Patent ID: 12220765

DESCRIPTION OF EMBODIMENTS

An embodiment of a butt-welded joint of steel materials according to the present invention will be described in detail below with reference toFIG.1toFIG.7. As illustrated inFIG.1andFIG.2, in a butt-welded joint1of steel materials (hereinafter also referred to simply as a “joint1”), according to this embodiment, base materials2,2are a pair of identical steel materials formed to have a cylindrical shape, and end portions2a,2aof the base materials2,2are coupled to each other by a welded portion3. Specifically, the welded portion3is formed such that end faces2b,2bof the end portions2a,2aof the base materials2,2are abutted against each other (positioned to face each other) to be in contact with each other, and welding is performed thereon annularly in a manner such that the welded portion3extends from surfaces (outer circumferential surfaces)2c,2cof the base materials2,2to an inner portion thereof along the end faces2b,2b, which are in contact with each other, and straddles the end portions2a,2a.

To be more specific, the welded portion3is formed in a manner in which keyhole welding is performed annularly on the end portions2a,2aof the base materials2,2from the surfaces (outer circumferential surfaces)2c,2c, and thereafter, heat conduction welding is performed annularly from the surface, in an overlapping manner, on the portion on which the keyhole welding has been performed. In this instance, the keyhole welding and heat conduction welding are both performed with radiation of, for example, a beam7having a high power density, as illustrated inFIG.3(a)andFIG.3(b); here, an instance in which a laser7is used will be described. In the keyhole welding, heating (first heating) is performed with a laser7having a high power density, and, accordingly, a depression (keyhole) is formed in the end portions2a,2aof the base materials2,2. The laser7travels to an inner portion of the base materials2,2through the depression, and, accordingly, deeper welding can be accomplished. In this instance, the portion melted by the keyhole welding is solidified by subsequent cooling to form a melted and solidified portion3d, and a hardness thereof is higher than a hardness prior to the welding.

On the other hand, in the heat conduction welding, a laser7having a lower power density than that for the keyhole welding is used. In the heat conduction welding, the surfaces2c,2cand their vicinities of the melted and solidified portion3din the end portions2a,2aare reheated (second heating) and accordingly remelted and resolidified to form a remelted and solidified portion5, and, concurrently, an inner region relative to the remelted and solidified portion5(a region at a greater depth from the surfaces) is modified by the reheating, without involving melting, to form a reheated solidified portion4. Thus, as a result of the keyhole welding and the heat conduction welding, the welded portion3is formed to straddle the end portions2a,2aof the base materials2,2.

That is, the welded portion3is formed of the melted and solidified portion3d, the remelted and solidified portion5, and the reheated solidified portion4. The melted and solidified portion3dis a portion formed as a result of melting and solidification of the end portions2a,2aof the pair of base materials2,2, the melting being caused as a result of first heating (keyhole welding) from the surfaces2c,2c. The remelted and solidified portion5is a portion resulting from remelting and resolidification of the melted and solidified portion3d, the remelting being caused as a result of reheating (heat conduction welding) of the melted and solidified portion3dfrom the surfaces. The reheated solidified portion4is a portion that is formed in an inner region of the base materials2,2relative to the remelted and solidified portion5(a region at a greater depth from the surfaces), and which has a structure resulting from a change in a structure of the melted and solidified portion3d, the change being due to the reheating, the change involving no melting. In this instance, the structure of the reheated solidified portion4is a structure modified by tempering the structure of the melted and solidified portion3d, which has been martensitized by the keyhole welding, by performing the heat conduction welding, and, therefore, a hardness is lower than that of the melted and solidified portion3d, which results in improved toughness. On the other hand, a structure of the remelted and solidified portion5is a structure resulting from the remelting of the melted and solidified portion3d, which is caused by the heat conduction welding, and the resolidification thereof, which is caused by the subsequent cooling, and, therefore, a hardness is higher than that of the reheated solidified portion4.

Note that the melted and solidified portion3dis deepest at a center in a width direction thereof (which is the location indicated by the dash-dot line ofFIG.2and which, in this embodiment, substantially coincides with the location where the end faces2b,2bof the base materials2,2are in contact with each other). A depth from the surface3aof the welded portion3(i.e., a surface5aof the remelted and solidified portion5) to a deepest portion of the melted and solidified portion3dis d0. Furthermore, the remelted and solidified portion5is also deepest at a center in a width direction thereof (which substantially coincides with the center in the width direction of the melted and solidified portion3d). A depth from the surface3aof the welded portion3(the surface5aof the remelted and solidified portion5) to a deepest portion of the remelted and solidified portion5is d1, which is smaller than d0. That is, the welded portion3, the melted and solidified portion3d, and the remelted and solidified portion5are formed such that the centers in the width directions thereof substantially coincide with one another, with symmetries being substantially formed about the centers, in the width direction.

