Have you ever wondered whether to choose alloy steel or stainless steel when purchasing castings, and which material offers better cost performance in specific working environments? This article provides a detailed comparison of alloy steel and stainless steel to help you select the material best suited for your components.
Understanding the material
Understanding the elemental composition and primary grades of these two materials will give you a fundamental grasp of their differences and their most common applications in investment casting, thereby helping you make a more informed choice.
What is Alloy Steel
Definition
Alloy steel refers to steel produced by adding one or more alloying elements to carbon steel to alter its mechanical, physical, or chemical properties, such as chromium, nickel, and manganese. These elements modify the microstructure of the carbon steel, giving alloy steel a strength far exceeding that of ordinary carbon steel.
Common Casting Grades
Alloy steels can be divided into two categories based on their alloy content: low-alloy steels and high-alloy steels.
Low-alloy steels refer to alloy steels with an alloy content of less than 5%. While maintaining good machinability and weldability, their mechanical properties can be enhanced through heat treatment. Major examples include AISI 4140 and AISI 8620.
High-alloy steel refers to steel with an alloy content of more than 5%. It significantly improves the material’s mechanical strength and includes die steels (such as H13) and high manganese wear-resistant steel (such as Hadfield steel or ASTM A128). The inclusion of high concentrations of chromium, molybdenum, or vanadium gives high-alloy steel exceptional abrasive resistance.
What is Stainless Steel
Definition
Since stainless steel contains at least 10.5% chromium, it is also classified as a high-alloy steel in metallurgy. However, in industrial procurement, it is generally treated as a separate category. The chromium in stainless steel could react chemically with oxygen in the air to form a dense passivation film on the surface of the casting, giving stainless steel exceptional corrosion resistance. To adapt to different environments, elements such as nickel (Ni), molybdenum (Mo), manganese (Mn), and titanium (Ti) are added to stainless steel to adjust its physical properties and performance.
Common Casting Grades
Based on its internal microstructure, stainless steel can be divided into five major categories.
Austenitic stainless steel, commonly known as 304 (CF8) and 316 (CF8M), contains high levels of chromium and nickel and is the most widely used corrosion-resistant material.
Martensitic stainless steel, mainly 410 and 440C, typically contains 11.5%–18% chromium and has a higher carbon content, making it a high-hardness, wear-resistant material.
Ferritic stainless steel, mainly 430, consists mainly of a ferritic microstructure, with a chromium content of 10.5%–30% and little expensive nickel, which is a cost-effective, oxidation-resistant material.
Duplex Stainless Steel, primarily 2205 (CD3MN), consists of 50% austenite and 50% ferrite in its microstructure; thereby, this material combines the corrosion resistance of austenite with the high strength of ferrite.
Precipitation-Hardening Stainless Steel, primarily 17-4 PH (CB7Cu-1). By adding elements such as copper (Cu), aluminum (Al), and niobium (Nb) to the alloy. Through hardening, this stainless steel achieves mechanical strength comparable to ultra-high-strength steel while maintaining excellent corrosion resistance similar to 304 (CF8). However, this material requires extremely precise manufacturing processes.
Alloy Steel vs. Stainless Steel: Core Differences
Comparing alloy steel and stainless steel from various perspectives can help you determine which material is better suited for your components.
Investment Casting Process
Both of these materials are used to manufacture components through the investment casting process, and their performance in investment casting significantly affects production costs.
Heat Treatment
The heat treatment of alloy steel primarily involves altering its properties by adjusting the morphology of carbides. This process begins with normalizing to relieve casting stresses, followed by quenching and tempering. By fine-tuning the tempering temperature, the material can achieve high strength and toughness.
In contrast, the heat treatment of stainless steel is more complex. As an example, austenitic stainless steel must undergo solution annealing after casting: heating to 1050°C or higher, followed by rapid water quenching. If cooling is too slow, corrosion resistance will decrease significantly, leading to intergranular corrosion in the casting—a risk greater than that associated with alloy steel.
Shell Firing
During the smelting of alloy steel, due to its high content of iron, a large amount of loose iron oxide scale tends to form on the surface of the molten metal, which in turn leads to scale and inclusions on the surface of the castings. To address this issue, strict control of pouring temperature and the cooling environment is typically required, which increases the complexity and cost of the casting process.
