AT902U1 - METHOD FOR PRODUCING SEAMLESS TUBES - Google Patents

Description

AT 000 902 Ul

The invention relates to a method for producing seamless tubes from materials that form a strongly elongated coarse grain structure by mechanical forming in combination with a recrystallizing heat treatment.

Such materials are dispersion-strengthened metals, metal alloys or intermetallic phases, which are used due to their excellent mechanical properties at high temperatures, in particular for thermally and mechanically highly stressed components. An important group of these materials are the so-called ODS superalloys based on iron, nickel or cobalt, in which oxide particles are used as dispersoids. The ODS superalloys extend the temperature range from heat-resistant materials to around 1350'C.

All these materials have in common that they achieve their good high-temperature properties, in particular their good creep resistance, through the formation of an elongated coarse grain structure. To form this coarse grain structure, the materials have to be mechanically formed with a high degree of forming and a subsequent annealing treatment for 2 AT 000 902 Ul

Be subjected to recrystallization. Due to this directional recrystallization, depending on the alloy and type of production, the materials show a rather pronounced directional dependence of the mechanical properties, for example the high-temperature creep resistance. The best strength properties are always observed in the direction of the main forming direction, i.e. parallel to the structural extension. In the directions orthogonal to this main forming direction, these materials have so far only shown comparatively unfavorable strength values at high temperatures.

The greatest mechanical stress in the circumferential direction is present for pipes subjected to internal pressure. The material tensions in the circumferential direction are twice as high as those in the longitudinal pipe direction. For pipes that are exposed to internal pressure and are exposed to high temperatures, the materials mentioned with an elongated coarse grain structure are suitable if it is ensured that the high microstructure in the pipe circumferential direction is set in order to fully utilize the heat resistance potential of these materials.

So far, seamless tubes made of the materials mentioned have been produced, for example, by deep hole drilling from bar stock or by hot extrusion with or without subsequent cold reduction, such as tube drawing or tube pilgrim step rolling. The disadvantage of this method, however, is that the main forming direction lies in the axial direction of the tube and thus also the strong structural extension or structural orientation lies in this direction, which is associated with a relatively low strength in the most stressed circumferential direction of the tube .

In " Development of Ferritic ODS Tubes for Heat Exchangers Operating Above 1100 ° C ", B. Kazimierzak, JM Prignon et al., &Quot; Structural Applications of Mechanical Alloying ", ASM, Ohio (1990), pp. 137-145 Hot extrusion process for the production of seamless pipes from ODS materials described, which show the desired elongated structure in the pipe circumferential direction.

A disadvantage of this method is that its successful application depends on a large number of parameters. The process, especially the temperature control, must be controlled within very narrow limits in order to obtain the grain stretching in the circumferential direction in a reproducible manner. This makes the process complex and expensive. In addition, the method severely restricts the tube formats that can be produced by means of the maximum available pressing forces of industrial extrusion presses, both upwards and downwards. The main disadvantage of this method, however, is that only relatively thick-walled tubes can be produced, which, for reasons of heat conduction for tubes in tube bundle heat exchangers or simply because of the increased cost of materials, often does not allow technically or economically satisfactory tube production despite the optimized structure. Today, hot extrusion with or without a downstream cold processing process can be used to produce pipes with outside diameters between 25 and 40 mm and wall thicknesses between 2.5 and 5 mm. 4 AT 000 902 Ul

The object of the present invention is to provide a method for the production of seamless tubes, in which a strongly elongated structure is oriented in the tube circumferential direction, which ensures good reproducibility of these microstructures in a simple manner and with which thin-walled tubes with wall thicknesses below 2.5 mm can be manufactured over a large diameter range.

The object is achieved according to the invention in that a cup-shaped or sleeve-shaped starting part is brought into the desired tube shape in the unrecrystallized state by pressure rolling in accordance with DIN 8583, T2 with a degree of deformation of at least 30%, and the material is subjected to intermediate annealing between individual deformation steps or final annealing is coarse-grained recrystallized after completion of the forming.

