Effect Of Phase Morphology On Hydrogen Embrittlement Resistance Of Duplex Stainless Steel

Hydrogen embrittlement(HE)is one of the reasons causing failures of equipment in hydrogen-containing environments.In the near future,the equipment for hydrogen energy production and utilization systems,such as hydrogen storage pressure vessels and hydrogen transport pipelines,will also face the threat of HE.Therefore,it is very important to improve the HE resistance of metallic materials for hydrogen-containing environments.Besides developing new HE-resistant materials,it is more convenient and economic to increase the HE resistance of materials on hand by means of modifying the microstructures.Ferritic(α)+austenitic(γ)duplex stainless steels(DSS)such as type 2205 and 2507 have been widely using in the oil and gas,petrochemical,marine and other industries,because of its higher strength,good weldability and excellent corrosion-resistance in Cl~--containing environments.However,the HE resistance of DSS is lower than austenitic stainless steels(ASS).There is a need to improve the HE resistance of DSS,making it possess both higher strength and HE resistance and thus capable of using in hydrogen energy systems.In the thesis,2205 and 2507 DSS was used.The effects of combined heavy cold-rolling and annealing treatment on the microstructural morphologies,mechanical properties and HE susceptibility were studied.The main research contents and conclusions are as follows:(1)Heavy cold-rolling was carried out for 2205 and 2507 steel sheets until a total thickness reduction of 90%.Afterwards,the cold-rolled sheets were annealed at 1050℃ for different durations.The effect of annealing duration on microstructure of 2205and 2507 DSS was investigated.The as-received specimen has a standard ferrite-austenite duplex-phase microstructure.90%cold-rolling deformation caused the two phases to be striped and continuous along the cold-rolling direction,and to be refined.By short-time annealing treatment(0.5 min and 0.75 min),the ferrite and austenite phases showed still continuous long strips along the rolling direction.However,with the increase of annealing time,the two phases’strips gradually broadened,while they started to break at their narrow sites,and the strips started to discontinuous,so that the austenite phases gradually showed island chains along the cold-rolling direction and the transverse direction.In addition,with the increase of annealing time,the grain size increased and the dislocation density decreased.(2)The effects of the microstructural difference caused by combining cold-rolling and annealing treatment on mechanical properties were studied.The cold-rolling strengthened the materials because of the increase of dislocations,the work-hardening and refinement of microstructures.Short-time annealing treatment decreased the work-hardening effect.However,the strength of the materials was still higher than the as-received materials and the elongation was improved.Long-time annealing treatment had no large effect on the mechanical properties in comparison with the as-received specimens,though it modified the microstructures.The results showed that the heavy cold rolling+short time annealing treatment could improve the DSS strength without reducing the plastic deformation capacity.(3)Based on electrochemical hydrogen pre-charging tests and tensile tests,the effects of phase morphology on HE of the two materials were investigated to show the feasibility of improving the HE resistance of DSS through modifying phase morphology.The results showed that the elongation of all specimens after electrochemical hydrogen pre-charging was reduced compared with the uncharged specimens,indicating the occurrence of hydrogen embrittlement.The HE susceptibility was as varied as the microstructural morphology.Short-time annealed specimens contained more dislocations,finer grain size,more small-angle grain boundaries,and thus more hydrogen traps.In addition,the austenite and ferrite phases were stacked in a continuous narrow strip alternating morphology,so the hydrogen had to diffuse through the austenite phase in order to entering into the interior of the specimen.As a result,the hydrogen diffusion coefficient was reduced,the depth of the embrittlement zone was shallow.Therefore,the hydrogen embrittlement sensitivity of short-time annealed specimens was low.The analysis shows that the short time annealing treatment after heavy cold rolling can impart not only higher strength but also higher hydrogen embrittlement resistance to the DSS.In the long-time annealed specimens,the austenite phase was discontinuous,showing a morphology of dispersed island chains.Therefore,hydrogen could diffuse along the ferrite by bypassing the austenite phase islands,and the diffusion coefficient was high,so the hydrogen embrittlement susceptibility was high.It is shown that the heavy cold rolling+short annealing treatment can reduce the hydrogen diffusion coefficient and improve the hydrogen embrittlement resistance of DSS by modifying the hydrogen trap density and the phase distribution morphology.