Heat treatment process of 45# steel

What is 45# steel?

45# steel is a medium carbon structural steel with good cold and hot workability, good mechanical properties, low price and wide sources, so it is widely used. Its biggest weakness is its low hardenability, and it is not suitable for workpieces with large cross-sectional dimensions and high requirements. The quenching temperature of 45# steel is A₃ + (30-50) ℃, and in practical operation, the upper limit is generally taken. A higher quenching temperature can accelerate the heating speed of the workpiece, reduce surface oxidation, and improve work efficiency. In order to homogenize the austenite of the workpiece, sufficient insulation time is required.

1. Quenching and tempering

Quenching and tempering is a dual heat treatment process of quenching and high-temperature tempering, aimed at ensuring that the workpiece has good comprehensive mechanical properties. In order to achieve good comprehensive performance of the quenched and tempered parts, the carbon content is generally controlled between 0.30% and 0.50%. During quenching, tempering and quenching, the whole section of the workpiece is required to be fully quenched, so that the workpiece can obtain a microstructure dominated by fine needle quenched Martensite. By high-temperature tempering, a microstructure mainly composed of uniformly tempered sorbite is obtained.
What is quenching?
Quenching is the process of heating a workpiece to a temperature above point AC₃ or AC₁ and maintaining it for a certain period of time. Then, the heat treatment process to obtain Martensite or (and) Bainite structure by rapid cooling at an appropriate rate.
Purpose: to obtain Martensite or lower Bainite structure, improve strength and hardness, so as to obtain the required properties after tempering at different temperatures.
(1) Quenching heating temperature
The quenching temperature is mainly determined based on the critical point of the steel in the Fe – Fe₃C phase diagram. Quenching heating temperature of hypoeutectoid steel: 30 ℃ -50 ℃ above AC₃ to fully austenitize the steel and obtain all Martensite structure after quenching. Quenching heating temperature of eutectoid steel and hypereutectoid steel: 30 ℃ -50 ℃ above AC₁ to obtain austenite and partial secondary cementite, and obtain Martensite (eutectoid steel) or Martensite plus cementite (hypereutectoid steel) structure after quenching.
(2) Quenching cooling
When quenching and cooling, in order to obtain Martensite structure, the austenite must be cooled at a critical cooling rate greater than Martensite. Rapid cooling will produce great quenching stress, leading to deformation and cracking of steel parts. Therefore, the most important problem in quenching process is to obtain Martensite structure, reduce deformation and prevent cracking.
Commonly used cooling media: Currently, the most widely used quenching cooling media are water and oil. In actual production, there are many cooling media used, and so far, no medium has been found that can fully meet the requirements of ideal quenching cooling speed. Water has a strong cooling ability and is the most suitable cooling medium for quenching carbon steel with less austenite stability. The cooling capacity of oil is smaller than that of water, therefore, using oil as a cooling medium in production is only suitable for quenching alloy steel with high stability of undercooled austenite.
(3) Quenching cooling method

• ① Single medium quenching is a quenching method that uses a quenching medium to cool down to room temperature. The advantage of this quenching method is that it is easy to operate and suitable for carbon steel and alloy steel workpieces with simple shapes. Carbon steel workpieces with simple shapes and larger sizes are often quenched with water, while small-sized carbon steel and alloy steel parts are generally quenched with oil. The disadvantage is that for large-sized and/or complex shaped workpieces, water quenching has a high tendency for deformation and cracking, while oil quenching has a low cooling speed and does not harden.
• ② Dual medium quenching is to immerse the workpiece into the medium with strong cooling capacity after austenitizing, and then transfer it into the medium with weak cooling capacity immediately when the structure is about to undergo Martensite transformation. Commonly used are “water oil” and “water air” dual medium quenching. This method can effectively reduce thermal stress and phase change stress, reduce the tendency of workpiece deformation and cracking, and can be used for quenching workpieces with complex shapes and uneven cross-sections. However, during operation, the residence time of the workpiece in water should be strictly controlled, and the operators must have rich experience and proficient skills.

