Hardened Steel: Hardening Methods, Properties, and Uses

Hardened steel

Hardened steel is carbon steel or alloy steel that has been subjected to heat treatment in order to improve the hardness and strength of the metal. Hardened steel has a higher brittleness and less ductility hence they are higher wear and tear resistance.

Steel can be hardened with these 4 main steps such as austenitizing, quenching, martensitic transformation and tempering. Carbon steel, alloy steel, tool steel, stainless steel, and high-speed steel can be used to harden steel.

Mianly steel hardening has 5 methods such as quenching and tempering, case hardening, precipitation hardening, cold working, and austempering. Among these 5 methods quenching and tempering, and case hardening are the most used method.

The advantages of hardened steel is based on its strength, hardness, wear, and tear resistance. Hardened steel has unique physical and chemical properties that are useful for applications. The hardness of the hardened steel can be measured using the Rockwell C, Brinell, and Vickers hardness tests.

Hardened steel has a higher wear, tear, and abrasion resistance. Hence this is more durable than nonhardened metals. Therefore hardened steel is used in various industries for different applications. When the hardness is increased, its brittleness also increases. Therefore hardened steel has few disadvantages, such as high brittleness, less workability, less corrosion resistance, less structure stability, and lower thermal conductivity.

What is Hardened Steel?

Hardened steel is any carbon steel or alloy steel that has been subjected to a heat treatment process to improve its hardness and strength. This heat treatment typically involves heating the steel to a specific temperature (austenitizing temperature), holding it there for a certain period of time, and then cooling it rapidly (quenching)  by using water, oil, or another medium. 

The rapid cooling transforms the steel’s microstructure to hard phase. After hardening, steel should be tempered. The tempering process will reduce the brittleness and improve the toughness of the hardened steel.

What are the Major Steps of Steel Hardening?

The major steps of steel hardening are austenitizing, quenching, martensitic transformation and tempering.

1 – Austenitizing: The steel is heated to a temperature where its crystal structure changes to austenite.

  • Carbon Steel: Typically heated to 800-900°C (1472-1652°F). 
  • Alloy Steel: Require different temperatures ranging from 800-1300°C (1472-2372°F), depending on the specific alloying elements.

2 – Quenching: After reaching the austenitizing temperature, the steel is rapidly cooled or “quenched” to trap the carbon within the structure and prevent it from precipitating out. The method of cooling can vary depending on the desired properties and type of steel. Common quenching mediums include:

  • Water: Offers rapid cooling but can be aggressive, leading to potential warping or cracking.
  • Oil: Provides slower cooling than water and can minimize distortions.
  • Air: Typically used for alloy steels designed to harden in air.

3 –  Martensitic Transformation: When steel is rapidly cooled, the austenite transforms into martensite.  Martensite is a hard and brittle phase. Therefore steel should be tempered.

4 – Tempering: Newly formed hardened steel is typically reheated to a lower temperature (tempered) to convert some of the martensites into more ductile phases. This process improves the toughness of the steel while slightly reducing its hardness.

What Steels Can Be Hardened?

steel for hardening

These 5 types of steel can be hardened such as carbon steel, alloys steel, tool steel, stainless steel, and high-speed steel.

  1. Carbon Steels: These are primarily alloyed with carbon. The carbon content determines the hardness potential.
    • Low Carbon Steel (Mild Steel): Contains up to 0.3% carbon.
    • Medium Carbon Steel: Contains 0.3–0.6% carbon.
    • High Carbon Steel: Contains 0.6–1.4% carbon.
  2. Alloy Steels: These are steels that contain other alloying elements in addition to carbon to achieve certain desirable properties.
    • 4140 steel contains around 0.4% carbon, 1% chromium, 1% manganese, 0.2% silicon, and 0.8% molybdenum.
    • 5160 steel (often used for making springs) typically contains 0.56–0.64% carbon, 0.7–0.9% manganese, and 0.7–0.9% chromium.
  3. Tool Steels: Tool steels are specialized steels designed to be made into tools, and they often contain elements like tungsten, molybdenum, vanadium, and chromium. Examples include:
    • D2 Tool Steel: Contains about 1.5% carbon, 12% chromium, 0.8% vanadium, 0.6% molybdenum, and 0.5% manganese.
    • O1 Tool Steel: Has roughly 0.9% carbon, 0.5% chromium, 0.5% tungsten, 1.1% vanadium, and 1.1% manganese.
  4. Stainless Steels: These are corrosion-resistant steels that contain a significant amount of chromium. When hardened, they can be used in tools, cutlery, etc.
    • Type 304 Stainless: Typically contains 18% chromium and 8% nickel (often referred to as 18/8).
    • Type 440C Stainless: High carbon stainless steel with around 1.1% carbon and 16–18% chromium.
  5. High-Speed Steels: Used for cutting tools, these steels can retain hardness at high temperatures.
    • M2 High-Speed Steel: Typically contains 0.85% carbon, 4% chromium, 5% molybdenum, 2% vanadium, and 6% tungsten.

