Stainless steel—ever wondered what makes this material so special? From your kitchen utensils to towering skyscrapers, stainless steel is everywhere, yet its unique properties often go unnoticed. Have you ever stopped to think about why it doesn’t rust, or how it stays so durable? In this article, we’ll dive into what stainless steel truly is, explore its fascinating qualities, and uncover why it’s a favorite in industries worldwide. Stick around, and let’s explore the magic behind this everyday marvel!
What Is Stainless Steel?
Stainless steel represents a family of high-alloy steels; it has been recognized primarily for its outstanding resistance to corrosion. Its naturally beautiful and durable surface has eliminated the need for additional treatments like plating or painting. This is often the reason that machinery manufacturers often do not treat the stainless steel material, to emphasize its natural attribute. For normal steel, sometimes called “black steel,” rust-proofing treatments must be done if the usability has to be maintained over an extended period of time.
Depending on the different media, the resistance of stainless steel to corrosion generally falls into two categories. One is stainless steel resistant to the weak corrosive medium of air, steam, water, and acid-resistant steel; it could bear the corrosion in such an aggressive chemical ocean of acids, alkalis, and salts. Although they are alike in chemical composition, both of them differ. The stainless steel, which is good in weak corrosive environments, may not resist chemical corrosion, while acid-resistant steel generally has good corrosion resistance in all conditions.
The alloying elements provide the great resistance of corrosion characteristic of stainless steel. The surface oxide film formed is unbroken, which shields the material from oxidation, thereby rendering the material serviceable over an enormous gamut of applications. Because of this combination of appeal and utility, combined with resistance, stainless steel finds broad applicability in several fields.
The History Of Stainless Steel
The first scientific investigations into the corrosion resistance of stainless steel were made at the beginning of the 20th century. The resistance of Fe-Cr and Fe-Cr-Ni alloys to corrosion was studied between 1904 and 1906 by L.B. Guillet and A.M. Portevin in France, and during 1907-1909 by W. Giesen in the UK. In Germany, around the same time several theories of stainlessness and passivation were presented by P. Monnartz and these formed a scientific basis for further development.
Development of the industrial stainless steel can be referred to 1912-1913, when H. Brearley in UK developed martensitic stainless steel with 12%-13% chromium, and between 1911-1914, C. Dantsizen in the US had come up with the development of ferritic stainless steel containing 14%-16% chromium and 0.07%-0.15% carbon. In the same period, E. Maurer and B. Strauss of Germany developed austenitic stainless steel that contained low carbon, 15%-40% chromium, and a small amount of nickel. In 1929, Strauss patented low-carbon 18-8 (Cr-18%, Ni-8%) stainless steel, which became a key milestone.
In 1931, E. Houdré invented the titanium-containing 18-8 stainless steel in order to make up the deficiencies in intergranular corrosion in sensitized 18-8 steel, which is what we know today as 1Cr18Ni9Ti or AISI 321. In the meantime, the Unieux laboratory in France discovered that adding ferrite into austenitic stainless steel could notably enhance its resistance to intergranular corrosion; thus, the duplex stainless steel came into being. In 1946, R. The martensitic precipitation-hardened stainless steel 17-4PH was developed by Smith and others in the US, which was followed by the development of semi-austenitic precipitation-hardened steels such as 17-7PH and PH15-7Mo.
During the 1940s and 1950s, Cr-Mn-N and Cr-Mn-Ni-N stainless steels were developed to save nickel, as well as ultra-low carbon (C≤0.03%) austenitic stainless steels. During the 1960s, other grades were developed such as duplex stainless steel of γ:α close to 1, extra high-purity ferritic stainless steel, and martensitic age-hardened stainless steel. These, though coming within the compass of the five major categories of stainless steel, represented a great enhancement of certain grades. As the performance requirements of stainless steel are constantly improving, some alloying elements have been added in recent years, including molybdenum, copper, silicon, nitrogen, manganese, niobium, and titanium, giving birth to new types of stainless steel such as high-chromium molybdenum ferritic steels and super austenitic steels. Since then, these steels have found extensive applications in such fields as nuclear energy, nitric acid, urea, food processing, and other industries.
How Stainless Steel Is Made
In the production of stainless steel, a manufacturer should have proper identification of the type of stainless steel that is desired to be produced. The reason being, the grade that is being intended will directly relate to the fraction of each constituent in the alloy such as iron, carbon, nickel etc. used in the making. These fractions may be a little less approximate, at times falling in the range between maximum and minimum due to inevitable impurities within every element constituting the alloy mixture.
