Microstructure And Mechanical Properties Of Laser Solid Forming And Repairing 300M Steel

300M ultra-high strength steel has been widely used in critical structural components for aviation and aerospace vehicles,such as high strength bolts,landing gear et al.due to its excellent lateral plasticity,fracture toughness and fatigue resistance.Until now,300M steel components are usually fabricated by die forging and then machining.Materials utilization is low,and its larger deformation resistance,which limits the 300M steel in the application of large components.Laser solid forming?LSF?,can realize the rapid near-net-shaping of the high performance complicated metallic parts with full-dense and without molds,can also be used for high performance repairing the large complex components,which has an important application prospects in manufacturing and repairing the complex 300M steel components.In present work,the microstructure characteristics,evolution and mechanical properties of laser solid formed and heat-treated 300M steel were investigated.Based on this,the microstructure and mechanical properties of laser forming repaired 300M steel had been studied,so as to lay an important scientific basis for the successful application of laser solid forming and repairing 300M steel technology in the fields of aeronautics and aviation.Main conclusions are as follows:?1?The microstructure and mechanical properties of as-deposited LSFed 300M steel have been obtained.The grain misorientation of as-deposited LSFed 300M steel has been distributed in two ranges:one range of misorientation angles being smaller than 10°and another zone of boundary misorientation being between 47°-60°.Low-angle boundary presents within the martensite laths and high-angle boundary presents within the martensite blocks.There are many differences in the phase distribution with different areas of the as-deposited samples.The as-deposited microstructure presents the tempered martensite,the mixture of martensite and bainite,and the bainite from the bottom to the top of the deposits.The microstructure lead to the ultimate tensile strength,yield strength and impact toughness of as-deposited LSFed 300M steel lower than those of the forgings.?2?The microstructure and mechanical properties of heat-treated LSFed 300M steel have been obtained.After quenching and tempering,the microstructure of the LSFed 300M steel is refined and uniform,and mainly consists of the mixture of tempered martensite,bainite and a small amount of retained austenite.Which results in the strength of heat-treated LSFed 300M steel improving and reaching the forgings standard.The impact toughness slightly less than the forgings standard.In addition,it is interesting that the hardness presents an incomplete linear correlation with the ultimate tensile strength and the yield strength for LSFed 300M steel.?3?The quenching treatment has a significant effect on the microstructure and mechanical properties of LSFed 300M steel.The dendritic segregation in LSFed300M steel gradually reduces with the increase of the austenitizing temperature,and even completely disappeares when the austenitizing temperature reaches 1050°C.The grain presents an obvious{111}<110>recrystallization texture.The time that the alloy elements completely become uniform in the interdendritic during quenching treatment is obtained in the basis of the diffusion behavior.The austenite grains and martensite packets of the LSFed 300M steel change little when the austenitizing temperature rises from 870°C to 950°C,which resultes in the yield strength decreasing slightly?declined to 1670 MPa from 1796 MPa?.But they significantly coarsen at the austenitizing temperature above 950°C,and the yield strength decreases dramatically from 1670 MPa to 1390 MPa.But the austenitizing temperature has no obvious influence on the hardness and ultimate tensile strength.Based on this,the relationship between the austenite grain growth and austenitizing temperature of the LSFed 300M steel have been obtained.?4?The microstructure of tempered 300M steel mainly consists of the tempered martensite,bainite and a small amount of retained austenite.The tempering temperature has little effect on the texture of the grain,the size of the martensite and bainite of LSFed 300M steel,but has a significant impact on the martensitic precipitation.With increase of the tempering temperature,?-carbide in the tempered martensite transforms to cementite.The hardness gradually decreases and fluctuates in a small range of 600-650 HV with increasing the tempering temperature.The ultimate tensile strength and yield strength increase first and then decrease.The ultimate tensile strength reaches the maximum value?1966 MPa?with tempering at 290°C,and the yield strength reaches the maximum value?1661 MPa?with tempering at310°C.However,the elongation and percentage reduction of area change little with the increase of the tempering temperature.?5?The fatigue crack growth rate curves of as-deposited and heat-treated LSFed300M steel move to the direction of lower?K with increasing the stress ratio?R?.The fatigue crack growth thresholds?FCGTs?of the LSFed 300M steel are significantly influenced by the original grain size and yield strength.This leads to the FCGT of as-deposited LSFed 300M stel higher than that of heat-treated LSFed 300M steel.The fracture mode of as-deposited LSFed 300M stel is quasi-cleavage fracture,and the fracture mode of heat-treated LSFed 300M steel is ductile fracture.?6?The microstructure and mechanical properties of laser forming repaired?LFRed?300M steel forgings with different areas?repaired zone,heat-affected zone and substrate zone?present an obvious heterogeneous microstructure.The microstructure of the repaired zone?RZ?has similar changes compared with the microstructure of as-deposited LSFed 300M steel.The microstructure of the heat-affected zone?HAZ?is uneven tempered martensite,and the microstructure of the substrate zone?SZ?is the mixture of martensite,bainite and a small amount of retained austenite.The tensile strength and impact toughness of the as-deposited LFRed 300M steel are much lower than those of the forgings standard.After quenching and tempering,the microstructure of the LFRed 300M steel with different areas become uniform and the various mechanical properties are improved significantly.The?_b,?_0.2,?,and?is 1965 MPa,1653 MPa,11.7%,38.4%,respectively.The distributions of stress and strain show that the strains of as-deposited and heat-treated LFRed 300M steel are much the same in during elastic stage but sharply increase at the RZ above the maximum ultimate tensile strength.This is due to the compatible deformation capability and strain hardening capacity of the 300M substrate higher than those of the LSFed 300M steel.The fracture mode of as-deposited LFRed 300M steel is quasi-cleavage fracture.Main crack deflects in next martensite packets during extending in the RZ.Secondary cracks also deflect in the high-angle grain boundaries.The fracture mode of heat-treated LFRed 300M steel is ductile fracture.