JPH05345928A - Post weld heat treatment for improving fatigue strength of weld joint - Google Patents

Abstract

PURPOSE:To improve joint fatigue strength by changing the structure of the toe of a low alloy steel weld joint by heat treatment. CONSTITUTION:Steels having Ac1 and Ac3 transformation points are welded, and the weld zone is heated up to a temp. in the intermediate temp. region between the Ac1 and the Ac3 transformation points and further cooled down to <=300 deg.C at >=10 deg.C/sec average cooling rate to undergo heat treatment. By the above heat treatment, a large amount of island martensite is formed in the weld zone, by which the propagation of fatigue crack is inhibited. By this method, the fatigue strength of the weld joint can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a transformed steel, especially a low alloy steel for welding having a tensile strength of 80 kgf / mm ² or less, by modifying the microstructure of the welded portion by heat treatment to obtain a welded joint. The present invention relates to a heat treatment method for improving fatigue strength.

[0002]

2. Description of the Related Art With the increase in size of welded structures, it is necessary to reduce the weight of the structure, and for this reason, the tensile strength of steel used for the structure is being advanced. On the other hand, in structures such as ships and machines that are subjected to repeated loads during use, consideration must be given to prevent fatigue fracture. In particular, the welded portion is apt to cause fatigue cracks most frequently because the stress concentration is likely to be high and the welded residual stress is often used. However, even if the strength of the base metal is increased, the fatigue strength of the welded portion hardly changes. Therefore, if the design stress is increased with the increase of the strength of the base metal, a fatigue crack easily occurs in the welded portion. For this reason, it becomes difficult to increase the design stress due to the increase in the strength of the base material.

According to the conventional research on fatigue fracture, there is almost no material factor affecting the fatigue strength of the welded portion, and the mechanical strength of the welded portion is improved by improving the mechanical factors such as reduction of stress concentration and welding residual stress. Fatigue strength has been improved.
That is, by grinding the weld toe and smoothing the shape of the toe by TIG welding to reduce the stress concentration, fatigue strength can be improved and the design stress can be increased as the base metal strength increases. It is the current situation.

[0004]

The problem to be solved by the present inventors is not to improve the fatigue strength by reducing the stress concentration and the residual welding stress, but by heat treating the microstructure of the welded part. The modification is to realize a method for improving the fatigue strength of the welded portion.

[0005]

In view of the above problems, the present inventors have made detailed investigations and studies on the relationship between the fatigue strength and microstructure of welded portions of low alloy steel having transformation, and as a result, We have found facts that are important for practical use. The most frequently used joint shapes in welded structures are butt welding and fillet welding. FIG. 1 schematically shows the positional relationship between the weld metal, the weld heat affected zone (hereinafter referred to as HAZ), and the base metal portion in these joints. It is a well known fact that fatigue cracks originate and propagate from the weld toe where stress concentration is most severe. As is apparent from FIG. 1, this toe is a HAZ microscopically close to the weld metal. Therefore, it is clear that the fatigue crack initiation from the toe and the initial stage of crack growth can be reproduced by subjecting the reproduced thermal cycle material that reproduces HAZ to the fatigue test. On the other hand, it is a well known fact that fatigue strength changes greatly depending on the stress concentration. In order to reproduce the fatigue crack initiation and propagation from the weld toe, it is necessary to reproduce the microstructure and also the stress concentration at the weld toe. From such a consideration, a small fatigue test piece having the shape shown in FIG. 2 was machined from the welded thermal cycle material, and the relationship between the microstructure and the fatigue strength was systematically examined. The small 3-point bending fatigue test piece shown in FIG. 2 was also subjected to stress concentration almost equal to the stress concentration at the toe.

Table 1 shows the tensile properties of chemical components and mechanical properties.
Strength is 50kgf / mm²Weld HAZ for high grade high strength steel
After applying the current heat cycle, and then applying heat treatment,
The small 3-point bending fatigue test piece shown below was processed and subjected to the fatigue test.
It was Here, the welding HAZ reproduction thermal cycle is a high frequency induction
It was applied using a thermal device. Maximum heating temperature is 1400 ℃
Then, the cooling time from 800 ° C. to 500 ° C. was set to 8 seconds. Well
In addition, the heat treatment conditions after welding include heating temperature from 600 ° C to 10
Change between 00 ° C, cooling after heating is water quenching
It was In Fig. 3, the heat treatment temperature is plotted on the horizontal axis, and the small 3-point bending fatigue
It is a plot of the fatigue limit stress range obtained in the labor test.
is there. Ac of this test steel₁And Ac₃The temperature is about
720 ° C and 850 ° C. From the figure, the heat treatment temperature is A
c₁And Ac ₃In the intermediate temperature range of
It can be seen that the fatigue limit is significantly increased.