Once these raw materials are gathered, the remaining manufacturing process for stainless steel can start. The key steps in this process are as follows:
Melting Of Raw Materials
Several stainless steel materials are placed in an electric furnace, used in most of the modern usages in manufacturing stainless steel. It is heated to their melting points, which may take up to 8 to 12 hours. When the metal reaches its melting point, the process of stainless steel manufacturing may proceed further.
Removing Excess Carbon
The hot metal is transferred to a VOD or AOD vessel for the further removal of carbon. The resultant variant of the alloy could be ‘standard’ or ‘low-carbon’, depending on how much carbon has been removed. Examples include stainless steel 304 and 304L, respectively. The impact will be such that the final product will differ in tensile strength and hardness.
Tuning Or Stirring
To improve the quality of the final product, the molten steel may be stirred to evenly distribute or eliminate certain components from the mixture. This process ensures that the stainless steel maintains consistent quality and meets the specifications set by end-users (such as Marlin Steel).
Profiling The Metal
As the stainless steel begins to cool, it goes through various shaping processes. Hot rolling occurs when the metal is still above its crystallization temperature. This process helps form the rough shape of the metal, typically used to produce metal billets or blooms. To achieve precise dimensions for the billets or blooms, the stainless steel can undergo cold rolling.
Heat treatment/Annealing
Annealing is a process of heating stainless steel and cooling under controlled conditions with regard to the removal of internal stresses and the mechanical properties changes. Annealed, the steel might be descaled not to damage the protective oxide layer.
Cutting And Shaping
Further in processing, after annealing, different operations of cutting and shaping are made to the stainless steel for an ideal final product of its application. The real cutting operations of the stainless steel will be depending on the size and shape of the billet/bloom, besides the desired form of the end product. For example, thick metal plates are cut by using mechanically large metal shears. Shapes can be cut from thinner metal plates by CNC punch presses or laser cutting machines. More often, Marlin Steel’s manufacturing team uses laser cutters and CNC punch presses to create custom cuts in metal out of sheet stock.
Application Of Surface Treatments
There are a number of surface treatments that the manufacturers of stainless steel can make to the billets, blooms, wire or whatever form before it is shipped to other manufacturers. The type of surface finish would of course depend on its application; however, the most common surface finish is to simply polish the surface to remove impurities and to make the surface smoother.
Different Types Of Stainless Steel
In general, general stainless steels are divided into three types according to metallographic structure: austenitic, ferritic, and martensitic stainless steels. With these three basic metallographic structures, for special needs and purposes, it further derived duplex steels, precipitation hardening stainless steels, and high-alloy steels with less than 50% iron composition.
Austenitic Stainless Steels: The matrix is majorly austenitic, which has a face-centered cubic crystal structure (FCC phase), non-magnetic, and is majorly cold-worked strengthened (which can have some magnetism, too). The AISI designates them with numbers 200 and 300 series such as 304.
Ferritic Stainless Steel The matrix Is mostly ferritic: it has a body-centered cubic (BCC) crystal structure; it is magnetic, can typically not be hardened by heat treatment, and can only slightly be strengthened by cold working. These have been designated 430 and 446.
Martensitic Stainless Steel: It has a martensitic matrix, which may be body-centered cubic or cubic and magnetic. Such steel is heat treatable, whereby its mechanical properties can be modified. The AISI classifies them as 410, 420, and 440. Martensite has an austenitic structure at high temperature and, upon cooling to room temperature at an appropriate rate, the austenitic structure is transformed into martensite – a process usually known as quenching.
Austenitic-Ferritic (Duplex) Stainless Steel: Besides containing both austenitic and ferritic phases in the matrix, the content of the less abundant phase is normally higher than 15%: magnetic and strengthened by cold work. Typical DUPLEX STS: 329 Heat-treatable (Maraging) Stainless Steels Higher mechanical properties in duplex steels are reached with strong improvements in the behaviour concerning the susceptibility to intergranular, chloride-stress, and pitting corrosions.
