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For carbon steel, under the same conditions, the supercooled austenite of eutectoid steel is the most stable, so the hardenability of eutectoid steel is the best in carbon steel. The further the difference between carbon content and eutectoid steel, the worse its hardenability. Corresponding to the actual steel number, T8 is the best hardenability of carbon steel

Why does quench penetration increase when melting fire temperature increases?

1. the higher the quenching temperature, the more fully the alloying elements are dissolved in austenite, thereby increasing the stability of supercooled austenite.

2. the higher the heat temperature, the obtained austenite grain may be coarser, thereby reducing the nucleation rate of supercooled austenite transformation, and correspondingly increasing the stability of supercooled austenite.

Any factor that increases the stability of supercooled austenite increases hardenability.

1.Why normalize grain refinement? How does it work? 1, first of all, it must be clear that this is the refinement of austenite grains. Normalizing fine grain refers to the original grain is relatively coarse, can be refined by normalizing process, such as hot deformation processing chaos, forging, rolling, forging and other normalization. The original grain is not coarse, normalizing can not be refined.

2. the role of normalizing is not only to refine the grain, there are other roles. Single analysis refining grains, then the austenite grain size at the end of normalizing insulation is basically the effect of normalizing refining grains. Note that it has nothing to do with the cooling rate behind, and the cooling rate of normalizing has an effect on other effects of normalizing.

3. the principle of normalizing grain refinement, mainly the use of appropriate austenitizing temperature "normalizing temperature) and holding time, the coarse grain structure re-austenitizing, re-nucleation and growth, to obtain relatively small austenitic grains.

4. the normalizing temperature and holding time provided on the material are the most conventional. Suitable for all functions of normalizing. There is no need to carry a condom in the actual work. For example, if you want to refine the grain, it is ok to appropriately reduce the normalizing temperature and shorten the holding time. Improve efficiency, reduce costs, achieve the purpose. 

Oh, that's technology. Relationship between carbon content and hardenability:

1. in the scope of the carbon content of the steel we are involved in, carbon belongs to the element that improves the stability of austenite, the carbon content increases, the stability of austenite also increases, the C curve shifts to the right, and the hardenability improves.

2. for hypereutectoid steel, because at the quenching temperature, carbon is not completely dissolved into austenite, and the determination of hardenability is the concentration of carbon in austenite, so the hardenability does not increase (or decrease) with the increase of carbon content

The example above is carbon steel

Effects of carbon content and quenching temperature on MS points:

1. in the range of carbon content of the steel we are involved in, carbon belongs to the element that improves the stability of austenite, and the carbon content increases, and the stability of austenite also increases, so the MS point moves down

2. for hypereutectoid steel, because at the quenching temperature, carbon is not completely dissolved into austenite, and the MS point is determined by the concentration of carbon in austenite, so it appears in the specific work, the actual position of MS changes with the austenitizing temperature.

3. The higher the quenching temperature, the higher the carbon concentration in austenite, and the MS point decreases

4. In addition, the quenching temperature increases, and the austenite grain size increases the MS point

Why normalize grain refinement? How does it work?

It should be clear that the temperature of normalization is in the austenitic region (including complete austenitizing and partial austenitizing temperature region). After the metal is processed to austenitizing, the metal grains undergo a phase transition process (that is, the transition of pearlite to austenite). The austenitizing process includes the nucleation of austenite, the growth of crystal nuclei, the dissolution of carbides and the process of austenitic homogenization. The process of grain re-nucleation and uniform growth is the process of refinement, but it should be noted that not normalizing can refine the grain, which is mainly related to the temperature of normalizing and the washing selection of holding time, the general principle of normalizing is high mixing "meat body temperature interval" short-term preservation source, To determine whether the holding time is appropriate according to the different components of the workpiece is based on the essence of the grain steel, the austenite grain size before cooling is actually the size of the grain after normalization, and the cooling does not affect the grain size. Several key elements for normalizing grain size are: heating temperature and holding time, heating coefficient (also known as heating rate), chemical composition of the metal (mainly alloying elements that promote or hinder grain growth), and the size of the original grain of the metal

Zhangzhou fire temperature increase why quenching increase?

