In recent years, with the continuous expansion of the market, the number of valve ductile iron parts in our factory has increased year by year, mainly including the valve body and bonnet of large-diameter butterfly valve and gate valve. The maximum diameter DN 1200 mm, the maximum casting weight about 1620 kg, the maximum part thickness of 67mm, the main wall thickness of 5mm, and the main brand is QT400-15. In the early days, ZHY casting ductile iron has been using heat treatment process to meet the product quality requirements. With long production cycle and high cost, it is decided to change to as cast ductile iron. Focusing on the process key point of obtaining high toughness in as cast state and according to the characteristics of castings, we take corresponding measures from three aspects: reasonably selecting the chemical composition of nodular iron, ensuring high temperature molten iron and strengthening the inoculation treatment process, and achieved good results.
From the production of ductile iron in recent years, the metallographic structure of valve castings is; Ferrite is 80% – 90%, cementite is less than 1%, spheroidization grade is 2-3, tensile strength is 410-5800 MPa, elongation is 10% – 16%, and the product quality requirements are met in most as cast conditions; The unqualified ones are scrapped or adjusted through heat treatment, which meets the internal quality requirements of ZHY casting valve ductile iron parts.
(1) Qualified as Cast Ferritic Ductile iron can be obtained by melting with cold air acid cupola, strengthening the control of melting process parameters and adjusting the production process.
(2) In smelting, it is necessary to ensure high molten iron temperature, control high carbon equivalent, and adopt multiple inoculation to strengthen the inoculation effect, which is a practical and effective measure to obtain as cast ferrite nodular iron.