热对金属有什么影响？

In metal processing and manufacturing, I often focus on a critical issue: how heat affects the properties of metals.

Different temperature conditions can trigger significant changes in the properties of metals, and heat has a profound impact on factors such as electrical conductivity, thermal expansion, and magnetism. Therefore, in the following sections, I will guide you through these effects and the role of heat treatment in enhancing metal properties, so you can become more familiar with the concept of heat treatment.

The Effect of Heat on Metals

Heat does not only affect the surface of metals. It penetrates deeply into their internal structure and properties. When metals are heated, temperature changes influence properties such as electrical conductivity, thermal expansion, magnetism, phase changes, grain growth, and yield strength. Heat treatment optimizes the hardness, wear resistance, and fatigue strength of metals by controlling temperature, enhancing their durability.

Surprised, right? Heat treatment has such a wide range of effects on metals. I’ve broken down some key insights about how heat impacts metals, and I hope you find them useful:

Electrical Conductivity

The electrical conductivity of metals refers to their ability to conduct electric current. As the temperature increases, the electrical conductivity of metals typically decreases. The rise in temperature intensifies the movement of electrons within the metal, which increases resistance and reduces the flow of current.

For example, at room temperature, copper has an electrical conductivity of 58 × 10^6 S/m, but at high temperatures (such as 200°C), its conductivity decreases by about 5%. Similarly, aluminum has a conductivity of 37 × 10^6 S/m, and its conductivity also decreases by nearly 5% with increasing temperature.

In precision machining, especially when working with electronic components made of aluminum or copper, the impact of the heating process on electrical conductivity must be considered. The potential reduction in conductivity at high temperatures can lead to performance issues. Therefore, temperature control is critical during the processing of high-precision electrical components to avoid failures caused by changes in conductivity.

Thermal Expansion

Thermal expansion is the phenomenon in which the volume of a metal increases due to the enhanced vibration of atoms or molecules as the temperature rises. The coefficient of thermal expansion varies among different metals, which directly impacts their dimensional changes at high temperatures.

For example, aluminum has a thermal expansion coefficient of 23.1 × 10^-6 /°C, while steel has a coefficient of 11.5 × 10^-6 /°C. This means that for every 1°C increase in temperature, aluminum will expand by 0.0231%, while steel will expand by about 0.0115%.

Thermal expansion can affect dimensional accuracy in precision machining. Specifically, when multiple metal components are assembled, differences in expansion rates between materials like aluminum and steel can lead to component misalignment. Therefore, when designing multi-metal structures or precision parts, it is essential to precisely control the machining temperature to avoid errors caused by thermal expansion.

Magnetism

The magnetism of metals is primarily influenced by temperature changes. When heated, metals, especially ferromagnetic materials, often experience significant changes in their magnetic properties. As the temperature increases, the magnetism of the metal may gradually weaken until it is completely lost. This phenomenon is closely related to the metal’s Curie point, which is the temperature at which the metal loses its magnetic properties entirely.

For example, the Curie point of iron is about 768°C, meaning that above this temperature, iron no longer exhibits magnetic properties. Cobalt has a Curie point of 1121°C, while nickel’s Curie point is 358°C.

For metal components used in high-temperature environments, such as motors, sensors, and electronic devices, changes in magnetism can affect their performance. For instance, changes in the magnetism of steel at high temperatures can influence its application in magnetic devices. Therefore, in CNC machining, it is crucial to precisely control the heating temperature during processing to ensure that the magnetic properties of the parts meet design specifications.

Phase Change

The phase change of metals refers to the transformation of their internal crystal structure at different temperatures. These changes directly affect the mechanical properties of the metal, such as hardness, strength, and toughness. The most common phase changes occur in steel.

For carbon steel, when the temperature reaches around 727°C, the steel transitions from ferrite to austenite. Further heating above 1100°C can transform it into high-temperature austenite. During the hardening process, steel undergoes a transformation from austenite to martensite as it cools, resulting in a significant increase in hardness.

The phase change that occurs when steel is heated to an appropriate temperature directly affects its cutting performance and machinability. In CNC machining, controlling the heating and cooling rates is critical, especially when processing hardened steel parts. Rapid cooling may lead to distortion or cracking.

