In forging processes, heating of workpieces is an extremely critical stage. This process plays a vital role in determining the final quality, mechanical properties, and service performance of the forged components. However, during heating, workpieces are prone to oxidation and decarburization, which can have far-reaching impacts on the quality and performance of the forged parts. These phenomena, if uncontrolled, can lead to significant material wastage, reduced surface quality, uneven microstructures, decreased hardness and strength, and even increased production costs.
This article will comprehensively explore the principles of oxidation and decarburization during forging heating, analyze their influencing factors, examine their impacts on forged components, and discuss effective preventive measures. The objective is to provide a scientific guide for forging production, ensuring high-quality output and maximizing efficiency while minimizing waste and material loss.
Principle of Oxidation: During the heating of a workpiece billet, the metal reacts chemically with oxidizing gases present in the furnace atmosphere, such as oxygen (O₂), carbon dioxide (CO₂), water vapor (H₂O), and sulfur dioxide (SO₂). This chemical reaction results in the formation of an oxide scale on the surface of the metal. Fundamentally, this is a diffusion-driven phenomenon. Specifically, iron atoms diffuse outward in the form of ions from the internal metal structure toward the surface, while oxygen atoms from the surrounding furnace atmosphere adsorb onto the metal surface and diffuse inward.
As a result, a three-layer oxide scale composed of different types of iron oxides forms on the surface of the forging:
Outer layer: Fe₂O₃, approximately 10% of the total oxide scale thickness
Middle layer: Fe₃O₄, approximately 50% of the total oxide scale thickness
Inner layer: FeO, approximately 40% of the total oxide scale thickness
The formation of this layered oxide scale can significantly affect subsequent forging processes. The thickness, adherence, and uniformity of the oxide layers are influenced by factors such as heating temperature, time, and the composition of both the steel and the furnace atmosphere. Understanding the oxidation mechanism is crucial for controlling surface quality and ensuring the structural integrity of forged components.
Principle of Decarburization: Decarburization refers to the phenomenon where the carbon content in the surface layer of steel decreases during heating. The process occurs when carbon in steel reacts at high temperatures with hydrogen or oxygen to form gases such as methane (CH₄) or carbon monoxide (CO). During decarburization, oxygen atoms diffuse inward into the steel, while carbon atoms diffuse outward toward the surface.
A decarburized layer forms only when the rate of decarburization exceeds the rate of oxidation. In weakly oxidizing atmospheres, the decarburization layer tends to be deeper. The carbon content in the decarburized layer is lower than in the normal bulk material, and the amount of cementite (Fe₃C) is also reduced. Consequently, the decarburized layer has lower hardness, reduced strength, and poorer wear resistance compared to the unaffected steel.
Decarburization is particularly detrimental in steels requiring high surface hardness or precise mechanical properties, such as high-carbon tool steels, bearing steels, spring steels, and high-speed steels. Understanding its mechanism is essential to take effective control measures during forging heating.
Impacts of Oxidation: The most direct consequence of oxidation is the waste of steel due to the formation of oxide scale, resulting in significant raw material loss. Forged parts with surface oxide layers may damage dies and molds during processes such as wire drawing, stamping, or die forging, leading to increased production costs, equipment wear, and reduced tool life.
During machining, oxide scale accelerates the wear of cutting tools, reduces processing efficiency, and shortens tool service life. For precision forging, the presence of surface oxide layers compromises surface quality, dimensional accuracy, and tolerances. Furthermore, oxide scale can interfere with subsequent heat treatment processes, leading to uneven microstructure, inconsistent hardness, and reduced mechanical properties.
In practical production, severe oxidation not only affects the immediate surface but also impacts the overall structural integrity and long-term performance of forged components, making oxidation control a key focus during forging heating.
Impacts of Decarburization: The decarburized layer has lower hardness and strength, which reduces the overall performance of the steel. Decarburization is particularly severe in high-carbon tool steels, bearing steels, high-speed steels, and spring steels, where surface hardness and toughness are critical for service performance.
For example:
2Cr13 stainless steel: Excessive heating or prolonged holding can decarburize the surface, causing premature formation of high-temperature ferrite on the surface. This significantly reduces surface plasticity and increases the risk of cracking during die forging.
Austenitic manganese steel: Surface decarburization results in uneven austenite distribution, which can lower strength during cold deformation and increase the likelihood of cracking due to non-uniform deformation.
Steel in general: Surface decarburization creates differences in microstructure and thermal expansion coefficients between the surface and the core. During quenching, these differences can generate high internal stresses, and because the surface strength is reduced, cracks may appear.
Bearing steel: Decarburized surfaces can form soft spots after quenching, leading to early contact fatigue failure during operation.
High-speed tool steel: Surface decarburization reduces red-hardness, impairing performance at elevated temperatures.
