Of the many grades of structural steel, 45 steel is the most widely used, especially in the hardware industry, with 45 steel components. The steel is a high quality carbon steel structural steel. Various heat treatments are available. It can be used not only as a structural member but also as a tool. For many years. Although this steel grade has accumulated rich experience in production and application, in the actual production, there will still be many misuses. As a result, the performance potential of the steel is not fully utilized, resulting in a lot of waste.
45 steel can be used as a mold template, but also can produce crankshafts, shafts, piston pins, jigs and other parts requiring high strength, but the surface of the mold template and parts is usually required to have high hardness and high wear resistance, No. 45 Although steel has general quenching properties, it can be hardened to HRC 42-46. At the same time, if the surface of the 45 steel is carburized and quenched, the required surface hardness can be obtained. The traditional heat treatment process of No. 45 steel mainly includes preparatory heat treatment, low temperature spheroidizing annealing, quenching, critical temperature quenching and high frequency quenching.
45 steel traditional heat treatment process1.1 preliminary heat treatment
No. 45 steel forgings are usually not annealed for two reasons:
First, if the annealing time is too long, it is easy to produce ferrite accumulation, resulting in tissue inhomogeneity;
Second, because the 45th steel forgings have a longer annealing cycle, the production efficiency is lower. For the 45th steel preparatory heat treatment, high temperature tempering and normalizing are generally used. 45th steel forgings are usually controlled within 724 °C, so that not only does the crystallization process not occur, but also the internal stress can be reduced, the hardness is greatly reduced, and the next cutting process is easy.1.2 Low temperature spheroidizing annealing
The low temperature spheroidizing annealing is a heat treatment method in which the workpiece is heated to a temperature below the eutectoid transformation temperature Acl and then cooled and cooled to obtain a spheroidized structure. When the temperature of the 5th steel forging is close to 724 ° C, it takes a long time. In the heat preservation stage, the spheroidal spheroids will change and become spherical pearlite. Its hardness is within 145HB, and its toughness is good, which lays a foundation for cold extrusion.
1.3. Quenching
Quenching of No. 45 steel is to heat the steel to the temperature above Ac3 (hypoeutectoid steel) or Ac1 (hyper-eutectoid steel). After heat preservation, it is placed in various cooling mediums (V cold should be greater than V). Martensite structure is obtained. Since the austenite stability of the 45 steel is relatively poor, it is necessary to rapidly quench and cool the martensite structure to obtain a high hardness after heating. 45 steel has good thermal conductivity. When quenching, it can be directly used in the furnace. It does not need preheating. The temperature is selected according to the relevant technical requirements of the workpiece. The general heating temperature is controlled between 860 °C and 820 °C.
1.4 critical temperature quenching
A large number of experiments have shown that when the 45 steel is quenched at a critical temperature of 780 ° C, it can obtain very fine austenite grains, which greatly improves the toughness and also significantly reduces the crack sensitivity. Some workpieces with widely different cross-sectional dimensions are prone to cracking during quenching, and the critical temperature quenching process can greatly reduce the probability of cracking.
1.5 high frequency quenching
High-frequency quenching is a metal heat treatment method in which an induction heating device is used to inductively heat a workpiece, rapidly heat the surface of the part, and then rapidly quench. When the high-frequency heating speed control range is between 200 and 1 000 ° C, the critical temperature is also increased accordingly. Therefore, the quenching heating temperature of the 45-gauge steel forgings is between 880 and 920 ° C, generally compared with other types. The steel height is about 80-1000 ° C and sometimes higher. In this way, the No. 45 steel is heated at a high speed by quenching, the microstructure is small, the stress is increased, and the forging can reach a hardness of 62-66 HRC, which has high wear resistance, strong fatigue resistance and small notch sensitivity. specialty.
