Recovery and Recrystallization

 

The process of recovery occurs, when a strain-hardened metal is heated to comparatively low temperatures, usually below (0.2 or 0.3) T melting point (Tmp). It comprises the relief of microstresses and partial elimination of distortion of the crystal lattice as a result of reduction in density of structural defects.

The ordinary set of mechanical properties does not usually exhibit any changes in the recovery process.

A further rise in temperature increases the mobility of the atoms, and when a definite temperature is reached, new equiaxed grains are formed (Fig. 4.4). The formation of new equiaxed grains in place of the oriented fibrous structure is called recrystallization treatment or primary recrystallization. As a result of recrystallization the effects of strain hardening are practically completely eliminated; and the properties approach their initial values. As may be seen in
Figure 4.4, the tensile strength and especially the yield point drop sharply upon recrystallization, and the ductility (d and j) increases. This reduction in strength and hardness can be explained by the elimination of lattice distortion and the drastic reduction in dislocation density (from 10¹⁰…10¹² to 10⁶…10⁸ cm^-2). The lowest temperature t_br, at which recrystallization begins and proceeds and at which metal is softened is called the temperature threshold of recrystallization.

The temperature t_br is about 0.4 T_mp for commercially pure metals. For pure metals this temperature is reduced to (0.1 to 0.3) T_mp and for solid solution alloys it is raised to (0.5 to 0.6) T_mp. As the first appoxitation this rule enables the temperature at which primary recrystallization begins to be determined. It is about -33°C for lead, about 270°C for copper and about 450°C for low-carbon steel. These temperatures separate cold and hot metal working.

Fig. 4.4. Effect of heating on the mechanical properties and structure of strain-hardened steel

 

To eliminate the effect of strain hardening metal is heated to higher temperatures than t_br to ensure a high rate of recrystallization and completion of the process. Such heat-treatment is called recrystallization annealing (it is carried out at temperatures from 650 to 750°C for steel).

Collective recrvstallization. When the process of primary recrystallization has been completed, subsequent heating leads to growth of recrystallized grains. This process is called collective recrystallization. In collective recrystallization period the mechanical properties are reduced because of large sizes of grains.

Overheating and Burning. Prolonged heating of steel at temperature above t¢ (Fig.4.4) leads to the formation of exceptionally large actual grains, both at these temperatures and after cooling to 20 °C. This effect is called overheating. Overheated steel has a coarse crystalline fracture and low mechanical properties. Overheating can be corrected by repeated heat treatment.

Heating steel to a higher temperature than that causing overheating, especially in an oxidizing atmosphere, leads to burning. This is accompanied by the formation of iron oxides along the grain boundaries. Burnt steel has a stony brittle fracture. Burning is an irremediable defect of steel.

 

Technological Plasticity

Technological plasticity is the main property required for metal forming processes. The plasticity of metals depends on:

- alloy composition and its structure;

- working temperature;

- scheme of deformation;

- rate of deformation.

Pure metals possess higher plasticity than their alloys. The less carbon contents, the higher is the plasticity of steel. When temperature increases the resistance of metal to deformation decreases, because of decrease in strength and hardness.

In every point of stressed body elementary very small cube may be picked out so, that plane stresses should be perpendicular to its plane surfaces. There are 9 cases of stressed states with one, two and three main stresses (Fig. 4.5). An alloy has higher plasticity when it undergoes compressing stresses and it has lower plasticity in case of tensile stresses.

The degree of deformation at first decreases, then raises with the increase in deformation rate.

 

Fig. 4.5. Possible cases of stressed state

 

Heating of Metals

 

There are three types of metal forming with respect to work temperature: cold forming, thermomechanical treatment and hot forming. The main type is hot forming, or hot plastic metal working. As we know, cold metal working causes strain hardening (sometimes it's favourable effect), decrease in plasticity and, as a result, cracks in billets appear. During hot plastic working metal has high plasticity, it does not undergo structural changes and strain hardening, capacity of forming machines and deformation forces required are less as compared with cold forming. But hot forming causes a scale formation, which leads to metal losses and dimensional accuracy decrease.

Heating temperatures for steel are (Fig. 4.6) by 100…200°C below solidus line AE and by 25…50°C above the line GSK, that is hot forming is carried out when steel has homogeneus austenite structure (hypereutectoid steel has structure: austenite + carbides).

Fig. 4.6. Heating temperatures for hot plastic metal working

 

According to a source of heat a distinction is made between flame (gas or oil) reverberatory furnaces and electrical resistance reverberatoryfurnaces. According to a working principle a distinction is made between bath and continuous furnaces (Fig. 4.7).

