Prediction of Aluminum Nitride Embrittlement in Heavy Section Steel Castings

A Case of Aluminum Nitride Embrittlement of Heavy Wall Cast Steel

“…Aluminum nitrides are known to precipitate on the primary austenite solidification grain boundaries under certain conditions and provide a brittle pathway for cracking [2][3][4][5]7]. The principal variables affecting the embrittlement mechanism are cooling rate, aluminum content, and nitrogen content [2][3][4][5]7].…”

“…Aluminum nitrides are known to precipitate on the primary austenite solidification grain boundaries under certain conditions and provide a brittle pathway for cracking [2][3][4][5]7]. The principal variables affecting the embrittlement mechanism are cooling rate, aluminum content, and nitrogen content [2][3][4][5]7]. It is also known that a similar embrittlement can be active in steel by precipitation of aluminum borides, aluminum carbo-borides, alloy carbides, or these precipitates combined with ferrite at the grain boundaries [2,3].…”

“…Aluminum is present in the steel normally as a residual from the deoxidation practice, and nitrogen can easily be dissolved in liquid steel from an uncontrolled atmosphere [4]. Aluminum nitrides precipitate at the primary solidification grain boundaries with C-curve kinetics, similar to chromium carbide precipitation (i.e., sensitization) in austenitic stainless steels or like pearlite formation in carbon steel normalization [5,7], hence the importance of cooling rate.…”

“…The size of these facets is much too large to correspond to the prior austenite grain boundaries associated with the immediately preceding heat treatment or weld transformation, which would be on the order of 0.0045 mm for ASTM grain size number 6 [1]. The appearance of the fracture surface is typically described as ''rock candy'' and has been associated with aluminum nitride (AlN) embrittlement of steel castings in the past [2][3][4][5][6][7]. Therein, the large size of the grain facets is due to fracture along the original primary solidification austenite grains or dendrites (as-cast), not the immediate prior austenite grain boundaries.…”

“…This is typical of aluminum nitride embrittlement [2,6] and distinguishes it from other forms of embrittlement such as temper embrittlement, tempered martensite embrittlement, and hydrogen embrittlement [6], which would exhibit relatively flat facets. The platelets are due to the AlN particles that form a semicontinuous network along the primary austenite solidification grain boundaries [2][3][4][5]7]. Micro-scale ductility in the form of small dimples was visible between the platelet locations.…”