Processing route of 45 steel
Quenching and tempering of 1 and 45 steel
45 # steel is a kind of 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 that the workpieces with low hardenability, large cross-section size and high requirements are not suitable for use.
The quenching temperature of No.45 steel is A3 (ferrite precipitates from austenite, that is, r-Fe? The starting line of a-Fe is 9 10? C-700？ ) C+(30~50)℃, the upper limit is generally taken in actual operation. Higher quenching temperature can accelerate the heating speed of workpiece, reduce surface oxidation and improve work efficiency. In order to homogenize the austenite of the workpiece, sufficient holding time is needed. If the actual charging capacity is large, it is necessary to extend the holding time appropriately. Otherwise, uneven heating may lead to insufficient hardness. However, if the holding time is too long, there will be unfavorable factors such as coarse grain size and serious oxidation decarbonization, which will affect the quenching quality. We think that the heating and holding time should be extended by 1/5 if the charge is greater than the amount specified in the process documents.
Because of the low hardenability of 45 # steel, 10% salt solution with fast cooling rate should be adopted. After the workpiece enters the water, it should be quenched, not chilled. If the workpiece is cooled in salt water, it may break. This is because when the workpiece is cooled to about 180℃, austenite rapidly transforms into martensite, resulting in excessive structural stress. Therefore, when the quenched workpiece is rapidly cooled to this temperature range, the method of slow cooling should be adopted. Because the outlet water temperature is difficult to grasp, it must be operated by experience. When the jitter of the workpiece in water stops, the outlet water can be cooled by air (it would be better if it could be cooled by oil). In addition, when the workpiece enters the water, it should be moving rather than static, and it should move regularly according to the geometry of the workpiece. Static cooling medium and static workpiece lead to uneven hardness and stress, which leads to large 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 large section is low, but it cannot be lower than HRC48. Otherwise, it shows that the workpiece is not completely quenched, and sorbite or even ferrite structure may appear in the structure, which remains in the matrix after tempering, and the purpose of quenching and tempering cannot be achieved.
The high temperature tempering of 45 # steel after quenching is usually at 560~600℃, and the hardness requirement is HRC22~34. Because the purpose of tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide. However, if the drawing requires hardness, the tempering temperature should be adjusted according to the drawing requirements to ensure hardness. If some shaft parts require high strength and hardness; However, some gears and shaft parts with keyways have to be milled and inserted after quenching and tempering, so the hardness requirement is low. As for the tempering holding time, it depends on the hardness requirements and the size of the workpiece. We think that the hardness after tempering depends on tempering temperature and has little to do with tempering time, but it must be tempered. Generally, the tempering and heat preservation time of workpieces is always more than one hour.
Quenching and tempering treatment of 2.40Cr steel
Chromium can improve the hardenability, strength and tempering stability of steel, and has excellent mechanical properties. Chromium steel should be used for quenched and tempered workpieces with large or important sections. But Cr steel has the second temper brittleness.
The quenching and tempering of 40Cr workpiece are specified by various parameters and process cards. Our experience in practical operation is as follows:
(40Cr workpiece should be oil-cooled after quenching. 40Cr steel has good hardenability, can be cooled and quenched in oil, and the deformation and cracking tendency of the workpiece are small. However, in the case of tight oil supply, small enterprises can quench workpieces with uncomplicated shapes in water, and no cracks are found, but operators should strictly control the import and export temperatures by experience.
(The hardness of 40Cr workpiece is still high after quenching and tempering, and the second tempering temperature should be increased by 20~50℃, otherwise the hardness will be difficult to decrease.
(3) After tempering at high temperature, the 40Cr workpiece with complex shape is cooled by oil neutralization and simple water in order to avoid the influence of brittleness of secondary tempering. After tempering and rapid cooling, if necessary, stress relief treatment should be carried out on the workpiece.
The level of operators is an important factor affecting the quality of quenched and tempered workpieces. At the same time, there are many reasons such as equipment, materials, pretreatment and so on. We believe that:
(1) The transfer speed of the workpiece from the heating furnace to the cooling tank is slow, and the water inlet temperature of the workpiece has dropped below the Ar3 critical point, resulting in partial decomposition and partial quenching of the workpiece, which can not meet the hardness requirements. Therefore, the cooling liquid of small parts should pay attention to the speed, and the cooling time of large parts should be mastered.
