In the process of heat treatment, fracture failure occurred, and it was found through inspection that the reason was the existence of mixed crystals.
In fact, in the mechanical manufacturing industry, mixed crystals are often found during metallographic observation after the heat treatment process.
Today, let’s talk about mixed crystals.
1. What is mixed crystal
Mixed crystal, as the name implies, is a mixture of different grain sizes.
Grain size is an indicator of the toughness of metal materials.
The higher the grain size is, the higher the material toughness is. On the contrary, the worse the toughness is.
If low-grade grains are mixed in the area of high-grade grains, the overall performance of the metal material will be reduced.
The larger the proportion of low-grade grains, the more unstable the overall performance of the material.
Generally, we consider that mixed grains occur when grains with different grain sizes of 3-4 grades appear in the structure at the same time.
When the proportion of large grains exceeds 10%, we have to pay attention to the early failure of mechanical parts caused by large grains.
The unstable factors caused by mixed crystal make it impossible to predict when the mechanical parts may fail, so mixed crystal is very unpleasant for heat treatment workers.
The metallographic picture of mixed crystal is shown in the following figure:
2. Causes of mixed crystal
There are two basic reasons for mixed crystals:
A. Alloying element segregation (uneven distribution of alloying elements)
B. Critical deformation (steel deformed, grain changed)
Focusing on these two reasons, it starts from steel smelting.
1. Steel smelting:
We all know that the steel smelting process is to use iron ore and various raw materials to finally smelt into molten steel through complex processes, and then cast to form ingots.
The steel plant is the most professional in the formation process of molten steel, and as the liquid phase molten steel, the degree of uniformity is natural, so no evaluation will be made here.
We only talk about the process of liquid steel condensing into ingots.
Ingot segregation is the most common form of segregation.
In popular terms, the reason for segregation in the ingot is very simple, that is, the alloy elements are easy to solidify first in the solidification process, while the places with few alloy elements will solidify later, which results in uneven distribution of alloy elements.
The most typical metallographic structure is dendrite segregation.
There are also some impurities. Slag inclusion will gather at a specific location with the rolling cooling process of molten steel.
The biggest problem of segregation is that it will lead to uneven distribution of alloy elements.
That is to say, it will lead to uneven distribution of carbon, chromium, nickel, molybdenum, aluminum and other alloy elements.
The uneven distribution of these elements will form relatively independent small regions with different chemical compositions.
Seriously, each area is a steel type.
At this stage, we can only see such grains, which is different from mixed grains. Are you still confused?
2. Steel rolling:
Rolling is a process from ingot to steel delivery.
The steel ingot becomes the bar, plate, wire rod, section steel, etc.
We need to reheat the steel ingot, and then pass N pass rolling to finally meet our needs.
Before rolling, the steel is generally subject to diffusion annealing.
The purpose of diffusion annealing is to homogenize the alloy elements of the steel.
As mentioned in the first paragraph, the segregation of alloy elements during solidification leads to uneven composition.
Such steel will have a big problem. Therefore, through diffusion annealing, the temperature is about 1200 ℃.
At this time, the activity of alloy elements increases, the range of activities increases, and diffusion movement will occur inside the steel, that is to say, running from the place with high concentration to the place with low concentration can improve the uniformity of steel.
At the same time, after all, the steel is still solid and has not become a liquid phase.
Although the alloy element has moved, it only improves the uniformity of the steel, which cannot be completely eliminated.
The rolling process is equivalent to the forging and extrusion process.
In this process, the steel has undergone heating, forging, extrusion, cooling, recrystallization, annealing, re extrusion and other processes.
Some defects of the original steel are gradually reduced in this process, and the degree of segregation of alloy elements is also gradually reduced.
If these two processes are perfect, there will be no problems behind them, but the reality is cruel.
In order to save costs and reduce costs, the steel plant will improve production efficiency in these two links, which may lead to insufficient diffusion annealing temperature and time, omitting annealing process and increasing forging ratio in the rolling process to improve production efficiency.
In this way, the defects of raw materials may be covered up, but they have not been eradicated, and even become more and more severe during the rolling process.
The reason for this change is described in 3.
3. Deformation problems (forging, extrusion):
After receiving the steel, the mechanical processing plant will generally adopt the hot forging and cold extrusion methods to make the workpiece preformed, and then finish machining, heat treatment and grinding to finally make the finished product.
In this process, the problem arises.
In fact, heating forging is the same as steel rolling mentioned in 2, except that the equipment is different, the compression ratio is different, and the product structure is different.
Cold extrusion is a process that uses the toughness of steel itself to produce plastic deformation without heating.
Both of these processes involve a problem of plastic deformation.
We know that the toughness of a metal refers to its deformation ability. Generally speaking, it refers to its ability to stretch or compress.
The longer it can be stretched, the better its toughness. The shorter it can be compressed, the better its toughness.
What happens to the grains during tension or compression? Let’s think about rubber bands.
At first, if the diameter of the rubber band is 10mm, if you stretch it 10 times its length, what is its diameter?
It is certainly not 1mm, but to illustrate the problem, everyone knows that it will become thinner. Continue to pull, and it will become thinner until you break it.
The deformation of metal is the process of grain change.