45 steel heat treatment common problems2.1 Low hardness
After the quenching of the 45th steel forgings, the hardness of the forgings should generally meet the requirements of HRC56-59. For forgings with large cross-sections, it should be larger than HRC48. There are four main reasons for this:
First, the carbon content of steel is low;
Second, in the quenching and heating stage, the required technical specifications were not met, the heating temperature was low or the holding time was insufficient, so that the austenite carbon and alloying elements in the forging structure were insufficient, and even unconverted pearlite remained. Or undissolved ferrite, resulting in the hardness index of the forging after quenching does not meet the requirements;
Third, the forging temperature is too high or the holding time is long, resulting in decarburization of the surface without achieving hardness;
Fourth, quenching and cooling are not in place, cooling is the final step of heat treatment, and it is the most important process of heat treatment. The quenching hardness of No. 45 steel can be converted into different microstructures at different cooling rates, and the quenching cooling is not in place, and its hardness will become low.2.2 longitudinal crack
The longitudinal crack is the axial crack that occurs, and the shape is thin and long, as shown in Figure 1.
Forgings of 45 steel with a diameter of about 8 mm are most likely to occur. Generally, the higher the carbon content, the greater the tangential tensile stress generated. When the tensile stress breaks through the strength limit of the forging, longitudinal cracks will form. The main causes of longitudinal cracks in No. 45 steel forgings are:
First, the way of installing the heating furnace is unreasonable, resulting in uneven heating of the forgings;
Second, when forgings are quenched, the temperature is higher and the internal and external stresses are large. At the same time, the causes of cracks in the 45th steel forgings are mainly caused by: the steel in the forging contains more low-melting harmful impurities, such as S, P, Bi, etc.; the size of the forging is selected in the quenching sensitive size range of steel. The cooling medium is much larger than the critical cooling rate of the forging.2.3 Transverse crack
The transverse crack is the crack generated perpendicular to the axial direction. Broken from the inside out. as shown in picture 2.
Horizontal stripes generally occur when they are not hardened and are induced by thermal stress. If the forging is hardened, the surface will be compressively stressed. The core is tensile stress, so that the maximum tensile stress is generated in the transition zone between the hardened layer and the non-hardened layer of the forging. When the tensile stress generated breaks through the tensile strength limit of the forging, the transverse crack will produce. There are three main reasons for the occurrence of transverse cracks in No. 45 steel forgings:
First, the drawing process and operation of the workpiece are unreasonable, such as too large a mold angle, no pickling, and uneven deformation of the inside and outside of the metal;
Second, carbonation occurs in the core of the workpiece, which causes a large difference in the plastic deformation ability of the inner and outer layers of the workpiece;
Third, there are inclusions in the workpiece.2.4 uneven hardness
After heat treatment of 45 steel, if the hardness is not uniform, the wear resistance will be reduced and the service life will be reduced. The main reasons for the uneven hardness of No. 45 steel are as follows:
First, the workpiece used itself has low hardenability;
Second, the surface of the workpiece remains with an annealed decarburization layer or a decarburization layer is produced when quenched and heated;
Third, the workpiece has a slow cooling rate after quenching and heating, and the classification, isothermal temperature is too high, the time is too long, or the cooling medium is improperly selected;
Fourth, the workpiece quenching medium contains too much impurities or aging;
Fifth, when the workpiece is quenched and cooled, the temperature is too high and the cooling is insufficient when the quenching medium is discharged;
Sixth, the workpiece is not tempered and the tempering temperature is too high.2.5 Surface formation of large carbon (chlorine) compound network
The reasons for the formation of large carbon (nitrogen) compound nets on the surface of 45 steel forgings are:
First, the carbon potential of the furnace gas is too high;
Second, the workpiece has a long time of infiltration;
Third, the cooling rate is too slow, and the network carbide is precipitated along the austenite grain boundary;
Fourth, the forging temperature is too high for forging, and the cooling is too slow after forging.2.6 Distortion
There are three reasons for the distortion of the 45th steel forgings:
First, the workpiece temperature is high during the quenching phase;
Second, the workpiece cooling method is unreasonable;
Third, the fixture design is unreasonable or improperly used.solution
Low hardness precautions
First, inspect the raw materials of the 45th steel forgings, understand the carbon content by grinding or looking at the spectrum or test components, select the raw materials that meet the technical requirements, and then remove the decarburized layer of the raw materials. Before the salt bath furnace is heated, deoxidation and slag should be carried out. The air furnace heating parts should be added with a protective agent (gas), and the condition can be heated by vacuum;
Second, according to the technical requirements of the 45th steel heat treatment process
Master the parameters such as temperature and time of the quenching and tempering process, and then select the appropriate quenching medium according to the technical requirements of the forgings;
Third, when the workpiece is water quenched, oil-cooled, water-cooled, grasp the time, not too short; at the same time, you need to be familiar with the performance of the heating furnace and the instrument.
