| Welding themal simulation technology is an important method to study the Coarse-Grained Heat-affected Zone(CGHAZ),however,there are various methods to formulating thermal simulation test schemes for different scholars.In addition,the optimized composition of nitrogen for improving the CGHAZ toughness of ship plate steel has not yet reached a unified conclusion.In this paper,the ANSYS finite element software is used to simulate the welding temperature field of large heat input welding,the thermal simulation test scheme is established according to the CGHAZ thermal cycle curve obtained from the simulation.Two kinds of ship plate steels with different nitrogen content were subjected to thermal simulation tests.And the microstructure and properties were studied by means of instruments such as CLSM,OM,SEM(EDS)and mechanical performance tests.The results are as follows:(1)The thermal simulation test scheme is established according to the thermal cycle curve obtained by welding finite element simulation,the test results are in good agreement with the actual welding results.In the thickness direction of the steel plate,the cooling rate of CGHAZ at different positions is basically the same.Along the welding direction,the farther the CGHAZ is from the welding starting point,the slower the cooling rate in the range of 800-500 ℃.(2)When nitrogen content increases from 59 ppm to 95 ppm,the number and size of TiN particles in the steel increase,the pinning effect on high-temperature austenite grains decreases.(3)In the heat input range of 220 kJ/cm-304 kJ/cm,the CGHAZ microstructures of 59 N steel and 95 N steel are polygonal ferrite,pearlite and granular bainite.The59 N steel CGHAZ has small grain size and high structural stability.The impact energy at-20 ℃ is always higher than that of 95 N steel.With the increase of heat input,the low temperature impact energy of 59 N steel CGHAZ first increases and then decreases.When the heat input further increases to 352 kJ/cm,the granular bainite in the 59 N steel CGHAZ disappears,the structure is composed of coarse massive ferrite and a small amount of pearlite,the average grain size is severely coarsened,and the low temperature impact toughness is further reduced.The CGHAZ structure type of 95 N steel remains unchanged,and the low temperature impact toughness is relatively stable.In addition,with the increase of heat input,the CGHAZ Vickers hardness of the two steels showed a downward trend,and the CGHAZ hardness of the 95 N steel was always higher than 59 N. |
