THE HEAT TREAT DOCTOR: Fundamentals of Press Quenching
Heat treaters know gears and bearing races are especially prone to dimensional changes during hardening and quenching, which can cause a number of problems during post-heat treatment manufacturing operations. Typically, additional stock allowances are needed to compensate for distortion so that parts can be machined to the proper finished dimensions. The objective of press quenching is to hold parts round and flat while they are being cooled, thereby reducing, though typically not eliminating, distortion. Let’s learn more.
Hardened or case-hardened gears or bearing races are either free or press quenched. The latter involves moving them individually out of the furnace by manual or robotic means into a quench press (Fig. 1). The transfer from the furnace to the quench press, with oil or polymer flowing, must be relatively fast - typically under 10 seconds. A high volume of quenchant, uniformly directed to all internal and external surfaces of the parts, is needed. Control of out-of-roundness to within 0.005” (0.127 mm) and out-of-flatness to within 0.001” (0.025 mm) is common from most industrial presses.
Critical ConsiderationsConsideration and regulation of the following press parameters has been found to help control distortion:
- Transfer time from furnace to press (consistent)
- Manipulator contact (area, duration)
- Part positioning on the die
- Die design
- Die pressure (clamping force, expander force) applied to hold the component
- Quenchant temperature
- Direction of quenchant flow
- Quenchant pressure
- Quantity of quenchant
- Location of points of contact on the component
- Duration of quenching (at various flow rates)
- Flow paths to and through the lower die for the quenchant to reach top and bottom simultaneously
- Die set maintenance and repair
- “Pulsing” feature (optional)
It is especially noteworthy to mention that old, worn or damaged tooling can be responsible for as much, if not more, distortion than part design or geometry. Quench dies should be routinely inspected for damage or wear and repaired or replaced as necessary. Also important is to check final dimensions after repair since one of the hidden dangers is that of altered or restricted quenchant flow.
The "pulse" feature is a popular option on presses as it allows a part to expand and contract normally while still controlling shape. Without it, stresses are induced because the part is not allowed to contract and expand. Pulsing reduces the friction caused by constant pressure and clamping on the race as it contracts during cooling. This friction promotes stresses that result in eccentricity and out-of-flatness. Properly applied, the pulsing technique finds the die in contact with the part throughout, but the pressure is released and re-applied every two seconds during the entire quench cycle. The expander pressure is normally not pulsed.
How the Press WorksA typical press-quench system (Fig. 2) will operate as follows. The component to be quenched is removed from the furnace (usually a continuous pusher or a rotary hearth furnace) and placed onto the lower die in the out position. Initiation of the automatic cycle moves the loaded lower die assembly into the center section of the machine. When this is fully advanced, the upper ram assembly and dies descend, with an expander centering the part just prior to the inner and outer dies locating on their respective pressure points. When the expander and dies are properly located, the ram holding them is latched in the down position and pressure is applied to all three. The inner die, outer die and expander usually have completely independent pressure controls, ranging from 0-24,000 lbf (0-107 kN) on the dies and 0-11,000 lbf (0-49 kN) on the expander. These are often regulated by hydraulic valves and monitored via pressure gauges. When used in a bore, the expander cone pushes out against the segmental lower die in order to hold the bore round and to size. The inner and outer dies help keep the component flat.
A guard usually completely encloses the upper dies, forms a quenching chamber and is attached to the upper ram moving up or down with it. When the upper ram latches, the quenchant pump starts supplying the chamber with fluid. The volume of quench fluid is normally controlled by a series of switches. In a typical press, each switch controls a solenoid valve supplying different flow volumes, often in the range of 50 gal/min (190 l/min), 100 gal/min (380 1/min) and 200 gal/min (758 l/min). Any combination may be used, up to a maximum of say 300 gal/min (1135 l/min). With all switches off, a minimal flow usually around 10 gal/min (40 1/min) is still maintained.
A circulation path within the press is created as the quenchant is pumped into the quench chamber through apertures around the outside diameter or through holes in the lower die. Quenchant fills the chamber around the component and flows out at the top. The elongated exit apertures may be fully open or closed to restrict flow, depending upon requirements. Timers used in conjunction with the flow-selector switches provide control of the duration of flow as well as volume, allowing a variety of flow/duration combinations.
Quality Assurance ConsiderationsIn addition to the actual press-quenching operation itself, there are several ways to assist in maintaining the parts as distortion free as possible. Component design and manufacturing methods are the most critical in minimizing distortion, but other factors are also important. In engineering, the design should consider distortion from the initial concept through all phases of production processing, including material considerations (type and source). For example, the forging process should be designed so that material flow minimizes stress patterns. Forgings should be normalized above the austenitizing temperature, and after machining, stress relief at subcritical temperatures is needed.
Part DesignIt is important in part design to avoid excessive or abrupt section changes that promote unequal heating and cooling rates. Fixturing and part support at temperature should also be considered during the design phase as creep from poor support is another major cause of distortion. The machinery employed in pressure quenching can normally hold twice the machining tolerances required of one area in its relationship to another. For example, if on a single plane the relationship between two surfaces is ± 0.002” (0.051 mm), then after quenching the best one can expect to attain is ± 0.004” (0.102 mm).
Part SamplingIn press quenching it is always a good idea to run a sample lot of parts prior to general production. In this way, the degree and nature of the dimensional changes can be observed. Possible changes in manufacturing operations can then be determined. Sampling also enables one to tell if specialized tooling is needed in conjunction with the press-quench machine for size control. Finally, metallurgical checks will allow determination of the optimized part microstructure as a function of setup parameters.
Record KeepingPress-quench die sets and associated components (plugs, expanders, rolls, etc.) should be marked in a suitable manner to identify them for use with certain part numbers. The press-quench setup data should be stored in a computer database or kept as a written record, including:
- Die numbers
- Plug numbers
- Quench time
- Flow rates
- Ram and expander pressure
- Pulse (on/off)
- Other pertinent data