Characterization of Hot Forging (Closed Die Forging)

Hot Forging – a classic

Hot forging comprises all forming processes that occur above the recrystallization temperature of a metal. The homologous temperature (TH) is used to distinguish it from the cold forming process.

 TH = 0.6 x TS (TS: Melting temperature of the material – all figures in K)

If the process temperature is above TH, the process is categorized as hot forging. This doesn’t necessarily mean that the material needs to be heated. In low-melting metals (e.g. lead), recrystallization occurs at room temperature.

 The characteristic effect of hot forming, is the significant material strength reduction (yield stress) at temperatures above TH. The forming component becomes a “doughy” consistency.

Recrystallization – the new formation of the metal crystalline lattice structure – is responsible for this. Through the degradation of the existing strain hardening (dislocation density) in the metal, the mobilization of dislocations (thermal activation) and the interchanging recovery and recrystallization processes occurring during and after the forging step, it is possible to achieve very high formability. Thus, hot forging is used when the goal is to achieve complex 3D geometries via forming. In addition, it enables the processing of difficult-to-form materials, which can be formed only with limitations when cold. Due to the strength reduction under hot forging conditions, the force and work demand of the processes can be lowered in comparison to cold forming.

The recrystallization is responsible, through the complete reformation of the microstructure, possibly multiple times, for the formation of a relatively fine-grained microstructure. It exhibits the optimal combination of strength and ductility. This circumstance qualifies hot forging as one of the most important manufacturing processes for the production of highly stressed safety components.


Hot Forging Processes

For the description of hot forging processes, we will concentrate on hot die forging, including the preforming operations such as bending, upsetting, and (hot) extrusion.

The following related topics are discussed separately:

Characteristics of Forgings

App. Finishing forging of a connecting rod with burr
Finishing forging of a connecting rod with burr

Hot forgings can be divided into forgings with and without flash.

Flash is necessary for the production of complex 3D geometries with very different local cross-sections and secondary design elements. The “land of flash” acts as a brake on the material flow and slows the flow of excess material. Thus, high internal pressure is built up in the die, which forces the remaining material into the die cavities.

 Flashless forgings are generally limited to axisymmetric components or components with cyclic-symmetric geometries.

Hot die forgings are usually manufactured in multiple steps. Semi-finished rolled products (round or rectangular sections) are used as blanks. In rare cases, continuous-cast material or profiled semi-finished products can be used.

Typical process chains include the following basic operations:

  • Cutting of the billets via billet shearing or sawing (cold or hot)
  • Heating to forging temperature
  • Preforming operations for material predistribution
  • (Buster, Rougher) Blocker and Finisher
  • Trimming (flash removal)
  • If applicable, punching and gaging (calibrating)
  • (Controlled) cooling
  • (If applicable, heat treatment, e.g. tempering)  
  • Final check

Preforming operations are used to minimize the flash percentage (which does not add to the value, but is represented in material costs). In this context, it is customary to refer to the material yield. It is calculated as follows:

Material yield = (weight of raw forging without flash and slug) / (input weight) * 100%

 Formel 'Material yield

The material yield ranges from 60% to 85% (with exceptions), depending on the complexity and component family. It should be noted that a stable trimming process requires a minimum flash percentage in the cross section. If the cutting ratio is too low, the cutting process is not robust, resulting in either rework or scrapping.

Hot forged products generally require additional processing steps to achieve ready-to-install components. Depending on the case, this can include different surface or heat treatments. Machining is always mandatory.

On one hand, this is necessary, because hot forging processes have to calculate a forging allowance due to conditions such as drafts, smoothing of sharp edges. On the other hand, the high process temperatures lead to mostly scaled/oxidized surfaces of the cooled forgings. Because of the locally varying thermal shrinkage of the forgings during cooling to room temperature, highly accurate manufacturing allowances are not feasible. Typical forging allowances can range from few tenths of a millimeter to several millimeters depending on the component size.

To reduce the allowance, “Near-Net-Shape”-components can be produced via warm forming. Ready to install contours can generally only be achieved with cold forming.

Due to the numerous requirements for forgings, approval processes from customers for forgings can take several months.


