ABSTRACT: Brass added a certain amount of lead element, significantly improve the material cutting performance, wear resistance, widely used in mechanical engineering in a variety of connectors, valves, valve stem bearings to maintain, among them, valve blanks, lock industry, Watch and clock industry are three important markets. However, lead is hardly dissolved in cu-zn binary alloy and is dispersed in the solid solution in the free state. The aggregation and distribution along the grain boundary can easily lead to the cracking of pb-brass during hot and cold working, in this paper, the mechanism of crack edge formation during cold and hot working of lead brass HPb59-1 was studied to control or reduce the crack edge.

Key words: Lead Brass; composition; hot working; crack control

Chinese picture classification: TF811 document identification number: A article number: 1002-5065(2020)13-0143-2

Lead brass, commonly known as free-cutting Brass, has excellent lubrication and wear reduction performance, the surface precision of its machined parts is very high. For a long time, the use of lead brass in the industry has been mainly made of sheet metal, with low processing efficiency and low production accuracy. Therefore, how to develop lead brass strip to replace the sheet metal in the use of effective means to improve efficiency, the production technology adopts the traditional three-stage processing technology of casting, hot processing and cold processing. The smelting and casting of lead brass is not difficult. Induction melting and crucible melting can be used, semi-continuous casting INGOT, sand mould and iron mould can be used, but hot-rolling and cold-rolling of lead brass are easy to produce edge cracking. Therefore, how to control or reduce the cracking edge of lead brass is the first difficult point to improve the yield of lead brass and the sustainability of the later parts processing. As far as lead brass HPb59-1 is concerned, the lead content is 0.9% ~ 1.8% , cold and hot pressure processing is OK, it is a widely used lead brass in the market at present, its characteristics are good machinability, good mechanical properties, can withstand cold and hot pressure processing, easy welding, brazing good, the general corrosion has good stability, but there is a tendency to corrosion cracking, mainly used in air-conditioning valves, hardware machinery. The composition of commonly used lead brass is shown in Table 1.

1. Study on crack edge control technology of lead brass

1.1 chemical composition control

PB is almost insoluble in cu-zn alloy when 1% PB is added into brass, and the hot working property of the material is poor because of the insoluble PB particle, but the PB can be distributed in the grain rather than in the grain boundary by phase transformation recrystallization (A-B) , the addition of some micronutrient can effectively refine the grains and make the lead distribution more uniform. In order to improve the uniform distribution of microstructure and composition of ingots, different micronutrient are added to the ingots, as shown in Table 2 below.

According to the above composition, the author made 4 batches of billet in the first round of casting (adding rare earth and Copper Boron each 2 batches) and carried out the hot rolling test under the same heating condition, in which adding rare earth ingot showed better hot working characteristics and rolled to 12mm thickness, there was no obvious edge crack, but the billet with copper and Boron was cracked and could not be processed further.

In the second round of Ingot test, the effect of adding rare earth elements on ingots was verified. A total of 14 ingots were produced, of which 10 ingots were added with rare earth elements and 4 ingots were not added with rare earth elements. 8 ingots added with rare earth elements after hot rolling were all passed, the pass rate is 80% , while 4 ingots without rare earth elements have 3 cracked edges, the pass rate is only 25% .

According to the results of the second round of tests, it is further confirmed that the addition of rare earth elements can improve the cast structure of lead brass, and that the addition of rare earth elements can effectively change the as-cast structure of lead brass under the same hot working conditions, improve the hot workability of materials.

Study on 1.2 hot rolling process

Although the addition of rare earth elements can effectively improve the hot-working properties of lead brass, during hot-rolling, some ingots still crack, and the hot-working process is discussed under the condition that there is no obvious difference in composition, after homogenizing heat treatment at different temperatures, compared with as-cast microstructure, the results of high power analysis are as follows:

The microstructure of the samples is βα Pb phase in the matrix. The 1# and 4# samples are in the as-cast state, and most of the Pb phase is distributed in α phase. After annealing at 600℃, the α phase changes from the original strip to the round block. With the increase of time, the round block of α phase grows somewhat, and most of the Pb phase is distributed in the grain boundary in a network, as shown in Fig. 1.

The hot-working process of the INGOT was initially determined by high-power analysis of the above structure. First, low-temperature homogenization annealing was used to distribute as many lead particles as possible in the grain boundary, which could effectively reduce the crack edge.

1.3 study on cold working technology

In this paper, the composition of lead brass and its hot-working technology are discussed, and the hot-working crack edge is effectively controlled, but lead brass has high cold deformation resistance, and the working hardening speed is fast, the material can still be broken, so the microstructure of the hot rolled strip blank and the annealed hot rolled strip blank are analyzed and compared to adjust the process.

The samples were prepared at high magnification and the microstructure was all matrix + phase + reinforced phase. After hot rolling, the microstructure of 1 # and 2 # samples is basically the same, while that of 3 # samples is that of hot rolled strip billet annealed in a bell-type furnace, the microstructure is basically the same, and the phase of 1 # and 2 # samples is chain-like along the processing direction. The phase directivity of 3 # samples (i. e. 1 # and 2 # samples) disappeared after annealing, and showed a network distribution. (figure 2)

According to the above analysis, homogenization annealing after hot rolling can effectively improve the microstructure of hot rolled strip, and the non-crack edge can be kept when the initial rolling reaches 6.0 mm after milling, which ensures the quality of the following cold working edge and the uniformity of the finished structure.

The microstructure of the rolled 2.0 mm strip after annealing was analyzed at high magnification. The microstructure of the Strip was all + + granular phase, the phase was irregular block and interphase distribution, the distribution of black granular phase was uniform, the Matrix was recrystallized, and the average grain diameter was 0.040 mm, no macrostructure defect was observed, and the finished product could meet the user's requirement.

2. Conclusion

By studying the composition, hot-working and cold-working process of lead brass, the author finds out the main causes of cracking edge in hot and cold-working process of lead brass, and changes the composition and hot-working process, the stable production process of lead brass has been formed, which has greatly reduced the production of the rolled edge of lead brass, but because of the alloy characteristics of lead brass, some slight edge cracks still appear in the batch production, and in the subsequent production, further control of the composition uniformity of the Ingot, the stability of the casting process and the uniformity of the heat treatment temperature are the main considerations for controlling the cracking edge of lead brass.

 Source: Chinanews.com by Zhu Mingyi and Zhang Juan