What Is Tempering?
Tempering, often called drawing, is a precise heat treatment process where metal components are heated to a specific temperature below their critical point. After reaching this temperature, they are held for a predetermined period before being cooled down gradually, typically in still air or inert atmospheres. This process aims to modify the metal’s mechanical properties to better suit its intended application.
Like annealing and normalizing, tempering alters the internal structure of metals, especially steel, to enhance characteristics such as toughness and ductility. It affects the entire component uniformly, although partial or localized tempering can be achieved through induction heating techniques. Tempered metals are particularly valued in applications requiring a balanced combination of strength and flexibility. Additionally, tempering can be employed to reduce the hardness of components that have been recently welded or heat-affected by other processes, alleviating localized stresses caused by high hardness zones.
While tempering is applicable to various metals, it is predominantly associated with carbon steels, which respond predictably to this treatment. The process is crucial in refining properties such as toughness, ductility, and hardness, making it a versatile tool in metalworking.
Stages of the Tempering Process
- Heating: The metal is uniformly heated to a temperature below its critical point, with careful control to prevent cracking or warping. Typical temperatures vary depending on the steel type—tool steels are tempered at around 200-300°C, spring steels at 300-400°C, and structural steels between 450-650°C.
- Dwelling: The material is held at the target temperature for a specific duration to allow the desired microstructural changes. This dwell time depends on the alloy composition, cross-sectional dimensions, and the targeted mechanical properties. Higher tempering temperatures and longer durations generally increase ductility and impact resistance but can reduce overall hardness.
- Cooling: The final cooling phase occurs in still air or controlled environments, influencing the microstructure and final properties. The cooling rate is critical to achieving the desired balance between hardness and toughness.
Heating Phase Details
During this initial stage, the material is heated in a furnace with controlled atmosphere—be it inert gases, vacuum, or controlled air—to prevent oxidation. The heating rate must be gradual to avoid thermal stresses. The temperature selected depends on the specific steel grade and the properties required; for example, tool steels are tempered at approximately 200-300°C, spring steels at 300-400°C, and structural steels at 450-650°C.
Dwelling (Soaking) Stage Explanation
Once the desired temperature is reached, the component remains in that state for a period tailored to its size and composition. This dwell time ensures thorough microstructural transformation, such as relieving internal stresses, reducing hardness, and improving toughness. Extended dwell times can lead to increased austenite retention or grain growth, so precise control is essential.
Cooling Stage Insights
Cooling can be performed naturally in still air or in controlled environments to influence the final microstructure. Slow cooling helps in achieving a tempered martensitic structure, leading to a combination of strength and ductility. The cooling method impacts residual stresses and the uniformity of properties throughout the component.
What Is Annealing?
Annealing is a comprehensive heat treatment process aimed at restoring or improving a metal’s ductility, reducing hardness, and relieving internal stresses. It involves heating the metal above its recrystallization temperature but below melting point, followed by slow cooling, which allows the internal structure to reorganize into a more stable and less brittle form.
This technique has been recognized since the Middle Ages, with its origins rooted in traditional blacksmithing practices. The term ‘annealing’ is believed to derive from the Middle English word ‘anelen,’ meaning to bake or set in fire. Historically, annealing was instrumental in transforming wrought iron and early steels, enabling more ductile and workable materials.
Annealing is essential for preparing metals for subsequent machining, forming, or welding, as it significantly reduces the risk of cracking and internal stress buildup. It restores the original physical properties of metals after deformation processes like cold working, improving their overall stability and performance.
The Annealing Procedure
1. Recovery Phase
During recovery, the metal is heated below its recrystallization temperature. At this stage, internal stresses and dislocations caused by prior deformation are gradually alleviated, which enhances stability without significant grain growth. This step improves electrical and thermal conductivity and reduces internal strain.
2. Recrystallization Phase
In this critical stage, the material is heated above its recrystallization temperature, prompting the formation of new, strain-free grains. The high temperature allows atoms to migrate, resulting in a microstructure that is softer, more ductile, and stress-free. The specific recrystallization temperature varies by alloy, typically around 50% of the melting point (Kelvin scale).
3. Grain Growth Phase
Once recrystallization has occurred, maintaining the material at high temperatures encourages grain growth. Controlled cooling afterward ensures that the grains remain uniform and stable, leading to improved ductility and toughness. Slow cooling facilitates the reduction of residual stresses and the formation of a refined, stable microstructure suitable for further processing.
Effective annealing requires meticulous control of temperature and time during each phase. For example, steel is commonly annealed at 700-900°C, while aluminum alloys are annealed at 200-400°C, tailored to achieve specific mechanical properties.
Comparing Annealing and Tempering
Although both are heat treatment techniques, annealing and tempering serve distinct purposes and follow different procedures. Annealing involves heating a metal to a temperature above its recrystallization point and then cooling slowly, primarily to enhance ductility and relieve internal stresses. In contrast, tempering involves heating a hardened steel to a lower temperature to decrease brittleness while retaining strength.
Both processes modify microstructures but are suited for different applications. Tempered steel offers high toughness and elasticity, ideal for structural, automotive, and machinery components, whereas annealed steel is softer, more ductile, and easier to work with, making it suitable for manufacturing household items, tools, and parts requiring shaping or machining.
Differences Between Annealed and Tempered Steel
Purpose and Outcomes
The primary goal of annealed steel is to optimize ductility and reduce internal stresses, resulting in a softer, more workable material. It involves restructuring the grain system to be more uniform and less brittle. Conversely, tempering aims to decrease the brittleness of hardened steel by transforming the microstructure to a tougher, more impact-resistant state, balancing hardness with ductility.
Application Examples
Annealed steel is extensively used in manufacturing processes that require bending, machining, or forming. It is ideal for tools, structural parts, and components that need to be reshaped without cracking. Tempered steel, on the other hand, is employed where durability and wear resistance are paramount—such as in gears, shafts, blades, and high-stress machinery parts. It is also prevalent in the production of cutting instruments like knives and swords, where a combination of hardness and toughness is crucial.
Industrial and Practical Uses
In industry, annealed steels are favored for their ease of machining and forming, making them suitable for manufacturing complex shapes and assemblies. Tempered steels are selected for their ability to withstand mechanical stress, impact, and wear, ensuring long-term performance in demanding environments.