Heat treatment is a critical process in metallurgy, used to modify the properties of materials such as metals and alloys to meet specific requirements for various applications. Heat treatment can enhance hardness, toughness, strength, flexibility, and corrosion resistance by subjecting materials to controlled heating and cooling cycles. 

The manipulation of parameters such as temperature, duration, and atmosphere during heating and cooling phases allows engineers and metallurgists to precisely tailor each material's microstructure and properties to meet the demands of particular applications. 

It's a fascinating blend of science and engineering that plays a pivotal role in manufacturing industries, ranging from automotive and aerospace to construction and beyond. Heat treatment operations can involve controlling both heating and cooling processes, often in combination, to achieve the desired material properties.

Quenching is the term used to describe the rapid cooling process employed during heat treatment. The material undergoes significant changes in its metallurgical, physical, and mechanical properties when we rapidly extract heat from it, typically through immersion in a quenching medium such as water, oil, or air.

Quenching is a crucial step in steel heat treatment, where the objective is to rapidly cool the austenitic phase (obtained by heating the steel to a specific temperature) to transform it into martensite. Martensite is a complex microstructure that increases the steel's hardness and strength. 

Below, Figure 1 shows a schematic ‘continuous cooling transformation diagram’ (CCT) where the cooling rate needs to be sufficiently rapid to avoid the transformation of soft steel products (like pearlite and bainite). This rapid cooling rate is typically achieved by immersing the steel into a quenching medium such as water, oil, or air, depending on the desired properties and the steel's composition.

Quenching of aluminum and other non-ferrous metals follows a similar principle to steel, involving rapid cooling from a high temperature to room temperature to maintain a supersaturated solid solution. This process is crucial for achieving desired properties, particularly for age-hardening alloys. This supersaturation is necessary for subsequent age hardening or precipitation hardening to occur effectively.

Quenching of aluminum and non-ferrous metals can be accomplished by immersion in a liquid quenchant, such as water or oil, or by exposure to air at high velocities (air quenching). The choice of quenching method depends on factors such as the alloy composition, part geometry, and desired properties.

Fundamentals of Quenching

During quenching, in practice, the surface of the steel parts cools faster than the center. Differential cooling rates between the surface and the center of steel parts during quenching can lead to non-uniform microstructures, resulting in differences in material properties. This phenomenon is often referred to as thermal gradients.

When the thermal gradient is significant, it can form different microstructures across the cross-section of the part. For instance, if the surface cools rapidly enough to form martensite while the center cools more slowly, bainite or even pearlite may form due to the slower cooling rate. This non-uniform microstructure can lead to variations “as quenched” in material properties such as hardness and strength between the surface and the center of the part.

This effect is illustrated in the schematic CCT diagram in Figure 2, considering the variable of the section thickness. The initial differences in microstructure between the surface and center of the part can lead to variations in the tempering response, and resulting mechanical properties.

Achieving the proper cooling rate during quenching is crucial to completely transform to martensite throughout the entire part, both at the surface and the center. This requires a ‘proper quenching process’ which means carefully controlling the quenching process to maintain uniform cooling rates across the part, allowing for consistent microstructures and mechanical properties.  

Several factors must be considered to achieve a ‘proper quenching process’ and desired cooling rates for total martensite transformation: 

1. Steel composition – steel hardenability 
2. Quenchant selection 
3. Part section thickness
4. Agitation 
5. Bath temperature

By carefully controlling these factors and tailoring the quenching process to the specific requirements of the steel composition and part geometry, manufacturers can ensure uniform transformation to martensite throughout the part.  Proper quenching practices are, therefore, essential for achieving desired material properties and ensuring the quality and reliability of the final product.

Unsure how to optimize your quenching process? Reach out to NUTEC Bickley for comprehensive guidance and support.

1. Steel Composition - Steel Hardenability

Increasing the alloy content in steel can enhance its hardenability, which refers to the ability of the steel to form martensite during quenching. Increasing hardenability reduces the critical cooling rate required for martensite transformation, allowing for faster cooling of the center of the parts and achieving full part transformation with more uniform hardness and mechanical properties.

Cost considerations are associated with using higher alloy content in steel. Alloying elements can significantly increase the cost of steel production due to the higher cost of raw materials and the more complex manufacturing processes involved. 

Therefore, it's essential to conduct a cost-benefit analysis to determine the optimal balance between material performance and production costs. The hardenability effect of adding alloys to steel composition is illustrated in Figure 3.

2. Quenchant Selection

The selection of quenchants plays a crucial role in the quenching process, and their thermal properties significantly impact quenching speed, severity, and cooling rates. The thermal properties—thermal conductivity, density, and viscosity—greatly influence how efficiently heat is transferred from the heated part to the quenching medium and how rapidly the part is cooled (cooling rates).

The combination of these thermal properties and factors such as specific heat capacity and boiling point determine the overall quench severity and cooling rates achieved during the quenching process. Manufacturers carefully consider these properties when selecting quenchants to ensure that the desired cooling rates and metallurgical transformations are achieved while minimizing the risk of distortion, cracking, or other defects in the quenched parts.

Quenchants can vary widely in their cooling rates, and selecting the appropriate quenchant depends on the desired cooling rate and the material being processed. Typical quenchants used are firstly water, mainly to obtain the fastest cooling rates. Less rapid cooling rates are obtained with water-based polymer quenchants and an oil-quenching process.  Air or other gas-quenching processes are used to achieve significantly slower cooling rates.  

