Ferritic stainless steel is one of the most commonly used alloys in powder metallurgy. A ferritic stainless steel powder is composed of three main components-iron (Cr), nickel (Ni), and molybdenum (Mo) (as well as sometimes containing niobium (Nb)). The microstructure diagrams typically used to illustrate metallurgical behavior of these alloys consist of a series of concentric circles. Each concentric circle typically corresponds to a certain solid phase-the most common solid phases being martensite, bainite, and ferrite. The martensite phase is the most commonly occurring, and is characterized by a hardness and strength of about 20-30 Rc. The bainite phase is slightly softer and has a tensile strength of about 40 Rc. Finally, the ferrite phase consists of metastable and stable lamellar structures and possesses slightly lower tensile strengths than the other solid phases.
The microstructure diagrams typically used to illustrate the metallurgical behavior of these alloys also identify grains and grain boundaries. Grains are individual crystalline units, usually between 20 and 500 angstroms in size, with specific orientations and distinct. Grain boundaries are essentially boundaries between these grains and thus separate the different crystalline units in a ferritic stainless steel powder. The energy stored in a grain boundary is an indicator of its structural integrity, which is an important factor in powder metallurgy. In addition, the surface energy of a grain boundary is also important for defining its strength, ductility, and formability.
Furthermore, the diagram also includes a diagram to indicate whether the alloy is ferritic or austenitic. Generally speaking, ferrite is a solid solution of two or more elements in iron and is characterized by a body-centered cubic (BCC) lattice structure. Austenite, on the other hand, is an alloy of more than two elements in iron, with a face-centered cubic (FCC) structure. The distinction between these two solid solutions is important to note as it can influence the mechanical properties of the alloy.
The graph also traditionally consists of a section that displays the alloy’s hardness range. This range is typically composed of two distinct points: the yield strength and the ultimate tensile strength. In the case of a ferritic stainless steel powder, the yield strength is usually lying around 260 Mpa, while the ultimate tensile strength is around 900 Mpa. By knowing this information, metallurgists can gauge the properties of the powder and determine the ideal parameters for forming/pressing/shaping the powder.
Finally, the traditional microstructure graph also includes an element known as the “formation temperature range”. This range is used to show the temperature range at which the alloy can form and at which these alloys have the optimal physical properties. The explanation of the formation temperature ranges vary dependent upon the individual alloys, with the formation temperature usually ranging from 800-1400°F. It is important to understand the formation temperature as it can help determine the amount of time and energy required to form the alloy.
In conclusion, the microstructure diagram used to illustrate the metallurgical behavior of ferritic stainless steel alloys typically contains several sections consisting of various components. These components include the solid phase diagrams to indicate the solid phases present, the grain diagrams to evaluate the grain size, grain boundary diagrams to evaluate the grain boundary strength, ferrite/austenite diagrams to distinguish between these two solid solutions, and the formation temperature range diagram to determine the temperature at which the alloy will form. Understanding the microstructure and behavior of ferritic stainless steel alloys is an important factor for powder metallurgy. By using this diagram, metallurgists will be able to form and shape the alloy according to their individual needs and potential applications.
