FEATURE ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2024 58 Q uenching of steel for hardening takes place after a material is heated through its phase transformation into the austenitic range and then rapidly cooled. This causes the austenite to complete a phase transformation into martensite. If the quench is not uniform, the component experiences a large temperature gradient throughout its geometry, resulting in thermal stresses and non-uniform transformation of the microstructure; a large deformation in relation to the pre heat treated geometry. In addition, a quenching stream that penetrates a batch of parts disperses unevenly and each part is cooled at a different rate depending on its position in the furnace. Manufacturers have attempted numerous methods to reduce distortion during quenching as it is difficult and costly to remove excess material after the component has been hardened. One of the most popular methods of distortion control is the press quench technique that quenches one part at a time instead of quenching in batches. Press quenching offers attractive results when it comes to distortion control. However, there are unattractive aspects of the process including safety concerns, handling of hot components, environmental (oil), and washing (oil removal) that require special handling and equipment to quench a component after heat treatment (Fig. 1). PRESS QUENCHING Press quenching is used when batch quenching causes too much distortion, so much so that it exceeds the additional material left for post heat treatment removal. These parts include case- and through-hardened 4D HPGQ: AN ALTERNATIVE TO PRESS QUENCHING FOR DISTORTION-FREE HEAT TREATMENT 4D high-pressure gas quenching ensures uniform heating and quenching for precise, repeatable results while eliminating distortion. Tom Hart* and Maciej Korecki* Seco/Vacuum, Meadville, Pennsylvania components (e.g., gears and rings). For components that are sensitive to hardening distortion, press quenching offers a versatile way to harden thin cross sections and large geometric parts. Press quenching hardens one part at a time, providing dimen- sional stability, controlled distortion, and repeatable results. A critical aspect of press quenching is the design and construction of the special dies. These dies are mechanically aligned to retain the hot plasticized part with pressure, restricting the desired features from distorting when quenching through its phase transformation. Oil flow across the part surface is also important to achieve the desired hardness and microstructure, and the dies must be designed to balance the need for die contact and proper oil flow over the part. Common dimensions that require distortion control are runout, parallelism, and concentricity, and when press quenching is done in a proper manner, precise tolerances can be achieved, 0.001 to 0.002-in., in relation to the pre heat treatment dimensions. HIGH PRESSURE GAS QUENCH (HPGQ) IN FOUR DIMENSIONS Furnace systems use various quenching processes and media to achieve desired metallurgical properties. Based on a material’s ability to be hardened, more aggressive cooling rates may be needed to reach the required hardness for the component’s end use. Figure 2 shows the three main types of quenching oil: fast, medium, and slow, in relation to the various quenching gases (N2, He, H2) used in thermal processing, and compares cooling rates for oil quenching versus gas quenching. Helium and hydrogen *Member of ASM International Fig. 1 — Illustration of a four-stage press quench machine[1]. Stage 1: Hot loading on lower die; Stage 2: Upper die compression and oil flow; Stage 3: Secondary (free) quench position; Stage 4: Discharge tank with conveyer for final quench. 10
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