It is known that a general property of steel materials such as chromium-molybdenum steel and carbon steel for machine structural use is that in a case where a carbon concentration thereof (i.e., a carbon content, specifically, a mass percentage of the carbon present in a base material) is high, a hardness of the steel material is high whereas a toughness thereof is low, and, on the other hand, in a case where the carbon concentration is low, the hardness of the steel material is low whereas the toughness thereof is high. Accordingly, to improve the fatigue strength of the joint1, in which steel materials are base materials, it is necessary to ensure that the carbon concentration of the steel material is within a specified range so as to prevent an instance in which one of the hardness of the base material and the toughness thereof is low. Hence, herein, a carbon concentration (carbon content) of the base materials2,2as a whole is specified to be 0.1 mass % or greater and 0.35 mass % or less.

From the results of an experiment, which will be described later, it was observed that in a case where base materials2,2having a carbon concentration as described above are used, a butt-welded joint1having a higher rotating bending fatigue strength than that of the base materials2,2can be obtained, provided that a width W0 of the melted and solidified portion3d, the depth d0 of the melted and solidified portion3d, a width W1 of the remelted and solidified portion5, and the depth d1 of the remelted and solidified portion5satisfy the relationships of formula (1) and formula (2) below.
0.46W0≤W1  (1)
0.14d0≤d1≤0.73d0  (2)

Furthermore, portions of the surface5a(i.e., the surface3aof the welded portion3) of the remelted and solidified portion5, which is formed by performing keyhole welding and heat conduction welding in an overlapping manner, have residual stress generated therein. The residual stress is a compressive stress in a center region in the width direction of the remelted and solidified portion5(i.e., a center region in the width direction of the welded portion3) and is a tensile stress in a region outside of the center region in the width direction. As a result, cracks are inhibited from forming in the center and its vicinity in the width direction of the welded portion3, in the surface5aof the remelted and solidified portion5.

Furthermore, in both the keyhole welding and the heat conduction welding, the welding is carried out in a manner such that a starting portion of the weld and a terminal portion thereof (a solidification terminal portion6, shown inFIG.16andFIG.18), with respect to a circumferential direction, overlap each other at the same location. In the keyhole welding, a recess due to the radiation of the laser7is formed in the terminal portion at the completion of the keyhole welding (seeFIG.17). A depth (maximum depth) h of the recess from the surface of the welded portion can be reduced by, in the heat conduction welding, remelting and solidifying the welded portion resulting from the keyhole welding (i.e., the melted and solidified portion3d), and, consequently, concentration of the stress that acts on the solidification terminal portion6can be inhibited. In addition, from the results of an experiment, which will be described later, it is desirable that the depth h of the recess from the surface3aof the welded portion3and the depth d1 of the remelted and solidified portion5have the following relationship.
0.32d1≥h(3)

Note that in the joint1, the end faces2b,2bof the abutted base materials2,2have a circular shape, but the shape is not limited thereto, and it is sufficient that the surfaces2c,2cof the abutted base materials2,2are disposed substantially in the same plane; for example, the end faces2b,2bof base materials2,2to be abutted against each other may have substantially the same shape and the same size.

EXAMPLES

Now, first examples of the present invention (test conditions 2 to 7 in Table 1 and Table 2) and second examples thereof (test conditions 10, 12, 14, 16, and 18 in Table 3 and Table 4) will be described in comparison with first comparative examples (test conditions 1 and 8 in Table 1 and Table 2) and second comparative examples (test conditions 9, 11, 13, 15, and 17 in Table 3 and Table 4), respectively. Specimens8used in the first and second examples were prepared as follows: as illustrated inFIG.4toFIG.6, a sample10, which served as the base material2, in which a hollow cylindrical body portion9awas integrally formed with a hollow end portion9b, which was tapered toward a distal end from the body portion9a, were used; and end portions9bof a pair of the samples10were butt-welded together by keyhole welding and heat conduction welding as described above.

In the first examples, laser welding conditions for the heat conduction welding were varied to change a size of the remelted and solidified portion5, and thus, a variety of physical property values of the welded portion3, which accordingly changed, were measured with the specimens8and evaluated. On the other hand, in the second examples, the carbon concentration (carbon content) of the sample10was varied, and thus, a variety of physical property values of the welded portion3, which accordingly changed, were measured with the specimens8and evaluated.

