On 2018-08-08 14:48:54
Metallic materials and heat treatment
In production, through heating, heat preservation and cooling, the steel will undergo solid-state phase transformation, so as to change its internal structure and improve its mechanical properties.
The workpiece is heated to Ac3 (Ac means the final temperature of all free ferrite transformed into austenite during heating, usually from 727 to 912 degrees) or Acm (Acm is the critical temperature line of complete austenization of hypereutectoid steel in actual heating) above 30~50 C, after heat preservation for a period of time, the metal heat treatment process is taken out from the furnace in air or water spray, spray or blow cooling.
Heat the steel to a certain temperature above AC3 or AC1, keep it for a certain time, and then take it out for water cooling or oil cooling to obtain martensite.
The austenitized workpiece is quenched into the molten salt with a temperature slightly higher than MS, and the isothermal holding time is enough to make the undercooled austenite undergo bainite transformation at a constant temperature. After the transformation is completed, it is taken out and cooled in air, which is called isothermal quenching.
The austenitized workpiece is quenched into the molten salt whose temperature is slightly higher or lower than Ms. after the internal and external temperature of the workpiece is even, it is taken out from the molten salt and cooled to room temperature in air to obtain martensitic structure. This treatment method is called staged quenching.
Single fluid quenching
The austenitized workpiece is put into a quenching medium until the end of transformation.
Double liquid quenching
The austenitized workpiece is first put into a cooling medium with strong cooling capacity for a certain period of time. When it is cooled to a little higher than MS, the workpiece is immediately taken out and put into another cooling medium with slower cooling capacity for cooling, so that it can be transformed into a martensitic heat treatment process.
A heat treatment process in which the quenched steel is heated to a temperature lower than the critical point A1, held for a certain period of time, and then cooled to room temperature.
When the quenched carbon steel is tempered at 500 ~ 650 ℃, the multiphase structure composed of coarse-grained cementite and polygonal ferrite is obtained.
When the quenched carbon steel is tempered at 350 ~ 500 ℃, the duplex structure of fine-grained cementite and acicular ferrite is obtained.
When the quenched carbon steel is tempered below 250 ℃, the supersaturated α solid solution and the dispersed carbide are formed.
It is a heat treatment process that the steel is heated to a certain temperature above or below the critical point and cooled with the furnace after holding for a certain time. It is one of the most widely used and most widely used heat treatment processes, and the annealing purposes of different kinds are different.
The hypoeutectoid steel workpiece is heated to 20-30 ° C above A3 for a certain period of time, and then it is isothermal at a certain temperature in the pearlite transformation range below ARL, so that it can be turned into pearlite and then it is put out of the furnace for air cooling. It can effectively shorten the annealing time, improve the production efficiency and obtain uniform microstructure and properties.
The heat treatment process of heating hypoeutectoid steel castings, forgings, weldments and hot rolled sections to 20-30 ° C above A3, holding for a certain period of time, and then cooling to 500-600 ° C in the furnace with air cooling. Its purpose is to refine grains, reduce hardness, improve cutting performance and eliminate internal stress.
The workpiece of hypereutectoid steel or alloy tool steel is heated to 20-30 ° C above ad, kept for a certain time, and then cooled to about 500 ° C in the furnace by air cooling (ordinary spheroidizing annealing) or cooled to 20 ° C below ARL for a certain time, and then cooled to about 500 ° C by air cooling (isothermal spheroidizing annealing) to obtain a kind of annealing process of granular pearlite. Its purpose is to reduce hardness, uniform structure, improve cutting performance, and prepare for quenching.
For important or alloy steel ingots or castings with uneven chemical composition such as dendrite segregation, in order to achieve the homogenization of chemical composition, it can be heated to the core 3 or above 150-300 ° C, which is an annealing process of slow cooling with the furnace after a long time of heat preservation. Because diffusion annealing needs to be heated at high temperature for a long time, austenite grains are very coarse. Therefore, it is necessary to conduct a full annealing or normalizing again to refine grains and eliminate overheating defects.
After cold deformation, the metal is heated above the recrystallization temperature for a proper time, so that the deformed grains can be transformed into uniform equiaxed grains again. This heat treatment process is called recrystallization annealing.
Stress relief annealing
In order to eliminate the residual internal stress caused by deformation processing, casting and welding process, so as to improve the dimensional stability of the workpiece and prevent deformation and cracking, the workpiece is slowly heated to 500-600 ° C along with the furnace, and then cooled to 300-200 ° C along with the furnace after a period of heat preservation.
Surface heat treatment of steel
It is a kind of heat treatment method that makes the surface of parts obtain high hardness and wear resistance, while the core still keeps the original good toughness and plasticity.
Carburizing is to make the carbon atoms penetrate into the surface layer of the workpiece and increase the carbon content of the surface layer, generally 1 = 0.8% ~ 1.05%. After carburizing, the workpiece is quenched and tempered at low temperature to achieve high hardness and wear resistance on the surface, while the center has enough strength and initial degree to achieve the purpose of external hard and internal initial.
