Defects and their elimination of stainless steel

General Characteristics

Stainless steel products are designed to work in aggressive environments at normal or high temperature. The main requirement imposed on stainless steels is corrosion resistance, which depends on the alloy composition, its structural state, aggressiveness of the corrosive environment and on 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 the corrosive substance into the deeper layers of stainless steel.

Chromium

Chromium is one of the strongest passivating agents in oxidizing environments. It forms the thinnest invisible chromium oxide film on the alloy's surface that is several atomic layers thick. The film density and corrosion resistance properties of stainless steel increase as the percentage of chromium increases. At 12-13% Cr the steel becomes stainless, i.e. resistant to atmospheric and soil moisture. Increasing the chromium content to 28-30% makes the alloy resistant in highly aggressive environments. Chromium is a ferrite-forming element, so steels with high chromium content (16-30%) are referred to the ferrite class. Their ferrite resistance makes them 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, non-load-bearing steels.

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. Growth of precipitations up to 100 nm (1000 A) and more leads to softening of steel.

Martensitic Steels

Martensitic chromium steels with low chromium content (12-18%) are used when both corrosion resistance and high strength are required. These steels can be hardened. Their carbon content is usually 0.1-0.4% (1X13-4X13 steels). The carbon content can be increased to 1% (9X18 steel) if necessary to obtain high hardness after hardening. Steels of this type are used for making knives, scissors, surgical tools, plastic molds, and steels with high carbon content are used for making ball bearings working in corrosive environments. But they are all unsuitable for high temperatures.

Alloying
As the temperature rises, the atoms' bonding in the crystal lattice weakens, the atoms' diffusive mobility increases and the strengthening phase (mainly chromium carbides) is reborn - enlarged, spheroidized, the alloying elements are redistributed between the solid solution and the strengthening phase. This leads to unstrengthening of stainless steel. Long-term preservation of strength at high temperature is assisted by the retardation of diffusion processes that is achieved through optimum alloying.

Casting technique

To improve the quality of the ingot surface and reduce titanium porosity, stainless steel has to be cast at high speed, the metal surface in the mold has to be protected by a reducing atmosphere or slag, and the metal jet during casting by inert gases.

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