FORGING PROCESS

FORGING PROCESS

1.INTRODUCTION
Forging is a metal mass that has been worked or brought to a configuration attained by controlled plastic deformation, by hammering, pressing, upsetting, rolling extruding, etc. Aluminum, Copper and their alloys, steels, titanium and many other alloys can be forged.
Forging can be produced either by hot working or cold working.


When the metal part is forged; individual grains are deformed, the Individual grains are recrystallized that is break up and form new grains.  Hot metal is plastic and deforms easily. At very high temperatures  grain growth, incipient melting, phase transformation and changes in
composition occur.
At lower hot temperature, the metal is more difficult to forge but resulting grain size may be finer and have higher mechanical properties

2. IMPORTANT FORGINOPERATIONS
2.1.Edging
Is shown in figure.
2.2.Fullering
Fullering, shown in the figure, is used to reduce the cross-sectional area of a portion of the stock. The metal flow is outside and away from the center of the die. e.g. connecting rod of an internal combustion engine.
2.3  Upsetting
Upsetting, as shown in figure, is the simplest operation.   Metal flows out equally in all directions so that ideally the final shape will be a cylinder of increased diameter and decreased height.  However, because of the friction between the dies and the metal, the cylinder flows to a Jesser extent at these interfaces.                                                       
2.4  Blocking
The forging die has the general shape of the finished part.   Details that might o struct the metal flow are omitted. Comers are rounded. The number of pro-forging steps and the number of the heating cycle depends on the complexity of the part. Planning of these steps and corresponding
dies requires considerable experience.
Bending, finishing, Impression and trimming to remove excess material or flash are other operations carried out In forging.   After this heat treatment is given to the forgings. After this coining operation is carried out for sizing, straightening, special Impression markings, and smooth surface finish shot blasting Is given for better surface finish.
2.5  Hammer and Press Forging
In drop hammers, quality depends on much skill of the operation.  In press forging, it is not so critical.   In drop hammer forging all operations from pre-forging to final shaping are done by multiple blows in several impressions. of the same die set.     In press. forging, performing operations are. concentrated in special. machines and,the_ press is used only for blocking.
FORGING OPERATIONS.

3.IMPERFECTIONS FROM THE INGOTS
Since the forging is made fro!ll the.billets, bar stock, etc. which have been obtained from the cast ingots many of the imperfections found in forging and problems that occur in service can be attributed to conditions that existed in the ingot.
3.1 Segregation  :
Tramp elements, dissolved gases, etc. are normally not distributed uniformly, In general, segregation occurs due to• solute•rejection at the solidification interface during casting.
Re-crystallization, which Involves breaking up of the grain structure to promote a more homogenous structure can partly correct segregation.   Segregation affects corrosion resistance, forging and joining characteristic mechanical properties, fracture toughness, and fatigue resistance.
3.2 Ingot Pipe
The presence of a shrinkage pipe will spoil the properties of the product.     Hot topping methods are used to min.imise this in which exothermic compounds are added to the top of the ingot during solidification.   It keeps the metal hot for a longer time and therefore reduces the depth of the pipe.
3.3  Forging Bursts
The substantial tensile stresses generated within the metal during forging cause tearing
of the material where it is weak.   Such proportions could be due to the presence of pipe porosity, segregation or Inclusions. This is more if the forging temperature Is too high.
3.4  High Hydrogen Content
It originates from moisture entrapped during melting and casting. This hydrogen diffuses at grain boundaries and Into boundaries between Inclusions and matrix.   This causes hydrogen embrittlement and produces small cracks. The propagation of crack can easily occur along these paths on the application of Impact load.
3.5 Non-metallic Inclusions
OriginalInclusions from castings as well as those inclusions which develop  In the billet during forging, especially oxide form Inclusions In forging.  nonmetallic inclusions In forging are
a frequent cause of failure In service. These lead to a reduction In mechanical properties as they
become the areas of stress concentration due to their heterogeneous nature in order to overcome the problem in forging, the forging quality must posses;
1.  Excellent Surface
2.  Freedom from segregation
3.   Uniformity of grain size
4.   Freedom from internal flaws
5.   Minimum non-metallic Inclusions
Surface defects that can be present are;:
a.  Seams b. Cracks
c.  Sand inclusions and oxide  scales
The core defects that can be present are :
a. Piping
b. Segregation of the Inclusions like S and other nonmetallic inclusions. c.   Blowholes
The quality of raw material can be checked by using special tests. In one method, a material with equal diameter and length is forged up to half its length. If defects like seam are present they open up and make forging to the desired level difficult task. The material may be rejected. NDT methods can also be used to check the seams and cracks.  Ultrasonic tests are suitable even to check the piping.
Sulfur prints are taken to check S segregation which Is very harmful.  Photographic paper Is soaked In dilute HCL at about 80°C for about 30 minutes and then brought Into contact with polished material. H2S Is formed. H reacts with silver halide and gives spots.
3.6  Anisotropy Caused by Forging
On plastic deformation preferred orientation can develop by crystallographic reorientation of grains during severe deformation and mechanical fiber which  Is brought about by the inclusions, voids, segregation and second phase constituents in the main direction of mechanical working.  These cause anisotropy of properties.

