GB2208392A

GB2208392A - Method and apparatus for the quenching and transformation of steel shot

Info

Publication number: GB2208392A
Application number: GB8816717A
Authority: GB
Inventors: John Moore; Brian Harding
Original assignee: BARTON ABRASIVES Ltd
Current assignee: BARTON ABRASIVES Ltd
Priority date: 1987-07-25
Filing date: 1988-07-14
Publication date: 1989-03-30
Anticipated expiration: 2008-07-14
Also published as: GB8717702D0; GB8816717D0; GB2208392B

Abstract

Austenitic steel shot at above 720 DEG C is fed into a fluidised bed of steel shot in a first container (20). Fluidising coolant air is passed through the bed to quench the steel shot to a temperature in the range of 250 DEG C to 450 DEG C. The quenched shot is fed to a static bed in a second container (12) where it is held at a temperature in the range of 250 DEG C to 450 DEG C for a length of time sufficient for it to develop a substantially bainitic structure. <IMAGE>

Description

METHOD AND APPARATUS FOR THE QUENCHING AND TRANSFORMATION OF STEEL SHOT Field of the Invention This invention relates to a method and apparatus for the continuous quenching and transformation of steel shot or steel balls (hereinafter referred to simply as steel shot).

Whilst it is to be appreciated that the invention is applicable to a multiplicity of different quenching and isothermal transformation processes, it is mainly concerned with treating steel shot of mean diameter 0.3 to 2.4 mm so that it undergoes a change of metallurgical structure from a normal austenitic structure to a substantially bainitic structure.

Background Art.

Hitherto, quenching and isothermal transformation of steel shot has been achieved principally by processes utilising a liquid bath to quench the shot.

In one process, the steel shot is introduced in its normal austenitic structure at a temperature of approximately 850"C into a water bath, the water temperature being controlled at approximately room temperature. Following quenching of the shot in said water bath, it is removed from the water bath and raised in temperature by any convenient means to the required transformation temperature.

The above-described water bath process possesses disadvantages. The rapidity of quenching into water is such that as much as 20% of the quenched shot is found to have cracked under the thermal stresses induced by quenching. In addition, the rapid quenching of he shot into water results in the formation of a martensitic steel structure.

Upon subsequent tempering of the quenched shot, at approximately 4800C, a tempered martensitic steel structure is produced, resulting in shot which is found to be less durable in service than shot composed of the above-mentioned substantially bainitic structure.

To overcome some of the above-mentioned disadvantages it has previously been proposed to quench the steel shot in a bath of liquid salts, the temperature of the said salt bath being controlled to 250 to 4800C.

Subsequent isothermal transformation of the shot is effected by holding the shot in the salt bath after quenching. This results in the steel shot achieving a substantially bainitic structure. The above described salt bath avoids transformation to the martensite phase, thereby reducing the proportion of cracked shot to approximately 5%.

However, the above-described salt bath process possesses disadvantages. Separation of the shot from the salt medium is difficult, and requires a separation operation which gives rise to a significant waste of expensive salts and to an associated effluent problem.

In addition, metallurgical limitations require that the time taken for isothermal transformation of the steel shot is at least twenty times longer than the time taken to quench the shot from its normal austenitic structure. Therefore the quenching capacity of a salt bath is severely restricted by a substantial volume of the bath being occupied in providing an environment for the isothermal transformation of the shot.

It has also been proposed to air cool steel shot rapidly in a cool air stream from above the austenite transformation temperature (Ar3) of the steel (about 7200C) to below the martensite transformation temperature (Mf) of the steel (about 1800C) in order to produce a martensitic structure. The shot is then tempered by re-heating to about 4800C. The resultant shot has a tempered martensitic structure having a Vickers hardness of about 400-520. Such a process is relatively inefficient thermally and leads to a product in which there is still room for improvement in terms of toughness.

An object of the present invention is to provide a method and apparatus in which the above disadvantages of the above-mentioned previously proposed processes can be obviated or mitigated.

Summary of the Invention.

