ارزيابي تاثير نيوبيم بر ريز ساختار و خواص مكانيكي فولاد زنگ نزن آستنيتي 301 فوق ريز دانه / نانو ساختار شده توسط فرايند مارتنزيت

چكيده انگليسي :

2 Evaluating Effect of Niobium on Microstructure and Mechanical Properties of Ultrafine Nanostructured AISI 301 Austenitic Stainless Steel through Martensite Treatment Maliheh Hasani Sadi hasani malihe@gmail com 26th April 2011 Department of Materials Engineering Isfahan University of Technology 84156 83111 Isfahan Iran Degree M Sc Language FarsiDr Abbas Najafizadeh Prof Supervisor E mail A najafi@cc iut ac irDr Mohammad Reza Toroghinejad Assoc Prof Supervisor E mail Toroghi@cc iut ac irDr Ahmad Kermanpur Assoc Prof Advisor E mail Ahmad k@cc iut ac irAbstractIn recent years a great attention has been paid on the development of high strength nanostructured steels However the lack of high ductility in the ultrafine nano grained steels restricts their wide spread use inconstructional applications In this study an ultrafine nano grained 301 austenitic stainless steel containing0 15wt niobium was fabricated by the thermomechanical martensite treatment This treatment includesstrain induced martensitic transformation by cold rolling followed by reversion annealing Effect of niobiumon the reversion behavior and mechanical properties of the ultrafine grained AISI 301 was investigated Forthis purpose ingots of the AISI 301 steel containing 0 15 wt niobium were prepared in vacuum inductionfurnace homogenized at 1200 C for 10h forged at 1000 1200 C and finally solution treated at 1200 C for6h After every stage specimens were quenched at water to prevent precipitation of second phases Theannealed specimens were then cold rolled at 0 and 25 C with different reduction in thickness up to 90 Thecold rolled sheets were then annealed at 700 900 C for 15 1800s Start and finish temperatures of thereversion transformation was determined by a dilatometric test The microstructures and mechanicalproperties were evaluated by means of optical and scanning electron microscopy with EDS analysis X raydiffraction ferritscope microhardness and tensile test The results showed that the amount of delta ferrite was decreased from 6 8 in the cast ingots to 3 5 0 51 and 0 12 after homogenization hot forging and solution annealing respectively The grain size after6 hrs solution annealing at 1200 C was 200 m The EDS analysis confirmed that the chromium carbides decomposed after solution annealing were precipitated at grain boundaries of the hot forged specimen Increasing thickness reduction decreasing rolling temperature and cross rolling resulted in an increase in thevolume fraction of martensite but a decrease in c In addition increasing annealing time and temperature ledto an increase in the volume fraction of reversed austenite A fully austenitic structure was observed in thespecimen annealed at 850 C for 300s with the austenite grain size of 130 19 nm By increasing theannealing time to 1800s the grain size was reached to 1900 170 nm The ultrafine grained steel annealed at850 C for 300s possessed an exceptional elongation of 46 high tensile strength of 1012 MPa due to bothnano sized precipitates and strain induced martensite that formed during tensile test The cold rolledspecimen at 0 C and annealed at 850 C for 300s illustrated the grain size of about 90 8 nm Increasing theannealing time and temperature resulted in a decrease in hardness due to the increase in volume fraction ofreversed austenite and grain growth Retardation in the reversion of this steel compared with the similarstainless steel AISI 301 without niobium was attributed to the niobium microalloying element Keywords 301 Austenitic stainless steel Martensite treatment Reversion annealing Ductility Ultrafine nano grain structure Niobium

حسني سعدي، مليحه

ارزيابي تاثير نيوبيم بر ريز ساختار و خواص مكانيكي فولاد زنگ نزن آستنيتي 301 فوق ريز دانه / نانو ساختار شده توسط فرايند مارتنزيت

آنيل بازگشتي , استحكام كششي , انعطاف پذيري , نيوبيم