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The present paper involves a fundamental research on microdomain yield behavior of an ultrahigh strength low alloy steel with high temperature tempered bainite.The smooth cylinder specimen was took from deep water mooring chain links from the steel with the chemical composition of 0.23C-0.25Si-0.70Mn-3.55 Cr+Ni+Mo-0.13 V+Nb+Ti(mass%),which was quenched from 1253K and then tempered at 873K.Its macroscopic yield strength is 1120MPa and the tensile strength is 1250MPa.In-situ neutron diffraction measurements of loading tension have suggested that a good linear elastic deformation can be kept up to 500MPa stress,and then(200)priority non-linear elastic strain,that is the yield of crystal lattice occur at 700MPa and the(110)non-linear elastic strain was found at 800MPa.The(200)and(110)nonlinear elastic strain increases gradually when the stress was further increased,however,the(211)kept its linear elastic deformation stage as before.The sub-microstructural analysis carried out using TEM and additional determine the nature and quantitative analysis has revealed that there are three kinds of alloy carbides:(1)-M3C cementites with an average particle size of less than 50 nm which inside laths and lath boundaries;(2)-M2C formed uniformly within the ferrites with a length of less than 200 nm and width of less than 20 nm;(3)ultra-fine high density MC cohered with matrix-Fe and its particle size is about 2 nm.The whole microdomain yield behaviour of the material was possibly influenced by the fcc-MC with high density.The results of CLT(constant load),SSRT(slow strain rate)and KIscc test of the present chain in seawater solution indicate,that threshold value of SCC(stress corrosion cracking)stress exceed 0.8 tensile strength and the chains KIscc value is double of KIscc value of 4340 steel type parts.MC not only form strong hydrogen trap,but also slow down microdomain yield likely by means of increasing yield strength of crystal lattice,thus reduce SCC sensibility of the steel.