Precipitation-Hardening Stainless Steel: The matrix can be either austenitic or martensitic, and it may undergo precipitation hardening through a specific heat treatment. These steels are designated by the AISI with 600 series numbers, such as 630 (17-4PH). Typically, austenitic stainless steels offer the highest corrosion resistance. Ferritic stainless steels are used in environments with low corrosion potential. Martensitic and precipitation-hardening stainless steels are applied in mildly corrosive conditions where high strength or hardness is required.
Different Grades Of Stainless Steel
According to American standards, stainless steel is generally categorized into three groups: the 200 series, the 300 series, and the 400 series. The most commonly used one of these is the 300 series; to some extent, both the 200 and 400 series are substitutes for the 300 series. Speaking strictly, the 400 series is not stainless steel but “stainless iron” since it does not contain nickel, hence it is magnetic, meaning it can easily be attracted by a magnet. While in the 200 and 300 series, there is nickel added to make them non-magnetic and cannot be attracted to a magnet. The 200 series contains less nickel, the 400 series has no nickel at all, while the 300 series contains most nickel. Thus, this series is mostly affected by fluctuation of nickel prices.
Here is the table for the 300 series stainless steels and their properties:
Model | Description |
301 | It has excellent ductility, making it ideal for forming products. It can be rapidly hardened through mechanical processing. Additionally, it offers good weldabilit |
302 | It offers the same corrosion resistance as 304 stainless steel, but its strength is enhanced due to a higher carbon content. |
303 | Made easier to machine than 304 by adding small amounts of sulfur and phosphorus. |
304 | General-purpose model, also known as 18/8 stainless steel. GB grade number is 0Cr18Ni9. |
309 | Better temperature resistance than 304. |
316 | Ranked as the second most commonly used steel after 304, SS316 is predominantly utilized in the food industry and for surgical instruments. The inclusion of molybdenum enhances its corrosion resistance, particularly against chloride corrosion. It is also referred to as “marine steel” and is frequently used in nuclear fuel reprocessing applications. |
321 | Contains titanium, which reduces the risk of weld corrosion, with other properties similar to 304. |
Below is a table outlining the properties of the 400 series stainless steels, which include ferritic and martensitic steels:
Model | Description |
408 | Offers good heat resistance but has weak corrosion resistance, containing 11% chromium and 8% nickel. |
409 | The cheapest model (UK and US), usually used for automotive exhaust systems, ferritic stainless steel (chromium steel). |
410 | Martensitic steel (high-strength chromium alloy) offers excellent wear resistance but has poor corrosion resistance. |
416 | Contains sulfur to improve machinability. |
420 | “Tool grade” martensitic steel, similar to early high-chromium steels such as Bessemer steel. Also used for surgical tools, can be made very shiny. |
430 | Ferritic stainless steel, commonly used for decorative applications like automotive trim, offers good formability but has limited resistance to temperature and corrosion. |
440 | High-strength tool steel with slightly higher carbon content, capable of reaching high yield strength and hardness up to 58HRC after proper heat treatment. Known as one of the hardest stainless steels, it is commonly used for razor blades. Available in four variations: 440A, 440B, 440C, and 440F (which is designed for easier machining). |
Properties Of Stainless Steel
Stainless steel possesses unique physical and chemical properties. These characteristics contribute to its exceptional performance across diverse industries and underline why it remains a preferred material worldwide.
Physical Properties Of Stainless Steel
Stainless steel has many physical properties that make it suitable for various purposes. Some of the key physical characteristics of stainless steel are discussed below.
- Density: The density of stainless steel ranges from 7.75 × 10³ to 8.05 × 10³ kg/m³. It infers that stainless steel is fairly dense compared to other metals, which is one of the factors contributing to its strength and durability.
- Ductility: The typical annealed austenitic stainless steel may show elongation up to a maximum of 70% before rupture, exhibiting excellent ductility. This property enables the material to undergo different forming processes such as bending and stretching to develop intricate shapes.
- Malleability: The material, when annealed, is very malleable. However, some grades of stainless steel work harden rather fast and become less malleable under stress. This shall be kept in mind when the material is being formed or shaped.
- Tensile Strength General Grades: The tensile strength of general grades ranges from 500 to 750 MPa, indicating the material’s ability to resist being pulled apart under tension.
- Yield Strength: Generally, stainless steel grades feature yield strengths that range between 500 to 650 MPa. Yield strength is a value that determines the stress required to cause a material to start deforming plastically; an indication of how much stress a material will bear before permanent deformation starts to occur.