Because the quenching temperature increases, the carbon and alloying elements dissolved into austenite increase, which can increase the stability of austenite, thereby improving the hardenability; Because the quenching temperature increases, the MS point will rise, but also improve the hardenability;

The quenching temperature increases the free energy difference between austenite and martensite, and it is easier to obtain more martensite during cooling.

What are the effects of carbon content and quenching temperature on MS point? Hypocopefold steel, with the increase of carbon content, will make the C curve shift to the right, so that the MS point drops, hypereutectoid steel mainly look at the quenching temperature, because the rate of fire temperature determines the concentration of C dissolved into austenite, so its MS depends on this c concentration.

The higher the quenching temperature, the higher the carbon concentration in the austenite, increasing the stability of the austenite, resulting in a decline in the MS point.

1. The influence of carbon content and quenching temperature on MS point?

For all steels, the MS point rises as the quenching temperature increases. However, the effect of carbon content on MS point is to decrease, which is often said that the C curve shifts to the right.

2. Quenching temperature increase Why quenching penetration increase?

The increase of the rate fire temperature makes the austenite grain coarse, and when the martensitic transformation occurs during cooling, the surface work required is small, so the degree of supercooling is reduced, and the energy required is small, so the quenching is improved.

3. Why normalize grain refinement? How does it work? Normalizing is generally used after tying, forging and other processes, when the grain of the workpiece is relatively large, at this time with normalizing, it can mention the role of refining the grain. The principle is that the heat preservation at high temperature homogenizes the grains, and then the slow cooling can refine the grains, and the principle of the four fires is basically the same.

4. The impact of carbon content on hardenability? Increasing carbon content shifts the C curve to the right, reducing the critical rate and improving hardenability. How can carbon equivalent [C] be used to determine the cooling mode? A: In the process of determining the heat treatment process of large forgings, we can determine whether we need water cooling, water quenching oil cooling or oil cooling according to the carbon equivalent of the workpiece. 

The specific principles are:

1. can completely water quenching conditions C% is less than or equal to 0.31% and [c] is less than or equal to 0.75%

2. can be carefully water quenching conditions c% is less than or equal to 0.32-0.36% and [c] is less than or equal to 0.75-0.88%

3. prohibition of water quenching conditions

c% is greater than or equal to 0.36% month [c] is greater than or equal to 0.88%

Factors affecting quenching deformation

There are many factors that affect quenching deformation, but there are mainly several aspects:

1. steel hardenability

If the hardenability of the steel is good, you can use a relatively moderate cooling rate fire medium, so the thermal stress is relatively small, and the permeability is good, the workpiece is easy to quench, and the role of the organizational stress and specific tolerance effect is relatively large.

2. Chemical composition of austenite

The lower the carbon content in austenite, the greater the effect of thermal stress. This is because the specific volume of low-carbon martensite is small, and the organizational stress is also small. Conversely, the effect of structural stress is greater. The yield strength of steel increases with the increase of alloying element content. In addition, due to the good permeability of alloy steel, the rate fire medium with relatively moderate cooling is generally used, and the rate fire deformation is also small.

3. quenching heating temperature heat heating temperature increase, not only the increase of thermal stress, but also due to the increase of hardenability, also increase the organizational stress, so the deformation is also increased.

4. quenching cooling speed

The greater the cooling rate, the greater the quenching stress and the greater the quenching deformation.

5. Original organization

The original structure mentioned here refers to the structure condition before the rate of fire, which has a wider meaning, including the registration of inclusions in steel, the band structure grade, the degree of component segregation, the orientation of the free carbide particle distribution, and the different structures obtained by different preparatory heat treatment.

Significance:

The significance of MS point and its influencing factors require deep supercooling for martensitic transformation. MS is the starting point of martensitic transformation, that is, the temperature at which the free energy difference between the two phases and M reaches the minimum chemical driving value required for the transformation. In other words, the MS point reflects the minimum degree of supercooling that martensitic transformation can undergo.

Influencing factors:

1. Chemical composition of austenite

2, stress and plastic deformation

3. Austenitizing conditions

4, there is a pre-martensite organizational transformation.

What is the substance of the phase transition?

The alloy has a stable state with lower free energy on each side of the phase transition temperature, and when the alloy crosses this temperature during heating or cooling, it will change from the original stable state to a new stable state, which is the essence of the phase transition.

Why normalize grain refinement? How does it work?

