Cast lead brass alloy has good mechanical properties and cutting performance and is widely used in sanitary ware, plumbing equipment, instruments, hydraulic valves, etc. Although the crystallization temperature range of cast lead brass is small, the mold filling ability is strong, and it has the degassing and deoxidation effects of zinc, etc., due to the fact that brass easily forms coarse columnar crystals during the solidification process and the casting structure is coarse, it is easy to heat crack during the casting process. Leakage and other phenomena occur during use, which affects its performance. Refinement of the structure of cast brass is one of the effective ways to improve hot crack resistance and density. At present, adding refiners is the main method to refine the structure of cast brass. The literature studies the effect of boron on the structure refinement of aluminum brass. Adding boron can effectively refine the cast structure of aluminum brass. The literature also studies RE It can refine the cast structure of aluminum brass. There are few studies on the effects of boron and rare earth on the structure and dezincification performance of cast lead brass. This paper studies the effects of rare earth and boron refinement treatments on the as-cast structure and dezincification corrosion properties of cast lead brass.

1. Test materials and processes

The test material grade is ZCuZn40Pb2 lead brass, and its chemical composition (mass fraction) is 36.4% Zn, 1.70% Pb, 0.62%, A1, and Cu balance. Rare earth with 10% RE. AI master alloy is added, and boron is added as Cu-4.68%B master alloy. A graphite crucible is placed in a well-type resistance furnace for alloy smelting. The smelting process of the test lead brass is as follows: the charge is melted, and when the temperature rises to 1030 degree , boron and rare earth refiners are added respectively using the bell jar method, fully stirred, and after 10 minutes of heat preservation, the pouring preheating temperature is 350 degree . The left and right metal type test rods and metal type round cup specimens. The metal round cup sample method is the simplest and most direct method for casting copper alloy companies to check whether the refinement process is good or not. The specific method is to pour a round cup sample, and after solidification and crusting, quickly pour out the unsolidified copper liquid in the center of the round cup sample to obtain a shell cup sample with a solidified crystal surface. By observing the morphological characteristics of the crystal surface to judge the effect of refinement processing. Dezincification test of cast lead brass: The corrosive solution is 1% CuCl2: solution, the solution temperature is (75±10) degree , the corrosion time is 24 hours, after the corrosion is completed, take it out, clean and dry, cut the sample from the center, and prepare the gold Phase samples were used to observe and measure corrosion dezincification depth using a microscope to explore the effect of refinement treatment on dezincification corrosion of cast lead brass. Adopt [PUS. Observe the microstructure under a PME3 metallographic microscope. The corrosive solution for the tissue is 5 gFeCl3+5 mlHCl+100 ml aqueous solution.

2. Test results and analysis

2.1 Effect of refiner on the structure of lead brass

Figure 1 shows the structure of lead brass without refinement and with boron and rare earth refinement. It can be seen that the structure of the unrefined metal mold cast lead brass is extremely coarse (Figure la). According to the copper-zinc phase diagram analysis, the cast structure should be phase (white) + phase (black) + Pb phase , the Pb phase is distributed on the phase or between the and phases as small black simple particles. The and phases in the as-cast unrefined structure are distributed in thick long laths. After the RE and B refinement treatment, the and phases were significantly refined. After the B refinement treatment, the coarse lath-like and phases disappeared and turned into short flaky or equiaxed grains (Figure 1b). Compared with the as-cast structure (Figure 1c), the structure of the chemically treated structure is significantly refined, and the and phases change from thick long laths to short thin laths. Therefore, for cast brass alloys, RE and B refinement treatments can significantly improve the as-cast structure of cast brass. Under this test condition, the structure refinement effect of cast lead brass treated with boron is significantly better than that of cast lead brass treated with boron. The effect of rare earth refinement treatment.

2.2 Effect of refiner on surface morphology and structure of shell cup sample

Figure 2 is the morphology of the crystal surface of the shell cup sample before and after the boron refinement treatment of liquid copper in the same furnace. It can be seen that the surface of the shell cup sample cast by unrefined copper liquid is rough (Figure 2a). From a macro perspective, it can be seen that there are many uneven small bumps on the crystal surface. Figure 3a shows the unrefined crystal surface and its vicinity. From the structure morphology, it can be seen that the structure of the rough surface is coarse columnar crystals, and the rough small protrusions are the columnar dendrites left at the liquid/solid interface when the copper liquid solidifies. Because the liquid between the unsolidified dendrites was poured during the test, out, so the columnar crystals formed at the front in the liquid are completely retained, forming a rough surface. Figure 2b is the surface of the boron-refined shell cup sample. Compared with the unrefined one, the surface is smooth and has no rough and small bumps. The surface optical microscope structure is shown in Figure 3b. The crystal surface is uneven, but the crystal surface is smooth. The structure of the smooth crystal surface shows equiaxed crystallization, so the refinement treatment can change the solidification structure of cast lead brass.

