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 nonconsumable, 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 oxidize 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 fluxesand 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 annulus 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 may be 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 oftemperature 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. Control is exercised by welding technique, jigs and fixtures, fabrication procedures, and final heat treatment. 

Translate the following Text in writing.

3. Complete the following sentences:

1. A characteristic feature of fusion welding is:

a) molten metal b) low-voltage discharge c) inert atmosphere 48

Furnace heating is usually employed in 

a) friction joining b) diffusion bonding c) ultrasonic joining

The consumable electrode is made 

a) negative b) positive c) neither

Total energy input in all welding processes is

a) is greater than required to produce a joint b) is smaller than required

to produce a joint c) equals to required to produce a joint

Reactions of most metals with the atmosphere or other nearby metals can 

1) improve the properties of a welded joint b) make the properties of a

welded joint worse c) never influence the properties of a welded joint

The most common gas used in gas-shielded metal-arc and gas-shielded

Tungsten-arc welding is

a) argon b) oxygen c) carbon dioxide

If not controlled, residual stress results in

a) precipitation processes in welded structures, b) freezing of the weldmetal c) bowing or distortion of the weldment

True or false?

1. There is always a welding pool in solid-phase welding processes.

2. Total energy input in all welding processes is greater than needed to

produce a weld.

3. Reactions of metals with the atmosphere or other nearby metals are

favorable to the properties of a welded joint.

4. Fluxes and inert atmospheres play a protective role and prevent oxidation.

5. The heat-affected zone is a region with unaltered properties.

6. Residual stress is present in all welded structures.

5. Answer the following questions:

Сварочное оборудование

"WELDING PROCESSES & EQUIPMENT"

(сварочное оборудование)

Read and remember:

You already know what kind of training you should go through to become

welder. Among other things, a welder should know about processes which

are similar to welding. Look through the vocabulary and the text below and

say what the main difference between welding and related metal joining

processes is.

WELDING PROCESSES & EQUIPMENT

Lead-in

You already know what kind of training you should go through to become

welder. Among other things, a welder should know about processes which

are similar to welding. Look through the vocabulary and the text below and

say what the main difference between welding and related metal joining

processes is.

Vocabulary

soldering        пайка; пайка мягким (легкоплавким) припоем

tinning          лужение; облуживание

leading          свинцевание

brazing          1) пайка твердым припоем (из меди и цинка) 2)

                 покрытие медью

sweat            паять, запаивать, припаивать (in, on)

gimmick          1) сложное приспособление Syn: gadget 2) а) прием,

                 трюк, уловка, ухищрение, хитрость

filler metal     присадочный металл, присадка

filler rod       присадочный пруток; присадочная проволока

heat buildup     теплообразование, тепловведение

heat distortion  деформация (материала) из-за теплового нагрева

stitch welding   прерывистая шовная сварка; точечная сварка

                 перекрывающимися точками; автоматическая

                 точечная сварка

Reading

     Text 1. Introduction to Welding Processes & Equipment

  Among the first things a new welder needs to understand, is what the

different kinds of welding processes and equipment are, and their application.

A quick rundown:

Terms:

Soldering: Bonding by melting a soft metal to the surface of pieces to be

joined. Low temperature. Good for joining dissimilar materials. Most

common solders are lead-tin alloys.

Tinning: A soldering process, where the surface of a metal is coated with

solder.

Leading: A form of soldering, solder is used to fill in the surface of metal.

Brazing: Similar to soldering, but uses a higher temperature to fuse the filler

metal to the work pieces. Stronger bond. (Includes "Silver Soldering") Work

heated to pre-melt temperatures.

Welding: Joining 2 similar work pieces by melting them together, usually

with an additional filler rod of some sort to take up space. Materials must be

similar.

Cutting: Work is heated to melting point and beyond, and "cut" by oxidizing

metal. (Literally burning it away).

Shield: A barrier to keep oxygen away from heated work to prevent

oxidation. Includes chemical coatings called flux (liquids, pastes, solids,

which may be vaporized into a barrier gas when heated), and inert gasses.

Oxidation of the surfaces will prevent proper bonding of the metals.

                            Gas Welding

Uses Flame from burning gas to create welding heat.

Propane torch: (Soldering, heating) Good for sweating pipes, starting fires,

and spending hours trying to heat frozen bolts, while the surrounding metal

gets just as hot.

Oxyacetylene torch: (Cutting, welding, brazing, soldering, leading) Most

universal and useful welding tool. (Uses Acetylene gas and Oxygen for hot

flame) With the right bits, rod, and technique, you can weld almost anything.

