JP4318599B2 - Method for producing brass material with excellent resistance to stress corrosion cracking - Google Patents

Description

  The present invention relates to a brass material excellent in stress corrosion cracking resistance, in particular, a brass material excellent in stress corrosion cracking resistance used suitably for a flare nut of an air conditioner, and a method for producing the same.

  The flare nut sandwiches the expanded copper tube together with the union, and is tightened with a predetermined torque to prevent leakage of the refrigerant gas. The flare nut is formed from a brass rod, and the applicable brass rod is JIS H3250 C3604. And the use of C3771 is prescribed.

  By tightening, residual stress acts as a tensile stress on the surface of the flare nut, and ammonia gas, which is rarely generated from the usage environment, acts as a corrosion medium, causing stress on the flare nut due to residual stress and condensed water due to tightening. Corrosion has been experienced.

  In particular, when the flare nut size is large, the tightening torque also increases, so the probability of stress corrosion cracking increases, so hot flare nuts with a diameter of up to 3/8 inch are hot. It is not possible to take the manufacturing process of removing stress by heat treatment after extrusion and cold working, and forming by cutting, and forging flare nuts with a diameter of 4/8 inch or more, hot forging The molding process. However, hot forging has a problem that the machining cost is remarkably increased as compared with cutting, and it is difficult to obtain good dimensional accuracy.

  In addition to residual stress, the crystal grain size and β phase ratio are known to affect the stress cracking resistance of brass materials. In the production of brass bars for flare nuts, while reducing residual stress, In order to reduce the β-phase ratio, after hot extrusion and cold processing, heat treatment is performed at around 475 ° C. where the α-phase ratio is highest in the equilibrium diagram, but when heated to a temperature of 450 ° C. or higher, the crystal Since grain coarsening is likely to occur, the stress corrosion cracking susceptibility cannot be sufficiently improved.

Several proposals have been made on methods for improving the stress corrosion cracking resistance of brass materials. As a brass material with excellent stress corrosion cracking resistance, which uses flare nuts as an application, the apparent Zn content Is 37 to 46 wt%, has an α + β crystal structure at room temperature, and has an area ratio of β phase of 20% or more and an average crystal grain size of α phase and β phase of 15 μm or less in the crystal structure. Brass has been proposed (see Patent Document 1), and is said to have high ductility, especially for high-speed external forces.
JP 2000-355746 A

  The present invention is based on the above-mentioned proposed brass material, and in order to obtain further improved stress crack resistance, further tests were conducted on the relationship between the alloy composition, crystal grain size, β phase ratio and stress corrosion crack resistance, As a result of investigation, the purpose was to achieve good stress cracking resistance when applied as a brass product used in a stress corrosion cracking environment, especially as a flare nut. Is to provide an advantageous brass material and a method for producing the same.

The manufacturing method of the brass material excellent in stress cracking resistance according to claim 1 for achieving the above object includes Cu: 57 to 61% (mass%, hereinafter the same), Pb: 1 to 3.7%. The Sn content is 0.35% or less, the Fe content is 0.50% or less , the balance is Zn and impurities, and after hot extrusion at a temperature of 450 to 600 ° C., cold processing is performed. It is brass made of α + β two phases at room temperature by processing into a rod shape and heat treatment at a temperature of 300 to 450 ° C., the average crystal grain size of α phase is 15 μm or less, and the average crystal grain size of β phase is 10 μm Hereinafter, the phase ratio of the α phase exceeds 80%.

Phase fraction of the α-phase is preferably in excess of 85%.

In addition, it is preferable that the maximum value of the group diameter of the β phase groups that are in contact with each other and be 15 μm or less .

The brass material according to the present invention is more preferably manufactured so that the Vickers hardness Hv satisfies Hv ≦ 420−5 × [Cu%] .

  According to the present invention, when applied to a brass product used in a stress corrosion cracking environment, in particular, as a flare nut, a brass material and a method for manufacturing the same are obtained that have good stress cracking resistance and are advantageous in terms of manufacturing cost. Provided.

  In the present invention, it is brass composed of α + β two phases at room temperature, the average crystal grain size of α phase is 15 μm or less, the average crystal grain size of β phase is 10 μm or less, and the phase ratio of α phase is 80%. It is important to reduce the β-phase ratio to less than 20%. By satisfying all of these conditions, stress corrosion cracking can be prevented even in large-diameter flare nuts with high tightening torque. It becomes. By reducing the phase ratio of the α phase to more than 85% and the phase ratio of the β phase to less than 15%, the stress corrosion cracking resistance is further improved.