It is a process of penetrating nitrogen into the surface of steel parts. The purpose of nitriding is to improve the hardness and wear resistance of the steel surface, as well as the fatigue strength and corrosion resistance.
It is a technology that uses special equipment to heat, melt or soften a solid material and speed up spraying it on the surface of the workpiece to form a special thin layer, so as to improve the corrosion resistance, wear resistance, high temperature resistance and other properties of the parts.
Physical vapor deposition (PVD method)
It is a vapor deposition method that uses physical methods to produce deposited atoms or ions, but there is no chemical reaction in the room.
Chemical vapor deposition (CVD)
It is a method that heat energy or radiation energy is input into the gas-phase reaction chamber filled with any pressure to make the gas-phase undergo certain chemical reaction, resulting in the deposition of a solid film on a specific surface of the workpiece.
Metal ion implantation
It is a treatment process that the ions of high energy beam are driven into the surface of metal materials to form a very thin near surface alloy, so as to change the physical, chemical and mechanical properties of the substrate surface.
After a certain period of time, the part is placed in a plating bath filled with special chemical agent. The process of obtaining a certain thickness of coating on the surface of the workpiece due to the electrochemical reaction between the chemical agents is called electroless plating.
Critical quenching diameter
It refers to the maximum hardenability diameter (i.e. the maximum diameter of semi martensite at the center) that can be obtained when the round bar sample is put into fire in a certain medium, expressed by d0.
Isothermal transformation refers to the rapid cooling of austenitized steel to a temperature below A1, so that the microstructure of undercooled austenite changes in the process of heat preservation, and after the transformation, it is cooled to room temperature.
Continuous cooling transition
After austenitizing, the steel is cooled continuously from high temperature to room temperature at a certain cooling rate, and the structural transformation completed in the process of continuous cooling is called continuous cooling transformation.
It is a kind of structure name of ferrous material, and it is supersaturated solid solution of carbon in a-Fe.
Lamellar martensite (acicular martensite)
It is a typical martensitic structure formed in medium, high carbon steel and high nickel iron nickel alloy. The spatial morphology of lamellar martensite is convex lens. Because the grinding surface of the sample is intersected with it, it is needle like or bamboo leaf like under the optical microscope. Therefore, lamellar martensite is also called needle like or bamboo leaf like martensite.
It is a martensite formed by austenite with low carbon content and a typical martensite structure in low carbon steel, medium carbon steel and stainless steel. Because the microstructure is composed of groups of laths, it is called lath martensite.
Stabilization of austenite
In the transformation temperature of martensite, if the cooling stops at a certain temperature, and the cooling continues after a period of time, the transformation of martensite does not start immediately, but only starts again after a period of time, and causes the corresponding increase of retained austenite, which is called the stabilization of austenite. Because it is caused by constant temperature stay, it is called thermal stabilization.
When austenite is cooled below the critical temperature, it is in an unstable state in thermodynamics, and decomposition transformation will occur during cooling. This kind of austenite, which exists and is unstable below the critical transformation temperature, is called supercooled austenite.
When austenite is supercooled to the temperature zone between the pearlite transformation temperature and the martensite transformation temperature, the transformation from shear transformation to short-range diffusion will take place. The transformation product is called bainite or bainite. That is to say, when the eutectoid austenite stays in the range of "nose" temperature to ^ point, bainite transformation will take place, forming a non lamellar structure bainite composed of ferrite and carbide.
Hardenability of steel
Hardenability of steel refers to the ability of austenitized steel to obtain martensite during quenching, and its size can be expressed by the depth of harden layer obtained by steel quenching under certain conditions. The deeper the hardenability layer is, the better the hardenability of the steel is.
Hardenability of steel
Hardenability refers to the highest hardness that can be achieved by the martensitic structure formed at a rate higher than the critical cooling rate under ideal quenching conditions, also known as hardenability.
Actual grain size
The austenite grain size obtained under a specific heating condition is called the actual grain size. The actual grain size is different from the initial grain size. The initial grain size is the grain size of austenite when it is just formed (i.e. its grain boundary just contacts), while the actual grain size refers to austenite which has been kept warm for a certain period of time. The actual grain diameter is larger than the initial grain diameter.
Tempering brittleness refers to the phenomenon that the toughness of quenched steel decreases after tempering. During tempering, the hardness and toughness of quenched steel decrease with the increase of tempering temperature, but there are two troughs in the relation curve between tempering temperature and impact toughness of many steels, one is between 200 ~ 400 ℃ and the other is between 450 ~ 650 ℃. With the increase of tempering temperature, the impact toughness decreases. Tempering brittleness can be divided into the first type and the second type.
High temperature temper brittleness
The brittleness of quenched steel in the temperature range of 500 ~ 650 ℃ is called high temperature temper brittleness, which is also called the second kind of temper brittleness. This kind of temper embrittlement mainly occurs in steel containing Cr, Ni, Mn, Si and other alloy elements.
Low temperature temper brittleness
The brittleness of quenched steel in the temperature range of 250 ~ 400 ℃ is called low temperature temper brittleness, also called the first type temper brittleness. This brittleness occurs in almost all quenched steels when tempered at about 300 ℃.