2. Quenching and tempering of 45# steel

45# steel is a medium carbon structural steel with good cold and hot workability, good mechanical properties, low price and wide sources, so it is widely used. Its biggest weakness is its low hardenability, and it is not suitable for workpieces with large cross-sectional dimensions and high requirements.
The quenching temperature of No. 45 steel is around 820-840 degrees Celsius, and in practical operation, the upper limit is generally taken. A higher quenching temperature can accelerate the heating speed of the workpiece and reduce surface oxidation. In order to homogenize the austenite of the workpiece, sufficient insulation time is required, usually 1 min/mm. If the actual furnace loading is large, the insulation time needs to be appropriately extended. Otherwise, there may be insufficient hardness due to uneven heating. However, if the insulation time is too long, the drawbacks of coarse grains and severe oxidation decarburization can also occur, affecting the quenching quality. We believe that if the loading capacity is large, the heating and insulation time needs to be extended by 1/5.
Due to the low hardenability of 45# steel, a 10% saline solution with a high cooling rate should be used. 45# steel is prone to soft spots during quenching. The water temperature should be less than 30 ° C. After the workpiece is filled with water, it should be quenched, but not cooled. If the workpiece is cooled in salt water, it may crack. This is because when the workpiece is cooled to about 180 ℃, the austenite rapidly transforms into Martensite, causing excessive structural stress. Therefore, when the quenched workpiece quickly cools to this temperature range, a slow cooling method should be adopted. Due to the difficulty in controlling the outlet temperature, it is necessary to operate based on experience. When the workpiece in the water shakes and stops, the outlet can be cooled by air (oil cooling is better if possible). In addition, the workpiece should be moved rather than static when entering the water, and regular movements should be made according to the geometric shape of the workpiece. The combination of stationary cooling medium and stationary workpiece results in uneven hardness and stress, leading to significant deformation and even cracking of the workpiece.
The hardness of quenched and tempered parts of 45# steel should reach HRC56-59, and the possibility of a large cross-section is lower, but it cannot be lower than HRC48. Otherwise, it indicates that the workpiece has not been fully quenched, and there may be sorbite or even ferrite structure in the structure. This structure is still retained in the matrix through tempering.

3. Tempering

Tempering is a heat treatment process where the workpiece is quenched and heated to a temperature below Ac₁, held for a certain period of time, and then cooled to room temperature. Tempering enables the workpiece to achieve the required performance.
(1) Tempering purpose
Steel is seldom used directly after quenching, because the structure after quenching is Martensite and residual austenite, and there is internal stress. Although Martensite has high strength and hardness, it has poor plasticity and high brittleness, and it is easy to deform and crack under the action of internal stress; In addition, the microstructure after quenching is unstable and can slowly decompose at room temperature, resulting in volume changes and deformation of the workpiece. Therefore, quenched parts must be tempered before use. The purpose of tempering is: ① To reduce or eliminate quenching internal stress; ② Stable organization, stable size; ③ Reduce brittleness and obtain the required mechanical properties.
(2) Changes in microstructure and properties during tempering
The microstructure transformation of quenched steel can be divided into four stages: decomposition of Martensite (below 200 ℃) → decomposition of residual austenite (200-300 ℃) → formation of cementite (250-400 ℃) → cementite aggregation and growth (above 400 ℃). As the tempering temperature increases, the internal stress in quenching continuously decreases or eliminates, the hardness gradually decreases, and the plasticity and toughness gradually increase.
(3) Common tempering methods

• ① [Low temperature tempering] (<250 ℃) Tempered Martensite is obtained after low temperature tempering. The purpose is to reduce the quenching stress and brittleness of steel. Tempered Martensite has high hardness (generally 58-64HRC), strength and good wear resistance. Therefore, low-temperature tempering is particularly suitable for workpieces with high hardness and wear resistance, such as cutting tools, measuring tools, rolling bearings, carburized parts, and high-frequency surface quenching.
• ② [Medium temperature tempering] (250 ℃ -500 ℃) The tempered martensite structure is obtained after medium temperature tempering. The steel has high Elastic Limit, high strength and hardness (generally 35-50HRC), good plasticity and toughness. Medium temperature tempering is mainly used for various elastic components and hot working molds.
• ③ [High temperature tempering] (>500 ℃) After high-temperature tempering, the tempered sorbite structure is obtained. The composite heat treatment process of workpiece quenching and high-temperature tempering is called quenching and tempering. After quenching and tempering, the steel has excellent comprehensive mechanical properties (generally hardness is 220-230HBS). High temperature tempering is mainly applicable to important machine parts such as crankshafts, connecting rods, bolts, automotive half shafts, machine tool spindles, and gears made of medium carbon structural steel or low alloy structural steel.

