2. Causes of mixed crystal
There are two basic reasons for mixed crystals:
A. Segregation of alloying elements (unequal distribution of alloying elements)
B. Critical deformation (deformed steel, altered grain)
Focusing on these two reasons, part of the steel casting.
1. Steel Casting:
It is well known that the steel smelting process involves the use of iron ore and various raw materials to eventually produce molten steel through a complex series of steps, which is then cast into ingots. Steel mills are the most professional in the molten steel forming process, and since molten steel in the liquid phase is naturally uniform, no evaluation will be carried out here. Instead, we will focus on the process of solidifying liquid steel into ingots.
Ingot segregation is the most common form of segregation.
In simple terms, the cause of segregation in the ingot is simple – alloying elements tend to solidify first during the solidification process, while areas with fewer alloying elements solidify later, causing an uneven distribution of alloying elements.
The most typical metallurgical structure produced by this process is dendritic segregation.
There are also impurities present, such as slag inclusions that accumulate in a specific location during the rolling and cooling process of the molten steel.
The main problem with segregation is that it results in an uneven distribution of alloying elements, including carbon, chromium, nickel, molybdenum, aluminum and others.
This uneven distribution forms separate regions with distinct chemical compositions, each of which can be considered a distinct type of steel.
At this stage, these regions can be identified as separate grains, distinct from mixed grains. Is this clearer now?
2. Steel Rolling:
Rolling is a process that transforms an ingot into its final form, such as a bar, plate, wire rod or steel profile.
The steel ingot is first reheated and then subjected to multiple rolling passes until it meets the desired specifications.
Before rolling, the steel is typically subjected to diffusion annealing.
The purpose of diffusion annealing is to homogenize the alloying elements of the steel. As discussed previously, segregation of alloying elements during solidification results in an uneven composition, leading to potential problems.
To solve this problem, the steel undergoes diffusion annealing at a temperature of approximately 1200°C.
At this temperature, the activity of the alloying elements increases and diffusion occurs within the steel, moving from areas of high concentration to areas of low concentration, thus improving the uniformity of the steel.
At the same time, the steel remains in a solid state and has not yet entered the liquid phase.
Although the alloying elements have moved, they only improve the uniformity of the steel and cannot completely eliminate segregation.
Rolling is similar to forging and extrusion processes.
During this process, the steel goes through heating, forging, extrusion, cooling, recrystallization, annealing, re-extrusion, among other procedures.
Some defects in the original steel are gradually reduced during this process, and the degree of segregation of alloying elements also decreases.
In an ideal scenario, these processes eliminate defects and reduce segregation, but in reality, steelmakers prioritize cost savings and efficiency.
To achieve this, they may decrease the diffusion annealing temperature and time, skip the annealing process, or increase the forging rate during the rolling process, which can cover up, but not eradicate, defects and lead to a further degree of greater segregation during the lamination process. .
This change is described in detail in the next section.
3. Deformation problems (forging, extrusion):
After receiving the steel, the mechanical processing plant typically uses hot forging and cold extrusion methods to form the part, and then completes machining, heat treatment and grinding to produce the final product.
However, problems may arise during this process.
Hot forging is similar to the steel rolling mentioned in section 2, but with different equipment, compression ratios, and product structures.
Cold extrusion, on the other hand, uses the toughness of steel to produce plastic deformation without heating.
Both processes involve plastic deformation problems.
The toughness of a metal refers to its ability to undergo deformation, which is typically thought of as its ability to stretch or compress.
The better the resistance, the greater the stretching capacity and the lower the compression capacity.
What happens to the grains during tension or compression? Consider a rubber band.
Initially, if the diameter of an elastic band is 10 mm and it is stretched 10 times its length, what happens to its diameter?
It certainly doesn't reach 1mm, but to illustrate the problem, everyone knows that it gets thinner. If you continue to stretch it, it will become even thinner until it breaks.
Metal deformation is a grain change process.
Before deformation, the grains are irregularly shaped, but are essentially ball-shaped.
With the application of external force, the grains are stretched like a rubber band and their space is compressed. They become thinner and thinner as the external force continues.
They were once a pile of potatoes, but now they have turned into a bundle of wheat stalks.
During this process, the grains appear to be unchanged, but upon closer inspection, their size has become much smaller.
It is important to keep in mind that appearances can be deceiving and not to be fooled by what is seen on the surface.
4. Heat Treatment:
Heat treatment is an intermediate process that cannot be seen or touched and cannot be immediately detected or adjusted during the process.
