Method and Apparatus for Forming a Titanium or Titanium Alloy Sheet Element With a Contoured Surface

The present invention relates to a process and apparatus for forming a sheet element with a contoured surface from a plane titanium or titanium alloy sheet by hot deep drawing in a press.

In certain cases, it is necessary to fabricate contoured-surface shapes, particularly cylindrical or frustroconical, for example, for fabricating tubes whose thickness may reach or exceed 10 mm, from titanium or titanium alloy, particularly the alloy containing 6 wt. % aluminum and 4 wt. % vanadium, generally known as TA 6V, which has good properties in terms of lightness (density 4.5), mechanical strength (tensile strength at least 900 MPa) and corrosion resistance, particularly to seawater. Such tubes or tube elements would be particularly suitable for connecting a drilling platform and drilling head to the deep sea bed, since steel tubes or tube elements would be too heavy. The alloy containing 3 wt. % aluminum and 2 wt. % vanadium, known as TA 3V, could also be suitable for such applications.

Forming such tubes from plane sheets first means forming the plane sheet into a contoured surface element, particularly cylindrical or conical with a center angle (angle formed by the planes passing through the axis and the extreme generatrices) of, for example, 180.degree. or 120.degree., followed by welding the elements obtained along generatrices to form the tube or a closed shell. The prior art confirmed by testing indicates that such forming cannot be done by a cold process since the mechanical properties of the sheet do not lend themselves to this deformation without becoming unusable. In particular, it is impossible to control precisely the dimensional characteristics of the fabricated part. However, hot forming is a relatively complex operation generally carried out by positioning the sheet in a tool which is itself preheated above the minimum temperature at which forming is to take place. The tool must be heated to the desired temperature before each forming operation, which necessitates a relatively slow rate of fabrication.

The purpose of the present invention is to provide a process for fabricating contoured-surface sheet elements, particularly cylindrical or conical, that is simpler and faster and which nonetheless produces elements of regular thickness and precise dimensions with no cracks. The forming process according to the invention is characterized by the plane sheet element being preheated to a temperature of at least 730.degree. C., by the preheated element being transferred to a non-preheated deep-drawing tool placed in the press soon enough for the temperature of the element to be still at least 700.degree. C. at the time of the deep-drawing operation, by transferring the deep-drawn element to a calibration tool and heating the deep drawn element to at least 650.degree. C. for a sufficient time for its definitive shaping, then allowing the so-processed element to cool to room temperature. The process of the present invention is particularly suitable for forming a sheet element 25 mm thick or less, and the deep-drawn element is preferably kept at a temperature of at least 650.degree. C. in the calibration tool for at least one hour.

For forming such a sheet element, according to the present invention, the temperature of the deep-drawn sheet element is brought to about 650.degree. C. in at least 5 hours and held at about 650.degree. C. for approximately one hour, allowed to cool in the calibration tool for at least 10 hours, and then is removed from this tool and allowed to air-cool.

The formed sheet element may, according to the present invention, be descaled by shot-blasting and, for this purpose, steel balls with diameters between 0.6 millimeters and 0.16 millimeters are used for shot-blasting.

After shot-blasting, according to the invention, the element is finished by being pickled in a cold bath of an aqueous solution with 15 to 40 wt. % nitric acid and 1 to 2 wt. % hydrofluoric acid, with the nitric acid/hydrofluoric acid weight ratio being higher than 10 and the solution containing less than 10 g/l of the total (iron+titanium) for 1 to 5 minutes followed by rinsing and drying.

If, after forming, blasting, and the finishing stage of chemical pickling, the element is left in storage for a relatively long period of time, a patina forms which negatively affects its appearance but not its properties. It can then be chemically pickled in a cold bath similarly to the finishing stage of chemical pickling.

The present invention also relates to apparatus for forming a contoured-surface sheet element from a plane sheet, with the apparatus included a combination of means for heating said plane sheet, a conveyor, a press, and calibration means, these various elements being distinct from each other.

The press may, in accordance with the present invention, be fitted with at least one insert, and, may have a furnace designed to receive the calibration means. Additionally, the mechanical descaling means may be provided for carrying out a descaling operation.

An apparatus for forming cylindrical sheet elements and a method for forming and descaling according to the invention is described hereinbelow as an example with reference to the accompanying drawings.