Figure 4 illustrates the typical cooling rates achieved with different more commonly used quenchants.

For example, Table 1 lists the typical values of several quenchants and quenching conditions. We can also see in Table 1 the differences between typical quenchants and the obtained heat transfer coefficients according to quenchant speeds.

By using higher-speed quenchants or adjusting the concentration of quenching solutions, manufacturers can increase the cooling rates across the entire part, achieving more uniform properties from the surface to the center. This approach is particularly beneficial for achieving full transformation to martensite or other desired microstructures in steel parts, resulting in consistent mechanical properties throughout the part.

For example, as illustrated in Figure 5, transitioning from a standard quenching oil to a high-speed oil can significantly increase the cooling rates during quenching.

In the case of using water-based polymer quenchants, cooling rates can be increased by reducing the concentration of the polymer solution, leading to faster and more uniform cooling.

Another exciting chart as a guide for quenchant selection is the sensitivity band. The schematic concept is illustrated in Figure 6.

Discover how we delivered cutting-edge heat treating solutions that exceeded expectations. Click here to read our success story and see the innovative technology in action.

3. Part Section Thickness

The section thickness of quenched parts significantly influences the cooling rates achieved at different locations within the part. Thicker sections tend to cool more slowly at the center than thinner sections and the surface, due to the insulation effect of the surrounding material. Therefore, a proper part geometry design must be carefully considered to ensure uniform cooling rates and avoid the formation of undesirable microstructures or mechanical property variations.

Part geometry and design play a critical role in the success of the ‘proper quenching process’, influencing cooling rates, microstructural transformations, and the occurrence of defects such as distortion and cracking. By carefully considering these factors and implementing appropriate design strategies, manufacturers can optimize the quenching process to achieve desired material properties while ensuring dimensional stability and part integrity. 

The complex shapes or intricate features may trap quenchants or inhibit heat transfer, leading to non-uniform cooling rates and potential distortion or cracking.  We can also see in Figure 6 the impact of heavy and thin sections on quenchant selection and agitation severity. 

4. Agitation 

The effect of agitation plays an essential role in the ‘proper quenching process’.  Agitation is critical in achieving uniform quenching and controlling cooling rates during the quenching process. Agitation ensures proper mixing of the quenchant, leading to uniform temperature distribution within the quenching tank and enhancing heat transfer between the quenchant and the quenched parts. This significantly impacts the cooling rates experienced by the parts and the resulting microstructure and mechanical properties.

In Table 1, we can also see the impact of the heat transfer coefficient value on the change in the quenchant velocity. 

Also, Figure 7 illustrates the impact on cooling rates according to the agitation or quenchant speed level.  As the severity of agitation increases, the vapor blanket stage decreases, and the cooling rate increases. The agitation level has little impact at the end of quenching during the convection stage.      

Proper agitation is essential for achieving uniform quenching and controlling cooling rates during heat treatment. Agility helps achieve consistent material properties and dimensional stability in quenched parts by ensuring uniform temperature distribution and enhancing heat transfer efficiency. 

5. Bath Temperature 

The bath temperature is another crucial factor for the ‘proper quenching process’, as it directly affects the heat transfer coefficient (HTC) and the cooling rates experienced by the parts being quenched.  The relationship between quenching bath temperature and heat transfer coefficient is inversely proportional: as the temperature of the quenchant decreases, the heat transfer coefficient increases, and vice versa.

The impact of the variable temperature is shown in Figure 8, where the cooling rate increases with lower bath temperatures and decreases with higher bath temperatures. Water is one of the available quenchants with high severity, especially with cold water. 

By adjusting the quenching bath temperature, manufacturers can tailor the cooling rates to achieve specific material properties while minimizing the risk of distortion, cracking, or other defects.

Aluminum Quenching

The quench rate is less critical for aluminum parts than for steel parts.  While the quench rate is less critical for aluminum than steel, the quench delay time is essential in aluminum quenching processes. The quench delay time is that between removing the aluminum part from the solution furnace and its immersion into the quenching medium.

To ensure optimal quenching results and achieve the desired mechanical properties in aluminum parts, manufacturers must carefully control the quench delay time and tailor the quenching process to the specific requirements of the alloy and product. This may involve optimizing furnace-to-quench times, quenching media selection, and process parameters to minimize quench sensitivity and achieve consistent and reliable results. For some aluminum products, the quench delay time may need to be as short as 7.5 seconds, while for others, it may be acceptable to have a slightly longer delay of up to 30 seconds.

Achieve optimal results in heat treating aluminum. Learn about different furnace configurations and their ideal use cases.

Conclusion

Through a comprehensive understanding and control of factors such as steel composition, quenchant selection, part section thickness, agitation, and bath temperature, manufacturers can optimize the quenching process, leading to uniform transformation to martensite and thus achieving the desired material properties. 

Even for non-ferrous metals like aluminum, the considerations are equally significant, although with a shift in focus towards quench delay time. Proper quenching practices, therefore, remain immensely crucial for ensuring the quality and reliability of the final product. 

This article provided an in-depth analysis of quenching processes, hoping to deliver an enlightening perspective on the importance of heat treatment in metallurgy.

Don't leave your quenching processes to chance. Get professional help from our knowledgeable team at NUTEC Bickley.

References:

• GE Totten, GE Bates, NA Clinton, Handbook of Quenchants and Quenching Ttechnology, ASM International 1993.
• ASM Heat Treater’s Guide 2nd edition. Practices and procedures for irons and steels 1995
• Houghton on Quenching.