Hot Forging: Products and target sectors

Application Fields for Hot Forging

The recrystallization accompanying the high temperatures and enhanced formability enables a precise adjustment of very fine-grained microstructure.   Strength-durability combinations can be specified more so than in any other forming process, qualifying hot die forging as a manufacturing process in all cases where high operating loads (static and dynamic) make special demands to the component. In general, such components are referred to as “Safety Critical Parts”. Consequently, the automotive and aerospace sectors represent the most important buyer markets for hot forgings. 


Hot Forgings for the Automotive Sector

App. Trimmed connecting rod forging.
Trimmed connecting rod forging.

Steel is predominantly used for automotive forging applications. Wrought aluminum alloys are on the rise due to the increasing lightweight construction demands; the use of magnesium is rare.

In the volume segment, components are forged in mostly medium to large-scale series (hundred thousand – 2 million). This enables the use of highly automated forging production lines with high output rates. In the premium sector and in commercial vehicle production, small to medium-sized batches are standard.

A number of typical product families are listed below. The examples are applicable for both passenger cars as well as commercial vehicles:

  • Engine parts: piston, connecting rods, cams, camshafts, crankshafts, valves, distributor housings, balance shafts
  • Driving gear: all types of pinions, synchronizer rings, CVT, …
  • Powertrain: Flange yokes and peg carriers, wheel hubs, driving shafts, universal joints, joint discs
  • Undercarriage: wheel carriers, axle journal, pivot bearings, axle stub, ball stubs, steering levers, steering levers, front axle beams, wishbones, ...
  • Brake system: Brake plates, brakes carriers, brake cylinders, brake handles

Hot Forgings for the Aerospace Sector

App. Aircraft turbine (Source: Steve Mann, Fotalia)
Aircraft turbine (Source: Steve Mann, Fotalia)

Application fields for forgings:

  • Fuselage components such as fittings for doors and wings, ribs, window frames
  • Engine with turbine blades, propulsion cone, discs, control levers
  • Fittings and guide rails for the empennage
  • Landing gear: carrier, fasteners, pins
  • Rotor systems: fittings, rotor head, gearbox cover

In the aerospace sector, high-strength and temperature resistant special materials such as nickel-based and titanium alloys are used, as well as lightweight construction materials such as wrought aluminum alloys and magnesium. 



Additional Application Fields

Abb. Offshore wind turbines
Offshore wind turbines


Furthermore, there are other important sectors and application fields in which forgings can showcase their strengths:

  • Construction industry
  • Mechanical engineering
  • Agriculture
  • Defense industry
  • Steel construction
  • Offshore Industry
  • Sanitary sector
  • Precision engineering and watchmaking industry
  • Manufacturing of forgings found in:
    • High quality hand tools
    • Tools for construction machines  
    • Household goods (i.e. knives, pans)
    • Medical implants and surgical instruments
    • Hydraulic units

Trends and Developments

App. View of a hot die forge
View of a hot die forge

To be able to keep up with the ongoing trend towards lightweight design, in addition to considering the feasibility of a process, or rather the manufacturability of a product, local product properties have to be considered in the FEM simulation. The palette of possible applications ranges from the forming of titanium micro components with allowances in micrometers to forgings weighing several hundred kilos or even tons. The increasing model depth (vertical scaling of process simulations) is posing new demands to the calculation precision of classic field sizes. To generate precise results, e.g. from models for the evolution of microstructures, thermal-physical boundary and starting conditions have to be dynamically adjusted in the sense of “closed-loop” feedback. The high quality and resolution required for microstructure models is increasingly forcing users to use fully coupled models that account for manufacturing process history. In addition, the stochastic distribution of input values must be taken into account to display real process behavior. The trend here is moving towards virtual descriptions of process capabilities and a robust design of manufacturing processes, while meeting increasing demands to forging’s dimensional accuracy and performance characteristics.

Complex models should be modeled as easy as possible and the calculation effort to information content ratio has to remain economical.

Simufact participates in numerous projects and task forces of associations and expert committees to find solutions to these problems. Further information can be found under the section Research and Innovation.


