Large, tall, long, easy, hot, big, cold, nice, bad, strong, short, wide, good, happy, high, low, busy, well, little, many, far
С инфинитивом без частицы to после глаголов
Elastomers are similar to thermoplastics but have sufficient cross-linking between molecules to prevent stretching and creep
RESINS
BASIC PRINCIPLES OF WELDING
GEAR
CONSTRUCTION OF AN AUTOMOBILE
TWO-STROKE AND DIESEL ENGINES
AC MOTORS
AUTOMATION IN INDUSTRY. FIXED AND PROGRAMMABLE AUTOMATION
HISTORY OF ROBOTICS
COMPUTERS
Навигация
BASIC PRINCIPLES OF WELDING
English language for technical colleges
393037
знаков
13
таблиц
0
изображений
6. BASIC PRINCIPLES OF WELDING
A weld can be defined as a coalescence of metals produced by heating to a suitable temperature with or without the application of pressure, and with or without the use of a filler material.
In fusion welding a heat source generates sufficient heat to create and maintain a molten pool of metal of the required size. The heat may be supplied by electricity or by a gas flame. Electric resistance welding can be considered fusion welding because some molten metal is formed.
Solid-phase processes produce welds without melting the base material and without the addition of a filler metal. Pressure is always employed, and generally some heat is provided. Frictional heat is developed in ultrasonic and friction joining, and furnace heating is usually employed in diffusion bonding.
The electric arc used in welding is a high-current, low-voltage discharge generally in the range 10-2,000 amperes at 10-50 volts. An arc column is complex but, broadly speaking, consists of a cathode that emits electrons, a gas plasma for current conduction, and an anode region that becomes comparatively hotter than the cathode due to electron bombardment. Therefore, the electrode, if consumable, is made positive and, if non-consumable, is made negative. A direct current (dc) arc is usually used, but alternating current (ac) arcs can be employed.
Total energy input in all welding processes exceeds that which is required to produce a joint, because not all the heat generated can be effectively utilized. Efficiencies vary from 60 to 90 percent, depending on the process; some special processes deviate widely from this figure. Heat is lost by conduction through the base metal and by radiation to the surroundings.
Most metals, when heated, react with the atmosphere or other nearby metals. These reactions can be extremely detrimental to the properties of a welded joint. Most metals, for example, rapidly oxidise when molten. A layer of oxide can prevent proper bonding of the metal. Molten-metal droplets coated with oxide become entrapped in the weld and make the joint brittle. Some valuable materials added for specific properties react so quickly on exposure to the air that the metal deposited does not have the same composition as it had initially. These problems have led to the use of fluxes and inert atmospheres.
In fusion welding the flux has a protective role in facilitating a controlled reaction of the metal and then preventing oxidation by forming a blanket over the molten material. Fluxes can be active and help in the process or inactive and simply protect the surfaces during joining.
Inert atmospheres play a protective role similar to that of fluxes. In gas-shielded metal-arc and gas-shielded tungsten-arc welding an inert gas—usually argon—flows from an tube surrounding the torch in a continuous stream, displacing the air from around the arc. The gas does not chemically react with the metal but simply protects it from contact with the oxygen in the air.
The metallurgy of metal joining is important to the functional capabilities of the joint. The arc weld illustrates all the basic features of a joint. Three zones result from the passage of a welding arc: (1) the weld metal, or fusion zone, (2) the heat-affected zone, and (3) the unaffected zone. The weld metal is that portion of the joint that has been melted during welding. The heat-affected zone is a region adjacent to the weld metal that has not been welded but has undergone a change in microstructure or mechanical properties due to the heat of welding. The unaffected material is that which was not heated sufficiently to alter its properties.
Weld-metal composition and the conditions under which it freezes (solidifies) significantly affect the ability of the joint to meet service requirements. In arc welding, the weld metal comprises filler material plus the base metal that has melted. After the arc passes, rapid cooling of the weld metal occurs. A one-pass weld has a cast structure with columnar grains extending from the edge of the molten pool to the centre of the weld. In a multipass weld, this cast structure maybe modified, depending on the particular metal that is being welded.
