What is pressure quenching?

  Pressure quenching is a kind of quenching process specially used to reduce the deformation of a workpiece of complex shape during heat treatment. Deformation in industrial heat treatment operations is caused by a number of independent factors. Some of these factors include the quality of the materials used to make the workpiece and its previous processing history; Residual stress distribution and previous heat treatment history; The non-equilibrium thermal stress and phase transition stress caused by quenching itself. As a result of these factors, high precision workpieces (such as industrial bearing rings and automotive spiral bevel gears) often exhibit unpredictable deformation during unconstrained or free oil quenching.

Pressure quenching is done in a carefully controlled manner, using specialized tools to create a concentrated force that restricts the movement of the workpiece and helps to minimize the deformation of the workpiece. If properly handled, this quenching method can usually achieve the relatively strict dimensional requirements specified in the industrial manufacturing specifications. It is commonly used in a variety of complex workpieces made of ferrous and non-ferrous alloys. Common steel alloys using pressure quenching usually include high carbon penetration quenched steel (such as AISI52100 and A2 tool steel) and low carburized carbon steels (such as AISI, 8620 and 9310).

  Carburized carbon steel in particular benefits from the process of pressure hardening due to its machining properties and its popularity in the automotive industry as well as in geared drives for industrial and consumer products. Ideally, during quenching, the transition temperature of the workpiece is uniform across the cross-section, so that the transition can occur evenly. However, in the carburized workpiece, the martensite transition temperature is not consistent across the entire cross-section. In the carburizing process, the carbon diffused to the surface of the parts produces a composition gradient, resulting in a gradient distribution of the transition temperature near the surface. During quenching, this gradient will promote or worsen the deformation problem of such a workpiece. This type of deformation is also caused by the non-uniformity of the microstructure of the matrix material (e.g., severely segregated material).In general, large thin-walled parts such as bearing rings with large apertures are more susceptible to the effects of these deformation problems than thick and heavy parts with compact geometry. Although pressure quenching does not eliminate these effects, its use helps to minimize such deformation problems.

The severity of the deformation during the heat treatment process strongly depends on the nature of the heat treatment process used on the workpiece. In order to minimize deformation during quenching, the heat dissipation of the parts shall be as uniform as possible. In the case of sudden changes in geometry, this is difficult to achieve. For example, in the same part, the thin section is adjacent to the thick section. A good example is a tooth on big or small gear. Compared with large gears and pinions, the surface area to volume ratio of teeth is larger, and they have a tendency to deform through “development” during quenching. Although such parts may produce unexpected deformation during free quenching or unconstrained quenching, this characteristic movement of gear teeth is highly repeatable in pressure quenching operations and can be taken into account in gear design to minimize the amount of grinding after quenching. As the workpiece is immersed in the quenching cooling medium, the gear teeth will cool and contract more rapidly than the adjacent thicker parts. As a result of this difference in cooling rates, the thinner and lighter parts of the workpiece tend to harden and contract rapidly while the rest of the workpiece remains inflated. Because the thicker parts cool and contract at a relatively slow rate, where the thicker parts join, their relative motion is hindered. The result is a thin section over a thick section

It develops more rapidly, resulting in temperature gradients and uneven tissue stresses. During pressure quenching, this problem is solved by selectively directing the quenching coolant to the thicker part and away from the thinner part in order to promote more uniform quenching. This has been achieved primarily through the use of specialized tools. By adopting this important measure, the deformation caused by the transformation can be minimized.

1 .Equipment

  In the early 1930s, quenching machines began to be widely used in the industrial production of the United States, mainly for the processing of ring gears in automobiles (including cars and trucks) (Figure 1).

▲ FIG. 1 A 64cm (25in) automatic quenching machine tool

Note: Manufactured in the early 1930s at the Gleason plant in Rochester, New York. The operator is quenching a finish

The operating large spiral bevel gear is removed from the lower die assembly.

These machines can be driven by hydraulic or pneumatic (depending on the design) systems and can use a variety of quenching and cooling media, most commonly oil. While the geometric design and optional features of these machines have changed significantly over the decades since their initial invention, their basic functions have remained the same. A representative form of a modern quenching machine tool is shown in Figure 2.

▲ figure 2 Gleason529 quenching machine in modern form

  The overall design consists of a number of basic components, including a vertical machine tool section, a control panel, a lower die table, tooling, and a base. Cooling devices are used to maintain the temperature of the quenching cooling medium within a specific narrow range, which may be part of a separate mechanical system, or may be used in a central vessel capable of connecting multiple quenching machines simultaneously. The vertical part of the machine includes the upper die top rod, hydraulic system branch valve box, hydraulic pipeline, solenoid valve, and valve, electrical panel control box. The control panel shows the various performance parameters that may need to be adjusted during the quenching cycle, as shown in Figure 3.

▲ FIG. 3 control panel shows typical quenching cycle

  The various parameters that need to be adjusted during the loop process

The base can be used as an oil storage tank for a quenching cooling medium and can also support the lower die assembly. Its schematic diagram is shown in FIG. 4.

