Defects Caused by Improper Heating of American Standard 304 Stainless Steel Pipes
Defects resulting from incorrect heating processes can be categorized into several types. First, there are surface-related issues caused by the interaction between the billet and the furnace atmosphere, such as oxidation, decarburization, carburization, sulfidation, and copper penetration. Second, internal structural problems like overheating, overburning, and incomplete heating may occur due to abnormal changes in the metal’s microstructure. Third, uneven temperature distribution during heating can lead to excessive internal stresses—such as thermal or structural stress—which may result in cracking of the billet. Here are some of the most common issues that arise during improper heating.
Decarburization
Decarburization occurs when the carbon content on the surface of the metal is reduced due to oxidation at high temperatures. This leads to a significant decrease in carbon concentration in the outer layer compared to the interior. The depth of the decarburized layer depends on factors like the steel composition, furnace gas, temperature, and holding time. Decarburization is more likely to occur in an oxidizing atmosphere, especially with high-carbon steels or those containing a lot of silicon. This defect weakens the mechanical properties of the part, including strength, fatigue resistance, and wear resistance.
2. Carburization
Forgings heated in oil furnaces often experience carburization on the surface or parts of it. In some cases, the carburized layer can be as thick as 1.5–1.6 mm, with carbon content reaching up to 1% (by mass), and even higher in certain areas. This can lead to the formation of ledeburite structures. The cause is typically poor mixing of fuel and air in the furnace, leading to incomplete combustion and the creation of a reducing, carburizing atmosphere near the billet. This results in an increase in surface carbon, which can negatively affect machining performance, making it harder for cutting tools to operate smoothly.
Overheating
Overheating refers to situations where the billet is heated beyond the recommended temperature range or remains in this range for too long, causing excessive grain growth. In carbon steels (both hypoeutectoid and hypereutectoid), this can lead to the formation of Widmanstätten structures. Martensitic steels may develop intragranular structures, while tool steels often show primary carbide network formation. In titanium alloys, overheating can produce distinct β-phase grain boundaries and elongated Widmanstätten structures. For alloy steels, overheating may result in a brittle, "stone-like" fracture. While some forms of overheating can be corrected through heat treatment, severe cases are often irreversible and significantly reduce mechanical properties, particularly impact toughness.
4. Overburning
Overburning occurs when the billet is heated excessively or held at high temperatures for too long. Oxygen and other oxidizing gases penetrate the grain boundaries, reacting with elements like iron, sulfur, and carbon. This causes the formation of low-melting-point oxides, weakening the intergranular bonds and drastically reducing the material’s ductility. If overburned, the metal may crack during forging or exhibit transverse cracks in the affected area. There is no strict temperature threshold between overheating and overburning, but overburning is generally identified by grain boundary melting or the formation of fishbone-like structures in high-speed steels. In aluminum alloys, overburning may create melting triangles and remelted globules. Once overburned, the material is usually unsalvageable and must be scrapped.
5. Heating Cracks
When large cross-sectional steel ingots or high-alloy materials with poor thermal conductivity are heated too quickly in the low-temperature stage, large temperature differences between the inside and outside of the billet can generate significant thermal stress. Combined with the low plasticity at these temperatures, if the stress exceeds the material’s strength limit, cracks can form from the center outward, potentially fracturing the entire section.
6. Copper Cracking
Copper cracking is another issue that can occur during heating, especially in steels that contain copper. When heated, copper tends to migrate to grain boundaries, weakening them and making the material more susceptible to cracking under stress. This is commonly seen in certain stainless steels and can lead to premature failure if not properly controlled. Proper furnace atmosphere management and temperature control are essential to prevent such defects.
