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.
Interstitial Contamination
Meaning
Chemical impurity state occurs when small non metallic atoms like oxygen, nitrogen, hydrogen or carbon occupy the gaps between larger metal atoms in a crystal. This interstitial contamination is a major concern in the production of reactive metals like titanium, zirconium, and tantalum because these elements have a high affinity for light atoms. When these small atoms wedge themselves into the lattice, they distort the crystal structure and restrict the movement of dislocations.
This leads to a significant increase in the hardness and yield strength of the metal, but it also causes a sharp drop in ductility and toughness. Controlling the levels of these impurities is vital for ensuring the reliability of high performance structural components.
Atomic Interaction
Mechanism of the contamination involves the diffusion of gas atoms from the surrounding atmosphere into the bulk of the metal during high temperature processing. Because the interstitial atoms are much smaller than the host metal atoms, they can move through the lattice relatively easily when the material is hot. Once trapped, these elements form a solid solution that is difficult to reverse through standard heat treatment.
In the case of interstitial contamination, the presence of even a few hundred parts per million of oxygen can make a titanium alloy too brittle for aerospace use. The atoms do not form separate phases but rather change the properties of the existing metallic phase by creating internal strain.
Mechanical Effect
Resulting changes in the material behavior are often permanent and can lead to catastrophic failure if not properly monitored. A metal suffering from high levels of interstitial contamination will exhibit a higher tensile strength but will fail suddenly without much plastic deformation. This brittle behavior is particularly dangerous in environments subject to impact or cyclic loading where toughness is the primary defense against crack growth.
For instance, hydrogen contamination can lead to hydrogen embrittlement where the metal cracks under very low stress. Regular chemical analysis is required to ensure that the material remains within the specified limits for all interstitial elements.
Processing Barrier
Preventing the pickup of these light elements requires a strictly controlled environment throughout the entire manufacturing chain. Melting and atomization of reactive alloys must be performed under a high vacuum or in a high purity inert gas atmosphere to avoid interstitial contamination. Even during subsequent steps like welding or heat treating, the metal must be shielded from the air to prevent the surface from absorbing gases.
The cost of manufacturing increases significantly as the requirements for purity become more stringent. Any breach in the protective atmosphere can ruin an entire batch of material, making process monitoring a critical part of the production workflow.