Research Of Heat Affected Zone On Microstructure And Performance Of 960MPa Grade High Strength Steel For Construction Machinery

With the rapid development of low alloy steel strength level, more problems caused by welding are exposed day by day. Recently, the efforts have been paid to improve or enchance the low toughness of heated affected zone and welded metal which can enchance the comprehensive mechanical properties that is the core scientific problems of its safe operation by researching crack initiation and crack propragation attracted a rapidly growing interest, how to In this paper, by using the thermal simulation technique, made the SH-CCT diagram of Q960 high strength steel, and analysis the microstructure and properties of the first pass welding thermal cycle and the sub region of reheated coarse grained region; by gas welding for the actual welding butt welding joint of Q960 high strength steel, studied the effect of heat input on microstructure and properties of welded joint which can provide the basic data and theoretical support for welding of Q960 high strength steel welding process design and engineering application.The main experiment results can be summarized as follows:(1) The investigations on the SHCCT of simulated CGHAZ combined with phase transformation kinetic for the Q960 high strength steel. Experiment results show that with the decrease of cooling rate range from 50℃/s to 0.5℃/s, transformation start temperature Ts and Tf is increased. The microstructure of the simulated CGHAZ shows mainly from parallel lath martensite to interlocked lath martensite and lath bainite mixed microstructure and to lath bainite and granular bainite. The hardness of the coarse grain zone decreases with the decrease of the cooling rate. Transformation fraction-Temperature curve of Q960 high strength steel in simulated CGHAZ is anti "S" shape, the relationship curve of Transformation fraction-Time is "S" shape, and the relationship curve of Transformation fraction rate-Time is "几" shape and experiment data is well fitted to J-M-A equation.(2) Thermal simulator is used to simulate single pass welding thermal cycle of CGHAZ. Cooling time in the range of　t8/5 is 10 s-150 s, with the increase of cooling time, the CVN energy in simulated CGHAZs first increase and then decrease. It is because of when the cooling time is 10 s, the microstructure of the simulated CGHAZ consists primary of low carbon martensite and a few parallel bainite lath, and when the cooling time is 20 s, the microstructure of the simulated CGHAZ is interlocked of lath martensite and lath bainite mixed structure, when cooling time t8/5 is above 40 s, the microstructure of the simulated CGHAZs consist lath bainite and granular bainite. These microstructural variation results in less high angle boundaries, which can effectively arrest crack propagation and improve toughness. High angle boundaries and effective grain size are positive linear proportion relationship with the CVN value. When investigated the relationship of M-A constituent and CVN energy of simulated CGHAZ at the cooling time is 40 s,60 s and 150 s indicate that CVN energy decrease with the increase of area fraction of M-A constituent. The damage of Large M-A constituent (Lmax>2 aspect ratio<4) on CVN energy is more severe than that of long strip M-A (aspect ratio> 4). CVN energy decreases with the increase of probability of large M-A constituent.(3) The microstructure of different regions in the reheated CGHAZ was simulated. Experiments is shown that the CVN energy in IC CHAZ was the lowest. It was because of the "necklacing" M-A constituents around the prior-austenite grain. The method how to improve the toughness of IC CGHAZ was conducted by changing the second peak temperature and cooling time. Experiment results was shown that with the increasing of the second peak temperature, the size o M-A constituents which Lmax is bigger than 2 and aspect ratio larger than 4 and area fraction are first increases then decreases. When the peak temperature is 800℃, the area fraction and block M-A constituents were the largest, the CVN energy was the lowest. With the increase of cooling time, area fraction and block M-A constituents gradually increased, the CVN energy in the middle cooling time has the highest value, which is mainly due to the middle cooling time IC CGHAZ in the interior of grains from ductile better present intertlocked lath martensite and lath bainite mix microstructure, with high misorientation of grain boundaries are relatively high, which can effectively arrest crack propragation and impact toughness improved.(4) The influence of heat input on the microstructure, high angle boundaries, M-A constituents and mechanical properties of welded metal and heated affected zone for Q960 high strength steel with the domestic GHS100. Experiment is shown that microstructure of both welded metal and coarse grain heat affected zone are composed of lath martensite and lath bainite. With the increase of heat input, the microstructure of the welded metal from consisting primary of low carbon martensite and a few parallel bainite lath, became more interlocked bainitic lath which is the ultimate reason of decided to weld metal toughness. The prior austenite grain size in CGHAZ is increased with the heat input increasing from 13 kJ·cm-1 to 18 kJ·cm-1. When heat input is 18 kJ·cm-1, the prior austenite grain size is increased sharply. With heat input increasing, the tensile strength of welded meatal decreases, yield strength of welded meatal increases at first decreases and then and impact toughness increases, while the impact toughness of heat affected zone at first decreases and then increases. Heated affected zone hardened and softened is not serious with heat input increasing.