Defects resulting from incorrect heating processes can be categorized into several types. First, there are surface-related issues caused by the interaction between the billet and the furnace atmosphere, such as oxidation, decarburization, carburization, sulfidation, and copper penetration. Second, internal structural problems like overheating, overburning, and incomplete heating may occur due to abnormal changes in the metal’s microstructure. Third, uneven temperature distribution during heating can lead to excessive internal stresses—such as thermal or structural stress—which may result in cracking of the billet. Here are some of the most common issues that arise during improper heating.
Decarburization
Decarburization occurs when the carbon content on the surface of the metal is reduced due to oxidation at high temperatures. This leads to a significant decrease in carbon concentration in the outer layer compared to the interior. The depth of the decarburized layer depends on factors like the steel composition, furnace gas, temperature, and holding time. Decarburization is more likely to occur in an oxidizing atmosphere, especially with high-carbon steels or those containing a lot of silicon. This defect weakens the mechanical properties of the part, including strength, fatigue resistance, and wear resistance.
2. Carburization
Forgings heated in oil furnaces often experience carburization on the surface or parts of it. In some cases, the carburized layer can be as thick as 1.5–1.6 mm, with carbon content reaching up to 1% (by mass), and even higher in certain areas. This can lead to the formation of ledeburite structures. The cause is typically poor mixing of fuel and air in the furnace, leading to incomplete combustion and the creation of a reducing, carburizing atmosphere near the billet. This results in an increase in surface carbon, which can negatively affect machining performance, making it harder for cutting tools to operate smoothly.
Overheating
Overheating refers to situations where the billet is heated beyond the recommended temperature range or remains in this range for too long, causing excessive grain growth. In carbon steels (both hypoeutectoid and hypereutectoid), this can lead to the formation of Widmanstätten structures. Martensitic steels may develop intragranular structures, while tool steels often show primary carbide network formation. In titanium alloys, overheating can produce distinct β-phase grain boundaries and elongated Widmanstätten structures. For alloy steels, overheating may result in a brittle, "stone-like" fracture. While some forms of overheating can be corrected through heat treatment, severe cases are often irreversible and significantly reduce mechanical properties, particularly impact toughness.
4. Overburning
Overburning occurs when the billet is heated excessively or held at high temperatures for too long. Oxygen and other oxidizing gases penetrate the grain boundaries, reacting with elements like iron, sulfur, and carbon. This causes the formation of low-melting-point oxides, weakening the intergranular bonds and drastically reducing the material’s ductility. If overburned, the metal may crack during forging or exhibit transverse cracks in the affected area. There is no strict temperature threshold between overheating and overburning, but overburning is generally identified by grain boundary melting or the formation of fishbone-like structures in high-speed steels. In aluminum alloys, overburning may create melting triangles and remelted globules. Once overburned, the material is usually unsalvageable and must be scrapped.
5. Heating Cracks
When large cross-sectional steel ingots or high-alloy materials with poor thermal conductivity are heated too quickly in the low-temperature stage, large temperature differences between the inside and outside of the billet can generate significant thermal stress. Combined with the low plasticity at these temperatures, if the stress exceeds the material’s strength limit, cracks can form from the center outward, potentially fracturing the entire section.
6. Copper Cracking
Copper cracking is another issue that can occur during heating, especially in steels that contain copper. When heated, copper tends to migrate to grain boundaries, weakening them and making the material more susceptible to cracking under stress. This is commonly seen in certain stainless steels and can lead to premature failure if not properly controlled. Proper furnace atmosphere management and temperature control are essential to prevent such defects.
The product range of Hongchuang hardware fasteners includes hexagon head screws, full thread and half thread hexagon socket head cap screws, hexagon wood screws, hexagon socket head cap screws, full thread and half thread hexagon socket countersunk head screws, hexagon socket head flange screws, set screws, etc. We provide products conforming to DIN, ANSI and JIS standards.
Stainless steel of grade 304 contains nickel of 8% and chromium of 18% and is the most commonly used grade for the production of fasteners. It possesses an excellent combination of strength, corrosion resistance and fabricability. Fasteners of this grade are being extensively used today in almost every industry.
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