Cluster-formula-based Coordinated Composition Relationship Of 304 Stainless Steel And Relevant Sensitizing Behavior

The industrial standards of steels cover wide elemental ranges.This is why in practical production each enterprise has to specify its own empirical composition standard,which makes the product quality control difficult.Taking the most popular stainless steel 304 as the typical example,this paper uses a cluster formula approach to interpret the standard composition and identify the coordinated composition relationship,and even to propose a more stringent one.Due to the similar atomic sizes of the major alloying elements Cr,Si,Ni,and Mn,and after classifying the elements into austenite stabilizer（Ni,Mn）and ferrite stabilizer（Cr,Si）,it is determined that the smallest structural unit that carries the alloy composition satisfies a 16-atom cluster formula（C,P,and S do not enter the formula）.This formula is then used to analyze the elemental ranges of the GB standards and the compositions reported in the literature.It is found that the real compositions fall within（Cr,Si）_3³.5-（Ni,Mn）_1.5±0.25-Fe_11.5±0.25（The numerical subscripts following each element in this cluster-formula indicate its atomic number）,much narrower than the standard one.Further,within the framework of the GB standards and in combination with the Cr and Ni equivalents,a new form of composition standard is proposed,which reflects the coordinated variations of the wt.%elemental ranges:1) when 18.0≤Cr+1.7Si≤19.0,0.6（Cr+1.7Si）-2.3≤Ni+1.06Mn≤2.5（Cr+1.7Si）-35.9;2)when 19.0≤Cr+1.7Si≤20.6,0.56（Cr+1.7Si）-1.59≤Ni+1.06Mn≤11.6;3)8.5≤Ni+30C+0.5Mn≤13.3;4)0.5（Cr+1.5Si）-（30C+0.52）≤Ni+0.5Mn≤2（Cr+1.5Si）-（30C+27.0）（wt.%）.This coordinated composition relationship for 304 steel provides a basis for the compositional control in practical production.It also shows the very first example to scrutinize the compositions of all steels.This work revisits the C-（Cr,Si）-（Ni,Mn）-Fe base system alloy using the cluster composition formula approach and constructs a cluster composition formula based on 304stainless steel,with the number of C atoms starts from 0.0125 and increases to 0.0625successively with a gradient of 0.0125,to design a cluster composition in accordance with the general formula（Cr,Si）_3.125-（Ni,Mn）_1.875-Fe₁₁.On the basis of this general formula for cluster composition and 0.0625 C atoms,a gradient of 0.1 Cr atoms was used,increasing to 3.1 and then decreasing to 2.7.The four alloy compositions of（Cr,Si）_3.225-（Ni,Mn）_1.875-Fe_10.9,（Cr,Si）_3.025-（Ni,Mn）_1.875-Fe_11.1,（Cr,Si）_2.925-（Ni,Mn）_1.875-Fe_11.2and（Cr,Si）_2.825-（Ni,Mn）_1.875-Fe_11.3were obtained respectively.Each sample was turned and melted five times using a vacuum arc melting system,followed by solid solution treatment with 2 h holding at 1150°C and water quenching,and finally sensitisation treatment with 1 h holding at 650°C to form three control groups in different states.By characterising the organisation and properties of the C-（Cr,Si）-（Ni,Mn）-Fe series alloys in different states,the best austenite stability was obtained for alloy with atomic number C of 0.05 with an alloy composition of0.067C-18.574Cr-0.393Si-10.675Ni-1.537Mn-Fe,wt.%,and the best mechanical properties and corrosion resistance were obtained for alloy with atomic number Cr of 3.1 with an alloy composition of 0.084C-19.386Cr-0.392Si-10.648Ni-1.533Mn-Fe,wt.%.