Ductile cast iron or SG iron material has different grades that vary according to the regional standards. If you take the US for example, the common material standard for SG iron is ASTM A536.  Here the mechanical properties are 0-40-18 (tensile strength 414 Mpa, yield strength 276 Mpa, elongation 18%). So, you need to ensure that the material grades of the mechanical properties required are properly communicated to the selected foundry with a specific standard so that the castings are manufactured based on your requirements. This said selecting the right grade of ductile iron is a tough task as it requires a certain level of engineering background. However, an experienced professional foundry should be able to take you through this process easily. Here are some factors you need to understand before choosing ductile iron material grades.

First check your 2D engineering drawings and technical specifications. Ensure it specifies the appropriate materials; note that the completed drawings should have details on material grade. Customer requirements may vary from only ductile cast iron (without any exceptional requirements as for its mechanical properties) to high requirements for strength, stiffness and toughness to very high strength, very high-pressure load capacity.  However, it must be noted that its elongation rate is just 3%, therefore its ductility is not resistant to impact. Therefore, you need to weigh its advantages and disadvantages before you decide which is the best available option. 

Certain material grades will not bend when subjected to higher pressure loads, whilst specific material grades will not break. This ductile cast iron type can typically withstand loads of high-pressure. So what you select boils down to customer-specific application and requirements.  Higher material grades come with higher tensile strength, but lower elongation.  If the ductile cast iron type has higher strength, the material will be brittle, and therefore exhibits low impact resistance.  Additionally, higher material grades will need greater production levels, and further process of heat treatment resulting in increased overall cost of castings.