Although the shape of the grains before deformation is irregular, everyone is still at peace, basically in the form of a ball.
With the arrival of external force, everyone is pulled like a rubber band, and the living space is squeezed. They can only become thinner and thinner with the external force.
It used to be a pile of potatoes, but now it has become a bunch of wheat.
In this process, we are still at peace, not much happens, and even you will be surprised to find that the grain size is super good.
From the thickness of potatoes to the size of wheat stalks, it is indeed much thinner.
But we should keep our eyes open and not be confused by appearances.
4. Heat treatment:
As an intermediate process, heat treatment can not be seen or touched, can not be detected immediately, and can not be adjusted during the process.
The product status can only be determined through process control and final inspection.
The problems arising from all the previous processes have all broken out in the heat treatment.
The carburizing and quenching processes of heat treatment are required to be heated above the austenitizing temperature of steel.
Therefore, the workpiece has to be heated to a temperature above AC3 for operation. In this process, many wonderful changes have taken place.
The ferrite lattice of body centered cubic is transformed into the austenite lattice of face centered cubic, and the amount of dissolved carbon, the amount of alloy elements incorporated and the diffusion of alloy elements will all occur in this process.
The boundary between grains will also be broken and recrystallized. The original grains will change and the grains will be reorganized.
The process of grain recombination is simply a process of energy competition, which means bullying.
Just like our current international situation, high-tech, nuclear weapons, and combat capabilities determine the size of a country.
The stronger the country is, the bigger it is. The smaller its capacity is, and the easier it is for a country to split.
Carbides formed by alloy elements are like strongholds in these countries, hindering the growth of grains.
On the other hand, in places where alloy elements are scarce, wherever they go, they are invincible, their territory is expanding, and grains are growing.
In order to ensure the deformation size of the product, the heating temperature should not be too high, which will lead to the restriction of the diffusion behavior of alloy elements.
If the heating temperature is too low to meet the austenitization, it will lead to the failure of phase transformation.
Therefore, the heating problem of heat treatment belongs to medium temperature heating, which is greatly restricted.
Generally, carburizing temperature is about 900-940 ℃, and quenching temperature is generally 30-50 ℃ above AC3 temperature.
These are the data left on textbooks.
Now let’s discuss the possible consequences of 1, 2 and 3 at this temperature.
a. Effect of alloy element segregation:
With the progress of austenitizing, there are different alloy element contents in different areas, which will lead to different austenitizing temperatures in these areas.
On the premise that the workpiece reaches the same temperature, some areas have begun to transform austenite, and some areas are still in the preparation process;
Some areas have already completed the transformation of austenite, while some areas have not yet completed, which will inevitably lead to the continuous growth of grains in those areas that have been transformed into austenite first, while the grains in those areas that have not yet completed the transformation are fine.
If the austenitizing is terminated at this time and the cooling is rapid, the coexistence of large and small grains will occur, and in serious cases, mixed grains will occur.
Most of the alloy elements will hinder the grain growth, such as V, Ti, Nb, etc;
Alloying elements will slow down the formation of austenite, such as Cr, Mo, W, etc;
Such elements will affect the grain size and play a role in refining grains.
At the same time, there are a few elements that can promote grain growth: such as Mn, P, etc.
If these elements segregate seriously in steel, mixed crystal may occur.
b. Influence of deformation during rolling, forging and cold working:
In this process, the grains are deformed due to pulling and extrusion, which makes the original grain boundary exist, but the energy is reduced.
As the heating temperature rises, the grains will recombine when the recrystallization temperature of the steel itself is reached.
At this time, the energy of the alloy element is more and more large, and two adjacent thin grain.
Elements that can only move in the grain originally become easier to break through the two grain boundaries, and they will take a shortcut.
Two thin grains will merge to form a large grain in a very short time.
With the change of heating temperature and heating time, these grains continue to grow until there is no energy to break through the grain boundary constraints.
At this time, many large grains have been formed.
This is not the case for all deformed grains. Only when the grains reach the critical deformation can they grow. In this way, there are some normal grains, so mixed grains are produced.
C. Effect of temperature:
The influence of forging process, heat treatment process temperature and time on the grain is also very large.
When the temperature is high and the holding time is long, the grains will grow.
This temperature depends on the material, and different materials have different limit temperatures.
The temperature of heat treatment is generally fixed, and the conventional carburizing temperature generally does not exceed 950 ℃.
At this temperature, most of the essentially fine grain steels will not change significantly.
However, it does not rule out that excessive temperature caused by parameter error or inaccurate temperature will lead to coarse grain of steel.
The grains produced by forging overtemperature are coarse, and the widmanstatten structure is generally found in the metallography after forging.
Widmanstatten can be eliminated by normalizing several times.
Essentially, the grain change caused by temperature can be compensated by normalizing.
However, it is generally not recommended to use widmanstatten if it appears in reality.
In conclusion, the most important reason for mixed crystal is element segregation, and it is difficult to eliminate it by heat treatment in the subsequent process.
Secondly, for each process that can produce deformation, it is necessary to pay attention to the grain size.
The grain size mixing caused solely by deformation can be improved through the heat treatment process.
If the grain has grown up, and the state is stable, and the alloy elements have precipitated on the grain boundary, it will also be troublesome to handle.