▲ FIG. 4 The oil flow from the oil pool to the cooling unit and then back to the quenching machine

  The vertical body is mounted from the front of the machine base and allows full access to the workpiece in the lower die, including placing the workpiece on the tooling to be quenched and removing the workpiece when the machine is in the “fetch” state after quenching.

During operation, the quenched workpiece is manually or automatically removed from a separate furnace (usually a box furnace, a continuous rotary furnace, or a push-rod furnace) and placed on the tooling of the lower die assembly. The full picture of the lower die assembly is shown in Figure 5.

▲ FIG. 5 The lower die assembly of quenching machine under the condition of “taking out”

Note: Spring pressurized central expander cone and independent grooved ring

It should be noted that the efficiency of the transport equipment from the heating furnace to the quenching machine is usually a key parameter in pressure quenching. Transfer time should be kept to a minimum to minimize heat loss. If this step takes too long, the result of delayed quenching may result in hardness related problems and undesirable transition products. After the workpiece is successfully placed on the lower die assembly, the machine begins to operate and the part is retracted into the center position under the upper hydraulic ejector assembly. The outer protection on the machine tool decreases as the assembly descends, and the middle eaves drive one (or more) internal expanders to contact the inner diameter of the workpiece at specified pressure points to maintain roundness at these positions (figure6).

▲ FIG. 6 Pressure quenching process

A) A hot gear is placed on the lower die assembly for pressure quenching

B) The center ejector rod and upper inner and outer die to drop down to contact with the parts

C) Start the timing cycle and the oil flow starts to enter the quenching chamber and around the parts

Each component of the rod assembly (center expander, inner and outer die) is controlled by three separate proportional valves, all monitored and controlled by pressure sensors. The preset pressure level is usually maintained by the expander throughout the quenching cycle, and in some machine tools with programming functions, this pressure level may change during the course of the quenching cycle. In the quenching process, the inner and outer die can be lowered so that it is in contact with the upper surface of the quenched workpiece to control the positioning, dishing, and planeness of the part. The flow of quench oil can be preset and pre-edited and then activated when the workpiece is quenched.

Figure 7 shows an example of a quenching oil cycle path established in a quenching chamber.

▲ FIG. 7 Central expander and quenching process

Schematic diagram of contact between the inner and outer die and parts

1- Mechanical protection device mounted on the upper die assembly

2- External upper die 3- internal upper die 4- Quenched parts

5- Lower die Assembly 6- Center expanders cone

Arrow line – Path of oil flow at quenching

Quenching oil is pumped to the quenching chamber through an opening around the outer diameter of the lower die. As the Chambers around the workpiece are filled, the quenching oil flows out of the top. If the tooling is designed properly, the best overall effect can be obtained by adjusting the direction of the quench oil spill workpiece. The extended opening at the outlet can be adjusted to restrict the flow of quench oil, or it can be opened completely for maximum flow, depending on the requirements of the part. The lower die is composed of a number of different grooved concentric rings

The maximum flow rate can be obtained by rotation, or the flow direction can be restricted to the quenching oil at the bottom of the part. During quenching, precise adjustment of these characteristics helps to minimize distortion due to uneven heat dissipation. In the quenching cycle, it is also possible to change the flow rate and duration of the quench oil by timing segments, in order to establish a well-defined quenching process for specific parts.

The lower die table is usually mounted on the cross-section of the rod and driven by hydraulic or pneumatic pistons. There is a CAM in the lower die assembly for adjusting the independent ring. By driving the CAM, these individual rings will be dished or tapered to better fit the required part geometry (see Figure 8).In order to establish proper contact with the quenched workpiece, a gasket is required below each ring. Another advantage of this structure is that the gasket can be cut and installed in a relatively fast and easy way. The proper support of parts is a key aspect of pressure quenching, in which die design plays a key role.

▲ FIG. 8:

a) Schematic diagram of the mechanism used to control the dish

This mechanism allows the lower die inner ring to be raised

Or lower (tighten) to compensate for the dishing error

b) Real die Assembly

It shows how to control the rise of this mechanism

Or lower independent rotary table with grooved ring

The oil quenching process consists of three basic stages:

1) In the initial vapor film stage, the oil vaporizes as soon as it touches the part, forming a vapor barrier around the part that ACTS as an effective insulating layer.

2) In the vapor transmission stage, the quench oil passes through the vapor layer and the heat transfer speed is faster.

3) In the convective stage, heat dissipation is mainly achieved through convective heat transfer.

To ensure uniform heat dissipation during the initial stage of quenching, the flow rate of the quenching medium must be sufficient to prevent vapor film formation. If bubbles form in the area surrounding the surface of the workpiece, the inhomogeneity of heat dissipation will result in unacceptable hardness changes and deformations. When the initial quenching stage is successfully eliminated, the flow velocity of the quenching cooling medium can be reduced. The final flow velocity distribution of the quench cooling medium specified for the part must be carefully selected to meet the requirements of hardness and geometry. Too slow a quenching rate will lead to delayed quenching, hardness change, and undesirable transition products. If the quenching cooling rate is too fast, the parts will be deformed and/or cracked. It is usually necessary to go through repeated tests to determine the proper flow velocity of the quenching cooling medium and to select the flow path of the quenching cooling medium around the parts. The success of quenching usually depends on the experience, knowledge, and skill of the machine operator.