According to one aspect of the present invention there is provided a method for the quenching and transformation of steel shot comprising the steps of (a) quenching the steel shot in a coolant gas flow under controlled conditions such as to cool the shot rapidly from above the austenite transformation temperature (Ar3) of the steel to a temperature which is in the range of 2500C to 4500C and which is suitable for nucleation of a substantially bainitic structure, and then (b) holding the shot at a temperature in the range of 2500C to 4500C for a length of time to produce a substantially bainitic structure, said quenching step (a) being effected at least partially in a fluidised bed of steel shot.

According to another aspect of the present invention, there is provided apparatus in which austenitic steel shot is to be quenched and transformed to a substantially bainitic structure, said apparatus comprising (a) a first chamber having means therein to enable maintenance of a fluidised bed of steel shot being quenched; (b) a steel shot inlet to the first chamber, said inlet being arranged to enable austenitic steel shot to be fed to the fluidised bed (c) coolant gas introduction means arranged to introduce coolant gas to the steel shot in use so as to effect quenching thereof from a temperature which is above the austenite transformation temperature (Ar3) of the steel at least partially within the fluidised bed; (d) quenched shot removal means arranged to remove quenched steel shot from the bed at a temperature in the range of 2500C to 450 C, and (e) holding means disposed to receive quenched shot from the removal means and arranged to hold the quenched shot at a temperature in the range of 2500C to 4500C for a length of time sufficient to produce a substantially bainitic structure.

The use of a fluidised bed of the steel shot involves no separation operations of steel shot in the quenching medium whilst at the same time permits a quality of quenching which is comparable to that achieved in a salt bath. The quenching and holding steps are inherently economical thermally because they do not involve cooling and re-heating. Additionally, the bainitic structure which is developed results in shot having greater toughness than tempered martensitic shot.

The residence time required of steel shot in the fluidised bed for adequate quenching is found to be of the order of one minute or less. Such quenching times, while being greater than corresponding quenching times in a salt bath, are nevertheless sufficient to ensure that metallurgical transformation of the cooling shot does not commence until a shot temperature suitable for the nucleation of a substantially bainitic structure is achieved. The residence time also depends upon the extent of quenching which takes place in the coolant gas flow before the steel shot reaches the bed.

The desired residence time is preferably achieved by removing the steel shot from the fluidised bed at an outlet location which is spaced laterally of the bed from a feed location at which the steel shot is fed to the fluidised bed. The residence time is that required for the shot to move laterally of the bed from the feed location to the outlet location.

The mean temperature of said fluidised bed of shot is maintained at a substantially constant level in the range 2500C to 4500C preferably 2500C to 4000C,. and more preferably 2500C to 3500C, by varying the temperature and flow of the above-mentioned fluidising gases, it being understood that the introduction of hot steel shot into said fluidised bed serves as a source of heat input thereto.

The steel shot in the fluidised bed is of the same or similar specification to that being quenched and the required depth of the bed can be maintained by removing steel shot from the bed at a rate equivalent to that at which it is being added thereto during quenching. The steel shot removed from the bed can be subjected to the holding step in a zone having an insulated housing which also encloses the fluidised bed of steel shot.

However, it is preferred for the fluidised bed of steel shot to extend at least partially around the holding zone so that the fluidised bed itself assists in limiting lateral heat losses from the holding zone.

Most preferably, the fluidised bed of steel shot is arranged to surround the holding zone completely in the horizontal direction.

The holding step is preferably effected in a non-fluidised or static bed, the mean temperature thereof being kept at a substantially constant level in the desired range by the introduction of a flow of pre-heated gases into said static bed.

Therefore, in continuous operation in a convenient embodiment, steel shot in its normal austenitic structure is introduced into a first container, thereby being mixed immediately with the fluidised bed of shot therein and quenched to a temperature close to the temperature of said fluidising bed within approximately one minute of mixing therewith. The continuous introduction of shot into said first container continuously displaces shot from the first container.

A system of baffles may be provided within the said first container for the purpose of ensuring that any individual granule of shot displaced from the first container has migrated to its point of displacement by a circuituous route and has therefore remained resident within the first container for at least the minimum quenching time of approximately one minute. It will be appreciated that individual granules of shot remaining in the fluidised bed for times longer than their quenching time will commence isothermal transformation to bainite within the fluidised bed. Shot displaced continuously from said first container passes immediately into a second container wherein shot is held for a residence time between 20 minutes and 2 hours, said residence time being the time required for satisfactory isothermal transformation of the shot to a substantially bainitic structure.Said shot is discharged from the base of said second container which therefore contains at all times in continuous operation a static bed composed of individual granules of shot continuously progressing from the top of said second container to the discharge location at the base thereof.

