FI98054B - Method for improving the fatigue strength of butt welding joints of sheets - Google Patents

Description

98054

METHOD FOR IMPROVING THE FATIGUE STRENGTH OF SHEET METAL WELDED JOINTS

The invention relates to a method as defined in the preamble of claim 1.

It is already known that the fatigue resistance of welds can be improved by various post-treatments such as grinding, ball bombardment, TIG treatment, etc., but the problem is that they significantly increase production costs and it is difficult to control the quality of their performance.

The lightening of structures has led to the use of ever stronger materials. Often, the potential weight savings have been limited because the fatigue strength of the weld metal is almost independent of the strength of the base material. The allowable tension level at the joint has determined the thickness of the entire plate field. On the other hand, the share of the joint in the structure is remarkably small, for example in ships.

20 The strength of high-strength steel in fatigue-prone structures cannot be fully utilized because the fatigue resistance of the welded joint is considerably lower than that of the base material alone. For example, for ferritic • ·. ** controlled rolled steel (RAEX 640), • ·. * ·· 25 with a yield strength of 640 MPa, the base material has a fatigue strength of 500 MPa / 1 million cycles. The fatigue strength of the butt weld joint is 200 MPa / 1 million cycles. The latter figure • · · · is decisive in the dimensioning. The fatigue strength of the butt joint is very close to the strength of the Fe 52 butt joint, »· · 30 so the high strength of RAEX 640 material cannot be achieved. take advantage of lightweight structures. A similar situation exists in welded aluminum structures.

The object of the invention is to eliminate the above-mentioned drawbacks.

»·· ·. ···. The method according to the invention is characterized by what is stated in claim 1.

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In the method according to the invention, the plates are joined together by welding. According to the invention, before welding, the end-welded edges of the plates are formed to a thickness greater than the thickness of the plate.

Taking into account the type of joint and its fatigue resistance compared to the fatigue resistance of the base material, the thickness of the edge is chosen so that the fatigue resistance of the joint is as good as or better than that of the thinner base material. With the method according to the invention, the required fatigue resistance can be achieved at lower production costs than alternative methods. With the help of edge thickening, the strength of high-strength steel in fatigue-prone structures can be better utilized. 15 By reducing the stress at the weld site by means of thickening, the fatigue strength of the whole structure is uniform. Edge thickening by molding is fast and at the same time a finished weld rail can be formed. The dome of the weld can be left or removed by machining while still maintaining the strength of the material. On the other hand, the raised dome is easy to sand.

In areas where the weight of the structure is of particular importance, the method according to the invention can achieve significant weight savings, since the total strength of the plate can be exploited.

One preferred application of the method according to the invention is shipbuilding with large. the plates are joined together by butt welds.

• The method according to the invention makes it possible to weld ship hulls from metal sheets, in which the basic condition is that the hull is considerably lighter and stronger than a conventional ship hull and in which high requirements are set for the fatigue resistance of welded joints.

. ·. The invention is particularly well suited for use. 35 in connection with all types of “fatigue-prone structures assembled with shirt welds. In addition to ships, such *. * Structures also include, for example, bridges and cranes made of casing beams, large storage tanks assembled from plates, etc.

In one embodiment of the method, the edge of the sheet is formed by rolling to blunt the edge of the sheet.

5 In one embodiment of the method, when forming the edge of the sheet from the edge thickness, a groove side of the weld is formed at the end of the sheet by molding it to an angle corresponding to the desired groove shape of the weld.

In one embodiment of the method, the transition area between the molded edge thickener 10 and the surface of the sheet is formed into a rounded shape that avoids discontinuities.

The invention will now be described in detail with reference to the accompanying drawing, in which Fig. 1 schematically shows a multi-plate plate field in which the plates are connected to each other according to an embodiment of the method according to the invention, Fig. 2 shows section II-II of Fig. 1. Wöhler fatigue strength curves.

Figure 1 shows a part of the side of the ship seen from the side. The side is formed of several rectangular shapes. . second plates 1, 2, 1 ', 2' connected to each other at the edges by butt welds.

• »• · · * ϊ 25 Figure 2 is a sectional view of an butt weld joint between plate 1 and plate 2 • ·, the fatigue strength of which has been improved in accordance with the invention.

