JPH06262385A - Improving method for fatigue strength of end part of weld by local quenching - Google Patents

Abstract

PURPOSE:To inexpensively improve the fatigue strength of an end part of weld only by adding a simple process at the time of welding. CONSTITUTION:At the time of welding a steel material, when the temperature of the surface of the end part of weld is >=200 deg.C and <=600 deg.C after welding, the surface of the end part of weld is cooled to <200 deg.C at >10 deg.C/sec. an average cooling speed by injecting refrigerant on the surface of the end part of weld. Consequently, a residual compressive stress generates on the end part of weld, and the fatigue strength of a welding member is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welding method for manufacturing a steel structure, and more particularly to a method for improving fatigue strength at a weld toe by local quenching.

[0002]

2. Description of the Related Art Fatigue failure of welded structures is caused by welding joints,
Of these, fatigue cracks occur at the weld toe where structural stress concentration and tensile residual stress exist, and most of these are caused by propagation of the cracks. Therefore, it is necessary to improve the fatigue strength of the welded joint in order to improve the fatigue strength of the welded structure, and several methods for treating the weld toe have been proposed for the purpose of improving the fatigue strength. The concept of the treatment of the weld toes is to alleviate the stress concentration that is the cause of the fatigue strength reduction of the above weld toes,
It can be roughly divided into two types for the purpose of controlling residual stress.

On the other hand, the toe treatment for the purpose of alleviating stress concentration is performed by cutting or grinding the weld toe with a grinder or the like, and by remelting the weld toe with TIG. Processing, a treatment for smoothing by remelting with plasma (Japanese Patent Publication No. 54-30386), a treatment for obtaining a smooth toe portion using a decorative welding rod, and the like.

On the other hand, as the toe treatment for the purpose of controlling the residual stress, an excessive load is applied in advance to generate a tensile stress exceeding the yield stress at the weld toe, and the residual stress of compression is removed after unloading. Preloading treatment to give, compressive residual stress by heating the entire joint and then quenching, rapid cooling treatment, peening to mechanically give compressive residual stress by hitting the weld toe with a wire, ball, etc. Methods such as processing are known.

Among the methods for imparting compressive residual stress, as a method for improving the fatigue strength of a welded joint using rapid cooling, a method of rapidly cooling the joint in water immediately after the molten zinc bath during galvanization of the joint is known. ing. This is to generate a compressive residual stress on the surface due to the temperature difference between the surface and the inside which is generated by putting the joint, which has been heated to a high temperature by the molten zinc bath, into water and rapidly cooling it.

Further, in fields other than steel structures, the welded joints of rails are disclosed in JP-A-3-277720, and the piping is disclosed in JP-A-56-130432. By this, a method for imparting compressive residual stress and improving fatigue strength is disclosed.

[0007]

All of the above-mentioned methods for treating the weld toe for the purpose of improving the fatigue strength have the additional process after the welding is completed, and have the drawback of increasing the cost. Further, regarding the application of the compressive residual stress by the preload, since the load for generating the stress equal to or higher than the yield stress must be applied to the joint part, there is a limit to the applicable scale of the welded joint.

Regarding the quenching treatment after plating, the application is limited to those requiring plating on joints, and not applicable to all joints.

Japanese Unexamined Patent Publication (Kokai) No. 3-277720 is limited to butt welding of rails and is not intended to improve the fatigue strength of welded joints of steel structures.
Further, since the cooling temperature is slower than that of the method of the present invention, the temperature at the start of cooling for imparting sufficient compressive residual stress is set to 500 ° C. or higher. There is a need to.

Japanese Unexamined Patent Publication No. 56-130432 relates to a welded portion of a pipe, and is not intended to improve the fatigue strength of a steel structure. Further, it requires local reheating, and the method is complicated.

The present invention is intended to inexpensively improve the fatigue strength of the weld toe portion only by adding a simple process at the time of welding.

[0012]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention rapidly cools a weld toe to generate a distribution of thermal stress on the surface and the inside, and after the natural cooling, compressive residual stress is generated on the surface. It is characterized in that the joint fatigue strength is improved by the generation. That is, the gist of the present invention is that, when welding a steel material, when the temperature of the weld toe surface after welding is 200 ° C. or more and 600 ° C. or less, by injecting a refrigerant onto the weld toe surface. Weld toe surface 10 ° C / sec up to a temperature of 200 ° C or less. There is a method for improving the fatigue strength of a weld toe by local cooling, which is characterized by cooling at an average cooling rate of. Here, the average cooling rate is defined as a value obtained by dividing the temperature difference of the weld toe surface at the start of cooling by the coolant and at the end of cooling by the cooling time.

[0013]

The present invention will be described in detail below.

After welding, the toe temperature is 200 ° C. or higher 600
The method of improving the fatigue strength by rapidly cooling the surface of the weld toe portion at a temperature of not higher than 0 ° C. is classified into the methods of imparting residual compressive stress among the methods of treating the weld toe portion described above.

Since the stress actually acting on the member is the sum of the internal stress such as residual stress and the external stress generated by the external force, under the repeated load in which the external force changes periodically,
Internal stress such as residual stress acts as an average stress. For this reason, the residual stress of compression lowers the stress ratio of the stress actually acting on the member, thereby improving the fatigue strength. For example, when the minimum stress during repetition is 0, that is, when the load with a stress ratio of 0 is applied, and the residual compressive stress is about 1/2 of the maximum value of the repeated stress, the stress ratio of the stress actually acting on the member is -1, which is 2 × 10 ^6.
Fatigue strength increases about 1.5 times as compared with the case where no residual stress exists.

The mechanism by which the residual compressive stress having the effect of improving fatigue strength is imparted to the weld toe by the method of the present invention will be described below.

