Introduction to Terms Related to the Processability of Metallic Materials

　　1: Castability (Moldability): This refers to the ability of a metallic material to produce sound castings through the casting process. Castability primarily includes fluidity, shrinkage, and segregation. Fluidity is the capacity of molten metal to completely fill the mold, while shrinkage measures the degree to which a casting contracts in volume as it solidifies. Segregation occurs when, during the cooling and solidification of the metal, differences in crystallization sequence lead to uneven distribution of chemical composition and microstructure within the material.

　　2: Malleability: This refers to the ability of a metallic material to undergo plastic deformation under pressure without developing cracks. It encompasses processes such as forging, rolling, stretching, and extrusion—whether performed hot or cold. The quality of malleability is primarily determined by the chemical composition of the metal.

　　3: Machinability (Cutting Ability, Mechanical Processability): This refers to how easily a metal material can be cut and shaped into a qualified workpiece using cutting tools. Machinability is typically assessed by measuring the surface roughness of the finished part, the allowable cutting speeds, and the rate of tool wear. It depends on various factors, including the metal’s chemical composition, mechanical properties, thermal conductivity, and degree of work hardening. As a general guideline, hardness and toughness are often used to roughly evaluate machinability—generally speaking, the higher the hardness of a metal, the more challenging it is to machine; however, even if the hardness isn’t particularly high, materials with high toughness can still prove difficult to cut effectively.

　　4: Weldability: Refers to the ability of a metallic material to adapt to welding processes. Specifically, it indicates how easily high-quality weld joints can be produced under given welding conditions. Weldability encompasses two key aspects: first, joining performance—how susceptible a particular metal is to developing welding defects under specific process conditions—and second, service performance—how well the resulting weld joint meets the requirements for practical application under those same conditions.

　　5: Heat Treatment

　　(1): Annealing: A heat treatment process in which metallic materials are heated to an appropriate temperature, held for a specific duration, and then cooled slowly. Common annealing processes include recrystallization annealing, stress-relief annealing, spheroidizing annealing, and full annealing. The primary purposes of annealing are to reduce the hardness of metal materials, enhance ductility for easier machining or forming operations, relieve residual stresses, promote uniformity in microstructure and composition, or prepare the material’s structure for subsequent heat treatments.

　　(2): Normalizing: This heat treatment process involves heating steel or steel components to a temperature 30–50°C above the Ac3 or Acm point (the upper critical temperature for steel), holding it at that temperature for an appropriate duration, and then allowing it to cool slowly in still air. The primary objectives of normalizing include enhancing the mechanical properties of low-carbon steels, improving machinability, refining grain structure, eliminating microstructural defects, and preparing the material’s microstructure for subsequent heat treatments.

　　(3): Quenching: This refers to a heat treatment process where steel components are heated above Ac3 or Ac1 (the lower critical temperature of steel), held at that temperature for a specific period, and then cooled at an appropriate rate to achieve a martensitic (or bainitic) microstructure. Common quenching techniques include salt-bath quenching, martensitic step quenching, bainitic isothermal quenching, surface quenching, and localized quenching. The primary purposes of quenching are to impart the desired martensitic structure to the steel parts, thereby enhancing hardness, strength, and wear resistance, as well as preparing the microstructure for subsequent heat treatments.

　　(4): Tempering: A heat treatment process in which steel components, after being hardened through quenching, are reheated to a temperature below Ac1, held at that temperature for a specific period, and then cooled down to room temperature. Common tempering processes include low-temperature tempering, medium-temperature tempering, high-temperature tempering, and multiple tempering cycles. The primary purpose of tempering is to relieve the internal stresses generated during quenching, ensuring that the steel component not only achieves high hardness and wear resistance but also exhibits desirable levels of ductility and toughness.

　　(5): Quenching and Tempering: This refers to a combined heat treatment process that involves both quenching and subsequent tempering of steel materials or components. Steel subjected to this process is known as tempered steel. Typically, it includes medium-carbon structural steels and medium-carbon alloy structural steels.