Grain Growth

Grain growth refers to the phenomenon where, as the metal is heated, atomic diffusion accelerates, leading to an increase in the size of the metal’s internal grains. The size of the grains directly affects the metal’s mechanical properties, especially its strength and hardness. Generally, as grains grow larger, the metal’s strength decreases, but its ductility and toughness increase.

Grain growth can be suppressed through rapid cooling (such as quenching), which increases the hardness and strength of the metal. When steel is heated above 900°C, the grain size increases significantly, leading to a reduction in yield strength and hardness.

Grain control is crucial when machining high-strength steels. By optimizing heat treatment processes, we can prevent excessive grain growth, ensuring that the strength and hardness of the machined parts meet design requirements. Grain control is a key factor in improving material performance, especially for high-strength components used in aerospace and automotive applications.

Yield Strength

Yield strength is the stress point at which a metal begins to undergo plastic deformation under external force. Heat treatment processes significantly influence the yield strength of metals by altering their microstructure, such as grain size, phase changes, and precipitation hardening, to either increase or decrease yield strength.

For steel, after tempering, the yield strength typically increases by 20%-30%. For example, in martensitic steel, yield strength can reach 500-1000 MPa, while after annealing, yield strength decreases to 200-500 MPa, but machinability improves significantly.

Metals must strike a balance between yield strength and machinability. Heat treatments (such as tempering and quenching) can enhance the yield strength of materials, especially for components that must withstand high loads, such as automotive engines and aerospace parts. Heat treatment processes increase the load-bearing capacity and durability of parts.

Hardness and Wear Resistance

Hardness refers to a metal’s ability to resist deformation under external force, usually measured using Brinell hardness (HB), Rockwell hardness (HR), or Vickers hardness (HV). Wear resistance, on the other hand, refers to the ability of a metal to resist wear when in contact with other surfaces. By applying different heat treatment processes, the hardness and wear resistance of metals can be significantly improved, extending their lifespan in high-strength and high-wear environments.

For example, through quenching and tempering, the hardness of steel can increase from HRC 25 to over HRC 55, significantly enhancing its wear resistance. After surface hardening treatments such as carburizing or nitriding, the surface hardness of steel can reach above HRC 60.

For components that require high wear resistance, such as gears, bearings, and cutting tools, selecting the appropriate heat treatment method during processing is essential. Improving hardness and wear resistance helps extend the service life of components, reduces wear, and decreases maintenance and replacement frequency.

Fatigue Strength

Fatigue strength refers to the maximum stress a metal can withstand under repeated loading or alternating stress without undergoing fatigue fracture. Proper heat treatment can significantly improve a metal’s fatigue strength, particularly in applications where the material is frequently subjected to cyclic loads or high-stress environments.

For instance, steel that has undergone tempering treatment can have its fatigue strength increased by 20%-50%, especially in high-strength applications such as aerospace and automotive industries. For components in aircraft engines, improving fatigue strength is crucial for extending their operational lifespan.

In CNC machining, optimizing heat treatment ensures that the manufactured parts possess high fatigue strength, especially for high-precision components used in fields like aerospace and automotive industries. In these sectors, a material’s fatigue strength directly affects the safety and lifespan of the product, making heat treatment a key step in enhancing its performance.

Oxidation Resistance

Certain specialized heat treatments or surface treatments, such as aluminizing, nitriding, or aluminum spraying, can significantly improve a metal’s oxidation resistance, especially when used in high-temperature environments. These heat treatment processes create protective oxide layers on the metal surface or alter its microstructure, thereby enhancing its durability in harsh environments.

For example, after heat treatment, high-temperature alloys such as nickel-based alloys form protective coatings that resist high-temperature oxidation, helping extend the life of components. These alloys are widely used in high-temperature environments, such as aerospace engines and gas turbines.

This enhancement in oxidation resistance helps significantly prolong the service life of metal parts in high-temperature or corrosive environments, reducing frequent maintenance and replacement, and improving product reliability and safety.

机械加工性能

By applying proper heat treatment, not only can the hardness and strength of metals be improved, but the efficiency, precision, and tool life in CNC machining can also be significantly enhanced. After heat treatment optimization, tool life can increase by over 30%, cutting forces can be reduced by 20%-30%, and machining accuracy can be improved by more than 10%.

