A detailed explanation of the transformations in steel during heating

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Transformations of steel during heating: In most heat treatment processes for steel, it is first heated to obtain austenite, and then cooled at different rates to cause the austenite to transform into various microstructures, thereby giving the steel different properties. Therefore, to master the laws of heat treatment, it is first necessary to study the changes that occur in steel during heating. I. The formation process of austenite during heating 1. Heating transformation of eutectoid steel As can be seen from the iron-carbon phase diagram, when steel is heated to a temperature above 727°C (the PSK line on the phase diagram, also known as the A1 temperature), pearlite transforms into austenite. This heating rate is very slow; the actual heating rates used in heat treatment are all higher than this slow rate. The temperature at which pearlite actually transforms into austenite is higher than A1, and this actual transformation temperature is defined as Ac1. Ac1 being higher than A1 indicates the presence of thermal lag; the faster the heating rate, the higher Ac1, and at the same time, the shorter the time required for the transformation from pearlite to austenite. Eutectoid carbon steel (with 0.77% C) has a pearlitic structure before heating; it is generally a lamellar structure in which ferrite phases and cementite phases are arranged alternately. During heating, the austenitization process can be divided into four steps, as shown in Figure 6-2. 2. Heating transformation of hypoeutectoid steel: First stage – formation of austenite nuclei. According to the Fe-Fe3C phase diagram, at temperature A1, ferrite contains about 0.0218% C, cementite contains 6.69% C, and austenite contains 0.77% C. During the transformation of pearlite to austenite, the original ferrite reorganizes from a body-centered cubic lattice into the face-centered cubic lattice of austenite, while the original cementite changes from a complex rhombohedral lattice to a face-centered cubic lattice. Therefore, the heating transformation of steel involves both the diffusion of carbon atoms and changes in the crystal structure. Based on energy and composition conditions, austenite nuclei form at the interface between the ferrite and cementite phases of pearlite (see Figure 6-2(a)); the more such phase interfaces there are, the more austenite nuclei will be formed. Phase 2: Growth of austenite. After the formation of the austenite nucleus, one side of it is in contact with cementite, while the other side is in contact with ferrite. As the transformation of ferrite occurs (with the reduction of the ferrite region), and as cementite dissolves (with the reduction of the cementite region), austenite continues to expand outward into the surrounding pro-ferrite region and cementite region, until all ferrite disappears and the austenite regions meet each other, forming individual austenite grains. Stage 3: Dissolution of residual cementite. Since the rate of transformation of ferrite into austenite is much higher than the rate of dissolution of cementite, a significant amount of undissolved \"residual cementite\" remains after complete transformation of the ferrite (see Figure 6-2(C)). A certain amount of time is required to maintain the temperature at that level so that all the cementite can dissolve. Stage 4: Homogenization of the austenite composition. Even if all the cementite dissolves, the composition within the austenite remains uneven: the austenite formed in the former ferrite regions has a lower carbon content, while that formed in the former cementite regions has a higher carbon content. A sufficient holding time is required for carbon atoms to diffuse fully so that the composition of the austenite can become uniform. The above analysis shows that for pearlite to transform into austenite and to homogenize the austenite composition, there must be two necessary and sufficient conditions: one is the temperature condition, which requires heating above Ac1, and the other is the time condition, which demands staying at a temperature above Ac1 for a sufficient duration. Under certain heating rates, the higher the temperature above Ac1, the shorter the time required for austenite formation and composition homogenization ； At a certain temperature (above Ac1), the longer the holding time, the more uniform the austenite composition becomes. It should also be noted that the change of austenite grains from small to large sizes is a spontaneous process; at certain heating temperatures above Ac1, an excessively long holding time will lead to the merging of austenite grains, resulting in an increase in their size. In contrast, when heated for the same duration, a higher heating temperature results in a significantly greater tendency for an increase in austenite grain size compared to the grain growth tendency at lower heating temperatures. Excessively large (or coarse) austenite grain sizes often lead to a reduction in the strength of steel after heat treatment. In engineering applications, it is desirable to have fine and uniformly composed austenite grains; to achieve this, one approach is to select the lowest possible austenitization temperature while ensuring uniformity in the austenite composition ； The second approach is to rapidly heat to a relatively high temperature and hold it for a short period, so that the formed austenite does not have time to grow, followed by cooling to obtain fine grains. In engineering, the size of austenite grain dimensions is defined as grain size, which is classified into 8 grades. Grades 1 to 4 correspond to coarse grains, grade 5 and above to fine grains, and grades above 8 to ultra-fine grains. The process of pearlite in hypoeutectoid