Support Hardening

Hardening

What is hardening?

Hardening is a heat treatment process aimed at increasing the hardness and strength of steel. This is achieved by heating the steel to a specific temperature (the austenitizing temperature) and then rapidly cooling it. This process changes the microstructure of the steel, increasing the material's hardness. Hardening is an essential stage in the manufacturing of many machined and industrial components, as it improves the parts’ wear resistance and service life.

Hardening Methods

  • Oil Quenching: The part is heated to the required temperature and rapidly cooled in oil. Oil quenching reduces the risk of cracking and distortion compared to, for example, water quenching.
  • Water Quenching: The part is heated and quickly cooled in water, producing an extremely hard surface but increasing the risk of cracking and distortions.
  • Air Hardening: Parts are heated and allowed to cool in air. This provides more even cooling and reduces cracking risks for certain steels.
  • Salt Bath Hardening: Salt baths are used to achieve uniform cooling, reducing warping and cracking. This method is especially suitable for hardening complex shapes and large parts.
  • Induction Hardening: Induction hardening is a surface hardening process where the part is rapidly heated by an alternating magnetic field and then quickly cooled. This method is particularly used for components exposed to heavy loads.
  • Case Hardening: Case hardening achieves high surface hardness while maintaining good ductility at the core. Case hardening techniques, such as carburizing, nitriding, and induction hardening, also induce compressive stresses at the surface, which improve fatigue strength.

Common Materials for Hardening

Tempering Steels

Tempering steels are alloy steels with a typical carbon content of 0.25–0.55%. These steels are used in parts that require high strength and toughness:

  • 42CrMo4: A highly durable alloy steel well-suited for structures and components subjected to heavy loads, such as machine shafts and gears.
  • 34CrNiMo6: Used for parts exposed to significant mechanical stress, such as drive shafts and power transmission components.

Carbon Steels

Carbon steels generally have a carbon content above 0.6% and are well suited for hardening and tempering:

  • C45: One of the most commonly used carbon steels for hardening, widely used in machine parts and shafts.
  • C60: Carbon steel offering better wear resistance due to its higher carbon content. It is especially used in applications requiring high fatigue strength.

Mold Steels

Mold steels are alloy steels specifically designed for making molds and tooling:

  • 1.2311 (P20): A widely used mold steel offering good machinability and hardenability, especially suitable for manufacturing plastic molds.
  • 1.2343 (H11): A high-temperature mold steel that resists extreme temperatures and thermal shocks, suitable for hot-working and die-casting molds.

Hardenable Stainless Steels

Stainless steels, such as martensitic stainless steels, can be hardened to achieve high hardness and good corrosion resistance:

  • X46Cr13 (1.4034): Martensitic stainless steel that, when hardened, offers good wear and corrosion resistance. Commonly used in cutting tools and surgical instruments.
  • X30Cr13 (1.4028): Offers good wear and corrosion resistance, typically used in knives and other cutting tools.

Hardenable Cast Irons

Certain alloy cast irons can be hardened to improve their mechanical properties:

  • GJS-500-7 (Ductile Iron): Ductile iron featuring embedded nodules to enhance strength and toughness. Ideal for power transmission parts and components subjected to heavy loads.
  • GJL-250 (Gray Cast Iron): Gray iron that can be hardened to improve wear resistance. Commonly used in engine parts and other wear-exposed components.

Machining of Hardened Steel

Machining hardened steel is challenging due to its high hardness. Tools used in machining include:

CBN grades (cubic boron nitride): CBN tools are specifically designed for machining hard materials, such as hardened steel. They offer excellent wear resistance and heat tolerance.

Key Considerations for Machinists Before Hardening

Before hardening, machinists must consider several important factors to ensure the final product meets required accuracy and quality standards:

  • Dimensional Changes: During hardening, the part's dimensions may change due to thermal expansion and contraction. These changes should be carefully anticipated, especially for critical tolerances.
  • Machining Allowances: Adequate machining allowances must be left during pre-machining for subsequent post-hardening machining stages. This may include leaving extra material for grinding to remove any irregularities caused by hardening.
  • Distortions: Hardening of long and thin parts can cause warping and deformation. These can be controlled by using suitable jigs and supports during the hardening process.

Tempering

After hardening, tempering is usually performed, where the hardened structure is re-heated at lower temperatures (150 ºC – 650 ºC). The purpose of this process is to remove the brittleness and stresses caused by quenching and to increase the toughness of the material. Tempering allows for achieving the desired hardness/toughness ratio, making the component more durable and long-lasting.

Summary

Hardening is a key heat treatment process in metalworking that improves the mechanical properties of steel, such as hardness and wear resistance. There are several hardening methods, including oil, water, air, and salt bath hardening, as well as specialized methods like induction and case hardening. Common hardenable materials are tempering steels, carbon steels, mold steels, hardenable stainless steels, and hardenable cast irons. Machinists must take dimensional changes and machining allowances into account before hardening to ensure the precision and quality of the final product. Stress relief through tempering after quenching improves the material's toughness and durability.