Comprehensive Data:

• Cutting force reduction: 15%-30%
• Machining efficiency improvement: 10%-15%
• Tool life extension: 25%-30%
• Machining accuracy improvement: 5%-10%

These data clearly show that optimizing heat treatment processes can significantly improve CNC machining efficiency and precision, while effectively extending tool life and reducing machining costs.

Below is a simple table to visually display the various effects of heat on metals:

Factors	Description	Reaction of Different Metals	Data Example
Electrical Conductivity	High temperatures increase electron activity in metals, reducing conductivity.	Conductivity of most metals decreases as temperature increases.	Copper: Conductivity at 20°C is approximately 59 MS/m, and at 100°C is around 57 MS/m.
Thermal Expansion	Metal expands when heated, and the expansion coefficient differs for various metals.	Aluminum has a higher expansion coefficient than steel, expansion differences need to be considered in precision machining.	Aluminum: Expansion coefficient is 22.2 × 10^-6 /°C Steel: Expansion coefficient is 12 × 10^-6 /°C.
Magnetism	The magnetism of metals changes with temperature, and ferromagnetic metals lose magnetism at high temperatures.	Iron is magnetic at low temperatures, but its magnetism weakens as temperature rises.	Iron: Curie point is around 770°C, above which it loses its magnetism.
Phase Change	The microstructure of metals changes with temperature, such as steel hardening.	Steel hardens when heated to a certain temperature.	Steel’s austenitizing temperature is 727°C, cooling to room temperature forms martensite.
Grain Growth	At high temperatures, the grain size of metals increases, leading to reduced strength. Rapid cooling can control grain size.	Metal grains increase in size at high temperatures, resulting in strength reduction.	Steel’s grain size can increase from 5μm to 50μm, hardness drops from 550 HV to 250 HV.
Yield Strength & Heat Treatment	Heat treatment can adjust the yield strength of metals and relieve stress.	Different heat treatment methods adjust the yield strength of metals.	After tempering, the yield strength of steel can drop from 900 MPa to 600 MPa.
Hardness & Wear Resistance Improvement	Heat treatment enhances hardness and wear resistance, extending service life.	Proper heat treatment increases the wear resistance of metals.	Hardened steel can reach 60 HRC, and after tempering, it’s around 50 HRC.
Fatigue Strength	Heat treatment improves the fatigue strength of metals, increasing durability.	Heat treatment can improve the fatigue strength of parts.	After aging treatment, the fatigue strength of aerospace alloys can reach up to 450 MPa.
Density	The density of metals may change during phase transitions, which needs to be considered for special applications.	Changes in density of different metals need particular attention in specialized applications.	Steel: 7.85 g/cm³. Aluminum: 2.70 g/cm³.
Oxidation Resistance	Heat treatment can improve the metal surface’s oxidation resistance, prolonging service life.	Processes like aluminizing and nitriding enhance oxidation resistance of metals.	Nitrided steel can improve oxidation resistance and increase its service life by 30%.
机械加工性能	Heat treatment improves metal cutting performance, reducing hardness for easier machining.	Annealed metals are easier to machine, while quenched metals are more difficult to process.	Annealed aluminum alloy has a hardness of 40 HB, while quenched steel can reach 60 HRC.

 

Twelve Common Heat Treatment Methods

The most common heat treatment methods include quenching, annealing, normalizing, tempering, surface hardening, nitriding, carburizing, and hot isostatic pressing. These methods primarily work by controlling heating, cooling, and temperature to alter the physical and chemical properties of metals.

Let me guide you through the applications and effects of these methods:

1. Quenching

Quenching is the process of heating metal to a high temperature (typically reaching the critical point or austenite region), followed by rapid immersion in a cooling medium (such as water, oil, or air). This rapid cooling process significantly increases the metal’s hardness and strength, but may also lead to brittleness.

Key features of the quenching process:

• Heating Temperature: The heating temperature for quenching is typically in the critical point or austenite region, with a common temperature range of 800-1000°C. The exact temperature depends on the metal composition. If the temperature is too high, it can lead to changes in the metal’s composition, affecting its properties.
• Cooling Medium: The choice of cooling medium affects the quenching result. Water provides a high cooling rate and is suitable for wear-resistant components. Oil offers a more moderate cooling rate, reducing the risk of brittleness, while air cooling is used when a lower cooling rate is needed to minimize internal stress.
• Effects: After quenching, the metal’s hardness and strength are significantly improved, but its brittleness increases. For thicker components, cracks and deformation may occur, so cooling rates need to be carefully controlled.
• Applications: Widely used in the manufacture of tools, cutting tools, bearings, gears, and other wear-resistant components, especially for parts requiring high hardness and strength.

