The Strength of Steel: what is it used for and why?

Steel is an alloy primarily composed of iron and carbon, with carbon content typically between 0.02% and 2.1% by weight. The carbon in steel is primarily responsible for its hardness and strength, while iron provides the basic structure and ductility.

Besides carbon, steel often contains other alloying elements that can modify its properties to suit various applications. Steel is categorised in different ways, leading to a larger number of distinct types that can be separated into different categories.

Carbon Steel

Carbon steel is a type of steel where the main alloying element is carbon. The carbon content in carbon steel typically ranges from 0.02% to 2.1% by weight. The properties of carbon steel depend largely on its carbon content and the heat treatment it undergoes. Carbon steel is widely used in various industries due to its strength, hardness, and affordability.

Most commercial Steels are classified in three groups:

  • Plain Carbon Steel
  • Low Alloy Steel
  • High Alloy Steel

 

Plain Carbon Steel is Steel that typically contains Iron with less than 1% carbon content, including small amounts of other elements such as Silicon, Manganese and Sulphur. Plain carbon can also be subcategorised:

  • Low – Low-Carbon Steels contain less than 0.30% carbon. Low-Carbon Steels are the most used grades, as they machine and weld well, as well as being more ductile than high-carbon steels.
  • Medium – Medium-Carbon Steels contain 0.30 to 0.45% of carbon, the increase in carbon means an increase in hardness and tensile strength.
  • High – Higher-Carbon Content steels usually contain 0.45 – 0.75%. Although the increase of carbon does increase the strength of the steel it also reduces its weldability due to martensite formation which occurs due to the rapid cooling process.
  • Very High – Steel containing up to 1.50% carbon content are typically used for products such as metal cutting tools, like high-carbon steels they require heat treatment before, during and after welding to reduce damage and maintain their mechanical properties.

Low Alloy Steel are typically designed for welding applications as their carbon content is usually below 0.25% and 0.15%. Some of these alloys include Nickel, Manganese, Molybdenum and Chromium. The addition of these elements can help improve strength and durability.

High Alloy Steel, the most commercially used high-alloy steel is Stainless Steel. Stainless Steel usually contains at least 12% Chromium, the addition of Chromium is responsible for the durability of the steel. Stainless Steel also shows fire resistance, due to its high strength retention factor at higher temperatures.

Stainless Steel

Stainless Steel can be further divided into categories:

  • Austenitic
  • Martensitic
  • Ferritic
  • Duplex
  • Precipitation Hardening (Ph Steel)

Austenitic Stainless Steel is great for welding but isn’t stable at room temperature, which then requires specific alloys to stabilize it such as carbon, manganese, nitrogen and nickel. The further addition of other alloys such a chromium, molybdenum and titanium can be incorporated into Austenitic steel to improve its corrosion resistance, strength and resistance at high temperatures.

Martensitic Stainless Steel is a commonly used steel that we see every day within our own kitchens as Martensitic steel is used for the creation of cutlery. It offers the least amount of chromium compared to other Stainless Steels but offers high hardenability and requires both pre- and post-heating when welding to prevent cracking.

Ferritic Steel contains around 12 – 17% Chromium with small amounts of Austenitic forming alloys. Due to the high corrosion resistance of Ferritic steel, it has many industrial applications in the field of food processing and water treatment, as well as additional uses within electrical and construction sectors.

Duplex Steel is a combination of Austenite and Ferrite, typically this mixed microstructure is of equal proportions. The combined structure results in a material that has high strength, greater than that of either Ferritic and Austenitic, as well as providing superior resistance to pitting, crevice corrosion, and stress corrosion cracking. Compared to austenitic stainless steels, this makes it ideal for harsh environments, particularly in chloride-rich conditions.

Precipitation Stainless Steel, also known as PH Steel, is a type of stainless steel that can be strengthened and hardened by heat treatment. This process causes the formation of fine particles (precipitates) within the metal matrix that stop dislocation movement, thereby increasing the materials strength and hardness. PH Steel combines the corrosion resistance of austenitic steel with the strength of martensitic and ferritic steels. PH Steel can also be treated to achieve a wide range of mechanical properties from high strength to high toughness, making it suitable for high stress applications in industries such as automotive industry, medical devices, aerospace and oil & gas industries.

