CHARACTERISTICS OF BASIC TYPES OF HEAT TREATMENT

The heat treatment process is an important operation in the technological manufacturing phase of many parts. Only through heat treatment is it possible to achieve high mechanical properties of steels, which ensures the normal operation of modern machine parts and tools.

1- ANNEALING OF STEELS
• Homogenization (Diffusion Softening)

Diffusion softening is generally applied in alloy steel ingots to improve the chemical splitting, mechanical properties, and to improve the dispersion of the builders. High temperatures are required for homogenization to occur.

• Recrystallization Softening

Recrystallization softening is applied to remove the hardening that occurs during pressure processing of metals before and between cold forming operations.

• Residual Stress Relief Smoothing

This type of softening is applied to relieve stresses that occur as a result of previous technological operations (casting, welding, cutting operations).

• Full Softening

Full softening is generally applied for sub-eutectic steels. Full softening is applied to forged and cast parts. In full softening, it is aimed to increase the mechanical properties of the cutting and processing capabilities.

• Isothermic Softening

Isothermal softening is applied to improve mechanical strength and obtain fine grain structure.

• Missing Softening

This type of softening is noticeable without full softening, and steels are heated to lower temperatures in incomplete softening. For sub-eutectic steels, incomplete softening is applied to relieve internal stresses and improve cutting through machining. However, only recrystallization takes place in incomplete softening (Austenite only transforms into pearlite and a very small amount of ferrite remains unchanged). For this reason, incomplete softening is applied for sub-eutectic steels if the hot mechanical treatment is done correctly and if coarse grains are formed. As a rule, incomplete softening is applied instead of full softening for supra-eutectic steels.

• Normalization

Normalization provides the recrystallization of steels and thus removes the coarse grained structure obtained in casting, rolling or forging and increases the strength. The normalization determination varies depending on the chemical composition of the steels.

2-HARDENING OF STEELS

Heating of sub-eutectic and super-eutectic steels to certain temperatures, keeping them for a certain time in order to complete the phase changes, and cooling them at a rate higher than the critical rate is called hardening. Carbon steels generally harden in water, alloy steels in oil or in another medium (lead or salt bath). Hardening is not the final heat treatment. Steels are tempered after hardening in order to obtain the desired mechanical properties in order to eliminate the brittleness and stresses that occur after hardening. In general, tool steels are subjected to hardening and tempering. In order to increase the wear resistance and strength of the hardness, structural steels are used to increase the strength (rupture resistance, yield limit), the hardness HB, the plasticity (elongation at break and cross section narrowing) and the tenacity (notch resistance).

• Nitration - Nitrocarburization

Nitriding is a surface treatment process. As a result of the diffusion of nitrogen to the steel surface, a hard layer with high wear resistance is formed on the surface of the material. Every material has a nitrating ability. Nitriding increases corrosion resistance and fatigue strength in some steels. In some steels, it reduces corrosion resistance. As a Combined Heat Treatment, the nitriding process is optimized according to the place where the mold will be used and the mold material, and the highest performance is provided. With the diffusion of carbon and nitrogen, a plate with high wear resistance is obtained on the material surface. The tenacity process, called salt nitration, is actually a nitrocarburization process, not a nitration process. , occurs by diffusion to the surface of the tool at the transformation temperature. This short cycle process increases the wear resistance of the material.

• Cold work tool steel hardening

Tool steels used in operations such as cutting, crushing, spinning and bending should be subjected to different heat treatment processes according to the place of use of the mold. The principle in the hardening of cold work tool steels is to ensure that the matrix structure consists of carbon and carbides. Due to their high carbon content, they generally have high hardenability. However, the residual austenite in the structure needs to be optimized by the cryogenic process and the applied tempering processes.

• Hot Work Tool Steel Hardening

In hot work tool steels, the austenitization temperature is selected according to the usage area of the mold. In order to get a good performance from hot work molds working in difficult conditions, especially the tempering temperatures should be selected correctly. A critical and follow-up heat treatment must be applied. Since the carbides formed between the grains reduce the toughness, the cooling process should be adjusted to minimize this formation.