The average oil temperature of pressure quenching is mostly 25~75℃ (75-165°F), depending on the nature of quenching operation, the type of quenching cooling medium used, parts materials, performance requirements after heat treatment, and so on. One measure to avoid damaging the sealing ring of the machine containing the quenched-cooling medium is to generally avoid the average temperature of the quenched-cooling medium being too high

60 ℃ (140 ° F). Proper routine maintenance of quench oil baths is important, but this is often overlooked during pressure quenching, resulting in unpredictable changes in the hardening of the materials treated in such systems. With the continuous use of a quenching cooling medium, the oil additive is gradually decomposed. Even if the quenching cooling medium is continuously filtered, fine particles will still accumulate with the extension of time. If not detected, this will result in an accelerated quenching rate, thereby jeopardizing the integrity of the oil quenching process. The viscosity, flash point, water content, sediment, and precipitation value of the quenching medium in the quenching tank should be monitored periodically according to the usage. Testing of quenched cooling medium shall be conducted at least once a quarter.

2. Deformation control factors

In general, in the process of pressure quenching, the basic key factors affecting the deformation of the workpiece are as follows:

1) The material quality of the workpiece and the previous processing process.

2) The distribution of residual stress of the workpiece and the preparatory heat-treatment process.

3) Unbalanced thermal stress and phase transition stress caused by quenching operation.

4) Steel type used and austenitizing temperature distribution.

5) Transfer time between the austenitizing furnace and quenching machine.

6) Type, quality, condition, and temperature of the quenching cooling medium used.

7) The direction and selectivity of the quenching cooling medium flowing through the workpiece.

8) Quenching duration at different flow rates.

9) Design, install and maintain appropriate quenching mold tooling.

10) The position of the pressure point on the workpiece.

11) The amount of pressure applied to maintain the workpiece geometry.

12) Pulses.

The last of these is a property unique to pressure quenching. During quenching, in order to minimize deformation, the inner and outer dies are usually pulsed to maintain the geometry of the part. The pulse characteristic periodically eases the pressure exerted by the inner and outer die, allowing the component to contract normally as it cools while maintaining the required part geometry. Without this feature, friction contact between molds will create stress, and components will not be allowed to contract as they cool. The pulse mode can effectively reduce friction contact and avoid the deformation caused by eccentricity and unevenness. When the pulse technique is correctly applied, the pressure is released while the mold is in contact with the part throughout the quenching cycle, and is then applied again at approximately 2s intervals. Although the internal and external modes are cyclic in this method, the pressure of the expander is generally not pulsed. Most of the pressure quenching machine tools used in industry today adopt this design characteristic, however, it is not the latest development. For decades, pulse technology has been an integral part of semi-automatic pressure quenching machine tools designed for high productivity. An example of one of these semi-automatic machine tools is shown in Figure 9.

▲ FIG. 9 USES the pulse principle of semi-automatic pressure

Schematic diagram of four positions of force-quenching machine tool

Each pressure quenching workpiece is required to correspond to a specific mold tooling design structure and machine tool Settings. In bearing rings and gears, aperture sizes and roundness are often maintained by expanding sectional dies. If the workpiece aperture is too small to support these section molds, a solid plug can be used instead to control the hole diameter and taper. The plug will be pressed out after quenching. It is important that when there are different positioning surfaces in the current die assembly, the dimensions between these positioning surfaces need to be maintained at a small tolerance. Failure to follow this rule can result in conflicting results and undesirable distortions. In addition to expanding the mold, shrinking the mold can also effectively maintain the geometric tolerance of the outside diameter, which is a key factor. A good example gears, whose thin spokes are connected to relatively thick gear teeth, bosses, and bearing diameters. Gears used in aerospace applications often contain several such properties that may cause uneven shrinkage in quenching. This can be effectively addressed by applying a compression load on the outer surface of the component.

The error in pressure quenching can be large.Φ, for example, a 230 mm (9) in Φ gear on the aperture in the condition of not quench the roundness error of 0.025 mm (0.001 in), pressure usually after quenching can reach 0.064 mm (0.0025 in). The same gear, when placed on the plate, is not allowed to have 0.05mm (0.002in.) feeler gauge clearance in any position between the plate and the gear surface. To Φ 460 mm (Φ 18) in gear, the gap should be less than 0.075 mm (0.003 in).If the factors listed above are properly handled (i.e. using high-quality forgings, normalizing correctly before machining, using sharp tools, following good machining operations, etc.), this strict error requirement can usually be achieved by pressure quenching.A extends is the use of roller pressure quenching hardening control (40) in length and 1020 mm Φ 200 mm (Φ 8 in) long cylindrical parts, shaft, crankshaft deformation. This technique USES rollers to carefully apply a controlled load on a hot piece as it rotates around its axis and the quench chamber is filled with a flowing quench cooling medium. Figure 10 shows a typical image of this highly specialized quenching machine tool.

▲ Figure 10 Worm diagram of press-in roll die quenching machine tool