The brass material for flare nuts for obtaining such characteristics contains a composition satisfying the component ranges of JIS H3250 C3604 and C3771, that is, contains 57 to 61% of Cu and 1.0 to 3.7% of Pb, In a composition range in which Fe is 0.50 % or less, Fe + Sn is 1.2% or less, and the balance is Zn and impurities, the Sn content is 0.35% or less and the phase ratio of the α phase exceeds 80%. It is desirable. If the Sn content exceeds 0.35%, the ratio of α phase becomes low and β phase increases, and the effect of preventing stress corrosion cracking is reduced.

  Even if the average crystal grain size of the β phase is 10 μm or less, stress corrosion cracking may occur if the maximum value of the group diameter D (see FIG. 1) of the β phase group in contact with each other exceeds 15 μm. It is preferable that the maximum value of the group diameter D of the β phase groups that are in contact with each other to be 15 μm or less.

  Brass bars used for flare nuts are subjected to stress relief annealing after hot extrusion and cold working to remove residual stress caused by cold working. If the thickness is insufficient, stress corrosion cracking occurs. Hardness is used as an index for evaluating whether sufficient stress removal has been performed.

  Specifically, the Vickers hardness Hv of the brass material after stress-relieving annealing needs to satisfy Hv ≦ (420−5 × [Cu%]), and this condition suppresses the occurrence of stress corrosion cracking. It becomes possible. In the case of a brass material composed of α + β two phases, even if the Cu concentration is the same, the α phase ratio differs depending on the heat treatment conditions, and the hardness varies depending on the α phase ratio. The α phase ratio shown is also taken into consideration, and stress corrosion cracking can be suppressed by satisfying this equation.

In brass materials , the addition of P has the effect of improving the dezincification corrosion resistance, and is particularly effective in suppressing the dezincification corrosion of the α phase. It also has the effect of miniaturizing crystal grains. On the other hand, since a part of P exists as a hard and brittle Cu ₃ P phase, addition of a large amount of P is not preferable from the viewpoint of cold workability. The preferable content of P is in the range of 0.02 to 0.07%, and the more preferable content range is 0.03 to 0.06%.

  The brass material according to the present invention is formed into a bar by hot extruding a billet and further performing cold working such as cold drawing. It is desirable that the hot extrusion temperature is low, and the resistance to stress corrosion cracking can be reduced by increasing the ratio of α phase and decreasing the ratio of β phase. The preferable hot extrusion processing temperature is 450 to 600 ° C., and the cross-sectional reduction rate in the hot extrusion processing is desirably 90% or more. If it exceeds 600 ° C., the crystal grain size becomes coarse and the effect of suppressing stress corrosion may decrease, and if it is less than 450 ° C., deformation resistance may increase and extrusion may become difficult. Furthermore, a preferable hot extrusion temperature is 570 ° C. or less, and stress corrosion cracking resistance superior to that of a hot forged material can be imparted by cutting after cold working.

  In the manufacture of the flare nut material, in order to satisfy the dimensional tolerance, it is desirable to perform cold working such as cold drawing after hot extrusion. Since residual stress is generated by cold working, annealing is performed to remove the residual stress. Generally, since a sufficient α-phase ratio cannot be obtained at the stage after hot extrusion, heat treatment is performed near 475 ° C. in combination with residual stress removal and increase of the α-phase ratio. Crystal grains are likely to be coarsened, resulting in a decrease in stress corrosion cracking resistance.

  In the present invention, by performing hot extrusion at a temperature of 450 to 600 ° C., the ratio of β phase can be sufficiently reduced in the hot extrusion stage. The heat treatment can be performed at a low temperature of 300 to 450 ° C. When the temperature exceeds 450 ° C, the crystal grains refined in the hot extrusion process are likely to become coarser, and the effect of suppressing stress corrosion cracking is reduced. Not suitable for.

  Examples of the present invention will be described below in comparison with comparative examples to demonstrate the effects of the present invention. These examples are one embodiment of the present invention, and the present invention is not limited thereto.

Example 1
Using free-cutting brass shavings as the main raw material, copper wire scraps, recycled zinc and lead scraps are blended to adjust the concentration of the component elements, and alloys having the composition shown in Table 1 are melted and cast, with a diameter of 203 to 294 mm. Ingots into billets. The obtained billet was hot-extruded into a hexagonal bar (cross section regular hexagonal bar) (see FIG. 2) having an opposite side dimension d: 16 to 32 mm at the temperature shown in Table 1, and then the cross-section reduction rate was 15%. Cold drawing was performed, and then heat treatment was performed at the temperature shown in Table 1.

  With respect to the obtained hexagonal bar (test material), the average crystal grain size, the ratio of α phase, the group diameter D and Hv of the β phase group that are in contact with each other and the stress corrosion cracking resistance were evaluated by the following methods. . The results are shown in Tables 4 and 5.