According to the invention, the cup-shaped or tubular starting part is applied to a mandrel on a conventional spinning machine and the outer diameter is reduced by means of one or preferably several spinning rollers in one or more overflows. This reduces the wall thickness and at the same time increases the length of the outlet cup or outlet sleeve, which creates a tubular body. In this way, pipes with wall thicknesses between 0.3 and 2.5 mm and diameters in the range from 20 to 450 mm can be produced.

The kinematics of the cylinder pressure rolling process is such that the material flow during the forming process takes place almost exclusively in the axial direction. In order to produce parts with 5 AT 000 902 Ul

In order to maintain the inside diameter, the tangential material flow (material flow in the pipe circumferential direction) must be reduced to a minimum by coordinating the dimensions of the starting part with the pressure rollers used and the number of overflows. It was completely surprising to the person skilled in the art that, despite the fact that the material flow during the cylinder pressure rolling mainly takes place in the axial direction, the tubes which are produced by the process according to the invention have an elongated coarse grain structure after the recrystallization annealing which runs in the tube circumferential direction. This is achieved by simple application of the cylinder pressure rolling within the known process limits without the use of additional or costly optimization of individual process parameters, so that good reproducibility of the pipes is ensured by the process according to the invention.

The process according to the invention also offers the advantage that high cross-sectional reductions per pass rolling pass and also high overall cross-sectional reductions can be achieved without heating and without intermediate annealing steps. This was not to be expected, especially when using the process with ferritic ODS materials, since these materials exhibit a brittle-ductile transition behavior, so that such materials normally have to be preheated to at least 60 - 100 * for the forming process in order to be sure after the forming to result in crack-free parts. The fact that the method according to the invention can be applied to practically all materials at room temperature without the workpiece and 6 AT 000 902 Ul

Mandrel must be prewarmed, there is an economical production and excellent reproducibility of the pipes. The good reproducibility concerns both the good dimensional stability and the metallurgical quality with regard to the coarse grain structure stretched in the pipe circumferential direction after the recrystallization annealing.

In addition to recrystallization annealing, intermediate annealing in the form of relaxation or relaxation annealing can also be inserted between individual deformation steps.

The method according to the invention is particularly advantageously applicable to materials in the form of metals, metal alloys or intermetallic phases which are solidified with oxide and / or nitride and / or carbide particles.

The oxides of one or more metals from the group of yttrium, aluminum, lanthanum, cerium and zircon have proven themselves in particular as solidifying particles.

A variant of a material which can be used according to the invention is to form the solidifying particles in the material in such a way that reactive metallic additives are added to the material, which are converted into the corresponding oxide, nitride or carbide particles during processing of the material and / or during heat treatment .

In particular, those based on iron, nickel or cobalt have proven themselves as metal alloys. 7 AT 000 902 Ul

The iron base alloys are those with the composition 6-30% by weight Cr, 0-10% by weight Al, 0-2% by weight Ti, 0-10% by weight Mo, 0-10% by weight W, 0 - 10% by weight of Ta, 0.1-2% by weight of one or more oxides from the group Y, Al, La, Ce and Zr, as well as Fe as the remainder and as nickel-based alloys, those with the composition 6-38% by weight of Cr , 0.1-7 wt% Al, 0-2 wt% Ti, 0-5 wt% Mo, 0-5 wt% W, 0-5 wt% Ta, 0.1-2 wt. % of one or more oxides from the group Y, Al, La, Ce and Zr, and Ni as the rest, in particular proven.

