3. Find out which sentences do not correspond to the con­tents ofthe text

1. The sand and ceramic moulds are reusable.

2. In permanent moulding the metal mould cannot be reused.

3. Permanent moulding is not a precision casting process.

4. In permanent moulding the only manual operations are setting cores, pouring metal into the mould and removing the solidified cast­ing from the mould.

4. Read the text and

л) find the answers to the questions

1. What advantages does the permanent mould process offer?

2. What steps do the processes for all metals involve?

b)find the English equivalents for the following

• очень хорошая чистота обработки поверхности

• высокие физические характеристики

• легкие сплавы

• значительные преимущества

• скорость охлаждения

• удаление отливки

• низкая стоимость и отсутствие дефектов в отливке

Theoretically, exceptionally good surface finish and high physical properties can be obtained by casting in a permanent mould, in case of

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light alloys, this is true. The permanent-mould process offers significant advantages at low cost and in casting soundness. However, few parts are adaptable, and the processes for all metals involve basically similar steps of pouring metal into the metal mould, controlling the cooling ratio and removing the casting after a predetermined cooling period.

5. Translate at sight

Процесс литья в постоянные формы используется для полу­чения значительного количества маленьких отливок. Процесс лег­ко поддается механизации. Заливка металла производится в ме­таллические формы, которые могут употребляться много раз. При литье в постоянные формы песок не употребляется совсем. На структуру металла и доброкачественность отливки большое влия­ние оказывают скорость охлаждения и процесс затвердевания ме­талла в форме. В настоящее время точные вычисления и научное проектирование форм способствуют получению доброкачествен­ных отливок.

Unit 12

DESIGNING

FOR CASTING SOUNDNESS

The proper design, coupled with sound heading and gating, is required to assure the casting be sound and free from de­fects. Because metal shrinks during solidification in a complex but predictable manner, a casting engineer must carefully plan the progress and rate of solidification in each section, so that shrinkage void will be concentrated in feed metal areas and net in,the casting proper.

The following is a brief and oversimplified description of the so­lidification process for metals.

In order to understand how metal freezes , it might be well first to follow what occurs during the heating and melting of steel.

If we examine the change of the specific volume of carbon steel (0,35 % C) at room temperature, we will notice that there is a 7.2 per

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cent expansion in heating this steel from temperature to its melting point. During the melting time there is an additional 3 per cent increase in volume of 1,6 per cent for each 100 °C of superheat. Now if we were to pour this liquid steel into a sand mould, the exact reverse proc­ess would take place during cooling and freezing. For example, as su­perheat of molten steel started to lose temperature, there would be a con­traction of approximately 1.6 per cent for each 100 °C superheat lost.

Since there is a decrease in volume of approximately 3 per cent as the steel transforms from the liquid metal to the solid, i. e. freezing occurs, it is necessary to provide the liquid metal for compensation if a shrink hole is not to form. Many of the problems encountered in pro­ducing a sound casting are due to the basic neglect of this fundamental law in design of casting.

In essence, two conditions must be met in order to avoid shrink­age defects associated with this decrease in volume during solidifica­tion. First, there must be a reservoir of liquid metal, which will remain liquid until the casting itself has frozen. This reservoir is usually pro­vided in the form of a heavy section of metal placed at a carefully cho­sen point on the casting. This appendage to the casting is usually called a «riser».

The second condition which must be met to avoid shrinkage de­fects is that there should be an open pathway for the liquid metal to flow to the point in the casting that required feeding metal. In other words, there must be directional solidifications towards the riser. The usual foundry practice is to make the mass of the riser greater than the heaviest equivalent section of the casting, that is, to feed so that it will remain liquid until the casting is solidified.

Exercises

1. Answer the following questions

1. What is required to obtain a sound and free-of-defects-casting?

2. Does metal shrink in a complex manner during solidification?

3. Can the progress and the rate of solidification of metal in each section be predicted and planned?

4. What becomes with the volume of metal which is heated from room temperature to superheat?

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5. What becomes with the volume of metal during cooling and freezing?

6. What is the decrease in volume manifested by?

7. What two conditions must be met in order to avoid shrinkage defects?

8. Why is it necessary to make the mass of the riser greater than the heaviest equivalent section of the casting?

1. casting defects

2. riser

3. void

4. contraction

5. expansion

6. superheating

7. shrinkage