JP2006297398A - Friction-welded member having excellent fatigue resistant property, and method for improving fatigue resistant property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a friction-welded member having an excellent fatigue resistant property, and further to provide a method for improving the fatigue resistant property. <P>SOLUTION: The friction-welded member is made of a steel material containing 0.1 to 0.8 mass% of C, 0.05 to 2.5 mass% of Si, 0.2 to 3 mass% of Mn, 0.005 to 0.1 mass% of Al, 0.001 to 0.02 mass% of N, and the remainder of Fe and unavoidable impurities, and has a tensile strength of 600 MPa or higher, wherein the compressive residual stress at the surface of a welded portion 3 is 50 to 90% of the tensile strength of the steel material. Further, the stress concentration in the welded portion is relaxed by reducing uneven portions and burrs in the welded portions by peening the surface of the welded portions of the steel material with an ultrasonic vibration terminal 5 vibrating at a frequency of 10 to 60 kHz with an amplitude of 0.3 to 50 μm. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a friction welded part having excellent fatigue resistance and a method for improving the fatigue characteristics thereof.

As a method of joining steel materials such as propeller shafts, friction welding method is used to join by joining the joining members and rotating one of them at high speed and pressing it using the diffusion of atoms generated between the joining surfaces. Is known, and is used when creating parts with parts that cannot be machined by joining or machining between different steel types.
This friction welding method is disclosed in Patent Document 1, for example.

A problem that arises when performing friction welding is that the burr produced by friction welding has a notch shape, and stress concentration is likely to occur and cracks tend to be generated, so that the fatigue strength of the joint is reduced.
In the above-mentioned Patent Document 1, a method for controlling the shape of the burr by regulating the inner diameter of the pipe to be joined and preventing the fatigue strength of the joined part from being lowered is disclosed.
However, in the invention described in Patent Document 1, since burrs themselves exist, there is a problem that stress concentration due to burrs is not lost and it is difficult to ensure sufficient fatigue strength.
JP 2004-141933 A

  An object of the present invention is to provide a friction welded part excellent in fatigue resistance and a method for improving the fatigue characteristics, which can solve the above-described problems of the prior art.

The present invention has been made as a result of intensive studies in order to solve the above-mentioned problems, and the gist of the present invention is the following contents as described in the claims.
(1) In mass%, C: 0.1-0.8%, Si: 0.05-2.5%, Mn: 0.2-3%, Al: 0.005-0.1%, N : Friction welding joint component comprising 0.001 to 0.02%, the balance consisting of Fe and inevitable impurities, and a member made of steel material having a tensile strength of 600 MPa or more, which is friction welding joined, A friction welding joint part having excellent fatigue resistance, wherein the compressive residual stress on the surface of the friction welding joint is 50 to 90% of the tensile strength of the steel material.
(2) The steel material is further mass%, Cr: 0.1-2%, Ni: 0.1-2%, Mo: 0.1-2%, Cu: 0.1-2%, Ti : 0.003 to 0.05%, V: 0.05 to 0.5%, Nb: 0.01 to 0.1%, or one or more of the above (1) Friction welded parts with excellent fatigue resistance as described in).
(3) Ultra-vibrating the surface of the joint part of a component obtained by friction welding the member made of the steel material having the composition described in (1) or (2) at a frequency of 10 to 60 kHz and an amplitude of 0.3 to 50 μm. A method for improving the fatigue characteristics of a friction-welded joint, wherein the stress concentration in the joint is reduced by striking with a sonic vibration terminal to reduce steps and burrs in the joint.

  According to the present invention, it is possible to provide a method for improving the fatigue characteristics of a friction welded part having high efficiency and excellent fatigue strength and a friction welded part, and as a result, the friction welded method is required to have fatigue strength. It can be applied to the joining of strength members, and there are significant industrially useful effects such as expansion of the application range of friction welding.