4.   DIFFICULTIES ENCOUNTERED IN HEATING
4.1  Formation of Slag
Chemical combination of oxide and furnace refractory can form slag at high temperatures. This badly affects the forging dies by causing wear. It also prevents the proper forming of the forging.  Reducing the atmosphere in the furnace will minimize oxide formation.   But this can form a sticky scale and pitted surface of the product. Because of this, the surface-atmosphere is oxidized (2% oxygen).
4.2  Overheating of the Stock
Overheating can be caused by prolonged hearing, high temperature, etc.   overheating reveals burning In the form of deeply embedded scale and Inter-crystalline cracks which can be detected by suitable NOT methods.
4.3  Crozzllng
In some forging, overheating  Is accompanied by a characteristic surface condition which Is revealed by the pick. long In acid. This Is referred to as a 'crocodile skin' for 'drizzling'.  It consists of a lnter-crystalline network of fine cracks or lines resembling crocodile skin.  This may be caused
by preferential heating at austenitic grain boundaries because of their heat affinity.  It could be
due to a certain type of furnace atmosphere. Normally steel gives ductile fracture but overheated or burnt steels give brittle fracture.
4.4 Metallographic Examination
It can reveal burning by showing a coarse grain size. Mechanical properties are also reduced. Overheating Is prevented by using the proper type of furnaces and regular temperature control. The choice of maximum forging temperature Is important.

5.  DIE DESIGN
In forging operation sufficient flash should form to achieve complete filling of the die. Parting line location and flash position are critical.  A parting line that divides the forging into two equal halves ensures minimum depth of flow of metal and also a minimum of excess metal. Such a parting line may not be always feasible.  Draft angles in the dies aid in metal flow to ensure filling and also help removal of the forgings from the die. Small radii can lead to cold shut formations.  They also shorten die life because of their erosion robs and bosses are difficult to fill. Large flat surfaces resist for deformation.

6.   DEFECTS ON FORGING These can be classified as;
1.   Deflects resulting from improper forging such as scans, cracks, taps, etc.
2.   Defects resulting from the melting practice such as dirt, slag blowholes.
3.   Ingot Defects: Piles, cracks, scabs, bad surface, and segregation.
4.   Defects resulting from improper heating and cooling such as burnt metal decarburized steel and flakes.
6.1  Defects due to Improper Forging
1.  Mismatch
2. Unfilled
3. Cold Shut
4. Overlaps
5. Wrinkles
6. Seams
7. Distortion
8. Cracks
9.   Buckling
6.1.1 Mismatch
Then two dies are not in perfect alignment mismatch will occur.   This can occur due to slipping of the die or careless mar1<1ng before die sinking. Die locking arrangement will reduce this.
6.1.2   Unfilled Forging
Improper filling of the die can be due to;
1. an insufficient amount of stock
2.  low temperature of forgings
3.  low speed of the ram
4.  Improper die design
5.  Friction between the die and the heated stock.
\t\lhile estimating the stock weight, the flash metal, scaling, decarburization, and waste due to shear cutting of the bar must be considered.  Sharp comers and deep cavities should be avoided.  The use of lubricants facilitates material flow.   Silicon-based compounds are found to be suitable. Graphite can also be used.    It should be fine powder with colloidal form in water. Pre-heating of the dies to 300 to 550°C can reduce machining as well as the stages involved in forging.
6.1.3  Cold Shuts
It is a discontinuity produced when two surfaces of metal fold against each other without welding completely. The defect can be due to the misplacement of the material.    If the material is less at one portion than in the next operation, the places where the material is more will flow towards the position where the material is less and these portions may not get welded up. Optimization of the billet section is necessary to avoid cold shuts and to get a good material flow. The billet diameter should be D instead of d.   If it is more, good material flow will not be obtained.
6.1.4  Overlaps
These occur at a portion where the material is more than required while adjoining portions have the right amount of metal. In some complicated shapes, the material has to be supplied in large quantities.  To compensate for this extra material, certain extra die cavities are provided in
the portion which is to be trimmed off. These are known as gutters.
6.1.5  Wrinkles
Wrinkles are similar types of surface defects as overlaps.   This type of overlapping occurs only on the surface.
6.1.6  Seams
Seams In raw materials open up and give rise to surface cracks in forging.
6.1.7 Distortion
These can occur during cooling from hot working temperatures due to residual stresses that are developed as a result of inhomogeneous deformation.   They may be pronounced in largo
complicated forging. They should be cooled slowly. Forgings may be buried in ashes for slow cooling.
6.1.8 Cracks
Surface cracks occur as a result of the excessive working of the surface at too low a temperature or as a. result of hot shortness. High sulfur in metal and/or in furnace atmosphere can cause hot shortness. Cracking at the flash of closed die forging is another surface defect since the cracking generally penetrates into the body of the forging when the flash is trimmed off.   Flash cracking can be avoided by Increasing the flash thickness or by re-locating the flash to the less critical region of the forging.
6.1.9 Hairline Cracking
These occur In large forgings particularly In alloy steels.  If they form in the early stages, they can be welded during subsequent forging.   If formed late, they pose a problem. These cracks are always Internal.  They are associated with segregation.  They can be detected by NDT methods. Better steel melting practices can reduce them.
6.1.10 Buckling
If the length of the unsupported stock is unusually greater than the diameter of the stock, buckling can occur. Apart from the forging defects, various defects such as oxidation and decarburization, quench cracks, distortion, and warpage, change in dimensions, soft spots, etc. can occur during heat treatment.

Calculation of Forging Reduction Ratio-
Forging reduction is the amount of cross-sectional reduction taking place during drawing out of a bar, billet or ingot. The original cross-section divided by the final cross-section is the forging reduction ratio (say for example 4:1)
If a 10x10cm bar is forged to size 5x5cm means reduction ratio 4:1 is achieved.

ASTM grain size number(n) is related with the number of grains that we can count in 100X magnification (N) by the relation, N=2^(n-1)  

So ASTM grain size number increases with decreasing grain size.

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