Effects Of Alloy Elements Addition On The Microstructure And Toughness Of Heat-affected Zone Of High-strength Steel

Steels with high strength and good toughness can be realized by the refin ement of their nicrostructures.However,the properties of coarse-grain heat-affected zone(CGHAZ)in high-strength low-alloy steel(HSLA)tend to be deteriorated due to the coarsening of microstructures during welding.The prior austenite size,acicular ferrite and M-A constituents are the most important factors affecting the microstructure and toughness of CGHAZ with high heat input welding.In this thesis,the effects of precipitates on austenite grain growth and the formation of acicular ferrite on grain refinement in the simulated coarse-grained heat-affected zone with high heat input welding were direct investigated by in situ observations.The effects of alloying elements on microstructure and impact toughness in the simulated coarse-grained heat-affected zone with high heat input welding were direct investigated.The results are listed below point by point:(1)The austenite grain grew by means of grain boundary movement in the heating,isothermal holding,and cooling process at high temperature.The growth of austenite grain stopped during the cooling process when the temperature was lower than 1150?.The specimen with small content of Ti form large number,small size and dispersive nano-scale TiN particles,which can inhibit grain boundary migration and refine the austenite grain effectively.(2)Multiple and single nucleation of acicular ferrite laths or plates were observed on the intragranular inclusions,which were complex oxides covered with outer layers of titanium nitrides and manganese sulfides.Meanwhile,the ferrite laths or plates were also nucleated on the broad face of pre-formed acicular ferrite laths or plates.Acicular ferrite laths or plates partitioned large austenite grains into many smaller and separate regions.The later formed bainite laths or plates nucleated on grain boundaries and grew into grain interior in the form of packets was cofined in those smaller and separate regions.The fine-grained mixed microstructures of acicular ferrite and bainite were attributed to the partitioning of austenite grains by the lath-like or plate-like acicular ferrite grains.Therefore,the crystallographic grain size of the mixed microstructures was much smaller than that of prior austenite grains.(3)Cu alloying is performed in low alloy high strength steel.When 0.45%Cu is added,CuS is well-matched with ferrite in the coarse-grained composite MnS during the simulation of different line energy welding thermal cycles,which improves the nucleation ability of inclusions induced acicular ferrite.Promote the formation of acicular ferrite interlocking structure by increasing the acicular ferrite content and improve the low temperature impact toughness.When 1.01%Cu was added,a large number of large-particle M-A components were formed in the coarse-grained zone structure,and the low-temperature impact toughness of the coarse-grained zone decreased.(4)Cr alloying is carried out in low alloy high strength steel.As the Cr content increases from 0.33%to 0.65%and 1.02%,the area percentage of acicular ferrite in the weld thermal simulation CGHAZ structure gradually decreases,while the area percentage and distribution density of the MA component vary with Cr content and welding.The increase in heat input increases.Under the simulated 100kJ/cm large-line energy welding condition,the coarse grain zone of 0.33%Cr sample has better low temperature impact toughness.However,with the increase of Cr content,the impact toughness of the sample at-20 oC is reduced under different heat input conditions,especially under the condition of large-line energy heat input.(5)Ni alloying is performed in low alloy high strength steel.At the heat input of 20 kJ/cm and 100 kJ/cm,the microstructures of the three samples with different Ni contents simulating CGHAZ are mainly composed of bainite,a small part of AF and M-A components.Although a small addition of Ni has no significant effect on the bainite microstructure,it significantly affects the formation of AF and M-A components.As the Ni content increases from 0%to 0.23%and 0.43%,the percentage of AF area in the welding thermal simulation CGHAZ increases gradually,and the area percentage of the MA component decreases.Therefore,as the Ni content increases,the impact toughness of the sample increases at low temperature.