The best mode for carrying out the present invention will be described in detail with reference to FIGS.
FIG. 1 is a diagram illustrating a friction welding member in which materials to be joined are friction-welded. An upper part (a) of FIG. 1 shows a side view and a lower part (b) shows a perspective view thereof.
In FIG. 1, 1 and 2 are members to be joined, and 3 is a joint.
In the friction welding, a stepped portion G of about several tens of μm as shown in FIG. 1 is generated after joining due to a deviation in the butt portion when the materials 1 and 2 are joined or a difference in dimensional accuracy between the materials 1 and 2 to be joined. To do.
Thus, if there is a level difference G at the joint, stress concentration occurs when a load is applied to the friction welding member, and the fatigue strength is significantly reduced.
FIG. 2 is a side view illustrating the friction welding material when burrs are generated. Even when the step is a negligible size of 1 μm or less, fatigue strength is significantly reduced due to the presence of burrs.
In order to prevent this decrease in fatigue strength, two measures are conceivable: reducing the step G of the joint portion to eliminate the stress concentration portion, or introducing compressive residual stress to the surface portion of the joint portion. .

The present inventors have found that it is possible to satisfy both of the above-mentioned two measures by hitting a steel material with an ultrasonic vibration terminal that vibrates ultrasonically. That is, it has been found that the fatigue strength of the friction-welded parts can be significantly improved by applying a large compressive residual stress to the surface of the joint 3 and reducing the steps that become the stress concentration part and removing burrs.
Therefore, the present invention reduces the level difference at the joint by hitting the surface of the joint 3 of the parts subjected to friction welding with an ultrasonic vibration terminal that vibrates at a frequency of 10 to 60 kHz and an amplitude of 0.3 to 50 μm. The stress concentration in the joint is reduced by removing burrs.

3 and 4 are diagrams illustrating an embodiment of the method for improving the fatigue characteristics of a friction welded part according to the present invention. FIG. 3 illustrates reduction of a step and FIG. 4 illustrates removal of a burr. In FIGS. 3 and 4, the left side (a) shows before the hitting process, and the right side (b) shows after the hitting process.
3 and 4, 1 and 2 are members to be joined, 3 is a joint, 4 is a burr, and 5 is an ultrasonic vibration terminal.
As shown in FIG. 3, by striking the surface of the joint 3 of the friction welding by the ultrasonic vibration terminal 5, the stress concentration of the joint 3 is eliminated by reducing the step G of the joint 3, so that the friction welding is performed. The fatigue strength of joined parts can be greatly improved.
Further, as shown in FIG. 4, by hitting the surface of the joint 3 of the friction welding joint with the ultrasonic vibration terminal 5, the stress concentration of the joint 3 is reduced by crushing and removing the burrs of the joint 3. As a result, the fatigue strength of the friction welded parts can be greatly improved.
The present invention is also effective when steps and burrs are generated simultaneously.

The reasons for limiting the present invention will be described below.
The reason why the portion subjected to the ultrasonic hammering treatment is limited to the surface of the friction welded joint 3 is that the fatigue failure is the main problem in the friction welded joint because it is the joint, and in particular, stress concentration occurs. It is effective to perform an ultrasonic hitting process on an extremely narrow region such as a stepped portion and a burr portion.
The reason why the frequency of the ultrasonic vibration terminal 5 is limited to 10 to 60 kHz is that the compressive residual stress applied to the steel material increases in this region. Similarly, the amplitude of the tip of the pin that vibrates ultrasonically is limited to 0.3 μm or more because sufficient compressive residual stress cannot be applied to the steel material with an amplitude below this. Although the residual stress increases as the amplitude increases, the plastic deformation becomes too large at 50 μm or more, and the dimensional accuracy of the parts decreases and the fatigue strength also decreases. Therefore, the upper limit of the amplitude is limited to 50 μm.

