If you want to improve the fracture toughness at low temperature, then you should consider alloy steels, including stainless steel grades, for the casting. Austenitic stainless steels (Type 304, Type 316, etc.) have excellent toughness even at -40 C. For a shaft material, you should consider grades like EN 24 (now called 817M40 in BS 970) or EN 26 (826M40) for improved toughness at the same hardness level. These steels are also appropriate for low temperature fasteners. ASTM A 320 is an industry standard for low-temperature fasteners, and some of the grades that are referenced for high-strength, quenched & tempered fasteners include:
SAE 4137 (roughly equivalent to EN 19A or 708M40)
SAE 4340 (1.65-2.00 Ni, 0.70-0.90 Cr, 0.20-0.30 Mo, so somewhere between EN 24 and EN 26)
low carbon, boron steel (approximately SAE 10B22 or SAE 15B22 which is similar to BS 3111 grade 9/1). This grade limited to fasteners less than 25 mm in diameter.