Coarse-grained bainite structure is produced by the simulated welding heat cycle, but the subsequent heating of Ac ₁ and Ac _{3 to} an intermediate temperature region partially produces austenite, and carbon is concentrated in the austenite. To do. It is easily inferred that the hardenability of austenite with an increased carbon concentration increases, and at least a part of this undergoes martensitic transformation during cooling. This island-shaped martensite is coarse grain H
It was confirmed that the AZ structure has a remarkable effect on improving the fatigue strength under stress concentration. FIG. 4 is a test piece that was not fatigue-fractured by performing a fatigue test in a stress range just below the fatigue limit for a test piece that was heated and cooled at a post-welding heat treatment temperature of 750 ° C., which is the condition showing the highest fatigue strength. As a result of observing the notched bottom of the specimen with a scanning electron microscope, an example of a microscopic crack found is shown. Microfatigue cracks are generated in bainite by repeated loading, but it is clearly seen that the cracks are prevented by the hard island martensite. The structure observed in white in the figure is island martensite.

In the experiment shown in FIG. 3, it was found that the time strength, that is, the number of repetitions until rupture in a constant load stress range was also increased by the heating / cooling heat treatment in the intermediate temperature range between Ac ₁ and Ac _3. There is. That is, this heating / cooling heat treatment also delays the propagation of fatigue cracks.
Therefore, it is clear that the island martensite has an effect of delaying the crack growth at a stress range level higher than the fatigue limit stress range, in addition to the effect of preventing the microscopic crack growth as described above.

That is, the essence of the present invention is that even if a microscopic fatigue crack occurs in the stress concentrated portion by generating island martensite in the coarse grain structure of the HAZ by post-welding heat treatment, The hard island martensite is used to prevent or delay the crack growth, thereby effectively improving the fatigue strength. Next, the appropriate range of the cooling rate after the heat treatment for forming island martensite was examined. FIG. 5 shows the results of an investigation of changes in the fatigue limit stress range when the heat treatment temperature after the simulated welding heat cycle was kept constant at 750 ° C. and the cooling rate during cooling after heating was changed. Test steel, welding H
The AZ reproduction heat cycle condition and the test piece shape were the same as those in FIG. However, the heat treatment after the welding HAZ reproduction thermal cycle was reproduced by a high frequency induction heating device.
From the figure, when the average cooling rate after heating by heat treatment is 10 ° C./sec or more, island martensite is generated, and the effect of improving the fatigue limit appears. Therefore, setting the average cooling rate after the heat treatment to 10 ° C./sec or more is an essential condition for improving the fatigue strength. However, the larger the cooling rate, the more island-like martensite is generated, and the effect of improving the fatigue limit becomes more remarkable. When ordinary low alloy steel is targeted, the martensitic transformation is almost completed by about 300 ° C. Therefore, cooling after heat treatment is 300 ℃
Is important, and the cooling rate at lower temperatures may be arbitrary.

The present invention constructed on the basis of the above findings will be summarized as follows. HA at the toe is the most prone to fatigue crack initiation and propagation in welded joints.
It is a Z organization. Due to the heat cycle during welding, a coarse-grained HAZ structure is formed near the toe. After welding, it is heated to an intermediate temperature range between Ac ₁ and Ac ₃ , and then cooled to 300 ° C or less at an average cooling rate of 10 ° C / sec or more to obtain coarse-grained HA.
Martensite is formed like islands in the Z structure. Even if a microcrack is generated under fatigue loading, the hard island martensite prevents the microcrack from propagating or delays the propagation to improve both the fatigue limit stress range and time strength. You can The island-like martensite has a strong effect of preventing and delaying the growth of microscopic cracks, and this effect also appears in a portion having stress concentration such as a weld toe. As is clear from the above-mentioned examination results, the austenite has Ac ₁ and Ac ₃ transformation temperatures, and austenite is partially generated in the intermediate temperature range of Ac ₁ and Ac ₃ , and at least austenite is cooled by cooling at an appropriate cooling rate. If the steel is partially martensitic transformed, the improvement in fatigue strength according to the present invention can be achieved.