FIG. 1 is a perspective view of two deep-drawn, calibrated cylindrical sheet elements;

FIG. 2 is a schematic perspective view of a furnace for heating sheet elements and a press for deep-drawing the heated elements into cylindrical halves;

FIG. 3 is a perspective view of a tool for hot calibration of the cylindrical halves; and

FIG. 4 is a perspective view of an insert.

In FIG. 1, halves 1 and 2 are shown in the position in which they would face each other with a view to welding along their longitudinal edges 4, 5, 6, 7 to form a tube element. The halves 1, 2 have substantial thickness which may exceed 12.7 mm, as can be seen from their semicircular ends 7, 8.

As shown in FIG. 2, a furnace generally designated by the reference numeral 11, for example an electric or gas furnace, preheats the plane sheet element, and, after heating to the desired temperature, a furnace door 12 is raised by means of counter-weight levers 13, 14. The preheated elements are placed on motorized conveyor generally designated by the reference numeral 15 which has rotating cylindrical elements 16, and conveyed to press generally designated by the reference numeral 17.

The press 17 has a fixed elongated punch 18 with an active part 19 integral with upper frame 20 and with a substantially semicircular straight section between two lateral shoulders. Below this punch 18 is a mobile die whose lower loading position (resting on base 21) is shown at 22 and whose upper deep-drawing position at 23 (dashed lines). It is obvious that it will not be a departure from the present invention for the punch 18 to be mobile and the die to be fixed. The punch 18 and/or mobile die may be fitted with inserts of variable thickness between which the plane sheet element is introduced. This insert or inserts allow(s) shells of different dimensions to be obtained from a given punch/die pair. Thus, these inserts are tools which adapt to the punch and/or the die to allow the geometric characteristics of the fabricated shell to be modified. It is obvious that the modifications that may be obtained are limited by the dimensions of the punch 18 and die in the absence of an insert.

FIG. 4 represents an insert 36 usable on die 22 and which, if it were used, would permit reduction of the external dimensions of the fabricated halves 1, 2.

After deep-drawing, the formed halves 1, 2 do not, however, have a fully semi-cylindrical shape as their radii of curvature are greater than the desired value. They must hence be subjected to calibration treatment at a temperature of at least 650.degree. C.

The halves 1, 2 to be calibrated, held in the press inserts, are introduced with the latter into the calibration apparatus shown in FIG. 3. The calibration apparatus comprises a base 24 and a punch 24A having a semi-cylindrical central part 25 and lateral shoulders. Half 26 is held tightly between base 24 and punch 24A by jacks 27, 28 supported by arches 29, 30 and resting on plane beams 31, 32. Flanges 33 maintain the compression of the tool. A handle 34 allows the arches 29, 30 to be raised and another handle 35 allows the tool assembly to be raised. This tool assembly and the halves 1, 2 are heated by placing them in a hot furnace with the temperature rise time being 5 hours, followed by a constant temperature of 650.degree. C. for at least one hour and cooling taking 10 hours in the furnace. The part and the inserts are removed from the furnace at approximately 200.degree. to 300.degree. C., then air-cooled. The part then has its final semi-cylindrical shape and it is removed from its inserts.

After calibration, the halves 1, 2 must be treated to remove the oxide scale. It has been shown that chemical descaling by immersion in a bath of nitric and hydrofluoric acids, even when hot, is ineffective and that immersion in an aqueous alkaline solution eliminates only the thin oxide scale. Immersion in a concentrated soda or concentrated soda and nitrate bath is more effective but poses safety problems and must be followed by neutralization in an acid bath. The preferred descaling method according to the invention is blasting with steel shot 0.06 to 0.16 mm in diameter until the oxide scale disappears. This mechanical descaling may be followed by final descaling by immersion in an aqueous solution of 15 to 40 wt. % (preferably 20 wt. %) nitric acid and 1 to 2 wt. % hydrofluoric acid, with the weight ratio between nitric acid and hydrofluoric acid being higher than 10 to limit the risk of hydrogen being absorbed by the alloy.

The solution must also contain less than 120 g/l of the total (iron+titanium) and must in any event be renewed as soon as the titanium content exceeds 10 g/l.

The length of immersion may range between 1 and 5 minutes, with 2 minutes generally being sufficient. After removal of the half from the final descaling bath, it is rinsed with water and dried.

If elements are to be made from contoured-surface sheet with a different shape, semi-frustroconical, for example, presses and calibration tools adapted to the desired shape will be used.