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Introduction to the classification of steel

[Summary] Castability (castability): refers to the properties of the metal material to obtain qualified castings by casting methods.

Introduction to the classification of steel

[Summary] Castability (castability): refers to the properties of the metal material to obtain qualified castings by casting methods.


1: Castability (castability): refers to the properties of the metal material to obtain qualified castings by casting methods. Castability mainly includes fluidity, shrinkage and segregation. Mobility refers to the ability of the liquid metal to fill the mould, shrinkage refers to the casting solidification, the degree of volume shrinkage, segregation refers to the metal in the cooling solidification process, due to crystallization successive differences in the internal chemical composition of the metal and the unevenness of the organization.

2: Malleability: refers to the metal material in the pressure processing, can change the shape without cracking performance. It includes the ability to hammer, roll, stretch, extrude and other processing in the hot or cold state. The malleability is mainly related to the chemical composition of the metal material.

3: Cutting processability (machinability, machinability): refers to the degree of difficulty of the metal material being cut and processed by the tool to become a qualified workpiece. Cutting processability is commonly used after processing the surface roughness of the workpiece, the allowable cutting speed and the degree of tool wear to measure. It is related to the chemical composition of the metal material, mechanical properties, thermal conductivity and processing hardening degree and many other factors. Usually hardness and toughness are used as a general judgement of good or bad cutting processability. Generally speaking, the higher the hardness of the metal material, the more difficult to cut, although the hardness is not high, but toughness, cutting is also more difficult.

4: weldability (weldability): refers to the adaptability of metal materials to welding processing. Mainly refers to the welding process under certain conditions, the ease of obtaining excellent welded joints. It includes two aspects: one is the combination of performance, that is, in a certain welding process conditions, the sensitivity of a certain metal formation welding defects, the second is the use of performance, that is, in a certain welding process conditions, the suitability of a certain metal welded joints for use requirements.

5: Heat treatment

(1): annealing: refers to the heat treatment process of heating metal materials to an appropriate temperature, maintaining a certain amount of time, and then slowly cooling. Common annealing processes are: recrystallization annealing, stress relief annealing, spheroidal annealing, complete annealing, etc. The purpose of annealing: mainly to reduce the hardness of metal materials, improve plasticity, in order to facilitate cutting or pressure machining, reduce residual stress, improve the organization and composition of the uniformity, or for the latter heat treatment to prepare the organization.

(2): Normalizing: refers to the process of heating steel or steel parts to Ac3 or Acm (the upper critical point temperature of steel) above 30 to 50 ℃, held for an appropriate period of time, after cooling in the still air heat treatment. The purpose of normalizing: mainly to improve the mechanical properties of low carbon steel, improve the machinability, refine the grain, eliminate organizational defects, for the latter heat treatment to prepare the organization.

(3): quenching: refers to the steel heated to Ac3 or Ac1 (steel under the critical point temperature) above a certain temperature, maintain a certain time, and then a suitable cooling rate, to obtain martensite (or bainite) organization of the heat treatment process. Common quenching processes are salt bath quenching, martensite quenching, bainite isothermal quenching, surface quenching and local quenching. The purpose of quenching: so that the steel parts to obtain the required martensite organization, improve the hardness, strength and wear resistance of the workpiece, for the latter heat treatment to prepare the organization.

(4): Tempering: refers to the steel parts after hardening, and then heated to a temperature below Ac1, holding time, and then cooled to room temperature heat treatment process. Common tempering process are: low-temperature tempering, tempering, high-temperature tempering and multiple tempering, etc.. The purpose of tempering: mainly to eliminate the stress generated by the quenching of steel parts, so that steel parts with high hardness and wear resistance outside, and has the required plasticity and toughness, etc..

(5): Tempering: refers to the steel or steel parts for quenching and tempering of the composite heat treatment process. The steel used in the tempering treatment is called tempered steel. It generally refers to medium carbon structural steel and medium carbon alloy structural steel.

