GB2397071A

GB2397071A - Quenching method and quenching chamber

Info

Publication number: GB2397071A
Application number: GB0400556A
Authority: GB
Inventors: Paul Francis Stratton
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 2003-01-13
Filing date: 2004-01-12
Publication date: 2004-07-14
Also published as: GB0400556D0; GB0300687D0; US20040182484A1

Abstract

A method of quenching small metal objects by tumbling them in a upward flow of quenching gas. The gas may be one or more of nitrogen, carbon dioxide, argon, helium or hydrogen, e.g. nitrogen containing up to 2 volume % of hydrogen. Quenching is performed in a chamber 2 which comprises a horizontal floor 4 with a plurality of openings 18 therein for distributing gas from a plenum chamber 14 into the chamber 2. The lower portion of the chamber 2 has a uniform cross-sectional area while the upper portion 10 has an increasing cross-sectional area, thereby allowing the quenching gas to decelerate. Typical metal objects quenched are washers, ball bearings, screws and small shafts.

Description

QUENCHING METHOD AND FURNACE

This invention relates to a method of quenching hot metal objects, typically metallic components of machines.

It is very well known that quenching a metal object (i.e. rapidly chilling the, object from a heat treatment temperature typically in the austenitic range to a much lower temperature less than 100 C and normally less than 50 C) can significantly improve its mechanical properties and characteristics.

Quenching is used to harden the object, or to improve its mechanical properties, or both, by controlling internal crystallization and/or precipitation, for example. Traditionally, quenching has been carried out using liquids such as water, oil or brine, either in the form of an immersion bath or a spray. In more recent years, gas quenching methods have been proposed. Gas quenching, as, for example, described in EP-A-1 108 793, has, in theory, the advantages of being clean, non-toxic and leaving no residues to be removed after quenching. Practical difficulties arise however. One of these practical difficulties is the need to provide jigs to support the objects being quenched.

GB-A-2 208 392 discloses a method for the quenching and transformation of steel shot. In a first stage of cooling the steel shot is fed at elevated temperature into a fluidised bed of the shot and air is passed through the bed to quench the shot to a temperature of 250 C to 450 C. The quenched shot is fed to a static bed in a second container where it is held at a temperature in the range of 250 to 450 C.

According to the present invention there is provided a method for the gaseous quenching of small hot metal objects, comprising tumbling the small hot metal objects in an upward flow of quenching gas distributed uniformly from a multiplicity of openings in a horizontal floor, the quenching gas flowing from the floor at a uniform tumbling velocity and being subjected to deceleration so - 2 to keep down the number of the objects carried away by the quenching gas, the tumbling being performed for a sufficient period of time for the objects to be quenched to a desired temperature.

The invention also provides a heat treatment furnace having associated therewith a quenching chamber for performing the method according to the invention, the quenching chamber having a horizontal floor, there being a multiplicity of openings in the horizontal floor for the distribution of quenching gas to the quenching chamber, wherein the quenching chamber has a region of constant cross-sectional area bounded at its lower end by the floor and at its upper end by a region of increasing cross-sectional area in the direction of flow of the quenching gas.

By tumbling the objects in the quenching gas, there is no need to provide any jigs to support them. Further, tumbling facilitates the achievement of uniform cooling necessary to minimise the creation of undesirable stress patterns in the objects.

The metal objects are typically engineering components, for example, washers, ball bearings, screws and small shafts and the like. They are preferably all of a size such that they can be readily levitated by a gas flowing at a subsonic velocity, preferably in the range of 15 to 150 m/s. Higher velocities may be used to tumble larger components.

The quenching gas may, for example, be nitrogen, which is readily and cheaply available commercially from the separation of air. The nitrogen preferably has added to it a small proportion (for example up to 2% by volume) of a reducing gas (for example hydrogen) so as to inhibit oxidation of the objects being quenched without creating a flammable gas mixture.

Instead of or in addition to nitrogen, argon, carbon dioxide, hydrogen or helium may be used as the quenching gas. Hydrogen and helium have superior heat transfer properties to the other gases. A mixture of two or more - 3 of nitrogen, carbon dioxide, argon, helium and hydrogen is another alternative quenching gas.

The arrangement of the quenching chamber enables the gas to undergo deceleration so as to keep down the number of the objects that are carried away by the quenching gas.

