Study In Low-Temperature Rapid Phosphating Process Used In Weld Joint Of PC Steel Strand

As high-grade structure material, 1860MPa prestressed concrete (PC) steel strand is widely used in modern construction, such as skyscrapers, large span bridges, and so on. It was made of high-carbon steel wires after drawing deformation. The bar to bar are connected by butt resistance welding in order to keep continuous drawing. Fracture often occurrs in the weld joint of the wire during drawing.Drawing fractures morphology were investigated and classified. Statistical results showed that the drawing fractures in weld joint were caused by three main reasons: 1) oxide inclusion brought about by welding process; 2) overheat structure caused by high temperature of heat treatment of welling process; 3) last but not lest, surface cracks occurred during drawing because of the destroy of lubricant coated layer in welding process. The surface crakes will also cause the fracture in the process of twisting. Thus a new phosphating process on weld joint surface is studied to improve the lubricant performance and avoid occurrence of surface cracks during drawing process.The effect of oxidative accelerant NaNO₂ and La(NO₃)₃ on potential (φ) vs time (t) relationships was studied. The growth process of phosphate coating was also observed. The result showed that NaNO2 can accelerate the growth of phosphate coating, and La(NO3)3 can accelerate the nucleation of phosphate. The better effect is obtained by using NaNO2 and La(NO₃)₃.The effect of process parameter on phosphating, such as surface conditions, activation process, time of phosphating and the component of solution on the weight and figure were tested. The experimental results showed that the best composition and process parameters are as follows, at 40℃and 10min, the composition of phosphating solution was H3PO4(>85%) 5ml/L,Zn(H₂PO₄)₂55g/L,Zn(NO₃)₂70g/L,NaNO2 0.4 g/L,La(NO₃)₃ 0.05g/L。The friction performance of the phosphate coating is studied as well. The result showed that the friction coefficient of the phosphate coating is about 0.4 when the load is 100N, but the un-phosphating sample is more than 0.7. The phosphate coating combined with sodium stearate and calcium stearate show a better fiction performance, and the friction coefficient is about 0.2.When the above phosphating process was used on the weld joint of wire, the result showed that the phosphate coating can eliminate surface transverse cracks.