Method and device for obtaining shaped test-pieces of steel as required in tensile under corrosion fatigue tests

Abstract

A method is described to prepare shaped steal test-pieces (1) for tensile under corrosion fatigue tests, which provides for contacting a test-piece (1) subject to rotation about its longitudinal axis and, each time, a disk (10) of sand paper, also rotating, the contact being ensured by the own weight of the test-pieces itself.

Description

[0001] The present invention relates to a processing method for preparing the steel specimens to be used in tests of tensile strength in a corrosive environment, as well as the device for carrying out such a method.

[0002] It is known that the tests undertaken by the materials to check their resistance to the different and hardest operating conditions, have to follow strict standards determining in any aspect the physical conditions at which the tests have to be carried out. In particular, to determine through laboratory tests the resistance of metals to peculiar types of rupture in environments wherein H2S is present, tensile and corrosion fatigue tests are provided being dictated by international standards EFC and NACE, especially the one called NACE™ 0177-96. Various test methods are described, always in a corrosive environment according to the mechanical stresses undergone by the specimen of material. The tests relating to the so-called “A method” according to which the specimen of material is subjected to tensile stresses until breaking, are particularly important. Obviously the sample or test-piece is defined in every dimensional feature thereof, and also all the testing conditions and the devices used to this purpose are detailed.

[0003] The test-piece, as shown (substantially as a scale-drawing) in FIG. 5, is of cylindrical shape in the central elongated portion 1a, having a reduced diameter with respect to the diameter of the two cylindrical end zones 1b. Also the minimum bending radius (R=15 mm) of the adjoining zone between the central cylindrical portion 1a with reduced diameter and the two end zones 1b is fixed.

[0004] It is known that one of the most critical features that a specimen must show for this type of test is the roughness degree which has to be less than 0.81 μm. To have a so high level of finish, it is thereby necessary to subject the specimens, already machined to the required sizes, to subsequent polishing and final lapping steps while providing a certain number of longitudinal finishing passes in sequence with disks obtained from sandpaper with a roughness from 80 to 250 GRIT (grains per square inch), then hand smoothing in longitudinal direction under water with four successive sandpapers having different grain sizes 250-400-600-1000 and finally hand lapping, still in longitudinal direction, with cloth and diamond paste with three different grades. 6-3-1 μm.

[0005] Obviously these manual operations are extremely delicate and require a notable expenditure of time and a large-scale employment of labour, which make particularly costly the preparation of such specimens designed to undergo destructive testing in a laboratory. Furthermore, the care required in carrying out these finishing steps is such to involve a high degree of experience and specialization by the operators, considering that possible inaccuracies found during the final check result in test-pieces not in conformity with the standards, which thereby shall be unavoidably discarded and destroyed to be replaced with other specimens for a new processing.

[0006] Therefore it is an object of the present invention that of providing a method of preparation of the metal specimens as defined above, with which the processing time and the labour employment can be reduced, with a lower rate of reworking.

[0007] It is also an object of the present invention that of providing a device capable of mechanically carrying out the surface finishing of the above-mentioned metal test-pieces by putting into practice the cited method by the use of a computerized system.

[0008] These and other objects are achieved through the features of the respective independent method and device claims.

[0009] Further objects, advantages and features of the device according to the present invention and of the method carried out this way will be made clearer by the following detailed description of a preferred embodiment, given by way of a non-limiting example with reference to the annexed drawings in which:

[0010] FIG. 1 shows a top plan view of an embodiment of the device according to the present invention;

[0011] FIG. 2 shows a cross-section view taken along line II-II of FIG. 1;

[0012] FIG. 3 shows a cross-section view along line III-III of FIG. 1;

[0013] FIG. 4 shows a fragmentary view, partially sectional, of the same device, taken in the direction of arrow A of FIG. 2; and

[0014] FIG. 5 shows a scale view of a test-piece in contact with an abrasive disk in a working position according to the inventive method.