Master the error value and the hardness value of the workpiece after quenching, so that the fatigue strength and comprehensive mechanical properties of the workpiece meet the requirements.Vertical line preventive measures
First, strictly inspect the raw materials of No. 45 steel, and do not use steel with excessive levels of harmful impurities;
The second is to use vacuum smelting as much as possible, refining outside the furnace or electroslag remelting the workpiece steel;
The third is to strengthen the improvement of the heat treatment process, using vacuum heating, protective atmosphere heating and full deoxidation salt bath furnace heating and stage quenching, austempering;
Fourthly, it can be changed into unintentional quenching for intentional quenching, that is, incomplete hardening, thereby obtaining measures such as lower bainite structure with high toughness, and the purpose of greatly reducing tensile stress is avoided, and longitudinal cracking of the workpiece is avoided.Horizontal stripes preventive measures
First, the workpiece is reasonably forged, the forging ratio (that is, the ratio of the length of the raw material to the diameter) is selected as much as 2.3, and the forging is double forged and forged, and the forged part is fired by the five-and-a-half fire. Carbides and impurities are fine and small, evenly distributed in the base of the workpiece steel, and the forged fiber structure is distributed around the cavity, which greatly improves the lateral mechanical properties of the module and reduces and eliminates the stress source;
The second is to correctly select the cooling rate and cooling medium. It should be faster than the Ms point of the workpiece, which is larger than the critical quenching cooling speed of the workpiece, so that the thermal stress generated by the supercooled austenite of the workpiece, the compressive stress generated by the surface layer and the tensile stress generated by the inner layer can cancel each other to prevent thermal stress. effect;
Thirdly, the slow cooling is performed between the Ms of the workpiece, the microstructure stress of the workpiece forming the quenched martensite is greatly reduced, the phase transformation stress is reduced, the total control stress of the workpiece is negative, and the transverse quenching crack is prevented;
The fourth is to use cL-l organic quenching medium. CL-1 organic quenching medium is an ideal quenching agent, which can not only control the reasonable distribution of hardened layer, but also reduce and prevent the quenching workpiece distortion. At the same time, it can also adjust the different concentration ratio of cL-l quenching agent to obtain different cooling speeds. Hardened layer distribution is required to meet the needs of different workpiece steels.
Uneven hardness prevention measures
First, it is necessary to correctly select a reasonable forging process and a spheroidizing annealing process to ensure that the workpiece has a good preliminary heat treatment structure;
Second, before the workpiece is heat treated, it must completely eliminate the rust and oxide on the surface of the workpiece. At the same time, pay attention to the protection during heating. Use vacuum heating or protective atmosphere heating as much as possible. Good carbon treatment;
The third is to correctly formulate the quenching and tempering process specifications of the workpiece, and the tempering should be sufficient;
Fourth, according to the technical requirements of the use of the workpiece, the cooling medium and cooling method are reasonably selected;
Fifth, the moisture content of the alkali bath should be strictly controlled. At the same time, the quenching oil should be filtered frequently and replaced regularly.
Sixth, the workpiece with high hardness requirements should be subjected to cryogenic treatment, and the workpiece steel after quenching should be subjected to surface strengthening treatment by nitriding or nitrocarburizing heat treatment.3-5 surface formation of large carbon (nitrogen) compound network preventive measures
First, reduce the carbon potential of the workpiece and shorten the strong osmosis time; if the depth of the permeable layer allows, diffusion treatment can be carried out in a lower carbon potential furnace;
Second, properly increase the quenching temperature;
Third, the workpiece can be subjected to a spheroidizing annealing of the layer.3.6 Distortion prevention measures
On the basis of adjusting the quenching process of the workpiece, rationally designing the fixture and improving the cooling conditions, the uniformity of the workpiece is guaranteed.
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