4. Tempering of 45# steel after quenching

(1) The heating temperature is usually 200 ℃, and the hardness requirement is HRC44-48. The tempering metallograph at 200 ℃ is tempered Martensite. If the drawing has hardness requirements, the tempering temperature shall be adjusted according to the drawing requirements to ensure hardness. Regarding the tempering insulation time, it depends on the hardness requirements and the size of the workpiece. We believe that the hardness after tempering depends on the tempering temperature and has little to do with the tempering time, but it must be penetrated back. Generally, the tempering insulation time of the workpiece is always more than one hour.
(2) There are two main reasons for the insufficient hardness of 45# steel after quenching:

• ① The heating temperature of 45# steel is too low or the insulation time is insufficient. In this state, the carbon and alloy element content of austenite in the structure is insufficient, and even there are still untransformed pearlite or undissolved ferrite in the structure, resulting in the hardness of 45# steel not reaching after quenching.
• ② The heating temperature of 45# steel is too high or the insulation time is too long, which causes decarburization on the surface of 45# steel and leads to a decrease in hardness.

Conclusion: 1. Quenching conditions affect the microstructure and properties of the sample. 2. The tempering temperature affects the microstructure and properties of the sample. 3. Carbon element affects the structure and properties of the sample.
45# steel is widely used in mechanical manufacturing and has excellent mechanical properties. Heat treatment can improve the mechanical properties, eliminate residual stress, and improve machinability of 45# steel. However, improper selection and operation of quenching heating temperature, cooling medium, and tempering temperature may lead to heat treatment defects. By studying the influence of different heat treatment processes on the microstructure and properties of 45# steel, the microstructure and mechanical properties of 45# steel under different heat treatment conditions are compared, Identify the influencing factors and patterns of the microstructure and mechanical properties of 45# steel, and test and analyze the hardness, wear resistance, and other properties after heat treatment. Finally, determine the optimal heat treatment process for manufacturing different parts of 45# steel.

Topic selection basis

45# steel is a high-quality carbon structural steel with relatively low hardness and easy machining. 45# steel can be used not only as a mold template, but also to manufacture parts with high strength requirements such as crankshafts, shafts, piston pins, and fixtures, making it widely used. In order to achieve the required mechanical, physical, and chemical properties of 45# steel tools, in addition to reasonable selection of materials and various forming processes, heat treatment processes are often essential. It does not change the appearance of 45# steel, and can fully utilize its potential through heat treatment, endowing it with various special properties required to improve the quality of 45# steel, extend its service life, and ensure safe and reliable machine operation. Subcritical quenching, as a strengthening and toughening process for 45# steel, is rapidly developing both domestically and internationally. On the one hand, it can improve the strengthening and toughening effect of 45# steel, hinder crack propagation, improve the distribution of harmful impurities, and improve the comprehensive mechanical properties of 45# steel; On the other hand, due to its lower heating temperature, it can reduce heating costs and improve energy utilization. Although heat treatment can improve the mechanical properties, eliminate residual stress, and improve machinability of 45# steel, improper selection and operation of quenching heating temperature, cooling medium, and tempering temperature may lead to heat treatment defects, making 45# steel unqualified or scrap, causing economic losses. If heat treatment defects cannot be detected in a timely manner, the use of 45# steel products with defects may cause major accidents. This article studies the effects of different heat treatment processes on the microstructure and properties of 45# steel, compares the microstructure and mechanical properties of 45# steel under different heat treatment conditions, identifies the influencing factors and laws of the microstructure and mechanical properties of 45# steel, and tests and analyzes the hardness, wear resistance, and other properties after heat treatment. Finally, the optimal heat treatment process for manufacturing different parts of 45# steel is obtained.

Experimental materials and equipment

1. Experimental equipment

(1) Heat treatment heating furnace:

• Box type resistance furnace;
• HR-1500 Rockwell hardness tester (Rockwell hardness C scale);
• Metallographic microscope and digital photography system polishing machine and metallographic sandpaper;
• Polishing machine and polishing fluid.

(2) Etching agent, alcohol, glassware, Cotton wool, filter paper, etc;

2. Experimental materials

Sample: diameter φ10mm, 15mm high 45# steel, T8 steel cylindrical small sample.

Experimental process

Heat treatment (quenching and tempering at 560 ℃), hardness measurement, and microstructure observation and analysis of 45# steel samples were conducted using a box type resistance furnace, Rockwell hardness tester, and metallographic microscope. The objective of this experiment is to obtain Martensite, which needs to be quenched and tempered to obtain good properties and qualified structures.

