Properties and defects of stainless steel

General characteristics

Products made of stainless steels intended for use in aggressive environments at normal or high temperatures. The main requirement for stainless steels is corrosion resistance, which depends on the alloy composition, structural state, the aggressiveness of the corrosive environment and from impact loads. The corrosion resistance is due to the formation on the surface of the product pestiviruses durable film that prevents the penetration of aggressive substances into the deeper layers of stainless steel.

Chrome

He is one of the most strong pestiviruses elements in oxidizing environments. On the surface of the alloy forms a very thin invisible film of chromium oxide thickness of a few atomic layers. The density of the film and corrosion properties of stainless steel increases with increasing chromium share. With 12−13% Cr steel becomes stainless, i.e., counter to the action of atmospheric and soil moisture. Increasing the chromium content up to 28−30% makes the alloy resistant to highly aggressive environments. Chrome is territooriumil element, so steel with high chromium content (16−30%) relates to ferritic class. Resistance of ferrite makes them impervious to quenching, so they are characterized by low strength properties, which limits their application. Purely ferritic steels on the basis of 25−28% Cr (X25, X28, etc.) with small additions of titanium or nitrogen (for grinding grain) is used as a heat-resistant, do not carry loads.

Strength and ductility

These qualities of the metal depends on many factors: crystal structure, bonding energy of atoms in the crystal lattice, metal purity, chemical composition, purity, boundaries, and other factors. According to modern concepts, the resistance to plastic deformation is mainly determined by the number of imperfections in the lattice structure, in the first place — the dislocation. High strength can be obtained if you reduce or, conversely, to increase the number of dislocations.

Hardening

In practical conditions of defect-free materials is not allowed, but hardening by increasing the number of dislocations and other imperfections is used very widely. Hardening stainless steels, which are solid solutions occurs by the interaction of impurity atoms not forming the main lattice of the solid solution — of the dislocations. However, more significantly hardens the steel block shifts superfine particles of the other phase, the resulting alloying and appropriate heat treatment. The greatest hardening corresponds to the fine structure of a second phase with a size of 20−50 nm (200−500 A), uniformly distributed throughout the volume of the grain. Growth allocations of up to 100 nm (1000 A) and leads to more softening of the steel.

Steel martensitic class

When both needed corrosion resistance and high strength, used chromium steels of martensitic class with low chromium content (12−18%). These steels can be hardened. The carbon content in them typically is 0.1−0.4% (steel 1X13−4X13). If you need after quenching high hardness carbon content may be increased up to 1% (steel 9X18). Steel of this type is used for the manufacture of knives, scissors, surgical instruments, molds for plastics and steel with high carbon content — for the manufacture of ball bearings operating in corrosive environments. But they are unusable at high temperatures.

Alloying
With increasing temperature the relationship of atoms in the crystal lattice is weakened, increasing the diffusion mobility of atoms and of the hardening phase (mainly chromium carbides) is reborn — grow, spheroidized, there is a redistribution of alloying elements between solid solution and hardening phase. This leads to a softening of stainless steel. Long-term preservation of strength at high temperature promotes the inhibition of diffusion processes, achieve optimal doping.

Casting technology

To improve the surface quality of the ingot and reduce porosity titanium, stainless steel have to spill, to protect the metal surface in the mold reducing atmosphere or slag, and the metal stream during casting, inert gases.

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