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Пермский национальный исследовательский политехнический университет
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Table 2.12 shows some examples of gasses solubility, typical inclusions occuring in the castings due to the gases and treatment £22j .

It is clear that hydrogen, oxigen, sulphur and carbon are the most likely examples of gas contamination in industrial cast metals.

 

 

 

 

Table

2.12

’Examples

of

gas

so lu b ilitie s

iz^various

metals,

typical gas

or

to lid

inclusions

that result

and methods

used to remove the inclusions [22] •

metal

©r al­

loy Gas group

*

Magnet Я

ium '

Equilibrium solubility Typical inc­ Examples of

H in cmrVlOOg at

lusions

due i -.methods

 

s .t .p . 0,S,C

and N in

to gases in

used to

en~

wt. %

 

 

melting

and

ntrol

or

to

 

 

casting

pr-

remove ga­

solidus

liquid us

ses

or

so­

ocesses

 

lid

inclu­

 

'MF J 1T ,J

 

 

sions.

 

 

18

 

 

Gas swept~

i

2G

ч

 

 

 

 

 

out by

chl­

 

 

 

 

 

 

 

 

 

 

orine

gas

 

 

 

 

 

or

decomp­

 

 

 

 

 

osition pro­

 

 

 

 

 

ducts

of

 

 

 

 

 

 

chlorides

 

 

 

 

 

such as C2

 

 

 

 

 

C W

V

 

 

 

 

 

 

0 civ

 

 

Table 2.12 ( contd. )

Nickel H 18.5

39.0

H^ and IhO

Degassing by delib­

 

 

 

e ra te oxidation fo—

 

 

 

llov;ed by

controlled

 

 

 

deoxidation.

0 .0 2

0.256 at

CO'jCO^ jlliO ,

Deoxidation with

 

eutectic, or couplex

oarbon,magnesium,

 

1456°C

oxides in

silicon ,

aluminium.

 

 

alloys

 

 

 

21.5 at

MS or

Kept low by careful

 

eutectic^ complex

selection

of charge

 

649eC

sulphides

materials

and con­

 

 

in alloys

trol during melting.

 

 

 

Some desulphurisat-

 

 

 

ion achieved by ad­

 

 

 

ding Mg or Ca.

0.552.2 at Graphite, eitectic^CO, CO,

1318°C

Iron

H 6.8

2 7.0

H^HgO

CO and COp kept low by controlling acc­ ess to oxigen sour­ ces.

During refining CQ, lowers hydrogen con­ tent ("carboriboil")

-o.oo; 0.16 at

complex

Some prevention by

eutectic, oxides,C6,

effective slag cover.

1525°C

COo

Deoxidation by Si,

 

 

ivIn,Al,Ti,Zr,Ca,Mg

 

 

or their combinati­

 

 

ons.

- 1X3 -

.Table 2.12. ( contd. )

Iron

S

<0.05

51.5 at

Jcouplex

Reduction in su]

(contd)

 

eutectic, sulphides

content by basic

 

 

 

988°C

treatment or met

 

 

 

 

 

 

GigуОз,0s.), ITeut

 

 

 

 

 

 

isation by add.lt

 

 

 

 

 

 

|(Bhi).

 

, ,

 

 

 

 

1

 

 

 

 

 

I

 

' N

0.011

0.C4Q

complex

During refining,

 

 

 

 

 

nitrides

lowers nitrogeni

 

 

 

 

 

 

tent (“ carbon bol

 

 

 

 

 

 

Neutralisation \

 

 

 

 

 

 

additions (A1,1

 

 

_

....

J

 

 

The phenomenon of

the

discontinuous drop in solii

lity may be

the reason

of an extensive

evacuation of dii

 

 

 

 

 

'*

V,'

olved

gases

from the

metal

that may cause the gas holes

porosity formation. From the point of view of castingj allurgy, the problems which arise when the amount of $

the liquid metal exceeds that which can be retained id ution in the solid state are of particular interest, Л concentration of the gas in the remaining liquid will crease as solidification progresses, and at a certain3 bubbles are nucleated and grow in the following manner

Н

^ 2

1&2 (gas)

 

 

 

or

 

 

 

 

 

 

H

+ О

^

HgO (gas)

 

 

The former is

an example of

the behaviour of a sin>»

pie molecular gas, and the latter that of the reaction

which may occur when two dissolved

gases

are present .The

gas bubbles formed may rise

to the

free

surface

and escape,

or more generally be trapped in the solidifying

alloy fo r ­

ming cavities ( voids or porosity

) of different size, sh­

ape and distribution.

