Understanding the Differences Between Quenching, Tempering, Nitriding, Cryogenic Treatment, and Oxidation

What Is Heat Treatment?

Heat treatment refers to a process that alters the internal microstructure of a workpiece or changes the chemical composition of its surface to improve its performance and usability. It is a critical technique in material processing.

When Is Heat Treatment Needed?

After receiving customer orders, mold steel is cut to the required size by the sawing workshop. It then undergoes a series of grinding or milling processes in the machining workshop. Depending on the requirements, molds that have been roughly machined may be sent back for heat treatment. The specific heat treatment process is chosen based on the material properties and the desired results.

Today, we will focus on five common heat treatment processes: quenching, tempering, nitriding, cryogenic treatment, and oxidation. Let’s break down each process and its purpose.

I. Quenching

1. What Is Quenching?

Quenching involves heating steel to its austenitizing temperature and then cooling it at an appropriate rate. This process transforms the microstructure within the workpiece’s cross-section into martensite or other unstable structures.

2. Purpose of Quenching:

1. Enhance Mechanical Properties of Metals

   • Increase the hardness and wear resistance of tools, bearings, etc.
   • Improve the elastic limit of springs.
   • Enhance the overall mechanical properties of shaft components.

2. Optimize Material or Chemical Properties of Special Steels

   • Improve the corrosion resistance of stainless steel.
   • Enhance the permanent magnetism of magnetic steels.

II. Tempering

1. What Is Tempering?

Tempering is a heat treatment process where quenched workpieces are reheated to a temperature below the critical point (AC1) and held for a specific duration. The workpieces are then cooled by a controlled method to achieve the desired microstructure and properties.

2. Purpose of Tempering:

1. Reduce Internal Stress and Brittleness
   Quenched workpieces have significant internal stresses and brittleness. Without timely tempering, they may deform or crack.

2. Adjust Mechanical Properties
   After quenching, workpieces are typically hard but brittle. Tempering allows for adjustments in hardness, strength, plasticity, and toughness, depending on the performance requirements.

3. Stabilize Dimensions
   Tempering stabilizes the microstructure, ensuring the workpiece does not deform during future use.

4. Improve Machinability
   For certain alloy steels, tempering can significantly enhance cutting performance.

III. Nitriding

1. What Is Nitriding?

Nitriding is a chemical heat treatment process in which nitrogen atoms diffuse into the surface of a workpiece under specific temperatures and in a controlled medium.

2. Purpose of Nitriding:

Nitriding imparts the following properties to the treated products:

• Excellent wear resistance.
• High fatigue resistance.
• Superior corrosion resistance.
• Enhanced high-temperature performance.

IV. Cryogenic Treatment

1. What Is Cryogenic Treatment?

Cryogenic treatment involves cooling metals to temperatures below -160°C. This process transforms nearly all the soft residual austenite into high-strength martensite.

2. Purpose of Cryogenic Treatment:

1. Eliminate Residual Austenite

   • After quenching and tempering, residual austenite typically accounts for 8%-20% of the microstructure.
   • Residual austenite is unstable and can transform into martensite under external forces or temperature changes. This may cause irregular expansion and reduce dimensional accuracy.
   • Cryogenic treatment reduces residual austenite to less than 2%.

2. Reduce Residual Stress

   • Cryogenic treatment stabilizes the metal matrix, increases strength and toughness, and significantly improves red hardness (high-temperature hardness).

V. Oxidation

1. What Is Oxidation?

Oxidation involves forming a dense, protective oxide film (primarily magnetic iron oxide Fe3O4) on the surface of steel components. This process provides anti-adhesion properties, and sulfide-treated surfaces (FeS film) exhibit both wear resistance and anti-adhesion characteristics.

2. Purpose of Oxidation:

• Enhance corrosion resistance.
• Improve high-temperature oxidation resistance.

Summary

The five heat treatment processes covered—quenching, tempering, nitriding, cryogenic treatment, and oxidation—each serve unique purposes:

• Quenching improves hardness and wear resistance.
• Tempering reduces brittleness, stabilizes dimensions, and adjusts mechanical properties.
• Nitriding enhances wear resistance, fatigue resistance, and corrosion resistance.
• Cryogenic Treatment stabilizes the metal matrix and improves strength and toughness.
• Oxidation adds a protective oxide layer for corrosion resistance and anti-adhesion.
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Choosing the right heat treatment process is critical for optimizing product performance and quality, especially in industrial and manufacturing applications.