Gas Atomization Processing Parameters for Tool Steel Powders
Gas atomization of tool steel powders requires maintaining melt superheat at 150°C above liquidus and gas-to-metal ratio between 1.8 and 2.5 to optimize yield.
Tundish Delivery Tube
Meaning
Ceramic or refractory component that guides the flow of molten metal from the tundish into the atomization nozzle ensures a stable and centered liquid stream. This tundish delivery tube is a critical interface between the bulk melting system and the high velocity gas jets that create the powder. It must withstand extreme temperatures and the erosive forces of flowing liquid metal without contaminating the alloy.
The geometry of the tube is designed to provide a consistent mass flow rate, which is necessary for maintaining a stable atomization process. Any failure or misalignment of this component can lead to an uneven spray, nozzle clogging, or a complete shutdown of the production run.
Flow Control
Diameter and length of the internal bore determine the volume of metal that enters the atomization zone per unit of time. A properly sized tundish delivery tube ensures that the melt stream is perfectly centered within the gas nozzle to maximize the efficiency of the fragmentation. If the stream is off center, it will interact unevenly with the gas jets, resulting in a wide and unpredictable particle size distribution.
The material of the tube must have low thermal expansion to prevent cracking when it is first contacted by the molten alloy. Some systems use heated tubes to prevent the metal from freezing inside the bore during the initial stages of the pour.
Thermal Stability
Refractory materials used for these tubes, such as alumina, zirconia, or boron nitride, are chosen for their ability to resist thermal shock and chemical attack. During the atomization of high melting point alloys, the tundish delivery tube may be exposed to temperatures well above sixteen hundred degrees Celsius. It must maintain its structural integrity and its dimensions over several hours of continuous operation.
Any erosion of the tube tip will change the flow characteristics and can introduce ceramic inclusions into the finished metal powder. The compatibility between the tube material and the specific alloy being melted is a major consideration for process engineers.
Material Compatibility
Selection of the correct refractory grade prevents unwanted chemical reactions that could alter the purity of the metallic product. Some alloys are highly aggressive and can dissolve certain ceramic binders, leading to the rapid degradation of the tundish delivery tube. For example, titanium and other reactive metals require specialized materials that do not react with the melt at high temperatures.
The tube is often considered a consumable part that must be inspected and replaced regularly to ensure the consistency of the powder quality. Improvements in ceramic technology have led to longer lasting tubes that allow for larger batch sizes and more efficient production cycles in the powder metallurgy industry.