Good for cutting anything from sheet metal to the turret off a tank, lead

filling, brazing (a sort of hard soldering process) welding plate, welding sheet

metal, welding aluminium, heating frozen bolts, or alternately cutting them

off, drilling holes in plate, welding cast iron, shrinking and forming steel, and

can double as a flame thrower in a pinch. Drawbacks are: Overheating of

some types of work, harder to control quality of some processes.

Oxy-propane: (Soldering, brazing, heating) A cheap compromise between

low cost and portable propane, and Oxy-Acetylene. Better than the former,

not as good as the latter.

                              Arc welding

Uses an electric arc to create welding heat.

Basic AC & DC arc welders (AC is cheaper) Uses flux coated steel (or

other) rods of various types for different jobs. Makes some of the best welds

on heavy gauge steels and cast iron. Cutting rods can make clean holes

through thick stock, and are about the only thing which can cut Kryptonite

bike locks. Very difficult to weld thin metals. You can also get a carbon arc

torch to use on an arc welder to braze. Eastwood's "stitch" welder is a

gimmick used on an arc welder to buzz the rod in and out, which may help on

thinner stock. (learning how to weld better, or going to a different process is

usually a better idea.)

MIG (Metal Inert Gas): A DC arc welding process which uses filler metal

fed in the form of a spool of thin wire, shielded by flow of inert gas (He,

Argon) instead of flux used in Arc. Very fast, much easier than Arc Welding,

with less heat buildup. Very good for sheet metal, due to minimal heat

distortion. Harder to weld thick stock, as welds are weaker due to poorer

penetration. The modern choice for steel body work, it can also be used for

Aluminium with Argon as the shield gas.

TIG (Tungsten Inert Gas): A high frequency AC arc process which uses a

tungsten electrode shielded by an inert gas to create a fine, controllable torch.

Uses a separate filler rod, as in Oxy-Acetylene welding. Capable of welding

very thin metals. About the best process for Aluminium, Stainless steel, and

other exotic stuff.

Resistance welding: includes spot welding: Uses the heat generated by

electricity flowing through work to melt and fuse. i.e.- put an electrode on

either side of 2 overlapped sheets of steel, turn on power. Metal in between

heats up, and melts together. An old favorite for assembling car bodies.

Plasma Cutters: Not a welder, but related. A high voltage arc is used to

superheat and ionize a stream of air to the "plasma" state. The stream of

plasma makes a rapid, clean, narrow cut with minimal heating of the work

piece.

After-reading activity

Answer the following questions:

1. What is the main difference between soldering and brazing?

2. What is used by welders to prevent oxidation?

3. What makes soldering advantageous before welding?

4. What welding processes are suitable for welding thin/thick metal plates?

5. What makes plasma cutting better than gas cutting?

Vocabulary

coalescence       соединение, слипание; сращение

filler material   присадочный материал

molten pool       ванна расплавленного металла, сварочная ванна

gas flame         газовое пламя

solid-phase       твёрдая фаза

ultrasonic        ультразвуковой

friction          трение

furnace        печь

diffusion      1) рассеивание

               2) диффузия

high-current   сильноточный

low-voltage    низковольтный, низкого напряжения

discharge      разряд

arc column     столб дуги

direct current (dc) постоянный ток

alternating current переменный ток

layer            слой; пласт; ряд

molten-metal   капля жидкого металла

droplet

flux           флюс

inert atmosphere инертная среда

annulus        тех. узкое кольцо (зазор и т. п.)

torch          сварочная горелка (для автоматической сварки –головка)

base metal     основной металл

grain          зерно

precipitation  осаждение

residual stress остаточное напряжение

Find the English equivalents for the following words and word combinations:

источник тепла, расплавленный металл, необходимый размер, сварной

шов, не нагретый металл, механические свойства, максимум

температуры, защищать   поверхности, быстрое охлаждение,

осуществлять контроль, препятствовать окислению, вступать в

химическую реакцию, термообработка, бомбардировка электронами,

зона термического [теплового] воздействия, общая потребляемая

энергия

***

Понятие о пайке. Виды пайки

1. Read the following Text:

Soldering

1. Soldering is employed when liquid tight joints are required. 2. They have comparatively low mechanical strength. 3. The tensile strength of a soldered seam may be from 3 to 10 kg per sq. cm. 4. The tensile strength of brazed joints is much higher (form 31 to 44 kg per sq. cm). 5. Metals may be soldered by means of a hand type soldering iron, a blowtorch and other methods. 6. Soldering iron may be plain, gas or electric. 7. Metals may be dipped in molten solder. 8. Sometimes the brazing and soldering of metals may use mechanized and automatic equipment. 9. Mechanized and automatic brazing methods include resistance brazing, arc brazing and others. 10. The selection of a mechanized brazing method should be based on the type of filler metal, weak material, size, of joined components.