The Niyama Criterion is a function based on Temperature Gradient and Cooling Rate. The criterion was developed by Dr. Niyama, a Japanese researcher studying shrinkage prediction in steel. Niyama found that the Temperature Gradient divided by the square root of the Cooling Rate corresponded to the presence of shrinkage porosity in steel castings. The lower the value, the higher the probability of shrinkage. If this number were 1 or above there was little or no shrinkage porosity in the castings. Niyama has been used extensively for shrinkage prediction in castings, until the use of more advanced calculations such as the Material Density Function.
The Niyama Criterion has been extended to alloys other than steel. Niyama is basically a prediction of directional solidification. Poor directional solidification is represented by a value of 0, good directional solidification by higher values. Ranges of critical values are:
Steels: 0 1
Cast Irons: 0 0.75
Aluminum: 0 0.30
Copper Base 0 1.30
The general idea is that the lower the value of Niyama, the worse the potential for shrinkage. A value of 0 is the highest probability of shrinkage, and as the value increases, the severity of the probable porosity decreases. Above the critical number, probability of porosity is low.
Niyama plots need interpretation, because they give low values in many areas where temperature gradients are low but shrinkage might not be likely, such as at the centerline of thin-wall areas and at the dividing line between the feeding areas of two risers. Also, Niyama is based only on heat transfer and does not take the effect of gravity into account. For most ferrous materials, Material Density (which DOES take gravity into account) is generally a better indicator. Niyama, however, is still used heavily in many non-ferrous applications (typically in aluminum alloys).