The paper describes the investigation of mechanisms of cast structure formation in Hadfield steel depending on the changes in the cooling rate of a casting in the following two temperature ranges: crystallization temperature(1,200-1,390 °С) and the temperature of excessive phase separation(560-790 °С). Changes in the cooling rate of the crystallization temperature range from 1.1 to 25.0 °С·s~(-1) result in the reduction of the average size of austenite grains from 266 to 131 μm. At the same time, the magnitude of developing shrinkage stresses changes from +195 to 0 MPa. When the cooling rate is higher than 16 °С·s~(-1), no shrinkage stresses are formed in the casting. Changes in the cooling rate of the casting in the temperature range of the excessive phase separation influence the number of phases, their morphology and chemical composition, the values of phase stresses, and the possibility of martensitic transformation. Changing in the cooling rate from 0.24 to 5.46 °С·s~(-1) results in the decrease of the amount of the excessive phase from 14.8% to 2.1%, which is composed of eutectic and carbides depending on the cooling rate, their quantitative ratio and morphology change. Such changes in the microstructure are reflected on the changes of value of developing phase stresses. When the cooling rate is 0.24 °С·s~(-1), it is +100 MPa, while the increase of the cooling rate to 1.4 °С·s~(-1) results in the decrease of tensile stresses to 0 MPa and their qualitative stresses change to compressive ones. Further increase of the cooling rate results in the increase of the value of compressive stresses. When the cooling rate is 5.5 °С·s~(-1), their value reaches-92 MPa. Martensite forming in the structure of Hadfield steel is possible if the cooling rate of the casting in the range of excessive phase separation is less than 0.25 °С·s~(-1). The paper describes the investigation of mechanisms of cast structure formation in the cooling rate of a casting in the following two temperature ranges: crystallization temperature (1,200-1,390 ° C) and the temperature of excessive phase separation (560 -790 ° С). Changes in the cooling rate of the crystallization temperature range from 1.1 to 25.0 ° С · s -1 result in the reduction of the average size of austenite grains from 266 to 131 μm. At the same time , the magnitude of developing shrinkage stresses stresses from +195 to 0 MPa. When the cooling rate is higher than 16 ° С · s -1, no shrinkage stresses are formed in the casting. Changes in the cooling rate of the casting in the temperature range of the excessive phase separation influence the number of phases, their morphology and chemical composition, the values ​​of phase stresses, and the possibility of martensitic transformation. Changing in the cooling rate from 0.24 to 5.46 ° С · s ~ (-1) results in the decrease of the amount of the excessive phase from 14.8% to 2.1%, which is composed of eutectic and carbides depending on the cooling rate, their quantitative ratio and morphology change. Such changes in the microstructure are reflected when the cooling rate is 0.24 ° С · s -1, it is +100 MPa, while the increase of the cooling rate to 1.4 ° С · s -1 results in the decrease of tensile stresses to 0 MPa and their version of the stress changes to compressive ones. When the cooling rate results in the increase of the value of compressive stresses. When the cooling rate is 5.5 ° С · s -1 ), their value reaches-92 MPa. Martensite forming in the structure of Hadfield steel is possible if the cooling rate of the casting in the range of excessive phase separation is less than 0.25 ° С · s -1.