How To Choose Molds When Forging Forgings And The Causes of Network Cracks

       How To Choose Molds When Forging Forgings And The Causes of Network Cracks

    Isothermal forging die structures are relatively complex. During design, manufacturing, and usage, one should fully consider the service life and efficiency of the isothermal forging die, aiming to minimize the cost of forging part production. Therefore, when selecting the isothermal forging process and designing the mold, the following principles and points to note should be observed.

    Select those components with complex shapes that are difficult to form during conventional forging, or require multiple heating cycles, as well as those with stringent organizational and performance requirements for isothermal forging.

    The choice between open-die or closed-die forging methods should be based on the structure, size, subsequent processing requirements, and equipment die-setting space of the forging. The overall design of the mold should meet the requirements of the isothermal forging infrared mold heating furnace process, be structurally reasonable, and be easy to use and maintain.

     The die-casting workpiece section should have dedicated heating, heat preservation, temperature control, and other devices, capable of reaching the required temperature for isothermal forging. Except for special forgings that require specific molds, the molds should be designed for universal use. The materials used for each part of the mold should be reasonably selected to ensure reliable performance of the mold parts at different temperatures. Isothermal forging molds operate at high temperatures, so to prevent heat loss and excessive heat transfer to the equipment, an insulating layer should be set between the mold base and the bottom plate. Additionally, water channels should be opened on the upper and lower bottom plates for cooling purposes. Electrical insulation should also be considered to ensure normal operation of the equipment and the safety of personnel working with infrared mold heating furnaces.

    Consider issues of guidance and positioning, as the isothermal forging die is placed in a heating furnace and it is impossible to detect if the die has shifted. Guidance devices should be considered on both the die frame and the module, with internal and external guidance devices coordinated. Additionally, positioning blocks should be designed when placing the raw material into the die to prevent misalignment.

     The crack depth is relatively shallow, generally about 0.01 to 1.5 mm, and appears in a radial pattern. It is also known as crazing, and the main causes include:

   (1) The raw material has a deep decarburization layer that was not removed during cooling machining, or the finished mold was heated in an oxidizing atmosphere furnace, causing oxidation and decarburization. 

   (2) The decarburized surface layer of the mold has a different carbon content and volume from the martensite in the steel base, leading to significant tensile stress during quenching. As a result, the surface metal often splits along the grain boundaries into a network pattern. 

  (3) The raw material is coarse-grained steel with large initial grains and large blocky ferrite, which cannot be eliminated by conventional quenching and remains in the quenched microstructure. Alternatively, if temperature control is inaccurate or instruments fail, resulting in overheat or even overburn, grain coarsening occurs, losing the bonding strength at grain boundaries. During quenching and cooling of the mold, carbides precipitate along the austenite grain boundaries, significantly reducing grain boundary strength, making the material brittle and prone to cracking along the grain boundaries in the form of a network under tensile stress, infrared heating furnace for molds.