Carbon vs. Alloy vs. Stainless: Steel Grades You Might Buy in China

There are basically three types of steel used in the manufacture of products:

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• Carbon steel
• Alloy steel
• Stainless steel

Each type of steel is made up of different amounts of iron and carbon, and in some cases additional alloys. Let’s take a closer look at these types of steel.

Which Type of Steel to Choose?

There are several factors to consider when choosing the right metal for your application:

• Hardness - the ability to resist abrasion, but also the difficulty of cutting or drilling.
• Strength - the amount of force required to bend the metal.
• Toughness - the ability to withstand stress (and not break).
• Formability - the ability of the metal to be shaped.
• Weldability - the ability to be welded (this is a function of melting point, thermal conductivity, etc.).

Types of Steel

1. Carbon Steel (Low & High Grades)

Carbon steel (an alloy of steel and carbon) will corrode but is hard. The more carbon in the steel, the harder it is. Low carbon steel is strong and tough, and can be case-hardened if necessary. High carbon steel can be heat-treated to make it much harder, but it tends to be more brittle and difficult to work with.

Common applications for carbon steel include tubes, plates, bolts, signs, furniture, fencing, and many other common metal parts, typically made from low carbon steel (also known as 'mild steel').

Professional kitchen knives, cutting tools in CNC machines, drill bits, saws, and masonry nails are all made with high carbon steel. The high hardness gives blades and cutting tools a sharp edge that lasts, but it also makes them brittle and more prone to breaking.

The disadvantage of high-carbon steel is that it is more expensive and harder to machine than alloys with less carbon. It is suitable when rust is not a concern, and the product doesn’t need to withstand tensile stress (it doesn’t bend easily and breaks more easily).

2. Alloy Steel

Alloy steel has additional chemical elements added to improve certain properties. Some of the most common alloying elements are manganese, nickel, chromium, molybdenum, vanadium, silicon, and boron.

The improved properties that alloy steels have over carbon steel include:

• Strength
• Hardness
• Toughness
• Wear resistance
• Corrosion resistance
• Hardenability

Common applications for alloy steels include construction and architecture where strength, toughness, and corrosion resistance are prerequisites. Jewelry, household items, cutlery, and cookware are also manufactured from alloy steel.

Steel alloys can be split into two categories: low alloy steels and high alloy steels. Low alloy steels have less than 8% total alloying elements in their composition. These steels have better hardness and wear resistance than carbon steel but tend to have less tensile strength. High alloy steels have more than 8% alloying elements and possess better properties than low alloy steels.

3. Stainless Steel

Stainless steel, an alloy of steel and chromium, doesn’t corrode easily but is not as hard. Any product that will be in constant contact with liquids is a good candidate for a steel alloy with high chromium content. Stainless steel is an alloy of steel with a minimum of 10.5% and up to 30% chromium, giving this steel its unique properties.

Stainless steel is divided into five categories:

a) Austenitic – Austenitic stainless steels are classed as the 200 and 300 series. The alloying elements consist of steel with 18% chromium, 8% nickel, and low carbon content. The most common steel produced is 304 stainless steel, commonly used for pipework, mining equipment, food and beverage, kitchenware, and architecture.

b) Ferritic – Ferritic stainless steels are plain chromium stainless steels where the chromium content can vary between 12% and 18%. These also have a low carbon content similar to the austenitic range. They are classified as the 400 series. This range of stainless steels is magnetic and has good ductility and corrosion resistance. Typical applications include heat exchangers, automotive fasteners, furnace parts, and heater parts.

c) Duplex – Duplex stainless steels contain high levels of chromium (between 18% and 28%) and nickel (between 4% and 8%). This dual high element level gives a mix of austenitic and ferritic structures, hence the name duplex stainless steel. Duplex stainless steel is generally twice as hard as plain austenitic or ferritic stainless steels. They have slightly better toughness and ductility properties than ferritic grades but not as good values when compared to austenitic grades. They have higher strength, good weldability and toughness, and high resistance to stress corrosion cracking. Typical applications include hot water tanks, brewing tanks, process plant equipment, and swimming pool structures.

