Production process of tin phosphor bronze

Due to the relatively high thermal brittleness of tin phosphor bronze, the current production of tin phosphor bronze sheet and strip products at home and abroad mainly adopts horizontal continuous casting to produce ingots, and performs long-term homogenization annealing treatment
After cold rolling, intermediate annealing, pickling, and rough and finish rolling, the crystallization temperature range of the finished tin phosphor bronze is relatively wide, between 835 and 1040°C, and it is not easy to form concentrated shrinkage cavities during solidification, and It is easy to produce porosity inside the casting.
Due to its large crystallization temperature range, when it enters the initial crystallization temperature, the first crystallized α solid solution gradually forms dendrites as the temperature decreases, while the tin-rich low melting point (tin melting point is
231.89℃) components are easy to segregate between dendrites or on the interface of adjacent grains, creating conditions for "reverse segregation" and forming porosity. Reverse segregation is one of the most notable features in the casting process of tin-phosphorus bronze. On the edge and surface of the ingot, the content of tin and phosphorus is higher than that in the center of the ingot. In severe cases, gray-white precipitates (commonly known as Tin sweat, the composition is Cu33Sn8 and Cu3P
In addition, the cold hardening rate of the alloy is high, and the number of annealing increases, which makes the production cycle longer.
Cause Analysis of Reverse Segregation of Tin Phosphor Bronze
Judging from the Cu-Sn equilibrium phase diagram, the solidification temperature of tin phosphor bronze is between 150 and 160 °C when 6% to 7% Sn is contained. When the crystal is solidified, it will not form concentrated shrinkage cavities, but will form dispersed shrinkage cavities or porosity.
In general, these scattered porosities are bridged during rolling, which has little effect on appearance and performance. However, under certain inappropriate process conditions, larger holes or band-like loose strips may be formed, and because these loose areas are the last solidified areas between dendrites, some low-melting point substances are easy to form segregation here. During rolling, the back of the billet is subject to tensile stress during repeated curling, and it is easy to crack along these loose areas to form cracks. Because the diffusion rate of Sn is very slow, the selective crystallization is carried out in the solidification temperature range, forming serious dendrite segregation. The first crystallization in the grain is the high melting point substance, and the dendrite that crystallizes later is the low melting point substance. The unevenness of the composition in the grain will cause a large difference in mechanical properties, especially the "cohesive force" between the dendrites. Weak side". During processing, the back of the strip is repeatedly subjected to tensile stress, and it is easy to crack along these "weak surfaces".
When C5191 is cooled and crystallized, due to the poor thermal conductivity, the temperature difference between the inside and the outside is large. After the hard shell is formed on the surface, the middle part is in a liquid state. Part of the Sn is extruded from the "channel" to the surface as reverse segregation. And the "channel" is easy to segregate non-equilibrium phases with higher Sn content, these non-equilibrium phases
Some of the phases are brittle and hard phases, which are easy to crack into cracks during rolling.