(2) The furnace charging quantity of the workpiece should be reasonable, and it is advisable to use 1~2 layers. Workpieces overlap each other, resulting in uneven heating and uneven hardness.
(3) The arrangement of workpieces entering the water should keep a certain distance. If it is too dense, the vapor film near the workpiece will be blocked and the hardness near the workpiece will be low.
(4) Open the furnace for quenching, not quenching at one breath. Depending on the temperature drop, the furnace should be closed halfway to raise the temperature again to make the hardness of the workpiece consistent after quenching.
(5) Pay attention to the temperature of the coolant. If the temperature of 10% brine is higher than 60℃, it cannot be used. The coolant must not contain impurities such as oil and mud, otherwise the hardness will be insufficient or uneven.
(6) If the unprocessed blank is quenched and tempered, the hardness will be uneven. If you want to obtain good quenching and tempering quality, the blank should be coarsened and the bar should be forged.
(7) Strictly control the quality, the hardness after quenching is low 1~3 units, and the tempering temperature can be adjusted to meet the hardness requirements. However, after quenching, the hardness of the workpiece is too low, and some even only have HRC25~35, so it must be quenched again. It must not be tempered only at medium and low temperature to meet the requirements of the drawings, otherwise it will lose the significance of tempering and may cause serious consequences.
Traditional heat treatment process of 45 steel
1. 1 preliminary heat treatment
No.45 steel forgings are usually not annealed for two reasons:
First, the annealing time is too long, which is easy to produce ferrite agglomeration, resulting in uneven structure; .
Second, because of the long annealing period and low production efficiency of forged rolled pieces of 45 steel. The pretreatment of 45 # steel generally adopts high temperature tempering and normalizing. The forging workpiece of 45 steel is usually controlled within 724℃, which not only does not produce crystallization process, but also reduces its internal stress and hardness, which is convenient for the next cutting.
1.2 low temperature spheroidizing annealing
Low-temperature spheroidizing annealing is a heat treatment method that the workpiece is heated to below the * * * transition temperature Acl, and then cooled slowly to obtain spheroidized structure. When the forging workpiece temperature of No.5 steel is close to 724℃, it needs long-time heat preservation to transform the flaky sphere into spherical pearlite, and its hardness is within 145HB, and its strength and toughness are good, which lays the foundation for cold extrusion.
1.3. Inhibition
Quenching of No.45 steel is to heat the steel to the temperature above Ac3 (sub-* * * steel precipitation) or Ac 1 (super * * * steel precipitation), and then put it into various cooling media (V cooling should be greater than V pro) to obtain martensite structure. Because of the poor austenite stability of 45 # steel, in order to obtain high hardness martensite structure, it is necessary to rapidly quench and cool after heating. 45 # steel has good thermal conductivity. It can be directly heated without preheating during quenching. The temperature is selected according to the relevant technical requirements of the workpiece, and the heating temperature is generally controlled between 860℃ and 820℃.
1.4 critical temperature quenching
A large number of experiments show that when 45 steel is quenched at the critical temperature of 780℃, very fine austenite grains can be obtained, which makes it tough.
Greatly improved, but also significantly reduced the crack sensitivity. Some workpieces with different cross-sectional sizes are prone to crack during quenching, and quenching at critical temperature can greatly reduce the probability of crack.
1.5 high frequency quenching
High-frequency quenching is a kind of gold that uses induction heating equipment to heat the workpiece, quickly heat the surface of the part, and then quickly quench it.
Belonging to a heat treatment method. High frequency heating speed control range 200? When the temperature is between 1 000℃ and s, its critical temperature increases correspondingly, so the quenching heating temperature of 45 # steel forgings is 880℃. Between 920℃, it is generally 80 ~ 1000℃ higher than other steel grades, and sometimes higher. In this way, 45 # steel is rapidly heated in high-frequency quenching, with fine structure and increased stress, which can make the forgings reach the hardness of 62-66 HRC, and has the characteristics of high wear resistance, strong fatigue resistance and low notch sensitivity.