: * · *: The endwise welded edges of each plate 1, 2 include edge thicknesses 3, 4 formed on the plates. The edge thicknesses 3, 4 are formed to be thicker than the thickness of the plate. The ends of the edge thickeners 3, 4 are • · ·

Hl in connection with molding, at the same time molded into corners corresponding to the shape of the weld rail, · · · * · "', in which case no machining step, milling, grinding, etc. is required. Edge packing: ***: Filling 35 sundas by molding is fast and at the same time • · ·. *. a finished weld rail is created.The rail sides can be shaped • · · \\ into any suitable shape corresponding to e.g. V-, I-, • · · • ·· »· 4 98054 U-, X-, etc. rail shape. The dome of the weld can be left as it is or removed by machining while still maintaining sufficient material strength.On the other hand, the raised dome is easy to grind.The transition area between the surfaces of the plates 1, 2 of the molded edge thickening 3, 4 and 5 is molded into a rounded shape

Figure 3 shows the so-called Viöhler fatigue strength curves corresponding to the axial stress state, in which in the double log rhythm scale the vertical axis has the stress o and the horizontal 10 axis the number of load changes, i.e. the load change number N. Each curve is obtained by selecting several stress values and making v The curves diagrammatically depict the fatigue strength of a substance as a function of load index.

15 The experimentally determined fatigue strength diagram of welded joints (SFS 2378) in Figure 3 can be used as a basis for dimensioning welded joints. The curves have been drawn within the framework of IIW (International Institute of Welding) using the results of 20 laboratory experiments available as source material. The obtained Wöhler curves are on the double logarithmic scale with the same slope corresponding to the value of the exponent of the crack growth law m = 3.0. Road-. . The resulting curve is represented by the equation Ν · Δσ3 = C. The constants C are given in • «·’ · ’· Table 4.

• · • »· *. *! 25 In Fig. 3, the curve I corresponding to the weld class (i.e., the annealing class) • · 100 represents, for example, a conventional butt weld joint, in which the plates are at the ends and the weld Γ **: the dome is left raised. Higher specific fatigue strength. * J *: is at the joint where the weld dome is machined to the level of the surface, which is represented by the specific fatigue curve II (weld class 125). An image from the standard SFS 2378 has been added • · ·

H1 from the corresponding Swedish standard specific fatigue • · · *** * curve III corresponding to the base material and the butt weld joint made by the method according to the invention. The basic material and the joint made according to the invention naturally have an even higher specific fatigue strength.

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Based on Figure 3, dimensioning calculations of the joint made by the method according to the invention can be made. The following is an example.

EXAMPLE 1 5 It is desired that the butt weld between the plates and the plate be evenly resistant to fatigue. What is the required thickening for the plate if the service life requirement is 1 million load changes? The plate has a thickness of 7 mm and a yield strength of 640 N / mm2.

10 Figure 3 gives the stress variation - base material 200 N / mm2 - butt joint 130 N / mm2 (weld class 100) The permissible stress variation of the base material is 200 N / mm2, so that the welding joint is equally strong with the base material 15 the joint area must be 200/130 = 1.55 -accumulative in relation to the basic substance. In this case, the total thickness of the thickening becomes 1.55 x 7 mm = 11 mm.

EXAMPLE 2 20 A butt joint between plates is required to withstand a fatigue nominal load of 130 N / mm2 for at least 2 million load changes. For cost reasons, the above. minimization of work steps and interim storage.

The strength required according to the invention is achieved by the following two steps: - V-welds are made on the edges of the plates to be welded by molding * [: - butt welding, '· 1; An alternative method would require the following three 30 work steps: - V-weld • · III grooves are made at the edges of the welded plates by milling • · ... »*. - shirt-nailing is performed, · · - the post-treatment of the weld at the edge line is carried out by TIG melting and it is ensured that the post-treatment of the weld is successful in the post-inspection.

* 1. Manufacturing a welded wire, for example by forming with a roller m · • · • · 6 98054, is a faster and cheaper work step than milling as an alternative. The thickening of the joint obtained by the same method according to the invention makes the post-treatment of the weld unnecessary.

The invention is not limited solely to the application examples presented above, but many modifications are possible while remaining within the scope of the inventive idea defined by the claims.

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Claims (5)

98054 A method for improving the fatigue strength of a butt weld of plates, in which method the plates are joined together by butt welding, characterized in that before welding the edges of the plates to be welded are formed to a thickness greater than the thickness of the plate. A method according to claim 1, characterized in that the edge of the plate is formed by rolling to press the edge of the plate. Method according to Claim 1 or 2, characterized in that when the edge of the plate is formed from the edge thickness, a groove side of the weld is formed at the end of the plate by shaping it at an angle corresponding to the desired groove shape of the weld. Method according to one of Claims 1 to 3, characterized in that the transition region 20 between the molded edge thickening and the surface of the plate is formed into a rounded shape which avoids discontinuities. : 5. A method according to any one of claims 1 to 4 method, characterized in that the welding • · · C. after the weld dome is machined away. • · · • · »· ···· · 1 2 · • · · · • · · ··· · · · · · ♦ ···» ··· «· · • · · · · · · • · ··· 2 · • · ♦ • 1 • · 3 98054