In contrast, stainless steel, due to its extremely high content of chromium, exhibits excellent resistance to high-temperature oxidation during melting. Although it requires a higher pouring temperature, when melted and poured in air, a dense, chromium-rich oxide film forms on the surface of the molten metal, effectively preventing further oxidation. Therefore, stainless steel can be melted directly to produce molten metal of very high quality, resulting in lower processing costs.
Surface Finish
Alloy steel is highly susceptible to decarburization during the cooling process of castings. This occurs because the carbon in the molten steel reacts with trace amounts of oxygen in the mold shell at high temperatures, resulting in a reduction of carbon content in the casting’s surface. To prevent decarburization, the cooling environment must be strictly controlled during the processing of alloy steel; in some cases, the decarburized surface layer must even be machined off during subsequent machining operations, which significantly increases the difficulty of processing.
In contrast, stainless steel has a low carbon content and, due to the presence of chromium, it can form a passivation layer. Therefore, unlike alloy steel, the surface finish of stainless steel castings poses virtually no risk of decarburization on their surfaces.
Mechanical Properties
Mechanical properties refer to the characteristics a material exhibits when subjected to force, such as hardness and toughness. Differences in mechanical properties determine a component’s service life and the possibility of fracture.
Hardness and Wear Resistance
Hardness directly determines the service life of components when they rub against each other or are subjected to abrasion from sand and gravel. In terms of hardness, both materials have their advantages, but alloy steel has the edge.
Before heat treatment, alloy steel has very low hardness, typically around 15–22 HRC, making it very easy to machine and greatly facilitating subsequent machining operations. During heat treatment, through tempering at different temperatures, the hardness can be precisely adjusted to meet your requirements within the range of 30–60 HRC, with a maximum of over 60 HRC. This excellent hardness enables it to effectively resist wear caused by sand and gravel.
In contrast, the austenitic stainless steel we most commonly use cannot increase its hardness through heat treatment and typically remains fixed at 150–180 HB (equivalent to 0–8 HRC). However, in the aspect of hardness, you can choose martensitic stainless steel. For example, 410 (CA15) can achieve a hardness of 40–45 HRC after quenching, while 440C, which is specifically designed for cutting, can achieve a maximum hardness of up to 62 HRC after quenching, which is fully comparable to alloy steel.
Although both martensitic stainless steel and alloy steel offer high hardness, you need to distinguish between them based on the component’s operating environment. If the component requires only extremely high hardness, we recommend that you choose alloy steel for its better cost-performance. However, if the operating environment also demands corrosion resistance, we recommend that you select stainless steel for its better chemical properties.
Impact Toughness and Low Temperature Performance
In terms of toughness, austenitic stainless steel demonstrates significant advantages. At normal temperature, the face-centered cubic (FCC) structure of austenitic stainless steel makes it highly resistant to brittle fracture, with an elongation rate reaching an impressive 60%. This high elongation allows the material to absorb energy through deformation when subjected to high-intensity impacts, thereby preventing fracture. Due to its highly stable internal structure, its toughness remains completely unchanged even at extremely low temperatures of -196°C.
Alloy steel, while possessing both high strength and high toughness, has low internal elongation and is highly prone to fracture when subjected to high-intensity impacts. Due to its low-temperature brittleness, alloy steel components lose their toughness when temperatures drop to -30°C or lower; if subjected to vibration under these conditions, they will fracture immediately.
Physical Properties
Physical properties refer to the inherent characteristics of a material, such as thermal expansion coefficient, magnetic properties, density, and so on. In investment casting and high-temperature applications, variations in these physical properties directly determine the complexity of the process and the level of precision.
Thermal Expansion Coefficient
In terms of thermal expansion coefficient, alloy steel is the undisputed winner.
Among stainless steels, austenitic stainless steel has a remarkably high thermal expansion coefficient due to its high nickel content, which is approximately 30% to 50% higher than that of alloy steel.
As the temperature rises, the thermal expansion coefficient also increases gradually. When stainless steel and alloy steel are both subjected to high-temperature conditions (such as 500°C), the rate of expansion for austenitic stainless steel becomes even more significant. Alloy steel (4140) has a thermal expansion coefficient of approximately 13.5 × 10⁻⁶/°C at 500°C. In contrast, austenitic stainless steel (316) soars to 18.2 × 10⁻⁶/°C at 500°C, which is more than 30% than alloy steel.
This coefficient of thermal expansion will cause massive tensile stresses to build up inside the casting as it cools. If your part design features abrupt wall thickness transitions, this makes it highly susceptible to thermal cracking and casting shrinkages at the joints. For alloy steel, however, this risk is much lower.