Common problems in heat treatment of 45 steel
1 low hardness.
The hardness of 45 steel forgings after quenching should generally reach HRC56? 59, for large section forgings, should also be
Greater than HRC48, mainly due to four reasons:
First, the carbon content of steel is low;
Second, in the quenching and heating stage, the required technical specifications cannot be met, the heating temperature is low or the heat preservation time is not enough, so that the content of austenitic carbon and alloying elements in forgings is insufficient, and even untransformed pearlite or undissolved ferrite remains, resulting in the hardness index of forgings after quenching not meeting the requirements;
Third, the forging has high heating temperature or long heat preservation time, and the surface is decarburized, which can not reach the hardness;
Fourthly, quenching and cooling are not in place. Cooling is the last process and the most important process of heat treatment. The quenching hardness of 45 steel can be transformed into different structures at different cooling rates. If quenching and cooling are not in place, its hardness will become lower.
2 longitudinal crack
Longitudinal cracks are cracks with axial trend and slender shape, as shown in figure 1
45 steel forgings with a diameter of about 8 mm are most likely to appear. Generally speaking, the higher the carbon content, the greater the tangential tensile stress.
When the tensile stress breaks through the strength limit of forgings, longitudinal cracks will form. The main causes of longitudinal cracks in 45 # steel forgings are:
First, the unreasonable installation of heating furnace leads to uneven heating of forgings;
Second, the temperature of forgings is high when quenching, and the internal and external stress difference is large. At the same time, the main reasons for cracking of 45 steel forgings are as follows
There should be: the steel in forgings contains more harmful impurities with low melting point, such as S, P and Bi; When the forging size is within the range of steel quenching sensitivity, the selected quenching cooling medium is much greater than the critical cooling rate of the forging.
3 transverse crack
A transverse crack is a crack perpendicular to the axial direction. Fracture from the inside out. As shown in figure 2.
Transverse striations generally appear when they are not hardened, which is caused by thermal stress. If the forging cannot be completely quenched, its surface will show compressive stress. However, the center of the forging presents tensile stress, so the maximum tensile stress will be generated in the transition zone between the hardened layer and the non-hardened layer of the forging. When the tensile stress exceeds the tensile strength limit of forgings, transverse cracks will occur. There are three main reasons for transverse cracks in 45 # steel forgings:
First, the drawing process and operation of the workpiece are unreasonable, such as the die angle is too large, there is no pickling, and the internal and external deformation of the metal is uneven;
Second, carburization occurs in the center of the workpiece, which makes the plastic deformation ability of the inner and outer layers of the workpiece quite different;
Thirdly, there are inclusions in the workpiece.
4 Uneven hardness
After heat treatment, if the hardness of 45 steel is uneven, the wear resistance will be reduced and the service life will be shortened. The hardness of No.45 steel is uneven.
The main reasons for consistency are as follows:
First, the used workpiece itself has low hardenability;
Secondly, the surface of the workpiece is left with an annealed decarburized layer or a decarburized layer generated during quenching and heating;
Third, the cooling speed of the workpiece after quenching and heating is slow, the grading and isothermal temperature is too high, the time is too long or the cooling medium is improperly selected;
Four, the workpiece quenching medium contains too many impurities or is aging;
Fifth, the temperature is too high and the cooling is insufficient when the workpiece is quenched and cooled;
Sixth, the tempering of the workpiece is insufficient and the tempering temperature is too high.
A large network of carbon (chlorine) compounds is formed on the surface of 5.
The reasons why a large area of carbon (nitrogen) compound network is formed on the surface of 45 # steel forgings are as follows:
First, the carbon potential of furnace gas is too high;
Second, the strong infiltration time of the workpiece is too long;
Third, the cooling rate is too slow, and the network carbide precipitates along the austenite grain boundary;
Fourth, the initial forging temperature is too high and the cooling after forging is too slow.
6 distortion
There are three reasons for the deformation of 45 # steel forgings:
First, the workpiece temperature is high in quenching stage;
Second, the cooling mode of the workpiece is unreasonable;
Third, the design of the fixture is unreasonable or improper.
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