1. Heat treatment of the sample

(1) Quenching
① Heating temperature
According to the purpose of heat treatment in this experiment, the quenching heating temperature was selected as 840 ℃.
② Insulation time
The diameter of this 45# steel sample is φ10mm small cylinder, with little difference in height and diameter.
③ Cooling medium
It can be seen from the continuous cooling transformation curve of 45 # steel that the critical cooling rate of carbon steel is very high, and water with strong cooling capacity (see Table 1 for the cooling diameter of common cooling medium) should be selected as the cooling medium to avoid the intersection of Cooling curve and C curve and obtain sorbite structure. The critical quenching diameter of 45 steel has been found as shown in Table 1. Choosing water as the quenching cooling medium can ensure that φ10mm cylindrical sample was quenched through.
Table.1 Cooling diameters of commonly used cooling media

Quenching medium	Static oil	20℃ water	40℃ water	20℃ 5％ NaCl aqueous solution
Critical diameter (mm)	10	20	16	21.5

Specific operation: put the sample on the Fire brick in the constant temperature zone of the box type resistance furnace, adjust the heating temperature, start timing when the thermometer displays 840 ℃, hold the temperature for 20 minutes, use the tongs to take out the sample and quickly put it into the cold water tank and stir it violently, so that the sample can be quenched thoroughly

(2) Tempering

① The required tempering heating temperature for the heating temperature experiment is 560 ℃ (tempering at a high temperature of 500-650 ℃ to obtain tempered sorbite structure)
② Cooling medium air for air cooling.
Specific operation: put the quenched sample on the Fire brick in the constant temperature zone of the 560 ℃ box resistance furnace with the temperature adjusted, and start timing after the furnace temperature is stabilized. After holding for 30min, clamp the sample with tongs and place it over the prepared refractory frame to cool to room temperature.

2. Sample hardness measurement

Grind the sample cooled to near room temperature on a Bench grinder to remove the surface oxide scale. Grind the surface of the sample flat with 360 # sandpaper, and then polish it with 400 #, 600 #, 800 #, 1000 #, 1200 #, and 1500 # sandpaper in sequence. Then, place the sample on the stage of the Rockwell hardness tester and measure the hardness of the sample using the Rockwell hardness C scale (the indentation head of the hardness tester is diamond, the range is 20-70HRC, and the loading load is 150kg). Take four points at different positions on the sample, the first point is not included in the data, and the last three points are included in the data. If the hardness values of the three points do not differ significantly, it indicates that the organization is relatively uniform. Finally, calculate the average of the three measured values.

3. Microscopic observation and photographic record

(1) Sample preparation

• ① Polishing of samples. Use a set of metallographic sandpaper (including 360 #, 400 #, 600 #, 800 #, 1000 #, 1200 #, and 1500 #) to polish the glass plate first coarse and then fine one by one. Pay attention to changing the polishing direction by 90 ° when replacing with a finer sandpaper, in order to observe the elimination of the original grinding marks. Finally, polish the sample on the polishing machine, paying attention to holding the sample with even and not excessive force.
• ② Polishing of samples. The sample is finely polished on a metallographic sample polishing machine to achieve a mirror like surface finish.
• ③ Display of microstructure. When the polished sample is directly observed under a microscope, there should be basically no grinding marks or pits, and grain boundaries, various phases, and structures cannot be observed. In this experiment, the chemical etching method is used. The etching solution (4% nitric acid alcohol) and pure alcohol are poured into a glassware respectively, and the Cotton wool is clamped with a bamboo clip, and the sample surface is rubbed with etching solution. When the bright mirror is light gray white, immediately flush with water.

(2) Observation and recording of microstructure
Observe the tissue of the prepared sample under a microscope at different magnifications of 40-400, and experience the impact of different magnifications on tissue observation. Select the appropriate magnification (200x, 400x) and use a digital photography system to digitally photograph the 45# steel sample.

Experimental results and analysis

1. Sample hardness and microstructure analysis

(1) Processing method: No processing

Structural analysis: Ferrite and pearlite, but smaller in grain size and higher in hardness compared to annealed grains.

Figure.1 200 × 45# steel original sample

Figure.2 400 × 45# steel original sample
(2) Treatment method: 840 ℃ quenching

Structure analysis: needle quenched Martensite and retained austenite.

Figure.3 200 × 45# steel quenching

Figure.4 400 × 45# steel quenching
(3) Processing method: 560 ℃ tempering and tempering

Structural analysis: a mixture of ferrite and coarse-grained cementite – tempered sorbite. Tempered sorbite appears as relatively coarse crystal particles under an optical microscope, with fine and uniform particles and small spherical carbides distributed within it. From the figure, it can be seen that the sample has a good etching effect, and the cementite particles can be clearly presented.

