination of resistance to

intergranular corrosion – Part 2: Corrosion test in a sulphuricacid/copper sulphate medium in the presence of copperturnings (Monypenny Strauss test)

materials by X- and gamma rays – Basic rules