3218

ribs have became comparatively cool. Due to thj.s some ten­

sile stresses w ill be developed-5n the body of the thick

part of the casting while conpressive stresses in tho ribs.

Tc correct and prevent this defect, the pattern is set, during the ramming of the drag-half mould, not perf­ ectly straight, but bent in the direction opposite to the warping. Bedplatesand baseplates, which, as cast, have usually a bottom side heavier than the top one, should be combered by setting the middle of the pattern below the enc*s

from the normal linear expansion, such prolonged heating may lead to permanent dimensional changes as a result of

stress re lie f or because some structural transformations

may occur at the higher temperature. In such cases to pr­ event undesirable changes of casting dimensions it is nec­

essary to insert previously either a stress re lie f or a

stabilizing heat treatment into a technological process of the casting treatment. The heat treatment of this kind is usually the cycling heating of castings up to the tempera­ ture determined by the constitutional or structural condi­

tions of an alloy. But it should be done with a rate of heating which does not give additional stresses.

Рог instance, large thin-walls steel castings, such as car

frames, can be warped at the annealing if their uniform heating is not ensured. Consequently, the procedure of

heat treatment must take into account specific features of castings and alloys of which they are made.

But these internal, residual or locked-up stresses

arise pot only from the temperature differences resulting from the unequal heating and subsequent cooling of parts of varying sections, but may be due to phase transformat»'*

ions in the alloy. They also may be relived by re-heating

the casting at a temperature and for a time sufficient to allow the necessary plastic deformation to occur and to

at the fir s t #ycle after which the cylinder diameter inc­ reased, in average, by 0,04 mm. But two next cycles were required to -stabilise the effect of the heat treatirent.

So dimensions of castings were stabilised before machining due to stress re lie f.

Although the most of castings do net need any heat

treatment, the stress relief is applied for some alloys as the only heat treatment following the casting process.

the changing of the casting design to equalize wall thic­ knesses and, hence, by the ensuring of an uniform crysta­ llization rate in a ll eross sections of the casting^

The correct design of a gate system and a dispersed input of the metal into a mould cavity as well as the cor­ rect use of ch ills w ill ensure the fulfilment of this con­ dition when the casting has parts of unequal thickness.

To prevent the casting warping it ought to endeavour to use mould and core mixtures allowing a solidifying and cooling alloy to shrink freely or restricting to this in a small degree. In general, ary measures decreasing int-

Cross .joint

Cross joint is a defect when dimensions of separa­

te casting parts ip planes perpendicular to a split plane are correct but these parts are displaced relatively to

-incorrect fixing of pattern halfs on cope and drag pattern platens }

-absence of the proper control above the condition

Accasionally a cross joint may be obtained due to cores shifting because they are not securely anchored or due to their warping during,drying and pouring an alloy. Dimensional ituhnou-acies due to core distortion or shift­ ing are practically serious, since they nay lead to a red­ uction in wall thickness but, at the same tine, they can­ not, as a rule, be detected by visual inspect ion of cast­ ings as the external dimensions are not affected.

Fig*2*3* Samples of a typical mould shift when a cope and drag do not match*

system and unsuccessful organization of molten metal str­ eam immediately in the mould cavity are the frequent rea­

son of this defect in thin walls castings* In this case heat losses from liquid metal to the mould, especially

from the front of the advancing stream, are too big. When the temperature of molten metal reaches the point of its

Poor venting of moulds and cores can have a cont:>*

ibutory effect. When metal enters a mould the pressure

of the air unside the mould builds up due to its heat ex­

pansion. Besides, new gasses occur in the result of an interaction of molten metal with mould materials.

Raising the ppuring temperature and reconsidering the position, size, and number of ingates w ill help to eliminate this defect. It also should be taken into acc­ ount that at the designing of a gating system it is nece­