HEAT TREATMENT EQUIPMENT FOR FASTENERS, METALWARE AND PARTS

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INTEGRATED HEAT TREATMENT PROCESS

 

 

 

Thermal treatment (heat treatment) of steel and alloys can be of the following types: annealing, normalizing, hardening, tempering.

  • Annealing - heat treatment of the metal whereby the metal is heated, and then slow cooled. Annealing can be of different types (type of annealing depends on the heating temperature, cooling rate of the metal).
  • Hardening - heat treatment of steel and alloys based on recrystallization of steel (alloy) when heated above the critical temperature; after sufficient exposure to the critical temperature for the completion of heat treatment followed by a rapid cooling. Hardened steel (alloy) has a non-equilibrium structure, so a different kind of heat treatment can be applied - tempering.
  • Tempering - heat treatment of steel, alloys is carried out after hardening to reduce or remove the residual stresses in steel and alloys, it increases the viscosity, which reduces the hardness and brittleness of the metal.
  • Normalization - heat treatment similar to annealing. Differences of these types of heat treatment (normalizing and annealing) is that in normalizing the steel is cooled in air (at annealing - in the furnace).

 

STEEL ANNEALING

Annealing - the process of heat treatment of metal, which is produced during heating, and then slowly cooling the metal. Moving of the structure of the non-equilibrium state to more equilibrium. Annealing of the first kind, its types: recovery of metals, recrystallization annealing, annealing for stress relief, solution annealing (also called homogenization). Annealing of the second kind – changing of the structure of the alloy by recrystallization near the critical points to obtain the equilibrium structure. Annealing of the second kind, its types: complete, incomplete, isothermal annealing. Annealed, its types, with respect to steel are shown below.

  • Recovery of steel - heating to 200 - 400C °, annealing to reduce or remove cold-hardening. As a result of the annealing, a decrease from the distortion of the crystal lattices of crystallites and the partial restoration of physical and chemical properties of the steel.
  • Recrystallization annealing of steel - heated to a temperature of 500 - 550C °; annealing to relieve internal stresses - heating to temperatures of 600 - 700C °. These types of annealing relieve the internal stresses of the metal casting from uneven cooling of parts thereof and the blanks processed by pressure (rolling, drawing, forging) using temperatures below critical. Due to recrystallization of the deformed grains new crystals grow closer to equilibrium, so the hardness of the steel decreases, and ductility and toughness increases. To completely remove the internal stress of steel desired temperature should be at least 600C °. Cooling after soaking at a predetermined temperature must be sufficiently slow: due to accelerated cooling internal stresses occur again.
  • Diffusion annealing of steel (homogenization) is applied when the steel has intracrystalline segregation. Alignment of the composition in the austenite grains is achieved by the diffusion of carbon and other impurities in the solid state, along with the self-diffusion of iron. As a result of the annealing, the steel becomes uniform in composition (homogeneous), so called diffusion annealing and homogenizing. Homogenization temperature should be high enough, however, should not be allowed burnout, grains melting. Assuming burnout, the oxygen in the air oxidizes the iron, penetrating into the interior of it, crystallites fragmented by oxide shells form. Burnout cannot be eliminated, so overheated blanks are defected. Diffusion annealing of steel usually leads to excessive enlargement of the grain, which should be corrected followed by full annealing (to the fine grain).
  • Complete annealing of steel is related to the phase of recrystallization, grain refinement at temperatures of points AC1 and AC2. Its appointment - improving steel structure to facilitate subsequent machining, stamping and hardening, as well as obtaining an equilibrium fine pearlite structure of the finished part. For full annealing steel is heated at a temperature of 30-50C ° above the line GSK and slowly cooled. After annealing, the excess cement (in hypereutectoid steel) and eutectoid cementite are in the form of plates, so called lamellar perlite
  • Upon steel annealing for lamellar pearlite blanks are left for cooling in the furnace, usually under partial heating of furnace by fuel, the cooling rate is no more than 10-20C ° per hour. Annealing is also achieved by grain refinement. The coarse structure, such as pro-eutectoid steel is obtained during solidification due to free grain growth (if cooling casting is slow), and as a result become overheated. This structure is called Widmanstatten pattern (on behalf of the Austrian astronomer A. Widmanstatten, opened in 1808 the structure of such meteoric iron). This structure gives the low strength to the blank. The structure is characterized by the inclusion of ferrite (white portions) and pearlite (dark portions) arranged in the form of elongated plates at different angles to each other. In hypereutectoid steel structure Widmanstatten pattern is characterized by excess cementite with line-type location. One of the results of annealing for lamellar pearlite is a fine-grained structure.
  • Incomplete annealing of steel is related to the phase of recrystallization only at points A and C1; incomplete annealing is applied after hot forming with fine-grained structure of the blank.
  • Annealing of steel for granular perlite is usually used for eutectoid, hypereutectoid steels to improve the ductility, toughness of steel and reduce its hardness. For granular perlite steel is heated above the AC1 then maintained briefly to completely dissolve the cementite in austenite. Then, the steel is cooled to a temperature slightly below the Ar1, kept at this temperature for several hours. The particles of remaining cementite serve as nuclei of crystallization of all remaining cementite which grows as round (globular) crystallites dispersed in ferrite. Compared with lamellar perlite granular perlite has substantially different properties such as lower hardness, but higher lamellosity and viscosity. This applies particularly to hypereutectoid steel, where all cement (as eutectoid, so the excess) is obtained in the form of globules.
  • Isothermal annealing - after heating and soaking the steel is rapidly cooled to a temperature below point A 1, and then held at this temperature until complete decomposition of austenite to pearlite, and then cooled in air. Application of isothermal annealing significantly reduces the time and increases productivity. For example, ordinary stainless steel annealing procedure lasts 13-15 hours and isothermal - only 4-7 hours.

