2.2 Selecting a Basic Forging Process

The choice among the various forging processes is driven by component size, production quantities, and component shape. The following guidelines usually apply.

1. When forgings are very large, when very few are required, or when delivery times are very short, open die forging is often the choice.
2. As shapes become more complex, and production quantities increase, impression die forging becomes the process of choice provided that the size does not exceed the capability of the impression die process.
3. Seamless rings may be made by open die forging over a mandrel, hot forging or ring rolling. Diameters less than one foot may be candidates for impression die forging. Diameters less than one foot up to 30 feet, in low to high quantities, are candidates for ring rolling. When diameters are too large for ring rolling, open die forging is the process of choice.

Table 2-1 Overview of Forging Alloys

Alloy Group	General Characteristics	Typical Applications
1. Steels	Most often selected for forgings
A.. Carbon	Wide range of grades and properties Most grades are readily forged	Nearly all market areas
B. Microalloy	Alternatives to quenched and tempered alloy steels; high strength and toughness without high treatment	Automotive, truck and off-highway
C. Alloy	Improved mechanical properties versus carbon steels	When carbon steels do not have the required properties
2. Stainless Steels	High corrosion resistance; more difficult to forge that carbon or alloy steels	Corrosion resistance and high temperature properties
A. Ferritic	Excellent corrosion resistance; good ductility; can be worked hot or cold
B. Austenitic	Highly resistance to acids; good toughness at cryogenic temperatures
C. Martensitic	Can be hardened and tempered; are magnetic
D. Special grades (e.g. PH, duplex stainless)	Combinations of high strength and corrosion resistance
3. Aluminum	Most easily forged into precise, intricate shapes; low density; generally heat treated; good corrosion resistance	Aerospace, automotive, truck, military components, sporting wear and accessories
4. Copper base	Excellent, corrosion resistance; excellent forgeability; good dimensional precision; low draft	Leakproof fittings, plumbing fixtures, gears, bearings pumps, valve bodies, non-sparking applications.
5. High Temperature Alloys	Good corrosion and oxidation resistance Good high temperature properties, particularly creep and low cycle fatigue	Gas turbine components
6. Titanium	High strength; low weight; high service temperatures, excellent corrosion resistance	Aerospace, chemical processing, prosthetics
7. Magnesium	Low density; low modulus of elasticity; requires special handling	Where minimum weight is required at relatively low service temperature

When the impression die process is selected, the hot, warm and cold processes may be considered. Table 2-2 gives broad guidelines for choosing among the three. Note that warm and cold forging may be used in combination.

When hot impression die forging is chosen, four options are available: blocker type forgings, finished forgings, near-net forgings, net shape forgings. These are illustrated in Figure 2-1.

Table 2-2 Preselection of Impression Die Forging Process for Steels

Factors of Choice	Type of Forging
	Cold	Warm	Hot
Steel Quality	CWQ/SBQ	SBQ	SBQ
Material costs (grade)	Higher	Medium	Low
Deformation Pressure	High	Medium	Low
Energy Costs	Low	Medium	High
Tolerances	Closest	Close	Generous
Tooling Cost¹	High	High	Lowest
Size Range	<28 gm (1 oz) to 23kg (50 lb)	110 gm (1/4 lb)² to 23 kg (50 lb)	Virtually Unlimited
Shape Restrictions	Limited	Less Limited	Virtually Unlimited

¹Excluding automation cost
²Often in combination with cold forming

• Blocker Type Forgings are generally forged in a single impression die, with generous finish allowance. This process is suitable for moderate production quantities. A rough rule of thumb for finish stock is at least 5 mm (0.2 inch) of machining envelope for each 300 mm (12 inches) of dimension for blocker type forgings made from steel. The allowance can be less for aluminum, and should be 25% to 50% more for difficult to forge heat resistant alloys.
• Finished Forgings are suitable for higher production quantities. They are forged with significantly less finish allowance than are blocker type forgings, and typical FIA guideline tolerances apply. Typical finish allowances are 1.25 to 2.5 mm (0.050 to 0.100 inch) plus draft, which varies from 3^o to 7^o.
• Near-Net Forgings are forged as closely as possible to the required dimensions of the finished part so that most surfaces require little or no machining. They are similar to finished forgings except they are closer to final configurations.
• Net Shape Forgings, sometimes called precision forgings, are forged on one or more sides to net shape requiring no further machining on at least one side. For example, tooth forms on gears up to 125 mm (5 inch) diameter are being forged to tolerances of ±0.10 mm (±0.004 inch), which is often close enough to eliminate gear cutting operations or to permit final grinding. The hole and back face are still finish machined in this case.

In some cases, product factors drive the choice of forging processes. For example:

Figure 2-1 Blocker, finished, near-net and net shape forgings

• Increased forging precision tends to drive up the cost of forging operations somewhat, and it usually reduces the cost of finish machining. As production quantities increase, the reduced cost of machining operations becomes a stronger offsetting factor to the higher tooling and processing costs. Appendices A, B, C and D give industry guideline tolerances for several forging processes.
• A very expensive raw material that is difficult to machine will suggest the most chipless process, and small production quantities might best be net shape forged. If the material is inexpensive and readily machined, open die forging in small quantities may be the optimum choice.
• Medium size to large rings can be made either by open die forging over a mandrel and finish machining or by a ring rolling process in which finish machining may or may not be required. Production quantities drive the choice. Open die forging may be an alternative when quantities are very low; ring rolling becomes more economical as quantities increase.
• Components that have features with rotationally symmetrical or axisymmetric shapes, such as splines, may be candidates for cold, warm or hot forging, depending on complexity and size.