Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1994-07-22
2001-05-08
Ip, Sikyin (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S596000, C148S598000, C148S599000
Reexamination Certificate
active
06228188
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process of heating and subsequently cooling at least one steel wire. An example of such a process is austenitizing the steel wire and subsequently cooling the steel wire to allow transformation from austenite to pearlite.
The term “steel wire” refers in what follows to a large range of carbon steel wires where transformation from austenite to pearlite may occur. A typical composition may be along the following lines:
a carbon content between 0.10% and 0.90%, preferably between 0.60% and 0.85%, a manganese content between 0.30% and 1.50%, a silicon content between 0.10 and 0.60%, maximum sulphur and maximum phosphorus contents of 0.05%. Other elements such as chromium, nickel, vanadium, boron, aluminium, copper, molybdenum, titanium may also be present; either alone or in combination with another element. The balance of the steel composition is always iron. All percentages expressed herein are percentages by weight.
The steps of heating the steel wire above the austenitizing temperature and subsequently cooling the steel wire to a temperature between 500° C. and 680° C. to allow transformation from austenite to pearlite are widely known and are commonly called patenting. Patenting is done to obtain an intermediate wire product (a so-called half-product, in contradistinction to a final product) with a metallic structure which allows further drawing without difficulties. The exact metallic structure of the patented steel wire as an intermediate wire product not only determines the absence or presence of wire fractures during the subsequent wire drawing but also determines to a large extent the mechanical properties of the steel wire at its final diameter.
In this way transformation conditions must be such that martensite or bainite are avoided even at very local spots on the steel wire surface. On the other hand, the metallic structure of the patented steel wire must not be too soft, i.e. it must not present too coarse a pearlite structure or too great a quantity of ferrite, since such a metallic structure would never yield the desired ulitmate tensile strength of the steel wire at its final diameter.
It follows that the second step of the patenting process, i.e. the cooling or transformation step, is very critical. Temperature ranges and cooling velocities must be so that the desired intermediate wire product is obtained.
The prior art has provided a plurality of ways to perform the transformation step, all of these ways having serious drawbacks.
Transformation may be done by means of a lead bath or of a salt bath. These embodiments have the advantage of giving the patented steel wire a proper metallic structure. Both require, however, considerable running costs. Moreover, both cause considerable environmental problems. And lead drag out brings about quality problems in the downstream processing steps.
Transformation may also be done in a fluidized bed. A fluidized bed may also give the patented steel wire a proper metallic structure. The investments needed for a fluidized bed installation are very high and the running and operating costs are even higher than for a lead bath. Moreover, fluidized bed installations may have a lot of maintenance problems.
Austenite to pearlite transformation may also be done in a water bath. A water bath has the advantage of low investment costs and low running costs. Water patenting, however, may give problems for wire diameters smaller than 2.8 mm and even becomes impossible for wire diameters smaller than about 1.8 mm.
SUMMARY OF THE INVENTION
It is an object of the present invention to avoid the drawbacks of the prior art.
It is a further object of the present invention to provide with a transformation process which has low investment costs, low running costs and which does not require much maintenance.
It is another object of the present invention to provide with a transformation process which gives patented steel wires with a proper metallic structure.
It is yet another object of the present invention to provide with a process which is suitable for transformation of steel wires with a diameter smaller than 2.8 mm, e.g. smaller than 1.8 mm.
According to the present invention, there is provided a process of heating and subsequently cooling at least one steel wire. The steel wire has a diameter which is less than 2.8 mm, e.g. less than 2.3 mm or less than 1.8 mm. The cooling is alternatingly done by film boiling in water during one or more water cooling periods and in air during one or more air cooling periods. A water cooling period immediately follows an air cooling period and vice versa. The number of the water cooling periods, the number of the air cooling periods, the length of each water cooling period and the length of each air cooling period are so chosen so as to avoid the formation of martensite or bainite.
The term “film boiling” refers to the stage of cooling by means of water, during which the steel wire is surrounded by a continuous and stable vapour film. This stage is characterized by a regular and relatively slow cooling.
The film boiling stage must be distinguished from two other stages which may occur during water cooling:
(i) the nucleate boiling stage where the stable vapour film disappears and where cooling is rapid and irregular;
(ii) the convective cooling stage where the water is in direct contact with the steel wires.
The stages (i) and (ii) must be avoided in the process according to the invention.
The term “water” refers to water where additives may have been added to. The additives may comprise surface active agents such as soap, polyvinyl alcohol and polymer quenchants such as alkalipolyacrylates or sodium polyacrylate (e.g. AQUAQUENCH 110®, see e.g. K. J. Mason and T. Griffin. The Use of Polymer Quenchants for the Patenting of High-carbon Steel Wire and Rod, Heat Treatment of Metals. 1982.3. pp 77-83). The additives are used to increase the thickness and stability of the vapour film around the steel wire. The water temperature is preferable above 80° C. e.g. above 85° C. most preferably above 90° C. e.g. around 95° C. The higher the water temperature, the higher the stability of the vapour film around the steel wire.
Water cooling is conveniently done in a water bath where the steel wire or steel wires are guided through via a horizontal and rectilinear path. The bath is usually of the overflow-type.
The term “water bath” refers both to a complete water bath taken as a whole and to that part of a complete water bath where the steel wire has been immersed.
It is possible to match the dimensions of the water baths to the number of steel wires so that—except for the starting up phase—energy does not need to be supplied to the water baths since the energy provided by the hot steel wires is sufficient to keep the water at the proper temperature. This reduces considerably the operating costs.
A further advantage and the working of the invention may be explained as follows.
The heat content of a wire is proportional to its volume, the volume being proportional to d
2
, where d is the diameter of the wire:
heat content=
C
1
×d
2
The surface of a wire is proportional to its diameter d:
surface=
C
2
×d
As a consequence, the cooling velocity, being proportional to the surface and inversely proportional to the heat content, is inversely proportional to the diameter d:
cooling velocity=(
C
2
×d
)/(
C
1
×d
2
) =
C
3
/d
The smaller the diameter, the greater the cooling velocity and the greater the chances for formation of martensite or bainite.
In this way transformation in water becomes difficult for wire diameters below 2.8 mm and becomes impossible for wire diameters below about 1.8 mm. The cooling velocity is that high that even by film boiling the “nose” of the transformation curve in a TTT-diagram is passed by. The result is the formation of martensite.
The invention makes patenting of steel wires with a diameter below 2.8 mm, e.g. below 1.8 mm (1.5 mm, 1.2 mm, 0.8 mm), possible by moderating the global coo
Meersschaut Dirk
Vanneste Godfried
Foley & Lardner
Ip Sikyin
N.V. Bekaert S.A.
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