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Material for May 7th 2010

Welding of Cast Irons

Dr. Jippei Suzuki

I think (or confident) that the cast iron should not be welded, because it is the material for

castings. The weldability of the cast iron is very poor, because its carbon content is very high.

Fig.K1

Fig.K1 is Fe-C system dual equilibrium diagram, which demonstrates two states. In iron

matrix, carbon exists in two types of state; one is Fe3C and another is graphite. The Fe3C

is the compound of Fe and C, of which the carbon concentration is 6.67mass% and graphite

is 100mass%C (free carbon), of which the crystalline structure is hexagonal. The state of

Fe3C is meta-stable and that of graphite is truely stable. Therefore, there are two phase

diagrams, one is Fe-Fe3C system (broken curve) and Fe-graphite system (solid curve). Some

curves and a line is only solid, because the graphote dose not exists. Usually, the steels

remain in the mata-stable state, because the barrier from Fe3C to graphite is very high.

The Fe-C alloy containing carbon of 2.14 6.67mass% (red arrow in Fig.K1)). There are

some types cast irons according to the stability of Fe-Fe3C/graphite.

Fig.K2

White cast iron (white pig iron)

Carbon exists in the form of Fe-Fe3C. Therefore, a large amoint of Fe-Fe3C decreases the

toughness of the material remarkably.

Mottled cast iron

Mottled cast iron is the mixture of white cast iron and gray cast iron.

Gray cast iron

Graphite particles in gray cast iron. Gray cast irons are divided according to the mi-crostructure of matrix; 1) pearlite, 2) ferrite/pearlite and 3) ferrite gray cast iron. In pearlite

and ferrite/pearlite cast irons, Fe-Fe3C exists in pearlite in the matrix.

Graphite becomes stable with increasing the cabon and silicon contents, and with slow

cooling rate. Therefore, mata-stable Fe-Fe3C is produced in the thin cross-section of the

castings.

Fig.K3

The effect of silicon is evaluated by the carbon equivalent; C.E. or the carbon saturation;

Sc.

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C.E. = T.C. +1

3(%Si + %P) T.C. : carbon content

Silicon has the similar influence as carbon for stabilization of graphite. Therefore, the

carbon equivalent is calculated from carbon and silicon contents. Phosphrus has the sameaction as silicon.

Sc =T.C.

4.3 0.3%Si

The carbon content of eutectic point in Fe-C system is 4.32mass%. This point is shifted

by addition of silicon. The Sc expresses the position of the cast iron against the eutectic

point. That is, it is hypo eutectic when SC < 1, eutectic when SC 1 and hyper eutectic

when SC > 1.

Fig.K4

Fig.K4 is Maurer structural diagram. In this case, the cast iron is heated to 1400 , and

then cast into the dry sand mold of 75mm in diameter. The microstructure chabges from

meta-stable to stable state with increasing carbon and silicon contents.

Fig.K5

Fig.K5 is Greiner-Klingenstein diagram. The microstructure changes from meta sta-

ble(white cast iron) to stable (ferrite+graphite) with incresing the thickness, because the

cooling rate become slow.

Fig.K6

Cast irons are classified according to some methods.

By luster of fracture surface

Gray cast iron graphite flake, White cast iron ledeburite+primary Fe3C or austen-

ite, Mottled cast iron

By Morphology of graphite

A. Flake graphite, B. Lump graphite, C. Spheroidal graphite By compositions

Low carbon cast iron, High carbon cast iron, High silicon cast iron, Alloy cast iron

By the microstructure of matrix

Ferrite cast iron, Pearlite cast iron, Austenite cast iron, Bainite cast iron(acicular cast

iron)

By mechnical properties

Normal cast iron(lower 300MPa), Tough cast iron(over 300MPa)

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Fig.K7

Fig.K7 shows the morphology of flake graphite. Usually, the graphite in gray casy iron is

shown in figure A.

Fig.K8

Spheroidal graphite is formed by adding cesium (Ce) or magnesium (or magnesium ally).

Flake graphite changes spherical shape. By the change in shape of graphite, the toughness

is improved and the forging can be applied. It is called as ductile cast iron.

Fig.K9

Malleable cast iron is made from white cast iron by heat treatment. The heat treatment

for making black heart malleable cast iron is devided into two steps. In the first step, the

cementite in ledebrite is decomposed to lump graphite, and the cementite in pearlite is done

during the second step. If without the second step, the pearlitic malleable cast iron can be

made.

Fig.K10

White heart malleable cast iron is also made from white cast iron. The product made

from white cast iron is heated in 900 1000 for long time with oxidant or oxide. The

surface of the casting is decarburized and the microstructure is ferrite. Interior portion of

the casting, cementite is decomposed to graphite.

Table K1 Generally it is difficult for cast iron to be welded because of its high carbon content and

low ductility. When the cast iron is melted and cooled rapidly, graphite is decomposed into

molten iron and large amount of cementite is formed during cooling process. That is, all

cast iron change to brittle white cast iron by rapid cooling.

Next, in the case of repairing the casting, oil remains in a gap between graphite and

matrix. Since this oil disturbs the welding, we should burn the surface of casting before

welding.

The ductility of cast iron is very poor. Thus, it is easy to occur the crack due to weldingthermal stress. Two methods are useful to prevent the cracl; peening and pre-heating.

Table K1 shows the weldability of cast irons.

Fig.K11

There are two ideas to weld the cast iron. One is the method with small heat input to

reduce the thermal stress. Another is the method with the preheating at high temperature

and the very slow cooling. In either event, the welding of cast iron is difficult.

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