The Organization And Properties Research Of A Marine Engineering High Strength And Toughness High Nitrogen Steel

With the development of China’s marine engineering, marine engineering steel areas need a lot of high strength and corrosion-resistant steel fasteners bolts,the studs of 10.9 above the level of high-strength steel we still need to imported from abroad now.This article is based on marine engineering bolted steel as the background,use the method of industrial production,produced a Cr20Mn18N0.8high nitrogen stainless steels with excellent comprehensive mechanical performance.Conducted solution treatment and aging annealing treatment to the HNS (analog self-annealing effect precipitation of nitrides in stud welding process).After that,conducted a series of micro-structure analysis,tensile performance test,corrosion resistance test,impact toughness test and fatigue test. Optimize the thermodynamic model stacking fault energy (SFE) calculations,researched the relationship between toughening mechanism HNS and stacking fault energy and combined the Schaeffler organization chart and stress-strain of stacking fault empirical formula to predict HNS"composition-organization-stacking fault energy-mechanical properties"quantitative relationship.Studies have shown that:(1)Cr20Mn18N0.8 nitrogen steel has a excellent mechanical properties, conditions of high fatigue strength, good resistance to pitting corrosion, which can meet the majority of marine steel performance requirements.(2)HNS yield strength reached 640Mpa after solution treatment,and the tensile strength reached 970Mpa,it also remains around 52% elongation and reduction of 43%;NSR (notch sensitivity factor) up to 1.22, has a excellent strength and toughness.Under the 4 degree and 8 degree skew angle tensile conditions, the yield strength remains at 729Mpa and tensile strength keep up at 1000Mpa, NSR remained at about 1.03. HNS fatigue limit was 330Mpa, more than the 304 stainless steel’s conditions fatigue limit 236Mpa, can satisfy the requirements of most marine steel fasteners. HNS impact absorption workup to 311J and 280J in room temperature and low temperature -40 ℃, the fracture is break off status dimples and belong to ductile, it has a good room temperature and low temperature impact toughness.(3)HNS yield and tensile strength of aging treatment basically equal to solid solution HNS, it also remains around 48% elongation and reduction of 36%,NSR (notch sensitivity factor) up to 1.16,and still has a good strength and toughness.Under the 4 degree and 8 degree skew angle tensile conditions,the yield strength and tensile strength also remains at a high level, NSR remained at about 1.15.With increasing of aging time, the HNS impact absorbed energy at room temperature and low temperature -40 ℃ dropped, low temperature -40 ℃ in more pronounced impact fractures break off status dimple-like crystal to crystal sugar gradually evolved;The XRD diffraction experiments show that after aging stretch of high nitrogen steel precipitates contain Cr2N diffraction peak.The HNS of aging 4h not only precipitates a large number of Cr2N diffraction peak but also have a certain amount of ε phase diffraction peak -mechanical twin crystal.It showed that,HNS occurred twin crystals induced plasticity effect (TWIP) in the process of tensile deformation, which also proved that why the HNS after aging treatment still have a good ductility.(4)Chemical soaked corrosion tests showed that,316L stainless steel case arises pitting in the short span of 24h, however,the HNS didn’t appear any corrosion.In NaCl solution liquid with a same corrosion current,electrochemistryl corrosion test show that,the electric potential of HNS was much higher than 316L stainless steel, the corrosion resistance of HNS was more than 316L stainless steel.(5)Use the improved thermodynamic model calculations, the stacking fault energy of solid solution treatment HNS was 14.73mJ/m2.Take the calculated values into the stacking fault energy with the yield strength and the corresponding strain empirical equation,the result shows that when yield occur,the stress and strain of the theoretical value and the actual experimental values are basically in the same.Combined solution treatment and tensile mechanical properties of HNS,find out the stacking fault energy influence the strength and toughness of the material, in a range of stacking fault energy, with the lower stacking fault energy, the yield strength of the material is increased, the elastic strain, elongation increases, the strength and toughness of material is increasing.