1. 4468 duplex stainless steel is a new steel grade of European corrosion resistant cast steel standard EN10283: 1998 and also a new cast duplex stainless steel type of nitrogen-containing austenitic and ferrite, which combines the advantages of ferritic stainless steel and austenite stainless steel. Its coefficient of thermal expansion is lower than that of austenitic stainless steel and it has excellent comprehensive properties in strength, plasticity, impact toughness and corrosion resistance in the cast stainless steel series. Owing to the high content of chromium and molybdenum as well as the strengthening effect of nitrogen on austenite, the yield strength and corrosion resistance of the alloy are significantly improved and the yield strength is 2 to 3 times that of austenitic stainless steel. These excellent properties make 1. 4468 stainless steel exhibit high corrosion and fatigue resistance in special environments. In foreign countries, due to the benefits of stainless steel investment casting, 1. 4468 duplex stainless steel is mainly used for casting high-grade building structure, where high strength and corrosion resistance are required.
Stainless Steel Investment Casting Process
The investment casting process of duplex stainless steel in stainless steel foundry is basically the same as that of austenitic stainless steel. The following characteristics should be considered in investment casting process of 1.4468 duplex stainless steel.
The solidification temperature range of duplex stainless steel casting is 125-140 °C, which is much wider than that of austenitic stainless steel casting (the latter is 25-40 °C). During the solidification period, there is microscopic shrinkage among dendrites, so it is more likely that its shrinkage hole is bigger than austenitic stainless steel and it is necessary to set a bigger pouring riser for feeding.
The molten steel has strong gas absorption capacity after 1.4468 stainless steel is smelted. And with N, the castings are prone to pores and need to be strictly controlled in the shell roasting, smelting and pouring processes. The shell roasting and the molten steel deoxidation should be sufficient, and it should be poured timely after the final deoxidation. The fluidity is slightly worse than 316L, but the melting point is lower than 316 and the casting temperature equivalent to 316L can be used. The control requirements for the pouring temperature are stricter than those of 316L. When the pouring temperature is too high, the surface of the casting is prone to burrs and pores. When the temperature is too low, defects such as spots or poor filling are likely to occur. Compared with the austenitic stainless steel, 1. 4468 stainless steel has a higher thermal cracking tendency. It is required to prevent thermal cracking defects in casting structure and shell’s withdrawing capability.
The free-linear shrinkage of 1.4468 stainless steel is 1.8%～1.9%, which is between the ferritic stainless steel and the austenitic stainless steel. It is about 0.5% smaller than the austenitic stainless steel. Compared with austenitic stainless steel, the changing range of shrinkage rate is bigger in the blocked state. When the full silica sol casting process is used to cast 200 mm glass connecting claws after measurement, the total shrinkage of the center distance is 2. 25% (the total shrinkage of austenitic stainless steel 316L is 3.0%), wherein the shrinkage of steel is only 1. 35%, while the total shrinkage of the central boss where the resistance is less is 2.7% and the shrinkage of steel is 1.8%.
Repair Welding of Castings
1.4468 stainless steel has excellent welding performance. For surface defects, such as shrinkage holes and pores, argon arc welding can be used for repair welding, but the composition of the solder should be the same or close to that of the casting.
Due to the existence of thick lath shape Widmanstatten structure, σ phase and carbide segregation, low plasticity and corrosion resistance, 1.4468 stainless steel investment castings need to improve its phase structure and comprehensive performance through solution treatment when they are under the condition of casting. Unlike austenitic stainless steels, duplex stainless steel castings should not be used directly without heat treatment.
Compared with austenitic stainless steel, such as 1. 4308 (304), 1. 440 (316), 1.4468 duplex stainless steel castings are more difficult to machine because they have high requirements in terms of stainless steel machine tools. The machining speed needs to be reduced by 20% compared with that of common austenitic stainless steels. Usually, the machining cost of duplex stainless steel castings is 20% higher than that of austenite stainless steel castings.