Note that regarding the sample10used, a full length thereof was 80 mm, an outside diameter of the body portion9awas 20 mm, an outside diameter of the distal end face of the end portion9bwas 14 mm, and inside diameters of the body portion9aand the end portion9bwere 12 mm.

The sample10used in the first examples was made of chromium-molybdenum steel material (SCM415) and contained 0.13 mass % to 0.18 mass % C, 0.15 mass % to 0.35 mass % Si, 0.60 mass % to 0.90 mass % Mn, 0.030 mass % or less P, 0.030 mass % or less S, 0.25 mass % or less Ni, 0.90 mass % to 1.20 mass % Cr, and 0.15 mass % to 0.25 mass % Mo. In the first examples, under the conditions in which a laser output, a welding speed, a focal point diameter (spot diameter) for the keyhole welding were fixed (i.e., the width W0 and the depth d0 of the melted and solidified portion3dwere fixed), the laser output, the welding speed, and the focal point diameter (spot diameter) for the heat conduction welding were varied to change the width W1 and the depth d1 of the remelted and solidified portion5; accordingly, the residual stress of the surface3aof the welded portion3and its vicinity, an average hardness of the welded portion3, the recess depth h of the solidification terminal portion6of the welded portion3, and the rotating bending fatigue strength of the prepared specimens8were measured.

In the keyhole welding and the heat conduction welding, a fiber laser welding machine was used. By radiating a laser7by using the welding machine, welding of the sample10, which served as the base material2, was carried out. The switching between the keyhole welding and the heat conduction welding was carried out by moving a condenser lens of the welding machine in a direction of an axis L of the joint1, that is, in a direction perpendicular to an abutting direction, thereby changing the focal point diameter of the laser7, which was radiated onto the abutted portion of the end portions9b,9bof the pair of samples10,10(the contact portion of the end faces). In performing the keyhole welding, which required a higher power density, a laser7having a narrowed, small focal point diameter was used, as illustrated inFIG.3(a). On the other hand, in performing the heat conduction welding, in which the power density needed to be reduced compared with that of the laser7for the keyhole welding, a laser7having a focal point diameter larger than that for the keyhole welding was used, as illustrated inFIG.3(b).

Furthermore, the specimens8of the first comparative examples (Conditions 1 and Conditions 8) were prepared from a sample that had the same shape and size and was made of the same material (SCM415) as the sample10used in the first examples. In this instance, the specimen8for Conditions 1 was prepared by butt-welding a pair of the samples10together, exclusively by keyhole welding. On the other hand, the specimen8for Conditions 8 was prepared by butt-welding a pair of the samples10together, under welding conditions in which the width and the depth of the remelted and solidified portion5were reduced compared with those of the specimens8of the first examples 1, by changing the laser output, the welding speed, and the focal point diameter for the heat conduction welding that was performed after the keyhole welding. In the first comparative examples, for the prepared specimens8, the residual stress of the surface3aof the welded portion and its vicinity, an average hardness of the welded portion3, the recess depth of the solidification terminal portion6of the welded portion3, and the rotating bending fatigue strength were measured. The welding conditions and the measurement results of the first examples and the first comparative examples are shown in Table 1 and Table 2 below.

TABLE 1WELDING CONDITIONSCARBONKEYHOLE WELDING CONDITIONSREMELTINGCONTENTFOCALCONDITIONSOFLASERWELDINGPOINTLASERTESTMATERIALOUTPUTSPEEDDIAMETERSHIELDINGOUTPUTCONDITIONSwt %(W)(mm/s)(mm)GAS(W)COMPARATIVE1SCM415850500.5NITROGEN—EXAMPLECO. 15EXAMPLE2wt %850EXAMPLE3850EXAMPLE4850EXAMPLE5600EXAMPLE6REMELTING400EXAMPLE7CONDITIONS600COMPARATIVE8350EXAMPLESHAPE OF MELTED ANDSOLIDIFIED PORTIONMELTEDWELDING CONDITIONSANDREWELTEDREMELTING CONDITIONSSOLIDIFIEDANDFOCALPORTIONSOLIDIFIEDWELDINGPOINT(mm)PORTION (mm)SPEEDDIAMETERSHIELDINGWIDTHDEPTHWIDTHDEPTH(mm/s)(mm)GASW0d0W1d1COMPARATIVE——NITROGEN11——EXAMPLEEXAMPLE501.51.050.23EXAMPLE502.21.010.16EXAMPLE500.90.950.33EXAMPLE501.50.820.15EXAMPLE501.50.460.14EXAMPLE500.40.750.73COMPARATIVE2000.40.350.08EXAMPLE

