Properties and defects of stainless steel
Technical Characteristics
The main requirement for stainless steels is corrosion resistance, which depends on the alloy composition, its structural state, the aggressiveness of the corrosive environment and the loads acting. Corrosion resistance is due to the formation, on the surface of the product, of a strong passive film that prevents the penetration of aggressive substances in the deeper layers of the material.
Chromium
Chromium is one of the strongest passivating agents in oxidizing environments. It forms a very thin, invisible oxide film several atomic layers thick on the surface of the alloy. The film density and corrosion resistance increase as the proportion of chromium increases. The 12-13% Cr content makes the steel stainless and resistant to atmospheric and soil moisture. Increasing the chromium content to 28-30% makes the alloy resistant in corrosive environments. Chromium is a ferrite-forming element, so alloys with high chromium content (16-30%) are referred to the ferrite class. The ferrite resistance makes the alloys immune to hardening, so they are characterized by low strength properties, which limits the field of their application. Purely ferritic steels based on 25-28% Cr (X25, X28, etc.) with small additions of titanium or nitrogen (for grain refinement) are used as heat-resistant, not load-bearing ones.
Strength and ductility
These qualities of metal depend on many factors: the crystal structure, the binding energy of the atoms in the crystal lattice, the purity of the metal, the chemical composition, the purity of the boundaries and other factors. According to modern concepts, the resistance to plastic deformation is determined mainly by the number of imperfections in the structure of the crystal lattice, primarily by dislocations. High strength can be achieved by reducing or, conversely, by increasing significantly the number of dislocations.
Strengthening
The question of creating defect-free materials under practical conditions has not yet been solved, but strengthening by increasing the number of dislocations and other imperfections is used very widely. The hardening of stainless steels, which are solid solutions, results from the interaction of impurity atoms - that do not form the lattice of the basic solid solution - with dislocations. However, shear blocking by highly dispersed particles of another phase that appear as a result of alloying and appropriate heat treatment hardens steel to a greater extent. The greatest strengthening corresponds to a fine structure with the second phase separation of 20-50 nm (200-500 A), evenly distributed throughout the volume of the grain. The growth of precipitations up to 100 nm (1000 A) and more leads to softening of the alloy.
Martensitic grades
When both corrosion resistance and high strength are required, chromium martensitic grades with a reduced chromium content (12-18%) are used that can be hardened. Their carbon content is usually 0.1-0.4% (grades 1X13-4X13). If it is necessary to obtain high hardness after hardening, the carbon content can be increased to 1% (grade 9X18). Alloys of this type are used for making knives, scissors, surgical tools, molds for plastics, and steels with high carbon content are used for making ball bearings operating in corrosive environments. But they are all unsuitable for high temperatures.
Alloying
As the temperature rises, the atoms' bond in the crystal lattice weakens, the atoms' diffusive mobility increases and the strengthening phase (mainly chromium carbides) is reborn - enlarged. The alloying elements are redistributed between the solid solution and the strengthening phase. This leads to softening of the stainless steel alloy. Long-term preservation of strength at high temperature is promoted by the retardation of diffusion processes that is achieved by optimum alloying.
Casting technique
To improve the quality of the ingot surface and to reduce titanium porosity, stainless steel has to be poured at high speed, the metal surface in the mold has to be protected with a reducing atmosphere or slag, and the metal jet has to be protected with inert gases during casting.
Furnizor
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