JP6668818B2 - Steel pipe torsion fatigue test method and specimen used for it - Google Patents

Description

  The present invention relates to a torsional fatigue test method for a hardened steel pipe base material such as an electric resistance welded steel pipe and a seamless steel pipe, and a test body used for the method.

  When a steel pipe is used for a mechanical structure, it may be used for a member to which a load is repeatedly applied by a torque in a rotating direction, and it is important to grasp the torsional fatigue characteristics of the steel pipe. In general, torsion fatigue characteristics are tested by welding a flange member to a pipe end, bolting the flange member to a test device, and repeatedly forcing the pipe end to apply a rotational force. At this time, in order to properly evaluate the base material, a portion having lower strength than the base material must be eliminated. This is because the applied stress concentrates on the low-strength portion, and the low-strength portion breaks before the base material breaks.

  During the torsional fatigue test, the pipe end and the flange jig must be smoothly joined so that there is no unevenness on the surface, but the steel pipe is joined by a method other than welding. In this case, stress concentrates at the end of the bolt hole, and fatigue fracture occurs from there. Therefore, in the torsional fatigue test, it is essential to weld the pipe end and the flange jig, but since welding always involves softening due to heat effect (HAZ), there is a problem that a lower strength part is generated than the base material. Was. This welding process results in a heat history equivalent to annealing for the base metal, and particularly in the case of a high-strength quenched steel pipe under development in recent years, the strength of the softened part is significantly lower than that of the base metal. Did not.

  Conventionally, as described in Patent Document 1, when a steel pipe is connected to a test apparatus bracket during a torsional fatigue test, the steel pipe is inserted into a bracket hole having an inner diameter of at most 0.6 mm smaller than the outer diameter of the steel pipe and then welded, so that compression residual due to external pressure is reduced. There has been proposed a method for evaluating a torsional fatigue characteristic by increasing a frictional force accompanying a stress. However, even in this method, welding is indispensable, and in the case of a high-strength hardened steel pipe in which a portion significantly softened by welding is generated, the application is difficult because the welded portion has almost the same diameter and breaks from the softened portion. Although a method of expanding the pipe end and joining it to a flange is also being studied, there is a possibility that even if a softened portion due to welding occurs, it can be covered with a large diameter. However, it is not technically easy to expand a high-strength material to a predetermined diameter, and there is a great concern that buckling or cracking will occur at the time of expanding the tube. In particular, it was difficult to uniformly expand a thick material of 5 mm or more, which was an effective method for relatively thin materials. In addition, the conventional expansion described here is a process of expanding the outer diameter and the inner diameter larger than the raw pipe, and does not control the wall thickness except for the thinning accompanying the expansion, that is, measures to increase the wall thickness. Means a machining process that is not performed.

  Conventionally, a torsional fatigue test cannot be performed on a single steel pipe due to such a problem, and the test has to be performed with the steel pipe incorporated in a mechanical structure. However, it takes time and cost to prepare test specimens, which requires special fastening jigs that depend on various shapes.In addition, since parts are designed without evaluating the fatigue characteristics of the steel pipe itself, Investigations tended to be insufficient, leading to delays in development, such as the need to return to design after the torsional fatigue test. From such a background, a method for evaluating the torsional fatigue characteristics of the base material of the steel pipe itself has been strongly desired.

JP-A-7-112273

  An object of the present invention based on the above-described problems is to provide a method for evaluating the torsion of a steel pipe base material itself and a test body used for the method, by appropriately joining even a high-strength hardened steel pipe to a flange member. .

The torsional fatigue test method of the present invention made in order to solve the above-mentioned problem uses a mold heated at both ends of a torsion test body made of a quenched high-strength steel pipe to a point A3 or more of the steel component. by upsetting the inner diameter than the steel base metal evaluating quenched after having the outer diameter larger then the outside diameter enlarged section without the tube expansion, integrated in the flange torsional fatigue testing machine after the entire circumference joined with joining flange member A torsional fatigue test is performed.

It is preferable that the enlarged-diameter portion is manufactured so that the external-magnification ratio η ≧ 5.5% and the length L _E ≧ 15 mm of the enlarged-diameter portion with respect to the range subjected to the torsion test.

