|
High Temperature Sintering
 MPP has developed considerable expertise in the area of high temperature sintering. MPP's Vice President of Technology, Dr. Chaman Lall, is an internationally recognized expert in both soft magnetism and high temperature sintering, having authored several papers on these subjects.
The Basics of High Temperature Sintering The implementation of high temperature sintering in the manufacture of ferrous powder metal alloys enhances the functional properties of parts, including tensile strength, ductility, impact strength, corrosion resistance, and soft magnetism. High temperature sintering is arbitrarily defined as processing above 1150° C (2100° F), since this is the practical limit for metal wire belts that are used to convey product in continuous sintering furnaces. The higher temperature processing is therefore performed in ceramic belt, ceramic pusher, ceramic walking beam, or vacuum furnaces. Traditional sintering at 1120° C (2050° F) yields entirely satisfactory powder metallurgy products for the vast majority of applications, but sintering at higher temperatures -as high as 1370° C (2500° F)- promotes additional particle-to-particle bonding and more complete alloying because of the higher diffusion rates. Particularly in the case of the high alloy steels and stainless steels that we use for many applications at MPP, the higher elemental diffusion rates permit the achievement of a more homogeneous structure, which enables the full value of the alloying ingredients to be achieved. The higher diffusion rates also increase the recrystallization and grain growth of the materials. The particle-to-particle bond area as well as more rounded porosity are promoted by higher diffusion rates. Each of these microstructural improvements leads to higher mechanical properties and ductility. Fatigue properties, in particular, are improved with higher temperature processing. In some alloy systems such as stainless steels and silicon steels, which contain Chromium and/or silicon, it is necessary to reduce the oxides of these elements in order to promote bonding of particles. The oxides are thermodynamically less stable at higher temperatures and can be more easily reduced to the metal form. Naturally, the dew points or oxygen levels need to be minimized during sintering to ensure that reduction of the oxides takes place. Also important is the need to maintain these conditions during cooling, since re-oxidation can occur at that stage of the process. In the case of stainless steels, the chromium is essential in providing the inherent corrosion resistance of these materials. If care is not taken, the element can be tied up in the form of an oxide at the grain boundaries and the depletion of the element from the matrix can lead to poor corrosion resistance. At MPP, we have invested considerable time, money, and energy in developing and controlling thermal profiles and other parameters in the high temperature sintering process. Benefits of High Temperature Sintering Higher temperature sintering has two valuable benefits for soft magnetic silicon steels. First, the higher temperatures promote reduction of the silicon oxides. Secondly, transient liquid phase formation increases the rate of diffusion, which increases densification and alloy homogeneity. As in all soft magnetic materials, the higher temperatures promote grain growth and dissolution of precipitates. Both of these phenomena serve to improve soft magnetic performance since "cleaner" and larger grains allow magnetic domain walls to move more easily under the influence of external magnetic fields. Much of the powder metallurgy industry uses mixtures of hydrogen and nitrogen as the protective atmosphere during sintering. While the hydrogen has the primary role of promoting reduction of oxides, the nitrogen can result in a few different outcomes. Nitrogen is not a true inert gas for processing steels since it does have some interaction with both ferrite and austenite phases. For structural applications, a small amount of nitrogen absorption is positive because it results in solid solution strengthening of the matrix. However, for soft magnetic and corrosion resistance applications, the gas can have a detrimental effect. In the case of stainless steels, the nitrogen readily combines with chromium, depleting it from the matrix and preventing it from providing the protective layer to the material that is so essential to corrosion resistance. In the 400 series stainless steels, there is the added complication that nitrogen dissolves in the austenite phase and, if the nitrogen content is sufficiently high, martensite formation can occur. In soft magnetic materials, nitrogen can form nitrides in the grains and grain boundaries, which pin the magnetic domain walls and lower performance. To some degree, the deleterious effects of nitrogen can be minimized by sintering at higher temperatures since the solubility of the element in austenite decreases as the temperature is increased. Ironically, the maximum solubility of nitrogen in steels occurs near 1120° C (2050° F). Therefore it is important to cool through this temperature range as quickly as possible in order to minimize the impact of nitrogen. At MPP, we have taken particular care to control the cooling phase of our high temperature sintering operations. High temperature sintering is essential when processing tool steels such as M2, D2 and T-15. In a similar manner to silicon steels and phosphorus steels, transient liquid phases are formed. The degree of liquid phase formation increases with temperature. Excessive amounts of liquid phase can lead to unacceptable levels of distortion and warpage, so there is a practical temperature limit that must be maintained. This is dependent upon the specific alloy being processed. For more information on high temperature sintering, go to Technical Articles.
|