As is clear from the principle of the present invention, the heat treatment method after welding may be any as long as it can realize the heating temperature and cooling rate of the present invention. Specifically, heating by inserting a heating furnace, heating by high-frequency induction heating, heating by a gas burner, or the like can be considered. Also, Ac ₁
Carbon concentrates in austenite in the intermediate temperature range between and Ac ₃ , but the time required for this is short because the diffusion of carbon is extremely fast. Therefore, the holding time in the intermediate temperature range between Ac ₁ and Ac ₃ is about several seconds, and the holding time may be arbitrary for practical use. The occurrence of fatigue cracks is a problem at the weld toe, and it is not necessary to heat the entire weld joint, and the effect of the present invention can be obtained by heating only the toe to a predetermined temperature. Further, the cooling method after heating may be arbitrary as long as an average cooling rate of 10 ° C./second or more can be obtained up to 300 ° C.

On the other hand, it is a widely known fact that the fatigue strength of a welded portion largely depends on the residual welding stress and that the reduction of the residual welding stress improves the fatigue strength. Since the post-weld heat treatment of Ac ₁ or less, which is generally widely performed, reduces the welding residual stress, the post-weld heat treatment improves the fatigue strength. However, as is clear from the above-mentioned examination results, the present invention provides coarse-grained HA in which fatigue cracks occur and propagate.
It is needless to say that the Z-structure is modified by heat treatment after welding to improve the fatigue strength, which is completely different in principle from the conventional method for improving fatigue strength by heat treatment after welding. Further, the heat treatment temperature of the present invention is different from the conventional post-weld heat treatment temperature for the purpose of reducing the residual welding stress.

[0013]

EXAMPLES Examples of the present invention will be described below. Table 2 shows the chemical composition and mechanical properties of the test steels with a tensile strength of 50 kgf / mm ² . Carbon dioxide welding was performed under the conditions shown in Table 3, and T
A fillet welded joint was created. From this welded joint, a three-point bending fatigue test piece having the shape shown in FIG. 6 was processed.

Table 4 shows the heat treatment conditions after welding and the fatigue limit stress range values obtained as a result of the fatigue test. Example a is the as-welded result. Examples b and c are the results of the heat treatment after welding according to the present invention, and a remarkable improvement in the fatigue limit is recognized as compared with the as-welded state. In Example d, the heating temperature is less than Ac _1, which is outside the scope of the present invention. Although the fatigue limit of the as-welded material is improved by reducing the welding residual stress, the effect is smaller than that of the present invention. In Example e, the cooling rate after the heat treatment after welding was allowed to cool in the atmosphere, so the cooling rate was out of the range of the present invention, and the improvement in fatigue strength was due only to the reduction of welding residual stress, and the effect was smaller than that of the present invention.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

[0018]

[Table 4]

[0019]

As described above, by performing the post-weld heat treatment according to the present invention, the microstructure of the weld heat affected zone can be modified and the fatigue strength of the welded joint can be improved.

[Brief description of drawings]

FIG. 1 is a schematic view of a welded portion.

FIG. 2 is a diagram showing the shape of a small-sized three-point bending fatigue test piece of a welded section heat cycle material.

FIG. 3 is a diagram showing a result of an examination of post-welding heat treatment temperature dependency of a fatigue limit of a reproduced heat cycle material.

FIG. 4 is a microscopic crack observed at the notch bottom of a test piece subjected to a fatigue test under stress just below the fatigue limit after heating the reproduced heat cycle material to an intermediate temperature range between Ac ₁ and Ac ₃ and then water cooling. FIG. 4 is a photograph of a metallographic structure by scanning electron microscopy showing the state in which is prevented from progressing by island martensite.

FIG. 5 is a diagram showing the results of an investigation of the cooling rate dependence of the post-welding heat treatment of the fatigue limit of the reproduced heat cycle material.

FIG. 6 is a view showing a fatigue test piece shape of a T-shaped fillet welded joint.

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[Procedure amendment]

[Submission date] February 4, 1993

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 2

[Correction method] Change

[Correction content]

[Fig. 2]

Claims (1)

[Claims] 1. After welding steel having Ac ₁ and Ac ₃ transformation points, the welded portion is heated to an intermediate temperature range between the Ac ₁ and Ac ₃ transformation points, and further 300 at an average cooling rate of 10 ° C./sec or more. A post-welding heat treatment method for improving the fatigue strength of a welded joint, which is characterized by cooling to below ℃.