Measurement of average grain size: Structure observation was performed on cross sections perpendicular to and parallel to the length direction of the test material, and α phase and β were cut by a cutting method in accordance with JIS using a structure photograph taken at 400 times. The average crystal grain size of the phase was measured.
α-phase ratio measurement: The α-phase ratio was measured by the cutting method in accordance with JIS, using the above-described structure photograph.
Measurement of group diameter D of β phase group: The group diameter D of β phase groups that were in contact with each other and were connected to each other by a cutting method in accordance with JIS was measured using the above-described structure photograph.

  Measurement of Hv: Vickers hardness (Hv) at a position of (1/6) D (D: radius of the inscribed circle) was measured according to JIS. Evaluation of stress cracking resistance: After processing the flare nut of the shape conforming to JIS B8607, tighten the test material with the median tightening torque specified in the same standard (see Table 2) and expose it under the conditions shown in Table 3 It was. In the evaluation of the flare nut after the test, cracks did not occur on both the copper tube side and the union side (◎), those with only extremely fine cracks (○), and cracks occurred, Those that did not penetrate the wall thickness were marked with (Δ), those that had cracks that penetrated the wall thickness were marked with (×), and cases where ◎ and ○ were passed.

  As can be seen from Tables 4 to 5, all the test materials according to the present invention have an average crystal grain size of α phase of 15 μm or less, an average crystal grain size of β phase of 10 μm or less, and a maximum value of β phase group diameter D. The ratio of α phase was 15 μm or less, and the ratio of α phase exceeded 80%, indicating excellent stress corrosion cracking resistance. In particular, test material No. with an α-phase ratio exceeding 85%. 2, 3, 5 and 9 showed very good stress cracking resistance.

Comparative Example 1
Using free-cutting brass shavings as the main raw material, copper wire scraps, recycled zinc and lead scraps are blended to adjust the concentration of the component elements, and alloys having the compositions shown in Table 6 are melted and cast, with a diameter of 203 to 294 mm. Ingots into billets. The obtained billet was hot-extruded into a hexagonal bar (cross section regular hexagonal bar) (see FIG. 2) having an opposite side dimension d: 16 to 32 mm at the temperature shown in Table 6, and then the cross-section reduction rate was 15%. Cold drawing was performed, and then heat treatment was performed at a temperature shown in Table 6.

  With respect to the obtained hexagonal bar (test material), the same method as in Example 1 was used, and the average crystal grain size, the ratio of α phase, the group diameter D, Hv of the β phase group in contact with each other and the stress corrosion cracking resistance Sex was evaluated. The results are shown in Table 7 and Table 8.

  As shown in Tables 7-8, the test material No. No. 11 has a high extrusion temperature. Since No. 13 had a high annealing temperature, stress corrosion occurred when the crystal grain size of the α phase increased and the tightening torque increased (note: the tightening torque increased as the flare nut diameter increased). Test material No. Since No. 12 had a low extrusion temperature, it was difficult to perform hot extrusion, and the test material could not be produced. Test material No. No. 14 has a low annealing temperature, and thus the residual stress is not sufficiently removed, the Hv equation cannot be satisfied, the β-phase ratio, the β-phase crystal grain size, and the β-phase group diameter D become large, and the stress resistance Corrosion cracking was inferior.

  Test material No. Since No. 15 had a large α phase crystal grain size, stress corrosion occurred when the tightening torque was increased. Test material No. No. 16 has a large group diameter D of the β phase. No. 17 has a large β-phase crystal grain size. Since No. 18 does not satisfy the Hv equation, stress corrosion occurred when the tightening torque increased. Test material No. Since No. 19 has a large amount of Sn and a low ratio of the α phase, the stress corrosion resistance is inferior.

It is a figure which shows the group diameter D of the beta phase group which contacts and mutually connects. It is a perspective view which shows the rod of a regular hexagonal cross section.

Claims (2)

Cu: 57 to 61% (mass%, the same shall apply hereinafter), Pb: 1 to 3.7%, Sn content is 0.35% or less, Fe content is 0.50% or less , the balance Brass made of Zn and impurities , hot extruded at a temperature of 450 to 600 ° C., then processed into a rod shape by cold working, and heat treated at a temperature of 300 to 450 ° C. The brass having excellent stress corrosion cracking resistance, characterized in that the average crystal grain size of the α phase is 15 μm or less, the average crystal grain size of the β phase is 10 μm or less, and the phase ratio of the α phase exceeds 80%. Material manufacturing method . The method for producing a brass material excellent in stress corrosion cracking resistance according to claim 1, wherein the Vickers hardness Hv satisfies Hv ≦ 420−5 × [Cu%] by stress relief annealing .

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