Design And Research Of High Strength And Ductility Hydrogen Resistant Stainless Steel Based On Supermartensitic Stainless Steel

With the increasing global energy demand,oil and natural gas exploitation is moving towards high-temperature,high-pressure,and high-corrosion deep oil and gas reservoirs,which puts forward higher requirements for tubing and casing materials.Martensitic stainless steel with good CO2 corrosion resistance and high strength has become an important candidate.However,martensitic stainless steel cannot be used in the service environments where the H2S partial pressure exceeds l0kPa,which becomes its Achilles heel.Improving the resistance to H2S stress corrosion(hydrogen embrittlement)of martensitic stainless steel is one of the urgent problems.The martensitic structure API 5CT P110 steel with low alloy content can be used in higher H2S partial pressure environment.It is essential to understand the hydrogen-induced fracture behavior and hydrogen embrittlement mechanism of martensitic stainless steel.This work first comprehensively studies the hydrogen embrittlement behavior of martensitic stainless steel from the perspectives of hydrogen diffusion and hydrogen concentration.Furthermore,based on the design of heterogeneous structure,high strength and plasticity hydrogen resistant stainless steels are designed and fabricated.The obtained conclusions are as follows:(1)The high hydrogen embrittlement susceptibility of 13Cr super martensitic stainless steel is mainly due to its high hydrogen concentration.IPZ hydrogen adsorption model analysis shows that 13Cr super martensitic stainless steel has a high hydrogen adsorption kinetic parameters,that is,13Cr super martensitic stainless steel has high tendency to adsorb hydrogen.The introduction of the precipitated austenite can improve the hydrogen embrittlement resistance of martensitic stainless steel because 1)soft austenite reduces the yield strength and hydrogen embrittlement susceptibility decreases with decreasing strength.2)The precipitated austenite can act as a hydrogen trap to reduce the effective hydrogen diffusion coefficient and thus hinder hydrogen diffusion to sites where stress is concentrated.The precipitated austenite will undergo a martensitic transformation,providing additional work hardening rate and ultimate tensile strength,which causes the yield ratio of martensitic stainless steel to decrease as the austenite content increases.At high hydrogen concentrations,a martensitic transformation is facilitated by hydrogen,resulting in local stress concentrations and high local hydrogen concentrations.Thus,hydrogen induced cracks are more likely to initiate or propagate in these regions,causing premature failure.(2)By eliminating the austenite memory effect and adjusting the internal stress in the retained austenite,the mechanical properties of the 15Cr super martensitic steel are optimized.The compressive stress in the retained austenite formed by quenching is not uniform,and its shear stress will cause the austenite to form stacking faults.Depending on the overlapping nature of stacking fault on the {111}?planes,stacking fault bundles,nano-twins and hexagonal close-packed martensite are formed.After annealing,the compressive stress transforms into hydrostatic compressive stress.The existence of hydrostatic compressive stress can stabilize the austenite and thus increase the elongation without decreasing the yield strength.Based on the principle of lowest energy,the reversed austenite usually satisfies the austenite memory effect.The reversed austenite has the same orientation or a twinning relationship as the original austenite.Cold rolling can provide needed strain energy to suppress the austenite memory effect and decrease the austenite transformation start temperature.This promotes significant reversed austenite nucleation and retards growth.(3)The 13Cr supermartensitic stainless steel was austenitized by adding 8wt.%Mn.Through the heterostructure design—remaining martensitie grain dispersed between lath austenite grains and micrometer sized austenite grains embedded inside ultrafine grains,a 125 ksi grade austenitic stainless steel with 33.2%total elongation is obtained.The austenitized structure effectively limit hydrogen uptake to a depth of?260 ?m from the surface,and the hydrogen induced threshold stress is increased by 40%to 860 MPa compared with 13Cr supermartensitic stainless steel.The dissolution of Mn oxides makes the austenitized 13Cr stainless steel have a faster initial corrosion rate.However,due to the coverage of Cr and Mo oxides,it still maintains a better corrosion resistance in acid-base solutions.In the acid solution,due to process of Mn oxides dissolution and Cr/Mo oxides coverage,the formed oxide film is not dense.It is susceptible to the attack of Cl-and caused the rupture of the passivation film.(4)A 15Cr-8Ni type austenitic stainless was designed by increasing the content of Cr and Ni in supermartentic stainless steel.Through the heterostructure design—Annealing twins split retained austenite grains,block martensite distributed in the retained austenite grains,fine-grained reversed austenite grains and residual martensite grains dispersed distribution—a 125 ksi grade austenitic stainless steel with 27.47%total elongation is obtained.The hydrogen induced threshold stress of designed A15Cr sample reached 890 MPa due to the low hydrogen diffusion rate(limiting hydrogen uptake to a depth of?124 ?m from the surface)and high critical hydrogen solubility(quasi-cleavage mode of fracture structure in hydrogen affected zone)of the austenitized structure.The A15Cr sample has good resistance to uniform corrosion and Cl-pitting because of the addition of corrosion resistant elements—Cr and Ni.