and hyper-eutectoid steels transforming into austenite upon heating is the same as that in eutectoid steels; that is, when heated above the Ac1 temperature, pearlite in both hypoeutectoid and hyper-eutectoid steels transforms into austenite. The difference lies in the transformation of ferrite in hypoeutectoid steel and the dissolution of secondary cementite in hyper-eutectoid steel. More importantly, the complete transformation of ferrite must occur above the A3 temperature (the GS line on the Fe-Fe3C phase diagram); taking heat lag into account, it actually needs to be above Ac3. The complete dissolution of secondary cementite requires a temperature above Acm (the ES line on the Fe-Fe3C phase diagram), and considering heat lag, this temperature should be above Accm. That is, for hypoeutectoid steel, the entire microstructure must be austenite after heating, which requires a temperature above Ac3; for hyper-eutectoid steel, the temperature must be above Accm. If hypoeutectoid steel is still heated only between the Ac1 and Ac3 temperatures, regardless of the heating time, the microstructure after heating will remain a combination of ferrite and austenite. When hypereutectoid steel is heated between the Ac1 and Accm temperatures, the resulting microstructure should consist of secondary cementite coexisting with austenite. The microstructural transformations during the cooling process after heating are merely conversions of austenite into other structures; the ferrite and secondary cementite do not undergo any transformation during cooling. Common defects that occur when steel is heated: (1) Oxidation. The oxidizing atmosphere during heating (such as air, O2, CO2, H2O, etc. in the atmosphere) oxidizes the steel, resulting in the formation of oxides such as FeO, Fe2O3, and Fe3O4 on the surface of the workpiece. Below a temperature of 560°C, relatively dense oxides such as Fe3O4 are primarily formed; these oxides can isolate the steel surface from oxidative atmospheres, preventing further oxidation of the steel surface. However, the austenitization temperature of steel is usually above 560°C; as a result, the steel oxidizes to form a loose oxide layer primarily composed of FeO. As the heating temperature increases and the heating time lengthens, the thickness of this oxide layer grows. This not only leads to increased wear of the steel but also causes the dimensions of the parts to decrease and their surfaces to become rougher. More importantly, it severely affects the quality of subsequent heat treatment processes. (2) Decarburization: The decarburization of steel occurs during its heating process, that is, the carbon in the steel is burned away, resulting in a decrease in the carbon content on the surface of the steel. Decarburization often occurs alongside oxidation; an oxidizing atmosphere is also a decarburizing atmosphere. Although H2 is a reducing atmosphere, it is also a decarburizing atmosphere. Generally, the higher the carbon content in steel, the more severe the decarburization. Due to decarburization, the carbon content on the surface of the steel parts decreases, which leads to a reduction in their mechanical strength, particularly in terms of fatigue strength, as well as a decrease in their wear resistance. (3) Overheating: Overheating of steel refers to a heating temperature that is higher than the normal level; as a result, the austenite grains in the steel become larger than normal, that is, the grains grow coarser. As a result, the plasticity, toughness, and strength of the steel decrease; at the same time, the deformation of the workpiece increases after heat treatment, which may also lead to heat treatment cracks and render the workpiece unusable. Overheated workpieces can generally be reheated at a lower temperature to refine the austenite grains again as a remedy. (4) Overheating refers to the phenomenon where the heating temperature is too high, causing the austenite grain boundaries or some of them to oxidize or even melt. As a result, the workpieces processed become very brittle; if they crack after just one forging step, the over-heated workpieces must be discarded as they cannot be saved, which is fatal. Methods to prevent heating defects: (1) Vacuum heating. Heating the workpiece in a vacuum is the most effective way to prevent oxidation and decarburization; it represents the trend in the development of heat treatment processes and is widely used in developed countries. The problem, however, is that the equipment required for vacuum heating is expensive, resulting in high processing costs. (2) Controlled atmosphere heating: During the heating of the workpiece, a certain protective atmosphere is introduced into the furnace to ensure that the steel is heated in an atmosphere that prevents decarburization, carburization, and oxidation. Practice has shown that it is an effective and reliable method; it is also a technique widely used in developed countries, and it represents one of the trends in the development of modern heat treatment. However, a generation device for creating a controllable atmosphere is required, and its application is limited by high costs as well as the limited availability of raw materials. (3) Salt bath heating: The workpiece is placed in a melted neutral salt solution for heating; the salt solution ensures thorough deoxidation, thereby minimizing or even eliminating oxidation during the heating process of the workpiece. The main problem is that the salt adhering to the workpiece is difficult to clean thoroughly; incomplete cleaning can lead to rust forming easily during storage and use. Furthermore, during operation the salt solution can explode when it comes into contact with water, and this may cause burns to the human body if not handled carefully; great attention must be paid to safety.