[0015] The method of the present invention intends to subject the steel specimen to be used in the tests of tensile-corrosion tests as above mentioned, already prepared at a size slightly larger than the final one (a few tenths of millimeter more), to the action of sandpaper disks brought successively, with finer and finer grain, into contact with the median zone, having a smaller diameter, of the specimen itself, driven into rotation about its longitudinal axis, while also the abrasive disk rotates about its axis which is perpendicular to the rotation axis of the specimen.

[0016] The latter is preferably resting on the sandpaper disk under the action of its own weight, preferably in a controllable way by means of suitable counterweights.

[0017] The preferred working position (as illustrated in FIG. 5) is that in which the test-piece specimen 1 to be worked, in horizontal position, is resting on an abrasive disk 10, also horizontal, with such an interference degree, whereby the disk profile enters into contact with the radiused zones at the ends of the central cylindrical portion 1a of the specimen itself, having a reduced diameter.

[0018] A preferred device to carry out the method of the present invention is the one represented in the enclosed drawings wherein the specimen 1 to be worked is rotatably mounted at the end of a swinging arm 2 which is hinged through a fulcrum 3 to a support 4. The specimen 1 can be driven into rotation about its longitudinal axis by means of a small motor 5 and a belt- or chain-driving system 5′.

[0019] Both the support 4 and motor 5 are mounted on a central table 6 of the device, around which there is rotatably mounted an annular-shaped outer support 7 of the sandpaper disks 10, 10′ . . . Bearings 8 are preferably provided along the inner wall of the rotatory annular support 7 to reduce as much as possible the friction during the rotation of the outer support 7 with respect to the central table 6. Obviously the fulcrum 3 is placed, as better shown in FIG. 1, at a central position of the device, along a diameter of the annular support 7, so as the swinging arm 2 ends at said support, innerly with respect of a median circular line C along which there are mounted a certain number (eight have been represented in FIG. 1) of supporting shafts 9 for the abrasive disks 10, equally spaced in suitable seats 14. In this way the test-piece specimen 1 (not shown in scale in FIG. 1, but with a slenderness much lower than its real value) will be brought to partially lean on the sandpaper disk 10 which at that moment is located at the working station in coincidence with the diameter of line C, being perpendicular to fulcrum 3, thereby to the longitudinal axis of rotation of specimen 1. As stated above, in this position the outer profile of disk 10 will preferably “match” the two zones merging from the central portion with smaller diameter to the cylindrical end zones of the specimen itself.

[0020] Still making reference to the drawings, 11 represents the motor controlling the rotation of each rotating shaft 9 and of the corresponding abrasive disk 10 when the annular support 7 has been brought to such a position that the same shaft 9 is located in front of the specimen 1, exactly at the position indicated as 10 in FIG. 1. The rotation of said rotatable support 7 of the abrasive disks 10 is ensured by a motor means stationary mounted, operating onto the periphery of the annular support through a driving means (12′) such as a belt.

[0021] In order to ensure all the other regulations which guarantee the exact positioning of specimen 1 to be prepared with respect to abrasive disk 10 at the working station, a mechanical jack 13 is provided which, passing with its rod 13′ through two overlapping base plates 17 and 18 with an intermediate gap 26, determines the correct positioning of the support table 6 for its horizontal adjustment about the diametral axis of the device as defined by the vertical plane passing through the median axis of the swinging arm 2. To this effect it is provided that the upper plate 17, on which the support table 6 is mounted, is hinged in 17′ on a plate 19 fixed to the lower plate 18 so as to allow, thanks to the gap or clearance 26, relative movements between the two plates according to arrows F, F′ and an immediate adaptation of the various support members to each other, in particular of table 6 and consequently of the fulcrum 3 for the swinging of the specimen carrier arm 2, also by means of a pair of springs 16, 16′.