The invention is explained in more detail below on the basis of examples and figures:

Show it:

Figure 1 cross section of individual sections of the tube produced according to Example 1 in 16 times magnification

FIG. 2 cross section of individual sections of the tube produced according to Example 2 in 16-fold magnification 8 AT 000 902 ul EXAMPLE 1:

An ODS alloy with the chemical composition 19% by weight Cr, 5.5% by weight Al, 0.5% by weight Ti, 0.5% by weight Y2O3 and the rest Fe was used in the following sequence to mechanically alloy the powder starting materials, hot isostatic compression of the powders, subsequent hot forming by round forging, boring the forged cylinder and finally overturning the inner and outer jacket surfaces, a sleeve-shaped blank with an inner diameter of 60 mm and a wall thickness of 5 mm was made. This blank was formed on a conventional 3-roller CNC spinning machine in only two overflows with the same inside diameter from the initial wall thickness to an end wall thickness of 1.25 mm. The pipe produced in this way showed excellent surface quality and dimensional accuracy. It was then examined for cracks using the dye penetration method. Finally, the tube was recrystallized in air at 1380 aC for one hour. During this annealing, a very thin and firmly adhering A ^ Oß layer was formed. A metallographic examination of the annealed tube revealed a coarse-grained recrystallization structure with a high structural extension in the tube circumferential direction corresponding to FIG. 1. 9 AT 000 902 Ul EXAMPLE 2:

A hollow format was produced from an ODS alloy of the same composition as in Example 1 by mechanical alloying of the powder starting materials, hot isostatic pressing and subsequent conventional hole pressing. This blank was then turned into a sleeve-shaped blank with an inner diameter of 60 mm and a wall thickness of 5 mm. The blank obtained in this way has a fine-grained structure. It was formed by spinning in only three overflows from the initial wall thickness of 5 mm to an end wall thickness of 0.7 mm. This corresponds to an overall degree of deformation of 84% relative cross-sectional decrease, which was achieved without heating and without intermediate annealing.

The tube was then examined for cracks using both the dye penetration method and the vortex current test. As a result, the tube was recrystallized in air at 1380 ° C for one hour. The microstructure shown in FIG. 2 was formed. 10th

Claims (7)

AT 000 902 Ul claim 1. Process for the production of seamless tubes from materials which, by mechanical shaping in combination with a recrystallizing heat treatment, form a highly elongated coarse-grain structure, characterized in that a cup-shaped or sleeve-shaped outlet part is recrystallized in a non-recrystallized state DIN 8583, T2 is brought into the desired tube shape with a degree of deformation of at least 30% and the material is coarsely recrystallized by intermediate annealing between individual deformation steps or by final annealing after completion of the deformation. 2. A method for producing seamless tubes according to claim 1, characterized in that a metal, metal alloy or intermetallic phase solidified with oxide and / or nitride and / or carbide particles is used as the material. 3. A method for producing seamless tubes according to claim 2, characterized in that the solidifying particles are oxides of one or more metals from the group of yttrium, aluminum, lanthanum, cerium and zircon. 11 AT 000 902 Ul 4. A method for producing seamless tubes according to claim 2, characterized in that the solidifying particles are formed from reactive metallic additives which are converted into the corresponding oxide, nitride or carbide particles during the processing of the material and / or during a heat treatment. 5. A method for producing seamless tubes according to claim 2, characterized in that a dispersion-strengthened alloy based on iron, nickel or cobalt is used as the metal alloy. 6. A method for producing seamless tubes according to claim 5, characterized in that the alloy of 6-30 wt.% Cr, 0-10 wt.% Al, 0-2 wt.% Ti, 0-10 wt.% Mo, 0-10% by weight W, 0-10% by weight Ta, 0.1-2% by weight of one or more oxides of the metals from the group Y, Al, La, Ce and Zr, and Fe as the rest. 7. A method for producing seamless tubes according to claim 5, characterized in that the alloy of 6-38 wt.% Cr, 0.1 - 7 wt.% Al, 0-2 wt.% Ti, 0-5 wt.% Mo, 0-5 wt.% W, 0 - 5 wt.% Ta, 0.1 - 2 wt.% Of one or more oxides of the metals from the group Y, Al, La, Ce and Zr, and Ni as the rest consists. 12th