The floor is preferably provided by a flat perforate member having an open area of less than 20%. The perforations are preferably uniformly spaced and are preferably all of the same size and shape. The perforations are preferably all of circular cross-section, having a diameter which may be selected according to the size of the objects to be quenched. The perforated member may comprise a solid plate with evenly spaced drilled apertures. Alternatively, at lower cost, it may comprise a sheet of expanded metal or woven metal (e.g steel) mesh having a ratio of solid area to open area of not less than 4:1. The smaller the size of the apertures the more uniform the gas field, but there is a tendency for very small apertures to become clogged, so it is preferred that each aperture has a diameter of at least 1 mm2. The perforated member is desirably sufficiently strong to support the objects at maximum loading in the event of the supply of gas being stopped or failing.

The floor typically defines the top of a plenum chamber to which the quenching gas Is supplied at an elevated pressure typically in the range of 2 bar to 20 bar.

The method according to the invention may be performed in association with the operation of a batch furnace, a semi-continuous furnace, or a continuous furnace Quenching methods and chambers according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a plan view of a quenching chamber according to the invention: - 4 Figure 2 is a section through the line 11-ll in Figure 1; Figure 3 is a section through the line lil-lil in Figure 1; Figure 4 is a graph Illustrating the variation of cooling rate of steel spheres at 600 C with diameter when they are quenched in nitrogen by the method according to the invention; and Figure 5 is a graph illustrating the effect of aspect ratio on the cooling rate of steel cylinders as 600 C for constant weights during tumbling in nitrogen in accordance with the invention.

Figures 1 to 3 of the drawings are not to scale.

With reference to Figures 1 to 3, a quenching apparatus comprises a quenching chamber 2 having a horizontal perforate floor 4 at its bottom and being open at its top 6. As shown in Figure 1, the chamber 2 is square in plan view. Alternatively, a cylindrical chamber 2 may be used. The chamber 2 has a lower region 8 contiguous to an upper region 10. The side walls 12 of the chamber 2 are vertical where they bound the lower region 8, but diverge at an angle of slope of 7 or less thereabove.

The floor 4 defines the top of a plenum chamber 14 with which a gas supply pipe 16 communicates. The pipe 16 is preferably coaxial with the vertical axis of the chamber 2. The chamber 2 may, if desired, be packed with a porous material such as steel wool to improve gas distribution.

The floor 4 is provided by a perforated plate or wire mesh. Its total open area is less than the open area of pipe 16 at its inlet to the plenum chamber 14.

The floor 4 has a multitude of orifices 18 formed therethrough. The orifices 18 - 5 are uniformly disposed and are all of identical size and shape, preferably being of circular cross-section.

The top 6 of the quenching chamber 2 may be fully open or partially closed. If partially closed, the partial closure should not disrupt gas flow from the plenum chamber 14 up through the quenching chamber 2. In general, the open area at the top 6 of the quenching chamber 2 is greater than the area of the floor 4.

In operation, components or objects to be quenched are typically discharged from, say, a batch furnace into the top of the device. Charging may be effected either mechanically or by hand. Quenching gas, for example nitrogen, typically containing a small amount of hydrogen to render it reducing but non-flammable, is introduced at elevated pressure typically in the range of 2 to 20 bar into the plenum chamber 14 from the gas pipe 16. The supply pressure is chosen so as to create a gas velocity through the floor 4 sufficient to levitate the components or objects being quenched. The number of components or objects that are quenched at any one time is selected to minimise collisions between the objects. The upward flow of quenching gas through the region 8 of the quenching chamber 2 is at an essentially uniform velocity. The velocity gently diminishes as the gas flows upwardly through the upper quenching region 10. The maximum quenching velocity at the bottom of the quenching chamber 2 is typically chosen to be greater than that needed to keep the objects or components to be quenched levitated. The minimum velocity at the top of the upper quenching region 10 is such as to ensure that essentially none of the objects or components are carried out of the top 6 of the quenching chamber 2 with the quenching gas. In practice, small local variations in the gas velocity cause the objects or components to tumble in the quenching gas. The tumbling of the objects or components is continued for a sufficient period of time for their temperature to fall from an initial 600 C or higher, typically 850 C7 to a temperature less than 100 C, typically less than 50 C. - 6

When they have been sufficiently quenched the objects or components are removed from the quenching chamber 2. Removal of the objects or components may be effected in any one of several different ways. For example, if desired, the lower region 8 of the quenching chamber 2 may be provided with a door (not shown). The gas velocity is reduced to zero in a controlled manner so as to avoid any severe impact of the objects or components on the chamber floor 4. The door is opened and the parts are removed. If the parts are so small that they would fall through the orifices 18 In the floor 4 a solid plate may be interposed between the floor 4 and the lower region 8 of the quenching chamber 2. Another alternative is to employ a detachable plenum chamber 14. A further alternative is to eject the objects or components through the top 6 of the quenching chamber 2. This can be done by increasing the gas supply pressure and hence the gas velocity at the top such that it exceeds the maximum velocity needed to levitate the objects or components. The ejected objects or components may be captured in a suitable device such as a baghouse or vortex separator. Ejection from the top has the advantage of eliminating the risk of any falling into the plenum chamber 14 but increases the consumption of quenching gas. If a door is provided for the discharge of the objects or components, the top 6 of the quenching chamber 2 may be closed so that when the door is opened the gas ejects the objects or components through the open door.

