When purchasing castings, have you ever wondered which material is best suited for your casting in the investment casting process? Which material offers the best cost-effectiveness? This article will explain why stainless steel holds a leading position in investment casting, helping you choose the most suitable material for your castings while reducing costs.
Types and Grades of Stainless Steel for Casting
There are different types of stainless steel. By understanding stainless steel through its characteristics and applications, you can choose the most suitable material in the shortest time, thereby significantly reducing your time costs, ensuring that your final product offers the best value for money and technical competitiveness.
| Type | Common Grades | Characteristics | Application |
| Austenitic Stainless Steel | 304（CF8） 316（CF8M） 316L（CF3M） | Corrosion resistance Non-magnetic Extremely ductile and tough | Food Industry Marine Environment Healthcare and Pharmaceutical Industry |
| Martensitic Stainless Steel | 410 420 440C | High hardness High wear resistance Strong magnetic properties | Mechanical Components High-Strength Cutting Tools
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| Precipitation-Hardening Stainless Steel | 17-4 PH 15-5 PH | High strength Corrosion resistance Deformation resistance | Aerospace Engineering Oil and Gas Engineering |
| Duplex Stainless Steel | 2205 2507 | High yield strength High resistance to stress corrosion | Chemical Industry Environmental Protection Paper Industry |
| Ferritic Stainless Steel | 430 409 444 | High antioxidant properties Excellent thermal conductivity Strong magnetic properties | Automotive Exhaust Systems Interior architectural Decoration |
Key Properties of Stainless Steel Castings
Different types of stainless steel vary in corrosion resistance and strength, but as the most common material used in investment casting, stainless steel has many common characteristics. Knowing these characteristics will give you a better appreciation of stainless steel’s high performance, enabling you to determine whether it is suitable for your casting’s operating environment and to more persuasively demonstrate the reliability of your components when recommending it to your customers.
Corrosion Resistance
The surface of stainless steel is covered with a dense, uniform, and self-healing chromium oxide passivation layer, which gives exceptional corrosion resistance. In particular, 316L stainless steel castings also exhibit resistance to pitting corrosion.
Temperature Resistance
Stainless steel, especially austenitic and precipitation-hardening stainless steel, retains structural stability and exceptional creep resistance even at temperatures above 800°C due to its cubic austenitic structure and resistance to deformation. Furthermore, the passivation film on its surface prevents severe oxidation of the stainless steel castings at high temperatures. Even at -196°C, it maintains excellent toughness and ductility under impact.
High Hygiene Grades
The passivation film on the surface of stainless steel gives it extremely high chemical inertness. This inertness prevents it from reacting chemically with food and medical solutions, unlike ordinary carbon steel, thereby greatly improving the purity of these liquids.
Why Investment Casting Is Perfect for Stainless Steel
Stainless steel has many advantageous characteristics, but its work hardening and high melting point make it unsuitable for most casting processes. However, these issues can be perfectly resolved through the investment casting process.
While the high melting point and work-hardening properties of stainless steel make it difficult for most casting processes, the lost-wax casting process entirely solves these bottlenecks. When stainless steel is used in investment casting, it can make stainless steel suitable for a wider range of applications and give your castings more favorable characteristics. It can directly eliminate potential scrap risks in your system, significantly shortening your product’s time to market.
Near-Net Shape
Austenitic stainless steel, particularly 304 and 316 stainless steel castings, is highly prone to work hardening during machining, which causes extra tool wear costs.
However, investment casting offers near-net-shape capabilities. By creating a wax pattern, pouring, and a series of processes, castings with complex internal cavities and flows can be produced directly, while maintaining strict stainless steel investment casting tolerances, such as CT4-CT6, and allowing for a minimum wall thickness of 1.5 mm to 2mm. With little CNC machining, machining costs and tool wear can be significantly reduced.
Reducing Welding Risks
Without using the investment casting process, manufacturers would have no choice but to assemble the parts by welding when the casting’s design is overly complex. However, because stainless steel is prone to intergranular corrosion during welding, the overall strength would be significantly reduced.
However, investment casting, also known as expendable mold casting, allows castings with complex structures to be made in a single step. Furthermore, because molten stainless steel has excellent fluidity, it perfectly fills thin-walled structures during pouring. The combination of stainless steel and investment casting reduces the risk of intergranular corrosion, low strength, and fracture.
High Refractoriness of the Mold Shell
Stainless steel has a very high melting point; for example, the melting point of 304 stainless steel is between 1400°C and 1450°C, and the pouring temperature in investment casting is often about 200°C higher than the melting point. In sand casting, it would be highly prone to chemical reactions when in contact with molten steel at such high temperatures, resulting in severe porosity on the casting surface.