- Tensile Strength : The tensile strength of precipitation-hardened stainless steels is remarkably higher, lying between 850 and 1,700 MPa. These steels have undergone some heat treatment to increase their strength. These are used where exceptionally high strength is required in applications.
- Yield Strength : The yield strength of precipitation-hardened stainless steels lies in the range of 520 to 1,500 MPa, making them ideal for applications involving very high stress levels where durability becomes the utmost priority.
- Melting Point: The melting temperature for stainless steel normally ranges from 1,370 to 1,420°C. The high melting point, as seen above, is important for the structural integrity of stainless steel under a temperature extreme.
- Thermal Conductivity: The thermal conductivity of stainless steel is generally poor, around 15 W/(mK), which makes it a poor conductor of heat. This might be advantageous for those applications in which retention or insulation of heat is required.
- Electrical Conductivity: Stainless steel is a poor conductor of electricity, too, with the general value of 1.33 m/Ωmm². For this reason, it cannot be used where high electrical conductivity is required but is useful in applications where the resistance to corrosion is more important than its electrical performance.
These physical properties make stainless steel a versatile material, widely used in construction, manufacturing, medical, and aerospace industries where strength, durability, and corrosion resistance are key requirements.
Chemical Properties Of Stainless Steel
Stainless steel is known for its excellent chemical properties, which make it resistant to a wide range of corrosive environments. Below are key chemical properties of stainless steel:
- Corrosion Resistance: Stainless steels exhibit varying resistance according to the different types of alloys, grade, and exposure to respective corrosive environments. Highly resistant in nonsalt, medium to good in salt, good to poor in hot-salty-acidic, and hot-caustic media.
- Chemical Resistance: Stainless steel usually has high to very high chemical resistance, with specific grades for more difficult operating conditions. The 316-type grades have particularly good resistance to acids and chlorides; hence, these materials find wide applications in chemical processing and pharmaceutical industries.
- Heat Resistance:Stainless steel exhibits excellent heat resistance, with certain grades, such as 330, able to withstand temperatures up to 2,000 °C. This makes them suitable for high-temperature applications, including furnaces, industrial machinery, and aerospace.
- Cold Resistance: Stainless steel reveals good properties at very low temperatures. Such as, in non-stabilized 304-grade steel, a temperature as low as -200 °C sustains its nature, hence widely used in cryogenic applications and environments of freezing condition.
- Sensitization Corrosion: During welding, sensitization corrosion occurs in grade 304 and 316 through the formation of chromium carbide at grain boundaries; hence, resulting in localized corrosions, the effect of which can be reduced by welding methods that involve low heat and using low carbon fillers.
It finds wide usage in those industries where stamina, performance in extreme conditions, corrosion resistance, chemical resistance, resistance to heat and cold, and precaution against sensitization are required due to the chemical properties of stainless steel.
Applications Of Stainless Steel
Does it ever come across to you how stainless steel could be part of one’s life? However, stainless steel has proved with facts that it has indeed infiltrated into various fields and has become an essential element in our lives.
Application Of Stainless Steel In The Field Of Medicine
Do you know which stainless steel products are applied in the medical area? Stainless steel needles, surgical knives, wheelchairs, IV stands, and medical tweezers, among many others, are made of stainless steel. The application to orthopedics is very important and daily. In addition, because of its excellent overall performance, advanced manufacturing technology, and affordability, the application of stainless steel in the medical area is expanding and becoming a major trend.
Application Of Stainless Steel In The Field Of Electronics
Still boiling water with fire? That’s so outdated. Try using a stainless steel kettle instead! Stainless steel kettles are much more convenient and efficient. Due to its unique properties, stainless steel has been widely applied in various other electronic fields. For example, modern water heaters now feature stainless steel outer shells, and stainless steel heating tubes are used in coffee machines. There are many other applications of stainless steel in electronics.
Application Of Stainless Steel In The Food Industry
In industries such as food processing, catering, brewing, and chemicals, surfaces must be easy to clean daily and frequently require the use of chemical cleaners. Stainless steel is the ideal material for this purpose due to its durability and ease of maintenance.The most representative applications of stainless steel in the food industry are stainless steel straws and stainless steel fluid pipes. However, stainless steel straws have been substituting plastic straws little by little because stainless steel straws have been deemed as much greener than plastics and follow the trend of contemporary social development. Stainless steel pipes for fluid conveying have flooded many restaurants. Compared to before, using stainless steel was always thought to be unhygienic, food-grade stainless steel has found a solid position in the food industry.