d) Martensitic – Martensitic stainless steels are plain chromium steels containing between 12% and 18%, as well as a relatively high carbon content of up to 1.2%. Martensitic grades have better corrosion (though not as much as austenitic grades) and wear resistance than other stainless steel grades and can be heat-treated to achieve high hardness values. These grades are magnetic in the annealed and hardened state. Typical applications include cutlery, cookware, surgical and dental instruments, springs, scissors, industrial blades, vehicle stampings, screwdrivers, pliers, and staple guns.

e) Precipitation Hardening – Precipitation hardening stainless steels (PHSS) are composed of chromium and nickel with at least one other alloying element (copper, aluminum, titanium, niobium, or molybdenum). PHSS grades offer an optimum combination of both martensitic and austenitic grade properties. Like martensitic grades, they are known for their ability to gain high strength through heat treatment while also possessing the corrosion resistance of austenitic stainless steel. The most well-known precipitation hardening stainless steel is 17-4 PH, named for its additions of 17% chromium and 4% nickel. It also contains 4% copper and 0.3% niobium. 17-4 PH is also known as stainless steel grade 630. Typical applications include dental drills and reamers, aircraft components, shaver heads, surgical needles, and aerospace applications.

Common Stainless Steel Grades

The three most common stainless steel grades we encounter in China are:

• 201 – inexpensive and very common
• 304 – the most common grade of stainless steel
• 316 – a more expensive grade with better resistance to corrosion

A Word About Stainless Iron

Stainless iron is sometimes used as well. The main difference lies in its composition, which includes less than 0.6% Ni or no Ni element, such as in 403 (12Cr12). It’s widely used in chemical and construction industries. Any magnetic iron alloy containing more than 12% chromium with a body-centered cubic structure is also known as stainless iron.

Most Common Heat Treatment Processes

Heat treatment is the process of heating and cooling metal without changing its physical shape. Different heat treatment processes can change the properties of steel, such as its hardness, toughness, and even soften it. These changes are determined by alterations in the steel’s microstructure.

Hardening – A steel with sufficient carbon content can be hardened by heating it up and rapidly quenching it. This process creates an austenite microstructure, which can be ferrite, martensite, or cementite.

Tempering – This process is carried out on carbon steels that have been hardened to reduce brittleness. The temperature of tempering depends on the desired result for the steel product. The lower the tempering temperature, the better the strength and hardness.

Annealing – Annealing involves heating steel past its critical temperature and letting it cool down very slowly. This results in the steel becoming more machinable and workable.

Normalizing – This process is similar to annealing, where the steel is heated and cooled slowly, typically by leaving it to cool in room temperature air. This gives the steel a microstructure of ferrite and cementite, which has higher strength and hardness but lower ductility.

Carburizing – Also known as case-hardening, carburizing involves infusing additional carbon into the surface of low carbon steel and then subjecting it to the hardening process. The outer carbon steel will have higher hardness, while the inner core remains tough.

How to Test a Metal to Ensure It Is the Right Grade?

The best way is to conduct both physical testing and chemical analysis.

The most common physical tests are:

• Tensile test (applying tension until failure)
• Hardness (or durometry) test, which can be done in different ways

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Chemical analysis is usually done with an X-ray fluorescence tester.

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Stainless Steel Grades

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Stainless Steel Grades

Stainless Steel refers to a group of steel alloys containing more than 12% chromium. Stainless steel is divided into the following grades:

Chromium is known for its high affinity for oxygen and forms a stable oxide film on the surface of the stainless steel (known as a passive oxide layer). This layer forms instantaneously in normal atmospheres, is self-healing, and rebuilds itself once removed. This film is the reason for stainless steel's high resistance to corrosion.

Note: Also see Maximum Operating Temperatures of Stainless Steel Grades.

Austenitic Grades

This group of stainless steels contains:

• 17% - 25% Chromium
• 8% - 20% Nickel
• A number of additional elements that help manufacturers achieve the required properties

Fully annealed conditions of this grade deliver a range of physical and mechanical properties. Mechanical properties can be further enhanced through cold working techniques.

Welding of these stainless steel grades can be done, with low carbon content resulting in fewer problems during welding than seen with the martensitic and ferritic grades. These stainless steels are normally non-magnetic but will become slightly magnetic when cold working techniques have been used.