Optimization of Material Yield

The input weight of forgings determines up to 60% of the manufacturing costs of forgings with flash. Therefore, the optimization of

material yield has one of the highest cost savings potential for simulation supported process analysis and optimization. The following example of a brass fitting should demonstrate this: 


Next to the flash reduction of almost 20%, switching from a double to a quadruple piece increased the output by a 100%. A positive side effect was the elimination of temperature spots, which would have resulted in increased scrap and rework rates in the original process chain.

The example impressively demonstrates, that through the cost leverage of material and output, the use of simulation products from Simufact can already be amortized in only one project.

Decreasing Die Costs

Another important cost driver is the cost of dies. Depending on the complexity and number of stages, they can constitute up to 20% of manufacturing costs. Early knowledge of die wear and die load can help the simulation user draw conclusions about the suitability of a process sequence. Often times, with targeted alterations of the process and die design, noticeable saving effects can be achieved quickly.

The following example shows how a non-optimized material flow and early die filling lead to premature die failure (forced die cracks), caused by high tension build-up in the die bottom.

The effect could be proven via simulation. After the introduction of corrective measures, calculated tool life was reliably and consistently achieved. Simultaneously, the input weight and the flash percentage were significantly reduced.

App. Real images of forced rupture in a double flange-die (finisher)
Real images of forced rupture in a double flange-die (finisher)
App. Maximum principal stress calculation in relation to filling
Maximum principal stress calculation in relation to filling

Our customers very successfully use Simufact’s simulation solutions for the analysis of potential forging errors. Unlike any other tool, numeric simulation is well suited to test initial process designs, identify potential sources of error and run through optimization loops, all before the first prototype is even forged.


Prediction of Forging Errors

The prediction of typical forging errors such as under-filling, forging laps/folds or piping, but also calculation of required integral values such as forces and energy/work (dimensioning of the forging aggregate) are all typical application fields for simulation software.

Refer to the following example:

App. Planned and simulated stage sequence of a fork head (ball clevis) with noticeable fold formation, already apparent in the pre-forging as well as under filling in the fork area.
Planned and simulated stage sequence of a fork head (ball clevis) with noticeable fold formation, already apparent in the pre-forging as well as under filling in the fork area.

In the initial design of the stage sequence, the customer might discover significant fold formation in the transition from fork neck to fork head as well as under filling in the fork area. A non-optimal design of the rolled preform would be the cause of this. Changes of the preliminary product (preform), as well as a modification of the stroke sequence of the hammers could be directly obtained from the simulation results.


Formation of Forging Fold

The optimization of material flow is shown here in an example of the forging of a fork head (ball clevis):

In the simulation, the original process design demonstrates a tendency towards fold formation in the flange area.


The stage sequence developed via simulation is not only without errors – the input weight could also be reduced.

(Source: Trinity India Ltd.)


The Hot Forging Module in Simufact Forming

Forged conrod - real and simulated
Simulation and real part - hot forging with Simufact Forming

Utilize the advantages of Simufact Forming for your hot forging processes.

Simufact Forming combines easy and intuitive usability with reliable predictions of forging processes. The simulation results are not only applicable to product and process development, but also during the continuous optimization process. The simulation already supports our customers in the quotation phase with quickly available feasibility studies. Simufact users achieve sustainable savings and competitive advantages such as:

  • Efficient material and energy use as the greatest cost saving potentials
  • Shifting expensive development and testing costs (physical tryouts) into the virtual environment
  • Decreasing costs of ensuring quality
  • Reduction of scrap and rework costs
  • Decreasing setup times and downtime in production
  • Creating new capacities for the new component development and innovation

The application module Simufact Forming “Hot Forging” principally serves bulk metal forming with starting temperatures above the recrystallization temperature.

In addition to  hot die forging, the module also covers all required and important side processes such as heating and cooling, cutting and trimming processes, and preforming operations (upsetting, bending). Supplementary processes of hot forging such as (hot) extrusion processes can also be modeled with this module. Together with the application module Rolling, reducer and cross wedge rolling operations can also be calculated.

 For a functional look at Simufact.forming Hot Forging, please refer to our product description: