Analysis of Casting Defects of Stainless Steel and Key Points of Production Process Control

Analysis of Casting Defects of Stainless Steel and Key Points of Production Process Control

The spraying of molten metal at riser occurred in the process schemes with pouring temperature higher than 1590 C, which resulted in large Blowhole in the castings. Through the analysis of the whole process, it is known that the spray of liquid metal is mainly caused by the large amount of gas from the casting mould and mud core. With the high temperature of molten steel, the resin decomposes and burns rapidly under the action of high temperature molten steel. During the pouring process, the gas emission is large, the gas solubility decreases and the volume expands rapidly during solidification. The temperature of riser is higher and the solidification is later. When other parts solidify, the gas rises from low temperature to high temperature, which makes the pressure under riser increase sharply and leads to larger stomata. On the premise of guaranteeing the pouring time, the spraying phenomenon of liquid metal was eliminated by controlling the pouring temperature in the range of 1550-1570 (?)

In the technological scheme of pouring temperature below 1540 C, a large number of air holes were found on both sides of the cooling chamber after casting dissection. The analysis shows that the main reason for the formation of the pore is the invasive pore, which is related to the pouring temperature and the structural characteristics of the casting. When the pouring temperature is too low, the solidification time is short, and the gas in the liquid metal can not float up and stay at the bottom of the casting. In addition, the pore in the outer wall of the cooling chamber is due to its bending shape. The vertical upper part is the inner wall of the mould, which hinders the floatation of the gas. By properly increasing the pouring temperature and shortening the pouring time, the pouring temperature was controlled in the range of 1550-1570 (?) C, and the air hole defects on both sides of the cooling chamber were eliminated.

In the process plan with casting time more than 20 seconds, the top of the supporting surface of castings is prone to air hole defects. After analysis, the defect is related to the long pouring time. Because the resin uses organic binder, the gas volume is large after pouring, and reaches the peak in a short time. When the mold is gassed in large quantities, the molten steel can not establish enough static pressure to prevent gas intrusion. In addition, the decomposition of resin after pouring will produce certain heat, the heat dissipation of sand mold is poor, the temperature gradient in solidification process is small, and it is difficult to form sequential solidification. Under the condition of incomplete sequential solidification, gas can hardly fully float to the top or riser. Once the detention position is not within the processing allowance range, it will be processed after processing. It is easy to have stomatal defects. By controlling the pouring time within 11-15 seconds, the top pore defects were effectively eliminated.

• 1 Splash Phenomenon
• 2. Air holes on both sides of cooling chamber
• 3 Supporting Face Top Blowhole

Key Points of Production Process Control

Because of the high technical requirements of casting quality, the whole process needs to be strictly controlled to ensure its quality. Combining with the above casting defects and many experimental studies, the production control points of stainless steel roof castings produced by bark resin sand were summarized and analyzed.

• (1) Following the principle of "low temperature quick pouring". Because the resin uses organic binder, the amount of gas is large after pouring. Reducing pouring temperature can reduce the total amount of gas. By rapid casting process, molten steel fills the cavity quickly. When the casting gas is released in large quantities, the static pressure of molten steel has been established enough to reduce the gas invasion.
• (2) The core is buried with exhaust rope, the upper box is tied with air holes and the passage is set aside to draw out the gas from the core. During the whole process, the core is basically completely covered by molten steel. If the gas is not discharged reasonably at high temperature, the casting will have blowhole defects.
• (3) timely ignition during pouring to reduce the pressure in the cavity. When the interfacial pressure exceeds the static pressure of metal, the gas invades the liquid metal through diffusion, and timely ignition can effectively reduce the pressure in the cavity.
• (4) Brush the alcohol-based paint 4-5 times in 2 hours after moulding. Resin sand has good collapsibility at high temperature, and requires high quality of coatings. It is easy to cause sand washing or sticking defects if the coatings are not compact or the adhesion of coatings is not strong. Brushing 4 to 5 times to ensure a certain thickness of coating layer can reduce the gas generated by resin decomposition into the metal liquid;
• (5) Shot blasting before use of chilled iron. When using the alcohol-based coating, the chilled iron part is not easy to ignite, and the surface pore is easily formed in the chilled iron part.
• (6) Riser cutting is done with electric welding rod or grinding wheel blade, which results in higher melting point due to higher chromium content, and it is difficult to use ordinary gas cutting riser.
• (7) Sand drop temperature should be controlled below 200 C. The martensitic stainless steel is accompanied by phase transformation during cooling. Most of the martensitic transformation is basically completed when the castings are cooled to about 200 C. The residual stress caused by volume shrinkage or volume expansion of phase transformation is very small, which can reduce the deformation of the castings.

Stainless steel is different from other materials. It must be cautious when casting. In addition to preventing defects in castings, it is also for efficient and energy-saving production of stainless steel precision casting. In particular, the following aspects must be kept in mind.

First of all, when casting stainless steel, the thickness of the casting can not be too thin, otherwise there will be white mouth phenomenon. Secondly, the inclination of the stainless steel casting can be larger, so as to facilitate the gradual removal, especially for the withdrawal mold. And the specific setting of the slope, also need to consider the type of alloy, casting height, surface location and other factors.

Because the heat dissipation rate of metal is faster, the small thick wall of stainless steel casting should be thicker than that of sand casting. On the premise of guaranteeing the casting to form smoothly, the heat can be emitted as soon as possible

Regardless of the size of the castings in precision casting, it is necessary to modify the shape of the castings after they are moulded. The purpose is to make the shape and size of the castings larger and meet the corresponding standards. This process can be used in the repair mode or more, the following several more commonly used can be used for reference.