Study On The Microstructure And Properties Of Electron Beam Welded Joints For Ultra High Strength Steel

30CrMnSiNi2A steel is a kind of low alloy medium carbon martensite ultra high strength steel.Due to it's good comprehensive properties,30CrMnSiNi2 A steel is widely used in such important areas concerning more of high stress and less of plastic requirement as wing spars,connecting bolts,landing gear,tube,et al.The published data of electron beam welded 30CrMnSiNi2 A steel technology is lack in virtue of the proprietary technology and technical blockade confidentiality reasons.There is a short of a systematic interaction research on the welding process,heat treating regime and mechanical properties,especially in fatigue property area.At the same time,it needs a detailed test data support during the application of an urgent demand of a force bearing component in the aviation area.For the reason above,there will be a theoretical and applied significance in this paper.This paper aims at an engineering raw material.First of all,the researches on the welding process parameters and five kinds of post weld heat treatment are carried out by the orthogonal test and control variables test.Subsequently,the influences of welding process parameters and heat treatment regime on morphlology, microstructure,welding defects,hardness profile,tensile property and impact properties at room temperature are studied in detail.Finally,high cycle fatigue test of base metal and welded joint at heat treatment state is carried out to test the ?-N curve and fatigue limit.Post weld heat treatment should be carried out for the EBW joint in the engineering application, so the paper obtains three kinds of welding process parameters and two kinds of post weld heat treating regime of 30CrMnSiNi2 A steel at the thickness of 14 mm.The largest influence factors of penetration,weld width and depth to width ratio are accelerating voltage+focus current,welding speed and welding beam,respectively.After the optimization of welding process parameters and heat treating regime,the microstructure of weld joint along transverse direction shows a gradual change of gradient,while,it shows little difference after heat treatment, tempered martensite mainly.The welding defects are caused by the defects in base metal,so it needs to strictly control the quality of base metal in the engineering application.In consideration of the micro hardness profile,tensile and impact properties,there is little difference between three kinds of welding process parameters and two kinds of quenching temperature obtained in this paper.The hardness profile of welded joint is lower,while it shows a smaller fluctuation at the heat treatment state.The hardness profile of quench and high-temperature temper state is lower,while,others are at a moderate state.The tensile strength and yield strength increase with the increase of tempering temperature at the quench and temper state,while the elongation,section shrinkage rate and impact toughness decrease.The fatigue limit of welded joint and base metal is 852 MPa and 834 MPa respectively;the median ?-N curve is 7.01×1028=N(S)7.8268 and 2.57×1029=N(S)8.0289 respectively.For the specimens of welded joint,the fracture location is weld seam at the low stress stae,while it is the base metal at the high stress state.The micro defect at local place is the main influence factor of the fatigue properties.