CORBETT, Michael R (46 Ridge Road, 4320 Umhlanga Rocks, ZA)
BELL, Leon McDougall (27 Beukes Avenue, Highway Gardens, 1609 Edenvale, ZA)
CORBETT, Michael R (46 Ridge Road, 4320 Umhlanga Rocks, ZA)
CLAIMS
1.
A steel tube drawing process in which a hot rolled seamless steel tube of relatively large OD and wall thickness is drawn down to a smaller OD and wall thickness, the process comprising at least one drawing stage in which the tube is located over a mandrel, the tube is induction heated and drawn over the mandrel and through a drawing die which reduces the OD of the tube, and the drawn tube is thereafter cooled.
2.
A process according to claim 1 wherein the tube is drawn over the mandrel and through the die at a constant speed.
3.
A process according to claim 2 wherein the constant speed is in the range
1.5m/min to 15m/min.
3.
A process according to claim 1 or claim 2 wherein, after drawing, the tube is allowed to cool to ambient temperature in the absence of forced cooling.
4.
A process according to any one of the preceding claims wherein the tube is induction heated to a temperature selected to allow normalisation of the steel during cooling.
5.
A process according to claim 4 wherein the tube is induction heated to a temperature selected, for the carbon content of the steel, to be at or just above the A3 line on the iron-carbon equilibrium diagram.
6.
A process according to claim 5 wherein the equivalent carbon content of the steel is approximately 0.7% by weight and the tube is induction heated to a temperature in the range 790 0 C to 820 0 C.
7.
A process according to any one of the preceding claims wherein the tube is drawn through the die on a draw bench and induction heating is applied to the tube as it enters the die.
8.
A process according to any one of the preceding claims when used to produce drawn tube for use in the production of extension drill steel.
9.
A process according to claim 8 wherein hot rolled seamless steel tube of 73mm OD and 14mm wall thickness is drawn down in multiple stages to produce steel tube of 39,2mm OD and 15mm ID.
10.
A process according to claim 8 wherein hot rolled seamless steel tube of 73mm OD and 14mm wall thickness is drawn down in multiple stages to produce steel tube of 32mm OD and 9mm ID.
11.
An apparatus for use in a steel tube drawing process in which a hot rolled seamless steel tube of relatively large OD and wall thickness is drawn down to a smaller OD and wall thickness, the apparatus comprising a drawing die through which the tube can be drawn, a mandrel over which the steel tube is located and which extends into the drawing die, an induction heater located adjacent an input side of the drawing die and which is operable to heat the tube as it enters the die, and means for drawing the tube over the mandrel and through the die, thereby to draw the tube down to a smaller OD and wall thickness. |
"STEEL TUBE DRAWING PROCESS AND APPARATUS"
FIELD OF THE INVENTION
THIS invention relates to a steel tube drawing process and apparatus. In one application the process and apparatus of the invention may be used to produce steel tube suitable for use in the production of extension drill steel, typically for use in rotary drilling operations, although other applications, for example hydraulic tube, are also envisaged by the invention.
BACKGROUND TO THE INVENTION
A conventional seamless tube mill is currently used to produce, from a steel billet, high alloyed, hot rolled seamless hollow steel bar, i.e. steel tube, stock which can used in the production of extension drill steel. The hot rolling process produces a hot rolled tube with an OD (outside diameter) and wall thickness nominally suitable for use in the production of extension drill steel. However the tube produced by the known hot rolling process may be dimensionally inaccurate. It may for instance have an irregular cross-sectional shape with a non-round, possibly generally polygonal internal cross-sectional profile, non-concentric inner and outer profiles and an inconsistent wall thickness. The irregular shape can give rise to stress concentration points which may contribute to early failure when the final product is used under load, for example as an extension drill steel in a drilling operation. To avoid such problems, it may be necessary to machine the rolled tube in order to achieve the required concentricity and dimensional accuracy, but this increases the final cost of the product.
In addition, the hot rolled tube may have a grain structure and mechanical properties which make it less than optimal for use in the production of extension drill steel. The tube may for instance be rather brittle with limited toughness and fatigue resistance.
Attempts have also been made to modify hot rolled steel tube stock by cold drawing it to the required, reduced OD and wall thickness so that it can be used in the production of extension drill steel. The cold drawing process involves a multi-stage procedure with the tube being drawn down to a smaller OD and wall thickness in each stage. However the cold drawn tube must be annealed after each stage in order to reduce undesirable work hardening effects. In the known process, a considerable period of time, possibly several days, is required for annealing between the successive drawing stages. As a result the cold drawing process is undesirably costly and time-consuming.