2. Annealing

Annealing involves heating the metal to a specific temperature, holding it for a period of time, and then cooling it slowly. The primary purpose of this process is to eliminate internal stresses and restore plasticity and ductility.

Key features of the annealing process:

• Heating Temperature: The heating temperature is typically within the metal’s recrystallization temperature range. For steel, annealing is usually done at 700-800°C, and for aluminum alloys, the annealing temperature is around 300-400°C. The temperature is adjusted according to specific material requirements.
• Cooling Method: Annealing requires a slow cooling process, usually through furnace cooling, air cooling, or controlled cooling in specific environments, to avoid generating new internal stresses and ensure uniformity in the material.
• Effects: Annealing significantly improves the machinability of metals, reduces work hardening, and makes the metal easier to shape and process. It restores the metal’s ductility and plasticity, aiding subsequent processing.
• Applications: Widely used in the processing of steel and aluminum alloys, especially when parts need good plasticity and ductility for machining. It is commonly used after sheet metal, welding, and cold working processes.

3. Normalizing

Normalizing is similar to annealing, but with higher heating temperatures and natural cooling in air. This process helps to homogenize the metal’s grain structure, improving its microstructure and enhancing mechanical properties.

Key features of the normalizing process:

• Heating Temperature: The heating temperature for normalizing is higher than annealing, typically between 850-950°C, ensuring full recrystallization of the metal’s grains. Normalizing helps to homogenize the metal structure and improve mechanical properties.
• Cooling Method: The metal is naturally cooled in air, leading to a slower cooling rate that ensures grain uniformity and improves the metal’s structure, reducing local hardness variations.
• Effects: Normalizing significantly improves the strength, hardness, and plasticity of steel, especially in large castings, by eliminating casting defects and enhancing the material’s overall properties.
• Applications: Primarily used for steel materials, particularly in steel heat treatments, to improve uniformity and mechanical properties. It is widely used in automotive components and structural steel parts.

4. Tempering

Tempering is a process of re-heating a workpiece that has been quenched, with the aim of relieving the internal stresses created during quenching, reducing brittleness, and increasing toughness.

The specific operations and effects are as follows:

• Heating Temperature: The tempering temperature typically ranges from 150°C to 650°C, adjusted based on the desired balance between hardness and toughness. Higher temperatures reduce hardness and increase toughness, while lower temperatures retain hardness.
• Cooling Method: After tempering, the workpiece is typically cooled in air. The cooling rate is slower to prevent excessive stress concentration, ensuring that the tempered metal has ideal mechanical properties.
• Effects: Tempering effectively adjusts the balance between hardness and toughness, reducing brittleness and enhancing the metal’s impact toughness. It is particularly suitable for the processing requirements of complex workpieces.
• Applications: Commonly used for parts that require high toughness and impact resistance, such as machine tools, cutting tools, springs, etc., especially in high-load, high-impact working environments.

5. Surface Hardening

Surface hardening involves locally heating the metal surface to form a hardened outer shell, while keeping the interior relatively softer. This process is mainly used to improve the wear resistance of the metal surface.

Key process details are as follows:

• Heating Method: Surface hardening is usually achieved through methods like high-frequency induction heating or laser heating. The heating rate is fast, and the cooling is rapid. The hardened layer typically has a depth of 0.5-3mm.
• Hardening Effect: After surface hardening, the metal surface exhibits high hardness, improving its wear resistance, while the interior remains relatively tough to avoid excessive brittleness. This enhances the part’s service life.
• Applications: Widely used for parts such as gears, shafts, and bearings, especially in applications requiring high surface wear resistance, such as high-load machinery and precision components.

6. Nitriding

Nitriding is a heat treatment process where nitrogen is introduced to the surface of the metal to form a nitrided layer. This process significantly improves the metal’s surface hardness, wear resistance, and corrosion resistance.