The torsion test body of the present invention made in order to solve the above-mentioned problem is a method in which both ends of a torsion test body made of a quenched high-strength steel pipe are installed by using a mold heated to A3 point or more of the steel component. In this method, the outer diameter is increased without increasing the inner diameter from the steel pipe base material to be evaluated, and then the outer diameter is increased, and then rapidly cooled, and the entire circumference is joined to the joining flange member .

It is preferable that the outer diameter enlarged portion has an outer diameter expansion ratio η ≧ 5.5% and a length L _E ≧ 15 mm of the outer diameter enlarged portion at the end with respect to a range subjected to a torsion test.

  According to the torsional fatigue test method of the present invention and the test specimen used in the method, a torsional fatigue test of the base material itself, which could not be realized conventionally, can be performed, and the characteristics can be grasped before designing the parts, so that the development resources can be greatly reduced. It becomes possible.

It is a figure which shows the structural example of a test body, the flange member for joining, and a testing machine. It is a graph which shows an example of the relationship between an outer diameter expansion ratio and Mises equivalent stress which arises in a tube outer surface. It is a graph which shows an example of the hardness distribution from the welding part after welding a test body with the joining flange member. It is a figure showing an example of upsetting.

Hereinafter, embodiments of the present invention will be described.
The properties of the welded portion depend on the strength of the base material after quenching, and a high-strength material inevitably involves softening of the welded portion. On the other hand, since the torsional rigidity is proportional to the fourth power of the diameter of the base tube, it is processed into a shape in which the outer diameter and the wall thickness are larger than a certain length at both ends of the base material, which is assumed to be softened, and the joining flange member is formed. To join. Thus, the outer diameter of the softened portion is effectively increased to increase the torsional rigidity and prevent the fatigue fracture, whereby the torsional fatigue evaluation of the base metal portion can be realized.

FIG. 1 shows the structure of a torsional fatigue test specimen. The torsion tester 10 includes a rotating motor 11 and joining flange members 12 and 13 joined to an end of the test sample 1. FIG. 1 shows a state in which the rotating motor 11 is connected to one (left) joining flange member 12 and the other (right) joining flange member 13 is fixed. An example of adding a torsional torque) is described. Here, twisting from a stationary state to a fixed angle and returning to the original stationary state is called a pulsating test, twisting from a stationary state to a fixed angle, passing through the stationary state and a similar constant angle in the opposite direction. The case of returning to a stationary state by twisting is called a swing test. The present invention is applicable to any of these. In the case where a rotating motor is connected to each of the joining flange members 12 and 13 for the purpose of an acceleration test or the like, and a load torque (torsion torque) is applied to both ends of the test body 1 in opposite directions. The present invention can be applied to any of them. Incidentally, the outer diameter OD ₁ of the target portion of the torsion test, to connect smoothly the outer diameter enlarged portion outer diameter OD ₂ of the EDP, provided a curvature portion like ER in FIG. 1, the outer shape changes smoothly It is preferable to do so.

In order to test the specimen 1, when the joining flange members 12 and 13 to be incorporated into the fatigue test apparatus and the specimen 1 are joined by welding or the like, the steel pipe is softened in the heat-affected zone as described above. for softening unit is not a whole weak portion (1) outside diameter enlargement ratio η and, above HAZ length of (2) it is preferable to set the outer diameter enlarging director of L _E. Hereinafter, a case of a hardened high-strength steel pipe of 150 ksi class, φ31.8 × t1.6 will be described as an example.
(1) Outer diameter expansion rate η
The material whose heat affected zone is softened is required to have a tube end outer diameter that can secure torsional rigidity that does not cause plastic deformation even under the above-mentioned load torque. When the outer diameter of the target portion of the torsion test is OD ₁ and the outer diameter of the outer diameter enlarged portion EDP is OD ₂ , the outer diameter expansion rate η is
η = (OD ₂ −OD ₁ ) / OD ₁
Is defined. In the torsional fatigue test, since the generated stress on the outer surface of the tube is the highest, the Mises equivalent stress σm of the outer surface of the tube caused by the load torque is set to, for example, in this example, the yield strength of the heat-affected zone to 500 MPa or less. It is preferable to increase the outer diameter by 5.5% with respect to the base tube diameter of 31.8 mm (FIG. 2). In this case, a pipe end outer diameter of φ34.0 mm or more is appropriate. The outer diameter of the joining flange member is preferably an outer diameter enlarged portion is EDP outer diameter OD ₂ or more, and more preferably the same.
(2) outside diameter-large director of L _E
At the time of joining with the joining flange member, for example, in the case of welding or the like, it is required that a portion softened by the influence of heat be covered by the outer diameter enlarged portion. Although in this example is about 10mm length of the softened portion caused by thermal influence (Fig. 3), when used in automotive structural parts, and components may also be more than 10mm, defining an outer diameter enlarging director of the L _E However, it is preferable that L _E ≧ 15 mm. In addition, in order to shorten the length of the softened portion due to heat, it is more preferable that the joining flange member is made of the same material as the evaluation steel pipe or that the joining method employs friction welding.