TABLE 2PHYSICAL PROPERTIESAVERAGERECESS DEPTH OFFATIGUERESIDUALHARDNESSSOLIDIFICATIONSTRENGTHSTRESS(Hv)TERMINAL(BASETEST(MPa)POSITIONPOSITIONPORTIONMATERIAL:CONDITIONSA1A2A3{circle around (1)}{circle around (2)}(mm)260 MPa)EVALUATIONCOMPAR-1135288152461—0.14130xATIVEEXAMPLEEXAMPLE2−263178934603870.01360○EXAMPLE3−2182401254683720.05350○EXAMPLE4−2822051154433840.06300○EXAMPLE5−1002621204703950.03300○EXAMPLE6−2902931944693990.03290○EXAMPLE7−188250130469362—270○COMPAR-8−452701444984580.02160xATIVEEXAMPLE

As shown in Table 1, in all of Conditions 1 to 8, the welding conditions for the keyhole welding were such that a laser output was 850 W, a welding speed was 50 mm/s, a focal point diameter was 0.5 mm, and nitrogen was used as a shielding gas for shielding the welding site from air. Accordingly, melted and solidified portions3dhaving a width W0 of 1 mm and a depth d0 of 1 mm were formed. Furthermore, regarding Conditions 2 to 8, the welding conditions for the heat conduction welding were adjusted such that a laser output was 350 W to 850 W, a welding speed was 50 mm/s or 200 mm/s, a laser focal point diameter was 0.4 mm to 2.2 mm, and nitrogen was used as a shielding gas. Accordingly, remelted and solidified portions5having different widths W1 and depths d1 were formed.

The residual stress of the welded portion3and its vicinity in the surface were measured by using an X-ray stress measurement method in which an X-ray having a specific wavelength was radiated onto a surface of the specimens8. As illustrated inFIG.6, under Conditions 1 to Conditions 8, the residual stress was measured at each of three points, namely, a measurement point A1, which was located on the surface3aof the center in the width direction of the welded portion3; a measurement point A2, which was located 1.5 mm to the proximal end side (one end side of the specimens8) of the sample10from the measurement point A1; and a measurement point A3, which was located 1 mm further to the proximal end side from the measurement point A2. In this instance, under Conditions 1, the residual stress at the center point in the width direction of the melted and solidified portion3dwas measured at the measurement point A1, and the residual stress at points where the structure had not been changed by the welding was measured at the measurement point A2 and the measurement point A3. Furthermore, under Conditions 2 to Conditions 8, the residual stress at the center point in the width direction of the remelted and solidified portion5was measured at the measurement point A1, and the residual stress at points where the structure had not been changed by the welding was measured at the measurement point A2 and the measurement point A3.

The results demonstrated that under Conditions 2 to 8, the residual stress at the measurement point A1 was a negative value, that is, a compressive stress was present at and near the measurement point A1. Accordingly, under Conditions 2 to Conditions 8, the formation of cracks can be inhibited at and near the measurement point A1 of the remelted and solidified portion5. Furthermore, under Conditions 2 to Conditions 7, the residual stress at the measurement point A1 was less than or equal to −100 MPa. Under these conditions, as will be described later, the rotating bending fatigue strengths of the specimens8were higher than that of a specimen11for comparison (i.e., the base material itself), which had the same shape and size as the specimens8and were seamlessly integrally formed of the same material (SCM415) as that for the sample10. On the other hand, it was demonstrated that in the specimen8for Conditions 1, the residual stress at the measurement point A1 of the melted and solidified portion3dwas a positive value, and, therefore, a tensile stress was present at and near the measurement point A1. Accordingly, under Conditions 1, the formation of cracks at and near the measurement point A1 cannot be inhibited, and, moreover, the formation and propagation of cracks may be promoted.

Regarding the hardness of the specimens8, the Vickers hardness of the base material that included the reheated solidified portion4and the remelted and solidified portion5of the specimens8was measured and evaluated. For the measurement of the Vickers hardness, a typical Vickers' microhardness tester was used. The specimens8were cut along the axial direction, and, on the cut surface, the Vickers hardness was measured at an interval of 0.1 mm in a longitudinal direction (the lateral direction inFIG.8toFIG.15) and in a transverse direction (the vertical direction inFIG.8toFIG.15). As shown inFIG.9toFIG.15, the results indicated that under Conditions 2 to Conditions 8, an average Vickers hardness value of the reheated solidified portion4was lower than an average Vickers hardness value of the remelted and solidified portion5.

