Stainless Steel Casting Parts
I. Introduction
Stainless steel castings play an indispensable role in numerous key industrial fields such as aerospace, automobile manufacturing, medical devices, and food processing, thanks to their excellent corrosion resistance, high strength, and aesthetic appearance. However, the problem of surface roughness is like a shadow that often looms over the quality of stainless steel castings, exerting many negative impacts on their performance, appearance, and service life. Thoroughly exploring the root causes of the surface roughness of stainless steel castings and carefully seeking practical and effective improvement methods are of crucial significance for promoting the wide application and continuous development of stainless steel castings in various industries. In this exploration process, as a well-known enterprise in the industry, EATHU, with its outstanding technology and rich experience, has demonstrated unique advantages and innovative practices in the field of stainless steel casting, providing valuable references and examples for solving the problem of surface roughness of stainless steel castings.
II. Multi-dimensional Exploration of the Causes of Surface Roughness of Stainless Steel Castings
(1) Intrinsic Structure of Materials: The Source of Roughness Caused by Pores
The microscopic structural characteristics of stainless steel materials constitute the internal basis for the surface roughness of castings. Stainless steel constructs a skeletal framework with a unique columnar crystal structure network, filled with refractory materials, various additives, and water. After undergoing the crucial process of drying and roasting, its surface will inevitably retain a large number of pores of different sizes and in disordered distributions. These pores are like tiny traps. During the critical moment of pouring molten metal, under the dual drive of gravity and pressure, the molten metal will penetrate deep into them along the channels of the pores. When the molten metal solidifies in the pores, it will form irregular protrusions and depressions on the surface of stainless steel, directly resulting in a rough texture on the stainless steel surface. And this phenomenon of molten metal penetration caused by the inherent pore structure of the material has become one of the root factors for the surface roughness problem of stainless steel castings, profoundly affecting the formation and evolution of the surface quality of castings.
For example, in the production of some stainless steel precision castings with extremely high requirements for surface quality, such as the casting of high-end medical device components or aerospace engine blades, even the surface roughness caused by tiny pores may lead to stress concentration, reduce fatigue strength, and even affect the biocompatibility or aerodynamic performance of the product. Therefore, deeply understanding and controlling the pore structure of stainless steel materials is crucial for solving the surface roughness problem.
(2) Core Link of Manufacturing Process: The Dual Influence of Casting Process Parameters
The casting process is undoubtedly the core hub link in the production process of stainless steel castings. Multiple process parameters included therein are like double-edged swords, having a significant and complex impact on the surface roughness of castings. The temperature of stainless steel, the pouring temperature and head of molten metal, the vacuum degree during pouring, etc. all fall within the scope of these influencing factors. Among these parameters, the mold temperature and pouring temperature have particularly prominent influences, just like two key chess pieces, playing a decisive role in the "chess game" that determines the surface roughness of castings.
As the temperature of stainless steel and the pouring temperature of molten metal gradually increase, the fluidity and filling performance of the molten metal are significantly improved, just like a surging river flowing more smoothly in the river channel. In this case, the molten metal can penetrate into the pores on the surface layer of stainless steel with stronger power and penetration ability, and the depth of penetration is greatly increased. When the molten metal solidifies in the pores, more and more obvious protrusions and depressions will be formed, thus causing the surface roughness of stainless steel castings to increase sharply. For example, when pouring large stainless steel valve castings at high temperatures, an excessively high pouring temperature will make the molten metal quickly fill the pores and solidify, forming dense, tiny granular protrusions on the casting surface, seriously destroying the flatness of the surface.
In addition, the pouring head and vacuum degree during pouring cannot be ignored. A larger pouring head will increase the pouring speed and pressure of the molten metal, making it more impactful during the filling process, easily leading to molten metal splashing and turbulence, and then entraining gases and impurities, affecting the surface quality of the casting. The vacuum degree during pouring is directly related to the gas content in the mold. If the vacuum degree is not properly controlled, the residual gas in the mold cavity cannot be effectively discharged during the filling process of the molten metal and will be trapped in the molten metal to form pores or bubbles. After these pores and bubbles burst, pits will be left on the casting surface, increasing the surface roughness.
(3) Investment Mold and Die: The Chain Reaction of Surface Quality
The surface roughness of the investment mold plays an extremely critical role in the formation process of the surface roughness of the entire stainless steel casting. It is like a mirror, directly reflecting the final surface state of the casting. And the surface roughness of the investment mold is closely related to the surface quality of the die, the investment mold method, and the process parameters, forming an interlocking causal chain.
The surface quality of the die is the cornerstone of the quality of the investment mold. If there are even slight flaws on the surface of the die, such as tiny scratches, sand holes, pits, or unevenness, these defects will be accurately transferred to the investment mold like precise molds. It's just like throwing a stone into a calm lake, and the ripples will spread across the entire lake. During the pouring process, these defects on the investment mold will further leave deep marks on the casting surface, becoming an important inducement for the increase in surface roughness. For example, in the casting of the stainless steel intake manifold of an automobile engine, a tiny scratch on the surface of the die may cause a protrusion at the corresponding part of the investment mold, and finally form an obvious ridge on the casting surface, which not only affects the appearance but also may affect the smoothness of the air flow.
The investment mold method and process parameters are like fine brushes, depicting the fine texture on the surface of the investment mold. Different investment mold methods, such as the traditional lost wax method, the advanced lost foam method, and the emerging 3D printing investment mold method, each have their own unique forming principles and characteristics, and the surface quality of the investment molds produced also varies. Meanwhile, in the process of making investment molds, the precise control of process parameters such as the temperature of wax liquid or molding materials, injection pressure, holding time, and cooling speed plays a vital role in ensuring the uniformity, smoothness, and dimensional accuracy of the investment mold surface. Even a tiny deviation of one of these parameters may trigger a series of chain reactions on the surface of the investment mold like the butterfly effect, and finally lead to the surface roughness of the casting exceeding the expected range.
(4) Alloy Characteristics Code: The Game between Thermal Conductivity and Grain Structure
The intrinsic characteristics of alloys are like a mysterious key hidden behind the surface roughness of stainless steel castings. Among them, thermal conductivity and grain structure are two key code elements, deeply affecting the final appearance state of the casting surface.
Some specific alloys have relatively low thermal conductivity. During the stainless steel casting process, after the hot molten metal is poured into the mold, it is difficult for the heat to dissipate quickly, and the cooling speed becomes extremely slow, just like a snail crawling. This slow cooling process provides sufficient time and space for the growth of grains, allowing the grains to grow wantonly. Eventually, the castings produced have coarse grains. The grain boundary grooves between these coarse grains are like the gullies on the earth's surface, with a depth that can reach 7μm or even deeper. The existence of these grain boundary grooves makes the casting surface present an obvious rough texture, seriously destroying the flatness and smoothness of the surface.
For example, when some high-temperature alloy stainless steel castings are used for gas turbine blades, due to the low thermal conductivity of the alloy, if not controlled, the coarse grains and grain boundary grooves on the blade surface will significantly reduce its high-temperature resistance and fatigue resistance, greatly shorten the service life of the blade, and may even cause safety accidents. Therefore, in-depth research on alloy characteristics and cracking the game code between thermal conductivity and grain structure are of great significance for improving the surface roughness problem of stainless steel castings.