Flame furnaces are used for big ingots, both electrical induction and contact heating devices are used for small billets.

 

Fig. 4.7. Chamber (bath) (a) and continuous (b) heating furnaces: 1 – bottom; 2 – billet; 3 – gas burner;

4 – lid; 5 – unload hole; 6 – flue; 7 – pusher; I – preheating zone (600…800°C);

II – zone of maximum heating (1250…1350°C); III – Standing zone (1200…1300°C)

 

Rolling

 

Rolling is the main method of metal forming. Bars, pipes, double tees, corners, sheets, balls, wheels and other products are manufactured using this method. Rolling is a method, in which revolving rolls are used for deformation of material.

A distinction is made between three rolling methods (Fig. 4.8): lengthwise (a), cross (b) and helical (c).

Fig. 4.8. The main methods of rolling: 1-roll; 2-billet; 3-mandrel

 

Let us consider the lengthwise rolling (Fig. 4.9). Sizes of a billet before rolling: L, B, H; after it 1, b, h, and L<1, B<b, H>h.

Fig. 4.9. Deformation of a billet in lengthwise rolling

Degree of deformation in general named drawing (extension) coefficient:

                                         (4.5)

 

Four main types of the rolled stock (products) are known:

- shape rolled stock;

- sheet rolled stock;

- pipe (tube) rolled stock;

- special rolled stock (periodical, balls, wheels and so on).

 

Tools and Equipment for Rolling. Metal is deformed between rolls, which may be plain
(Fig. 4.10a) or size (Fig. 4.10 d), i.e. with grooves on their surfaces. Two grooves of size rolls form a roll pass. The pass may be open (fig 4.10 b) or closed (Fig. 4.10 c).

Fig. 4.10. Milling rolls and passes: 1 – barrel; 2 – neck; 3 – capture

 

Equipment for rolling is named a mill (Fig. 4.11).

According to the number and arrangement of roll stands, a distinction is made between: duo mill; three-high mil; double duo mill, or four-high mill; multiroll mill; universal mill.

Rolling technology. Duo rolling mills, as a rule, are used to manufacture half-finished products: blooms and slabs. The mill, named blooming, is used for squeezing of ingots to obtain blooms with cross section from 150x150 to 450x450 mm. Then blooms are used for production shape rolled stock. The mill named slabing is used to produce slabs, which have cross section form 65x1600 to 300x2600 mm and then are used for sheet rolled stock manufacture.

Fig. 4.11. Scheme of a mill: 1-bed of roll stand; 2-4-pressing device; 5,6-support and working rolls;

7-pinion stand; 8,10-clutch; 9-reduction gear; 11- electrical motor

 

Three-high mill has two passes and works similar to duo mill.

Double duo and multiroll mills provide the use of working rolls with small diameter, and by this to reduce deformation zone volume without bending of the rolls.

To receive a required cross section of rolled product bloom or slab is passed through systems of passes (Fig.4.12).

Welding pipes are produced from skelp (Fig. 4.13). Process includes forming of plane half finished article (skelp) in a pipe, welding and finishing operations. Skelp is deformed at temperature 1300…1500°C and thereby furnace welding is mainly employed. Electric arc welding also may be used.

Fig. 4.12. Production of double-tee bar

Fig. 4.13. Process of welding pipes production

 

Extrusion of Metals

 

Extrusion is a comparatively new and foremost process among the industrial methods by which metal is wrought into useful forms. Essentially it is a process of converting a block of solid metal into a continuous length of uniform cross-section by forcing it to flow under high pressure through a die orifice, which is shaped to impart the required form to the product.

In the majority of cases pressing is a hot working operation, so a billet is heated to provide a suitable degree of softness and plasticity, but sometimes (e.g. for plastic metal) the cold process may be used.

The sketches in figure 4.14 serve to illustrate the essential principle of the process. At the same time, it also explains the distinction between two methods of working, known as direct and inverted extrusion.

6

7

.

Fig. 4.14 Direct (a, b) and inverted (c) extrusion: 1, 2- ram; 3-billet; 4-container;

5-die; 6-needle (mandrel); 7 - article

 

The needle is used when a hollow product must be produced.

This method is used for manufacturing of steel and non-ferrous alloys:

- bars from 3 to 250 mm in diameter (thickness);

- pipes from 20 to 400 mm in diameter;

- different shapes which can't be produced by rolling.