2.3 Effect of refinement treatment on dezincification performance of cast brass

Figure 4 shows the effect of refinement treatment on the dezincification depth of cast lead brass. Due to dezincification corrosion, the surface structure is different from the white layer inside. This is because in the corrosive environment, the zinc on the surface corrodes, the concentration decreases, the zinc-rich phase ( phase) in the structure decreases, and the phase increases, resulting in an obvious structure. difference. The dezincification depth of unrefined lead brass is relatively large (Figure 4a). The average depth of the dezincification layer of the lead brass is 17000um, followed by the quantitative measurement of the dezincification layer. The depth of the dezincification layer of the rare earth refined treatment is second (Figure 4b). The average depth of the zinc layer is 132.2um. The depth of the dezincification layer treated with boron refinement is the smallest (Figure 4c). The average depth of the dezincification layer is only 57.2um through quantitative measurement. This shows that the refinement treatment can effectively suppress the dezincification corrosion of cast brass while refining the as-cast structure. In particular, the effect of boron refinement treatment on inhibiting dezincification corrosion of cast brass is more obvious. Regarding the mechanism of dezincification corrosion of lead brass, there are currently two main mechanisms: one is dissolution. The redeposition mechanism and the preferential dissolution mechanism of zinc may sometimes be the result of the combined action of the two. The preferential dissolution mechanism of zinc believes that during the corrosion process of brass, zinc on the alloy surface is preferentially dissolved from the brass, generating double vacancies. The double vacancies diffuse into the interior of the alloy, and the zinc alloy components diffuse to the surface until dissolved. Dissolve. The redeposition mechanism believes that brass dezincification corrosion includes two possibilities: one is that copper and zinc are dissolved on the anode at the same time. When the copper ions in the solution reach a certain concentration, the copper ions are reduced to metallic copper and deposited on the surface. , as an additional cathode to accelerate the dissolution of zinc in the alloy; the other is that within a short period of time at the beginning, zinc preferentially dissolves into the solution. As zinc diffusion becomes difficult, copper and zinc will dissolve at the same time, accompanied by the back-deposition of copper. . Boron refinement treatment can inhibit the dezincification of brass, mainly because boron is added to ZCuZn40Pb2 lead brass, which can fill the grain boundaries and double vacancies, increase the bonding force there, and hinder the migration of grain boundaries and double vacancies. channel, reducing the migration speed of zinc, thereby reducing the corrosion rate of ZCuzn40Pb2 lead brass. RE refinement treatment can eliminate impurities in the brass matrix. When ZCuZn40Pb2 lead brass contains impurity elements such as O and S, the electrode potential difference between them and the matrix is large, forming a primary battery and accelerating corrosion. Rare earths can interact with O, S and other impurity elements. S generates high melting point, low density rare earth compounds [81], which easily float into the slag, thereby purifying the matrix and reducing the corrosion rate. When rare earths are added to brass, an extremely thin and dense layer of rare earth oxides will be formed on the surface. It can prevent the diffusion of Zn and Cu atoms into the solution, reduce the dissolution rate, thereby delaying corrosion and improving the dezincification corrosion performance of the alloy.

3.Conclusion

(1) RE or B refinement treatment can refine the as-cast structure of ZCuZn40Pb2. Refinement treatment can change the thick long lath structure of brass into short thin laths or equiaxed crystal structure. The refining effect of B refining treatment is better than that of adding rare earth refining treatment. Refinement treatment can change the surface of the shell cup sample from rough to smooth. The rough surface layer structure is columnar crystals, and the smooth surface layer structure is equiaxed crystals.
(2) RE or B refinement treatment of ZCuZn40Pb2 alloy can inhibit dezincification corrosion of cast brass. The dezincification inhibition performance of B refinement treatment is more obvious than that of RE refinement treatment.