Hot die materials are burdened with a formidable task, which is an elaborate way of saying high temperatures and higher pressures are at work. The forging process applies compressive force and heat to the tools used in forging processes, so deformation stress is a major issue. Fortunately, heat treatment techniques extend the lifespan of the die by injecting its material profile with a much-needed hardness boost.
Hot Die Materials Endure
Two forces assail this material. Firstly, contact force takes a big chunk out of the metal’s working life. Heat extremes also cause fatigue in the die. But it’s the cyclical nature of this event that really takes its toll, for the work is conducted again and again until the die material is aged and no longer able to function. The resulting thermal wear factor, therefore, combines with mechanically-induced fatigue to cause die failure.
Solving Die Manufacturing Costs
When the forging process is cycling, the header and die configuration work the softened alloy perfectly, but the cycle does inevitably impose a cost. The workpiece conducts energy, obviously, so the die ages. Pressure extremes have a similar effect, so the forging tools are destined to fail, which is unless they’re properly heat treated and hardened. A wide range of services come to the rescue when hot die tools require heat treatment, but even the best toughening service is only as good as its source material.
Hot Die Materials: Selecting Alloys
In general, a methodical tooling approach will select an alloy that retains its hardness rating at high temperatures, and this toughness factor won’t be affected by temperature variances. This baseline requirement provides a material backbone for the heat treatment process, so existing properties are intelligently targeted and empathized while any undesirable mechanical characteristics are minimized.
Heat Treatment for Tools Used in Forging Processes
Now that the alloy has been reinforced with tungsten or vanadium, chromium or molybdenum, it’s time for the hardening and tempering stage. Elevated heat-to-strength retention factors harden the material, but that hardness rating isn’t merely skin deep. It goes deep, deep into the cross-sectional area of the tool block so that uniform mechanical resistance is assured throughout the die material’s structure.
Forging processes use dense metal alloys that are further exposed to advanced heat treatment techniques. The baseline properties of the selected metal are deeply transformed by the process, so material conductivity is also a major characteristic, one that requires proper assessment if the hardening technique is to toughen the entire cross-sectional area of the tool.
As we discuss carbonitriding, expect to be carried through a heat treatment process that relies heavily on the control of local atmospheric variables. In particular, as the label implies, carbon and nitrogen atoms are injected into the controlled environment. Undoubtedly, this is an essential technique, one that surface hardens base steel, but how does it work?
Carbonitriding Fundamentals
When a soft metal part is heated, its mechanical properties become accessible and readily manipulable. Imagine doubling down on this engineering science by controlling the atmosphere around the part. That’s what this technique does. It uses a branch of engineering science known as gaseous diffusion to incite surface absorption. Essentially, the air around the steel becomes saturated with carbon. The diffusion process takes over, resulting in an interstitial layering effect. Finally, the part is removed from its gas-filled furnace. Case hardened, the newly heat treated surface sports 0.07mm to 0.75mm of wear-resistant martensite.
Nitrogen Infused Hardness
The fundamentals go a long way towards describing the basics, but this heat treatment technology is far from simple when it’s applied in practice. The source of the two gas mediums represents an initial hurdle. Decayed methane or propane provide the carbon component. Meanwhile, nitrogen is also introduced as an ammonia-based gas. The nitrogen increases the hardness quotient and deepens the case beyond what can be achieved by the nitrogen-less carburizing technique. Additionally, only material hardness is affected. No additional layers are deposited, so dimensional stability is not altered in any way.
Two Roads to Success
A furnace with a gas-infused atmosphere has been used as our case study, but one other method exists in a fully-fledged carbonitriding arsenal. The second version dispenses of gasses so that liquids can take on the burden of the case hardening task. Here, metal components are lowered into a special salt bath. The cyanide-heavy salt then acts as the liquid diffusion mechanism. Again, carbon is released with nitrogen. This mode of operation delivers uniform coverage, but it is best suited for small-scale batch processing work.
Relatively lower furnace temperatures are employed when carbonitriding commences. A purely carburized case requires upward of 900°C of heat, for example, whereas the addition of nitrogen lowers this temperature to approximately 800°C. In exchange for lower energy expenditure and the addition of ammonia, the process grants the low carbon steel a much harder and thicker surface layer, one that doesn’t change the dimensions of the worked part. Commonly worked products include gears, pistons, and shafts.