　　(6) Chemical Heat Treatment: This refers to a heat treatment process where metal or alloy workpieces are placed in an active medium at a specific temperature and held for a certain period, allowing one or more elements to diffuse into the surface layer. This process alters the material's chemical composition, microstructure, and properties. Common chemical heat treatment techniques include carburizing, nitriding, carbonitriding, aluminizing, and boronizing. The primary objectives of chemical heat treatment are to enhance the hardness, wear resistance, corrosion resistance, fatigue strength, and oxidation resistance of steel components' surfaces.

　　(7) Solid Solution Treatment: This refers to a heat treatment process where an alloy is heated to a high temperature within the single-phase region, held at that temperature long enough to ensure complete dissolution of any excess phases into the solid solution, and then rapidly cooled to produce a supersaturated solid solution. The primary purposes of solid solution treatment include enhancing the ductility and toughness of steels and alloys, as well as preparing the material for subsequent precipitation hardening processes.

　　(8): Precipitation Hardening (Age Hardening): This is a heat treatment process where solute atoms in an oversaturated solid solution tend to cluster together, forming solute-rich zones and/or precipitating fine particles that are uniformly dispersed throughout the matrix, thereby enhancing the material's strength. For example, austenitic precipitation-hardened stainless steels, after undergoing solution treatment or cold working, can achieve exceptionally high strength when subjected to precipitation hardening at temperatures ranging from 400–500°C or 700–800°C.

　　(9) Aging Treatment: This refers to a heat treatment process where alloy components, after undergoing solid solution treatment, cold plastic deformation, or casting and forging, are either held at elevated temperatures or maintained at room temperature for an extended period. During this process, their properties, shape, and dimensions may change over time. If the treatment involves heating the component to a higher temperature and maintaining it there for an extended duration, it’s called artificial aging. On the other hand, if the component is simply left at room temperature or exposed to natural environmental conditions over a long period—resulting in gradual changes—it’s referred to as natural aging. The primary objectives of aging treatment include relieving internal stresses within the material, stabilizing its microstructure and dimensions, and enhancing mechanical properties.

　　(10) Hardness Through-Thickness: This refers to the characteristic that determines the depth of hardening and the distribution of hardness in a steel material under specified conditions. Whether a steel has good or poor hardenability is typically indicated by the depth of the hardened layer—specifically, the greater the depth of this layer, the better the steel's hardenability. Steel hardenability primarily depends on its chemical composition, particularly the presence of alloying elements that enhance hardenability, as well as factors like grain size, heating temperature, and holding time during the quenching process. Steels with excellent hardenability allow for uniform mechanical properties throughout the entire cross-section of the component, while also enabling the use of quenchants that minimize thermal stresses during quenching, thereby reducing deformation and cracking risks.

　　(11): Critical Diameter (Critical Quenching Diameter): The critical diameter refers to the maximum diameter of a steel bar that, when quenched in a specific medium, will result in either fully martensitic or 50% martensitic microstructure throughout its core. For certain steels, the critical diameter can typically be determined through hardenability tests conducted either in oil or water.

　　(12): Secondary Hardening: Certain iron-carbon alloys, such as high-speed steel, require multiple tempering processes to further enhance their hardness. This hardening phenomenon is known as secondary hardening, and it occurs due to the precipitation of specific carbides and/or the transformation of austenite into martensite or bainite.

　　(13): Temper Embrittlement: This refers to the embrittlement phenomenon that occurs when quenched steel is tempered within certain temperature ranges or slowly cooled through these ranges after tempering. Temper embrittlement can be categorized into two types: Type I and Type II temper embrittlement. Type I temper embrittlement, also known as irreversible temper embrittlement, typically happens at tempering temperatures between 250°C and 400°C. Once the material is reheated, the embrittlement disappears, and subsequent tempering within this same temperature range will no longer induce brittleness. Type II temper embrittlement, or reversible temper embrittlement, occurs at higher temperatures ranging from 400°C to 650°C. If the material is reheated and the embrittlement vanishes, it must then be rapidly cooled—quickly enough to avoid lingering in the 400°C–650°C range or allowing slow cooling, as prolonged exposure or gradual cooling could trigger the embrittlement process again. The occurrence of temper embrittlement is closely linked to the alloy elements present in the steel. For instance, elements like manganese, chromium, silicon, and nickel tend to increase the susceptibility to temper embrittlement, while molybdenum and tungsten have the opposite effect, helping to mitigate this phenomenon.