What are the 5 Methods of Steel Hardening?

The most used 5 methods for steel hardening are quenching and tempering, case hardening, age hardening, cold working, and austempering. These 5 methods are based on the specific application, desired properties, and type of steel.

  1. Quenching and Tempering
    • Quenching: Steel is heated to a certain temperature (austenitizing temperature) and then rapidly cooled in a quenching medium such as water, oil, or air. This process forms martensite, a hard structure in steel.
    • Tempering: After the quenching, the steel is brittle. Therefore it should be tempered. Tempering involves reheating the quenched steel to a lower temperature and then cooling it. This process increases the toughness of the steel.
  2. Surface Hardening or Case Hardening
    This process hardens only the surface of the steel, leaving the inner core soft and tough. Below is the process of the case hardening.
    • Carburizing: Steel is heated in the presence of a carbon-bearing material (like charcoal or carbon monoxide). Carbon is absorbed into the surface, and then the steel is quenched to harden it.
    • Nitriding: Introducing nitrogen to the steel surface using ammonia gas or other nitrogen-bearing mediums. It is done at lower temperatures than carburizing and results in a hard, wear-resistant surface.
    • Cyaniding: Steel is heated in a cyanide bath, where it absorbs carbon and nitrogen. The steel is then quenched.
    • Carbonitriding: A combination of carburizing and nitriding processes.
    • Flame Hardening: Steel is heated rapidly by direct exposure to a high-temperature flame, then quickly quenched.
    • Induction Hardening: Electromagnetic induction is used to heat the steel’s surface rapidly. It is then quenched, usually in water.
  3. Precipitation Hardening (Age Hardening):
    • Used mainly for certain stainless steels and non-ferrous alloys. The steel is heated, allowing some elements to dissolve, and then cooled quickly. A secondary, low-temperature heat treatment causes these elements to precipitate, increasing hardness and strength.
  4. Cold Working:
    • Steel is deformed (e.g., by rolling, drawing, or bending) at room temperature. This deformation causes an increase in hardness and strength due to the movement of dislocations within the steel’s crystal structure.
  5. Austempering:
    • Steel is quenched from the austenitizing temperature in a bath held at a temperature where bainite (a combination of ferrite and cementite) can form. This results in a balance of toughness and strength.

What are the 8 Advantages of Hardened Steel?

Hardened steel advantages

The advantages of hardened steel is based on the characteristics such as high wear resistance, fatigue resistance, corrosion resistance, higher strength, higher hardness, less deformation, and higher load capacity. Each of these advantages has specific benefits.

  1. Improved Wear Resistance: Hardened steel can increase its wear resistance. Hence it can last longer under abrasive and erosive conditions than softer steel.
  2. Increased Strength: Hardeed steel can significantly increase the tensile and yield strengths of steel, making it more resistant to deformation under load.
  3. Improved Fatigue Resistance: Hardened steel often has a greater resistance to repeated bending, loading, and other stress-related activities. Therefore hardened steel longer service life.
  4. Higher Hardness and Stiffness: Hardened steel is suitable for applications like cutting tools (drills, cutters, knives) where a sharp edge or high stiffness is essential.
  5. Resistance to Plastic Deformation: Hardened steel resists permanent shape changes better than softer steel. This is especially useful in precision applications where maintaining dimensional accuracy is essential.
  6. Improved Surface Finish: Hardened process provide a smooth and polished surface, which can be beneficial for moving parts and aesthetics.
  7. Corrosion Resistance: Some hardening methods, especially those that result in the formation of certain oxide layers or passive films, can enhance a steel’s resistance to corrosion.
  8. Increased Bearing Load Capacity: Hardened steel can handle a higher loads. This can reduce the size and weight of machinery components.

What are the Properties of Hardened Steel?

Hardened steel is steel that has undergone a specific heat treatment process to improve its mechanical properties. The primary goal of hardening is to increase the hardness and strength of the steel. As a result, hardened steel has the following properties.

  1. Hardness: Hardened steel have  64-68 HRC hardness. The specific level of hardness depends on the type of steel and the hardening process used. 
  2. Tensile Strength: Hardened steel have a higher tensile strength of 1300 – 1650 MPa.
  3. Yield Strength: Hardened steel has a higher yield strength of 950MPa compared to its annealed or normalized state.
  4. Wear Resistance: Wear resistance have improved than the non-hardened steel type.
  5. Reduced Ductility: Hardened steel is more brittle than non-hardened regular steel
  6. Fatigue Resistance: Higher fatigue resistance than non-hardened steel types.
  7. Potential for Residual Stresses: The shape can be distorted during the quenching.
  8. Magnetic Properties: Hardened steel have its ferromagnetic properties.
  9. Corrosion Resistance: The hardened steel has a higher resistance to corrosion.

What are the Testing for Hardened Steel?

hardness values hardened steel

The hardness of hardened steel can be tested using these 3 methods such as Rockwell hardness test, Brinell hardness test, and Vickers hardness test.

