Knowledge

NO 1. Introduction

Steel containing a certain amount of special alloy elements is called alloy steel. After adding a certain amount of alloy elements to steel, it acquires certain special mechanical and physical-chemical properties to meet the comprehensive needs of national economic development.

 

The purposes of alloying steel:

1. Strengthen ferrite and increase the strength of steel before quenching.

2. Enhance the stability of austenite, improve the hardenability and dispersion of the microstructure of steel, and increase the mechanical strength of parts after heat treatment.

3. Improve the strength of steel at medium and high temperatures (thermal strength, etc.).

4. Enhance the corrosion resistance of steel (acid resistance, heat resistance, oxidation resistance, etc.).

5. Endow steel with certain special physical properties (magnetism, electrical conductivity, diamagnetism, elasticity, toughness, and expansion properties, etc.).

Most high-alloy steels used in industry can be processed by pressure, but some steels have very low forgeability and even lose the possibility of pressure processing.

The forgeability of steel is closely related to its chemical composition and plasticity at high temperatures. There are many factors affecting forgeability. Let's study the forgeability of various types of steel used for different purposes respectively.

 

NO2. Classification of Alloy Steels

Alloy steels can be classified according to their uses into: alloy structural steels, alloy tool steels, and steels with certain special physical properties.

1. Alloy structural steels: Usually contain a small amount of carbon (0.1~0.2%C) and 1~2% alloy elements. Most of these steels have high plasticity. Low-alloy structural steels have the same deformation resistance at high temperatures as ordinary carbon steels and can withstand large plastic deformations. However, during the initial forging of steel ingots, since the cast structure has not yet been broken, the first few forgings should be done lightly to prevent cracks at the grain boundaries and the exposure of subsurface bubbles to the surface for oxidation and inability to be forged together. When a certain deformation amount is reached, the hammer force should be increased rapidly to forge to the required size.

Nickel-chromium structural steels have a very serious problem of adhering oxide scales on the surface, especially the low-carbon ones, sometimes very thin and tough oxide scales appear. During forging, it is necessary to pay attention to their removal. On the one hand, it affects the surface quality of the forged part, and on the other hand, it increases the difficulty of heat treatment and cutting processing.

2. Alloy tool steels: They are divided into stamping tool steels, cutting tool steels, and measuring tool steels, etc. Stamping tool steels contain 0.7~2.0% carbon and 1~10% alloy elements. This type of steel has high hardness and sufficient plasticity.

Cutting tool steels contain 0.7~2.0% carbon and 1~25% alloy elements. This type of steel has extremely high hardness and good wear resistance.

The forgeability of this type of steel is relatively poor because it contains a high amount of carbon and a large number of alloy elements, which increases the deformation resistance of the steel and reduces its plasticity. Therefore, when forging this type of steel, it is necessary to strictly control the forging temperature. When the forging temperature is too high, it is easy to produce "overheating" and "overburning" defects; when the final forging temperature is too low, the plasticity drops sharply and cracks occur. The forging temperature range is very narrow, and rapid and agile operation is required.

3. Steels with certain special physical, chemical, and mechanical properties (such as stainless steel, heat-resistant steel, acid-resistant steel, heat-strength steel, and diamagnetic steel):

The alloy element content in this type of steel can be as high as 30% or more. Due to the high content of alloy elements, the plasticity of these steels is reduced to varying degrees. At the same time, the mechanical properties and other performance requirements for this type of steel are relatively high. Therefore, it is very important to reasonably formulate and strictly follow the forging process regulations.

For example, the forging process of diamagnetic retaining rings is relatively complex. The forging ratio and deformation process requirements are also very strict. Some countries use cold deformation after hot forging to increase the strength of the retaining ring, while China uses hot deformation followed by semi-hot deformation and other processes.

 

II. Classification by microstructure after normalizing (see Appendices 1 and 2):

There are pearlite steel, martensite steel, austenite steel, ferrite steel and ledeburite steel (or cementite).

1. Pearlite steel:

It contains a small amount of alloying elements (total less than 5-7%), and different amounts of carbon.