Normalizing process is the process of steel heating, insulation and cooling. Before normalizing, the material structure generally has coarse structure and uneven composition quality problems. Normalizing heating and insulation process is mainly to allow the original organization to carry out sufficient austenite homogenization and stabilization, which is also the general fire temperature is higher than the rear room hot external processing temperature, the austenite particles are large, so the fire heating and insulation can not be directly refined grains, the subsequent air cooling is the key to refine the organization. The greater the air cooling rate, the greater the degree of supercooling of austenite, the more co-folding nuclei and the shorter the precipitation time, the smaller and finer the precipitation amount, the more eutectoid volume, and the more nucleation of eutectoid structure, the more easily the remaining eutectoid composition of austenite is decomposed into multiple pearlite structures, and the smaller the spacing of pearlite sheets, the finer the structure.

What is temper brittleness? How to prevent and eliminate the tempering brittleness of steel?

A: The phenomenon of brittleness increase in the tempering process is called tempering brittleness. When some alloy steels are tempered at 250-400 degrees and 500-550 degrees, there is a reduction in impact toughness, which is the so-called temper brittleness.

As for the causes of temper brittleness, the current research is not enough, and the causes of temper brittleness are generally considered as follows.

Tempered steel at 250 degrees to 400 degrees tempering brittleness is because: first, at 300 degrees tempering, the residual austenite is transformed into martensite, and the horse is hard and brittle, so the brittleness of the workpiece increases: second, at such a temperature, the carbide precipitation from martensite is granular or sheet distributed in the grain boundary, thus reducing the impact toughness of the steel. This type of tempering brittleness is called the first type of tempering brittleness, which is inherent in the tempering process of steel and cannot be eliminated and reduced. Therefore, tempering in this temperature range should be avoided as much as possible during the heat treatment process. If the steel contains C,M,, and other elements, when tempering at a temperature of 500-550 degrees, if the tempering is slowly cooled in this temperature range, it will increase the brittleness. According to some research data, the reason for the increase in brittleness is that when the steel is slowly cooled at 500-550 degrees, the carbides and oxides in the steel are precipitated along the grain boundaries, and the impact toughness of the steel is reduced. This type of temper brittleness is called the second type of temper brittleness. It can be prevented by quick cooling after tempering.

Because fast cooling will bring new stress to the tempered workpiece, a low temperature tempering should be carried out for important parts to eliminate the internal stress caused by fast cooling. If the workpiece is improperly cooled after tempering, and the second type of tempering brittleness is produced, it can be reheated to the original tempering temperature, held for a period of time, and then quickly cooled

Eliminate

In the tempering process, as the tempering temperature rises, the plasticity continues to increase, while the impact toughness is not a straight line rise, in the low temperature 250-400 degrees tempering and high temperature 450-650 degrees tempering range, toughness will decline, which is tempering brittleness. The former is called low temperature tempering brittleness and the latter is called high temperature tempering brittleness. Low temperature tempering brittleness is also called irreversible tempering brittleness or the first type of tempering brittleness. This tempering brittleness occurs in both carbon steel and alloy steel, which may be related to the precipitation of continuous carbide flakes on the grain boundary or subgrain boundary during low temperature tempering, and is also brittle for low temperature tempering

Only by avoiding tempering within the brittleness temperature range can it be prevented.

Alloy steel may produce high temperature tempering brittleness when tempered in the range of 450-650 degrees, which is also known as reversible tempering brittleness or the second type of tempering brittleness. In the following three cases :1, slow cooling in the range of 450650 degrees, 2, after tempering in the temperature range above, and then reduced to the temperature range of tempering, 3, in the temperature range of tempering brittleness for a long time, even if the subsequent fast cooling will produce tempering brittleness.

The temper brittleness is most sensitive to chromium steel, manganese steel and chrome-nickel steel, and the temper brittleness can be reduced when a small amount of D or rare earth elements are added. The occurrence of high temperature tempering brittleness is mainly related to the segregation of Buddha, Tin, phosphorus and other elemental elements on the original austenite grain boundary during tempering, thus reducing the fracture strength of the grain boundary. For high temperature tempering brittleness can be eliminated by short time heating above 600 degrees. Rapid cooling after tempering at a brittle temperature prevents high-temperature tempering brittleness