Cold Forging vs Hot Forging

Cold forging shapes metal at or near room temperature, and the part work-hardens [1]. Hot forging works the metal above its recrystallization point, high enough to avoid strain hardening [10]. Cold forging holds closer tolerances but needs higher forces, costlier tooling and larger quantities, and suits smaller parts [1][2][4][10]. Hot forging needs less force and has virtually no size limit [1][10].

Side by side

The Forging Industry Association’s comparison for steel parts [10]:

	Cold	Warm	Hot
Deformation pressure	Very high	Medium	Low
Tolerances	Closest	Close	Generous to close
Tooling cost	Highest	High	Lowest
Energy cost	Low	Medium	High
Material cost	Higher	Medium	Lower
Size range	Smallest (1/4–20 lb)	Small to medium (1/2–30 lb)	Virtually unlimited

Warm forging heats the workpiece to a temperature significantly below the typical hot forging temperature [1].

What changes with temperature

Tolerances. The lower temperature gives greater dimensional accuracy [1]. Cold forming reaches ±0.001 to ±0.005 in., against ±0.020 in. for hot forming [10]. Cold forgings need no draft angles to release them from the tooling [4].
Force. A hot workpiece flows more easily and needs less force [1]. Cold, the metal flows less well and higher forces are needed [1].
Scale. A cold workpiece is too cool for scale to form [1].
Strength. Cold forging work-hardens the part [1]. Hot forging diminishes work hardening or avoids it entirely [1].
Strokes. Cold forgings are almost always made in several forming strokes. If the material reaches its formability limit, it is annealed before the next stroke [6].
Size. Commercial cold forgings typically weigh less than 23 kg (50 lb) [4].

Which process fits which job

Quantity. Cold-forging tooling typically costs five to ten times as much as tooling for equivalent hot impression-die forgings, but it lasts much longer [2]. FIA recommends quantities approaching 10,000 lb of parts per month to offset the tooling cost; with materials more expensive than steel, cold forging can be economical at lower quantities [10].
Shape. Relatively small parts that are rotationally symmetrical, need high strength and precision, and are made in larger quantities are candidates for cold forging [3]. As shapes get more complex and quantities rise, impression-die forging becomes the process of choice [3].
Very large parts, few parts: open-die forging is the typical choice [3].
Tooling and setup costs are spread over the order, so their effect on the piece price falls as the quantity rises [9].

Materials

Every major alloy group used for hot forging includes alloys suitable for cold forging [5]. The more alloy content, the less deformation is possible in one stage [5]. Cold-forging steels are generally carbon and lower-alloy grades with less than 0.45% carbon; 300 and 400 series stainless, aluminum alloys up to 6061, brass and bronze are also cold forged [10].

Forging titanium

Titanium alloys are more difficult to forge than most steels [8]. FIA gives 1,700 to 2,300 °F (925 to 1,260 °C) as the forging range for steels and for titanium and nickel alloys [7].
Processing is done with special care to limit surface contamination by oxygen, carbon or nitrogen [8].
Ti-6Al-4V (Grade 5) is the most common titanium forging alloy [8].
For Ti-6Al-4V made by casting, the β-transus is about 995 °C; forging below it is used much more often than forging above it [11].
Titanium forgings typically need 20% to 25% greater tolerance than steel forgings [10]. They can be forged to precision tolerances [8].

Open die and closed die

Open-die forging is a hot process that uses standard flat, “V” or swage dies [1].
Impression-die (closed-die) forging uses a pair of matched dies with contoured impressions [1].
Compared with impression-die forging, open-die forging has higher per-piece costs, less dimensional precision and more finish machining [1].

Forgings from Laube

Laube sources custom metal forgings to customer drawings through its supplier network, and supplies titanium bar in Grades 1, 2, 3, 4, 5, 9 and 23. Send your drawing for a quote.