These are Irons alloyed with high percentages of Nickel and Chromium. Basically they are the iron equivalents of Stainless steel in their properties, but alas not their looks. Two basic types are available based on the type of carbon. The graphite appears as either flake, as in grey iron or spheroidal as in SG irons. The SG versions having greater mechanical properties than the flake. Both types, because of the high Nickel, have similar properties of heat resistance and good resistance to salt water and other corrosive materials. Chromium gives the resistance to erosion. Ni Resist can exhibit low levels of magnetic permeability i.e. they can be non magnetic. Main uses : sea water pump and valve parts and high temperature ovens / furnaces. 

The Flake graphite grades are denoted with a prefix of 'F'
The SG niresist grades are denoted with a prefix 'S'

Flake grades have a tensile of 170 to 240 N/mm.sq
SG grades are stronger with a tensile of 370 to 490 N/mm.sq

Selection of General Engineering Grades
Part of Austentitic Cast Iron BS 3468: 1986 
ASTM A436 & ASTM A439 439 A 439 436 A436
MIL G-858B