STEEL COLD FORGING
Cold forging is a metal forming process where a slug or blank is deformed at (or at near) room temperature. Cold forged parts can be produced using one or more single station vertical mechanical or hydraulic presses, or automated multi-station cold forming machines where the pre-formed part is transferred from one station to the next at high speed with each station performing a specific cold forming process.
The many Processes used in Steel Cold Forging
A complete cold forging production process usually comprises several processes to produce the final part shape. Depending on the intricacy of the shape to be formed these may include any of the following:
- forward or backward extrusion
- cold heading
The cold forming processes do not cause any structural changes to the metal which maintains all its original mechanical and tensile characteristics. Cold forging is a very cost effective metal forming method for mass produced parts with respect to other forging methods. The benefits of a good cold forging sequence include:
- a reduction in waste material
- better dimensional control
- reduced use of energy
- and the production of a net shape or near-net part with little or no machining requirements
A good forging sequence should have a correct die filling sequence that virtually eliminates scrap and produces a part that is free of defects or folds. The forging sequence is however not easy to ace even for the most experienced process designer who has to rely on “trial and error” until the perfect sequence is set up, during which time experienced workers need to intervene in the production process to carry out manual adjustments and calibrations. There are many things that can go wrong in the cold forming process. For example the incorrect distribution of deformation between the various stations can cause the compression of parts that are already filled, overloading the machine and jeopardizing the quality of the molds.
This is why an accurate engineering simulation of the entire production design process has become so important.
Why Numerical Simulation can improve the Cold Forging Process
A complete virtual simulation study of the cold forging process can save companies a lot of time and money, as the entire process can be optimized eliminating the costly trial and error phase altogether.
The numerical simulation of steel cold forging needs to account for some characteristics that are peculiar to cold steel forging:
- both the material to be forged and the dies are made of steel. The software model therefore should make use of a deformable die rather than a rigid die
- carbides such as tungsten carbide and titanium carbide are often the material of choice for the inserts in order to improve the finishing of the part. These provide pre-stress to the tool stack and improve the tools total life. It is therefore important that the software package used is able to accurately simulate different materials with the same tool as well as the methods by which these inserts are mounted in the tool stack (via press-fitting or shrink-fitting)
Virutally Simulating the Cold Heading Process
High volumes of steel screw and bolds are often manufactured using the cold heading process. A process that uses multi-stations and automated presses to transform a wire into a complex part at high speed to produce up to 400 pieces per minute. Your simulation software should allow you to:
- group the full sequence of operations into a single step
- since the material used for the wire blank normally undergoes heat treatment such as annealing and has already been drawn, the model must be able to capture the initial boundary conditions of the material. A user-defined stress-strain curve can be used to specify the steel properties after heat treatment using the built-in database of materials
- produce an accurate simulation of the wire drawing process to obtain a correct initial surface strain distribution and cross section for the wire blank
Critical Operations in the Forging Sequence
Critical operations include:
- flash trimming – the numerical material model must have a choice of fracture criteria and the mesh must be sufficiently fine along the trimming line in order to properly capture material failure
- thread forming – involves the initial cylinder being rolled between two tools which can be flat (combs) or cylindrical. The software model must be able to simulate several rotations of the cylinder, each one with a small deformation, in an incremental fashion (finding a good balance between accuracy and computational run time)
Other Steel Cold Forging Processes that can be simulated include:
- sheet metal forming such as stamping, drawing, deep drawing and bending
- ring rolling
Thanks to virtual simulation, all the expensive, necessary adjustments required during a new product design cycle can be avoided, thus eliminating costs related to prototype molds and press down-time required for quality control. For existing production runs, a posteriori simulations can test potential changes to the mold and the effect of such changes on the quality of the workpiece without the need for expensive physical tests.
The main advantages of Numerical Simulation in Cold Forging Process Design
The engineering simulation of the design and production of cold forged products allow engineers to:
- test different set-ups, including changes in the billet dimensions, billet initial positioning, dies shape, operations sequence without having to create real dies and perform any real test
- increase interaction with the customer during the design process, who can evaluate and approve the proposed changes in the part and/or process on the basis of objective evidence provided by the software
- improve part quality, from its shape, to the elimination of surface and internal defects (folds)
- reduce scrap material (waste)
- improve the die total life by reducing stresses caused by punches/inserts insertion
- improve die wear by calibrating the die assembly to obtain the correct pre-loading on the inserts surface
- to make the correct press choice, avoiding overloading on the press components
- contribute to the time-to-market reduction of the final product
How can EnginSoft help your Simulation Engineers?
EnginSoft engineers use Transvalor ColdForm as the reference software tool for the design of cold forging processes. The introduction of ColdForm in the R&D department of any design company allows a deeper understanding of the dynamics of material flow between the dies and the criticalities of the specific process at hand.
EnginSoft engineers have extensive experience in the simulation of all operations related to the production of steel parts that are cold forged. EnginSoft’s services in Cold Forging simulation range from taking on the entire customer project, to taking on a secondary role assisting your in-house simulation engineering teams in building their own expertise, to simply selling the software package and possibly offering engineer training in the use of the product.