Figure.5 200 × 45# steel tempered

Figure.6 400 × 45# steel tempered
The hardness of the sample was measured to be 28HRC. Based on its hardness and microstructure morphology, it can be determined that tempered sorbite has been generated. Therefore, the heat treatment process developed in the experiment can obtain the required microstructure, and the hardness experimental values also comply well with the manual.

2. Hardness test data

Table.2 Hardness of 45# Steel in Different States

Group	1	2	3	4	Average
Hardness
Original hardness	22	26	20	24	23
Hardness after quenching	51	53	52	56	53
Hardness after tempering	23	26	31	27	28

Hardness unit: HRC

Table.3 Hardness of T8 Steel in Different States

Group	1	2	3	4	Average
Hardness
Original hardness	35	38	40	37	35
Hardness after quenching	65	62	63	60	62.5
Hardness after tempering	59	60	57	60	59

Hardness unit: HRC

3. The effect of quenching on the performance of the sample

(1) The influence of quenching temperature
The quenching heating temperature of 45# steel should be 30-50 ℃ above Ac₃, and 840 ℃ is selected as the quenching temperature to obtain fine and uniform austenite grains, and fine Martensite structure can be obtained after quenching. If the temperature is too high above Ac₃, such as 1000 ℃, the austenite grain will be coarsened. After quenching, the Martensite will be coarser, the brittleness will increase, and the hardness will decrease. The hardness of coarse-grained Martensite is higher than that of fine-grained Martensite. If heated in the two-phase zone between Ac₁ and Ac₃, such as after quenching at 770 ℃, the high hardness Martensite is mixed with low hardness ferrite, resulting in insufficient hardness (see Table 3) and reduced mechanical properties.
(2) The influence of quenching medium
The commonly used quenching media and their cooling capacity are shown in Table 3 As shown by the commonly used quenching media and their cooling capacity, it can be seen that water has a large cooling capacity. However, if the cooling speed is too fast in the low-temperature zone, the workpiece is prone to quenching and cracking. In addition, the cooling capacity of water is sensitive to temperature changes, and as the water temperature increases, the cooling capacity sharply decreases. The cooling speed of oil throughout the entire process is smaller than that of water, and the cooling speed is suitable in the low temperature zone, but the cooling capacity is very low in the high temperature zone.

Table.4 Common Quenching Media and Their Cooling Capacity

Quenching medium	Cooling rate (℃/s)
	In the range of 650~550 ℃	In the range of 300~200 ℃
Water (18 ℃)	600	270
Water (26 ℃)	500	270
Water (50 ℃)	100	270
Mineral oil	100	20

The critical cooling rate of carbon steel is large. Generally, the quenching medium with strong cooling capacity, such as water, can be used to obtain the microstructure of all Martensite. If oil is selected as quenching medium, because of its low cooling rate, the Cooling curve will intersect the “nose tip” of the CCT curve, and a small part of troostite structure will be obtained during the transformation process. Because troostite nucleates along the original austenite grain boundary and forms a network structure, the troostite network+Martensite microstructure will be obtained at room temperature, which will reduce the strength and significantly reduce the hardness (see Table 4. Different tempering temperatures and measured hardness values of 45# steel).

4. Effect of tempering on the sample

(1) The Effect of Tempering Temperature on the Structure of 45# Steel
The room temperature structure of steel after quenching is Martensite and residual austenite, both of which are metastable phases. Once heated, the atomic diffusion ability strengthens and spontaneously transforms into stable phase ferrite and cementite. With the increase of tempering temperature, the transformation can be roughly divided into five stages: ① segregation of carbon atoms in Martensite; ② Decomposition of Martensite; ③ Transformation of residual austenite; ④ Transformation of carbides; ⑤ The aggregation and growth of carbides α Phase recovery and recrystallization.
45# steel is tempered at a low temperature of 150-350 ℃ to obtain tempered Martensite structure. Tempered Martensite shows dark stripe lamellar structure under optical microscope. After low temperature tempering, only carbon atoms are segregated, which is not significantly different from quenched Martensite, but tempered Martensite is more vulnerable to corrosion than quenched Martensite, so the microstructure is darker than quenched Martensite.
After moderate temperature tempering at 350-500 ℃, tempered troostite structure is obtained. As Martensite decomposes and Supersaturation solid solution carbon atoms precipitate cementite, the cementite aggregates, grows up and spheroidizes in strip shape α There is a distribution of fine-grained cementite on the surface, but it is difficult to distinguish under an optical microscope.
Tempering at a high temperature of 500-650 ℃ yields tempered sorbite structure. At this point α Phase recrystallization occurs, gradually replacing needle like ferrite with equiaxed ferrite α Phase. Its microstructure is a composite structure composed of fine-grained cementite and equiaxed ferrite, and the cementite particles can be distinguished under an optical microscope.
If 45# steel is tempered between 650 ℃ and A₁, the granular cementite will become significantly coarsened, resulting in a granular pearlite structure.