The condition of the product can only be determined through process control and final inspection.
However, the problems that arose from all the previous processes become apparent during heat treatment.
The heat treatment process requires carburizing and quenching to be heated above the austenitizing temperature of the steel.
Therefore, the part must be heated to a temperature above AC3 for operation to occur. During this process, several important changes occur.
The body-centered cubic ferrite network transforms into the face-centered cubic austenite network, and the amount of dissolved carbon, incorporated alloy elements, and the diffusion of alloy elements occur during this process.
The boundaries between grains are also broken and recrystallized, causing the original grains to shift and be rearranged.
The cereal recombination process is essentially an energy competition, similar to the current international situation, where the size of a country is determined by its high technology, nuclear weapons and combat capabilities.
The stronger a country is, the larger it becomes, while a weaker country is more prone to fragmentation.
Likewise, carbides formed by alloying elements act as strongholds on the grains, hindering their growth.
In areas where alloying elements are scarce, they are unstoppable and their territory expands, causing grains to grow.
To ensure the desired deformation size of the product, the heating temperature must be maintained at a moderate level to restrict the diffusion behavior of alloying elements.
If the heating temperature is too high, it may result in phase transformation failure, and if the temperature is too low to achieve the required austenitization, it will also lead to phase transformation failure.
Therefore, the heating problem in heat treatment requires medium temperature heating, which is highly restricted.
Typically, the carburizing temperature is around 900-940°C and the quenching temperature is 30-50°C above the AC3 temperature.
These are textbook values, and now let's consider the possible consequences of the previous sections at these temperatures.
The. Effect of segregation of alloying elements:
As the austenitization process progresses, different areas exhibit varying contents of alloying elements, leading to different austenitization temperatures in those areas.
Even if the part reaches the same temperature, some areas have already started the austenite transformation while others are still in the preparation phase.
Some areas have already completed the austenite transformation, while others have not yet, resulting in continued grain growth in areas that have already transformed to austenite and fine grain growth in areas that have not yet completed the transformation.
If austenitization is stopped at this point and cooled rapidly, a coexistence of large and small grains will occur and, in severe cases, mixed grains may form.
Most alloying elements such as V, Ti, Nb, etc. hinder grain growth.
Alloying elements such as Cr, Mo, W, etc. will delay the formation of austenite.
These elements can affect grain size and play a role in grain refining.
On the other hand, there are some elements, such as Mn and P, that can promote grain growth.
If these elements segregate seriously in the steel, mixed crystals may occur.
B. Influence of deformation during rolling, forging and cold working:
During the pulling and extrusion process, the grains are deformed, which reduces the energy at the original grain boundaries.
As the heating temperature increases, the grains will recombine when the recrystallization temperature of the steel is reached.
At this time, the energy of the alloying element becomes larger and two adjacent fine grains can easily fuse together.
Elements that were previously restricted to movement within a single grain can now break through the boundaries of the two grains and take a shortcut to merge the two fine grains into one large grain in a very short time.
As the temperature and heating time continue to change, these grains continue to grow until there is no energy to break through the grain boundary restrictions.
At this point, many large grains have formed.
However, not all deformed grains reach the critical strain required for growth. This results in some normal grains, leading to the formation of mixed grains.
C. Effect of temperature:
The forging process and the temperature and time of the heat treatment process have a significant impact on the grain.
When the temperature is high and the waiting time is long, the grains will grow.
This temperature limit depends on the material, and different materials have different temperature limits.
Heat treatment temperatures are generally fixed and the typical carburizing temperature does not exceed 950°C.
At this temperature, most fine-grained steels do not undergo significant changes.
However, excessive temperature due to parameter errors or inaccurate temperature measurement can result in coarse-grained steel.
The grains produced by high-temperature forging are coarse, and a Widmanstatten structure is often found in metallography after forging.
Widmanstatten can be eliminated by normalizing several times, and the grain change caused by temperature can be compensated by normalization.
However, it is generally not recommended to use a Widmanstatten structure if it appears in reality.
3. Summary:
In short, the main reason for mixed crystal is element segregation, and it is challenging to eliminate this through heat treatment in downstream processes.
Furthermore, it is essential to pay attention to grain size during any process that produces deformation.
The particle size mixture caused exclusively by deformation can be improved through heat treatment.
However, if the grain has already grown and stabilized, and alloying elements have precipitated at the grain boundary, it may be difficult to resolve the problem.
