Duplex stainless steel refers to the stainless steel that about half of the solution structure is ferrite phase and the other half is austenite phase and it is generally required that the minimum mass fraction of the less one is 30%. Duplex stainless steel combines the excellent toughness and weldability of austenitic stainless steel with the high strength and chloride stress corrosion resistance of ferritic stainless steel, meanwhile it has good local corrosion resistance and comprehensive mechanical properties. So, duplex stainless steel castings are widely used in petroleum, natural gas, fertilizer, papermaking, food equipment, marine environment, energy and environmental industries, light industry and food industry. Investment casting of duplex stainless steel is considered when the parts have complicated shapes and are difficult to form by other processing methods such as pump wheel or valve body.
Duplex Stainless Steel Casting Process
As the top leading stainless steel casting manufacturer in China, we well know the each detail of duplex stainless steel casting process. Below we will introduce how duplex stainless steel castings are made in CFS Stainless Steel Foundry.
1.1 Wax Pattern Manufacturing
The production of wax pattern is an important part of the duplex stainless steel casting process and the quality of the wax patterns determines the quality of the final castings directly. Firstly, to prepare the pattern material, the paraffin and stearic acid are mixed at a ratio of 50% each and then placed in a water wax cylinder to be heated to 90°C until they are become liquid. The wax sheet is processed by a horizontal wax sheet machine, the wax liquid and the wax sheet are placed in a mixer at a ratio of 1:1, after it becomes a uniform paste wax, then placed in an incubator to keep warm at a temperature of 45 to 48°C. Pre-heating the pressing machine to 15~25°C and applying a thin layer of release agent on the surface of pressing machine to take out the investment mold from it. The parting agent can be 10# transformer oil or turpentine. The pasted wax material is pressed into the pressing machine with a pressure of 0.3~0.5 MPa. The temperature is kept at 20~25°C and the wax pattern is pressed in the pressing machine according to the size and thickness of the mold. With a suitable pressing time , the wax pattern will have sufficient strength and accurate size.
1.2 Shell making
To make a good shell is indispensable for obtaining high-quality stainless steel investment castings. The shell coating is divided into a surface layer and a reinforcement layer. The coating applied on the surface of the wax pattern is surface layer coating and its function is to accurately replicate the shape of wax pattern surface and forms a dense and smooth cavity surface. The reinforcement layer coating is also called back layer coating, which is used to thicken and strengthen the shell, so that the shell has excellent strength and air permeability.
The investment casting shell is mainly composed of binder and refractory material. There are three types of binders: silica sol, ethyl silicate and water glass. Ethyl silicate binder has been rarely used due to the high cost and environmental pollution, the water glass binder is mainly suitable for castings with low surface quality requirements. The silica sol is selected as the binder owing to the complex shape and high surface requirements of duplex stainless steel castings.
The surface coating is mainly composed of bonding agent (silica sol), refractory powder, surface lubricant and defoamer, etc.The refractory powder may be selected from fused corundum, zircon powder or fused silica, polyoxyethylene alkyl alcohol ether can be selected as the surface lubricant and defoamer can be selected from silicone resin defoamers.The transition layer coating is mainly composed of bonding agent (silica sol), refractory powder, wetting agent and defoamer. The above coating materials are added to the cylinder according to the following order: silica sol→wetting agent→refractory powder→defoamer and stirred with L-type mixer. The viscosity of the coating can be determined by using the flow cup after it was stirred evenly and reached the time specified by the process.
The sanding process is also required after the wax patterm is dipped in coatings on purpose that sanding is used to fix the coatings with sand and make the shell has sufficient strength, air permeability and deformability. The sanding type of the surface layer should be consistent with the surface layer of refractory powder. The suitable particle size is 100/120 mesh and the reinforcing layer is made of 30/60 mesh and 16/30 mesh mullite powder from the inside to the outside. After sanding, the shell needs to be dried under the specified temperature and humidity. The surface layer is generally self-drying for a longer time and the reinforcement layer is air-dried for 2~3 hours of each layer.
1.3 Shell Dewaxing & Roasting
With high pressure steam method, the steam dewaxing kettle can be used for dewaxing after the shell coating is completed. In order to remove residual wax, moisture and other volatiles in the shell, the shell is roasted after dewaxing and the roasting temperature is usually 850. ~1100°C, it should last for a specified time. The well-roasted shell should have good air permeability and its appearance is white color or light white.
Pouring refers to the process of pouring a predetermined chemical composition and a molten metal liquid into a shell. The gravity casting method is the commonly used method and it is adopted in most of the duplex stainless steel products , namely it is poured at a high temperature after the shell is taken out from the roaster. Owing to the high temperature of the shell, the metal cools slowly and flows in the shell and has good filling effect, which can greatly improve the quality of castings. The solidification range of duplex stainless steel (125-140°C) is much higher than that of austenitic cast steel (25-40°C), so there is microscopic shrinkage among the solidified dendrites and therefore, the bigger riser is adopted in duplex stainless steel castings . In addition to the pouring from the bottom, an assisted pouring gate should be provided for the thick-walled large castings to improve the feeding heat efficiency of the riser. The directionality of the solidification and crystallization of the casting is strong and the geometrical shape of the casting should be smooth designed in order to avoid tearing and solidification cracking due to the high thermal gradient. The molten steel has strong suction capacity and the casting is prone to defects such as pinholes and subsurface bubbles. Therefore, it is necessary to pay attention to the following items during the smelting: 1) excellent deoxidation state; 2) to control the tapping temperature and pouring temperature; 3) after pouring, to clean the casting after cooling for a suitable period of time.
1.5 Casting Cleaning
Generally ,the casting shell is removed by mechanical unshelling method or high-pressure water sand cleaning method after the duplex stainless steel casting is cooled and the gating and risering as well as the process rib are cut off by a cutting machine or plasma or other equipment. Then the residual root of the gating and risering is grinded by an angle grinder. The sticky sand and oxide scale on the casting surface and in the internal cavity are cleaned with the ball throwing method or sand blasting method and finally, aimed at the defects, the final treatment is conducted and form a qualified casting under the premise of meeting the technical requirements of the casting.
Duplex Stainless Steel Casting Company
CFS Stainless Steel Foundry is the professional company for various stainless steel investment castings including duplex stainless steel castings (2205, 2207, etc). Due to the advantages of stainless steel casting in China, many overseas companies from USA, UK, Canada, Australia, South Africa, etc turn to China like CFS to purchase castings. If you would like to source a good supplier for your duplex stainless steel casting item from China, pls get in touch with us!