Spheroidal Cast Iron for Works at Low Temperature – Requirements, Structure, Properties, Control of the Production Process

Abstract

Two standardised grades of spheroidal cast iron determined in standard EN PN 1563 – 1997 as: EN-GJS-350 – 22LT (T = –40°C) and EN GJS 400 – 18LT (T = –20°C) are intended for work at low temperatures: –20 and –40oC. The main mechanical property of these cast iron grades is a high impact strength at a work temperature down to: –40°C. A series of controlled melts was performed to optimise the production technology of spheroidal cast iron, which in as-cast state is characterised by ferritic matrix (the best without any pearlite), fine precipitates of nodular graphite and high purity (without non-metallic inclusions). Variable structures of metal charges and various spheroidisation techniques (the modification methods) (slender ladle with a tight cover – Tundish technology as well as the technology with cored wire) were applied in the research. In order to obtain refinement of graphite precipitates and to achieve the ferritic matrix multistage inoculations of technologies were applied. Cast iron was subjected to refining to limit non-metallic inclusions since they decrease the impact strength. The production process of cast iron was controlled by the thermal derivative analysis at the stage of initial cast iron and after its secondary metallurgy (modification and inoculation). It was pointed out, that the reproducible production of cast iron for work at low temperatures was only possible when all elements of the technological process were strictly adhered to. It was pointed out, in the hereby paper, that: it should be strived to maintain Si content not higher than 2.50÷2.60%, which at producing spheroidal cast iron is sometimes difficult and requires using a lot of pig iron in the metal charge. For a fast assessment of the cast iron quality, concerning its impact strength, the proposed – in the hereby paper – index quality (IQu) can be applied. It is determined on the bases of measuring the cast iron hardness and propagation velocity of ultrasound wave.

Keywords

• impact strength
• ferritic ductile iron
• low temperatures
• work-of fracture