| In order to meet the requirements of high temperature,high pressure and other aggressive environments,multi-component alloying is applied to obtain stainless steels with good high-temperature oxidation resistance and good mechanical properties.The addition of alloying elements can affect the austenite stability of stainless steels.The effects of alloying elements on the austenite stability,oxidation properties and mechanical properties should be considered comprehensive.At present,the most widely used method of phase prediction is the composition equivalent method.However,there is still lack of in-depth study on the composition equivalent of stainless steels.On the other hand,in the composition design of Alumina-Forming Austenitic(AFA)stainless steels,there is lack of quantitative analysis tools to study the effect of chemical short-range-order structure on the austenite stability,and also to match the multi-component alloying elements.This thesis studied the austenite stability of stainless steels through composition equivalent method and short-range-order structural model.Newly developed phase-boundary equivalent coefficients were calculated based on Fe-M binary phase diagrams.A novel parameter,which evaluates the absolute austenite stability,was proposed and applied to the analysis of common stainless steels.The cluster-plus-glue-atom model was introduced to study the short-range-order structure of AFA stainless steels.The cluster formula was obtained by the composition analysis of existing AFA stainless steels and was then confirmed by matching main alloying elements.Based on the cluster formula,microalloyed AFA stainless steels were designed for high-temperature austenite stability and good oxidation resistance.The main results of this thesis are as follows:1.The composition-equivalent method of stainless steels was understood via the slope of the phase boundary line separating ? and ?+? phase zones in Fe-M(M stands for any alloying element)binary phase diagram.By analyzing prevailing equivalent coefficients and Schaeffler constitution diagram,it is concluded that the value of equivalent coefficient is affected by the element content,and Schaeffler diagram can be regarded as a special representation of non-equilibrium phase diagram of multi-element alloy system.The slope of?/(?+?)phase boundary is used to evaluate the ? stabilizing efficiency of any alloying element in steels.Its negative or positive sign indicates ? stabilizer or ? stabilizer,and its value represents the stabilizing efficiency.The prevailing equivalent coefficients are well interpreted using the slope ratios of the alloying elements divided by that of Ni or Cr,after analyzing over one hundred common stainless steels.This composition-dependent slope of?/(?+?)phase boundary well explains the empirical equivalent coefficients and can be used to calculate the phase-boundary equivalent coefficients of any alloying element in steels.2.According to the decomposition path of austenite in Fe-M binary phase diagram,a novel ? decomposition vector,?M,was proposed to effectively evaluate the austenite stability of stainless steels.The ? decomposition vector was defined starting from the pure Fe point(0,912),pointing to the nearest point(XMend,TMend)that ? phase starts to decomposition to a phase rather than a phase.This parameter evaluates the absolute ? stability of any alloying element,and also correct the phase-boundary equivalent coefficient of Co which was calculated by the slope of the ?/(?+?)phase boundary.A total ? decomposition vector of stainless steels,?s,can be calculated by the weighted summation of ? decomposition vectors of each alloying element.This ?s reflects the austenite stability of stainless steels and was proved to be effective by analyzing more than one hundreds of common stainless steels.3.The compositions of the existing AFA stainless steels were analyzed by introducing cluster-plus-glue-atom model.Based on that,the austenitic AFA steels targeting high-temperature structural stability were designed,and the cluster formula of this kind was determined by studying the designed alloys.In order to form single-phase austenite,it is necessary to match the contents of Ni and Al.190 heat resistant stainless steels which are protected by Al2O3,were analyzed by a previously developed cluster-plus-glue-atom model.A 16-atom-cluster formula,including 1 center atom,12 shell atoms and 3 glue atoms,simplified as[Al1-Fe12]-Cr3,was proved applicable to these steels.By considering the complementation of Ni and Cr equivalent,two groups of AFA stainless steels were designed as:AlxSi0.05Nb0.15-Fe8.7Ni3.0Mn0.3-Cr3.6-xMo0.2(x=0.8,1.0 and 1.1)and Al1Si0.05Nb0.15-Fe11.7-yNiyMn0.3-Cr2.6Mo0.2(y=3.2,3.4,3.7 and 4.0),with a constant C of 0.1 wt.%.These two series have constant Ni with increased Al(instead of Cr)and constant Al with increased Ni(instead of Fe),respectively,and were used to study the effect of Ni and Al on austenitic stability under solutionized(1250?/1.5 h)and the same solutionized then aged(800?/24 h)conditions.The microstructural characterization shows that,in 16-atom cluster formula,with constant Ni of 3.0,the matrix is single-phase austenite when Al is 0.8;while ferrite is formed when Al is 1.0 and 1.1.With constant Al of 1.0,the matrix remains single-phase austenite when Ni ranges from 3.2 to 4.0.In 16-atom cluster formula,Al0.8(2.45 wt.%)and Al1(3.08 wt.%)need Ni3.0(20.00 wt.%)and Ni3.2(21.43 wt.%)to avoid the formation of ferrite,respectively.The ideal cluster formula of AFA stainless steels is identified as[(Al,Si,Nb)1-(Fe,Ni,Mn)12](Cr,Mo,W)3,which describes the average distribution of atoms in alloys.4.Based on the determined 16-atom-cluster formula which solved the matching of major alloying elements,AFA stainless steels targeting high-temperature oxidation resistance were designed by minor-alloying.The 800? air-oxidation resistance of 0.08 wt.%C alloy series satisfying cluster formula[(Al0.89Si0.05NbxTa0.06-x)-(Fe11.7-yNiyMn0.3)]Cr3.0-z(Mo,W)z,x=0.03 or 0.06,y=3.0 or 3.2,z=0.07 or 0.2,were tested.All samples are graded as complete oxidation resistance level according to Chinese standard HB 5258-2000,as their oxidation rate and oxidation-peeling mass are generally below 0.1 g/m2 × h and 1.0 g/m2,respectively.In alloys without Ta and W,a Cr2O3-type oxide layer is formed on the surface and Al2O3 particles of sizes up to 4 ?m are distributed beneath it.In contrast,in Ta/W-containing alloys,a continuous protective Al2O3 layer is formed beneath the outer Cr2O3 layer,which prevents internal oxidation and provides the lowest weight gain(0.0059?0.0093 g/m2×h).Instead of internal Al2O3 particles,AlN is formed in Ta/W-containing alloys.The W-containing alloy possesses the thinnest internal nitride area,indicating the good inhibition effect of W on nitrogen diffusion. |
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