Unit 7.6: Hardenability

Hardenability is the ability of a steel to harden in depth when it is subjected to heat treatment (quenching).

In simple terms, it is the capacity of steel to develop hardness not only on the surface but also inside the material.

Explanation

When steel is heated to a high temperature and then rapidly cooled (quenched), its structure changes and hardness increases.

However, the hardness may not be uniform throughout the material.

• Some steels become hard only at the surface

• Some steels become hard deep inside the material

The ability of steel to form hardness through its cross-section is called hardenability.

Important Note

Hardenability is different from hardness.

Thus, a material may have high hardness but low hardenability.

Factors Affecting Hardenability

1. Alloying Elements

Alloying elements greatly increase hardenability.

Important elements include:

• Chromium

• Nickel

• Molybdenum

• Manganese

• Tungsten

These elements slow down the cooling transformation and allow deeper hardening.

2. Carbon Content

Carbon increases the maximum hardness of steel.

However, it has a limited effect on hardenability compared to alloying elements.

3. Grain Size

Larger grain size increases hardenability.

Because larger grains slow down phase transformation during cooling.

4. Cooling Rate

Rapid cooling (quenching) promotes formation of martensite, which increases hardness.

Different quenching media include:

• Water

• Oil

• Air

• Brine

Measurement of Hardenability

Hardenability is commonly measured using the Jominy End Quench Test.

In this test:

1. A heated steel specimen is cooled at one end by water jet.

2. Hardness is measured at different distances from the quenched end.

3. The variation in hardness indicates hardenability.

Importance of Hardenability

Hardenability is important in components that require deep hardness and high strength.

Examples include:

• Gears

• Shafts

• Axles

• Bearings

• Machine components

Higher hardenability ensures uniform mechanical properties throughout the material.

Examples of Steels with High Hardenability

Alloy steels are widely used where deep hardening is required.

Engineering Applications

Materials with high hardenability are used in:

• Automotive parts

• Heavy machinery

• Power transmission systems

• Structural components

These applications require strength and wear resistance throughout the section.

Exam-Focused Points

Important for JE / AE mechanical exams:

• Hardenability = ability of steel to harden in depth during quenching.

• It is different from hardness.

• Alloying elements increase hardenability.

• Grain size affects hardenability.

• Measured by Jominy End Quench Test.

• Alloy steels have high hardenability.

Example Competitive Exam Questions

Question: What is hardenability?
Answer: The ability of steel to harden in depth during heat treatment.

Question: Which test is used to measure hardenability?
Answer: Jominy End Quench Test.

Question: Name one factor affecting hardenability.
Answer: Alloying elements.

Question: What is the difference between hardness and hardenability?
Answer: Hardness is resistance to indentation, while hardenability is the ability of steel to harden through its depth.

Quick Revision Summary

• Hardenability = depth of hardness after quenching.

• Different from hardness.

• Influenced by:

  • Alloying elements

  • Grain size

  • Cooling rate

• Measured by Jominy End Quench Test.

• Alloy steels have higher hardenability than plain carbon steels.