Research On Microstructure,Ductility And Toughness Of A New Low Cost Ferritic Stainless Steel Welding Joints

12%Cr content stainless steels are characterized by ultra-low carbon and nitrogen and a small amount of Ti and Nb addition to guarantee good weldability and corrosion resistance.The welding joint of this kind of steel exhibits great inhomogeneity in microstructure and mechanical properties after welding thermal cycles thus HAZ（heat affected zone）is the weak link of the welding joint.Ductility and toughness especially subzero toughness decreases greatly and it is the major bottle neck factor for wide application of this type of stainless steel.In this thesis,12Cr stainless steels produced in China,in conformance to 1.4003 grade of EN10088,were thoroughly investigated.The 2^st generation 12Cr stainless steel grades were developed based on the results of investigation of welding processes and weldability of 1^st generation steels,in order to improve the weldability of the grades.Welding thermal simulation,heat treatment,optical microstructure,SEM,TEM and EDS were used to study the characterization of microstructure.The hardness indentation,tensile and impact testing were used to study the mechanical properties.Many valuable results on the evolution of microstructures and mechanical properties of the grade were acquired and toughening mechanism for martensite was discovered,thus revealing the means to improve weldability of 12Cr F+M（ferrite+martensite）dual phase stainless steels substantially.The main results and conclusions are as follows:（1）The first generation 12Cr stainless steels are mainly composed of ferrite at room temperature for lower austenitizing elements as well as dual stabilizing elements Nb and Ti addition,and the mechanical properties of welding joints decrease notably because of coarse ferrite grains.Welding thermal simulations based on the measured welding thermal curves of HAZ were carried out with GLEEBLE 1500 to investigate the microstructure and mechanical properties of micro zones of HAZ.The results show that HTHAZ（high temperature HAZ,peak temperature ranging from 1200℃to melting point）composed of coarse ferrite grains exhibits brittleness even at room temperature,while LTHAZ（low temperature HAZ,800-1200℃）composed of fine martensitic grains demonstrates lower DBTT of 0℃or so.（2）The second generation 12Cr stainless steels were developed by adjusting K_ff（kaltenhuaser ferrite factor）from 8.14 to 7.54,leading to increase of proportion of martensite at room temperature,in the view of improving the weldability of the steels.Heat treatment experiments of base metal were conducted at the temperatures ranging from 600℃to 1000℃,and the effect of heat treatment parameters on the microstructure and mechanical properties evolution of the steels was investigated and concluded.In addition,the intrinsic relationships between heat treatment and welding thermal cycles was analyzed and proposed.（3）The innovative double heat treatment procedure was developed and applied for welding joints,in which the welding joints were held at temperature just below A_c1 for a short time（subcritical heat treatment）after they were recrystallized atα+γzone and fine grain martensite formed at room temperature.As a result,the impact toughness of the samples increased to 80 J/cm³ at-40℃.The brittle non–equilibrium martensite transforms to a type of microstructure with high toughness at subzero temperature,and the microstructure possesses excellent stability as well.Further investigation on toughening mechanism shows that dislocation density in martensite decreases greatly through subcritical heat treatment.In addition,small angle（≤10°）subgrain boundaries aggregate and merge into middle（10-15°）and large subgrain boundaries（≥15°）,thus decrease the density of defects in martensite.The factors above are essential to decrease the energy in martensite phase thus increase the ductility and toughness of the steels.The optimized PWHT procedure is holding at 780℃for 3t minutes,here,t standing for thickness of steels,then followed by rapid cooling.（4）Systematic investigation on microstructure and mechanical properties evolution of 12Cr dual phase stainless steel was carried out and microstructural and mechanical features and evolution rules of low carbon low nickel martensite were revealed.The way to acquire tough martensite in 12 Cr stainless steels through heat treatment was discovered and dynamic feature of the transformation was investigated through various experiments.The relationships among mechanical properties,microstructure feature,holding temperature and time were discovered.The finding was of important theoretical and practical significance to improve comprehensive properties of martensite in stainless steels,hence promoting further development and extensive application of low chromium stainless steel grade.