Stainless Steel Used In The Automotive Sector
Stainless steel’s use in the automotive field perhaps accounts for its biggest application today. Today, it is known to be one of the most emerging automobile fields where the uses of stainless steel are on high demand. For today, the applications for the making of a car, frame, car bodies, fuel tanks, stainless steel, and auto adornments, involve stainless steel in it. Due to the huge demand for stainless steel in the automotive industry, it has become one of the main driving forces for the development of stainless steel.
Benefits And Limitations Of Stainless Steel
While stainless steel offers significant advantages, like durability and low maintenance, it has limitations, such as higher costs. Weighing these factors helps in deciding its suitability for specific projects
Benefits Of Stainless Steel
It has widely been remarked that stainless steel is extremely resistant to rust, corrosion, and heat. These characteristics make this material especially good for items intended to function in harsh conditions. The material can bear with its load through the most violent weather or high temperatures. Because it can be recycled, it’s very friendly to the environment; being fully recyclable, it’s also reusable with no loss in properties, thus making it really ecologically viable. Another important advantage of stainless steel is its hygiene: it is easy to clean and maintain, which makes it a very good material for medical and culinary purposes where sanitation is an issue.
Limitations Of Stainless Steel
While stainless steel has many attributes, there is a flip side to all these advantages as well. A couple of the serious drawbacks include its generally high initial cost compared with other materials and the fact that sometimes it may turn out to be uneconomical for specific applications. Second, stainless steel is more difficult to process by machining or welding; one needs to have specialized instruments and expertise, which can enhance the production cost and time, especially in customized intricate designs.
Conclusion
Stainless steel is a very important material in modern industries because of its outstanding durability, resistance to corrosion, and aesthetic appeal. Its unique properties, such as strength, heat resistance, and low maintenance, make it invaluable in a wide range of applications, from construction to healthcare. From structural components and medical devices to kitchen utensils, stainless steel continues to play a vital role in advancing technology and improving everyday life, proving its lasting importance in the global market.
Final Thoughts
As a worker with 15 years of CNC machining service experience, I deeply understand the unique advantages and challenges of stainless steel in machining. Its resistance to corrosion, high temperature, and strong strength make stainless steel increasingly popular in industries like aerospace, medical, and food processing. For example, our company once processed a batch of stainless steel surgical tools for a medical device manufacturer, where high precision and surface smoothness were required. Through precise CNC machining, we ensured each component met strict quality standards and had flawless surfaces, meeting the client’s functional needs.
However, machining stainless steel also presents challenges, especially with harder grades like 304 and 316, which can cause tool wear. Therefore, Ultirapid has accumulated extensive experience in using high-quality cutting tools and optimizing machining parameters to efficiently and safely complete complex stainless steel machining tasks. Our technical expertise and years of experience allow us to provide reliable solutions for our clients.
Faqs
Why Is Stainless Steel So Hard To Machine?
Stainless steel is difficult to machine due to its hardness, work hardening tendency, and heat retention. The material’s toughness increases tool wear, requiring specialized cutting tools like carbide or high-speed steel. Additionally, as stainless steel hardens during machining, it demands precise control of speed and coolant to prevent further hardening and maintain surface quality. These factors make stainless steel more challenging to machine compared to softer metals.
What Speed Do You Need To Mill Stainless Steel?
When milling stainless steel, the cutting speed varies depending on the specific grade and the tool used. For austenitic stainless steel (like 304 or 316), cutting speeds typically range from 100 to 150 feet per minute (30-45 meters per minute). Martensitic and ferritic grades require slightly slower speeds, around 80-120 fpm (24-36 meters per minute). It’s crucial to adjust the cutting parameters based on the tool, machine, and material to ensure efficient machining and tool longevity.
What Is The Best Machining Grade Of Stainless Steel?
The best machining grade of stainless steel depends on the specific requirements of the application. 304 stainless steel is a popular choice for general machining due to its good balance of machinability, corrosion resistance, and versatility. For easier machining, 303 stainless steel, which contains sulfur to enhance machinability, is ideal. However, for high strength and wear resistance, 17-4 PH stainless steel is preferred, offering excellent machinability while maintaining superior mechanical properties.