The base metal adjacent to the weld, or the heat-affected zone, is subjected to a range of temperature cycles, and its change in structure is directly related to the peak temperature at any given point, the time of exposure, and the cooling rates. The types of base metal are too numerous to discuss here, but they can be grouped in three classes: (1) materials unaffected by welding heat, (2) materials hardened by structural change, (3) materials hardened by precipitation processes.
Welding produces stresses in materials. These forces are induced by contraction of the weld metal and by expansion and then contraction of the heat-affected zone. The unheated metal imposes a restraint on the above, and as contraction predominates, the weld metal cannot contract freely, and a stress is built up in the joint. This is generally known as residual stress, and for some critical applications must be removed by heat treatment of the whole fabrication. Residual stress is unavoidable in all welded structures, and if it is not controlled bowing or distortion of the weldment will take place.
Arc welding
Shielded metal-arc welding accounts for the largest total volume of welding today. In this process an electric arc is struck between the metallic electrode and the workpiece. Tiny globules of molten metal are transferred from the metal electrode to the weld joint. Arc welding can be done with either alternating or direct current. A holder or clamping device with an insulated handle is used to conduct the welding current to the electrode. A return circuit to the power source is made by means of a clamp to the workpiece.
Gas-shielded arc welding, in which the arc is shielded from the air by an inert gas such as argon or helium, has become increasingly important because it can deposit more material at a higher efficiency and can be readily automated. The tungsten electrode version finds its major applications in highly alloyed sheet materials. Either direct or alternating current is used, and filler metal is added either hot or cold into the arc. Consumable electrode gas-metal arc welding with a carbon dioxide shielding gas is widely used for steel welding. Metal transfer is rapid, and the gas protection ensures a tough weld.
Submerged arc welding is similar to the above except that the gas shield is replaced with a granulated mineral material as a flux.
Weldability of metals
Carbon and low-alloy steels are the most widely used materials in welded construction. Carbon content largely determines the weldability of carbon steels. Low-alloy steels are generally regarded as those having a total alloying content of less than 6 percent. There are many grades of steel available, and their relative weldability varies.
Aluminum and its alloys are also generally weldable. A very thin oxide film on aluminum tends to prevent good metal flow, however, and suitable fluxes are used for gas welding. Fusion welding is more effective with alternating current when using the gas-tungsten arc process to enable the oxide to be removed by the arc action.
Copper and its alloys are weldable, but the high thermal conductivity of copper makes welding difficult. Metals such as zirconium, niobium, molybdenum, tantalum, and tungsten are usually welded by the gas-tungsten arc process. Nickel is the most compatible material for joining, is weldable to itself, and is extensively used in dissimilar metal welding of steels, stainless steels and copper alloys.
Похожие работы
... compound-shortened word formed from a word combination where one of the components was shortened, e.g. «busnapper» was formed from « bus kidnapper», «minijet» from «miniature jet». In the English language of the second half of the twentieth century there developed so called block compounds, that is compound words which have a uniting stress but a split spelling, such as «chat show», «pinguin ...
... of job. Because I suppose if than marriage he must provide his family. And the young coups must that they meant for understand each other. Customs and holidays in English – speaking countries. - Customs and holidays in the urban community are of great importance for everyone. - I think no matter who the man might be he must know customs and holidays his country. What holidays do you ...
... was the first writer in Europe to formulate communist principals as a bases of society. The Renaisense in England. The prideses of Shakespeare. The most brilliant period of English literature was in the second half of the 16’th and begining of 17’th centure.Sometimes it’s called “Elizabethen age” after quen Elizabeth 5. England had become a geat world ...
... like that: what kinds of animals eat grass?» The boy brightened up. «Animals!» he exclaimed, «I thought you said admirals.» Unit 9 Grammar: 1. Passive Voice. 2. Пассивные конструкции характерные для английского языка. 3. Формы инфинитива. I. Language Practice 1. Practise the fluent reading and correct intonation: — Helölo, Tom! — Helølo, Nick ö. Here you ø are at ...
0 комментариев