The above described methods of discharging the components or objects are all suitable for use with batch or semi continuous furnaces. If the furnace is a continuous one, then an arrangement of two or more quenching apparatuses of the kind shown in Figures 1 to 3 of the drawings may be employed. The continuous furnace may be arranged periodically to discharge into a first quenching chamber. When that chamber is fully loaded the components are diverted to a second quenching chamber. The quenching of components in the first chamber is then completed and the components are withdrawn. On completion of the withdrawal, components are now sent from the furnace to - 7 the first chamber while quenching continues and is completed in the second chamber. The cycle may thus be repeated continuously. The capacity of the quenching chambers is determined in accordance with the throughput of components and the necessary residence time of the components to complete quenching.

There is no need to employ any intermediate particulate fluidised medium to facilitate heat transfer between the metal objects and the quenching gas, and it Is preferred that no such medium be employed.

The quenching method according to the invention is further illustrated by the

following examples:

Example 1

The first example is the quenching of small cylinders manufactured from steel (density 7840 kg/m3). Each cylinder is 3 mm diameter and 30 mm long. Its weight is therefore 0.001568 kg. The quenching gas is nitrogen.

The general formula for the gas velocity required to levitate the component is: V = [(2xMx9.81) / (AxD)] 05 [1] Where V is the gas velocity in m/s, M is the mass of the component in kg, A is the projected area of the component in m2, and D is the density of the gas in kg/m3 The gas velocity therefore depends on the area projected by the component.

For the cylinder the minimum projected area is a circle of area m2 where r is the radius of the component, and in this example equals 0.0000069 m2. The - 8 density of nitrogen is 1.1605 kg/m3 at 21.1 C and 1 atmosphere pressure.

The maximum required gas velocity is therefore 62 m/s.

The component's maximum projected area is a rectangle of area 2rh, where h is the height of the cylinder, and in this example equals 0.00009 m2. The minimum required velocity is therefore 17.2 m/s.

To achieve this velocity difference the area of the chamber 2 increases 3. 6 times between the maximum and minimum velocity points If components are not to hit the bottom of the chamber or be ejected at the top. This is the equivalent of a 1.9 times increase in the diameter of a circular cross section chamber. At the maximum design slope, this implies that the distance between the upper and lower velocity points is 3.66 times the diameter of the bottom of the chamber.

Calculation suggests that the heat transfer coefficient for this component will be of the order of 120 to 22 W/m2/ C.

Example 2

Example 2 is of the quenching of cylinders manufactured from the same material as Example 1 and with the same aspect ratio but a 10 times larger diameter.

Calculation using equation [1] gives minimum and maximum velocities of 55. 8 and 199.2 m/s for this component. All the aspects of the quench chamber design are thus the same as for Example 1 except for the maximum gas velocity. Calculation suggests that in this case the heat transfer coefficient will be in the range 300 to 330 w/m2/ C. - 9 -

Example 3

Example 3 is the quenching of rectangular plates again manufactured from the same material as Example 1. Each plate is 20 mm wide, 100 mm long and 2 mm thick.

Calculation using equation [1] gives minimum and maximum velocities of 16. 3 and 115.2 m/s for this component However, in this case the ratio between the maximum and minimum velocities is 7.07 implying a ratio of minimum to maximum diameter of the quench chamber of 2.66 times which with a 7 slope gives a height of 6.76 time the diameter of the base.

Example 4

Example 4 is the quenching of gear shafts again manufactured from the same steel as Example 1. Each shaft is 400 mm long and has a diameter of 40 mm. Part way along the length of the shaft (the exact position does not affect the calculation) there is a gear 100 mm diameter and 50 mm wide.

For this example the maximum and minimum gas velocities are 118.5 and 76.2 m/s. This would put it into the range of quenching rate 200 to 400 w/m2/ C. As the minimum and maximum velocities are very similar the quench chamber need only be 1.25 times its bottom diameter at the top 1. 02 times the diameter above the base.

Example 5

Example 5 is the quenching of spheres of the same density material 25 mm diameter (a typical large ball bearing). Applying Equation [1] to this object we find that there is only one velocity for this object as it has an aspect ratio of one. The levitation velocity is 47 m/s. -

Example 6

Example 6 is the quenching of 2 mm diameter spheres. This would need a levitation velocity of only 13.3 m/s.

