Knowledge

NO3. The Influence of Alloying Elements on the Plasticity of Alloy Steels

 

Most alloy steels can be processed by pressure, that is, they have plasticity. However, with the increase of alloying elements, the deformation resistance generally increases, the plasticity decreases, and the temperature range of plasticity shortens, causing difficulties in pressure processing. The following studies the influence of various alloying elements on the plasticity of steel respectively.    

                                          

Carbon (C)

Carbon is an element commonly present in steel. As the carbon content increases, the steel's resistance to deformation rises and its plasticity decreases. However, steel with a carbon content as high as 1.4% C can still be rolled and forged smoothly. Steel with a carbon content as high as 2.2% C can only be rolled and is very difficult to forge, to the extent that it is practically impossible.

For example, in Soviet welding factories, 9V20 steel is used, with the following chemical composition: 1.8-2.2% C, 0.35% Si, 0.35% Mn, 0.015% S, 0.03% P. Ingots weighing 200 kilograms can be rolled normally at 1000°C. There are no particular difficulties in rolling 100×100 mm steel billets of the same composition. The influence of alloying elements on the oxidation resistance of steel is related to the carbon content in the steel, that is, to the relationship between these alloying elements and carbon. Many alloying elements added to steel may form carbides or dissolve in ferrite and austenite. When the content of alloying elements formed in steel is not high, these elements will dissolve in ferrite or austenite, forming carbon-containing solid solutions. Other alloy forms are Fe₃M₄C, where M represents a certain alloying element.

When the content of alloying elements that promote carbide formation is high, complex carbides are formed. For example, when the chromium content is greater than 2%, Fe₃Cr₃C or Fe₄Cr₂C is formed; when the chromium content is greater than 10-12%, Cr₇C₃ is formed. If tungsten and molybdenum are present in the steel, Fe₃W₃C or Fe₄W₂C or Fe₄Mo₂C is formed. These formed Fe-Cr or Cr-C compounds, as well as tungsten carbide and molybdenum carbide, need to be heated to a temperature significantly above the critical point to dissolve into austenite. All carbides are characterized by high hardness, brittleness, high melting point (such as WC), and low plasticity. The presence of carbide particles in the steel structure has a significant impact on the wear resistance of tool steel. Because the changes in the steel structure are closely related to the precipitation and coagulation of carbide particles. Some steels have carbide segregation - that is, the local accumulation of a large amount of carbides, similar to the presence of non-metallic inclusions in steel. In tool steel, it can cause blade peeling during use, and it is even more unacceptable in bearing steel.

Due to the above reasons, alloy steels with high carbide content are more difficult to forge. Generally, forging such steel requires a large deformation amount and a forging hammer with sufficient striking energy. Because the uniformity of the distribution of carbide particles mainly depends on the degree of deformation, striking energy, forging method, as well as strict control of the starting forging temperature and cooling method.                                                

 

 

Manganese (Mn)

Manganese is one of the most abundant elements in China, so how to utilize manganese to smelt various new steel grades is a very realistic issue. In 1957, China was the first to forge steel and produce 25MnSi high-strength reinforcing bars through large-scale melting, and mastered the production process of 16 manganese. In addition, it also carried out the formulation work of 16MnAl and 16MnAlTi, etc.

Manganese is a component of carbides and forms Mn₃C carbide with carbon, which is more stable and stronger than iron carbide (cementite).

There are no pure carbides of manganese in steel, but rather complex carbides of the FeMn₃C type.

Manganese can improve the plasticity of steel because it combines with sulfur in steel to form MnS, which replaces the thermal brittleness phenomenon of FeS. Manganese sulfide has a higher melting point (1620℃) and exists in the form of spherical particles in steel, unlike iron sulfide (FeS, melting point 988℃) which is distributed in a network form at the grain boundaries. Manganese sulfide does not cause thermal brittleness at high temperatures.

 

Manganese is highly sensitive to overheating in steel. According to the data from Obele and Goffel, manganese has no significant effect on the forgeability of pearlitic steel (with carbon content of 0.2% C and manganese content less than 0.5% Mn, and with carbon content of 0.8% C and manganese content greater than 12%). The thermal conductivity and critical transformation point of austenitic manganese steel (with carbon content of 0.2% C and manganese content greater than 12% Mn, and with carbon content of 0.8% C and manganese content greater than 7% Mn) are reduced. When the forging temperature of manganese is too high, coarse-grained structure is formed, making forging difficult (degrading forgeability). Conversely, when the forging temperature is lowered, fine-grained structure appears, improving forgeability.    

                                          

Nickel (Ni)

Nickel has a strong ability to absorb gases during the smelting of steel, especially hydrogen. When the hydrogen content in steel is too high, it will cause a large number of bubbles. At the same time, when the primary crystals grow coarsely, cracks will form along the grain boundaries. These two defects are the causes of forging cracks.

The effect of nickel is opposite to that of manganese. Nickel can promote the distribution of sulfides in a pasty form along the grain boundaries. Therefore, in nickel steel, when the sulfur content increases, cracks will occur during forging or rolling. In high-nickel steel, heating in a furnace gas containing sulfur is most undesirable. The formed sulfides penetrate into the grain boundaries of the steel, causing surface cracks during forging or rolling.

In high alloy steel, the higher the nickel content, the worse the surface quality of the steel after forging. Nickel is an element with weak affinity for oxygen. Due to the strong oxidation of iron and the local increase of nickel on the surface of the steel, the surface layer becomes red and brittle.                                                  ​​​​​​​

Chromium (Cr)

Chromium is a silvery-white metal with a slight bluish tint, featuring high hardness and a melting point of 1825°C. It is a strong carbide-forming element. In pearlitic steel, carbides do not start dissolving into the solid solution until above the A₃ point temperature. In martensitic steel, carbides do not begin to decompose until 1200°C. In high-carbon and high-chromium steels, carbides are particularly stable and only dissolve in liquid alloys.

Chromium promotes the formation of large grains (columnar crystals) in ferritic manganese steel, which is mainly characteristic of austenitic steels (containing high chromium and high nickel-chromium). During cooling, internal shrinkage cracks form along the coarse grain boundaries. Such shrinkage cracks are often found in a large portion of low-carbon steels.

High-alloy steels, such as X12 steel, can be quenched when cooled in air, or chromium-nickel steels (3-4% Ni and 1-1.5% Cr) often develop surface cracks. Therefore, forging such steels is very difficult and prone to scrapping.

 

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