Rockwell hardness test measures the permanent indentation depth produced by a force/load on an indenter. The test starts with a minor load followed by a major load. The depth of indentation determines the hardness value.

  1. Rockwell Hardness Test:
    Rockwell is commonly used for steel, brass, and aluminum parts.
    • Principle: Measures the permanent indentation depth produced by a force/load on an indenter.
    • Procedure: Starts with a minor load followed by a major load. The depth of indentation determines the hardness value.
    • Indenters: Diamond cone for harder materials (HRC) and steel ball for softer materials (HRB).
    • Rockwell Hardness Test values for hardened steel – 64 HRC – 68 HRC
  2. Brinell Hardness Test:
    • Principle: Measures the diameter of the indentation left by an indenter under load.
    • Procedure: A known force is applied to a hardened steel or carbide ball of known diameter pressed into the material’s surface.
    • Indenters: Hardened steel ball or carbide ball.
    • Brinell Hardness Test values for hardened steel – 600 HB – 900 HB
  3. Vickers Hardness Test:
    • Principle: Measures the diagonal length of the square indentation left by a diamond pyramid indenter under load.
    • Procedure: A diamond pyramid with a square base and an angle of 136° between opposite faces is pressed into the material under a specific load. The length of the diagonals of the impression is measured.
    • Indenters: Square-based diamond pyramid.
    • Vickers Hardness Test values for hardened steel – 900 HV

What are the Industries and Applications of Hardened Steel?

Hardened steel has higher durability and resistance to wear and abrasion. Therefore hardened steel is used in numerous industries. Below are the 12 most industries that hardened steel use mostly.

  1. Automotive Industry:
    • Gear components
    • Camshafts and crankshafts
    • Bearings
    • Transmission systems
    • Suspension and steering components
    • Brake components
  2. Aerospace Industry:
    • Landing gear components
    • Bearings and shafts
    • Fasteners and connectors
    • Gear systems
  3. Tool Manufacturing:
    • Cutting tools like drills, end mills, and taps
    • Dies and molds for forging and casting
    • Machine tool bits
    • Knives and blades
  4. Mining and Construction:
    • Drilling bits
    • Excavator teeth
    • Crusher jaws
    • Chain links and sprockets for heavy machinery
  5. Defense and Firearms:
    • Gun barrels
    • Bullet casings
    • Components of armored vehicles
    • Bayonets
  6. Agriculture:
    • Blades for tractors and harvesters
    • Plow components
    • Chains and sprockets for various machinery
  7. Railroad Industry:
    • Rails and clips
    • Bolts and screws
    • Axles
  8. Energy:
    • Components of wind turbines
    • Drill bits for oil and gas extraction
    • Components for hydroelectric turbines
  9. Medical Industry:
    • Orthopedic implants like screws, plates, and rods
    • Surgical instruments
  10. Bearings Industry:
    • Roller bearings
    • Ball bearings
    • Thrust bearings
  11. Mechanical Equipment:
    • Springs
    • Bolts, screws, and other fasteners
  12. Homeware and Appliances:
    • Blades for blenders and food processors
    • Knife blades
    • Scissors

What are the Disadvantages of Hardened Steel?

The disadvantages of hardened steel is based on higher brittleness, less workability, different levels of corrosion resistance, higher cost, less dimensional stability, and limited impact resistance. These disadvantages cause to limit the application range, and find alternatives to the hardened steel.

  1. Higher Brittleness: One of the most significant drawbacks of hardened steel is its brittleness.
  2. Difficulty in Cutting or Machining: Hardened steel is much more challenging to machine, cut, or drill than softer steel.
  3. Susceptibility to Embrittlement: Processes like case hardening can sometimes lead to phenomena such as hydrogen embrittlement or temper embrittlement, where the material becomes even more brittle and may fail unexpectedly.
  4. Difficult to Weld: Welding hardened steel requires specific skills and carefully selected filler materials to avoid weakening the material or introducing stresses that could lead to failure.
  5. Different Types of Corrosion Resistance: Depending on the specific composition and hardening method, some hardened steels may not have as good corrosion resistance as other materials or treatments.
  6. Higher Cost: High-quality hardened steel products are generally more expensive than their softer counterparts.
  7. Less Dimensional Stability: The hardening process introduces significant amounts of energy and changes within the material. If not properly controlled, this can lead to warping or other dimensional instabilities.
  8. Limited Impact Resistance: Depending on the hardening process and the steel’s composition, some hardened steels may have reduced resistance to repeated impact, leading to surface cracking or spalling over time.
  9. Environmental Considerations: The chemicals and energy used in some hardening processes can have environmental impacts that need to be managed carefully.
  10. Lower Thermal Conductivity: Hardened steel usually has a lower thermal conductivity compared to non-hardened steel, which can be a disadvantage in applications where efficient heat transfer is required.
  11. Wear on Opposing Surfaces: While the wear resistance of hardened steel itself is a benefit, it may lead to increased wear on opposing or mating surfaces, particularly if they are made of softer materials.