The Basic Austenitic Grades:

T302

The original general-purpose 18/8 (18% Chromium/8% Nickel) stainless steel grade. The majority of other stainless steel forms have been developed from this grade. T302 exhibits the following physical properties:

• Excellent ductility
• Excellent welding characteristics

T303

This grade was specially developed for machining, especially in automatic screw machines. Here, sulfur or selenium is added to deliver excellent free machining properties and non-seizing properties. However, the addition of sulfur or selenium reduces the steel’s resistance to corrosion slightly below that of T304. Not a recommended grade for welding, this grade is non-hardenable.

T304

Delivering the best all-around performance, this stainless steel grade is one of the most versatile and widely used. With lower carbon content, this grade shows a slightly higher resistance to corrosion than T302. Following welding, it has lesser susceptibility to inter-granular corrosion.

T304L

A low-carbon stainless steel, this grade has a similar resistance to corrosion as T304 but superior inter-granular corrosion resistance after welding stress-relieving processes. This grade is recommended for parts fabricated through welding processes that cannot be annealed. Products made using this grade are limited to a maximum service temperature of 426°C.

T310S

Specifically developed for high-temperature services (up to ± 1,100°C) where high creep strengths are required, this grade, however, is not recommended for prolonged use as brittleness may occur. This grade is also non-magnetic when annealed as well as when cold-worked.

T316

A marine alloy, this grade includes 2% to 3% molybdenum, which improves resistance to corrosion. T316 has superior resistance to chemical corrosion agents and higher resistance to marine corrosion compared to other grades in this category. T316 also has better creep resistance and strength at high temperatures compared to T304 and greater properties for work hardening. Other applications of this alloy include the textile, chemical, and paper industries.

T316L

A low-carbon grade with corrosion resistance similar to that of T316, it exhibits better inter-granular corrosion resistance after stress relieving and cold working processes. Parts manufactured from this grade are limited to service temperatures no higher than 426°C. This grade is recommended where fabricated parts cannot be subsequently annealed.

T321

This grade is T304 stabilized with the addition of titanium. Titanium is added to five times the carbon content to prevent inter-granular corrosion and offer scale resistance at high temperatures (up to 850°C). T321 has slightly lower corrosion resistance than T304 and will not be recommended where bright or mirror polishing is required.

T347

T347 is a modified T304 alloy stabilized by adding tantalum and columbium. Recommended for fabricated parts produced through welding techniques that cannot be annealed, the columbium content offers:

• Resistance to inter-granular corrosion after stress relieving treatments or welding
• Prevention of harmful carbide precipitation at the grain boundaries

This alloy is similar in composition to alloy T321.

Martensitic Grades

These stainless steel grades contain:

• 12% - 14% Chromium
• 0.08% - 2.00% Carbon

Due to their high carbon content, martensitic grades respond well to heat treatment for various mechanical strengths, such as hardness. Extreme care is required during welding processes. Note that heat-treated martensitic grades result in useful combinations of mechanical properties and corrosion resistance.

T409

A construction stainless steel, T409 is intended for use in structural applications where mechanical properties and corrosion resistance are more critical than appearance, such as automotive exhaust systems.

T410

A heat-resistant and general corrosion-resistant steel, it is easily machined and forged. T410 delivers good cold working properties and is not recommended in situations where severe corrosion occurs. One of the most affordable corrosion-resistant stainless steels, it is commonly used for cutlery.

T420

With higher carbon content than T410, T420 has higher hardness (± 500 Brinell) and optimal corrosion resistance when hardened or tempered. It is magnetic in all conditions.

T431

A nickel-bearing stainless steel, T431 is designed for heat treatment processes to achieve high mechanical properties. It is also magnetic in all conditions and has better corrosion resistance than T410 and T430.

Ferritic Grades

Ferritic grades contain:

• A minimum of 17% Chromium
• 0.08% - 2.0% Carbon

Higher chromium content leads to increased corrosion resistance at high temperatures, but ferritic grades have a lack of mechanical properties as they cannot undergo heat treatment. These grades are magnetic, and welding should be done with care.

T430

A heat-resisting stainless steel, this alloy exhibits better resistance to corrosion and heat than T410. Non-hardenable with excellent weldability, T430 has only mild cold working properties due to the high chromium content. This alloy does not require annealing.

Maximum Operating Temperatures of Stainless Steel Grades

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