Low temperature mechanical properties of 460 MPa polar ship steel and its welded joints
The steel used for polar ships and its welded joints must be safely served at low temperatures.In order to study the effect of polar low temperature environment on the properties of ultra-high strength polar ship steel and its welded joints,the 460 MPa strength grade low temperature steel with thickness of 54 mm for polar ships was pro-duced by thermo-mechanical control process(TMCP),and welding tests were conducted by submerged arc automat-ic welding(SMA)process.The microstructure of base metal,welded line and welding heat affected zone(HAZ)was analyzed.The tensile strength,fracture toughness and fatigue limit strength of base metal and welded joint were tested at low temperature.The results show that the microstructure of base metal is mainly composed of polyg-onal ferrite(PF),pearlite(P)and granular bainite(GB).In the welded joint prepared by submerged arc automatic welding,the center structure of the weld is mainly composed of acicula ferrite(AF)and granular bainite(GB),the welding fusion line is clear,the coarse grain heat affected zone(CGHAZ)structure is mainly bainite(B)with coarse grains,and the fine grain heat affected zone(FGHAZ)shows a normal microstructure.The temperature directly af-fects the mechanical properties of base metal and its welded joints used in polar ships.With the decrease of tempera-ture,the strength of base metal and welded joint increases and the impact work decreases.Under the extreme low temperature environment of-70 ℃,the yield strength of base metal can reach more than 500 MPa.At-10 ℃,the characteristic values δm of crack tip opening displacement(CTOD)of base metal,welded line and heat affected zone are 1.18,1.11 and 0.98 mm,respectively.With the decrease of temperature,the fracture resistance of the material decreases significantly.At-40 ℃,the CTOD characteristic values δu of base material,welded line and heat affected zone are 0.62,0.70 and 0.32 mm,respectively.In the extreme low temperature environment of-70 ℃,brittle fracture occurs in the samples of the heat affected zone.The fatigue property of base metal is better than that of wel-ded joint,and the tensile strength and the fatigue ultimate strength of the material can be increased at low tempera-ture.It is necessary to pay attention to the welding process and welding quality of the low temperature steel used in polar ships,so as to improve the actual low temperature application performance of the welded joint.
polar ship steelTMCPlow temperature propertyheat affected zonecrack tip opening displacement(CTOD)