In contrast, investment casting employs a silica sol process, and the mold shells are made of high-strength refractory materials. This ensures that the materials within the mold shell do not react chemically with the stainless steel during pouring. Consequently, the chemical composition of the casting remains uncontaminated, and the corrosion-resistant structure of the stainless steel is preserved.
Preventing Intergranular Corrosion
During the cooling process after pouring, carbon within stainless steel castings gradually diffuses outward, reacting with chromium, the key element responsible for corrosion resistance. They would form chromium carbides, precipitate at grain boundaries, leading to intergranular corrosion on the casting surface.
In the stainless steel investment casting process, solution heat treatment can completely dissolve these carbides. Subsequent water quenching further solidifies the internal microstructure, preventing the reformation of carbides. This ensures that chromium is uniformly distributed across the casting’s surface, allowing the surface to regain a chromium passivation film and restore its corrosion resistance.
Improving Surface Finish
The investment casting process not only addresses issues with stainless steel but also provides high surface purity for your stainless steel castings. This is achieved through a series of post-treatments, including shot blasting, pickling and passivation, and polishing. These processes could remove fine burrs and pits from the surface while reforming the passivation film.
The excellent surface finish reduces the adhesion of food residues when stainless steel castings are used in piping castings for the food and hygiene industries, thereby minimizing bacterial growth. It also reduces marine organisms’ adhesion when castings are used in pump and valve castings in marine environments, ensuring stable fluid flow rates.
Stainless Steel vs. Other Metals
Materials such as carbon steel, cast iron, and aluminum alloy can also benefit from the investment casting process; however, their characteristics and suitability are far inferior to those of stainless steel. Understanding the differences between stainless steel and these metals will help you better understand the advantages of stainless steel and make more informed decisions when selecting materials for your castings.
Stainless steel vs Carbon steel
Although the initial cost of carbon steel is much lower than that of stainless steel, its properties are far inferior.
- Corrosion Resistance
Carbon steel does not contain elements such as chromium or nickel in stainless steel, so it lacks the passivation layer on the surface. As a result, when it is exposed to air or certain corrosive media, it would rust quickly.
- Temperature Resistance
Carbon steel performs worse than stainless steel at both high and low temperatures. At temperatures exceeding 400°C, carbon steel undergoes rapid high-temperature oxidation, causing its mechanical strength to drop sharply; at temperatures below -20°C, carbon steel becomes cold-brittle and fractures rapidly upon impact.
- Risk of Decarburization
Unlike stainless steel, which contains a variety of elements, carbon steel contains a large amount of carbon. During the smelting and cooling processes, surface carbon reacts with atmospheric oxygen and is lost, resulting in a significant decrease in the casting’s hardness and strength.
Stainless steel vs Cast iron
Compared with stainless steel, which is used to produce high-precision castings, cast iron is often used to manufacture large structural components due to its inherent properties and low cost, and it is rarely used in the investment casting process.
- Toughness
Stainless steel exhibits extremely high toughness at both high and low temperatures. However, cast iron contains a large amount of flake graphite, which makes it prone to brittleness and fracture under severe impact.
- Surface Finish
Cast iron contains high levels of carbon and silicon, which are prone to reacting with the mold shell under the high temperatures in the investment casting process. This characteristic would lead to surface roughness in castings typically ranging from Ra 12.5 μm to Ra 25 μm or worse, while stainless steel can achieve a surface roughness of Ra 3.2 to Ra 6.3 μm. This characteristic of cast iron is incompatible with the high precision required in investment casting, so it is typically produced only through sand casting.
Stainless steel vs Aluminum
Aluminum alloys are commonly used in the aerospace industry due to their light weight, high thermal conductivity, and good machinability. However, stainless steel offers advantages that aluminum alloys cannot match under extreme operating conditions, such as high temperatures and pressures.
- Mechanical Strength
Aluminum alloys have lower density and lower absolute strength, which makes them highly prone to fracture in operating environments with high pressure and high impact. Stainless steel, particularly martensitic stainless steel, can withstand extremely high mechanical stress and is less prone to fracture.
- High-temperature Resistance
Aluminum alloys have excellent thermal conductivity but very low heat resistance. With a low melting point of around 600°C, the mechanical strength could decrease significantly in operating environments exceeding 200°C; therefore, they are not commonly used in fire safety components. In contrast, stainless steel, with its high melting point and resistance to high-temperature oxidation, is an excellent heat-resistant material.
