Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2000-12-13
2003-04-29
Ip, Sikyin (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
66, 66
Reexamination Certificate
active
06554926
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods of quenching heated metallic objects.
BACKGROUND OF THE INVENTION
It is very well known that quenching a metallic object (i.e., rapidly chilling the object from a heat treatment temperature in the austenitic range to a much lower, usually room, temperature) can significantly improve its mechanical properties and characteristics. Quenching is used to harden the object and/or to improve its mechanical properties, by controlling internal crystallisation 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 spraying system. In more recent years, gas quenching methods have been developed. Gas quenching has the advantages of being clean, non-toxic and leaving no residues to be removed after quenching, however difficulties have been encountered in achieving similarly high quenching rates as are provided by more conventional liquid quenching processes.
Quenching is a high speed process, requiring the heat within the object to be drawn away at a high heat flow density through the cooled surface of the object. It is usually desirable for the quenching of the object to be uniform, so that the quenched object has uniform surface or internal characteristics, however, uniformity of quenching is difficult to achieve in most quenching techniques, due to various factors, principally Leidenfrost's phenomenon. The quenching effect of any quench system is usually characterised in terms of the Grossman quench severity factor, H; for liquid quenchants such as water or oil, H usually falls in the range 0.2 to 4. Such high values of H are not easily attainable using gas quenching; when quenching using gas, the cooling intensity can be increased using several different means; increasing the quenching pressure; increasing the velocity at which the gas is sprayed on to the object; choice of gas (nitrogen is less preferable than helium, which is less preferable than hydrogen, because of their respective heat transfer coefficients, although helium and hydrogen are expensive compared to nitrogen); optimising the gas flow conditions and enhancing the turbulence, and enhancing the cooling of the gas.
Gas quenching employing multiple cooling gas streams comprising mainly nitrogen, argon and/or helium at pressures up to 60 bar has been practised in vacuum furnaces, and its characteristics for quenching bulk components are well known. More recently the gas quenching of single or small groups of components which had been heated in either vacuum or conventional atmosphere furnaces has been proposed. To eliminate the need to cool the furnace structure, these techniques involve the transfer of the object to be quenched to a specially designed cold chamber, as is known in the art.
In order to meet the criteria for uniform quenching of a single object or component it is necessary for the quenchant to reach the surface of the object uniformly. In practical gas quenching processes this implies that gas which has been heated through contact with the object must also leave the surface uniformly (so that further fresh, cold gas can reach the surface to continue the quenching process); therefore discrete amounts of arriving and departing gas must exist. Theoretically these amounts would ideally be infinitely small, but practical considerations necessitate that they be as large as possible so far as is consistent with substantially uniform heat transfer.
A second factor affecting quenching uniformity is the interaction of the individual gas streams. It has been shown that, for constant mass flow and a stream width (d) to distance between the gas nozzle orifice and the surface of the object (a) ratio of four, the heat transfer coefficient reaches a maximum when the distance between adjacent gas streams (b) is three times the stream width (d). The turbulence formed at the edges of the gas streams as they impinge on the object surface is known to have a significant effect on the transfer of heat, however the form and size of these turbulent areas is difficult to predict due to the complex interaction between the gas streams.
A further factor affecting the uniformity of gas quenching is that although the velocity of the gas striking the object surface should be as high as possible, and as near perpendicular to the surface as possible, the velocity and angle of incidence relative to the surface of the gas streams must also be as uniform as possible, as the heat transfer coefficient is dependent on both of these. It has been suggested that, to maximise the heat transfer coefficient and to minimise the interaction factor between adjacent gas streams, the distance (a) between the gas nozzle orifice and the surface should be as large as possible so far as is consistent with the loss of velocity of the gas stream over distance. For example, U.S. Pat. No. 5,452,882 proposes that, in order to achieve a quench severity factor, H, of between 0.2 and 4, a plurality of gas streams of diameter d should be directed towards the object to be quenched from nozzles (of diameter d) spaced at a distance between 2 d and 8 d from the surface of the object and with a distance between adjacent nozzles, b, of between 4 d and 8 d. There is a continuing need to provide an efficient and economic gas quenching process capable of high quench severity and of substantial uniformity.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a method of quenching a heated metallic object comprising discharging a plurality of discrete gas streams from a plurality of nozzle outlets such that the gas streams impinge substantially uniformly over the outer surface of the object, wherein the distance (a) between each nozzle outlet and the outer surface of the object against which the associated gas stream impinges is less than or equal to half the diameter (d) of the nozzle outlets.
For the avoidance of doubt it should not be inferred from the use of the word “diameter” that the invention is limited to gas streams of circular cross section; the present invention extends to gas streams of any cross-sectional shape, the “diameter” of these being calculated through assuming that the cross-sectional area of a non-circular gas stream, for the purpose of putting this invention in to practice, is in fact circular. Thus the word “diameter” where used herein should be interpreted as meaning the diameter of a circular gas stream or the theoretical diameter of a circular gas stream which has an equal cross-sectional area to a non-circular stream. For such small distances between nozzle outlet and the object, the cross-sectional area and the “diameter” of the gas stream remains substantially constant throughout its transit between nozzle outlet and the object, and equal to the cross-sectional area and the “diameter” of the nozzle outlet.
The nozzle outlets may be of substantially equal cross-sectional area, or the area of the nozzles may vary, provided that the total area of nozzles per unit area of the object to be cooled remains substantially constant. It may, for example, be advantageous to have different nozzle areas in order to quench an object having a complex or convoluted surface shape or configuration.
We have discovered from investigating the complex interaction of the gas streams that there is an unexpected and surprisingly large and rapid increase in the heat transfer rate at very small values of the distance between the gas stream nozzle outlet and the surface of the object (ie where a≦0.5 d), when the areas of high turbulence produced at the edges of the nozzles interact with the surface of the object to maximise the transfer of heat to the gas and to produce more uniform cooling. Also, as will be described further below, a method in accordance with the invention is demonstrably capable of providing a substantially uniform quench, as a varied quench, as desired.
The method of the invention also enables quench rates to be achieved which are equivalent to conventional oil quench
Cohen Joshua L.
Ip Sikyin
Pace Salvatore P.
The BOC Group plc
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