As shown inFIG.8, a measurement of the Vickers hardness associated with Conditions 1 revealed that the numerical value of the Vickers hardness of the melted and solidified portion3d, which resulted from melting by keyhole welding and solidification, was higher than the numerical values of the Picker's heights of other portions of the joint. Presumably, this is because the structure of the melted and solidified portion3dhad been martensitized by the keyhole welding. Furthermore, at the site of (0.1 mm, 0.7 mm) and the site of (0.2 mm, 0.5 mm), which corresponds to (vertical, lateral), shown inFIG.8, the Vickers hardness of the joint was 660 Hv, which was a very high numerical value compared with those of the other sites. Presumably, this is because the two sites were located at or near the boundary between the melted portion associated with the keyhole welding and the heat affected zone, which was hardened under the influence of the heating for the keyhole welding, and, therefore, a cooling rate after the keyhole welding was fast, and, consequently, the structure at and near the boundary was martensitized.

Furthermore, as shown inFIG.16toFIG.19, the recess depth h of the solidification terminal portion6of the welded portion3is a maximum height difference of a crater (recess), which was formed in a region onto which the laser was finally radiated when the base materials2,2were welded together. The recess depth h of the solidification terminal portion6was 0.14 mm under Conditions 1, in which keyhole welding was exclusively performed, whereas the recess depth h was 0.01 mm to 0.06 mm under Conditions 2 to Conditions 8, in which heat conduction welding was performed after keyhole welding. Furthermore, under Conditions 2 to Conditions 8, the recess depth h of the solidification terminal portion6of the remelted and solidified portion5and the depth d1 of the remelted and solidified portion5had the relationship of formula (3) mentioned above.

Hence, by performing heat conduction welding after keyhole welding, the recess depth h of the solidification terminal portion6can be reduced, and as a result, concentration of the stress that acts on the solidification terminal portion6can be inhibited.

Regarding a rotating bending fatigue test (ISO 1143:2010) for measuring the rotating bending fatigue strength, an ONO-type rotating-bending fatigue tester of the four-point loading type was used. In the tester, both ends of the specimen were held by the distal ends of a pair of spindles, and the load at which breakage occurred (i.e., a maximum value of the cyclic stresses that acted on a center portion (welded portion3) in the axial direction of the specimen8) in a case where rotation was performed 20 million times at a rotational speed of 2000 rpm was measured. Furthermore, to evaluate the measured rotating bending fatigue strength of the specimens8, the rotating bending fatigue strength of the above-mentioned specimen11for comparison was measured in a similar manner. The result was that under Conditions 1, the rotating bending fatigue strength of the specimen8was a value lower than that of the rotating bending fatigue strength of the specimen11for comparison. Presumably, this is because in a case where keyhole welding was exclusively performed, the structure of the welded portion was martensitized and, consequently, had a fragile construction.

Furthermore, in the cases of Conditions 2 to Conditions 7, that is, in the cases where the width W0 of the melted and solidified portion3d, the depth d0 of the melted and solidified portion3d, the width W1 of the remelted and solidified portion5, and the depth d1 of the remelted and solidified portion5simultaneously satisfied the relationships of formula (1) and formula (2) mentioned above, the rotating bending fatigue strengths of all the specimens8were higher than the rotating bending fatigue strength of the specimen11for comparison (that is, the rotating bending fatigue strength of the base material itself (base material strength). Presumably, this is because as a result of performing heat conduction welding in an overlapping manner on the portion on which keyhole welding had been performed, the reheated solidified portion4, which was formed in an inner region relative to the remelted and solidified portion5in the welded portion3(a region at a deeper location with respect to the surface3a), had a low hardness and, therefore, had a high toughness compared with the remelted and solidified portion5, which was formed in a region closer to the surface3aof the welded portion3, and, consequently, even if cracks had been formed in the surface3aof the welded portion3, the cracks would not have easily propagated to an inner portion. On the other hand, under Conditions 8, the rotating bending fatigue strength of specimen8was a value lower than that of the rotating bending fatigue strength of the specimen11for comparison. Presumably, this is because the energy density of the laser in performing the heat conduction welding was lower than those for the other conditions, and, consequently, the reheated solidified portion4was not formed deeply into an inner portion of the welded portion.

From the measurement results described above, it was determined that under Conditions 2 to Conditions 7, the fatigue strength was improved, because the rotating bending fatigue strengths of the specimens8, which were prepared by butt-welding together the samples10,10, which served as the base materials2,2, were higher than the rotating bending fatigue strength of the specimen11for comparison (base material itself), which was integrally formed of a single base material. Furthermore, it was determined that under Conditions 1 and Conditions 8, the fatigue strength was not improved, because the rotating bending fatigue strengths of the specimens8, which were formed by butt-welding together the pair of samples10,10, were lower than the rotating bending fatigue strength of the specimen11for comparison.