Key process details are as follows:

• Heating Temperature: Typically carried out at temperatures between 500-550°C. Lower temperatures help form a high-quality nitrided layer, avoiding excessive oxidation and ensuring the appropriate thickness and hardness of the nitrided layer.
• Atmospheric Conditions: The metal is heated in a nitrogen-rich atmosphere (such as ammonia gas), allowing nitrogen to penetrate the surface and form the nitrided layer, thus improving wear and corrosion resistance.
• Effects: The nitrided layer can achieve hardness levels above HV1000, with excellent corrosion and wear resistance, significantly extending the service life of parts. It is especially suited for high-load, high-speed components.
• Applications: Commonly used for surface treatment of automotive engine components, precision tools, and mechanical parts, particularly for components that work in high-load and high-temperature environments, such as bearings and gears.

7. Carburizing

Carburizing is a process where metal is placed in a carbon-rich gas environment at high temperatures, allowing carbon to penetrate the surface of the metal, thereby increasing its surface hardness.

• Heating Temperature: Typically performed at temperatures between 900-950°C, allowing the metal surface to absorb carbon. The depth and hardness of carburization can be controlled by adjusting the time based on the component’s requirements.
• Effects: After carburizing, the metal surface has high hardness, while the interior retains relatively high toughness. This makes carburizing widely used for components requiring a high-strength surface and good internal toughness.
• Applications: Commonly used for parts such as gears, shafts, and gear shafts, especially in applications requiring high surface hardness and good impact resistance, such as automotive transmission systems and mechanical drive components.

8. Hot Isostatic Pressing (HIP)

Hot Isostatic Pressing is a process that involves treating metal materials under high temperature and high pressure, aiming to eliminate internal pores and defects, thereby enhancing the material’s density and uniformity.

• Temperature and Pressure: During the HIP process, the metal material is heated to high temperatures (typically between 900-1200°C), while isostatic pressure (100-200 MPa) is applied. This process effectively eliminates porosity and material defects.
• Effects: The process removes defects such as porosity and cracks, improving the overall density and strength of the metal. Metal treated with this process exhibits significantly improved mechanical properties and higher uniformity.
• Applications: Widely used in aerospace and high-precision component manufacturing, especially suitable for producing high-performance materials such as titanium alloys and superalloys, commonly used in spacecraft and medical equipment.

9. Quenching and Tempering

Quenching and tempering is a comprehensive heat treatment process where the metal is first quenched and then tempered at high temperatures to balance hardness, strength, plasticity, and toughness.

• Process: The metal is first heated to the critical temperature (typically 30-50°C above Ac3 or Ac1) and then rapidly cooled to form martensitic structure. It is then tempered, usually at temperatures between 500-650°C, to optimize the mechanical properties of the workpiece.
• Effects: Through quenching and tempering, the hardness, strength, plasticity, and toughness of the metal are balanced, ensuring good overall mechanical performance. This process is particularly suitable for parts requiring high strength and toughness.
• Applications: Widely used in the processing of various mechanical parts, especially those requiring high strength, toughness, and wear resistance, such as automotive parts, mining equipment, and construction machinery.

10. Aging Treatment

Aging treatment is a process where the metal is heated to release internal stresses, stabilize dimensions, and enhance the material’s physical properties. There are two types of aging: artificial aging and natural aging.

• Process: Artificial aging involves heating the metal at high temperatures and holding it for a specific time, allowing alloying elements in the metal to precipitate and improve its performance. Natural aging, on the other hand, involves placing the metal at room temperature, gradually releasing internal stresses.
• Effects: Aging treatment effectively eliminates internal stresses in the metal and improves its dimensional stability, which is beneficial for manufacturing precision components. It also significantly enhances the hardness and strength of the material.
• Applications: Widely used in aerospace, precision machinery, and other industries. Particularly important for the application of alloy materials like aluminum and titanium alloys in precision machining, aging treatment is a key step in maintaining long-term stability of parts.

11. Chemical Heat Treatment

Chemical heat treatment involves changing the chemical composition of the metal surface to optimize its performance. Common processes include carburizing, nitriding, and carbonitriding.

• Process: The metal is heated in a specific chemical medium, allowing it to absorb certain elements (such as carbon or nitrogen) to form an alloyed layer, thereby improving surface hardness, wear resistance, and corrosion resistance.
• Effects: This process significantly enhances the surface properties of the metal without affecting its core characteristics. It is often used for components that require high surface hardness and corrosion resistance.
• Applications: Carburizing is commonly used for gears, shafts, and tools, while nitriding is often used for gasoline engine components and precision tools.