(6): chemical heat treatment: refers to the metal or alloy workpiece placed in a certain temperature of the active medium insulation, so that one or several elements into its surface layer, in order to change its chemical composition, organization and performance of the heat treatment process. Common chemical heat treatment processes are: carburizing, nitriding, carbonitriding, aluminizing, boriding, etc. The purpose of chemical heat treatment: mainly to improve the surface hardness of steel parts, wear resistance, corrosion resistance, fatigue strength and oxidation resistance, etc.

(7): solution treatment: refers to the alloy heated to a high temperature single-phase area maintained at a constant temperature, so that the excess phase is fully dissolved in the solid solution after rapid cooling, in order to obtain a supersaturated solid solution heat treatment process. The purpose of solid solution treatment: mainly to improve the plasticity and toughness of steel and alloys, for precipitation hardening treatment ready, etc.

(8): precipitation hardening (precipitation strengthening): refers to the metal in the supersaturated solid solution solute atomic polarization area and (or) by which the off-solution particles are diffusely distributed in the matrix and lead to hardening of a heat treatment process. Such as austenitic precipitation stainless steel in solid solution treatment or after cold working, in 400 ~ 500 ℃ or 700 ~ 800 ℃ for precipitation hardening treatment, can obtain a very high strength.

(9): Ageing treatment: refers to the alloy workpiece by solution treatment, cold plastic deformation or casting, forging, placed at a higher temperature or room temperature to maintain, its performance, shape, size and change over time heat treatment process. If the workpiece is heated to a higher temperature and aged for a longer period of time, the aging treatment process is called artificial aging treatment, if the workpiece is placed at room temperature or natural conditions for a long time and the aging phenomenon occurs, called natural aging treatment. The purpose of aging treatment, eliminate the internal stress of the workpiece, stabilize the organization and size, improve mechanical properties, etc.

(10): Hardenability: refers to the characteristics that determine the depth of hardening and hardness distribution of steel under the specified conditions. Steel hardenability is good and poor, commonly used to indicate the depth of hardened layer. The greater the depth of the hardened layer, the better the hardenability of the steel. Hardenability of steel mainly depends on its chemical composition, especially the alloying elements containing increased hardenability and grain size, heating temperature and holding time and other factors. Hardenability of good steel, steel parts throughout the cross-section to obtain uniform and consistent mechanical properties as well as the quenching stress of steel parts can be used to reduce deformation and cracking quenching agent.

(11): critical diameter (critical hardening diameter): critical diameter refers to the steel in a medium after quenching and cooling, the heart to get all martensite or 50% of the maximum diameter of the martensite organization, some steel critical diameter can generally be obtained through the oil or water hardening test.

(12): secondary hardening: certain iron-carbon alloys (such as high-speed steel) must be tempered several times before further increasing its hardness. This hardening phenomenon, known as secondary hardening, it is due to special carbide precipitation and (or) due to participation in the transformation of austenite into martensite or bainite.

(13): Tempering brittleness: refers to the tempering of quenched steel at certain temperature intervals or from the tempering temperature slowly cooled through the temperature interval of the embrittlement phenomenon. Tempering brittleness can be divided into the first type of tempering brittleness and the second type of tempering brittleness. The first type of tempering brittleness, also known as irreversible tempering brittleness, mainly occurs in the tempering temperature of 250 ~ 400 ℃, after reheating brittleness disappeared, repeated in this interval tempering, no longer brittle, the second type of tempering brittleness, also known as reversible tempering brittleness, occurs at a temperature of 400 ~ 650 ℃, when reheating brittleness disappeared, should be rapidly cooled, can not stay in the 400 ~ 650 ℃ zone for a long time or slow cooling, otherwise the catalytic phenomenon will occur again. The occurrence of temper brittleness and steel alloying elements, such as manganese, chromium, silicon, nickel will produce temper brittleness tendency, while molybdenum, tungsten has a tendency to reduce temper brittleness.


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