Season Cracking | Caustic Embrittlement | Bauschinger Effect | Work Hardening
What is Season Cracking?
Season cracking results when the material is subjected to the combined effect of internal stresses and inter-crystalline corrosion due to the corrosive media in an industrial atmosphere. Internal stresses left in any object for an extended period, after cold working, are susceptible to corrosion. The cracks typically pass around the inter-crystalline grains.
The fractured surfaces are then separated by the tensile stresses produced due to the effect of season cracking, e.g., alpha brass (60% copper and 40% zinc) is susceptible to inter-granular cracking in an ammonia atmosphere. This can be prevented by annealing the brass at 200-3000C.
A simple test to check the material against season cracking is carried out by immersing the specimen in an aqueous solution containing 1% mercurous nitrate and 1% nitric acid. If the specimen is under the stressed condition, it will crack within a few minutes.
The effect of season cracking is also observed in boiler steel plates around the punched holes under the combined action of stress and a high concentration of hydroxyl ions in the environment. This is known as caustic embrittlement and can be prevented by annealing the plates at 6500C, or the addition of phosphates prevents caustic cracking.
Stress corrosion of austenitic stainless steel occurs in a chloride environment. The crack, in this case, is transgranular. It propagates across the grains. The addition of molybdenum or increased nickel content in the stainless steel reduces this type of corrosion.
Inter-granular corrosion can be prevented in 3 ways:
(a) By reducing the carbon content in the stainless steel to less than 0.50%;
(b) By quenching the stainless steel to prevent chromium carbide precipitation;
(c) By adding strong carbide forming elements such as Nb or Ti so that carbon precipitates as niobium carbide or titanium carbide and not as chromium carbide. An improperly heat-treated 18/8 stainless steel may show inter-granular corrosion due to chromium carbide precipitation at the grain boundary or due to prolonged exposure to high temperature.
It is observed that the yield strength of the material, during plastic deformation, increases in the direction of plastic flow when subjected to force beyond the elastic limit. The plastic deformation will start at lower yield stress if the stress is applied in the opposite direction. This is because under the reversed load, the residual stresses caused by initial deformation increase the stress. This phenomenon is known as the Bauschinger effect.
When a material is subjected to a gradual tensile load, it produces higher stress than the yield stress. Again, if the load is removed gradually and the material is subjected to a compressive load, it will show compressive stress lower than the tensile stress with the permanent strain instead of zero strain, when the load is removed totally.
The same phenomenon is observed whether the material is first subjected to compressive loading and then to a tensile load. The tensile stress observed is less than the compressive stress. The stress reduction is due to the presence of residual stress even after the load is removed, which is the cause of dislocations. The reduction in compressive (tensile) stress after the tensile (compressive) loading of the specimen is known as the Bauschinger effect. It is seen in the metals after cold working.
Work Hardening (Strain Hardening)
The increase in tensile strength and Hardness due to the cold working of a metal is called ‘work-hardening’ and is due to permanent deformation. Initially, a minor amount of plastic deformation, above the elastic limit, results in a substantial increase in work-hardening. Therefore, it decreases rapidly until fracture takes place.
After that, the rate of work hardening decreases rapidly until fracture takes place. It decreases ductility and electrical conductivity. It is denoted as & hard, ¾ hard, and extra hard, for example, copper, aluminum, nickel, lead, tin, and zinc, etc., as they can crystallize at room temperature.
The development of certain lattice directions in the grains of metal, which has undergone severe deformation, with the principal direction of flow, is known as Preferred Orientation. It signifies similar ion orientation in all the crystals of a specimen of metal. It is crucial with respect to the physical and mechanical properties of the section as a whole.
Plastic deformation produces a preferred orientation of the grains of a metal which has been mechanically worked. The formation of strongly preferred orientation results in an anisotropy (variation in properties in different directions) in mechanical properties. Even though the individual grains of metal are anisotropic w.r.t mechanical properties when these grains are combined randomly into polycrystalline aggregate, the aggregate’s mechanical properties tend to be Isotropic (i.e., uniform in all directions).
The preferred orientation resulting from deformation is greatly dependent on the slip and twinning systems available for deformation. But it is not affected by process parameters like dying angle, rill speed, roll diameter, etc. The simplest preferred orientation is produced by the rolling or drawing of a wire. In an ideal wire, a definite crystallographic direction lies orientation to the wire axis and the preferred orientation is symmetrical around the wire.