Magnetic Properties
In the aspect of magnetic properties, your choice depends on the operating environment of your parts. Alloy steels, due to their internal microstructure consisting of ferrite and martensite, exhibit strong magnetic properties at normal temperatures; they are easily attracted by magnets and are also easily magnetized.
In contrast, the magnetic properties of stainless steel vary depending on the grade. For example, austenitic stainless steel is theoretically non-magnetic or only slightly magnetic, but the other four types of stainless steel, which have internal structures composed of ferrite and martensite,e are all magnetic. If you need components that will not interfere with the existing magnetic properties of your equipment during operation, austenitic stainless steel, specifically grades 304 and 316, would be your best choice.
Chemical Properties
Chemical properties are also a major factor affecting the service life of components. Although both alloy steel and stainless steel are alloys, differences in their chemical compositions result in different behaviors when exposed to corrosive media such as air and seawater.
Corrosion resistant
The primary difference in their chemical properties lies in their corrosion resistance. This is due to variations in their elemental composition. When exposed to oxygen in the air, alloy steel will react with oxygen to form a layer of iron oxide because of its higher iron content, commonly known as rust. This layer of rust is unable to withstand external corrosion, and this chemical oxidation is unstoppable.
In contrast, stainless steel, due to its higher chromium content, reacts with oxygen to form a dense passivation film that prevents the penetration of corrosive substances. Furthermore, if this passivation film is scratched or worn during use, it s self-repairs to form a new layer.
When exposed to different chemical media, the two materials exhibit distinct behaviors.
| Chemical media | Alloy Steel | Stainless Steel (Take 316 as an example) |
| Atmosphere and Freshwater | It oxidizes and rusts very easily, so it must be protected from the air by painting, electroplating, or applying rust-preventive oil. | It has almost no chemical reactions and retains its metallic luster over the long term. |
| Seawater | Chemical corrosion, such as pitting, may occur. | Because 316 material has molybdenum and nitrogen, it exhibits exceptional resistance to pitting corrosion. |
| High-temperature oxidation | At temperatures above 450°C, severe high-temperature oxidation occurs, resulting in surface decarburization and the formation of large amounts of slag and scale. | Even at high temperatures ranging from 600°C to 800°C, the passivation film on the surface remains stable, demonstrating exceptional high-temperature stability. |
| Strong acid | A violent displacement reaction will occur, the metal will dissolve rapidly, and release a large amount of hydrogen gas. | It exhibits extremely high chemical inertness and effectively resists corrosion by acidic solutions. |
As can be seen, stainless steel is resistant to corrosion from most industrial acids and alkalis, as well as atmospheric and seawater. If your components need to come into direct contact with chemical fluids or corrosive media, stainless steel is your best choice.
Applications
Alloy steel and stainless steel are suitable for use in many applications. Understanding which they are best suited for will greatly assist you in selecting the right material.
Marine Environment
In marine environments, marine castings made of stainless steel are superior to those made of alloy steel in the vast majority of cases.
Alloy steel will undergo severe pitting corrosion when exposed to oxygen in seawater, causing components to corrode and flake off rapidly. It can only be used in seawater when taking extreme protective measures, such as applying anti-corrosion chemicals. However, even with a protective chemical coating on the surface, alloy steel is only suitable for constructing large structures such as ship hulls or the frameworks of offshore drilling platforms.
Although stainless steel can form a passivation film, you must still exercise caution when selecting it, as chloride ions in seawater are the nemesis of some stainless steel grades. For example, 304 stainless steel casting cannot withstand this type of corrosion. However, 316 stainless steel casting, due to the addition of molybdenum, offers excellent resistance to pitting corrosion caused by chloride ions in seawater and is highly suitable for environments with flowing seawater.
Pump&Valve Components
In the pump and valve industry, alloy steel and stainless steel account for half of the market. The choice of material by a pump impeller casting manufacturer depends specifically on the application of the pump or valve.
If the pump casting, pump housing casting, or valve casting is intended for marine or chemical applications involving highly corrosive fluids, if it features an impeller casting exposed to high-velocity fluid flow, or if it is intended for the food or medical industries, where components must not contaminate the medium. For these various purposes, stainless steel is recommended. Its corrosion resistance, exceptional toughness, and ability to remain non-contaminating make it well-suited for these applications.
