Duplex stainless steel, refers to a type of stainless steel in which ferrite and austenite each about 50%, generally less phase content also need to reach 30%. In the case of low C content, the content of Cr is 18%-28% and the content of Ni is 3%-10%. Some steels also contain alloying elements such as Mo, Cu, Nb, Ti and N.
This kind of steel has the characteristics of both austenite stainless steel and ferrite stainless steel. Compared with ferrite stainless steel, both the plasticity and toughness is higher, no room temperature brittleness, resistance to intercrystalline corrosion and welding performance are significantly improved, but also has 475℃ brittleness and high thermal conductivity of ferrite stainless steel , as well as superplasticity. Compared with austenitic stainless steel, the strength is high, and the resistance to intergranular corrosion and chloride stress corrosion is obviously improved. Duplex stainless steel has excellent corrosion resistance and is also a ni-stainless steel. Thus, duplex stainless steel is also widely adopted in investment casting process.
Duplex Stainless Steel Casting Process
Step 1: Wax Pattern
The manufacturing of wax patterns is a key part to start duplex stainless steel casting process. The quality of the wax patterns determines the quality of the final duplex stainless steel castings directly.
Firstly, to prepare pattern materia and mix the paraffin and stearic acid according to the proportion of 50% each, and then put them in a water bath dewaxing tank to heat to 90ºC until it turns into a liquid state. The wax coating and wax sheet are put into the mixer in a 1: 1 ratio and to be stirred into a uniform pasted wax material, and then placed in a thermostat and kept at a temperature of 45 ~ 48ºC.
To preheat the pattern to 15 ~ 25ºC, and apply a thin layer of mold release agent on the surface of the pattern so as to take out the investment pattern. The mold release agent can be 10 # transformer oil or turpentine. The pasted wax material is pressed into the pattern with a pressure of 0.3-0.5 MPa, and the room temperature is kept at 20 ~ 25ºC. The mold is maintained in the pattern according to the size and thickness of the mold. A scientific holding time can ensure the wax pattern has sufficient strength and accurate dimensions.
Step 2: Shell Making
A well-made shell is essential to obtain high-quality duplex stainless steel castings. The shell coating is divided into a surface layer and a reinforcement layer, and the surface coating is applied on the surface of the wax pattern. Its role is to accurately make the surface shape of the wax pattern and form a compacted and smooth cavity surface. The interlayer coating is also called the back layer coating. Its role is to thicken and strengthen the shell so as to make the shell mold with good strength and air permeability, etc.
The shell of investment casting is mainly composed of two parts: a binder and a refractory material. There are three types of binders: silica sol, ethyl silicate and water glass. Ethyl silicate adhesives have been rarely used due to the high cost and environmental pollution . Water glass adhesives are mainly suitable for castings that have few requirements in surface quality. Silica sol is selected as the binder owing to the to the complex shape of duplex stainless steel castings and high surface requirement.
The surface layer coating is mainly composed of binder (silica sol), refractory powder, surface lubricant, defoamer, etc. The refractory powder can be selected from fused alumina, zircon powder or fused quartz and the surface lubricant can be polyoxyethylene Alkyl alcohol ether and defoaming agent can be organic cinnamon resin defoamer. The transition layer coating is mainly composed of binder (silica sol), refractory powder, wetting agent, and defoamer. Add the above coating materials in the order of silica sol-wetting agent-refractory powder-defoamer into the barrel and stir evenly with an L-type mixer until the time specified in the process, and use a flow cup to determine the viscosity of the coating.
A sanding process is also required after the wax pattern is dipped in the coating. The purpose of sanding process is to fix the coating with sand particles and make the shell mold have sufficient strength, air permeability and deformability. The sanding type of the surface layer should be consistent with the refractory powder of the surface layer, and the appropriate particle size is 100/120 mesh. The reinforcement layer uses 30/60 mesh and 16/30 mesh mullite powder from the inside to the outside. The sanded shell mold needs to be dried at the specified temperature and humidity. The surface layer is generally self-drying for a longer time, and the reinforcement layer is air-drying. Generally, each layer can be dried for 2-3 hours.
Step 3: Dewaxing and Roasting Shell
The shell can be dewaxed by the method of high pressure steam after it is coated, and the shell is roasted after dewaxing so as to remove the residual wax, moisture and other volatiles in the shell mold. Usually, the roasting temperature is 850ºC ~ 1100ºC and is kept for a specified time. A well roasted shell mold should have good air permeability, and the appearance of the shell is white or light white.
Step 4: Pouring
Pouring is a process in which molten metal with a predetermined chemical composition and temperature is poured into the shell mold while it is hot. The commonly used method is gravity casting. This method is also applied in most duplex stainless steel products, it means the shell mold is poured at a high temperature after being taken out of the roasting furnace. Due to the high temperature of the shell mold and the metal cools slowly in the indium shell mold and has good fluidity and perfect filling effect, which can greatly improve the quality of castings. The solidification range of dual-phase stainless steel (125 ~ 140ºC) is much larger than that of austenitic cast steel (25-40ºC), so there is microscopic shrinkage between solidified dendrites. Therefore, a larger riser should be applied in duplex stainless steel castings. In addition to bottom-pouring, auxiliary gates should be set for large-scale thick-walled castings to improve the heat efficiency of riser feeding.
The direction of solidification and crystallization of the casting is obvious. Attention should be paid to the geometric design of the casting. Tearing and solidification cracks due to high thermal gradients should be avoided. The molten steel has strong property of air-absorbing ability, and the casting is prone to defects such as pinholes and subsurface bubbles. Therefore, it is necessary to pay attention to: 1) good deoxidation status; 2) to control the appropriate tapping temperature and pouring temperature; 3) after pouring, to cool for an appropriate amount of time before cleaning
Step 5: Casting Cleaning
Usually, the casting shell is cleaned by the method of mechanical stripping or high-pressure water sanding after the casting is cooled. The pouring riser as well as process ribs are removed with a cutter or plasma equipment, then use an angle grinder to grind the remaining part of the pouring riser on the casting, and to clean the sticky sand and mill scale on the surface and inner cavity of the casting with the method of shot blasting or sand blasting. With the repaired defects, qualified duplex stainless steel castings are obtained.
CFS Foundry are skilled in making duplex stainless steel castings with alloys like 2205, 2207, etc. We are happy to work closely with our customer on all the duplex stainless steel casting project. Any inquiry on duplex stainless steel casting, pls do not hesistate to contact us!