However nucleation and growth of

the bubbles are possible only under the condition that the pressure of gas is controlled by the following general

condition

(

Fig.2.18

) :

 

 

 

 

Pg

^

Pa + Ph *

2 6

t

 

 

r

 

Where P©-

the

internal pressure of the gas as defined by

 

 

the

equilibrium constant;

 

P -

 

the

atmospheric pressure;

 

Pjj—the

hydrostatic pressure

of a metal column

above the appropriate point, which is determined as t

(с)

Pig.2.19, Typical variations in (a) size,(b)shape and (c) distribution of cavities In cast structures due to formation oi simple molecular and compound gases du­ ring solidification or in tlie solid state* Solid incl­ usions follow similar patterns of distribution in the cast structure [22J

Foundry Department

"Casting defects and

Dr* V* Bastraicov*

measures of their prevention*1*

may be concentrated sufficiently

in

the

reeidual liquid,

to react, e.g . with hydrogen or

carbon,

and give

rise to

gaseous products towards the end of

the

freezing

process*

In some cases cavities may originate due to the vo­ lumetric contraction on freezing and they then act as free space volumes into which the gas diffuses when it exceeds the solubility lim it.

Both the macro-and micro-pattern of gas cavity dis­ tribution (Pig.2.19) are related to the type of crystal forming, and the direction of solidification . Por example, they my forla between the arms of dendritic crystals, but when formed at the end of solidification they are mainly located at the boundaries between crystals,

Ih addition to the gases dissolved in alloys during

melting further gassing of castings can occur during the

mould fillin g stages

of

the casting process. 3h general,

gas holes

can arise

in

castings from the

following

number

of causes

t

 

 

 

 

1- due to

unsatisfactory quality of a liquid metal because

of its

high gas saturation and the extensive extraction

Of the

gas during

the solidification

process.

Gases

may be

absorbed in a molten metal in

t

 

^a) furnace from the atmosphere and from wet charges;

(b)the furnace or ladles from incompletely dried linings or wet tools, and

(c)passing through air from furnacesto ladle or from ladle to mould.

Cooling in the mould the metal evalves dissolved ga­ ses forming gas holes situated across the whole casting 85 small gas bubbles or pinholes as it has been said above,

2- as the result of chemical reactions taking place in the molten metal during cooling and solidification , for example, an origin of blow holes in cast steel ingots as the result of the well-known reaction s

PeO Ф C = Pe + CO

Blowholes may also occur as the result of the rea­ ction between manganese and sulpher at their high content in the metal.

In steel castings subsurface pinholes can result fro; incomplete deoxidation of the molten metal.

3- due to high gas creatability of moulding end core mix­ tures as the result of the reaction of the molten metal with the mould or core materials. In some cases fumes

- 1X9

or hydrocarbons are generated when the molten metal comes into contact with the oil-binder of the core . ha other cases surface reactions cause subsurface porosity or pin­

holes. in aluminium alloys containing more than about

1 per cent of magnesium, a reaction tends to occur betw­

een the magnesium of the alloy

and the water vapor of the

mould :

 

 

Mg + H^O s MgO 4*

Hg

(gas)

Hydrogen-filled pinholes

at

the surface or beneath

it are the result. Some copper-base and other alloys a±e subjected to similar defects unless proper precautions are observed;

4— Because

of low permeability of moulds and cores and

their insufficient or incomplete ventilation. These

phenomena may be caused by different

sources.

They are*

(a) the us© of fine

moulding sands,

(b)

the high content

of clay

or

binder,

(c)

the high degree

of overramming

the

mould,

(d)

the lack

i f

vents or their blocking

due to

mi­

smatching or due to penetration of the metal or paste into vents , etc.

All of this hardens the removal of gases being formed during the fillin g the mould with molten metal and after that. Whenever the rate of evolution of gas exceeds the

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