Definition - What does Intergranular Cracking mean?
Intergranular cracking is a form of corrosive attack that progresses preferentially along grain boundaries. Intergranular cracking is known for being able to cause corrosion, which is referred to as intergranular corrosion or intercrystalline corrosion. In intergranular corrosion, the cracking that is seen occurs along the grain boundaries due to the presence of a tensile stress and it is this activity that we refer to as intergranular stress corrosion cracking (IGSCC).
Intergranular cracking is a result of local differences in the composition of a metal as a crack propagates along the grain boundaries of a material, usually where these grain boundaries are weakened. Intergranular cracking can potentially occur in a wide variety of materials, including steel alloys, copper alloys and ceramics. In metals with multiple types of organized lattice patterns, when one lattice ends and another begins, the fracture is known to change its direction to follow the new grain. Intergranular cracking is increased by strain homogenization that results from coarse grains.
Intergranular cracking is also known as intergranular corrosion, intergranular corrosion cracking, intergranular stress corrosion cracking (IGSCC), intercrystalline corrosion, interdendritic corrosion or intergranular attack.
Corrosionpedia explains Intergranular Cracking
Intergranular cracking is not visible on the surface and is very destructive. It spreads through the interior of the metal along the grain boundaries, reducing the strength and destroying the ability of the metal to be formed or shaped. Positive identification of this type of corrosion usually requires microstructure examination under a metallurgical microscope, although sometimes it is visually recognizable.
Metals and Alloys Susceptible to Intergranular Cracking
Among the metals affected by this type of corrosion are stainless steel, certain magnesium alloys and the copper-bearing aluminum alloys. Intergranular corrosion occurs in certain grades of stainless steel when the steel is heated, as with welding. In this case, brittleness results, causing the metal to crack near the weld. For this reason, a post-weld heat treatment is needed before reinstalling stainless steel parts that have been welded.
Causes of Intergranular Cracking
Intergranular cracking will likely occur when there is a hostile environmental influence and is favored by larger grain sizes and higher stress. Intergranular cracking could possibly occur over a wide range of temperatures.
There are several processes that can lead to an intergranular fracture, such as:
- Micro-void nucleation and coalescence at inclusions or second-phase particles located along grain boundaries.
- Grain boundary crack and cavity formation associated with elevated temperature stress rupture conditions.
- De-cohesion between contiguous grains due to the presence of impurities at grain boundaries and in the presence of hydrogen and liquid metals.
- Stress corrosion cracking associated with chemical dissolution along grain boundaries.
- Cyclic loading when the material has lost its ability to accommodate plastic deformation between contiguous grains leading to grain boundaries.
Prevention of Intergranular Cracking
Intergranular cracking can be prevented through the use of:
- Low carbon grade stainless steels.
- Use of stabilized grades alloyed with titanium or niobium. Titanium and niobium are strong carbide formers. They react with the carbon to form the corresponding carbides, thereby preventing chromium depletion.
- Use of a post-weld heat treatment.