It is particularly preferred for the first container to surround the second container in the horizontal direction and for transfer from the first container to the second container to occur via a weir.

It is to be understood that where reference is made above to specific values of temperature and time, such values are given for guidance only and are not in any sense to be taken as limiting.

The present invention possesses the under-mentioned advantages.

1. The quality of quenched steel shot achieved with the provided fluidised bed is comparable with that achieved with a salt bath, insofar as a proportion of the order of 5% of the quenched steel shot is found to have cracked under quenching, while the metallurgical condition of the quenched steel shot is suitable for the nucleation of a substantially bainitic structure leading to improved toughness compared with a tempered martensitic structure.

2. No separation of the steel shot from a quenching medium is required, resulting in a greater continuity of operation and a reduction in the floor space usage required for a quenching and isothermal transformation process.

3. The apparatus is inexpensive to install and maintain and requires no regular replenishment of constituent material.

4. The apparatus allows for a wide range of throughputs of steel shot. The time allowed for transformation of steel shot can be controlled over a wide range by varying the rate of discharge of shot from the holding bed, and by providing a holding bed of sufficient volume to allow a wide range of residence times of shot therein.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a part-sectional side elevation of an apparatus for the quenching and transformation of steel shot according to the present invention, and Fig. 2 is a top plan view of the apparatus of Fig. 1 with a top cover thereof omitted.

Referring to the drawings, the apparatus comprises a housing 10 of circular cross-section with a reduced area base section 12 including a frusto-conical portion 14 having a cone angle of 600 leading to a vertical outlet pipe 16. An annular jacket 18 surrounds the wall of the housing 10 above the base section 12 so as to define an annular first chamber 20 defining a fluidised bed zone. The first chamber 20 has a floor 22 below which is an annular plenum chamber 24 arranged to be fed with fluidising gas via a tangential inlet pipe 26. The floor 22 has an annular fluidising gas outlet 28 which serves (a) to direct fluidising gas admitted into the first chamber 20 downwardly towards the floor 22 and (b) to prevent steel shot from falling through the gas outlet 28 into the plenum chamber 24.

The first chamber 20 has an upper gas outlet 31 extending tangentially therefrom in the opposite direction to the inlet pipe 26. The apparatus further includes a gravity feed inlet pipe 32 which is arranged to receive hot steel shot directly from an austenitising furnace (not shown). The inlet pipe 32 opens into the first chamber 20 at a location which is above the surface of the fluidised bed of shot which is maintained therein in use. At the diametrically opposite side of the apparatus to the inlet pipe 32, there is provided a steel shot outlet 34 fitted with an inclined (250 to horizontal) chute 36.

The position of the outlet 34 effectively determines the height of the fluidised bed.

Adjacent the bottom end of the frusto-conical portion 14 there is provided an auxiliary hot air feed pipe 38.

The whole apparatus is closed by a top cover 40' and is mounted on a frame 42 via a pair of roller supports 44 and a load cell 45 (Fig. 2) for monitoring the weight of steel shot in the apparatus throughout the treatment.

The pipe 16 leads to a conveyor 46 for feeding treated steel shot away at a predetermined and controllable rate which substantially matches the rate of feed of shot to the apparatus through the inlet 32.

In use, steel shot at above the Ar3 temperature passes under gravity through the pipe 32 and is allowed to fall into the fluidised bed established in the chamber 18. At the same time the fluidising and quenching air, after having passed through the bed, from the outlet 28, travels upwardly through the descending shot and partially quenches the latter as it falls towards the bed. The air then passes out of the first chamber 20 through the outlet 31. During this time, the partially quenched steel shot reaches the fluidised bed and is further quenched by the air and also as a result of contact with colder shot already in the bed.

There is a generally horizontal flow of shot circumferentially of the bed from the location at which the shot enters the bed from the inlet 32 to the outlet 34. It is this horizontal flow which determines the average residence time of the shot in the fluidised bed. Upon reaching the outlet 34, the shot overflows therethrough onto the chute 36 and falls into the base section 12 of the housing 10. In the base section 12, a static (i.e non-fluidised) bed of steel shot is maintained. Air is introduced via pipe 38,for accurate temperature control but at a rate which is insufficient to cause fluidisation of the shot. The shot progresses slowly downwardly towards the outlet pipe 16 and, during this time, is transformed to the desired substantially bainitic structure. The depth of the static bed and the feed rates of the air and shot are such as to maintain the required temperature therein.