 

STEEL HARDENING

There are different types of hardening with polymorphic transformations of steels and without polymorphic transformations, for the majority of non-ferrous metals. Material affected by hardening becomes harder, but brittle, less elastic and viscous, if you make more repeats of heating and cooling. To reduce the brittleness and increase the toughness and ductility after hardening polymorphic transformation is applied. After hardening without polymorphic transformation aging is applied. When you leave the material some reduction in hardness and strength can be noticed.

Depending on the heating temperature, tempering is divided into complete and incomplete. In case of complete hardening the material is heated at 30 - 50 ° C above the GS for hypoeutectoid, eutectoid steel, as for hypereutectoid PSK line, in this case, the steel structure becomes austenite + cementite and austenite. With incomplete hardening heating above the PSK diagram is carried out that leads to formation of the excessive phase. Incomplete hardening is usually used for tool steel. Hardening is removed by tempering the material. For some products hardening is performed partially, for example in manufacturing of the Japanese katana, only the cutting edge of the sword is hardened.

Hardening environments

While hardening for supercooling of austenite up to martensite transformation temperature of rapid cooling is required, but not throughout the entire temperature range, but only within 650-400 ° C, i.e. in the temperature range at which austenite is least stable fastest transformed into cementite feritno- mixture. Above 650 ° C the rate of austenite transformation is small, and therefore, the mixture can be cooled by hardening in the temperature range slowly, but certainly not enough to initiate loss of ferrite or austenite to pearlite transformation.

Action mechanism of the hardening medium (water, oil, aqua-polymeric hardening medium (Termat) and cooling in salt solutions) is as follows. At the moment of immersing articles into the hardening medium around the formed film superheated vapor occurs, cooling is performed through the steam jacket, i.e. relatively slowly. When the surface temperature reaches a certain value (defined by the composition of the hardening liquid) in which the steam jacket is broken, the liquid begins to boil at the surface of the component, and cooling quickly.

The first stage of relatively slow boiling is called the stage of film boiling, the second stage of rapid cooling - nucleate boiling stage. When the surface temperature of the metal below the boiling point of the liquid, the liquid cannot boil, and slow cooling. This stage is called convective heat transfer.

Methods of hardening

  • Hardening in a cooler - heated to a certain temperature part is immersed in a hardening fluid, where it remains until completely cooled. This method is used for hardening of simple parts of carbon and alloy steels.
  • Intermittent hardening in two mediums - the method is used for hardening high-carbon steels. Detail is firstly rapidly cooled in fast cooling medium (e.g. water), and then slowly cooled (oil).
  • Laminar hardening is carried out by intensive sprinkling of a part by water jet and usually it is used when it is necessary to harden the part. In this method a steam jacket is formed that provides better hardenability than a simple water hardening. This hardening is usually done in the inductors at HDTV facilities.
  • Graded hardening - hardening at which the item is cooled in a hardening medium, having a temperature above the martensite point of the steel. Upon cooling and exposure in this medium hardened steel must have temperature of hardening bath at all points of the section. This is followed by a final, usually slow cooling during which hardening takes place, i.e. the transformation of austenite to martensite.
  • Isothermal hardening. In contrast to the graded hardening isothermal hardening requires maintaining of the steel in the hardening medium until complete isothermal transformation of austenite.

