STEEL HOT FORGING
Closed die hot forging also known as impression forging is perhaps the most commonly used forging process for mass produced steel parts. The forging process makes use of two or more die parts which come together tightly with an appropriately sized billet placed inside. The steel billet is heated at high temperature and undergoes plastic deformation, consequently taking the shape of the die. If an accurate billet size is used, there will be no wastage; otherwise any excess metal pushes itself out of the die cavities to form what is called a flash which can be later removed by machining.
Closed die steel hot forging is a highly repeatable forging process with mass production output in the order of hundreds and thousands of parts. It is so repeatable because the production process parameters such as:
- the furnace temperature
- the transfer process of the billet till it touches the die surface and then of the forged part to the following station for machining
- the kinematics of the press
can all be kept constant. Consequently closed die forging was one of the first forging processes to be successfully reproduced using Virtual Simulation techniques.
When defects such as laps or shrinkage occur in this type of process they tend to recur in a systematic manner. The causes of such defects can often be traced back to the original forging process design such as the size and position of the initial billet, or in the case of multi-process operations they can be traced back to a poor choice in the pre-form station. In the pre-forming phase an adequate amount of metal must be moved close to the final forged product to facilitate the next operations with a minimum flow and deformation of the material.
Closed die forging uses machinery such as:
- drop hammers
- vertical and horizontal rolling mills
- universal mills with complex kinematics
When a part is finished, with the exception of near-net-shape parts, the parts are normally machined to remove the scraps and are sometimes also twisted and coined before they receive a thermal treatment such as quenching.
Using Virtual Simulation in the Forging Design Process
The final mechanical characteristics of a product are the result of the entire forging process. It is for this reason that an effective virtual simulation study must cover the entire forging process so that the information derived from one part of the study can be fed as input to the part that follows. This is especially important in the case of highly automated forging processes.
When the forging process is done using a drop hammer or a screw press, where the parts are massive and the forged part is formed in more than one operation, the simulation needs to include all the cooling phases, including cooling resulting from workpiece to die contact or simply from air drying. Once the desired quality standards have been defined, it is possible to estimate the force needed to obtain the desired shape and to verify the stress that the operation puts on the dies in order to identify areas at risk of cracking.
The Advantages of Virtual Simulation of the Hot Forging Process
Some of the reasons why it is always advisable to create a complete virtual simulation for each hot forging workpiece and the tools used before it goes into mass production are:
For the workpiece the simulation is able to:
- predict filling defects (laps and cold shuts)
- predict superficial defects (folds, surface cracking)
- predict the grain flow pattern and micro-structure
A virtual simulation of the tools used in the forging process will provide:
- an analysis of their deformation and stresses
- a prediction of their wear
Additionally a virtual simulation of the press used can provide an estimate of:
- the maximum forging load, to be compared with press characteristics
- press deflection assessment
both of which can be useful for press design.
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