3218

Fig,2#15, Scheme for the		calculation of	stresses
in the	casting	with Jjindered	shrinkage
r aH iron		m аПеаНе iron

Fig* 2.16, Variation of cast iron mechanical prop» erties. depending on teiqperature.

Foundry, Department	"Casting	defects and
Dr, V,3aetralafi£/,	measures of	their prevention **

- ки.

These conditions are (

1* high mechanical and thermal hindering to the contracting metal due to the casting design and mould technology j

2, The formation of grain boundry inclusions which

decrease fastening strength among crystals;

3, features of alloys and their chemical composition;

4, large primary crystallization and transcrystall­ ization;

5* high coefficient of shrinkage and high shrinkage velocity of metal in the given range;

6, the availability of blowholes and porosity which are the cause of stresses concentration.

The most important reason of the hot crack format­ ion belongs to the mechanical hindering to contraction which is especially high just after solidification while sand mixture which prevents shrinkage ds not sufficiently heated and do not loose their strength.

Of course, the formation of these cracks takes plade

in the weakest parts. Usually hot tears	occur in	the th i-
Ckest parts and in places of connections	of thick	and thin

cross-section parts of a casting ( Fig,2.14- b,d,h). Any

local hot	spot may be the	reason of this defect, Especi­
ally it is	dangerous for	thin - walls castings.

ernali on macro - or on micro - scale but				in a ll cases	they
have a black* hardly oxidized surface of				an irregular	sha­
pe as	a result of intensive	oxidation at		high temperatures,
	In order to eliminate	this	defect formation several
steps	are necessary depending on		reasons	of their origin*

They are }*=-

1. to have a liquid alloy of a sharply given ehemica l composition without hurmful additions or with them, but their amount must not exceed the proper limits established for this alloy j

2, to	eliminate reasons causing a hindered shrinks
age of a casting in the mould during the solid­
ification and cooling ( to use soft moulding
and core mixtures with organic additions, burn­
ing down and creating porosity in the mould,					due
to	that the	p liability and collapvSibility		of	the
mould and aore hardly increasee
They may be s
cellulose,		wood saw dust, peat, etc.	The	amount
of	them is	changed in ranges 10-20% in volume			or
up	to 5% in weight depending on its		composition);

3* to eliminate rapid transitions from thin to thick cross -sections of the casting, to make smooth

and feeders and ingot noulds made of cast iron

inclined to cradling before tlie use |

3- to preheat casting made of cast iron and allo­

yed or high carbon steels up to the temperature

250 - 300°C before either welding or flame* cut­

ting out.

Gas holes

Sometimes castings are affected by gas holes and

porosity. Gas holes are voids in casting walls having

a clean, smooth, deoxidized or sligh tly7oxide tinted sur­

face. Gas holes, as any unsolidness of a casting, deer*» ease its strength and, hence,are an inadmissible defedt of the casting. Castings with the gas holes are usually rejected.

Gases in a casting metal can be in view of both solutions and chemical combinations.

Gases are usually soluble in solid and liquid metal but they are almost universally more soluble in liquid metals than in solid ones. .2 ссЛ ability of a gas in molten metals is submitted to Si-verts* law «