During the stainless steel forging process, proper heating control, mold design, and operational guidelines are crucial for ensuring the quality of the forged parts. The formation of forging defects is typically the result of multiple factors working together. Therefore, understanding the causes and taking preventive measures is key to ensuring the performance of the forged parts. Below is a detailed analysis of the common causes of defects in stainless steel forging and their preventive measures.
Issues in the Heating Process
In stainless steel forging, the heating process is a critical step that determines the final product's quality. Different types of stainless steel materials have varying sensitivities to heating temperature and time. Improper control of these factors can lead to a series of issues.
1. Grain Growth
Ferritic stainless steel is prone to grain growth during heating. If the grains become too large, it can lead to brittleness, reducing impact toughness and affecting mechanical properties. To prevent grain growth, avoid excessive heating temperatures and strictly control the high-temperature holding time to prevent over-heating and grain expansion.
2. Excessive Austenite Phase Formation
For austenitic duplex steels and martensitic steels, excessive heating temperatures can promote the formation of too much austenite, which reduces the alloy's processability and increases the risk of forging cracks. To prevent this, control the heating temperature and avoid prolonged exposure to high temperatures.
3. Sulfur Infiltration
If sulfur infiltrates stainless steel, it can form low-melting point NiS or Ni+Ni3S2 eutectic compounds with nickel. These sulfides often accumulate at grain boundaries, leading to intergranular cracking. To prevent sulfur infiltration, ensure the purity of the raw materials and control sulfur content through appropriate smelting processes.
Mold Design and Operational Issues
Improper mold design and non-compliant operations are common sources of defects during forging. Proper mold design and operational procedures can significantly reduce defect occurrences.
1. Uneven Metal Flow
During forging, excessive shear stress between the metal flow area and the deformation-resistant area can cause defects, especially when lubrication is inadequate. Overly fast metal flow can also lead to local overheating, resulting in excessive austenite formation and material brittleness. To avoid this, optimize mold design, improve lubrication conditions, control metal flow speed, and ensure uniform plastic deformation at each stage of forging.
2. Mold Precision and Guide Problems
The precision of the mold and guide system is crucial for the accuracy of the forged parts. Large gaps in equipment guide rails or improper mold adjustment can cause the forged parts to misalign along the split line. To avoid this, adjust the guide rail gap, ensure mold precision, and try to align the mold cavity center with the striking center to improve the accuracy of the forged parts.
3. Surface Defects on Forged Parts
Surface defects, such as localized pits, are often caused by severe oxidation of the billet or the adhesion of slag impurities to the surface. To prevent this, strictly control heating time, remove oxidation scales and impurities from the billet surface, and ensure the mold cavity is clean. Inspect the billet surface before forging and remove any impurities that may affect forging quality.
Issues in the Cooling Process
The cooling process has a significant impact on the quality of stainless steel forged parts, particularly in controlling cooling speed and temperature range. Different types of stainless steel are prone to different defects during the cooling process.
1. Cooling Cracks
Martensitic stainless steel is sensitive to the cooling process. If the cooling speed is too fast, cooling cracks may occur. This is due to changes in thermal and organizational stresses during cooling, which can cause cracking. To prevent this, cool martensitic stainless steel forged parts slowly. A common method is to cool them in sand pits or slag to slow the cooling speed and ensure uniform cooling, reducing the risk of cracks.
2. Carbide Precipitation and Embrittlement
Austenitic stainless steel is prone to carbide precipitation (Cr23C6) along grain boundaries if it stays in the 480-820°C range for too long, increasing the risk of intergranular corrosion. To prevent this, minimize the time austenitic stainless steel forged parts stay in this temperature range and cool them to room temperature promptly. Ferritic stainless steel should be cooled quickly to avoid prolonged exposure around 475°C, preventing embrittlement.
Common Forging Defects and Solutions
Various forging defects may arise due to improper operation, equipment issues, and mold design flaws. Below is an analysis of common defects and their solutions.
1. Oversized Forged Parts
Oversized forged parts, especially in the height direction, are usually caused by overly heavy billets, low heating temperatures, or insufficient hammering force. To resolve this, avoid using overly heavy billets, control heating temperatures properly, and ensure adequate hammering force to guarantee accurate dimensions of the forged parts.
2. Misalignment and Bending Deviations
Misalignment along the split line or bending deviations along the centerline of forged parts are often caused by large gaps in equipment guide rails or mismatched molds. Solutions include adjusting the guide rail gap, improving mold precision, and repairing worn or deformed molds to ensure accuracy during the forging process.
3. Surface Crushing Damage
Surface crushing damage on forged parts usually occurs when the billet is not positioned properly, or when repeated hammer strikes occur. To prevent this, strictly control operational guidelines, ensure coordination between equipment movements and operators, and avoid repeated hammer strikes.
4. Over-Acid Pickling
Excessive acid pickling can cause pitted surface defects on stainless steel forged parts. This is typically caused by overly concentrated pickling solution or excessive pickling time. To prevent over-acid pickling, prepare the pickling solution properly and control pickling time to ensure a clean surface without defects.
Conclusion
The formation of defects in stainless steel forging is the result of multiple factors, including heating, mold design, operational procedures, and cooling processes. By properly controlling heating temperatures and times, optimizing mold design, following strict operational guidelines, and employing scientific cooling control, defects can be effectively reduced, ensuring product quality and performance. In actual production, detailed management of the forging process and technological improvements are key to enhancing the quality of stainless steel forged parts.