Based on these indicators, eta> 0 and and that the diameter OD ₂ of Mises equivalent stress is greater than the evaluation matrix of the surface, L _E> designed to be a length of the heat affected zone from the pipe end by bonding By doing so, a preferable torsional fatigue test can be realized.

To manufacture such a test body, there is a method of upsetting a pipe end. This consideration of the outer diameter and wall thickness and test conditions of the test body, the η and L _E required for specimen was calculated, previously manufactured mold in advance篏合. The test body is heated at both ends or one end at a time, fitted into a mold, and formed into a shape along the mold. However, when the specimen is a quenched type high-strength steel pipe, it is desirable to heat the end to a point A3 or more of the specimen component and to rapidly cool after forming. FIG. 4 shows an example of upsetting.

  In the present invention, the inner diameter is not enlarged, but it is assumed that the inner diameter is slightly increased by the above-described processing method. In this case, an inner diameter expansion rate of about 20% (｛(inner diameter after processing−inner diameter after processing) / inner diameter before processing｝ × 100 (%)) is allowable.

The test body manufactured in this manner is joined to a flange member and incorporated into a flange torsion fatigue tester. Examples of the joining method include, but are not limited to, friction welding and arc welding. Since the friction welding is a solid-phase diffusion, since the temperature of the joint surface is how the melting temperature of the steel material, the heat affected zone is short, on which can be a relatively small L _E, that can be joined regardless of the thickness There are features. Although in arc welding is possible joining between the test body and the flange member, not must take large relatively L _E, there are cases where a homogeneous junction is difficult by the thickness. In the case of welding such as arc welding, it is preferable to continuously weld the entire circumference. However, any state may be used as long as it can withstand the torque of the test condition even intermittently. For example, 50% or more of the entire circumference is welded. It is preferable if it is.
Hereinafter, examples of the present invention will be described.

  In the present embodiment, a case where welding is used as an example for joining the test piece and the joining flange member will be described. Here, a torsional fatigue test of a hardened high-strength steel pipe of 150 ksi class, φ31.8 × t1.6 is taken as an example. This steel pipe has a yield strength of 1000MPa and a tensile strength of 1200MPa in the quenched part, a yield strength of 500MPa and a tensile strength of 650MPa in the unquenched part. The fatigue test condition is a load torque of 800 N · m assuming a torsional fatigue test of a member for an automobile.

As a reference example of making a test specimen by shaving, two types of single-diameter, single-wall steel pipes with an outer diameter of φ3 × t2.7 × 400 mmL and φ34 × t9.0 × 400 mmL from an ingot manufactured by laboratory melting. By shaving, the evaluation base material portions were processed to φ33.8 × t2.7 and φ33.8 × t9.0, respectively. Thereafter, the welded portion with the joining flange member was machined so as to have an outer diameter of φ31.8 except for both pipe ends of 15 mm each so as to satisfy the above-described conditions of the pipe expansion ratio and the outer diameter enlarged portion length. In order to smoothly connect the outer diameter OD _{1 of the} portion to be subjected to the torsion test to the outer diameter OD ₂ of the outer diameter enlarged portion EDP, a curvature of about ₂ mmR was provided so that the outer shape changed smoothly. This is to suppress stress concentration at the time of a torsional fatigue test due to an extreme shape change.

