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The presented work aims to construct a plasticity diagram for SCH20 cast iron, which is necessary to predict the ultimate deformation values of this material under deformation conditions with significant negative values of the coefficient of the stress state. A theoretical model of the composite cast iron sample deformation during its joint compression with plastic materials has been developed, influencing hydrostatic pressure values and the stress-deformed state index along with the wall thickness. The calculations performed according to the developed model made it possible to determine the stress-deformed state parameters and create the necessary conditions for determining the deformation accumulated before fracture at various negative values of the coefficient of the stress state. A technique is proposed for experimental studies of the composite sample deformation, including a test program and the development of a device for compressing the studied samples. Experimental studies of a composite sample compression consisting of SCH20 cast iron and L63 brass confirmed theoretical calculations results and made it possible for the first time to obtain significant plastic deformations of cast iron with negative values of the stiffness coefficient of the stress state. The studies carried out made it possible for the first time to construct a cast-iron plasticity diagram, including the area where deformation occurs at significant negative values of the stiffness coefficient of the stress state.