In contrast with metal processing by cutting, cold forging does not produce swarf and thereby saves materials and eliminates waste. In addition. Cold forging results in improved material strength because the process of forging using a mold does not involve cutting or dividing the metallic fibres. Generally it is said that the molds used in cold forging are very expensive, and that development expenses are high, so the technique is not cost effective unless it is used for mass production.
Cold forging offers a unique and special advantage when it comes to production of Blind parts requiring internal splines or profiles.
The process is best suited for components using low carbon steels.
The basic advantages of Cold Forging vs Hot Forging are
a. Hot forging is done at 1000 – 1100 C
i. Uneven heating will cause warping and shrinking of final shape
b. The surface will have a heavy decarbonization due to this extreme heat
i. Will need to be proof machined to qa depth of 2-3 mm in order to remove this extra decarb layer
The disadvantages of cold forging vs hot forging are
c. Cold Forging requires multiples steps of annealing (based on requirement)
d. Components must be symmetrical along the axis for uniform stress concentrations
e. Comfortable tolerances that can be achieved is of the order of 100m
f. If it is difficult to machine, use cold forging. If machining is the easier step, use machining.
g. Only if the carbon content is less than .35 - .4% cold forging is feasible and would need 2to3 stages of cold forging to get to the final shape
h. Al and other soft ductile metals are easier to cold forge, provided they don’t crack under stress.
i. The repeatability is generally very good for cold forged components
j. The initial x-section of the slug vs the final x-section of the component should be 2.5 times more as a general guideline. If it is more the slug is more likely to crack.
k. For backward extrusion, the depth of the extrusion should be around 2 to 3 times the diameter. If it is more than 3 times the diameter, the punch might break.
Schematic Cold Forging Process
l. The cold forging die is usually composed of two type of steels, the outer and inner
i. The outer is usually a ductile mild steel material.
ii. The inner is usually of hard HSS variant
m. The HSS steel portion is kept under compression in this
i. Basically the HSS is kept in the red portion, to allow more than twice the possible strain.
ii. In addition, the tolerances are taken at the lower end or even below the accepted minimum in order to compensate for elasticity of the dye.
o. Life of the die, punch, and lubricant coating thickness would vary depending on the product
p. Please note that the outside and inside are two separate rings.
i. This is to prevent a stress concentration which can cause breakage.
Various materials that can be cold forged include
i. SAE 1006, 1008, and especially 1010 are good materials
ii. EN2A, EN8, and EN1A(which isn’t highly suitable as it tends to crack under loads)
iii. S10C, S15C, SCM415, SCM240
iv. 16MnCr5, 20MnCr5
v. SAE 8620
vi. Essentially various impurities should be controlled to be within the levels as shown below .
1. Cr upto .9% by wt
2. Mo upto .4% by wt
3. Ni upto 1% by wt
4. Most texts claim that Si can be allowed up to .4% by wt. But, in our experience, we find 2% as an acceptable maximum for good results
r. Stainless Steels
i. SUS 302, 304, 305, 316, 405, 420, 431
s. In the case of Aluminium, maximum amount of Mg. that can be allowed is .3to4% by wt.. Excess Mg.can cause cracking
i. SAE 6061,6063,6351,6066 are all good qualities
t. Brass of 70-30 grade and Copper that is oxygen free (electrolytic copper) are also good materials for Cold Forging.
u. Phosphorus and Sulfur can be 0.05% by wt. MAX
i.These are added for chip breaking and machinability
Basic design considerations
ii. This piece is not a good candidate for cold forging
iii. It would be cheaper just to machine this piece rather than to cold forge
iv. The biggest advantage of Cold forging is the saving in material
v. This cross section is a much better piece to cold forge than turning
vi. The material lost in turning is far greater than what it would cost in cold forging
General volumes need to be on the order of at least 5k per month to compensate for the investment into the die as a general rule.