An important feature of hot working is that it provides the improvement of mechanical properties of metals. Hot-working (hot-rolling or hot-forging) eliminates porosity, directionality, and segregation that are usually present in metals. Hot-worked products have better ductility and toughness than the unworked casting. During the forging of a bar, the grains of the metal become greatly elongated in the direction of flow. As a result, the toughness of the metal is greatly improved in this direction and weakened in directions transverse to the flow. Good forging makes the flow lines in the finished part oriented so as to lie in the direction of maximum stress when the part is placed in service.
The ability of a metal to resist thinning and fracture during cold-working operations plays an important role in alloy selection. In operations that involve stretching, the best alloys are those which grow stronger with strain (are strain hardening) - for example, the copper-zinc alloy, brass, used for cartridges and the aluminum-magnesium alloys in beverage cans, which exhibit greater strain hardening.
Fracture of the workpiece during forming can result from inner flaws in the metal. These flaws often consist of nonmetallic inclusions such as oxides or sulfides that are trapped in the metal during refining. Such inclusions can be avoided by proper manufacturing procedures.
The ability of different metals to undergo strain varies. The change of the shape after one forming operation is often limited by the tensile ductility of the metal. Metals such as copper and aluminum are more ductile in such operations than other metals.
I. Перепишите предложения, определите степени сравнения прилагательных или наречий, переведите на русский язык.
1. London is the largest city in Europe.
2. He is the youngest in our family.
3. She speaks English better than he.
4. Tree is one of the largest and oldest plants on the Earth.
5. This farm has the poorest soil in our region.
II. Перепишите предложения, определите видовременные формы глаголов и укажите их инфинитив, переведите предложения на русский язык.
1. He did not go to the country.
2. We will go to the library in the evening.
3. We went home as it was cold.
4. They had done much work by 7 o'clock.
5. They have seen them this month.
III. Перепишите предложения, подчеркните в каждом из них модальный глагол или его эквивалент, переведите предложения на русский язык.
1. I am to be at the library.
2. She is to come.
3. You must look through this journal.
4. May I take your dictionary?
5. He could play chess well.
IV. Перепишите предложения. Подчеркните причастия, установите функции каждого из них (определение, обстоятельство, часть сказуемого). Переведите предложения на русский язык.
1. Preparing for the examination she worked at the library.
2. Speaking she entered the room.
3. It was not snowing.
4. A reading student is my friend.
5. I am going to the library now.
V.Прочитайте и устно переведите весь текст. Письменно переведите абзацы 2, 5