(2) The Effect of Tempering Temperature on the Hardness and Strength of 45# Steel

Table.5 Different Tempering Tempering Temperatures and Measured Hardness Values of 45# Steel

Group	1	2	3	4	5
Tempering temperature (℃)	200	300	400	500	600
Hardness of tempered 45 steel (HRC)	53.6	50.9	40.1	24.5	21.53

From Table.4 Different tempering temperature and measured hardness values, the tempering hardness of quenched steel decreases with the increase of tempering temperature. When 45 # steel is tempered below 200 °C, the hardness decreases slowly. This is due to the precipitation of a large number of ε carbides from α solid solution, which increases the plastic deformation resistance and delays the decrease of hardness. Tempering at 200-300 °C, due to the decomposition of retained austenite into tempered martensite, the hardness decreases slowly. Tempering above 300 °C, as ε carbide becomes cementite, coherent damage and cementite aggregate and grow, the hardness decreases rapidly.

(3) Taking 45# steel and T8 steel as examples, analyze the effect of carbon content on the hardenability of steel.

Table.6 Hardness of 45# steel and T8 steel

	Original hardness	Quenching hardness	Tempering hardness
45 steel	23	53	28
T8 steel	35	62.5	59

Hardenability refers to the highest hardness that steel can achieve after quenching, mainly related to the carbon content of the steel. In this experiment, 45# steel and T8 steel obtained fine Martensite structure after normal quenching, and their hardness values were 53HRC and 62.5) HRC respectively. Comparing these two data shows that as the carbon content of the steel increases, the hardenability increases. Because the structure obtained from quenching of steel is Martensite, the hardness of Martensite mainly depends on the carbon content. With the increase of carbon content, the hardness of Martensite increases, which is mainly due to the Solid solution strengthening effect of carbon. In addition, with the increase of carbon content, the transformation points Ms and Mf of Martensite decrease, which promotes the occurrence of self tempering phenomenon and makes carbide precipitate to produce aging strengthening. Therefore, increasing the carbon content of steel can improve its hardenability.

Conclusion

Based on the experiments conducted and their results, the following conclusions can be drawn:

• 1. Quenching conditions affect the microstructure and properties of the sample. When the quenching temperature and cooling rate (select effective cooling medium) are appropriate, fine Martensite structure will be generated, which will have high strength, good plasticity and good mechanical properties after tempering. When the quenching temperature is low, incomplete quenching occurs. The structure is martensite + ferrite, with low strength, low hardness and poor mechanical properties. When the quenching temperature is high, coarse austenite will be formed. Due to the heredity of the structure, coarse grained martensite will be formed after quenching. The cooling rate is too fast, resulting in huge internal stress and the possibility of quenching and cracking. The cooling rate is too slow, and the formed Martensite is incomplete, and pearlite is formed (pearlite, sorbite, troostite).
• 2. The tempering temperature affects the microstructure and properties of the sample. According to the tempering temperature, it can be divided into low temperature tempering, medium temperature tempering, and high temperature tempering (different steel grades correspond to different temperatures, and generally the more alloy elements, the higher the temperature). Production of tempered Martensite, tempered troostite, tempered sorbite. Tempered Martensite has the smallest grain and the highest hardness and strength; Tempered troostite grains are between the two, with moderate hardness and strength, and according to data, they have excellent elasticity; Tempered sorbite Flat noodles is the thickest, with the lowest strength and hardness, but it has higher plastic toughness.
• 3. Carbon element affects the structure and properties of the sample. Carbon atom can play a role of Solid solution strengthening. For the formation of Martensite, the greater the strength and hardness of the matrix, the less easy the formation of Martensite, thus reducing the Ms point. At the same time, the strength and hardness of the final quenched structure increased due to the Solid solution strengthening effect.