The shape of the tip of the ultrasonic vibration terminal 5 is not limited, but a hemispherical shape, a bowl shape, a bowl shape, and the like are conceivable, but there is no particular limitation in the present invention. However, in the case of the hemispherical tip shape, when processing the joint portion of the round bar-round bar joint, the projections and the projections are brought together, so the processing may become unstable. The best is a bowl-like shape that combines a convex and a concave, but a convex pin as shown in FIG. 3 or FIG. 4 is preferable because the manufacturing cost of the ultrasonic vibration terminal increases.
As the curvature of the tip of the ultrasonic vibration terminal 4 increases, the stress concentration coefficient of the joint after completion of the processing decreases, but the processing takes time because the volume to be crushed increases. For this reason, the curvature is preferably 1 mm to 20 mm, but not particularly limited. When the curvature of the pin tip is greater than the step, the tip of the pin does not reach the bottom of the step at the start of processing, but as the processing proceeds, the shape of the step changes due to plastic deformation and becomes equal to the curvature of the pin tip.
When the material to be joined is a round bar as illustrated in FIG. 1, it is desirable to perform a striking process over the entire circumference of the joint.
The reasons for limiting the preferable ranges of the steel material component, strength, and residual stress constituting the friction welded part to be subjected to the impact treatment by the ultrasonic vibration terminal described above will be described below.

<Reason for limiting steel components>
C is an element effective for strengthening steel, but if it is less than 0.1%, sufficient strength cannot be obtained. On the other hand, if added excessively, toughness decreases, so the upper limit of the amount added is 0.8%.
Si is effective as a steel strengthening element, but less than 0.05% has no effect. On the other hand, if added in excess, the toughness and machinability deteriorate, so the upper limit of the amount added is 2.5%.
Mn is an element effective for strengthening steel, but if it is less than 0.2%, a sufficient effect cannot be obtained. On the other hand, if added in excess, the toughness and machinability deteriorate, so the upper limit of the amount added is 3%.
Al is an element effective for deoxidation of steel and refinement of crystal grains, but if it is less than 0.005%, there is no effect. On the other hand, if added in excess, the machinability decreases, so the upper limit of the amount added is 0.1%.
N is an element required for precipitation strengthening by generating V carbonitride or Nb carbonitride, but if it is less than 0.001%, a sufficient effect cannot be obtained. On the other hand, if added excessively, the toughness deteriorates, so the upper limit of the amount added is 0.02%.

Cr, Ni, Mo, and Cu are all elements that increase strength without impairing toughness when added in appropriate amounts. Cr, Ni, Mo and Cu are all less than 0.1%, and there is no effect, and if over 2%, the toughness is greatly deteriorated. Therefore, the lower limit of the addition amount is 0.1% and the upper limit is 2%. And
Ti is an effective element because it produces nitrides and carbides and increases the strength by precipitation strengthening. Furthermore, since the Ti nitride remains without dissolving at high temperatures, it is effective in preventing austenite coarsening during heating. If the content is less than 0.003%, these effects do not appear. If the content exceeds 0.05%, the toughness deteriorates, so the lower limit of the amount added is 0.003% and the upper limit is 0.05%.
V is also an effective element because it produces nitrides and carbides as well as Ti, and the strength is increased by precipitation strengthening. However, in order to enjoy the effect, addition of 0.05% or more is necessary. On the other hand, if added excessively, the toughness deteriorates, so the upper limit of the amount added is 0.5%.

Nb is also an effective element because it produces nitrides and carbides as well as Ti, and the strength is increased by precipitation strengthening. However, if it is less than 0.01%, sufficient effects cannot be obtained in order to enjoy the effect. On the other hand, if added excessively, toughness deteriorates, so the upper limit of the amount added is 0.1%.
In addition to these elements, Pb, S, Bi and the like may be added as elements for improving machinability, and such cases are also included in the present invention.
In addition to the above elements, unavoidable impurities such as P and S are included, but such cases are also included in the present invention.

<Tensile strength>
The reasons for limiting the tensile strength of the steel materials constituting the friction welded parts of the present invention will be described below.
In steel materials having a tensile strength of 600 MPa or less, a sufficient fatigue strength improvement effect cannot be obtained at 50%, which is the lower limit of the compressive residual stress of the joint, so the lower limit was set to 600 MPa.
<Residual stress>
The reason for limiting the residual stress in the joint portion of the steel material constituting the friction welding component of the present invention will be described below.
In a steel material having a strength of 600 MPa or more, a sufficient fatigue strength improvement cannot be observed with a compressive residual stress of 50% or less of the tensile strength, and imparting a compressive residual stress of 90% or more of the tensile strength, Since this is difficult in the present invention, the upper limit was made 90%.