Degree of deformation (extension) is the highest, with respect to other methods, from 10 to 80, because metal is compressed in three directions. Dimensional accuracy is higher compared to rolling. The extrusion is carried out by means of powerful hydraulic presses.

The advantages of the method are

- intricate shapes may be produced;

- brittle materials may be deformed, because metal is compressed in three directions and plasticity is high;

- high dimensional accuracy of product.

The disadvantages are the following:

- rapid wear of tools;

- large pressing rest of metal in container.

 

Drawing

 

This process is used for reducing the cross section of shape rolled stock, wire and pipes by drawing half finished article through special tool, named reducing die, or draw hole.

Figure 4.15 represents schemes of drawing the bar and pipe. Procedure is usually executed without heating.

Degree of deformation (drawing coefficient) is the smallest, because tensile stresses act within a billet: l=1/L=1.25…1.45

Cold deformation promotes strain hardening of metal. To prevent an article's fracture the recrystallization annealing is carried out after a few drawing operations.

The drawing is used for production wires, which have diameter from 0.002 to 10 mm, pipes with diameter from 0.2 to 500 mm, bars with diameter from 3 to 150 mm and intricate shapes.

Fig. 4.15. Schemes of drawing of a bar (a) and a pipe (b):
1-reducing die; 2-billet (half-finished article)

 

The drawing provides a very high dimensional accuracy and surface quality, because the drawing die is polished and deformation is mainly cold.

The drawing die is made of tool steel (large sizes), hard-facing alloys (middle sizes) and technical diamond (small sizes for wire less than 0.2 mm in diameter).

Two types of drawing mills are used (Fig. 4.16):

Fig. 4.16. Chain (a) and drum (b) mills; for a: 1 -die-support; 2-billet; 3-reducing die; 4-6-capture device and carriage; 8, 10-sprockets; 9-chain; for b: 1-wire; 2, 4, 6, 8, 10-drums; 3, 5, 7, 9-reducing dies; 1 1-motor; 12-gears

- drum mill for wire production;

- chain mill for rods and pipes production.

Hammering

 

Hammering, or forging is the method of hot metal forming in which metals are deformed by universal tools, such as hammer, anvils, block heads and others.

There are two types of hammering:

- hand (smith) forging;

- machine forging.

Only the second type of hammering is used in machine industry. A half-finished product of hammering is named a forging. Mass of forgings ranges from a few grams to 250 ton. The main advantages of hammering are:

- simple universal tools are used for manufacturing of different articles;

- heavy forging may be produced only by hammering.

Main Hammering Operations (Fig. 4.17). Upsetting is conducted under impact loading P of a billet located an the anvil. As a result the height of a billet is reduced.

Degree of deformation is determined by the following formula:

                                                                   (4.6)

where F₀ and F₁ are initial and final cross sections correspondingly;

H and h are initial and final heights correspondingly.

 

Heading (Fig. 4.17d) is local upsetting of the upper part of a billet (e.g. to produce head of a bolt).

Spreading is a process of making the billet wider and correspondingly thinner.

Drawing (Fig. 4.17a) is a forging operation for elongation of the billet or a part of it.

Ring rolling or paying out (Fig. 4.17c) means increasing in diameter of a ring, which undergoes hammering by block head and a rotation around cylindrical mandrel.

Piercing (Fig. 4.17e) is making a hole in the billet.

Fig. 4.17. The main hammering operations: a, b – drawing; c – ring rolling; d – heading;

e – piercing; f – bending; g – twisting; h – cutting off

 

Chopping or cutting off (Fig. 4.17h) is separating of one part of half-finished article from another one.

Bending and twisting operation are shown in Fig. 4.17f and 4.17g correspondingly.

Hammers (smith forging hammers) and hydraulic presses are used in forging process. The forging hammer is a machine of impact action. The impact is produced by mass of falling parts. A compressed air, steam and electrical energy are used in forging hammers.

Let us consider design of a pneumatic forging hammer (Fig 4.18). When compressed air is given through hole 4 a piston 6 moves up in a cylinder 5. The piston lifts a piston rod 8, a slider 10 and a block head 11, which are assembled together.

Fig. 4.18. Forging hammer: 1 – handle; 2 – traction; 3 – valve; 4 – hole (pipe); 5 – cylinder; 6 – piston;

7 – sliding surface; 8 – rod; 9 – body; 10 – slider; 11 – block head; 12 – smithy; 13 – anvil block

 

During the next part of the working cycle compressed air is released away and all moving parts named falling parts (the piston, the rod, the slider and the block head) fall down to perform deformation of a workpiece. This workpiece is placed on a smithy (swage anvil) 12, which is mounted on anvil block 13, located, in turn, on a basement. In steam-air hammer a bedframe 9 is mounted separately from the anvil.