Decarburization also affects the heat treatment response. A decarburized surface may not transform to martensite fully during quenching, resulting in insufficient hardness and compromised wear resistance. Consequently, preventing decarburization is critical to maintaining the functional properties of forged components.
In the forging process, oxidation and decarburization are key factors affecting the quality and performance of forged parts. Understanding the causes of these phenomena and their influencing factors is essential for implementing effective preventive measures. The following are the main factors that affect oxidation and decarburization:
Heating temperature is the primary factor influencing the rate of oxidation. Higher temperatures increase the diffusion rates of both metal atoms and gaseous atoms, leading to faster oxidation and thicker oxide scales. Observations indicate:
At 200–500°C, steel surfaces only form a thin oxide film.
At 600–700°C, significant oxidation occurs, forming visible oxide scale.
Above 850–900°C, the oxidation rate rises sharply, creating thick oxide layers.
Heating time also has a major influence: longer exposure at high temperatures increases oxygen diffusion and oxide scale thickness. High-temperature exposure, especially above 900°C, accentuates these effects.
For decarburization, higher temperatures and longer holding times generally exacerbate surface carbon loss. When the temperature exceeds 1000°C, strong oxidation can initially suppress decarburization. However, at even higher temperatures, the loss of protective oxide scales accelerates decarburization significantly.
The composition of the furnace atmosphere plays a critical role. Strongly oxidizing atmospheres composed mainly of O₂, CO₂, H₂O, and SO₂ promote thick oxide scale formation. In contrast, reducing atmospheres containing gases such as CO and H₂ can prevent or greatly reduce oxidation.
Regarding decarburization: water vapor is the strongest decarburizing agent, followed by CO₂ and O₂, while hydrogen (H₂) has the least effect. Increasing CO concentration reduces decarburization tendency. Heating in neutral or weakly oxidizing atmospheres is effective for limiting carbon loss from the surface.
Steel composition significantly affects oxidation and decarburization. For oxidation: carbon content above 0.3% slows oxidation. Alloying elements such as chromium (Cr), nickel (Ni), silicon (Si), and molybdenum (Mo) form dense protective films on the surface, preventing oxygen diffusion and reducing oxidation rate. Steels containing Cr and Ni above 13–20% exhibit minimal oxidation.
For decarburization: higher carbon steels are more prone to carbon loss. Certain alloying elements such as carbon (C), tungsten (W), silicon (Si), and aluminum (Al) increase decarburization tendency, whereas chromium (Cr) and manganese (Mn) inhibit it. Nickel (Ni) and vanadium (V) have minimal influence on decarburization.
In forging production, the negative effects of oxidation and decarburization on the quality of forged parts are evident. To ensure the performance and precision of the forged components, it is crucial to reduce oxidation and decarburization. Fortunately, by implementing a series of scientific and reasonable measures, these phenomena can be effectively controlled, thereby improving production efficiency and product quality.
Rapid heating, minimizing exposure time at high temperatures, is essential for reducing oxidation and decarburization. Complete fuel combustion and minimal excess air reduce residual oxygen, while maintaining slight positive furnace pressure prevents cold air ingress. Reducing water content in fuel also helps control oxidation.
Protective media such as gases, liquids, or solids can significantly reduce oxidation and decarburization.
Gaseous media: Controlled atmospheres using inert gases (N₂, Ar) or reducing gases isolate the workpiece from oxidizing gases.
Liquid media: Glass baths or salt baths form protective films on the surface, preventing oxidation.
Solid media: Graphite powder or applied glass lubricants can shield the surface effectively.
Advanced heating technologies, such as suspension or bright heating, are also effective in limiting oxidation and decarburization.
Maintaining a neutral or protective furnace atmosphere reduces oxidation and decarburization. Well-deoxidized salt bath heating produces a lower decarburization tendency compared to ordinary box furnace heating. Careful control of furnace gases can significantly slow both oxidation and carbon loss.
During hot forging, if production pauses, the furnace temperature should be lowered. For long interruptions, billets should be removed or cooled within the furnace. Minimizing intermediate annealing frequency and temperature reduces high-temperature exposure, lowering oxidation and decarburization risk.
High-temperature protective coatings or coverings prevent oxidation and decarburization. These protective films isolate the metal surface from oxidizing gases, effectively reducing the formation of oxide scales and carbon loss.
Oxidation and decarburization are common phenomena during forging heating, with serious effects on quality, mechanical properties, and service life of forged parts. A thorough understanding of their principles, influencing factors, and preventive measures allows effective control of these phenomena, ensuring high-quality forged components, reducing raw material wastage, and improving production efficiency and economic benefits. In practice, multiple preventive measures should be applied based on production conditions and workpiece requirements, optimizing the heating process to guarantee high-quality forging output.