 

STEEL TEMPERING

Steel tempering softens the effect of hardening, reduces or eliminates the residual stress, increases the viscosity decreases hardness and brittleness of steel. Accommodation is made by heating the parts to hardened martensite to below the critical temperature. Thus, depending on the heating temperature condition can be prepared martensite, troostite or sorbitol tempering. These conditions differ from the corresponding tempering states structure and properties: during hardening cementite (in troostite and sorbite) is obtained in the form of elongate plates as the lamellar perlite. And when tempered it turns grainy as in the granular perlite.

The advantage of dot structure is more favorable combination of strength and ductility. At the same chemical composition and hardness of steel with the same dot structure it has a significantly higher relative narrowing y, toughness a n, increased elongation d, yield stress s t compared to steel with lamellar structure.

Hardening martensite has unstable tetragonal lattice, tempering martensite - stable centered cubic lattice of alpha-iron.

Accommodation is divided into low, medium and high, depending on the heating temperature.

To determine the temperature of products tempering annealing colors table is used. Thin film of iron oxides gives the metal a variety of rapidly changing colors - from light yellow to gray. This film appears if purified from dross steel product is heated to 220C °; by increasing the heating time or temperature of the oxide film becomes thicker and its color changes. Annealing colors appear as on crude and hardened steel.

At low temperature tempering (heating to a temperature of 200-300C °) in the steel structure there is mainly martensite, which, however, varies lattice. Also, precipitation of carbides of iron starts from a solid solution of carbon in alpha-iron and the initial accumulation of small groups. This entails a reduction in hardness and an increase in plastic and viscous properties of the steel, as well as reducing internal stresses in parts. For low temperature tempering items are held for a certain time typically in oil or salt baths. If for low tempering items are heated in air, the temperature control is performed by using annealing colors appearing on the surface of the item. The appearance of these colors is associated with the interference of white light in the films of iron oxide on the surface of parts during heating. In the temperature range from 220 to 330C ° depending on the film thickness the color changes from light yellow to gray. Low tempering is used for cutting, measuring tools and gears.

During medium (heating in the range of 300-500C °) and high (500-700C °) tempering martensite state turns respectively to troostite or sorbitol state. The higher tempering, the smaller hardness of tempered steel and the greater its ductility and toughness. When tempered steel receives the best combination of mechanical properties, improving indicators such as strength, ductility and toughness, that’s why high tempering after hardening it for martensite is used for heat treatment of forging dies, springs, leaf springs, and the highest - for many parts subject to high stress (for example, car axles, engine connecting rods).

For some grades of steel tempering is performed after normalization. This relates to a fine-grained alloy of pro-eutectoid steel (especially nickel) having high viscosity and therefore poor machinability of cutting tool. To improve the machinability normalizing of steel at elevated temperature is performed (up to 950-970C °), whereby it acquires a large structure (defining better processability) and simultaneously increased hardness (because of the low critical hardening rate of nickel steel). In order to reduce hardness the high tempering of this steel is performed.

 

NORMALIZATION OF STEEL

Normalization is cheaper operation than annealing, because furnaces are used only for heating and holding the product at a temperature of heating, and cooling is performed outside the furnace. Furthermore, normalization accelerates heat treatment process. Thus, it is profitable to replace annealing by normalization. However, it is not always possible because the hardness of some steels increases after normalization more significantly than during annealing. Mild steel is recommended to be subjected to normalization, since it has almost no difference in the properties after annealing and normalizing.

Steels containing more than 0.4% carbon, obtain increased hardness after normalization. These steels are better to be annealed. In practice also such steels are often subjected to normalizing annealing in place, and then tempered at a high temperature of 650 - 700C ° for reducing hardness. Normalization is used to produce a fine-grained structure in castings and forgings, to eliminate internal stresses for preparation of steel structure for hardening.