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The term HEAT TREATMENT mean all the processes in which an object or material in the solid state deliberately warmed and cooled in a certain way to obtain the desired properties. Heat treatment influence the mechanical properties such as strength, hardness, ductility, impact strength, wear resistance. In many cases it is associated with the change of structure viz. neededto achieve state of equilibrium. Because the attainment of equilibrium during phase changes in thesolid state is completely determined by diffusion, the outcome will be decisive heat treatment,which will affect the course of diffusion. Size of diffusion is influenced by temperature, and at a temperature at which diffusion can take place even more. And that way influence of diffusiondivided heat treatment into two basic groups:

  1. Ways of heat treatment which are difusion more support or hinder her just a little: ANNEALING

  2. Ways of heat treatment which are difusion significantly hinder or completely stop: Quenching

Types of annealing according to the type of annealed material

  • Annealing of steel
  • Annealing of cast iron
  • Annealing of non-ferrous metals

Annealing

The essence of annealing is the heating part of the annealing to temperature (for differentmaterials and different methods of annealing), remaining (endurance) at this temperature for some time and then usually very slow cooling. In steel for fasteners is the annealing temperature just below of 721 ° C, where the material will remain for several hours and then cooled slowly so thatthere is a softening. Changes to the structure of hard lamellar pearlite to soft globular pearlitewhich structure becomes the ideal material for cold forming.

Overview of ways annealing steel

Types of annealingMethodAnnealing 
  temperature (°C)
Without overcrystallizationSoft
Recrystallization
Againflake
To reduce tension
680 to 790
550 to 700
650 to 700
500 to 650
With overcrystallizationHomogenization
Normalization
Isothermal
Solvent
1000 to 1300
750 to 950
700 to 850
  over 950

 

Normalization annealing (with overcrystalization)

It is the first of the annealing ways in which is happend overcrystalization. Heating to a temperature of 800-920 ° C with sufficient endurance at this temperature in order to achievehomogeneous austenite and subsequent slow cooling in still air. The result is the relatively fine-grained structure with higher strength, while removing the non-uniformity resulting from the previous structure forming.

 

Annealin for reduction of tension

It is used to reduce internal tensions that arise in such products after machining, hot forming attemperatures slightly above the finishing 721 ° C after uneven rapid cooling of the air etc. Afterheating to 500-600 ° C and endurance at this temperature (1 to 10 hours. according to the sizeand shape of components), subsequent slow cooling to achieve the ideal structure of the materialfor cold forming. This heat treatment is used before the manufacture of cold formed bolts, screwsstrength 4.6 and 5.6.

Quenching and tempering

Quenching

The purpose of quenching is to increase the hardness of steel. If the steel with a minimum content of carbon 0.3% is heated to a temperature of 800 ° C (depending on the type of steel), maintained for at this temperature and then cooled with water, oil, air, salt bath, (which suppresses theformation of ferrite and pearlite) preserved so unstable austenite at temperatures below 500 ° C is transformed to bainite or hard, but very brittle martensite. Achived strength depends on the percentage of C (the more the steel is harder - for many martensite hardness does not increasebeyond a 0.8% C) and percentage of martensite, which is the critical cooling rate created in the core material. For very thin alloy steel screws to the critical temperature is reached in the core. The problem is that the martenzit core is not possible cool quickly enough (due to thermalconductivity), therefore it is necessary to add alloying elements: boron, manganese, chromium, nickel, molybdenum, supporting and hardening to reduce the critical cooling rate. Choosing the right environment has a quenching effect on the cooling rate on the surface. Bolts are mainlyquenched in oil (water is indeed effective, but may also cause deformation of quenchedcomponents).

Quenched steel to martensitic structure has considerable internal tension and besides, that has great hardness is also very fragile. In this state it is applicable only very rarely.
To reduce internal stress and thus the fragility (or get tough structure), is appropriate anneal steelafter hardening. It's heating up to a tempering temperature. For the tempering temperature to 200° C, the strength decreases slightly but decreases much more fragile. At temperatures above 200 °C, a complete disintegration of martensite to ferrite and cementite in very fine form.
This structure is characterized by strength, high resistance and toughness.

Combinated quenching a tempering

Takes place at temperatures from 340 ° C to 650 ° C so that it follows a combined quenching andtempering in quick succession, which is usually one of the most widely used for thermal processingof fasteners. The optimal result is a high tensile strength and sufficient resistance (stiffness), theconnecting member to better resist outside forces. That is why through a combined quenching andtempering heat treated strength bolts also 8.8, 10.9, and 12.9

Surface hardening

In previous methods of heat treatment the thing was to achieve refined steel with high strengthand good notch toughness (highly stressed structural components). Or we tried to obtain a highhardness at relatively low toughness (tools). However, in some cases, especially in structuralcomponents subjected to friction, we require simultaneously high strength and high surfacehardness (see for example, screws). So we want to obtain a good surface wear resistance and corecomponents of great resistance to shock. 

Surface hardening
Surface quenchingChemical-heat treatment of steel
by flameinductioncase-hardeningnitriding

nitrocarburizing

 

Surface quenching

For this treatment is suitable carbon steel with content of carbon between 0.45 to 0.60%, which are already considerable quenching hardness. Steels with such a content of carbon in featuresatisfactory toughness and core strength especially in those cases when a component of thesurface quenching normalized either annealed or heat treated. Which means That the corecomponents that are not affected by surface quenching, retains the properties of the previous heat treatment. According to the method of heating the surface layer to distinguish austenizationtemperature surface hardening:

  1. by flame:  (mostly used oxy-acetylene torches, cloudy layer is> 2mm)
  2. induction:   surface layer is heated by current middle and high frequencies with an inductor(coil of a small number of threads of copper tube, through which flows cooling water. passes through the inductor an alternating current of frequency, are formed of alternating magnetic field induction components of eddy currents in the same frequency, which the heated part.selecting frequency can affect the depth of warm layers. It is used mostly for special applications for hardening weaknesses.

 

Chemical heat treatment of steel

Involves a many ways of processing in which saturate the surface of steels of various elements to achieve desired properties, such as fire resistance, corrosion resistance, wear resistance, etc.

  1. Case-hardening: most widely used method of chem-thermal processing. The surface of the object from mild steel (with carbon content max 0.2%) is the prin saturation carbon in solid,liquid or gaseous environment over austenizační temperature (in itself only austenite dissolvescarbon) content of the C 0.7 - to 0.9% . Opacity of this layer reaches a high hardness,toughness while maintaining the core. Carburizing layer is 0.5 to 1.5 mm thick.
  2. Nitriding: steel surface saturation with nitrogen, which reacts with iron and other elementsbeing added intentionally (especially Al and Cr). They are created by hard nitrides, which causea considerable increase in hardness. runs at 500 to 600 ° C.
  3. Nitrocarburizing: is the saturation of the surface carbon and nitrogen simultaneously incyanide salt bath at temperatures from 750 to 850 ° C, or a gas with the addition of ammoniaat temperatures from 800 to 880 ° C. When the temperature of the carbon is higher, than thecarbon saturation will be higher and vice versa. Then the components tempered in oil, which reduces stress.
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