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Chapter 11: Problem 19

In your own words describe the following heat treatment procedures for steels and, for each, the intended final microstructure: full annealing, normalizing, quenching, and tempering.

Short Answer

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Question: Describe the four heat treatment procedures for steels and their intended final microstructures. Answer: The four heat treatment procedures for steels are full annealing, normalizing, quenching, and tempering. Full annealing results in a soft, ductile ferrite-pearlite microstructure by heating steel above its upper critical transformation temperature and slowly cooling it. Normalizing refines the grain structure, resulting in a fine-grained pearlite microstructure with interspersed ferrite grains, by heating steel slightly above its upper critical transformation point and air cooling. Quenching produces a hard and brittle martensitic microstructure by rapidly cooling steel after heating, while tempering relieves stresses induced by quenching and improves toughness and ductility, resulting in a tempered martensite microstructure.

Step by step solution

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1. Full Annealing

Full annealing is a heat treatment process in which steel is heated to a temperature above its upper critical transformation temperature and then slowly cooled, typically in a furnace. This allows the steel to fully transform into austenite and then, during slow cooling, into a soft, ductile ferrite-pearlite microstructure. The purpose of full annealing is to eliminate internal stresses, improve the steel's ductility and toughness, and refine the grain structure for improved machinability.
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2. Normalizing

Normalizing is a heat treatment process where steel is heated to a temperature slightly above its upper critical transformation point and then cooled in open air. This rapid cooling results in a fine-grained pearlite microstructure with ferrite grains interspersed. The purpose of normalizing is to refine the grain structure, improve ductility and toughness, and relieve internal stresses. Compared to full annealing, normalizing usually achieves a stronger, harder, but less ductile material.
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3. Quenching

Quenching is the process of rapidly cooling steel after heating it to a temperature above its upper critical transformation point. This is usually done by immersing the hot steel in a quenching medium such as water, oil, or air. The rapid cooling causes the austenite to transform into martensite, a very hard and brittle microstructure. The primary purpose of quenching is to obtain high levels of hardness and wear resistance in the steel.
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4. Tempering

Tempering, often performed after quenching, is a heat treatment process that involves reheating the steel to a specific temperature below its lower critical transformation point and then allowing it to cool slowly. This process leads to precipitation of tiny carbide particles in the martensitic matrix, resulting in a tempered martensite microstructure. The purpose of tempering is to relieve stresses induced by quenching and to improve the steel's toughness and ductility, making it less prone to cracking or breaking while maintaining an elevated level of hardness and strength.

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Most popular questions from this chapter

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