Today, we're sharing a comprehensive overview of terms related to the processing properties of metallic materials.
1. Castability (Moldability)
This refers to the ability of a metallic material to produce a qualified casting using casting methods. Castability mainly includes fluidity, shrinkage, and segregation. Fluidity refers to the ability of molten metal to fill a mold; shrinkage refers to the degree of volume shrinkage during solidification; and segregation refers to the unevenness of the internal chemical composition and structure of the metal caused by differences in the timing of crystallization during cooling and solidification.
2. Forgeability
This refers to the ability of a metallic material to change shape without cracking during pressure processing. It includes the ability to perform forging, rolling, stretching, and extrusion processes in hot or cold states. The quality of forgeability is mainly related to the chemical composition of the metallic material.
3. Machinability (Machinability, Machinability)
This refers to the ease with which a metallic material can be machined into a qualified workpiece using a cutting tool. Machinability is commonly measured by the surface roughness of the machined workpiece, the allowable cutting speed, and the degree of tool wear. It is related to many factors such as the chemical composition, mechanical properties, thermal conductivity, and work hardening degree of the metallic material. Hardness and toughness are usually used as a rough indicator of machinability. Generally speaking, the higher the hardness of a metallic material, the more difficult it is to cut; even if the hardness is not high, if the toughness is great, cutting is still difficult.
4. Weldability (Weldability)
This refers to the adaptability of a metallic material to welding processes. It mainly refers to the ease with which a high-quality welded joint can be obtained under certain welding process conditions. It includes two aspects: first, the bonding performance, that is, the sensitivity of a certain metal to forming welding defects under certain welding process conditions; and second, the service performance, that is, the suitability of a certain metal welded joint to the service requirements under certain welding process conditions.
5. Heat Treatment
(1) Annealing: This refers to a heat treatment process in which a metallic material is heated to an appropriate temperature, held for a certain time, and then slowly cooled. Common annealing processes include: recrystallization annealing, stress-relief annealing, spheroidizing annealing, and full annealing. Annealing aims to reduce the hardness of metallic materials, improve their plasticity to facilitate machining or pressure processing, reduce residual stress, improve the homogenization of microstructure and composition, or prepare the microstructure for subsequent heat treatment.
(2) Normalizing: This refers to the heat treatment process of heating steel or steel parts to 30℃～50℃ above Ac3 or Acm (the upper critical point temperature of steel), holding it for an appropriate time, and then cooling it in still air. The purpose of normalizing is mainly to improve the mechanical properties of low-carbon steel, improve machinability, refine grains, and eliminate microstructural defects, thus preparing the microstructure for subsequent heat treatment.
(3) Quenching: This refers to the heat treatment process of heating steel parts to a temperature above Ac3 or Ac1 (the lower critical point temperature of steel), holding it for a certain time, and then cooling it at an appropriate rate to obtain a martensitic (or bainitic) microstructure. Common quenching processes include salt bath quenching, martensitic graded quenching, bainitic isothermal quenching, surface quenching, and local quenching. The purpose of quenching is to obtain the desired martensitic structure in steel parts, improve the hardness, strength, and wear resistance of the workpiece, and prepare the microstructure for subsequent heat treatment.
(4) Tempering: This refers to the heat treatment process where steel parts are hardened, then heated to a temperature below Ac1, held for a certain time, and then cooled to room temperature. Common tempering processes include low-temperature tempering, medium-temperature tempering, high-temperature tempering, and multiple tempering. The purpose of tempering is mainly to eliminate the stress generated during quenching, so that the steel parts have high hardness and wear resistance, as well as the required plasticity and toughness.
(5) Quenching and tempering: This refers to the combined heat treatment process of quenching and tempering steel or steel parts. Steel used for quenching and tempering is called quenched and tempered steel. It generally refers to medium-carbon structural steel and medium-carbon alloy structural steel.