Now, the second examples of the present invention will be described in comparison with the second comparative examples. In the second examples, a sample10, which served as the base material2, was formed from carbon steel for machine structural use, and specimens12, which were obtained by butt-welding a pair of the samples10,10together by keyhole welding and heat conduction welding, were used. In this instance, the sample10used had the same shape and size as that used in the first examples. Furthermore, as the carbon steel for machine structural use that formed the sample10, S10C, S15C, S20C, S25C, and S35C were used. The S10C contained 0.15 mass % to 0.35 mass % Si, 0.30 mass % to 0.60 mass % Mn, 0.030 mass % or less P, 0.035 mass % or less S, and 0.08 mass % to 0.13 mass % C. The S15C contained the same mass percentages of Si, Mn, P, and S as the S10C and 0.13 mass % to 0.18 mass % C. The S20C contained the same mass percentages of Si, Mn, P, and S as the S10C and 0.18 mass % to 0.23 mass % C. The S25C contained the same mass percentages of Si, Mn, P, and S as the S10C and 0.22 mass % to 0.28 mass % C. The S35C contained the same mass percentages of Si, P, and S as the S10C, 0.60 mass % to 0.90 mass % Mn, and 0.32 mass % to 0.38 mass % C.

On the other hand, specimens12of the second comparative examples were prepared by butt-welding together, exclusively by keyhole welding, a pair of samples10,10, which had the same shape and size as the sample10used for each of the welding conditions of the second examples and were made of the same carbon steel for machine structural use. For each of the specimens12of the second examples and each of the specimens12of the second comparative examples, the residual stress of the surface3aof the welded portion3and its vicinity, the average hardness of the welded portion3, the recess depth h of the solidification terminal portion of the welded portion3, and the rotating bending fatigue strength were measured and evaluated. The welding conditions and the measurement results of the second examples and the second comparative examples are shown in Table 3 and Table 4 below. Note that regarding S45C, which contained the same mass percentages of Si, Mn, P, and S as the S35C and 0.42 mass % to 0.48 mass % C, cracks were formed in the welded portion at a stage when the melted portion resulting from the keyhole welding was solidified, and thus a tendency for cracking was exhibited; therefore, it was determined at this stage that the fatigue strength was evidently low, and, accordingly, the variety of measurements and associated evaluations were not conducted.

TABLE 3WELDING CONDITIONSCARBONKEYHOLE WELDING CONDITIONSREMELTINGCONTENTFOCALCONDITIONSOFLASERWELDINGPOINTSHIELDINLASERTESTMATERIALOUTPUTSPEEDDIAMETERGOUTPUTCONDITIONSwt %(W)(mm/s)(mm)GAS(W)COMPARATIVE9S10C850500.5NITROGEN—EXAMPLECO. 1 wt %EXAMPLE10850COMPARATIVE11S15C—EXAMPLECO. 15 wt %EXAMPLE12850COMPARATIVE13S20C—EXAMPLECO. 20 wt %EXAMPLE14850COMPARATIVE15S25C—EXAMPLECO. 25 wt %EXAMPLE16850COMPARATIVE17S35C—EXAMPLECO. 35 wt %EXAMPLE18850SHAPE OF MELTED ANDWELDING CONDITIONSSOLIDIFIED PORTIONREMELTING CONDITIONSMELTEDREMELTED ANDSOLIDIFIEDANDSOLIDIFIEDWELDINGPOINTPORTION (mm)PORTION (mm)SPEEDDIAMETERSHIELDINGWIDTHDEPTHWIDTHDEPTH(mm/s)(mm)GASW0d0W1d1COMPARATIVE——NITROGEN——EXAMPLEEXAMPLE501.51.050.23COMPARATIVE————EXAMPLEEXAMPLE501.51.050.23COMPARATIVE————EXAMPLEEXAMPLE501.51.050.23COMPARATIVE————EXAMPLEEXAMPLE501.51.050.23COMPARATIVE————EXAMPLEEXAMPLE501.51.050.23