The primary crystallization and dendritic segregation of pearlite steel are relatively weak and can disappear even with a small deformation. This is because the primary crystal shell is very thin and can be eliminated by heat treatment. Therefore, dendritic segregation has no obvious harmful effect on the forging process of this steel.

However, it must be pointed out that this is only completely correct when the steel has high purity and good deoxidation. When the content of non-metallic oxides and sulfides and other inclusions is high (they often concentrate at the grain boundaries), the plasticity of the steel is greatly reduced and cracks occur.

Therefore, the plasticity of pearlite steel is largely determined by the exclusion of gases and the degree of deoxidation, as well as the reduction of the content of inclusions.

2. Martensite steel:

It contains a large amount of alloying elements (total more than 10-15%), with a carbon content of less than 0.5-0.7%. When the standard sample is cooled in air, it can be completely quenched into martensite structure.

This type of steel has high deformation resistance and low plasticity, and the primary crystallization is difficult to break. The eutectoid transformation temperature of this type of steel is lower. Therefore, heat treatment cannot change its original structure or break the primary crystal shell. When forging this type of steel, attention should be paid to the forging temperature and deformation. Improper handling can easily lead to scrap.

3. Austenite steel:

It contains a large amount of alloying elements that expand the austenite zone, that is, elements that expand the γ zone. These elements include: carbon, nitrogen, copper, nickel, manganese and cobalt. There is no critical transformation point for austenite. It remains in the austenite state during heating.

The primary crystals of austenite steel are very stable and cannot be eliminated by heat treatment. Only with a considerable degree of deformation can the primary crystals be broken. Therefore, pressure processing plays a significant role in improving the properties of this type of steel.

4. Ferrite steel:

It has a relatively high content of alloying elements and a low carbon content (not more than 0.2-0.4% C). This type of steel contains a large amount of alloying elements that increase A₃ and decrease A₄, that is, elements that narrow the γ zone. These elements include: silicon, phosphorus, aluminum, molybdenum, tungsten and titanium, etc.

This type of steel also has no allotropic transformation. It remains in the α-iron state before melting. Therefore, general heat treatment methods cannot change the original cast structure or dendritic segregation of this steel or improve its mechanical properties.

The dendritic segregation of ferrite steel (stainless steel, heat-resistant steel) is not very obvious and has little effect on plasticity. The primary crystals are easily replaced by recrystallization after deformation.

However, ferrite steel is prone to form ring-shaped cracks during solidification and cooling. To improve the plasticity of high-chromium and nickel-chromium ferrite steel, nitrogen can be added to the steel to refine the grains and eliminate dendritic segregation.

5. Cementite steel:

This type of steel has a high carbon content and contains a large amount of elements that form cementite, such as titanium, vanadium, tungsten, molybdenum and chromium. This type of steel often forms ledeburite during casting, so the primary crystallization of this type of steel is also very stable. Heat treatment can hardly break the network distribution of ledeburite. To reduce dendritic segregation and improve plasticity, it is necessary to ensure that the cast ledeburite is broken and the cementite is evenly distributed throughout the metal volume. For example, the primary crystallization and dendritic segregation of nickel-chromium ledeburite steel (X13H7C2, X18H25C2) break very slowly during deformation. Even after the forging ratio reaches 7-9, this structure can still be seen at the cross-section. Pre-heat treatment before forging can partially break the ledeburite network of the primary crystals and improve the structure after forging. However, due to the high carbon content and the formation of complex carbides with alloying elements, the steel becomes brittle, making the forging of this type of steel very difficult. High-speed steel and X12 steel are more typical.

Therefore, based on the ease of formation of primary crystals and dendritic segregation, all alloy steels can be classified into two categories:

The first category: In the microstructure of the steel, the primary crystals of pearlite and austenite can be easily broken during heat treatment or hot working. This includes all pearlitic structural steels, some tool steels, pearlitic stainless steels and heat-resistant steels, etc.

The second category: The microstructure of the steel contains stable ferrite, carbides and ledeburite networks. The primary crystals of this type of steel cannot be broken by heat treatment and are even difficult to break during pressure working. All ledeburite steels, martensitic steels, austenitic steels (stainless, heat-resistant and acid-resistant), high-speed steels and alloy tool steels belong to this category.

 

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