• Oxidation

It is especially applied to prevent sticking and increase wear resistance in hot work tool steels. It is not applicable to all types of molds. The knowledge and experience of the heat processor gains importance in this regard. In addition, the oxidation process can also be applied specifically to increase the cutting life of high speed steels after the special nitration process.

• Cryogenic process

With the help of new technologies and the development of production processes, as well as cooling liquids, heat treatments at low temperatures are possible. The cryogenic process decreased to -196°C (-320°F), making the heat treatment more efficient and trouble-free. Thus, with this process, which can be applied to molds that are not likely to crack, the mold life is increased by 4 times; high wear resistance; high toughness, low friction on the surface can be achieved. In addition, the risk of cracking after wire erosion is minimized.

• High Speed Tool Steel Hardening

Heat treatment of high speed tool steels is a critical issue. In particular, the austenitization temperature and time must be determined very well. Preheating processes must be done completely. In addition, at least three tempering processes must be performed.

HARDNESS CONVERSION TABLE FOR STEELS



EFFECT OF ALLOY ELEMENTS ON STEEL STRUCTURE

• CARBON (C)

The element with the main hardening effect in steel is carbon. Each increase in carbon content increases the hardness and tensile strength of the steel in the hot rolled or normalized state. But it weakens its flexibility, ability to be forged, welded and cut.

• MANGANESE (Mn) 1244°C

Manganese improves the resistance of steel. It slightly weakens its flexibility. It has a positive effect on forging and welding. Manganese hardness and resistance-enhancing properties depend on the amount of carbon. The effect of manganese in high carbon steels is greater than in low carbon steels. Manganese increases the quenching depth and improves its resistance to corrosion.

• SILICUM (Si)1410°C

Silicon is an element found in all steels, such as manganese. In steelmaking, some silica from iron ore or bricks with furnace lining enters into the steel itself. The term silicon steels is used for steels with more than 0.40% silicon in their composition. Silicon increases mechanical resistance and specific gravity in steel castings. Although the presence of silicon in the steel negatively affects the flexibility, it increases the tensile strength by 10 kg/mm and the yield limit at a similar rate for each 1% increase. Since steels with 14% silicon are resistant to chemical reactions, these steels cannot be forged.

• PHOSPHORUS (P) 118°C

In general, phosphorus in steel is known to be harmful. In high quality steels, the percentage of phosphorus is kept as 0.030• 0.050 at most.

• SULFUR (S)

It is an element that is considered as undesirable foreign substances such as phosphorus in cases where it is not possible to increase the machinability of the steel. Normally the allowable amount is limited to a maximum of 0.025-0.030%. As a result, sulfur makes the steel brittle. And it strengthens its rolling.

• CHROME (Cr) 1920°C

Chromium is an alloying element that increases the strength of steel, but adversely affects its flexibility to a very small extent. Chromium increases the heat resistance of steel. It prevents scaling. The high amount of chromium in it increases the resistance of the steel against corrosion and wear.

In chromium stainless steels, as the chromium content increases, the ability to weld decreases. Chromium produces the most stable carbide. An increase of approximately 8• 10 kg/mm² in tensile strength is observed in response to an increase in the percentage of chromium per one percent in steel. Although not within the same ratio, the notch resistance decreases even if the yield limit increases.

• NICKEL(Ni) 1453°C

Nickel increases the resistance of steel less than silicon and manganese.
Chromium nickel steels are resistant to rusting and heat. It increases the notch resistance of machine building steels especially at low temperatures. Nickel refining and cementation increases the resistance of steels, and austenitic steels are a suitable alloying element for corrosion and scaling resistant steels.

• Molybdenum (Mo) 2610°C

Molybdenum increases the tensile strength of steel, especially its heat resistance and its ability to be welded. Steels with a high amount of molybdenum become more difficult to forge. Molybdenum is used more often with chromium. The effect of molybdenum is similar to wolfram.

• VANADIUM (V) 1730°C

It is one of the most important elements that is added in a small amount but provides a large property change. In particular, it increases the resistance to impact. It usually ensures that the inserts are cutting for a long time.

• Wolfram (W) 3380°C

It is a precious element with positive properties. It is mostly used in high speed and hot work steels.