| Super low carbon martensitic stainless steels are widely used for hydraulic turbines,valve bodies,pump bowls,compressor cones,impellers,and high-pressure pipes in power generation,offshore oil and gas and petrochemical industries,because of their high strength,excellent plasticity and toughness,good corrosion resistance and weldability.According to the increasing severity of corrosion problems in oil field,the strength of super 13Cr products can reach up to 110 ksi,what is more,it has excellent corrosion resistance to high concentration of CO2 and a trace of H2S.But when the temperature exceeds 150 ?,its capacity of corrosion resistance to CO2 decreases and the strength cannot be enhanced further.With the exploration and development of bore,ultra-deep well,the environment under the shaft becomes increasingly harsh,it demands better corrosion resistance and higher strength more than 125ksi to make oil well products.In this paper,we start from the alloy composition project and design six different component steels based on Super Martensitic Stainless Steel 00Crl5Ni6Mo2.We will research how Nb-N micro-alloying,the content of Mo and whether it has Mn,Cu or not influences the microstructure transformation and mechanical properties,the corresponding relation of them by observing the microstructure,testing their mechanical properties after normalizing and tempering and find out a calculation formulation of Ms fitting Super Martensitic Stainless Steel.The principal results are as follows:1.A calculation formulation of Ms fitting Super Martensitic Stainless Steel is got by regression calculation:Ms(?)?539-450(%C+%N)-30(%Mn)-12.7(%Cr)-17.8(%Ni)-22(%Mo)-10(%Cu)-8(%Si)-11(%W)+56(%Nb)The error doesn't beyond ± 10? between measured temperature and theoretical temperature calculated by the empirical formula.2.The microstructure of six kinds of super 15Cr martensitic stainless steel normalized are lath martensitic and they consist of tempered martensite,reversed austenite and carbonitride after tempering at a high temperature region between 590? and 650?.At lower tempering temperature,film-like reversed austenite forms prior at martensite interface or original austenite grain boundary,when the tempering temperature gets higher,the volume fraction of reversed austenite increases.3.In the whole tempering process at a high temperature and cooling after heat preservation,six kinds of super 15Cr martensitic stainless steel have the highest strength after tempering at 590?.With the tempering temperature higher,the yield strength show a downward trend firstly slow,quick to reduce and the rangeability is about 150-400MPa,the strength of extension reduce evenly and slowly at a little rangeability,basically keeping in 1 OOMPa,the elongation and the impact absorbing energy slowly first and then faster present a rising tendency.The rangeability of the elongation keeps within 3%and the impact absorbing energy doesn't surpass 40J.The changing rule of toughness is counter to the strength and has positive correlation with the volume fraction of reversed austenite under room temperature.Thus it can be seen that the reversed austenite existing between the laths of martensite of the tempering steel as a tough phase can observably improve the toughness on the promise of few damage to the strength and enhance the comprehensive mechanical property of super martensitic stainless steel.4.In the high corrosion resistant Super Martensitic Stainless Steel,the austenite stabilized element Ni,Mn,N,Cu need to be added properly into the steel to overcome the adverse effect of Mo promoting the generation of ? ferrite;And Mo strongly suppresses the formation of the reversed austenite in the tempering microstructure so that the yield strength can be obviously improved.Cu not only can enhance the corrosion resistance of Super Martensite Stainless Steel but also can restrain the 5 ferrite and facilitate the reversed austenite as Mn,thus they both marginally reduce the yield strength.The existing of N effectively restrains the formation of ? ferrite and forms special nitride with the micro alloying element Nb to enhance the strength of the steel,but N and C both can markedly reduce the martensite starting temperature. |
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