Hot forging is a process that shapes steel after it has been heated above its recrystallization temperature. Above that point, new grains form as the steel is worked, so it does not work-harden during forging and flows under less force than cold steel. The part can still be hardened afterward by heat treatment, such as quench and temper. The Forging Industry Association puts the normal hot working range for steel at 1,900 to 2,300°F (about 1,040 to 1,260°C). In that range, a press can turn a solid slug of steel into a gear blank, a bearing race, or a wheel nut in a few blows.
Hot forging is worth evaluating when a steel part is machined from bar or cast today and the annual volume is high enough to cover the cost of dies. MacLean-Fogg’s hot forging and machining business in Savanna, Illinois, forges those blanks on horizontal presses and takes them through the machining and secondary operations a finished part needs.
Table of Contents
- What Is Hot Forging?
- Types of Forging by Temperature
- How the Hot Forging Process Works, Step by Step
- What Hot Forging Does for Strength and Fatigue Life
- Near Net Shape and Material Use
- Secondary Operations That Finish a Hot Forged Part
- Hot Forged Parts and the Industries That Use Them
- Disadvantages of Hot Forging and When It Is the Wrong Fit
- How MacLean-Fogg Hot Forges Components
- Frequently Asked Questions
Key Takeaways
- Hot forging shapes steel above its recrystallization temperature, where it flows under less force and does not work-harden while it is being forged. The Forging Industry Association lists the normal hot working range for steel as 1,900 to 2,300°F.
- On a horizontal hot forging press, heated steel moves through several forging stations in one machine and leaves close to its finished shape.
- Forging develops grain flow that improves tensile strength, ductility, impact toughness, fracture toughness, and fatigue strength (Forging Industry Association).
- In University of Toledo testing of crankshafts, forged steel showed 36% higher fatigue strength than ductile cast iron (New Equipment Digest).
- Hot forging pays for its dies at higher annual volumes. At lower volumes, machining from bar may cost less.
What Is Hot Forging?
Forging shapes metal by pressing or hammering it between dies, and the work temperature decides which type of forging it is. The Forging Industry Association’s Product Design Guide for Forging explains that the hot workpiece temperature improves plastic flow and reduces the force required to work the metal. Lower forging force lets hot forging produce larger and more complex steel parts than cold forging. The industry uses “hot forging” and “hot forming” for the same work, and MacLean-Fogg uses hot forging for both the process and its presses.
Types of Forging by Temperature
The types of forging are usually grouped by temperature into three bands, and each one behaves differently on the press. Cold forging is performed at or near room temperature, where the steel work-hardens, no scale forms, and the lower temperature promotes greater dimensional accuracy (FIA). Warm forging heats steel below its recrystallization temperature, from about 1,000°F up to just below the hot working range (FIA Glossary of Forging Terms). Hot forging runs in the hot working range, where deformation pressure is lowest and tolerances are the most generous of the three.
| Process | Typical Steel Temperature | Deformation Pressure | Tolerances |
|---|---|---|---|
| Cold forging | Room temperature | High | Closest |
| Warm forging | About 1,000°F (540°C) to just below 1,900°F (1,040°C) | Medium | Close |
| Hot forging | 1,900 to 2,300°F (about 1,040 to 1,260°C) | Low | Generous |
Sources: Forging Industry Association, Glossary of Forging Terms and Product Design Guide for Forging.
The right band for a part depends on its size, material, tolerances, and annual volume. MacLean-Fogg’s article on hot forging vs cold forging [URL to come] covers that decision in detail.
How the Hot Forging Process Works, Step by Step
The five steps below follow the hot forging process from bar to finished component. The first three produce the forged blank on a horizontal multi-station press, and heat treatment and machining finish it.
Step 1: Heat the Steel Bar
The process starts with a steel bar in the grade the print calls for, such as SAE 5120 or 8620 alloy steel for gear and shaft blanks. On a horizontal hot forging press, the bar is heated on its way into the machine. MacLean-Fogg heats the steel by induction to about 2,200°F (about 1,200°C), which falls within the hot working range for steel. Induction heating generates an electric current inside the metal itself, and the temperature has to stay consistent from part to part because it changes how the steel flows in the dies.
Step 2: Shear the Bar Into Slugs
Inside the machine, the heated bar is sheared to length, and each slug carries the volume of steel the forging needs. Getting the slug volume right matters because too little steel leaves the dies underfilled and too much overloads the tooling.
Step 3: Forge the Part Through Multiple Stations
The slug then moves through a series of forging stations inside the same machine. Transfer fingers carry the part from station to station, and the dies and punches at each station move the steel closer to its finished shape. Presses of this type are used for high-volume automotive parts such as gears, bearings, hubs, and fasteners. MacLean-Fogg runs smaller parts at up to 180 pieces per minute, with the rate depending on the part and the press.
Step 4: Heat Treat and Shotblast
After forging, parts cool and go to heat treatment when the print calls for it. Heat treatments for MacLean-Fogg’s hot forged parts include normalizing, quench and temper, and isothermal annealing, matched to the part and print, and gear blanks are normalized as required before machining. After heat treatment, parts are shotblasted to clean the scale from the surface.
Step 5: Machine and Finish
A hot forged part leaves the press close to its finished shape, with stock left on the features that carry tight tolerances. CNC turning, drilling, broaching, tapping, and shaper cutting bring those features to print. Some parts ship as forged, and others continue through machining, finishing, and assembly.
See the Press Ranges and Machining Capacity
The Hot Forging Capabilities Sheet lists the press envelopes, machining capacity, heat treatment, secondary operations, and certifications for MacLean-Fogg’s hot forging and machining business.
