ارزيابي متغيرهاي فرايند مارتنزيت بر ريز ساختار خواص مكانيكي فولاد زنگ نزن آستنيتي 201 فوق ريز دانه/نانو ساختار

چكيده انگليسي :

Investigation of Process Parameters of the Martensite Treatment on the Microstructure and Mechanical Properties of the Ultrafine Nano Grained 201 Austenitic Stainless Steel Mohammad Moallemi m moallemi@ma iut ac ir Date of Submission 2011 03 5 Department of Materials Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language Farsi Supervisor Abbas Najafizadeh a najafi@cc iut ac ir Ahmad Kermanpur ahmad k@cc iut ac ir Abstract Production of ultrafine nanostructured steels with simultaneous high strength and good toughness has been the center of attention in recent years Martensite process is one of the advanced thermo mechanical processes for production of ultrafine grained austenitic stainless steel This process is carried out by cold rolling of steel and formation of strain induced martensite followed by reversion of microstructure to the ultrafine grained austenite The aim of the present project was to evaluate the process parameters of the martensite treatment for production of the ultrafine nano grained 201 stainless steel The 201 austenitic steel slabs were produced by casting in permanent mold followed by homogenization at 1200 C for 15 h The slabs were then hot forged in the temperature range of 1150 1200 C In order to reduce secondary phases the forged specimens were annealed at 1150 C for different time periods Both one step and two step martensite processes were applied on the specimens The annealed samples were cold rolled at 10 0 and 25 C followed by reversion at 750 900 C for 15 1800 s Microstructures were characterized by light and scanning electron microscopes Phase characterizations were conducted by feritscope and XRD methods A mathematical model was used to predict the behavior of austenite to martensite transformation behavior during the rolling process Mechanical properties of the final products were examined by hardness and tensile tests The results revealed that homogenization at 1200 C caused an enormous grain growth with considerable reduction in the amount of delta ferrite Although grain refinement was occurred during hot forging the volume fraction of delta ferrite was increased This increase was attributed to the released adiabatic heat from the localized molten zones in steel during hot forging process The saturation strain of martensite fraction in ambient temperature was 0 4 It was found that the lower cold rolling temperature and higher initial austenite grain size resulted in a smaller martensite saturation strain A good agreement was found between the predicted amounts of the strain induced martensite from mathematical model and the experimental data Based on the mathematical model the maximum rate of austenite to martensite transformation was accorded in 0 1 strain The results from the reversion process showed that in early stages the martensite fraction was first decreased but it was then increased eventually This increase was because of the formation of carbides that caused an increase in the Ms temperature producing the formation of thermal induced martensite during quench process The combination of the maximum reversion along with the smallest grain size was achieved in the one step martensite process at 850 C The least grain size for this specimen was about 90 nm with the ultimate tensile strength of 1520 MPa and elongation of 27 The grain size in the two step rolling process after the secondary reversion at 850 C for 60 s was about 260 nm The ultimate tensile strength and elongation were 1285 MPa and 32 respectively Keywords Advanced thermomechanical process Stainless steel Cold roll Reversion annealing Martensite

معلمي، محمد

ارزيابي متغيرهاي فرايند مارتنزيت بر ريز ساختار خواص مكانيكي فولاد زنگ نزن آستنيتي 201 فوق ريز دانه/نانو ساختار

فرايند ترمومكانيكي پيشرفته , نورد سرد , آنيل بازگشت