SUMMARY OF THE INVENTION
According to the present invention there is provided a steel tube drawing process in which a hot rolled seamless steel tube of relatively large OD and wall thickness is drawn down to a smaller OD and wall thickness, the process comprising at least one drawing stage in which the tube is located over a mandrel, the tube is induction heated and simultaneously drawn over the mandrel and through a drawing die which reduces the OD of the tube, and the drawn tube is thereafter cooled.
The tube may be drawn over the mandrel and through the die at a constant speed, possibly in the range 1.5m/min to 15m/min.
The tube may be allowed to cool to ambient temperature in the absence of forced cooling.
Preferably the tube is induction heated to a temperature selected to allow normalisation of the steel during cooling. Typically the tube is induction heated to a temperature selected, for the carbon content of the steel, to be at or just above the A3 line on the iron-carbon equilibrium diagram. For an equivalent carbon content of approximately 0.7% by weight the tube will be induction heated to a temperature in the range 79O 0 C to 820 0 C.
Conveniently the tube is drawn through the die on a draw bench and induction heating is applied to the tube as it enters the die.
In one application of the process, when used to produce drawn tube for use in the production of extension drill steel, hot rolled seamless steel tube of 73mm OD and 14mm wall thickness may be drawn down in multiple stages to produce steel tube of 39,2mm OD and 15mm ID or 32mm OD and 9mm ID.
Another aspect of the invention provides an apparatus for use in a steel tube drawing process in which a hot rolled seamless steel tube of relatively large OD and wall thickness is drawn down to a smaller OD and wall thickness, the apparatus comprising a drawing die through which the tube can be drawn, a mandrel over which the steel tube can be drawn and which extends into the drawing die, an induction heater located adjacent an input side of the drawing die and which is operable to heat the tube as it enters the die, and means for drawing the tube over the mandrel and through the die, thereby to draw the tube down to a smaller OD and wall thickness.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:
Figure 1 diagrammatically illustrates apparatus used in a process according to the invention, at the commencement of a drawing operation;
Figure 2 diagrammatically illustrates the same apparatus during the drawing operation;
Figure 3 shows a cross-sectional view of the drawing die used in the apparatus seen in Figures 1 and 2; and
Figure 4 shows a conventional iron-carbon phase diagram.
DESCRIPTION WITH REFERENCE TO THE DRAWINGS
In Figure 1 the numeral 10 indicates a length of high alloyed, hot rolled seamless, hollow steel bar stock, i.e. tube, which in this example has an OD of 73mm, an ID (internal diameter) of 45mm and a radial wall thickness of 14mm. Using the process of the invention, it is desired to draw the tube 10 down to a smaller OD and wall thickness which will render it suitable for the production of extension drill steel to be used in rotary earth drilling operations, for example core drilling operations.
The process of the invention is carried out on a draw bench 12 which is generally of a type used in cold tube drawing processes. The illustrated draw bench 12 includes a carriage 14 which can be driven by means of a chain 16 and which is fitted with a clamping apparatus 18. In use, the clamping apparatus is clamped to a smaller diameter extension piece 20 which is welded to a leading end of the tube 10 and which projects through a drawing die 22.
The tube 10 is located on a fixed mandrel 24 and is supported for axial movement by means of rollers 26. The coil 28 of a high frequency induction heating unit 30 is arranged about the tube 10 close to the die 22.
In this example, the tube 10 has a significant nickel and molybdenum content, and has an ultimate tensile strength of approximately 900MPa and a yield strength of about 400MPa.
The induction heater operates at suitable power output to elevate the temperature of the steel of the tube 10 to a value above the A3 line on the iron-carbon equilibrium diagram. A typical iron-carbon phase diagram, in which the A3 line is indicated by the numeral 27, is shown in Figure 4.
The carriage 14 is driven at a suitable drawing speed, in the direction of the arrow 32, so the steel tube 10 is accordingly pulled or drawn over the mandrel and through the die at this speed. Figure 1 illustrates the tube 10 at the commencement of the drawing procedure and Figure 2 illustrates it during the drawing procedure.
Prior to the commencement of the drawing operation, the external surface of the tube 10 is lubricated with a suitable lubricant. Lubricant may also be applied to the mandrel.
