Copper alloy for welded tubes

Abstract

 A copper-based alloy for welded tubes, containing 25 to 40% zinc, 0.005 to 0.070% phosphorus, 0.05 to 1.0% each tin and aluminum, all by weight, and the balance copper.
Welded tubes made from this alloy have improved corrosion resistance and less sensitivity to weld cracking.

Description

BACKGROUND OF THE INVENTION

[0001] This invention relates to copper alloys for use in making welded tubes with excellent resistance to corrosion and weld cracking. This invention also relates to a welded tube produced by welding such copper alloy member.

[0002] Thin-walled copper alloy tubes, fabricated by high-frequency resistance welding or high-frequency induction welding rather than by traditional tube-making processes, have increasingly come into use in recent years. This tendency is most pronounced in the field of radiator tubes. The ordinary lock seamed tubes are being replaced by tubes welded by the high-frequency resistance or induction technique that meets the modern requirements for lower cost and higher production efficiency. However, the copper alloy tubes welded in that way have a common disadvantage of far less corrosion resistance of the welds than the remainder, because of their peculiar welded structures. This is a serious limitation to the use of the welded copper alloy tubes, particularly in the ever aggravating service environments. In addition, the copper alloy tubes fabricated by the high-frequency induction or resistance welding are susceptible to weld cracking, a defect inherent to either welding process. In the light.of these difficulties of the existing tubes to be overcome, there is a demand for materials which can be welded with better corrosion resistance and less sensitivity to weld cracking.

BRIEF SUMMARY OF THE INVENTION

[0003] The present invention has resulted from studies made in view of the foregoing. Under the invention, copper alloys for applications as welded tubes with improved corrosion and weld cracking resistance have now been developed which comprise 25 to 40% zinc, 0.005 to 0.070% phosphorus, 0.05 to 1.0% each tin and aluminum, all by weight, and the balance copper and unavoidable impurities. It is preferable that the grain size is adjusted to be not more than 0.015 mm by final annealing for enhanced weld cracking resistance.

BRIEF DESCRIPTION OF THE DRAWING

[0004] FIG. 1 is a cross sectional view of a tube for a weld cracking test; and FIG. 2 is a schematic view illustrating an arrangement for testing the weld cracking of a test tube by a weight dropped inside a heating furnace.

DETAILED DESCRIPTION OF THE INVENTION

[0005] The actions of the alloying elements in the copper alloys of the invention and the grounds on which the proportions of the constituents and the grain size of the alloys are confined within the specified ranges will be explained below.

[0006] Copper and zinc, which form two major component materials of the copper-base alloys according to the invention, are both excellent in corrosion resistance, workability, mechanical strength, and also in thermal conductivity. The zinc proportion is restricted to the range specified because less than 25 wt% zinc will adversely affect the workability of the resulting alloy whereas more than 40 wt% zinc will cause β-phase precipitation in the copper-zinc alloy and impair the corrosion resistance and cold workability of the objective alloy. The phosphorus content should be in the range of 0.005 to 0.070 wt% because if it is below the lower limit no improvement in corrosion resistance will result and if the amount exceeds the upper limit an indication of intercrystalline corrosion will appear despite improved general corrosion resistance. The tin amount is defined to be in the range of 0.05 to 1.0 Wt% because less than O.D5 wtt tin will not impart added corrosion resistance, especially to the welds of welded products, while no more beneficial effect on the corrosion resistance will be achieved by the addition in excess of 1.0 wtt, which is the saturation point. Exactly the same numerical range applies to aluminum. Again the addition of less than 0.05 wt% will not improve the corrosion resistance, especially of the welds, and the larger addition of more than 1.0 wt% aluminum will merely saturate and will no longer enhance the corrosion-resisting effect. As explained above, phosphorus confers corrosion resistance on the resulting alloys and the addition of tin and aluminum renders the welds of the alloys corrosion-resistant. The reason for which the grain size of the alloy after final annealing is limited to 0.015 mm or below will now be clarified.

