Defects Caused by Improper Heating Processes in American Standard 304 Stainless Steel Pipes
When heating stainless steel pipes, especially American Standard 304, improper procedures can lead to various defects. These defects are typically categorized into three main types: 1) surface chemical changes due to the furnace atmosphere, such as oxidation, decarburization, carburization, sulfidation, and copper penetration; 2) internal microstructural abnormalities like overheating, overburning, or insufficient heating; and 3) thermal stress-induced cracks caused by uneven temperature distribution within the billet. Here are some of the most common issues encountered during the heating process.
Decarburization
Decarburization occurs when carbon at the surface of the metal is oxidized during high-temperature exposure, resulting in a significant reduction in carbon content on the outer layer compared to the interior. This phenomenon is commonly seen in steels that are heated in an oxidizing environment. High-carbon steels and those with high silicon content are particularly prone to this issue. The depth of the decarburized layer depends on factors such as the steel composition, furnace gas, temperature, and holding time. Decarburization weakens the mechanical properties of the material, including strength, fatigue resistance, and wear resistance.
Carbon Increase
In oil-fired furnaces, forgings may experience carbon increase on their surfaces. This happens when the fuel and air mixture is incomplete, creating a reducing atmosphere near the nozzle. As a result, carbon deposits form on the surface, sometimes reaching a thickness of 1.5–1.6 mm with carbon content up to 2% (by mass). This can lead to the formation of ledeburite structures, which negatively impact machining performance and cause tool wear.
Overheating
Overheating occurs when the billet is exposed to temperatures above the recommended range or held for too long in the forging or heat treatment zone. This leads to grain coarsening and abnormal microstructures. For example, hypoeutectoid or hypereutectoid carbon steels may develop Widmanstätten structures, while martensitic steels may show intragranular etching. Tool steels might exhibit carbide network formation, and titanium alloys could show β-phase grain boundaries. Overheated materials often suffer from reduced mechanical properties, especially impact toughness. Some forms of overheating can be corrected through subsequent heat treatments, while others are irreversible.
Overburning
Overburning is a more severe condition than overheating, where the billet is heated beyond the critical temperature, causing oxidation and even melting at the grain boundaries. This destroys intergranular cohesion and significantly reduces the material's ductility. If overburned, the metal may crack during deformation or develop transverse cracks. In carbon steels, this can lead to melted grain boundaries and fishbone-like ledeburite structures. In aluminum alloys, melting at the grain boundaries and remelting balls may appear. Once overburned, the part is usually scrap and cannot be salvaged.
Heating Cracks
Heating cracks often occur in large-section ingots or high-alloy steels with poor thermal conductivity. If the heating rate is too fast in the low-temperature stage, large thermal stresses can develop due to temperature differences between the inside and outside of the billet. Combined with low plasticity at lower temperatures, these stresses can exceed the material’s strength limit, leading to cracking from the center outward.
Copper Crisp
Copper crisp is a defect that appears when copper penetrates the surface of the steel during heating. This usually happens in environments where copper compounds are present, such as in certain furnace atmospheres or in contact with copper tools. The presence of copper lowers the melting point of the steel and can cause brittle cracks, especially during subsequent forming or heat treatment processes. Preventing copper contamination is essential to avoid this type of defect.
When heating stainless steel pipes, especially American Standard 304, improper procedures can lead to various defects. These defects are typically categorized into three main types: 1) surface chemical changes due to the furnace atmosphere, such as oxidation, decarburization, carburization, sulfidation, and copper penetration; 2) internal microstructural abnormalities like overheating, overburning, or insufficient heating; and 3) thermal stress-induced cracks caused by uneven temperature distribution within the billet. Here are some of the most common issues encountered during the heating process.
Decarburization
Decarburization occurs when carbon at the surface of the metal is oxidized during high-temperature exposure, resulting in a significant reduction in carbon content on the outer layer compared to the interior. This phenomenon is commonly seen in steels that are heated in an oxidizing environment. High-carbon steels and those with high silicon content are particularly prone to this issue. The depth of the decarburized layer depends on factors such as the steel composition, furnace gas, temperature, and holding time. Decarburization weakens the mechanical properties of the material, including strength, fatigue resistance, and wear resistance.
Carbon Increase
In oil-fired furnaces, forgings may experience carbon increase on their surfaces. This happens when the fuel and air mixture is incomplete, creating a reducing atmosphere near the nozzle. As a result, carbon deposits form on the surface, sometimes reaching a thickness of 1.5–1.6 mm with carbon content up to 2% (by mass). This can lead to the formation of ledeburite structures, which negatively impact machining performance and cause tool wear.
Overheating
Overheating occurs when the billet is exposed to temperatures above the recommended range or held for too long in the forging or heat treatment zone. This leads to grain coarsening and abnormal microstructures. For example, hypoeutectoid or hypereutectoid carbon steels may develop Widmanstätten structures, while martensitic steels may show intragranular etching. Tool steels might exhibit carbide network formation, and titanium alloys could show β-phase grain boundaries. Overheated materials often suffer from reduced mechanical properties, especially impact toughness. Some forms of overheating can be corrected through subsequent heat treatments, while others are irreversible.
Overburning
Overburning is a more severe condition than overheating, where the billet is heated beyond the critical temperature, causing oxidation and even melting at the grain boundaries. This destroys intergranular cohesion and significantly reduces the material's ductility. If overburned, the metal may crack during deformation or develop transverse cracks. In carbon steels, this can lead to melted grain boundaries and fishbone-like ledeburite structures. In aluminum alloys, melting at the grain boundaries and remelting balls may appear. Once overburned, the part is usually scrap and cannot be salvaged.
Heating Cracks
Heating cracks often occur in large-section ingots or high-alloy steels with poor thermal conductivity. If the heating rate is too fast in the low-temperature stage, large thermal stresses can develop due to temperature differences between the inside and outside of the billet. Combined with low plasticity at lower temperatures, these stresses can exceed the material’s strength limit, leading to cracking from the center outward.
Copper Crisp
Copper crisp is a defect that appears when copper penetrates the surface of the steel during heating. This usually happens in environments where copper compounds are present, such as in certain furnace atmospheres or in contact with copper tools. The presence of copper lowers the melting point of the steel and can cause brittle cracks, especially during subsequent forming or heat treatment processes. Preventing copper contamination is essential to avoid this type of defect.
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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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