(6) Chemical heat treatment: This refers to the heat treatment process where metal or alloy workpieces are placed in an active medium at a certain temperature and held to allow one or more elements to penetrate into their surface layer, thereby changing their chemical composition, microstructure, and properties. Common chemical heat treatment processes include carburizing, nitriding, carbonitriding, aluminizing, and boronizing. The purpose of chemical heat treatment is primarily to improve the surface hardness, wear resistance, corrosion resistance, fatigue strength, and oxidation resistance of steel parts.
(7) Solution treatment: This refers to a heat treatment process in which an alloy is heated to a high-temperature single-phase region and held at a constant temperature to allow the excess phase to fully dissolve into the solid solution, followed by rapid cooling to obtain a supersaturated solid solution. The purpose of solution treatment is primarily to improve the plasticity and toughness of steel and alloys, and to prepare for precipitation hardening.
(8) Precipitation hardening (precipitation strengthening): This refers to a heat treatment process in which solute atoms in a supersaturated solid solution agglomerate and/or precipitate out and disperse into the matrix, leading to hardening. For example, austenitic precipitation-hardening stainless steel, after solution treatment or cold working, can achieve very high strength by undergoing precipitation hardening treatment at 400℃～500℃ or 700℃～800℃. (9) Aging Treatment: This refers to the heat treatment process in which the properties, shape, and dimensions of an alloy workpiece change over time after solution treatment, cold plastic deformation, casting, or forging, followed by placement at a higher temperature or maintenance at room temperature. If the workpiece is heated to a higher temperature and aged for a longer period, it is called artificial aging treatment. If the workpiece is placed at room temperature or under natural conditions for a long period, the aging phenomenon is called natural aging treatment. The purpose of aging treatment is to eliminate internal stress in the workpiece, stabilize its microstructure and dimensions, and improve its mechanical properties.
(10) Hardenability: This refers to the characteristics that determine the depth of hardening and hardness distribution of steel under specified conditions. The hardenability of steel is often expressed by the depth of the hardened layer. The greater the depth of the hardened layer, the better the hardenability of the steel. The hardenability of steel mainly depends on its chemical composition, especially the presence of alloying elements that increase hardenability, grain size, heating temperature, and holding time. Steel with good hardenability allows for uniform mechanical properties across the entire cross-section of the steel component, and allows for the use of quenching agents with low quenching stress, thus reducing deformation and cracking.
(11) Critical Diameter (Critical Hardenability Diameter): The critical diameter refers to the maximum diameter of steel after quenching in a certain medium, at which the core obtains a complete martensite or 50% martensite structure. The critical diameter of some steels can generally be obtained through hardenability tests in oil or water.
(12) Secondary Hardening: Some iron-carbon alloys (such as high-speed steel) require multiple tempering processes to further increase their hardness. This hardening phenomenon is called secondary hardening, which is caused by the precipitation of special carbides and/or by their participation in the transformation of austenite into martensite or bainite.
(13) Temper Brittleness: This refers to the embrittlement phenomenon of quenched steel after tempering within certain temperature ranges or slow cooling from the tempering temperature through that temperature range. Temper brittleness can be divided into first-type temper brittleness and second-type temper brittleness. The first type of temper embrittlement, also known as irreversible temper embrittlement, mainly occurs at tempering temperatures between 250℃ and 400℃. After reheating, the embrittlement disappears, and repeated tempering within this range does not result in embrittlement again. The second type of temper embrittlement, also known as reversible temper embrittlement, occurs at temperatures between 400℃ and 650℃. After reheating, the embrittlement disappears, and the temperature should be cooled rapidly. Prolonged exposure or slow cooling within the 400℃–650℃ range will cause catalytic re-occurrence. The occurrence of temper embrittlement is related to the alloying elements in the steel. For example, manganese, chromium, silicon, and nickel tend to cause temper embrittlement, while molybdenum and tungsten tend to reduce this tendency.