[0007] Our investigations on possible causes of cracks in the welds formed by high-frequency induction and resistance welding have revealed that the welds in contact with molten base metal are embrittled at the grain boundaries so that the welds crack at slight impacts. After further investigation specifically as to this phenomenon it has now been found that the grain size plays such an important role that the phenomenon can be substantially controlled by reducing the grain size.

[0008] In brief, the grain size of the objective alloy after final annealing is limited to 0.015 mm or less because the size exceeding the limit tends to cause weld cracking.

Examples

[0009] Alloys of the compositions shown in Table 1 were prepared by melting. The solidified alloys were hot rolled and then, with suitable annealing, cold rolled to sheets one millimeter thick. After final annealing at varied temperatures, the sheets were tested. Welded test pieces for corrosion resistance tests were made by butt welding the 1-mm-thick alloy sheets of the compositions in Table 1 by the T_IG method. The corrosion resistance test was conducted by dissolving 1.3 g sodium hydrogen carbonate, 1.5 g sodium sulfate, and 1.6 g sodium chloride in one liter of water, and immersing each welded test piece in the solution kept at 88°C, with introduction of 100 ml air per minute, for 240 hours.

[0010] The depths of dezincification corrosions that occurred in the welds and the base metals were determined. On the basis of the values so obtained as the criteria, the corrosion resistances of the test alloys were evaluated. The results are given in Table 2.

[0011] 

[0012] The test on the resistance to weld cracking due to embrittlement of the grain boundaries of the weld in contact with the molten base metal was performed in the following way. The 1-mm-thick sheet of each alloy of the composition in Table 1 was worked into a tubular form as illustrated in Fig. 1. The tube had the inside diameter a of 20 mm and the outside diameter bof 22 mm. The test tube was dipped for 3 seconds in a molten metal of the same composition kept at a temperature of its melting point plus 50°C. The tube was taken out of the bath into a heating furnace and was subjected to an impact test in a test arrangement as shown in Fig. 2 while the metal deposited on its surface was still in the molten state. As shown in Fig. 2, the test tube 1 was placed on a supporting table 2. A falling weight 3 of 200 gw was set above the test tube at a distance C of 50 mm. The weight was freely falled in the direction d against the test tube. The ringlike cross sectional contour of the test piece was inspected under a microscope to see if there had occurred any intercrystalline crack.

[0013] In this manner the resistance of each test alloy to weld cracking was evaluated. Table 3 shows the test results.

[0014] 

[0015] It will be appreciated from Tables 2 and 3 that the alloys of the invention have improved weld cracking resistance as well as excellent resistance to dezincification corrosion at both the base metal and the weld.

[0016] The dezicncification corrosion reached the depths of 70 to 125 µm in the base metals and 165 to 413 µm in the welds of comparative alloy test pieces (Noa. 1-3 and 19-21), whereas the alloy test pieces of the invention (Nos. 4-18) were corroded only 8 up to 15 µm deep in the base metals and only 15 up to 63 µm deep in the welds, indicating the superiority to the ordinary alloys in the resistance to dezincification corrosion.

[0017] Table 3 shows that, of the test alloys according to the invention all of which are exceedingly resistant to dezincification corroaion, those having grain sizes of not greater than 0.015 mm will not undergo intercryatalline cracking upon subjection to impact while in contact with molten base metal and prove less susceptible to intercrystalline embrittlement and more excellent in weld cracking resistance than the alloys of grain sizes in excess of 0.015 mm.

Claims

1. A copper alloy for the manufacture of welded tubes consisted essentially of:

2. A copper alloy for the manufacture of welded tubes consisted essentially of:

with the grain size adjusted to be not more than 0.015 mm by final annealing.

3. A welded tube manufactured by welding a copper alloy material consisted essentially of:

4. A welded tube manufactured by welding a copper alloy material consisted essentially of:

with the grain size adjusted to be not more than 0.015 mm by final annealing.

Drawing