stellite6 alloy powder laser surfacing technology in steam generator blades (Tech Center of Shanghai HY Industry Co., Ltd)
Performance of key parts plays an important role in the long-term stable operation of equipment. Wear and corrosion are the two main failure forms of key mechanical parts, and the resulting losses are very huge. According to a research report by the National Materials Policy Council of the United States, the United States spends a huge amount of nearly 100 billion US dollars every year to make up for the loss of parts caused by friction and wear. The material part of the loss is 20 billion US dollars. In the United Kingdom in Europe, the annual loss due to friction and wear is more than 500 million pounds; in the 1980s, a survey in China found that the loss caused by corrosion in China was more than 40 billion yuan at that time; in the 1990s, China comprehensively investigated the economic industry. The analysis of the loss caused by friction and wear in production points out that this loss accounts for 1.8% of the gross national product. As we all know, friction, wear and corrosion are the process of material loss on the surface of parts, and some other failure mechanisms of materials also start from the surface. Using surface protection measures to delay and control the damage of the surface has become an effective method to solve the above problems. While solving these problems, it also promotes the formation and development of surface engineering science and surface technology.
SUS403 stainless steel has austenite structure at high temperature, and martensitic structure after quenching. It is mainly used to manufacture tools, generator blades, bearings and other components that serve in harsh environments. Due to the thin size and the erosion of high temperature steam, the leading edge part of the steam generator blade that works for a long time at high temperature is prone to failure (wear and cavitation). In order to improve the performance of the failure part of the blade, the stellite6 alloy lath is usually welded to the leading edge of the blade by silver-based brazing or TIG welding. In addition, there have been studies using the plasma surfacing method to try surfacing stellite6 alloy powder to the leading edge of the blade. However, due to the low bonding strength of brazing and the dispersion of heat sources in TIG arc and plasma arc, the welding deformation of the blade after surfacing welding is large, which makes the welding seam formation difficult to control and the production efficiency is low, so it is difficult to meet the service performance of the blade. Require.
Advantage of the laser surfacing process is that it can form a surfacing layer with a composite functional structure, low dilution rate, and small welding deformation, and it is easy to obtain a high-quality and wear-resistant surfacing layer through the surfacing process of rapid heating and cooling. In addition, by optimizing the laser processing parameters (such as defocusing amount, welding speed and powder feeding amount, etc.), the dilution rate of the surfacing layer can be flexibly controlled to meet the performance requirements. Therefore, in the field of manufacturing, laser surfacing technology has developed rapidly in recent years. The Japanese automobile industry has applied laser surfacing technology to the production of automobile engine inlet and outlet valves and valve seats, while the Japanese nuclear power industry has applied laser surfacing technology to the production of valve parts for complete sets of equipment. Due to the high energy density of the laser beam and the easy control of heat, it is especially suitable for precision surfacing of parts and surfacing of thin plate parts.
1 stellite6 alloy powder laser surfacing Test method
1.1 Test material
Shape of the steam generator blade is shown in Figure 1. The material of the test blade is SUS403 stainless steel, and its chemical composition is w(C)=0.15%; w(Si)=0.5%; w(Mn)=1.0%; w(=)0.030%; w(P)=0.040%; w(Ni)=0.60%; w(Cr)=13%; the balance is Fe. The surfacing welding adopts stellite6 alloy powder (size is 58μm～212μm), and its chemical composition is w(C)=1.1%; w(Cr)=28.3%; w(Si)=1.3%; w(W)=4.3 %; w(Ni)=1.6%; w(Fe)=2.0%; the balance is Co.
1.2 Test equipment
Part of the laser surfacing head is shown in Figure 2. The heat source is a semiconductor laser (LDF-4000 manufactured by LASERLINE, Germany) with a rated power of 4 kW. The powder feeder used TWIN10-SPG (manufactured by Sulzer Metco Ltd). Using Ar as the powder feeding gas, the
The stellite6 alloy powder is delivered to the surfacing area. The adjustment of the powder feeding speed is realized by the change of the rotation speed of the powder feeder disc. The faster the disc rotates, the more powder is fed. This paper adopts the lateral powder feeding method, in which the diameter of the nozzle of the feeding head is 2.0mm. The blade is clamped on the rotating mechanism by a special clamp. During surfacing, the surfacing manufacturing of the blade is completed through the combined motion of the surfacing head and the rotating mechanism.
1.3 Process parameters
During laser surfacing, proper preheating and insulation of the blades are required. The laser output power is 2.4 kW, the surfacing speed is 1 m/min, the defocusing amount is +10 mm, the powder feeding amount is 16.6 g/min, the protective gas (Ar) flow rate is 30 L/min, and the powder feeding gas flow rate is 4 L/min, the overlap ratio of the surfacing layer is 50%. In addition, proper heat treatment of the blade is required immediately after surfacing. There are two layers of surfacing on the blade. The size of the first surfacing layer is about 122 mm × 13 mm × 1 mm, and the size of the second layer is about 112 mm × 12 mm × 1 mm.
2 stellite6 alloy powder laser surfacing Experimental results
Through the analysis of the performance indicators of the laser surfacing layer of the steam generator blade, the following conclusions can be drawn:
Cellular crystal growth, dendrite growth and equiaxed crystal growth can be observed in sequence at the junction of the first layer and the second layer of laser surfacing welding; due to the influence of secondary heating, the heat-affected zone (HAZ) can be observed in sequence. To fusion zone, coarse grain zone, mixed grain zone and fine grain zone.
The microstructure of the surfacing layer is a hypoeutectic structure, and its primary crystal phase is composed of Co-rich γ austenite; while the eutectic structure is composed of Co-rich γ austenite and complex carbides (Cr23C6, Co3W3C) , CoCx and WC, etc.).
After laser surfacing, the average hardness of the steam generator blades is increased by two times. On the one hand, it is due to the existence of carbide hard phase in the surfacing layer, and on the other hand, due to the fast heating and cooling speed of laser surfacing, which makes the The structure of the surfacing layer is small, resulting in a fine-grain strengthening effect.
Surfacing layer contains a large number of complex carbide hard phases, which not only improves the hardness of the blade, but also increases the wear resistance of the blade after surfacing by 7 times compared to the wear resistance of the base metal.