Brief introduction of technical performance terms of metal materials

Brief introduction of technical performance terms of metal materials

1: Castability (castability): refers to the properties of metal materials that can be used to obtain qualified castings by casting methods. Castability mainly includes fluidity, shrinkage and segregation. Fluidity refers to the ability of liquid metal to fill the mold. Shrinkage refers to the degree of volume shrinkage when the casting solidifies. Segregation refers to the inhomogeneity of the chemical composition and structure of the metal due to the difference in crystallization sequence during the cooling and solidification of the metal. .

2: Forgeability: refers to the performance of metal materials that can change shape without cracking during press processing. It includes the ability to perform hammer forging, rolling, stretching, extrusion and other processing in hot or cold state. The quality of forgeability is mainly related to the chemical composition of metal materials.

3: Machinability (machinability, machinability): refers to the degree of difficulty for metal materials to become qualified workpieces after being cut by tools. Machinability is often measured by the surface roughness of the workpiece after machining, 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 metal materials. Hardness and toughness are usually used as a rough judgement of machinability. Generally speaking, the higher the hardness of metal materials, the more difficult it is to cut. Although the hardness is not high, the toughness is greater and the cutting is more difficult.

4: Weldability (weldability): refers to the adaptability of metal materials to welding processing. Mainly refers to the difficulty of obtaining high-quality welded joints under certain welding process conditions. It includes two aspects: one is the bonding performance, that is, the sensitivity of a certain metal to form welding defects under a certain welding process condition, and the other is the use performance, that is, under a certain welding process condition, a certain metal welding The applicability of the joint to the requirements for use.

5: Heat treatment

(1): Annealing: refers to a heat treatment process in which metal materials are heated to an appropriate temperature, kept for a certain period of time, and then slowly cooled. Common annealing processes are: recrystallization annealing, stress relief annealing, spheroidizing annealing, complete annealing and so on. The purpose of annealing: mainly to reduce the hardness of metal materials, improve plasticity, to facilitate cutting or pressure processing, to reduce residual stress, to improve the uniformity of structure and composition, or to prepare for the subsequent heat treatment.

(2): Normalizing: refers to the process of heating steel or steel parts to 30-50°C above Ac3 or Acm (the upper critical point temperature of steel), keeping it for an appropriate time, and cooling it in still air. The purpose of normalizing is to improve the mechanical properties of low-carbon steel, improve machinability, refine grains, eliminate structural defects, and prepare for the subsequent heat treatment.

(3): Quenching: refers to heating the steel to a temperature above Ac3 or Ac1 (the lower critical point temperature of steel), keeping it for a certain period of time, and then obtaining martensite (or bainite) at an appropriate cooling rate The heat treatment process of the organization. Common quenching processes include salt bath quenching, martensite graded quenching, bainite austempering, surface quenching and partial quenching. The purpose of quenching: to obtain the required martensite structure of the steel, improve the hardness, strength and wear resistance of the workpiece, and prepare the structure for the subsequent heat treatment.

(4): Tempering: refers to a heat treatment process in which steel parts are quenched and then heated to a temperature below Ac1, kept for a certain period of time, and then cooled to room temperature. Common tempering processes are: low temperature tempering, medium temperature tempering, high temperature tempering and multiple tempering. The purpose of tempering: mainly to eliminate the stress generated by the steel during quenching, so that the steel has high hardness and wear resistance, and has the required plasticity and toughness.

(5): Quenching and tempering: refers to the compound heat treatment process of quenching and tempering steel or steel parts. The 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: refers to a heat treatment process in which a metal or alloy workpiece is placed in an active medium at a certain temperature to make one or several elements penetrate into its surface to change its chemical composition, structure and performance. Common chemical heat treatment processes are: carburizing, nitriding, carbonitriding, aluminizing, boronizing, etc. The purpose of chemical heat treatment: mainly to improve the hardness, wear resistance, corrosion resistance, fatigue strength and oxidation resistance of the steel surface.

(7): Solution treatment: refers to the heat treatment process in which the alloy is heated to a high temperature single-phase zone and kept at a constant temperature, so that the excess phase is fully dissolved in the solid solution and then quickly cooled to obtain a supersaturated solid solution. The purpose of solution treatment: mainly to improve the plasticity and toughness of steel and alloy, and prepare for precipitation hardening treatment.

(8): Precipitation hardening (precipitation strengthening): refers to a heat treatment process in which the solute atom segregation zone of the metal in the supersaturated solid solution and/or the dissolution of the particles are dispersed and distributed in the matrix to cause hardening. For example, austenitic precipitated stainless steel can be subjected to precipitation hardening treatment at 400-500°C or 700-800°C after solution treatment or cold working to obtain high strength.

(9): Aging treatment: refers to the heat treatment process in which the alloy workpiece is solid-solution treated, cold plastically deformed or cast, forged, placed at a higher temperature or kept at room temperature, and its performance, shape, and size change with time. If the workpiece is heated to a higher temperature and aging for a longer period of time, it is called artificial aging. If the workpiece is placed at room temperature or stored for a long time under natural conditions, the aging phenomenon is called natural Aging treatment. The purpose of aging treatment is to eliminate the internal stress of the workpiece, stabilize the structure and size, and improve the mechanical properties.

(10): Hardenability: refers to the characteristics that determine the hardening depth and hardness distribution of steel under specified conditions. The hardenability of steel is good and bad, which 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 alloying elements that increase the hardenability, grain size, heating temperature and holding time and other factors. The steel with good hardenability can make the entire section of the steel piece obtain uniform mechanical properties and the quenching agent with small quenching stress of the steel piece can be selected to reduce deformation and cracking.

(11): Critical diameter (critical hardening diameter): Critical diameter refers to the maximum diameter at which all martensite or 50% martensite structure is obtained in the core after the steel is quenched in a certain medium. The critical diameter of some steels The diameter can generally be obtained by the hardenability test in oil or water.

(12): Secondary hardening: some iron-carbon alloys (such as high-speed steel) need to be tempered many times before further increasing their hardness. This hardening phenomenon, called secondary hardening, is due to the precipitation of special carbides and/or due to the transformation of austenite into martensite or bainite.

(13): Tempering brittleness: refers to the embrittlement of quenched steel in a certain temperature range or slow cooling from the tempering temperature through this temperature range. Tempering brittleness can be divided into the first type of temper brittleness and the second type of temper brittleness. The first type of temper brittleness is also called irreversible temper brittleness. It mainly occurs when the tempering temperature is 250～400℃. After the reheating brittleness disappears, the tempering is repeated in this interval and no brittleness occurs. The second type of tempering Brittleness, also known as reversible temper brittleness, occurs at a temperature between 400 and 650°C. When the brittleness disappears after reheating, it should be cooled quickly. It should not stay for a long time or slowly cool in the range of 400-650°C, otherwise catalysis will occur again. The occurrence of temper brittleness is related to the alloying elements contained in steel, such as manganese, chromium, silicon, and nickel, which will have a tendency to temper brittleness, while molybdenum and tungsten have a tendency to weaken temper brittleness.