If your pump or valve is designed to withstand pressures of tens or even hundreds of kilograms, or if it requires frequent opening and closing and must endure severe friction, we recommend choosing ultra-high-hardness alloy steel. This material ensures dimensional accuracy for these high-pressure pump components, prevents part deformation, and extends the service life of the parts.
Automotive & Heavy Construction
In both of these applications, components are subjected to severe impacts, high-frequency vibrations, and abrasion from gravel and sand every day. Therefore, alloy steel with high hardness is the best choice. However, in certain specific situations, stainless steel is the optimal choice.
For automotive castings that must withstand extreme pressure and severe vibration, as well as the undercarriages of heavy construction machinery that endure abrasion from sand and gravel, alloy steel with excellent hardness and strength prevents cracking under wear and ensures a long service life for the components.
However, for engine exhaust manifolds and turbocharger impellers that must operate continuously at high temperatures between 600–900°C, stainless steels, such as CF8 (the cast equivalent of 304 stainless steel), which offer exceptional resistance to high-temperature oxidation and creep, are more suitable for these components than alloy steel.
Furthermore, for hydraulic rods and sealing valves in construction machinery that are exposed to the open air and mud, while hardness is required, corrosion-resistant stainless steel is more essential. This helps prevent leaks in sealing valves caused by rust to a certain extent.
Final Thoughts
Both alloy steel and stainless steel castings are composed of various added elements, but their properties and applications differ significantly. Choosing the right material for your castings is crucial to your project. While alloy steel offers lower short-term costs and higher strength, stainless steel castings often provide a higher long-term return on investment in environments with high corrosion, high pressure, or extreme temperatures. If you want to find an excellent stainless steel foundry, please contact us. Pingheng Machinery has specialized in stainless steel investment casting for 27 years, ensuring strict airtightness, and provides high-quality castings for a wide range of applications.
FAQ
Q1. Which is more cost-effective to procure, alloy steel or stainless steel?
Generally speaking, the procurement cost of stainless steel is significantly higher than that of alloy steel. This is primarily because stainless steel contains large amounts of chromium and some expensive metals such as nickel and molybdenum, and it exhibits work-hardening properties, resulting in higher machining costs. However, from a Total Cost of Ownership (TCO) perspective, stainless steel often yields a higher ROI because of its long life span under corrosion application.
Q2. In applications that need to be subject to severe stress and frequent vibration, which has a longer service life, alloy steel castings or stainless steel castings?
Alloy steel castings have a longer service life. This is because, after heat treatment, the strength of alloy steel will increase significantly, and its fatigue limit can reach 40%–50% of its tensile strength. In contrast, stainless steel has lower strength and is prone to microcracking under frequent stress.
Q3. In the machining of investment castings, what are the specific comparative metrics for the “machinability” of alloy steel and stainless steel castings?
Assuming that the easier it is to cut, the higher the machinability, the machinability index for alloy steel is approximately 55%–65%, indicating that it is easy to machine and offers high machining efficiency. In contrast, the machinability index for stainless steel is approximately 35%–45%. Its high toughness makes it difficult to break, and its work-hardening characteristics make it more difficult to cut.
Q4. In terms of dimensional accuracy, what is the difference between alloy steel and stainless steel?
Under the same investment casting process, the dimensional accuracy of both materials is essentially the same, with both capable of meeting ISO 8062-3 DCTG4 to DCTG6 grades. However, because alloy steel will form a porous oxide scale and is highly susceptible to decarburization, its surface roughness is slightly higher than that of stainless steel after shot blasting.
Q5. What is the difference in machining allowances for investment castings made of stainless steel and alloy steel?
Generally, alloy steel requires a much larger machining allowance than stainless steel, which is because alloy steel will form a decarburized layer. In order to ensure this layer is completely removed during subsequent machining, a machining allowance of 1.5 mm to 2.5 mm on one side is generally required. In contrast, stainless steel has no risk of decarburization, so machining only needs to account for dimensional accuracy adjustments, typically 1 to 1.5 mm, which significantly reduces waste of raw metal materials.
Q6. Why is it mandatory to use stainless steel for food and medical applications, rather than alloy steel treated with electroplating for rust prevention?
This is because surface coatings are at risk of failure under prolonged exposure to fluid flow. Furthermore, if the electroplated coating becomes damaged or peels off, the flaking coating poses a risk of heavy metal contamination. In contrast, as a globally recognized hygienic-grade material that complies with FDA standards, stainless steel will not leach harmful substances even if the surface is damaged.