Download the Hot Forging Capabilities Sheet
What Hot Forging Does for Strength and Fatigue Life
As the steel flows through the dies, its grain structure is refined and the grain lines follow the outline of the part. The Forging Industry Association lists the properties that grain flow improves: tensile strength, ductility, impact toughness, fracture toughness, and fatigue strength.
In a part cut from rolled stock, grain runs in the rolling direction and does not follow the outline of the part. FIA notes that forgings generally show better directional properties and fatigue performance than parts machined from bar or plate because of grain flow. In a casting the grain is random, and FIA describes porosity as difficult to eliminate in castings while forgings are free from it.
In a University of Toledo study of crankshafts for the Forging Industry Educational and Research Foundation and the American Iron and Steel Institute, forged steel showed 36% higher fatigue strength than ductile cast iron (New Equipment Digest, 2008). The test crankshafts came from a single-cylinder engine, so the figure describes that comparison.
Near Net Shape and Material Use
A near net forging comes off the press with some surfaces that need little or no machining and others left as forged. Near net forgings also use lower draft angles, so less material ends up in the part.
Machining a near net forging is limited to the features that need it, such as bores, faces, and diameters with tight tolerances. For parts machined from bar, FIA points out that processing and material costs are generally high because much of the purchased stock is removed.
Secondary Operations That Finish a Hot Forged Part
Few hot forged parts go straight from the press into an assembly. Two part routes from MacLean-Fogg’s hot forging and machining business show what usually happens next.
Gear Blank
The blank is hot forged, normalized as required, and CNC machined 100% all over. Critical dimensions are inspected, and the blank is packed with rust protection and shipped ready to hob.
Securex® Wheel Nut
The nut body and the washer are both hot forged. The nut and washer are assembled and tapped, and then coated to the application’s requirements.
Other Secondary Operations
Beyond these two routes, the Savanna operation handles broaching, drilling, shaper cutting, CNC threading, facing, and fastener assembly, matched to the part. Secondary cold forging and coining are done on single-blow horizontal and vertical machines.
Hot Forged Parts and the Industries That Use Them
Horizontal hot forging fits steel parts that are round or close to round and run in high annual volumes, especially parts that carry load in fatigue. Those parts go into automotive and electric vehicles, heavy-duty trucks and trailers, agriculture, construction, recreational vehicles, mining, military and defense, and industrial machinery. Parts MacLean-Fogg hot forges include:
- Transmission gear and shaft blanks, including gear blanks shipped ready to hob
- Bearing races
- Suspension ball joint housings
- Securex wheel nuts and other nuts
- Mining roof support components
- Industrial hot forged components and custom parts
Disadvantages of Hot Forging and When It Is the Wrong Fit
Hot forging needs dies, and FIA notes that setup and tooling costs are spread over the quantity produced, so their effect on piece price falls as order quantity increases. On a small program that cost is spread over fewer parts, so machining from the bar may cost less.
Size and geometry set other limits, since a part has to fit the press, and the press table below lists each machine’s envelope. Features that need tight tolerances or a fine finish will still be machined after forging, because hot forging holds wider tolerances than cold forging.
Heating the steel adds its own costs. The Forging Industry Association’s design guide rates energy costs for hot forging steel as high, and heated steel forms scale on its surface, which is why hot forged parts are shotblasted after heat treatment. FIA’s glossary also notes that decarburization, the loss of carbon from the steel surface during heating, is usually present to a slight extent in steel forgings, and that excessive decarburization can result in defective products.
An engineer considering hot forging for a part can check four things against the current print and route. They are how much material comes off the part today, whether the geometry can be forged, the annual volume, and what machining would remain after forging.
How MacLean-Fogg Hot Forges Components
MacLean-Fogg has hot forged parts in Savanna, Illinois, since 1976. The operation runs 11 Hatebur horizontal hot forging presses across four platforms, from the AMP 20S for smaller parts such as wheel nuts to the AMP 50XL for parts up to 104 mm OD and 2.0 kg. Typical annual volumes run from 50,000 to more than 1 million parts, depending on the component and manufacturing route. Materials include carbon steels, alloy steels, and copper, specified to the part and application.
| Hatebur Press | Qty | Max OD | Max Length | Max Weight | Typical Parts |
|---|---|---|---|---|---|
| AMP 20S | 5 | 40 mm | 35 mm | 0.17 kg | Securex wheel nuts, smaller gear blanks, and other nuts |
| AMP 30S | 4 | 67 mm | 55 mm | 0.70 kg | Pinion and small sun gear blanks, nuts, mining components, and other parts |
| HM 35 | 1 | 75 mm | 55 mm | 0.90 kg | Pinion and sun gear blanks and other parts |
| AMP 50XL | 1 | 104 mm | 95 mm | 2.00 kg | Large pinion and sun gear blanks, mining components, nuts, and other parts |
Listed values are maximum capability limits and may not be achievable together. Part feasibility depends on geometry, material, tooling, and forging requirements. Source: MacLean-Fogg Hot Forging Capabilities Sheet.
Forging and machining sit in the same business, so engineers can set the forged shape around the machining that follows. More than 150 CNC lathes handle parts up to 200 mm in diameter and 120 mm long. The engineering team includes dedicated tool design engineers for forging and machining who work in simulation and CAD software, along with program managers and product launch experts, and a quality lab handles CMM inspection and metallurgy. The facilities are IATF 16949 and ISO 14001 certified. Engineering work starts before tooling is built, in a design for manufacturability review where forging, machining, and inspection are planned around the finished part.
FAQs
Talk to an Engineer About a Hot Forged Part
If a steel part is machined from a bar today, it may be a candidate for hot forging. Send the print, material, annual volume, and current manufacturing route, and MacLean-Fogg engineers will review whether hot forging fits and what machining would remain.