12. Blued Treatment

Bluing treatment (also known as blackening treatment) is a process where a metal surface is treated with a chemical solution to form a black oxide film. This process not only enhances the metal’s corrosion resistance but also improves its appearance and surface hardness.

• Process: The metal workpiece is immersed in a solution containing chemical agents (such as sodium hydroxide, sodium nitrate, etc.), heated to a specific temperature, allowing the metal surface to form a dense oxide layer. This oxide layer is typically made of ferric oxide (Fe₃O₄) and appears black or deep blue.
• Effects: Bluing treatment effectively reduces oxidation on the metal surface, prevents rust, and extends the lifespan of components, particularly in humid environments. The treated surface has a uniform, attractive sheen, which meets the aesthetic requirements of many components.
• Applications: Widely used in mechanical parts, tools, instruments, automotive components, etc. It is especially suitable for steel parts processed by machining, such as bolts, gears, bearings, and nuts, improving their durability and corrosion resistance. Bluing treatment is also often applied to precision parts that require an aesthetically pleasing surface, such as certain instrument parts and electronic components.

FAQs

What is the impact of heating on metals?

In daily life, heating affects the physical properties of metals, such as hardness, strength, and ductility. Through heat treatment, metals can be made more suitable for different working environments.

Does heating weaken metals?

Heating, if not properly controlled, can weaken a metal’s strength. Excessive heating or improper cooling may lead to brittleness in the metal, but with proper heat treatment, its strength and performance can be enhanced.

How does heat affect metal strength?

As the temperature increases, the strength of metals typically decreases, especially at higher temperatures. However, through proper heat treatment, the strength and hardness of metals can be enhanced to achieve optimal performance.

Will metals melt at high temperatures?

Yes, every metal has a melting point, and when the temperature exceeds this point, the metal transitions from a solid to a liquid state. For example, iron has a melting point of 1538°C, aluminum around 660°C, and copper about 1083°C. Metals will melt above these temperatures.

What happens to steel as the temperature increases?

As steel is heated, it undergoes phase transitions and grain growth. When the temperature rises, steel changes from ferrite to austenite, which enhances ductility and plasticity, but reduces strength and may affect processing performance, especially in high-precision and high-strength applications.

What are the strongest heat-resistant metals?

Tungsten, molybdenum, and tantalum are the strongest heat-resistant metals. Tungsten has a melting point of 3422°C and is commonly used in aerospace and other high-temperature environments. Molybdenum and tantalum have melting points of 2623°C and 3017°C, respectively, and are widely used in high-temperature components and chemical reactors.

Does heating cause metals to shrink?

When metals are heated, they generally expand rather than shrink. The increased atomic vibration leads to an increase in volume. Different metals have different coefficients of expansion aluminum, expands more than steel. When cooled, metals contract, so it’s important to account for thermal expansion and contraction.

Does heating cause metal to deform?

Heating metals can lead to deformation. The heating process loosens the metal’s lattice structure, weakening the atomic bonds and making it more prone to plastic deformation. This property is widely used in hot-working processes such as forging and stamping.

Does metal stretch when heated?

Yes, as metals are heated, the activity of their atoms increases, causing expansion and possible stretching or deformation. Aluminum, in particular, is more prone to stretching at high temperatures compared to steel. It’s crucial to control heat treatment temperatures to avoid affecting the dimensional accuracy of the product.

Why do engineers “heat treat” materials?

Heat treatment allows engineers to precisely control the heating, soaking, and cooling processes. This can effectively improve the metal’s hardness, strength, toughness, and wear resistance. For example, in parts that require high strength and wear resistance, heat treatment can enhance the hardness of the metal, making it more durable. In cases where excellent machinability is needed, annealing can release internal stresses, making the metal easier to process.

Conclusion

The effects of heat on metals are more complex than we might think. Every time I work with metals, I realize that temperature changes not only affect physical properties but also determine how the metal performs in applications. With proper heat treatment, we can precisely adjust the hardness, strength, and wear resistance of metals to meet specific requirements. Understanding these changes allows for greater precision in metal processing and makes existing challenges easier to overcome.