The provision of a hot fluidising zone around a static bed zone helps to prevent lateral heat losses from the latter so that it is easier to maintain the desired temperature in the static bed zone. Additionally, the vertical height of the apparatus can be minimised.

The above-described arrangement of annular fluidising zone surrounding a static bed zone in the apparatus is considered to have uses in other fields where materials are to be treated in or using fluidised beds.

Claims

1. A method for the quenching and transformation of steel shot comprising the steps of (a) quenching the steel shot in a coolant gas flow under controlled conditions such as to cool the shot rapidly from above the austenite transformation temperature (Ar3) of the steel to a temperature which is in the range of 2500C to 4500C and which is suitable for nucleation of a substantially bainitic structure, and then (b) holding the shot at a temperature in the range of 2500C to 4500C for a length of time to produce a substantially bainitic structure, said quenching step (a) being effected at least partially in a fluidised bed of steel shot.

2. A method as claimed in claim 1, wherein the coolant gas flow is supplied to the underside of the bed so as to act as the fluidising gas.

3. A method as claimed in claim 1 or 2, wherein the holding step (b) is effected by feeding the shot from the quenching step to a static bed of the shot, and maintaining the required temperature in the bed by passing gas therethrough.

4. A method as claimed in claim 3, wherein the shot from the quenching step (a) is fed continuously to the top of the static bed whilst the quenched and transformed shot is removed continuously from the bottom of the static bed.

5. A method as claimed in any preceding claim, wherein the shot is quenched in step (a) to a temperature in the range of 250 C to 4000C.

6. A method as claimed in any preceding claim, wherein the shot is held in step (b) at a temperature in the range of 2500C to 400eC.

7. A method as claimed in claim 1, substantially as hereinbefore described.

8. Steel shot having a substantially bainitic structure when quenched and transformed by a method as claimed in any preceding claim.

9. Apparatus in which austenitic steel shot is to be quenched and transformed to a substantially bainitic structure, said apparatus comprising (a) a first chamber having means therein to enable maintenance of a fluidised bed of steel shot being quenched; (b) a steel shot inlet to the first chamber, said inlet being arranged to enable austenitic steel shot to be fed to the fluidised bed; (c) coolant gas introduction means arranged to introduce coolant gas to the steel shot in use so as to effect quenching from a temperature which is above the austenite transformation temperature (Ar3) of the steel at least partially within the fluidised bed (d) quenched shot removal means arranged to remove quenched steel shot from the bed at a temperature in the range of 2500C to 4500C; and (e) holding means disposed to receive quenched shot from the removal means and arranged to hold the quenched shot at a temperature in the range of 2500C to 4500C for a length of time sufficient to produce a substantially bainitic structure.

10. Apparatus as claimed in claim 9, wherein the holding means comprises a second chamber in which a static bed of the steel shot is maintained in use, and pre-heated gas inlet means in the second chamber, said inlet means being arranged to feed pre-heated gas to the static bed.

11. Apparatus as claimed in claim 10, wherein the second chamber has an outlet for steel shot below the static bed and an inlet for steel shot above he static bed, said inlet being connected with the quenched shot removal means.

12. Apparatus as claimed in claim 9, 10 or 11, wherein the steel shot inlet of the first chamber is disposed above the fluidised bed in use and arranged to allow the steel shot passing therethrough to fall under the action of gravity into the bed.

13. Apparatus as claimed in claim 9, 10, 11 or 12, wherein the quenched shot removal means includes a baffle system for inhibiting the passage of steel shot through the fluidized bed in use to the holding means, thereby to ensure a sufficient average residence time in the fluidized bed to effect quenching.

14. Apparatus as claimed in claim 9, 10, 11, 12 or 13, wherein the coolant gas introduction means is disposed in the first chamber so as to effect, in use, fluidisation of the bed of steel shot therein.

15. Apparatus as claimed in claim 9, substantially as hereinbefore described.