[0022] For the adjustments in the horizontal plane, with possibility of rotation about a central vertical axis passing through the fulcrum 3 and perpendicular thereto, thus defining the exact positioning of the median axis of the swinging arm 2, thereby of the specimen 1 with the axis of shaft 9 (or seat 14) in correspondence with the working station, a pair of adjusting screws 15, 15′ are provided which are suitable to move along small rotation angles in the horizontal plane, respectively clockwise or counter-clockwise, the swinging specimen carrier arm, so that the specimen, already positioned on a horizontal plane or anyhow perfectly parallel to that of the supporting annular 7, thereby also of the abrasive disk 10 with which it has to come into contact, reaches the correct position to start with the surface finishing step. A counterweight 20, provided at the opposite end of the swinging arm 2, with respect to the specimen carrier end, can be adjusted to vary its weight, i.e. the force due to the gravity applied in correspondence with the contact zone between specimen and abrasive disk. The movement of rotation from motor 11 to the shaft 9 on which the abrasive disk 10 is traditionally mounted, is preferably transmitted, as shown in FIGS. 2 and 3, as described in the following.

[0023] Upon completion of the above-described regulations, with the adjusting screws 15, 15′ shifting the specimen 1 exactly to the desired position, and having obtained the precise horizontal orientation of table 6 by means of the jack 13, when motor 11 is already rotating, both motor 5 for rotating specimen 1 resting on disk 10 owing to its own weight and a control cylinder 21 are operated, the latter having its rod pivotedly mounted at the end of a lever 22, in its turn linked in 22′, causes the lever to swing (counter-clockwise rotation when the rod retracts, as illustrated in FIGS. 2, 3). A lever portion 23, protruding laterally, can advantageously fit into a corresponding recess of the outer wall of the annular support 7 to block the same at a given position during the operation. At the same time the opposite end 24 of lever 22 thrusts upwards an idle pin 25 driven to rotate by motor 11, brings the same to mesh with the lower end of shaft 9, which is thus caused to rotate. This way the motor 11 can continuously rotate, thus driving therewith the idle pin 25 which can drive the shaft 9 only when the cylinder 21 is operated. As already seen above, this determines at the same time the annular support 7 to stop at the correctly registered position and consequently the specimen 1 with respect to the disk 10 mounted on the shaft 9.

[0024] It will be noted that when a new specimen 1 begins to be worked, the counterweight 20 is the lowest in order not to balance the contact force due to the specimen weight and to render more efficient the surface finishing with the first abrasive disk 10 having coarser grain, like previously for the manual working according to the prior art. Subsequently disks of finer grain are brought against specimen 1 at the working station, possibly increasing the counterweight in 20 to reduce the abrasive force applied and the quantity of material removed, thus gradually obtaining the lowest roughness desired. Near the working end, when abrasive disks of slightest roughness are used, and substantially all the material in excess has been removed and the central useful zone of cylinder with reduced diameter is become longer to the cost of the end portions with larger cross-section, abrasive disks of larger diameter can be used to ensure the matching with the radiused end zones.

[0025] The necessary timing, controls of the rotation speeds of the test-piece specimen and/or the abrasive disks, as well as the movements of the annular support 7 to bring a subsequent disk 10′ to the working station are preferably controlled by means of a pre-settable computerized program.

[0026] Finally, it will be possible to complete the method according to the present invention with steps of final measurement of the roughness obtained (up to 0,1 μm) by providing at the same time the device as above described and illustrated with the necessary measuring means.

Claims

1. A method to obtain shaped steel specimens, as required in tests of tensile under corrosion tests to subject the same to the final surface finishing steps to bring them to the lowest values of roughness required, characterized by comprising the step of keeping in rotation about its longitudinal axis the steel specimen (1) already machined at its central portion (1a), to reach slightly larger sizes than the prescribed final size, by bringing the same to contact with a rotating sandpaper disk (10) in a plane tangent to the outer surface of the central length (1a) of said specimen (1), the contact being ensured by the specimen own weight.