These examples shown that the larger the aspect ratio, the larger the difference in minimum and maximum velocities and the larger the difference between the area of the floor 4 and the top 6 of the quenching chamber.

Design of the chamber needs therefore take into account the largest aspect ratio part to be processed almost regardless of its size. The size and the number of components to be processed simultaneously needs to be taken into account when determining the bottom diameter size. The quench chamber 2 is preferably slightly deeper than required by these calculations to allow for the effects of momentum during tumbling adding to both upward and downward motion.

It is somewhat counterintuitive to find that using the method according to the invention larger, heavier objects with larger aspect ratios have a higher heat transfer coefficient than lighter spherical objects or even lighter high aspect ratio objects. However, heat transfer coefficient is not the only fact that needs to be taken into consideration. The absolute amount of heat to be removed also affects the cooling rate. Thus for the same heat transfer coefficient smaller parts cool faster.

Cooling rates at 600 C are set out below. - 11

Example Estimated average Thermal Surface area Cooling heat transfer mass at (m2) Rate ( C/s) __ coefficient (W/m2/ C) 600 C (J/ C) 1 170 1.16 0.0002865 25

__ _ _

2 320 1230 0.0297 4.6 3 170 25.3 0.00448 18 4 300 4836.5 0.0597 2.2

_ _

200 47 0.00196 5 6 80 0.024 0.0000125 25 It is possible to calculate the cooling rate that can be achieved for all shapes.

However, the results for some typical shapes are shown below in Figures 4 and 5. The heat transfer coefficient used is an estimated average based on a combination of different gas flow conditions during tumbling. The calculated cooling rates are for convective heat transfer only and do not include the effects of radiation heat loss which are relatively small at 600 C. Although most steels are quenched from about 850 C it is the cooling rate at around 500 C where the cooling rate is most critical with regard to the crystal structures formed.

It has been shown that a low alloy steel (SAE 8620) will fully harden when gas quenched with a cooling rate of 20 C/s or higher. Thus, for example, it can be seen from Figure 4 that spheres of this material will fully harden if their diameter is less than approximately 3 mm. Other materials have lower critical cooling rates. For a material with a critical cooling rate of 15 C/s for example from Figure 2, all cylinders weighing 1 g or less will be fully hardened.

It can be understood from the foregoing that for lower hardenability materials the method according to the invention is restricted to lighter components. The method is particularly applicable to these light components as it produces - 12 even quenching in an environmentally friendly medium without the need for complex jigging. If higher hardenability materials are used then much heavier components can be processed. The method according to the invention is particularly applicable to larger complex parts of high hardenability such as spiral gears of BS970:708A42 steel or high load capacity carburised gears of BS970:835M15 steel where uniform quenching is required, especially as there is no need for a jig to maintain the part in the same position during quenching. - 13

Claims

1. A method for the gaseous quenching of small hot metal objects, comprising tumbling the small hot metal objects in an upward flow of quenching gas distributed uniformly from a multiplicity of openings in a horizontal floor, the quenching gas flowing from the floor at a uniform tumbling velocity and being subjected to deceleration so as to keep down the number of the objects carried away by the quenching gas, the tumbling being performed for a sufficient period of time for the objects to be quenched to a desired temperature.

2. A method as claimed in claim 1, in which the quenching gas is nitrogen.

3. A method as claimed in claim 2, in which the quenching gas contains up to 2% by volume of hydrogen.

4. A method as claimed in any one of the preceding claims, in which the uniform tumbling velocity is in the range of 15 to 150 m/s.

5. A method as claimed in any one of the preceding claims, in which the floor is provided by a flat perforate member having an open area of less than 20%.

6. A method as claimed in claim 5, in which the perforations are uniformly spaced and are all of the same size and shape.

7. A method as claimed in claim 6, in which the perforations are all of circular cross-section. - 14

8. A method as claimed in any one of the preceding claims, in which the floor defines the top of a plenum chamber to which the quenching gas is supplied at an elevated pressure.

9. A method as claimed in claim 8, in which the elevated pressure is in the range of 2 bar to 20 bar.

10. A method as claimed in any one of the preceding claims, wherein the metal objects are engineering components.

11. A heat treatment furnace having associated therewith a quenching chamber for performing a method as claimed in any one of the preceding claims, the quenching chamber having a horizontal floor, there being a multiplicity of openings in the horizontal floor for the distribution of quenching gas to the quenching chamber, wherein the quenching chamber has a region of constant cross-sectional area bounded at its lower end by the floor and at its upper end by a region of increasing cross-sectional area in the direction of flow of the quenching gas.

12. A furnace according to claim 11, wherein the furnace is of a batch, semi-continuous or continuous kind.