Effect Of Thermal Cycling On Microstructure And Mechanical Properties Of Low Carbon Bainite Weld Metal

In recent years,the development of iron and steel materials is very rapid.Their mechanical properties and excellent welding performance are greatly improved and widely used.Ultra-low carbon bainitic steel is one of them.However,it is very necessary to improve the weld performance to match the performance of the base metal for steel with such excellent performance.Therefore,it is very meaningful to study the relationship between microstructure and mechanical properties of weld metal for rolling process and welding process of this excellent bainitic steel.The microstructure and mechanical properties of the weld metal are not only affected by the alloying elements,but also by the welding process parameters.Usually in the multi-layer and multi-channel welding process,coarsening of the grain will occur in the reheated zone of the weld metal,which leads to deterioration of the impact toughness.We can predict microstructure and mechanical properties of the weld metal through its continuous cooling transition(CCT)curve.In this paper,the Gleeble-3800 thermal/force simulation system was used to simulate the thermal cycling of the low-carbon bainite weld metal with different Ni content at different continuous cooling rates.The change of expansion amount with the temperature during the thermal cycle was measured.The microstructure and microhardness of welds under different thermal cycles were measured.The continuous cooling transformation(CCT)curves and microstructure transformation curves of the weld metals were plotted.The thermal simulation processes for measuring CCT curves were applied to the impact samples of these two kinds of bainite weld metal.Then Charpy impact tests were done at room temperature and low temperature(-50℃)respectively.The impact toughness at different continuous cooling rates was measured and the law of change of the impact toughness of bainite weld metal at different cooling rates is plotted.The effects of different Ni contents on the microstructure transformation after cooling and the austenite grain size were studied.The thermal simulation test of bainite weld metal,the observation of microstructure in the thermal simulated coarse grain area,the microhardness test and the thermal simulation Charpy impact test results show that: With the cooling rate increasing,the changes in thermal simulation coarse grain area of bainite weld metal with 0%Ni are as follows: PF→AF(small amount)+GB.The hardness increases on the whole.But the rising speed of hardness curve is different due to changes of microstructure’s type and proportion.The trend of impact toughness at room temperature and-50 ℃ were consistent.The impact toughness is poor and little change in the low cooling rate range,improves rapidly in the medium cooling rate range and decline slowly after reaching highest value in the high cooling rate range.the changes in thermal simulation coarse grain area of bainite weld metal with 4%Ni are as follows: GB+PF(small amount)→BF+AF(small amount)→M.The hardness declines firstly then increases.When GB contains the most least block M-A,the hardness reaches the minimum.The impact toughness at room temperature increases firstly then decreases.When GB contains the least large block M-A that is at medium slow cooling rate,room temperature impact toughness reaches the maximum.The impact toughness at-50℃ increases firstly then decreases too and reaches the maximum when the content of BF is the most that is at medium high cooling rate.The effect of Ni content on the bainite welds metal shows that the addition of alloying element Ni in bainite weld metal can strongly delay the transition of ferrite and decrease the phase transition temperature range of bainite and expand cooling rates range of bainite formation.BF forms more easily and M can form at high cooling rates.The original austenite grain size in thermal simulation coarse grain area of 4% Ni content of the weld metal is less than 0% Ni content of the weld metal’s.It shows that the addition of an appropriate amount of alloying elements Ni can play a role in refining grain.Ni can improve the low temperature toughness of the weld metal due to the formation of more BF and grain refinement.But high Ni content can result in the segregation of elements and the formation sulfide and phosphide at slow cooling rate so that defect appears and impact toughness become worse.At higher cooling rates,the increasement of the hardened phase martensite content also leads to defect and worse impact toughness.The side effect of high Ni content is: at the slower cooling rate is likely to cause the segregation of elements to form sulfide and phosphide,resulting in tough fracture fracture defects,resulting in deterioration of the impact toughness,at a greater cooling rate of hardening phase The martensite content increases,resulting in plastic deformation of the fracture surface appears on the cleavage surface and defects,making toughness worse.