Precipitation hardening stainless steel offers a unique combination of high strength, hardness and high corrosion resistance, which makes it ideal for a wide range of demanding applications and its ability to tailor its mechanical properties through heat treatment provides versatility, allowing this steel to be used in various industries where performance and reliability are needed.

Tool Steel

Tool steel is a type of carbon and alloy steel specifically used in the production of tools. The suitability of tool steel comes from its hardness, resistance, and resistance to deformation at elevated temperatures.

Tool steel has different types and properties used for different applications:

High Speed Steel (HSS) – High Speed Steel is a subset of tool steels, mostly used for cutting tools and it is very durable, as well as maintaining strength at high temperatures, which allows for faster cutting speeds.Welding

The typical composition of HSS includes elements such as Tungsten, which enhances the hardness of the steel, Chromium which provides its resistance to corrosion, and Cobalt, which contributes to the steels ability to retain its hardness at high temperatures, typically up to 600℃. Due to its tolerance to high temperatures, HSS is typically used for applications such as drill bits, milling operations, saw blades, gear cutters and reamers which makes it an ideal steel for the use in engineering and machining processes.

Hot Work Tool Steel – is specifically designed to withstand high temperatures and repeated heating and cooling cycles. Capable of retaining strength and hardness at elevated temperatures which makes it suitable for hot working processes.

As types of steel ranges in grades, common steel grades such as H13 is the most used hot-work steel, as it provides a combination of toughness and heat resistance. H11, a similar steel grade to H13 is also used, as it provides better toughness but lower resistance to temperature. H21 contains a high tungsten content, providing a superior heat tolerance and wear resistance but less toughness than H13. Hot Work Tool Steel is mostly used in the hot forging process, used to shape metal parts as well as die-casting dies, to produce parts from molten metals like aluminium, magnesium and zinc.

Cold Work Tool Steel –  is designed for applications where the tool operates at or near room temperature. This type of steel is optimised for high wear resistance, toughness and the ability to hold sharp edges, making it ideal for cutting, shaping and forming applications. Cold Work Tool Steel is highly resistant to abrasion, making it ideal for cutting and forming operations as well as its dimensional stability which helps maintain its size and shape during heat treatment and use.

Cold-Work Tool Steel is not suitable for high temperatures, it loses hardness and strength at elevated temperatures. Higher grades can also be more prone to chipping or cracking if not tempered properly. The steel itself is essential in industries that require precise cutting, shaping and forming at room or low temperature, making it useful for applications in rolling mills, for shaping metal sheets and bars and to produce knives and blades.

Shock-Resisting Tool Steel –  are designed to withstand large amounts of impact and shock loads. The steels are used in applications where the tool is subjected to severe impact, or sudden loading.

The shock resisting aspect of this steel is due to the addition of Silicon within its composition, which enhances the toughness and resistance to shock and impact. The shock resistance of these tools makes them useable in a wide range of applications and are essential in industries such as machining, industrial, building and construction as the steel can be machined into tools such as chisels, hammers and jackhammer bits, and riveting tools.
Metal Quenching

Water Hardening Tool Steel – also known as W-Series is a type of steel that can be hardened when quenching it in water. It is known for its high carbon content, with provides increased hardness and wear resistance, however it is more prone to cracking and distortion during the quenching process. The higher carbon content allows the steel to withstand significant abrasions while still maintaining a sharp edge but does have moderate toughness, making it more brittle compared to other tool steels.

Oil Hardening Tool Steel – also known as O-series, is a type of steel that is hardened by quenching it in oil. Compared to water quenching, quenching the steel in oil reduces the risk of cracking and distortion. Oil hardening tools offer a good balance between hardness and wear resistance.

O-Series can be hardened to significant levels, typically around 57-64 HRC after heat treatment, as well as operating well in applications in which abrasions and wear are common.

 

 

 

 

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