2. A method according to claim 1, characterized in that the abrasive disk (10) used is centrally registered with respect to the cylindrical median length (1a), of smaller diameter, of the specimen (1), with its profile contacting the end zones at which said central length (1a) merges with the cylindrical end zones (1b) of larger diameter of the specimen itself.

3. A method according to claim 1 or 2, comprising successively further steps of surface finishing with abrasive disks (10) having finer and finer grain size.

4. A method according to anyone of the preceding claims, wherein the first step of surface finishing is carried out with a sandpaper disk having grain size from 80 to 250 GRIT, while for the subsequent working steps sandpaper disks are used having grain size from 250 to 1000 GRIT until reaching roughness values of less than 0.81 μm, preferably down to 0.1 μm.

5. A device for the surface finishing of steel specimens to be used in tests of tensile strength combined with corrosion, characterized by comprising a swinging arm support (2) of a specimen (1) rotatably held at the ends to rotate about is own longitudinal axis, there been provided a motor (5) for driving such a rotation, being mounted on the same support of a hinge fulcrum (3) of said swinging arm (2), the specimen (1) carrier end of which extends itself, with lowered arm, until a contact zone within an annular support (7) with the center on said fulcrum (3), substantially parallel to the longitudinal axis of rotation of specimen (1), said contact zone or working station being located between the inner periphery of said annular support (7) and an intermediate circumference (C) comprised between said inner periphery and the outer one, said circumference (C) having thereon, equally spaced apart from each other, a number of vertical seats (14), each of which is capable to house a shaft (9) for carrying an abrasive disk (10), whereby there is interference between the profile of disk (10) at the working station and said specimen (1) on its support (2) when the latter is lowered due to the own gravity force of said specimen.

6. A device according to claim 5, characterized by the fact of comprising driving means (12) for rotating, about a vertical central axis passing through the center of said circumference (C) of said annular support (7) with respect to said fulcrum (3) fixed on a support (4) mounted on a central support table (6) with respect to said outer annular support (7), also said motor (5) being mounted on that table (6).

7. A device according to claim 5 or 6, characterized by further comprising rotation driving means (11) mounted at a fixed position in correspondence with the said working station coincident with one of said seats (14) when brought, through the rotation of said support (7), in front of a specimen (1) on its swinging arm support (2).

8. A device according to claim 7, characterized in that means (22, 25) are provided to be driven by an actuating means (21) to transmit the rotation from said driving means (11) to a shaft (9) supporting an abrasive disk (10) and fitted in said seat (14) at the working station.

9. A device according to claim 8, characterized in that said actuating means (21) is made as a cylinder, the rod of which drives the end of a lever (22) the opposite end of which is capable of thrusting an idle pin (25) driven into rotation by said actuating means (11) to mesh the same with the said shaft (9) and drive the latter into rotation about its axis.

10. A device according to claim 9, characterized by further comprising on said lever (22) stop means (23) for blocking said annular support (7) with said shaft (9) at the exact position of working station at the same time of said shaft (9) being driven by said actuating means (21).

11. A device according to any of claims 6-10 characterized by comprising means (15, 15′) for adjusting the position, in its plane, of said support table (6) by means of small angle rotations in opposite directions about said central vertical axis.

12. A device according to any of claims 6-11, characterized in that said support table (6) is in turn mounted on base plates (17, 18) overlapping with an intermediate gap (26), one of which (17) can swing around a horizontal median axis coincident with the diameter of said circumference (C) passing through said working station, there being provided jack means (13) to cause such a swinging movement and a hinge means (19) between said two plates (17, 18), there being further provided springs (16, 16′) housed within said plate (17) in contact with the said support table (6) to automatically register the positioning of the latter on the base plates.

13. A device according to claim 5, characterized in that an adjustable counterweight (20) is provided at the end of the swinging arm support (2) which is opposite to the end on which the specimen (1) is rotatably mounted.