Cold forging is a metal forming process of hammering or squeezing a piece of metal between two dies below recrystallization temperature. Cold forged products are known to have superior strength and durability compared to casting and machining. The increased strength is attributed to strain hardening; while increased toughness is from grain flow modification.
Strain hardening causes a ductile and malleable metal to become harder and stronger while being subjected to plastic deformation. As the name suggests, this process is done well below recrystallization temperature or the absolute melting temperature of the metal. This is usually done at room temperature. Temperature can be slightly elevated to increase the ductility of the metal while being formed.
Grain flow control, on the other hand, causes the product to resist cracking and have longer fatigue life. Cold forging, like other forming processes, changes the grain flow direction to align along the contour of the product.
The Strain Hardening Mechanism
Strength gained from cold forging can be better explained by analysing the effects of plastic deformation. When metals plastically deform, a part of this deformation energy is converted to heat while the remaining is stored internally. Most of this stored energy is associated with dislocations. As the metal is plastically deformed through cold forging, dislocations in the metal’s crystal structure multiplies. The addition of new dislocations forces them to be closer together. The resulting increase in dislocation density creates a resistance from motion by other dislocations. In other words, a resistance to produce more dislocation develops which makes the metal harder, but less ductile.
The degree of plastic deformation can be referred to as percent cold work. The graphs below show how cold working affects yield and tensile strength and ductility of various metals. Note that for 1040 steel, 20% cold work increases its yield strength by more than 60%.
The ductility, on the other hand, suffers greatly as the metal hardens. There is a limit on how much plastic deformation a material can take. Eventually, fracture and cracking will take place.
In order to mitigate the loss of ductility, annealing is applied before the cold forging process or in between forging repetitions known as intermediate anneals. Annealing produces an opposite effect to strain hardening where it increases ductility and reduces hardness. This additional heat treatment process makes it possible to produce a desired shape along with the increased mechanical properties imparted by the hardening process, without the risk of cracking while working the metal.
The Advantage of Favourable Grain Flow
Control of grain flow is also a major advantage from cold forging. This is achieved by aligning the grain flow according to the direction of applied loads and possible locations and orientations of cracks on the product. This is done by carefully designing the dies and the pressure required, either through one press or by progressive forming. The metal is oriented accordingly which produces grain flow perpendicular to the direction of potential cracking or fracture, or parallel to the direction of tensile or compressive stress. This results in a “crack-arrestor” configuration. An analogy can be observed on a piece of wood. Cracking will not easily propagate perpendicular to the direction of wood grains or the wood cell fibres.
Compared to casting and machining, grain flow follows the contour of forged products. In forging, weak points are minimized along the bends and curves where stress is usually concentrated. Casting produces no grain orientation; while machining does not modify the initial grain flow of the metal. Thus, both casting and machining has no inherent strengthening mechanism in its process.
References:
1) Callister Jr, William D., Rethwisch, David G. Materials Science and Engineering: An Introduction, John Wiley and Sons, 2014
2) ASM International. Forming and Forging, Volume 14 of the 9th Edition Metals Handbook, ASM International, 1996
3) Black, J T., Kohser, Ronald A. DeGarmo’s Materials and Processes in Manufacturing 11th ed., John Wiley and Sons, 2012
