| From the 1980 s to this century,the research on high nitrogen stainless steel is one of the hot spots in the research of structural materials.Because of its excellent corrosion resistance,it is considered to have broad application prospects in marine and petrochemical industries.Due to the non-magnetic properties,high strength and high impact toughness of high nitrogen austenitic stainless steel,it has become an important material for power station retaining ring steel and high-quality oil drill collars.Nitrogen-controlled and high-nitrogen stainless steels have developed into important categories of stainless steel families.This paper takes 0Cr20Mn18Mo2Ni1.8N0.8 high nitrogen steel for non-magnetic drill collars as the research object.After solution treatment,tensile properties and impact properties at different deformation rates are tested,and the deformation microstructure is observed.Scanning electron microscopy Tensile and impact fracture analysis were carried out to summarize the fracture mechanism of the test steel;through compression experiments with different deformations,the deformation structure was observed and analyzed,the samples after compression deformation were annealed,the metallographic structure was observed,and the recrystallization annealing temperature was determined.Stress-relieving annealing provides reference data;aging and different process annealing treatments are carried out on the test steel respectively to observe the characteristics of aging precipitation;fatigue experiments of 0Cr20Mn18Mo2Ni1.8N0.8 steel under different loads are carried out to calculate the fatigue strength,draw its S-N curve,and combine the field The microscopic mechanism of fatigue crack initiation,propagation and unstable fracture of 0Cr20Mn18Mo2Ni1.8N0.8 steel was determined by SEM fracture analysis.The results show that:(1)After precision forging,solid solution or low temperature annealing,the grains in the single-phase austenite structure are equiaxed,a small amount of austenite grains distribute deformation twins,and the austenite matrix distributes deformation twins.High temperature stable compound in the form of coarse salt particles.(2)After precision forging and solution treatment,the test steel has high strength,ultra-high plasticity and ultra-high impact toughness,and the highest impact toughness value is nearly 380 J;in the solid solution plate tensile test,the fracture mode is plastic fracture.(3)With the increase of compressive deformation,a large number of twin crystals are distributed in the structure,the twin crystal region has an obvious outline,and the twin crystal is slightly bent under the action of internal stress.The recrystallization temperature of high nitrogen steel with 15% deformation is above 1100?,and the recrystallization temperature of high nitrogen steel with 30% deformation is in the range of 850 to 1000?and lower than 1000?.For the high nitrogen austenitic stainless steel after solid solution,the nucleation and growth of aging precipitation occurs preferentially at the grain boundary,and there is no obvious precipitation in the grain.As the amount of deformation increases,intragranular deformation substructures such as dislocation cells increase,and crystal defects increase.(4)The test steel meets the material mechanics and processing performance requirements of high-strength non-magnetic drilling tools,and can be developed as a product to replace imports.The conditional fatigue limit of forged high nitrogen steel is865.25 Mpa,and the conditional fatigue limit of solid solution high nitrogen steel is736.10 Mpa.According to the S-N curve fitting results,the fatigue properties of forged alloys at room temperature are generally higher than those of solid solution high nitrogen steels.The fatigue source of forged high nitrogen steel is precipitates or inclusions,while solid solution high nitrogen steel is the uneven stress distribution at the edge of the sample.However,the fracture rate of solution high nitrogen steels is lower than that of raw wrought alloys.Forged high nitrogen steel has higher strength and therefore longer fatigue life.But it is more prone to sudden failure,while solid solution alloys are more stable. |
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