TABLE 4PHYSICAL PROPERTIESRECESS DEPTH OFRESIDUALAVERAGESOLIDIFICATIONBASETESTSTRESSHARDNESS (Hv)TERMINALFATIGUEMATERIALCONDI-(MPa)POSITIONPOSITIONPORTIONSTRENGTHEVALU-STRENGTHTIONSA1A2A3{circle around (1)}{circle around (2)}(mm)(MPa)ATION(MPa)COMPARATIVE9−179139107379—0.1240○200EXAMPLEEXAMPLE10−209110953933530.05260○COMPARATIVE11−44237159460—0.1170x230EXAMPLEEXAMPLE12−3632111404553600.05260○COMPARATIVE13−20213139463—0.1180x260EXAMPLEEXAMPLE14−3222251304353650.05330○COMPARATIVE15−442581515430.1290○270EXAMPLEEXAMPLE16−3282741495604150.05310○COMPARATIVE17−18921395671—0.1370○330EXAMPLEEXAMPLE18−1652771346604530.05400○

The residual stress of the surface3aof the welded portion3was measured by using a measurement method similar to that for the first examples. In this instance, under Conditions 9, Conditions 11, Conditions 13, Conditions 15, and Conditions 17, which are for the comparative examples, the residual stress at the center point in the width direction of the melted and solidified portion3dwas measured at the measurement point A1, and the residual stress at points where the structure had not been changed by the welding was measured at the measurement point A2 and the measurement point A3. Furthermore, under Conditions 10, Conditions 12, Conditions 14, Conditions 16, and Conditions 18, which are for the examples, the residual stress at the center point in the width direction of the remelted and solidified portion5was measured at the measurement point A1, and the residual stress at points where the structure had not been changed by the welding was measured at the measurement point A2 and the measurement point A3.

The results demonstrated that under all of Conditions 9 to 18, the residual stresses at the measurement point A1 were negative values, that is, a compressive stress was present at and near the measurement point A1. Furthermore, the residual stresses at the measurement point A1 of the specimens12under Conditions 10, Conditions 12, Conditions 14, and Conditions 16, that is, the residual stresses at the measurement point A1 of the specimens12resulting from the heat conduction welding, which was performed after the keyhole welding, were negative values smaller than the residual stresses at the measurement point A1 of the specimens12under Conditions 9, Conditions 11, Conditions 13, and Conditions 15, respectively, that is, the residual stresses at the measurement point A1 of the specimens12resulting from the keyhole welding exclusively performed. Accordingly, under Conditions 10, Conditions 12, Conditions 14, and Conditions 16, in which heat conduction welding was performed after keyhole welding, the formation of cracks at and near the measurement point A1 can be inhibited to a further degree than under Conditions 9, Conditions 11, Conditions 13, and Conditions 15, in which keyhole welding was exclusively performed.

Note that in the cases of Conditions 17 and Conditions 18, that is, in the case where the specimen12was formed of S35C, the residual stress at the measurement point A1 in the example (Conditions 18), in which heat conduction welding was performed after keyhole welding, was a negative value slightly greater than the residual stress at the measurement point A1 in the comparative example (Conditions 17), in which keyhole welding was exclusively performed. However, considering the fact that the value was a negative value smaller than those of Conditions 13, Conditions 15, and Conditions 17, which are other comparative examples, inhibition of the formation of cracks at and near the measurement point A1 can also be expected for the example of Conditions 18, in which S35C was used, as with the other examples in which S10C to S25C were used.

Regarding the hardness of the joint1under Conditions 9 to Conditions 18, the Vickers hardness of the base material that included the reheated solidified portion4and the remelted and solidified portion5of the specimen12was measured and evaluated in a manner similar to that for Conditions 1 to Conditions 8 of the first examples. As shown inFIG.21,FIG.23,FIG.25,FIG.27, andFIG.29, the results were that under Conditions 10, Conditions 12, Conditions 14, Conditions 16, and Conditions 18, an average Vickers hardness value of the reheated solidified portion4was lower than an average Vickers hardness value of the remelted and solidified portion5. Furthermore, as shown inFIG.20,FIG.22,FIG.24,FIG.26, andFIG.28, under Conditions 9, Conditions 11, Conditions 13, Conditions 15, and Conditions 17, the Vickers hardness of the melted and solidified portion3dwas a numerical value higher than that of other portions of the joint that had not been melted by the keyhole welding.

As shown inFIG.27, under Conditions 16, at the site of (0.3 mm, 0.4 mm), which corresponds to (vertical, lateral), the hardness was 726 Hv, and at the site of (0.3 mm, 0.5 mm), the hardness was 655 Hv; the numerical values were very high compared with those of the other sites. Presumably, a reason that the Vickers hardness was high at the above-mentioned two sites is similar to a reason that a site having a high hardness was formed under Conditions 1. That is, presumably, this is because these two sites were located at or near the boundary between the melted portion associated with the keyhole welding and the heat affected zone, which was hardened under the influence of the heating for the keyhole welding, and, consequently, the structure at and near the boundary was martensitized.

