Warm Forging

Brief Introduction to Warm Forging

Warm forging refers to the metal forming process where a workpiece is heated to the temperature that is above its hardening temperature but below the temperature where the scales form. The process fills the gap between the closer tolerance yet sometimes expensive cold forging process and the lower precision hot forging process. Such a process is implemented to fabricate close tolerance constituents in steel or other metal alloys that were not achievable or possible by cold forging.

Warm forging is also being utilized to fabricate constituents very close to the final shape that was formerly made by hot forging with generous finishing allowances. Components such as shafts, gears, and automotive front wheel drive tulips are mostly made by means of warm forging. Warm forging enables the conversion of certain types of components that are not able to be hot or cold forged. 
The warm forging manufacturing process happens within the temperature range of 650 °C and 1000 °C, depending on the used material and the type of component. This is typically above the work hardening temperature of the work-piece and below the temperature at which scale forms. If the forging temperature is below the transformation point of 740-770 °C, the material should go through preliminary heat treatment. This temperature range is utilized for parts with medium convolution, which don’t require heat treatment after forging. The forging temperature is above the transformation point for parts with greater convolution. Even though this temperature range is close to the one of hot forging, in respect of technology warm forging quite resembles cold forging.
The warm forging process is set down somewhere between the cold and hot forging techniques, combining all their superiorities as precision, material usage, surface quality, and flexible shapes but generally demands high engineering skills (tools design, cooling) and a dedicated forging press. The practice of warm forging operations requires abundant knowledge in engineering as well as in die design (including temperature regulation) and slug preparation so that the results come as desired and the die life can be maintained. All key process components such as raw material quality, slug weight (net shape without flash), slug preparation, temperatures, and maintenance key parameters have to be accurately controlled.

Warm Forging Machine and Material Consideration

Since warm forging temperatures are typically lower than those utilized in hot forging, the work-piece is less flexible during this process and complicated shapes are harder to fabricate. Warm forged components also have a lower level of tolerance than cold forged ones. Among all the metallic materials, steel alloys are the most commonly used in the warm forging process. Warm forging makes it easier to deform steel alloys than in cold forging and can have finer tolerances than those made with hot forging.

These superiorities contributed to the increased use of the warm forging manufacturing method, especially when creating a warm forging machine-like high-volume automotive machine. The machine employed as a warm forging machine must be engineered to accomplish an appropriate tooling life to improve cost-savings. This machine should be compatible with the quality and size of the work-piece materials that will be forged using this fashion. Dies should be well maintained throughout their operational lives to increase lifespan.

Pros and Cons of Warm Forging

1. Surface quality is good
2. Greater work-piece ductility than cold forging
3. Can meet high-volume part demands
4. Flexible shapes
5. Lower forging press loads than cold forging

1. Rigid temperature control
2. Lower accuracy than cold forging
3. Energy costs associated with heating
4.Greater forging press loads than hot forging
5. Requires skilled engineering to design appropriate tooling

Cold Forging

Cold forging can be categorized into two types: impression die forging and truly closed die forging. Lubricant and circular dies are sometimes involved in such a process which is performed at room temperature. Carbon and standard alloy steel forgings are the most usual cold-forged. Parts are typically symmetrical and rarely exceed 25 lb.

The primary dominance is the material savings achieved through precision shapes that require little finishing. Utterly contained impressions and extrusion-type metal flow yield fruitful, close-tolerance components. The production rate of a cold forging process is very high and the tooling life is longer compared to those of warm forging and hot forging. 

Cold forging typically improves mechanical attributes, but the improvement is not helpful in several ordinary forging applications; the key advantage is still the economy of the process. Cold forging techniques are known to have the best dimensional precision when in comparison with hotter forging methods. This feature is owing to the fact that the work-piece is deformed near the net-shape of the final product.

Cold forgings do not have to be cooled down after forging, since they are already made near their hardening temperatures. Hot and warm forgings can experience shrinkage as they solidify, a phenomenon that is not a problem in cold forging. The strain from friction between the work-piece and the machinery utilized to deform it can give rise to the temperature of the work-piece to reach up to 250°C even though the work-piece stays in a solid-state in cold forging. Consequently, the cold forging fashion is usually utilized with softer metals such as copper alloys, aluminum, and low alloy steels that often weigh less than fifty pounds. In other words, softer metals are easier to shape at lower temperatures and demand less forging pressure.

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