Ono type rotary bending test pieces (JIS Z 2274, No. 1 test piece, symbol 1-6) were cut out from the steel components shown in Table 1, and cut at 1/2 L positions. The cut surface was mirror-polished, and one test piece was rotated at 2000 rpm with a nitrogen atmosphere and a surface pressure of 10 MPa, and bonded for 10 seconds. A step of 70 μm to 150 μm and a burr of 20 to 100 μm occurred at the joint of the test piece.
A specimen subjected to the ultrasonic treatment of the present invention and a comparative material subjected to no treatment or out-of-range treatment were prepared for this test piece, and an Ono-type rotary bending fatigue test was conducted to determine the fatigue strength. It shows in Table 2. The residual stress measurement values in Table 2 are obtained by separately preparing a test piece that has not been subjected to a fatigue test and measuring the residual stress of the joint surface layer. The residual stress is measured using X-rays, and the intensity of diffracted X-rays is measured and obtained from the half-value width of the peak intensity.
Further, the step G in Table 2 indicates the maximum value among the measured steps at five points obtained by equally dividing the test piece in the circumferential direction.
The sample without ultrasonic impact treatment has only obtained a fatigue strength that is less than ¼ of the tensile strength. This is because the residual stress in the vicinity of the friction welding joint is a tensile residual stress, and the steps and burrs generated by the friction welding joint reduce the fatigue strength. By performing an appropriate ultrasonic striking treatment, it is possible to eliminate steps and burrs and introduce a compressive residual stress to obtain a fatigue strength of about half the tensile strength.
From the above, it has been found that the present invention is effective when a significant improvement in fatigue strength is observed as compared with the comparative material.

It is a figure which illustrates about the level | step difference which arises when a to-be-joined material is friction-welded, (a) is a side view, (b) is a perspective view. It is a figure which illustrates about the burr | flash produced when the to-be-joined material is friction-welded. In this invention, it is a figure which illustrates embodiment of the fatigue-characteristics improvement method of friction welding components by reducing a level | step difference, (a) shows before impact processing, (b) shows after impact processing. In this invention, it is a figure which illustrates embodiment of the fatigue characteristic improvement method of friction welding components by removing a burr | flash, (a) shows before impact processing, (b) shows after impact processing.

Explanation of symbols

1, 2 To-be-joined member 3 Joining part 4 Burr 5 Ultrasonic vibration terminal G Step difference in joining part

Claims (3)

% By mass
C: 0.1-0.8%
Si: 0.05 to 2.5%,
Mn: 0.2-3%,
Al: 0.005 to 0.1%,
N: 0.001 to 0.02%
A friction welding joint part obtained by friction welding welding a member made of a steel material having a balance of Fe and inevitable impurities and having a tensile strength of 600 MPa or more, and compressive residual on the surface of the friction welding joint A friction welding component having excellent fatigue resistance, wherein the stress is 50 to 90% of the tensile strength of the steel material. The steel material is further in mass%,
Cr: 0.1 to 2%,
Ni: 0.1 to 2%,
Mo: 0.1 to 2%,
Cu: 0.1 to 2%,
Ti: 0.003 to 0.05%,
V: 0.05-0.5%
Nb: 0.01 to 0.1%
The friction-welded part having excellent fatigue resistance according to claim 1, comprising one or more of the following.   The surface of the joint portion of a component obtained by friction welding the steel member having the component composition according to claim 1 or 2 is hit with an ultrasonic vibration terminal that vibrates at a frequency of 10 to 60 kHz and an amplitude of 0.3 to 50 μm. A method for improving the fatigue characteristics of a friction welded part, characterized in that stress concentration in the joint is reduced by reducing steps and burrs in the joint.

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