Aluminum in steel castings may come from two primary sources. First, aluminum is introduced to the steel during the deoxidation process. Before pouring into the mold, the steel must be deoxidized to reduce the oxygen content of the melt. If the oxygen levels remain high, then the oxygen will react with carbon to generate CO creating small poros- ity or pinholes in the casting. Although several elements which have a higher oxidation potential than carbon can be used, aluminum is the most common choice. Upon the in- troduction of aluminum in a liquid steel melt, the oxygen binds to it forming alumina, Al2
to be removed as slag. Excess aluminum is required to pre- vent porosity during solidification and to counteract the ad- ditional pickup of oxygen through pouring. Aluminum may be picked up from exothermic insulating binder sleeves sur- rounding risers or exothermic riser topping. This additional aluminum pickup in risers is found in the cast material close to the sleeve material.
O3
For an electric arc furnace melting practice with an effective oxygen blow the average nitrogen is 80 ppm.5 Nitrogen is difficult to tightly control.
Nitrogen dissolves into the steel on any exposed surface to the air. The total amount of nitrogen pick up is directly re- lated to the time that the liquid metal is exposed to the air. In longer melting practices with no oxygen boil, such as in induction melting, the nitrogen content can be as high as 200 ppm.4
One additional complicating factor in the formation of AlN precipitate is macro-segregation. Macro-segregation is the movement of element species due to the micro-segregation and convective flow in the casting during solidification. Micro-segregation is the local rejection of elements from the solid to the liquid during solidification. Macro-segrega- tion may have an important influence on the final composi- tion locally of the aluminum and nitrogen. This effect is not considered in the work to follow and instead only the average composition of the aluminum and nitrogen is used in the calculation of the potential formation of AlN phase. Further consideration of the influence of macro-segrega- tion on AlN precipitation is left for future work.
, and floats to the surface
Method
The theoretical Hannerz chart shows the relationship of the maximum aluminum content and maximum nitrogen content for different cooling rates before embrittlement. This chart allows the user to determine whether the cast- ing is at risk of embrittlement based on known casting conditions. But this chart faces several limitations. First, the temperature range in which the cooling rate is calcu- lated is not specified. The cooling rate can vary as much as an order of magnitude in the temperature range during the precipitation of AlN phase. Second, the Hannerz chart given in his original work was based on an approximate evaluation of the Temperature-Time-Transformation (TTT) diagram. The paper itself records that “To obtain the more exact values every value on the curves should be multiplied by 1.35”.3
The original chart based on the
approximate evaluation of the TTT has been shown to be not conservative enough in the prediction of AlN phase in heavy section castings.1
The more accurate equation from Hannerz paper is shown as follows:
Equation 1
concentration of B in the matrix, CA of A in the precipitate AB, Q1 for A in austenite, Q2
where CA
is the concentration of A in the matrix, CB
is the
AB is the concentration is the activation energy
is the activation energy for reac-
C is the factor in the solubility product for the reaction A+B->AB, R is the ideal gas constant, k is the cooling rate. The values for each of these material properties are given in Table 1.
tion A+B->AB, x is the half-thickness of the precipitated plate, D0
Table 1. Constants Used to Calculate Equation 1 and the TTT Curves
is the diffusion coefficient for A in austenite,
28
International Journal of Metalcasting/Summer 10
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