Furthermore, the recess depth h of the solidification terminal portion6of the welded portion3was 0.1 mm under Conditions 9, Conditions 11, Conditions 13, Conditions 15, and Conditions 17, in which keyhole welding was exclusively performed, whereas the recess depth h was 0.01 mm to 0.06 mm under Conditions 10, Conditions 12, Conditions 14, Conditions 16, and Conditions 18, in which heat conduction welding was performed after keyhole welding. Furthermore, the numerical values of the recess depth h of the solidification terminal portion6of the remelted and solidified portion5and the depth d1 of the remelted and solidified portion5were 0.05 mm for h and 0.23 mm for d1 under all of Conditions 10, Conditions 12, Conditions 14, Conditions 16, and Conditions 18. Accordingly, the recess depth h of the solidification terminal portion6of the remelted and solidified portion5and the depth d1 of the remelted and solidified portion5had the relationship of formula (3) mentioned above. Hence, regarding the second examples, too, by performing heat conduction welding after keyhole welding, the recess depth h of the solidification terminal portion6can be reduced, and as a result, concentration of the stress that acts on the solidification terminal portion6can be inhibited.

Regarding the rotating bending fatigue strength, specimens12formed of the respective materials, S10C to S35C, were prepared, and, in a manner similar to that for the first examples, the specimens12were mounted to the ONO-type rotating-bending fatigue tester, and the load at which breakage occurred (i.e., a maximum value of the cyclic stresses that acted on a center portion (welded portion3) in the axial direction of the specimen12) in a case where rotation was performed 20 million times at a rotational speed of 2000 rpm was measured. Furthermore, to evaluate the measured rotating bending fatigue strength of the specimens12, the rotating bending fatigue test was similarly conducted on each of specimens13for comparison, which had the same shape and size as the specimens12and were seamlessly integrally formed of the respective materials, S10C to S35C, and thus, the rotating bending fatigue strength of the base material itself (base material strength) was measured.

The results were that in all of the specimens12of Conditions 10, Conditions 12, Conditions 14, Conditions 16, and Conditions 18, the width W0 of the melted and solidified portion3d, the depth d0 of the melted and solidified portion3d, the width W1 of the remelted and solidified portion5, and the depth d1 of the remelted and solidified portion5satisfied the relationships of formula (1) and formula (2) mentioned above, and thus, rotating bending fatigue strengths that were higher than those of the integrally formed samples13(i.e., the base material itself) were achieved. Presumably, this is because, as with the specimens8of the first examples, in the specimens12of the second examples, the reheated solidified portion4, which was formed in an inner region relative to the remelted and solidified portion5in the welded portion3, had a low hardness and, therefore, had a high toughness compared with the remelted and solidified portion5, which was formed in a region closer to the surface3aof the welded portion3, and, consequently, even if cracks had been formed in the surface3aof the welded portion3, the cracks would not have easily propagated to an inner portion.

From the measurement results described above, it was determined that under Conditions 10, Conditions 12, Conditions 14, Conditions 16, and Conditions 18, which were for the second examples, the fatigue strength was improved for all of the steel materials having a carbon concentration (carbon content) in a range of 0.1 mass % to 0.35 mass %, because the rotating bending fatigue strengths of the specimens12, which were prepared by butt-welding together the samples10,10, which served as the base materials2,2, were higher than the rotating bending fatigue strength of the specimen13for comparison (base material itself), which was integrally formed of a single base material. On the other hand, under Conditions 9, Conditions 15, and Conditions 17, which were for the second comparative examples, the rotating bending fatigue strengths of the specimens12were higher than the rotating bending fatigue strengths of specimen13for comparison, and, therefore, it can be determined that the fatigue strength was improved; however, in the cases of Conditions 11 and Conditions 13, the rotating bending fatigue strengths of the specimens12were lower than that of the specimen13for comparison, and, therefore, it cannot be said that the fatigue strength was improved.

Accordingly, regarding the specimens12obtained by exclusive keyhole welding, it cannot necessarily be said that the fatigue strength was improved for all of the steel materials having a carbon concentration (carbon content) in a range of 0.1 mass % to 0.35 mass %.

REFERENCE SIGNS LIST

1Butt-welded joint2Base material3Welded portion3dMelted and solidified portion4Reheated solidified portion5Remelted and solidified portion6Solidification terminal portion8,12Specimen10Sample11,13Specimen for comparison