Understanding the TCO in Stainless Steel Casting
Compared to carbon steel and cast iron, stainless steel offers significant advantages in corrosion resistance and high-temperature, high-pressure operating environments. However, the purchase price of stainless steel is typically 30–50% higher than that of carbon steel and 20–40% higher than that of cast iron.
Its higher upfront costs are primarily due to raw materials and the investment casting process, but the total cost is often lower than castings made from other metals. Understanding which processes save your total costs will help you pay more attention to the initial investment in stainless steel and reduce unnecessary costs later. This further demonstrates that choosing stainless steel investment casting is the revenue-generating solution that truly maximizes long-term profits.
Near-Net Shape
Although the initial tooling costs for investment casting are high, its near-net-shape properties could make the price reasonable. Because it allows stainless steel castings to be produced in a single step with minimal machining, effectively preventing the increased machining costs associated with the work-hardening characteristic in stainless steel. Furthermore, the use of an expendable casting process eliminates the need for welding, significantly reducing the risk of intergranular corrosion during welding and minimizing the costs associated with post-weld heat treatment.
Post Surface Treatment
In the investment casting process, there are many surface treatment steps after the stainless steel castings are completed, and these account for a portion of your costs. However, after undergoing a series of surface treatment processes, such as shot blasting, pickling, passivation, and polishing, not only does the surface roughness can achieve Ra 3.2– 6.3 μm, but the original passivation film on the surface is also optimized. This ensures the stainless steel’s corrosion resistance and reduces the risk of increased costs due to rust caused by damage to the passivation film.
Service Life
The costs of stainless steel raw materials and the investment casting process are significant. However, this process fully utilizes the properties of stainless steel, such as corrosion resistance and temperature resistance. Not only do these processes reduce later costs, but they also extend the service life of stainless steel castings, eliminating the need for frequent recycling and recasting.
Critical Applications for Stainless Steel Castings
With its properties and compatibility with investment casting, stainless steel can be used in many fields. Knowing these fields can help you quickly select the appropriate material for your operating environment.
Chemical Industry
The presence of a stainless steel passivation film gives it exceptional corrosion resistance, making it widely used in the chemical industry, including in chemical pipe-fitting castings, flange castings, and pump housing castings.
Marine Industry
Stainless steel castings, particularly 316L stainless steel castings, have exceptional resistance to chloride ion pitting corrosion in seawater due to their molybdenum content and are widely used in pump castings, fittings, pump impeller castings, valve body castings, and other components.
Food and Pharmaceutical Industries
Stainless steel castings are highly inert, which prevents them from leaching iron ions when they react with acidic substances. What’s more, their exceptional surface finish achieved through investment casting makes them widely used in food-grade piping, valve castings, and medical devices.
Final Thought
Now that you understand why stainless steel is widely used in investment casting because of its properties and compatibility with the process. If you’re looking for a reliable stainless steel investment casting manufacturer, please contact us. Pingheng Machinery, with its customized one-stop service, provides high-quality stainless steel castings with long service life.
FAQ
Q1. What quality control and non-destructive testing are used for stainless steel castings?
Yes, we perform 100% airtightness testing, hardness testing, and non-destructive testing to comprehensively verify that the stainless steel castings meet the required standards and to check for issues such as casting shrinkage and porosity.
Q2. Can I use carbon steel initially and replace stainless steel later?
Yes. But carbon steel will need to be replaced after only a short period of use in corrosive environments. The downtime costs and re-purchase costs associated with replacement are often 5 to 10 times higher than those for stainless steel. While saving on initial costs, you should also consider the total cost over the long term.
Q3. What is the difference between 304 stainless steel and 316 stainless steel?
The main difference between them is that 316 stainless steel contains 2％-3％ molybdenum, which gives it greater resistance to pitting corrosion than 304 stainless steel. 316L is an ultra-low-carbon variant of 316; it does not exhibit carbide precipitation at high temperatures, thereby reducing the possibility of corrosion in castings made from it.
If you want to know more about this, please click “304 vs 316 Stainless Steel – the Ultimate Guide for Beginners and Experts”
Q4. How to verify the chemical composition and dimensional accuracy of precision castings?
We use standard spectrometers and HEXAGON coordinate measuring machines to ensure that the elemental composition and dimensional accuracy of our stainless steel castings meet required standards, ensuring a long service life.
Q5. How is the total cost of stainless steel castings calculated in investment casting?
There is a general formula for calculating the total cost of stainless steel castings: TCO = Procurement + Machining + Installation + Maintenance + Downtime + Scrap – Residual Value. If you want to know more about the TCO of stainless steel in investment casting, please click “The Ultimate Guide to Investment Casting Costs: What Determines Your Quote?”
