Oxidation and Decarburization in Forging Heat Treatment

 
In forging, heating the metal blank is a critical step. During this process, oxidation and decarburization are common and significant phenomena that can affect the quality of forgings and pose challenges for subsequent operations. Understanding the mechanisms, influencing factors, and prevention methods for oxidation and decarburization is crucial to improving the quality of forgings.

Oxidation in Forging Heating

Oxidation occurs when the surface of the forging reacts with oxidizing gases in the furnace (such as O₂, CO₂, and H₂O) during heating, leading to the formation of scale. For example, when steel is heated, iron reacts with these gases to form three types of oxides: ferrous oxide (FeO), magnetite (Fe₃O₄), and ferric oxide (Fe₂O₃), which create distinct oxide layers:

Outer layer: Ferric oxide (Fe₂O₃), accounting for about 10% of the scale thickness.
Middle layer: Magnetite (Fe₃O₄), making up about 50%.
Inner layer: Ferrous oxide (FeO), contributing about 40%.

Oxidation is essentially a diffusion process: iron atoms diffuse outward to the surface while oxygen from the furnace gas diffuses inward. At high temperatures, this diffusion intensifies, resulting in thicker scale. Several factors influence the extent of oxidation:

1. Material Composition

Different steels oxidize at varying rates under the same conditions. For example, low-carbon steel is more prone to oxidation than high-carbon steel, as the latter generates more CO during heating, which reduces the formation of iron oxides. Alloying elements such as chromium (Cr), nickel (Ni), and aluminum (Al) can form a dense oxide film on the surface, limiting further oxidation.

2. Furnace Gas Properties

Furnace gas can be oxidizing, reducing, or neutral, depending on the air supply during fuel combustion. Oxidizing gas (containing O₂ and CO₂) accelerates oxidation, while reducing gas (containing CO and H₂) helps minimize it. Electric resistance furnaces, typically operating in air, create an oxidizing environment.

3. Heating Temperature and Duration

The higher the temperature, the faster the oxidation. For instance, at 200-500°C, only a thin oxide film forms, but at 600-700°C, oxidation becomes more intense, and scale begins to develop. Beyond 850-900°C, oxidation accelerates significantly, leading to thicker scale. Longer heating times, particularly at high temperatures, also promote more extensive oxidation.

Decarburization in Forging Heating

Decarburization refers to the reduction of carbon content on the surface of steel when it reacts with oxidizing gases (such as O₂, CO₂, and H₂O) during heating. This is a two-way diffusion process: oxygen from the furnace gas diffuses inward, while carbon from the steel diffuses outward, forming a decarburized layer with reduced carbon content. Like oxidation, decarburization is influenced by various factors.

1. Chemical Composition of the Steel

The higher the carbon content, the more pronounced the tendency for decarburization. Certain alloying elements, such as silicon and aluminum, can intensify decarburization, while chromium and manganese can effectively inhibit it. Nickel (Ni) and vanadium (V) have a smaller impact on decarburization.

2. Furnace Gas Properties

Water vapor has the greatest impact on decarburization, followed by CO₂ and O₂, while atmospheres rich in CO can suppress it. Heating in a neutral or mildly oxidizing furnace gas can reduce decarburization.

3. Heating Temperature and Duration

Higher temperatures lead to more severe decarburization. However, at temperatures above 1000°C, intense oxidation can overshadow decarburization, though if the scale peels off, decarburization may become more pronounced again.

Effects of Oxidation and Decarburization

Oxidation and decarburization can negatively impact the performance of forgings during heating.

1. Impact of oxidation

Oxide scale can increase wear on the forging dies and reduce the surface finish of the forgings. Residual scale can also accelerate wear on cutting tools during machining, reducing production efficiency.

2. Impact of decarburization

The decarburized layer weakens the strength and wear resistance of the surface. If the thickness of the decarburized layer exceeds the machining allowance, it can compromise the final properties of the forging.

Preventing Oxidation and Decarburization

To minimize oxidation and decarburization during heating, several measures can be taken.

Rapid heating: When possible, use rapid heating to shorten the time spent at high temperatures, reducing the occurrence of oxidation and decarburization.
Control furnace gas: Adjust the air supply to limit excess oxygen and reduce the amount of oxidizing gases in the furnace. Minimizing the moisture content in the fuel can also lower the amount of water vapor in the furnace gas.
Maintain furnace pressure: Keeping a slight positive pressure in the furnace can prevent cold air from entering, reducing the oxidizing atmosphere.
Use oxygen-free or low-oxygen heating techniques: Employ protective atmospheres or vacuum heating techniques to effectively prevent oxidation and decarburization.

By adopting these practices, oxidation and decarburization during the heating process can be significantly reduced, leading to improved surface quality and overall performance of forgings, while enhancing production efficiency.

Conclusion

In summary, oxidation and decarburization during the heating process are key factors that affect the quality of forgings. By understanding their mechanisms and implementing effective protective measures, such as controlling heating speed, adjusting furnace gas composition, and using protective atmospheres, the negative effects of these phenomena can be minimized. This ensures better surface quality and mechanical properties of forgings, contributing to more efficient and competitive production processes in modern forging operations.