Nice to know - simply explained
Heute: Interkristalline Korrosion (IK-Korrosion); auch Kornzerfall genannt
Austenitic chromium nickel steels and ferritic chromium steels have a tendency to carbide precipitation, preferably at the grain boundaries, when subjected to appropriate heat treatment. These chromium-rich carbides cause a reduction of the free chromium in the vessel in the region of the grain boundaries. If the chromium content in this area falls below the resistance limit of approx. 12% chromium, the steel can no longer self-passivate.
In the presence of an electrolyte, which can be acids but also just water, a galvanic element is formed which dissolves the less noble zones (anode) along the grain boundaries. If the whole vessel is covered with a coherent matrix of carbides along the grain boundaries, the individual grains are dissolved out
Sensitization
In practice, sensitization of the steel occurs due to heat treatment in the critical area which leads to carbide precipitation. The critical temperature zones for austenitic stainless steels is 450°C to 900°C and for ferritic steels after a high annealing temperature above 900°C and correspondingly slow cooling. The holding time in the critical temperature zone is largely determined by the carbon content. Such conditions can arise, among other things, during stress relieving, slow cooling of larger components or after welding.
Avoiding grain decay
By lowering the carbon content, the risk is basically reduced from the metallurgical side. One example is the double attestation of the well-known material 1.4301 with the material number 1.4307. According to EN 10088-1, both materials have the same alloy content, except in the carbon content. For material 1.4301 a maximum carbon content of 0.07% and for material 1.4307 a maximum of 0.03% is specified.
Stabilizing the carbon content by adding titanium or niobium also reduces the risk. Titanium and niobium bind the carbon more strongly.
By appropriate heat treatment after sensitization of the steel, carbide formation on the grain boundaries can be reversed. A prerequisite is rapid cooling. For austenitic materials, the annealing temperature is 1050-1100°C and for ferritic materials between 750 – 850°C.