Measures to Prevent Failure of Strength of Lead Brass Valve Shells (Part one)

Summary
From the aspects of the grade selection and composition control of the raw material, hot forging and specifications of annealing processes, and installation precautions. Various measures to prevent the failure of the strength of lead brass valve shells are expounded. It is pointed out that strict control of the heating temperature of hot forging and rational adoption of the annealing process are the key measures to prevent valve shell materials from coarse grains and stress corrosion cracking.
 
1. Overview
Lead brass not only has good corrosion resistance and antibacterial properties, but also has high strength, good plasticity, easy cutting, and is widely used in the manufacture of various valves. At present, the materials used for civil low-pressure valves are generally lead brass in the construction of municipal infrastructure such as urban water supply and drainage, pipeline gas, heating and HVAC in China. The most important performance of the valves used in the above fields is strength. However, defects such as shell cracks and fractures occur during production or subsequent use due to improper selection of valve materials or irregular processing, installation and use, making the strength of the entire valve fail and eventually leading to leakage of the medium, causing water accumulation, fire and other hazards. The failure of valve strength will not only bring inconvenience to the service life of users, but also cause energy waste, economic loss and even danger to life. The grade selection and composition control of raw materials for lead brass valve shells, hot forging and specifications of the annealing process, installation precautions are explained, and various improvement measures to avoid failures of the strength of the valve are proposed.
 
2. Specifications of the forging process 
The forging lead brass valve shell is a particularly important link in manufacturing valves. Through forging, raw materials can be obtained with finer grains, which can improve their mechanical properties and prolong the service life of products. Take HPb59-1 as an example to introduce the influence of the forging process on the metallographic structure and properties of lead brass, as well as the method of standardizing the forging process.
 
2.1 The heating temperature and holding time
HPb59-1 is a ternary alloy, which is composed of α phase (Cu-based solid solution) and β-phase (CuZn-based solid solution) and lead ions distributed in dots. It has good plasticity and malleability at high temperatures. The most important thing in the forging process is the selection of the heating temperature. If the temperature is between 250 and 650°C, the lead brass will be in the brittle zone at medium temperature, the plasticity will decrease significantly, and cold cracking will easily occur in the forging process. When the temperature exceeds 740°C, that is, the heating temperature exceeds the transition point from α+β to β, the mechanical hindrance to the migration of the α-to-β phase grain boundary will be lost, and the β grains will become larger, which will greatly reduce the plasticity of lead brass. There will be serious defects such as peeling and cracking, which eventually lead to the failure of the strength of the valve shell. A reasonable forging heating temperature should be 10 to 20°C lower than the transformation temperature of the α plus β or β, that is, it is more reasonable to control the initial forging temperature between 710 to 730°C.
 
Since the actual copper content has a great influence on the metallographic transformation of lead brass, the heating temperature of the copper rod should be appropriately increased in combination with the increase in the actual copper content of the material during heating. In addition to HPb59-1, common lead brass grades for valves include grades with a Cu content of more than 61% in GB/T29528-2013. The copper content of related lead brass grades has a wide range, so the forging heating temperature is 720 to 800°C. In actual production, for every 1% increase in the actual copper content of the material, the heating temperature can be increased by 30 to 50°C. In addition, when heating, it is necessary to pay attention to quickly passing through the temperature range of a large number of α phase transitions, and try to avoid this temperature range for heat preservation. Since the β grains are very unstable at high temperatures, if the holding time is long, the β grains will increase sharply, and partially decompose during forging, and supersaturated α solid solution will precipitate on the grain boundaries, which will affect the high-temperature plasticity of lead brass, eventually leading to cracking. Therefore, when the lead brass is heated, the principle of uniform heat penetration should be used, and the holding time should not be too long.
 
2.2 Heating methods
Common heating methods for forging lead brass valve shells include kerosene heating, natural gas heating and intermediate frequency furnace heating. In the heating process of kerosene and natural gas, materials are usually piled up in a disorderly manner, which is prone to problems such as uneven heating, impermeable heating, and excessive partial temperature. The intermediate frequency furnace is widely used in forging heating of lead brass valve shells due to its advantages of fast heating speed, minimal oxidation, and superior working environment. In addition, the materials are heated in an array in the furnace, making the heating temperature very uniform. In terms of temperature control, it is necessary to equip temperature measuring devices such as infrared thermometers and optical temperature probes to accurately control the initial forging and final forging temperatures of the product, and to identify under-temperature or over-burned copper in the hot forging process in a timely manner and remove it. In the past, the qualitative method of judging the heating temperature by observing the color of the heating material with the naked eye by the operator is not advisable. In addition, in the heating process, secondary heating should be avoided to cause the grain boundaries to be melted, otherwise, the metallographic phase will become coarse and subsequent cracking will occur.
 
2.3 Other considerations
When forging, the size and placement of the bar should be reasonably selected in combination with the shape of the shell to minimize its deformation, and the weight of the feeding should be precisely controlled to avoid thick edge due to excessive material, or irregular shape of the forging due to lack of materials. The heating furnace should be cleaned regularly. When loading the material. When cutting the material, it is necessary to ensure that the two ends of the copper are flat and free of burrs to avoid defects such as debris inclusions, folding, and peeling during hot forging. In the hot forging process, the release agent should be properly and evenly sprayed to ensure that the lead brass is smoothly formed in the mold. Rapid cooling should not be used for forging to avoid the increase in brittleness and internal stress of the blank. Natural air cooling should be used to reduce the internal stress of the blank.