This invention concerns the field of heat-treating of materials, which involves rapid cooling (also called quenching) at the end of a high-temperature cycle. Rapid cooling is employed when the material being treated exhibits desired phase transformations during rapid cooling from high temperatures. The most common goal of heat treatment in current commercial applications is improved hardness.
Many heat treatment processes are carried out in vacuum furnaces. During the quenching step of a heat treatment cycle, it is often desirable to provide an atmosphere comprising gases that are inert with respect to the material being treated. (The material being treated is also referred to herein as the “heat load” or “HL”). Helium (He) and argon (Ar), or blends thereof, are commonly-used inert gases for this application. Mildly-reactive gases, or blends of inert gases and mildly-reactive gases, are technologically-acceptable and provide a less costly alternative. Nitrogen (N2) and hydrogen (H2) are examples of mildly-reactive gases used in this application, which can be mixed together or provided with secondary gas additions such as carbon dioxide (CO2) or argon.
One common method for conducting a quenching step is the introduction of a cooling gas, which is then circulated inside the vacuum furnace and a water-cooled heat exchanger. Use of highly conductive gases, such as hydrogen and helium, and/or high molecular weight gases, such as argon and carbon dioxide, as the cooling gas can result in desirable cooling rates, but such gases are impractical for many applications. For example, use of helium is often cost-prohibitive. The cost of a helium recovery and recycling system can exceed the cost of a simple, single-chamber vacuum furnace. Use of hydrogen introduces operational risks (due to its flammability) and requires highly trained, reliable operators and dedicated supply and furnace systems. In addition, achieving desired cooling rates with gases introduced at ambient temperature requires the quenching step to be carried out at a relatively high pressure, e.g. 15-35 bars, and the cooling gas to be circulated at a relatively high velocity. This pressure range requires a robust furnace structure that is significantly more expensive than similar structures that offer cooling pressures between 6-12 bars. High-velocity cooling gas flow may result in an undesired, directional, and non-uniform cooling of a heat load that leads to unacceptable dimensional distortion of treated metal parts.
Another approach to increasing cooling rates involves the use of cryogenic fluid in liquefied or cryogenic vapor form. As compared to a cooling gas introduced at non-cryogenic temperatures, a cryogenic fluid will enable increased heat flux from a heat load by virtue of an enlarged temperature difference between the load and the cooling medium. Cryogenic fluids have been substituted for water in heat exchangers used to cool the cooling medium in a quenching step. Liquefied cryogenic gases such as liquid nitrogen (LIN) have been used as the cooling medium. This approach benefits from the enthalpy of liquid boiling as it is injected into the vacuum furnace. Unfortunately, the heat capacity of the cryogenic fluid and the latent heat of LIN that can be injected into a vacuum furnace of a specific volume are insignificant when compared to the heat accumulated in a metal load that must be rapidly removed. Increasing the mass of cryogen injected into a furnace and, thus, increasing the cooling effect, is possible by increasing the quenching pressure. As noted above, however, this approach requires the use of furnaces that can operate at a higher pressure, which is significantly more expensive. Another limitation on existing methods of injecting cryogenic fluids is the inability to rapidly inject cryogenic fluids that tend to rapidly boil and choke injection points or nozzles located inside the hot furnace because they are commonly delivered in a saturated vapor condition.
Accordingly, there is a need for an improved quenching method that provides the heat capacity necessary to quench the material being treated at a lower cost than existing methods.