The higher the mass of the anvil block is, the higher the efficiency of an impact and work of the hammer will be. The capacity of hammers is defined by mass of the falling parts. It ranges from 50 to 1000 kg. Forge hammers are used for producing forgings that have mass up to20kg.

Hydraulic presses (Fig. 4.19) are machines of static action. When oil is given into a main cylinder 10 under pressure from 20 to 30 MPa a plunger 6 with traverse 3 and head block 2 is moved down and the head block deforms a work. The plunger 6 and the head block 2 are raised by cylinders 9 and pistons 5.

Hydraulic presses have deforming force from 5 to 600 MN.

Forging manufacturing process. Process of manufacturing a forging comprises the following operations (steps):

- drawing of the forging is developed on the base of drawing of a finished part;

- the machining allowances, laps and tolerances are point out on the forging’s drawing (Fig.4.20);

- the shape and weight of required billet is determined;

- the necessary equipment is selected;

- the sequence of forging operations should be specified.

Let's assume that the part be done has shape of a lever with a jaw (Fig. 4.21a) and the billet has shape of a cube (Fig. 4.21.b). The sequence of operations to manufacture a part are shown in Fig. 4.21c…i. As a result, the required forging is received.

 

Die Forging

The main disadvantages of forging process are:

- low output;

- low dimensional accuracy;

- difficulty in receiving forgings of intricate shapes.

 

Fig. 4.19. Hydraulic press: 1 – base; 2 – block head; 3 – traverse; 4 – column; 5 – rod; 6 – plunger;

7 – traverse; 8 – piston; 9 – raising cylinder; 10 – main cylinder; 11 - oil feed

 

  

a                                        b

Fig. 4.20. Drawings of the gear (a) and the fording (b)

 

 

       a                             b                 c                 d                                  e

 

                   f                             g                             h                                    i

 

Fig. 4.21. Technological process of manufacturing of a lever with a jaw: a – article; b – billet;

c, f, g - drawing; d, e, h - necking; i - bending and necking

 

To settle these problems die forging is used. There are two types of die forging: hot and cold ones. The hot die forging has a wide application in the machine industry. The cold die forging is used, mainly, for manufacturing the forgings of alloys which have high plasticity.

The essence of die forging is that a forging is manufactured by means of a metal die, which as a rule consists of two parts. When the parts are moved to meet together by a hammer or a press a billet is transformed into forging. The manufacturing die forging process is similar to a hammering one.

Two methods of hot die forging are distinguished (Fig.4.22):

- with flash (die is "open");

- without flash (die is "closed").

In the first case mass of a billet must be equal or exceed mass of the forging. When the billet will be squeezed-inside the die cavity, named impression; the surplusmetal will be squeezed into the ring flute, forming a flash. Then it is necessary to remove a flash by cutting.

In the second case mass of the billet must be equal to the mass of the forging. So we need to control the mass of the billet very carefully.

Fig. 4.22. Die forging process in open (a) and closed (b) dies: 1 – forging;

4, 6 – forging with flashes 2 and 3; 5 – billet for forging; 7, 8 – parts of the die; b: 1, 2 – parts of the die

 

Dies may have one impression for manufacturing the simple forging or several impressions for manufacturing the intricate forging. In the latter case the process consists of a sequence of die forging operations, first, in roughing impressions to draw or bend the billet and then in intermediate and finishing impressions to produce a finished forging (Fig. 4.23).

Finishing operations of die forging are as follows:

- flash cutting;

- film cleaning;

- dressing of forgings (correction of their shape) in hot or cold conditions by presses;

- heat treatment;

- cleaning from scale;

- calibration (coining) to receive more precise dimensions; calibration may be plane or volume one (Fig. 4.24).

Equipment for hot die forging includes hammers, crank presses, horizontal forging machines, hydraulic and screw presses, etc.

 

Fig. 4.23. Multi-impression die and operations of forging: 1 – fullering; 2 – drawing (extending);

3 – bending; 4 – blocking; 5 – finish fording

Fig. 4.24. Plane (a) and volume (b) calibration (operation of calibration undergo sizes indicated

on the sketches)

 

Hammers do not allow to obtain high dimensional accuracy of forgings because parts (halves) of dies are not interconnected and their replacement is very high.