  As an example of upsetting, two types of single pipes with outer diameters of φ31.8 × t1.6 × 420mmL and φ31.8 × t8.0 × 400mmL prepared by electric resistance welding are upset, and both ends are Each 15 mm was processed to φ33.8 × t2.7 and φ33.8 × t9.0. Although a mold is required for the upsetting process, a region having a smaller diameter than the end of the tube is a mold of a bamboo split type which is tightly adhered by applying a bolt or a high load. The tube end is preheated at about 1000 ° C, and molded by bringing it into close contact with the fitting die. At this time, both ends may be heated and molded simultaneously, or one end may be molded at a time. In the present example, a quenched high-strength steel pipe was targeted, so that the test specimen was put into water immediately after forming and rapidly cooled. As a result, the strength of the test specimen in the upsetting process became substantially uniform.

On the other hand, the joining flange member was made of a material of SCM435, and was formed as an integrated processed product having a joining flange member formed at the opposite end for connection with a test apparatus with 12 bolts. These joining flange members were subjected to friction welding (remaining heat pressure: 2 kg / mm ² , remaining heat time: 5 seconds, friction pressure: 5.0 kg / mm ² , frictional allowance: 3 mm, upset pressure: 10 kg / mm ² , upset time) : 5 seconds, rotation speed: 1000 rpm, and twist: 5 mm), or by performing full-circle arc welding, and joining with a test piece, and incorporating the test piece into a torsional fatigue test of the test piece.

  In the comparative test shown in Table 1, a case 1 in which the steel pipe of φ31.8 × t1.6 was not expanded, and a case where the outer diameter expansion ratio was set to 3% which is 5.5% or less which is necessary for calculation by the same processing as above. Cases 2 and 3 and cases 4 and 5 where the length of the outer diameter enlarged portion was 7.5 mm including the heat-affected zone at the time of joining were performed. As a result, the case 1 that was not expanded was damaged by the HAZ softened portion generated at the time of joining with the flange earlier than the evaluation base material was damaged during the torsional fatigue test. In cases 2 to 5, although the occurrence of microcracks was observed on the surface of the softened portion of the HAZ, the test itself was able to perform a normal torsional fatigue test. On the other hand, for a φ31.8 × t1.6 steel pipe, in the case of satisfying the specified outer diameter expansion ratio and the outer diameter expanded part length, the evaluation base material part was broken first regardless of the processing method and joining method. The normal torsional fatigue test was completed.

  For a thicker φ31.8 × t8.0 steel pipe, we attempted to expand the outer diameter φ31.8 → φ33.8 and inner diameter φ19.8 → φ21.0 using a plug as a comparative example. Small cracks occurred on the outer surface, and the fatigue test was not performed. This is because the torsional stress was concentrated at the site of the outer surface crack and was broken earlier than the evaluation base material. On the other hand, the sample prepared by cutting and upsetting did not have any processing problems, and the torsional fatigue test was normally performed.

  Table 1 shows the test results. Based on whether plastic deformation or cracking occurs in a region other than the region subjected to the torsional fatigue test of the test specimen, the quality of the result is visually judged.

1: specimen 10: tester 11: rotating motor 12, 13: bonding flange member ER: curvature portion EDP: outer diameter enlarged section OD _1: outer diameter of the target portion of the torsion test _OD 2: EDP outer diameter
L _E : Length of enlarged outer diameter part

Claims (4)

The both ends of the torsion specimen made of a quenched high-strength steel pipe are subjected to upsetting using a mold heated to the A3 point or more of the steel component without expanding the inner diameter from the steel pipe base material to be evaluated. A torsion fatigue test method for a steel pipe, characterized in that the outer diameter is increased, the diameter is increased, the pipe is quenched, and the entire circumference is joined to a joining flange member. 2. The steel pipe according to claim 1, wherein the outer diameter enlarged portion is manufactured with an outer diameter expansion ratio η ≧ 5.5% and a length L _E ≧ 15 mm of an outer diameter enlarged portion with respect to a range to be subjected to a torsion test. 3. Torsional fatigue test method. The both ends of the torsion specimen made of a quenched high-strength steel pipe are subjected to upsetting using a mold heated to the A3 point or more of the steel component without expanding the inner diameter from the steel pipe base material to be evaluated. A specimen for torsion fatigue test of a steel pipe, characterized in that the diameter is increased and an outer diameter is enlarged, which is quenched, and is joined to a joining flange member all around. With respect to the range where the outer diameter enlarged portion is subjected to the torsion test, the outer diameter expansion ratio η ≧ 5.5%, the length L of the outer diameter enlarged portion is _E The test body for torsion fatigue test of a steel pipe according to claim 3, characterized in that the test body is manufactured at ≧ 15 mm.