Referring to Figure 3 it can be seen that the fixed mandrel 24 has a mandrel rod 24.1 carrying a slightly enlarged mandrel head 24.2 which extends to a position flush with the output side 22.1 of the drawing die 22 and which has an external diameter 24.3 corresponding to the ID of the drawn tube. The die itself has an arcuately tapered die passage 22.2 with an internal diameter 22.3 corresponding to the OD of the drawn tube. In this example the diameter 22.3 defines a bearing length 22.4 which is approximately equal to 40% of the OD, i.e the bearing length 22.4 = 0.4OD.
In this example, the tube 10 is drawn through the die 22 at a constant speed, i.e. the carriage 14 moves at a constant velocity throughout the drawing operation. In other examples, the drawing speed may be varied during the drawing operation. The drawing speed may for instance be increased or decreased at the beginning and end of the operation compared to an intermediate period during which the major part of the tube is drawn through the die.
As the drawn tube emerges from the output side of the die 22 it immediately commences cooling in the ambient air. In this example, after the tube has been drawn fully through the die it is allowed to cool in air to ambient temperature with no forced cooling applied. This may be achieved by rotating the tube on support rollers (not shown) to ensure that ambient cooling is applied uniformly to the external surface of the tube.
In other examples, forced cooling may be applied to the drawn tube in order to create special properties, depending on the final use to which the tube is to be put.
In the case of extension drill steel it may for example be desired to draw the initial tube 10 down to an OD of 39,2mm or 32mm, with an ID of 15mm or 9mm and a wall thickness of 12.1mm or 11.5mm respectively, since these are dimensions commonly used in extension drill steels employed in rotary core drilling operations. In order to achieve the desired final dimensions it will typically be necessary to drawn the original tube down in several successive stages with the OD, ID and wall thickness of the tube being incrementally reduced during each stage. In practice, after a series of tubes have been drawn down in each stage and have cooled down thereafter, the draw bench is re-equipped with a smaller die and mandrel prior to a repetition of the drawing process in the next stage.
Persons skilled in the art will recognise that the selection of the steel temperature to be just above the A3 line in a conventional iron-carbon equilibrium diagram allows a hot drawing procedure to take place but also permits normalisation of the steel during the subsequent cooling step, thereby reducing undesirable work hardening effects. It is accordingly believed that the process of the invention will produce drawn steel tube which is less brittle and generally has better mechanical properties, for example ultimate tensile strength and yield strength than that produced by the known cold drawing process described above.
An added advantage of the process of the invention as exemplified above is the fact that the initial, irregular cross-section of the standard hot drawn seamless hollow bar stock, i.e. the tube 10, is converted by the dies and mandrels used in the successive drawing stages to a regular cross-section in which the OD and ID are within close tolerances. The regular cross- section which is produced can reduce the creation of stress concentration points which could perhaps give rise to premature failure under load of, for instance, extension drill steel produced from the drawn tube.
In the example described above, the steel tube 10 is subjected to heating by a single induction coil 28 as it enters the die 22. In other embodiments, two or more induction heaters may be arranged, typically in series, to elevate the steel temperature to the required level for hot drawing and subsequent normalisation to take place when the tube is cooled to ambient temperature.
In one particular example of the invention, where the steel of the tube 10 has an equivalent carbon content of the order of 0.7% by weight, the induction heater may be arranged to elevate the temperature of the tube 10, as it enters the drawing die, to a value in the range 79O 0 C to 820 0 C and a constant drawing speed in the range 1.5 to 15 m/min is used. These parameters may however vary depending on the carbon content of the steel and the required mechanical properties.
It has been found that steel tube produced by the process exemplified above has a surface finish and close tolerance geometry superior to that of hot rolled products. The steel tube produced by the process of the invention also has a fine grain structure allowing the drawn product to exhibit higher levels of toughness and fatigue resistance than hot rolled products.
In addition to the above-mentioned advantages, it is pointed out that the rapid, substantially instantaneous heating of the steel tube 10 to the required temperature as it enters the die 22, by means of high frequency induction heating, has the advantage that undesirable carburisation, surface hardening and/or surface oxidation effects are limited. The resulting, normalised drawn steel product is accordingly more tractable to any subsequent heat treatment which may be required in producing extension drill steel or other final product.
It should also be noted that while specific mention has been made of a chain drive system to draw the tube through the die, other examples may
employ other drawing mechanisms, such as hydraulic or rack and pinion mechanisms.
Next Patent: SINGLE MAGNETIC TRACK TWO AXIS POSITIONING SYSTEM