When the surface of the weld toe, which has been heated to a high temperature by welding, is rapidly cooled by the injection of a refrigerant, a temperature difference occurs in which the surface has a low temperature and the inside has a high temperature. Due to this temperature difference, a distribution of thermal stress is generated, in which the surface is pulled and the inside is compressed. The tensile stress generated on the surface of the weld toe portion due to this thermal stress exceeds the tensile yield stress if the temperature difference between the surface and the inside is sufficiently large, and causes tensile plastic strain. After that, when it is naturally cooled and the whole is brought to a normal temperature state, the tensile plastic strain region is constrained by the surroundings, and a compressive residual stress is generated on the surface of the weld toe portion.

In order to generate a tensile stress equal to or higher than the yield stress on the toe surface due to the above-described action, it is necessary to cause a temperature difference of a certain amount or more between the surface and the inside of the weld toe portion. At this time, the surface temperature of the toe portion needs to be 200 ° C. or higher. Further, if the cooling rate is slow, the entire joint is cooled and the effect becomes small. Therefore, in order to effectively cause a temperature difference above a certain level between the surface and the inside, the average cooling rate is 10 ° C / sec. It is necessary to be above.

Further, the toe temperature at the start of refrigerant injection is 60
If the temperature is 0 ° C. or higher, the surface temperature rises due to the heat inside the welded portion after quenching, and the effect of the present invention is not exerted. Therefore, the toe temperature at the start of refrigerant injection needs to be 600 ° C. or lower.

Regarding the refrigerant, 10 ° C./sec. If the above average cooling rate can be secured, liquid nitrogen, liquid argon,
Anything such as water may be used, but it must be nonflammable or explosive as a matter of course. Also, it is desirable to avoid those that adversely affect the steel material.

[0021]

[Examples] For a cross fillet joint (Fig. 1) and a turn welded joint (Fig. 2), a load control swing fatigue test (stress ratio R =
0) and T-shaped fillet joints (FIG. 3), a three-point bending fatigue test was performed on the weld toe treated by the method of the present invention and the as-welded one. The treatment according to the present invention is performed at an average cold speed of 20 ° C./sec. Was cooled with liquid nitrogen.
The test steels shown in Table 1 were used, and the shapes of the test pieces were shown in Tables 2, 3, and 4, respectively. The test was conducted at room temperature in the atmosphere. For comparison of fatigue strength, 2 × 10 ⁶ times strength obtained from SN curve was used.

The test results are shown in Table 5. In Table 5, "no treatment (natural cooling)" means that the material was left to cool in the atmosphere after welding. The joint subjected to the welding toe treatment by the method of the present invention has 20% to 60% higher weld strength than the natural cooling, although there are differences depending on the shape of the weld joint, the plate thickness, and the steel type. It can be seen that the toe treatment by the method of the present invention is effective in improving fatigue strength.

[0023]

[Table 1] Properties of sample steel ──────────────────────────────────── Steel grade number Yield stress ( MPa) Tensile strength (MPa) Elongation (%) ──────────────────────────────────── I 305 450 32.1 II 412 550 28.0 III 476 631 26.8 ──────────────────────────────────── ─

[0024]

[Table 2] Dimensions of cruciform joint specimen ──────────────────────────────────── Joint No. L W t1 H t2 ──────────────────────────────────── 1 800 80 25 25 60 12 2 2 800 80 25 60 12 3 800 80 10 10 60 6 4 800 80 20 20 60 10 5 800 80 80 40 60 20 20 6 800 80 10 60 60 6 7 800 80 20 20 60 10 8 8 800 80 40 60 60 20 9 800 800 80 14 60 6 14 60 80 80 6 ────────────────────────────────────

[0025]

[Table 3] Dimensions of test specimens for turning welded joint ──────────────────────────────────── Joint No. L W1 t1 H W2 t2 ──────────────────────────────────── 11 800 80 80 25 60 60 100 12 12 12 800 80 12 60 100 100 12 13 800 80 80 25 60 100 12 14 14 800 80 12 60 60 100 12 ─────────────────────────────── ──────

[0026]

[Table 4] Dimensions of T-shaped joint specimen ──────────────────────────────────── Joint No. L W t1 H t2 ──────────────────────────────────── 15 500 100 100 20 60 10 10 16 500 500 100 40 60 20 17 500 500 80 80 60 40 18 500 500 100 20 60 10 19 500 500 100 40 60 20 20 20 500 100 100 80 60 40 ─────────────────────── ─────────────

[0027]

[Table 5]

[0028]

INDUSTRIAL APPLICABILITY The method of the present invention is a very simple and inexpensive treatment method in which only the weld toe portion is rapidly cooled immediately after welding, and it can be easily applied to large-scale structures.
Moreover, the residual compressive stress can be efficiently applied to the weld toe. As a result, the fatigue strength of the welded member can be improved.

[Brief description of drawings]

FIG. 1 is a view showing a cross fillet welded joint shape. (A)
Is a front view and (b) is a plan view.

FIG. 2 is a view showing a shape of a turn-welded welded joint. (A)
Is a front view and (b) is a plan view.

FIG. 3 is a diagram showing a T-shaped fillet welded joint shape. (A)
Is a front view and (b) is a plan view.

Claims (1)

[Claims] 1. When welding a steel material, when the temperature of the weld toe surface is 200 ° C. or more and 600 ° C. or less after welding,
By injecting a refrigerant onto the surface of the weld toe, the surface of the weld toe is heated to a temperature of 200 ° C. or lower at 10 ° C./sec. A method for improving the fatigue strength of a weld toe portion by local cooling, which comprises cooling at the above average cooling rate.