The crank press (Fig 4.25) consists of electric motor 1, belt transmission 2-3, gear transmission 5-6, crank 8, plunger 10 and table 9 of variable height. Two parts of the die are fixed to the plunger and the table and slide along guiding columns. This ensures high dimensional accuracy of forgings.

Fig. 4.25. Cinematic scheme of a crank hot-forging press: 1-electric motor; 2 – pulley;

3 – fly-wheel; 4 – shaft; 5, 6 – gears; 7 – friction coupling; 8 – crank; 9 – table;

10 – plunger; 11 – rod; 12 – brake

 

Advantages of die forging process: high productivity (dozens and hundreds of forgings per hour); high accuracy.

Disadvantages:

- complex and expensive tools, which may be used only for definite articles;

- mass of work (forging), as a rule, are not more than 30 kg; for manufacturing of heavy forgings powerful equipment is required.

Cold die forging is mainly used for manufacturing the forgings of alloys with high plasticity (Al, Mg, Cu, sometimes, low carbon steel). There are three basic types of the process: cold forming, cold extrusion and cold upset forging.

Cold forming process is similar to hot die forging, but it is carried out at a room temperature.

Cold upset forging process is carried out by cold upsetting automatical machines, which are similar to horizontal forging machines, but have higher productivity (from 20 to 400 forgings per minute).

Cold die forging requires higher deforming forces. After it metal acquires strain hardening, but the method offers higher, than hot die forging, dimensional accuracy, surface quality and absence of scale losses.

 

Stamping

Stamping or sheet stamping is a cold method of the plastic metal working. A strip or a tape may be used as a billet (half-finished article). Such parts as a watch hand, car bodies, rocket shells are manufactured using stamping.

Stamping provides high productivity: sometimes, a part per second. Alloys of high plasticity (Cu, Al, Mg, Ti) low carbon steel, plastic, leather and other materials undergo stamping.

All stamping operations are carried out by special tools and machines (stamps and presses).

Stamping operations may be classified as forming and separating.

The separating operations are used for cutting of a sheet in tapes and pieces by shears. The shears may be of three main types: straight-blade, guillotine and circle shears (Fig. 4.26).

 

Fig. 4.26. Separating operations of sheet stamping: a-cutting by guillotine shears;

b-cutting by circle shears; c-blanking; d-piercing; e-notching; 1 – upper knife; 2 – lower knife;

3, 8 – billet; 4 – size restrict; 5 –fixer; 6 – lower die; 7 – upper die; 9 – part (stamping); 10 – waste

 

Length of the cutting line for straight-blade and guillotine shears is not in excess than horizontal length of the shears L.

The length of the cutting line (of the tape) is not limited when the circle shears are used. The cutting out (blanking) and piercing are making external and inner contours of a part, correspondingly.

Forming operations are plotted in Fig. 4.27.

All these and other operations are made by dies of complex construction. Stamps consist of upper dies, bed dies, upper and bed plates, columns, bushings, springs and other parts. Upper plate is fastened to plunger of a press, bed plate is fasten to a table of the press.

 

Fig. 4.27. Forming operations of sheet stamping: a-straightening; b-bending; c, d, e-draw-forming,

f-spread forming, g-flanging; 1 – upper die (punch); 2 – lower die; 3 – billet.

 

WELDING

Welding is a technological method of manufacturing of permanent joints and facing the surfaces of parts if required. Parts of various materials may be welded. Welding is carried out in the air or other gases and vapors, under water, in vacuum.

The first welding method was found 4...5 thousand years ago, when man produced iron from iron ore in open fire, using charcoal as a fuel and reducer. During hammering to remove residual charcoal the pieces of iron were joined with each other. This was blacksmith, or forging (hammer) welding.

Then man noticed that during hammering at room temperature two pieces of gold (it possesses high plasticity) might be joined too. This was a cold welding. It happened approximately 3 thousand years ago.

Approximately at this time the foundry welding was introduced into practice. It was performed in the following way: liquid metal (brass) was poured between two brass parts and welded them together.

These three methods of welding have been used for long time and only in 1882 Russian engineer M. Benardos suggested arc welding with non-consumable carbon electrode.

In 1888 Russian scientist N. Slavjanov worked out a method of the arc welding with metallic (consumable) electrode. In 1902 gas welding was suggested in France. So, the application of welding has rapidly expanded in the recent 100 years. About 60 welding methods are employed now.