1 Introduction The air compressor produced by our factory is a reciprocating piston type high 1 air compressor, which is graded and compressed. The piston rod described here is at the first stage. Its end is connected to the aluminum piston, and the other end is connected to the piston. It is driven by the connecting rod and the crankshaft to reciprocate in the cylinder. The piston rod carries the alternating tension and compressive stress with different angles of the crankshaft rotation. Stress, which requires the piston rod not only has a certain strength, but also has a certain degree of inertia. Our factory produces piston rods in order to reduce the core of the core is hollow, there are two pieces welded together. Also considering the processability of welding, 45 steel is used. The piston rod is an important moving part in the air compressor, and the machining process is complicated, and the process flow and the structure of the piston rod before the quenching and tempering are complicated.
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Before the piston rod is tempered, the structure of the piston rod from the top of the piston shows that the left side of the shape is drilled with a hole (1) for the welding and quenching of the air outlet, and it needs to be blocked after the heat treatment. On the right side of the shape, there is a width of 6,1 around the outer circumference of the piston rod is a girth weld. From the shape, we can also see that in the middle of the piston rod, the height of 38mm is only 4mm. The shank at both ends is 22nlm thick.
2Technical requirements and stress analysis Because the piston rod is required to carry tensile stress and compressive stress during the movement, comprehensive mechanical properties are required. It is required to be quenched and tempered at the current heat treatment. The hardness requirement is Han 28,32. The workpiece shall not have internal defects such as cracks in the central flood hole due to magnetic particle inspection. The workpiece is first welded, and a large internal stress is generated after welding. The thickness of the two side walls of the weld is not equal. This causes a cooling rate not to occur in both, resulting in a large thermal stress. This positional force caused by welding is not as good as it is, and it is eliminated, which may cause cracks. The workpiece of the welding 7 passes through the hoist and the semi-finished car to generate a certain machine plus force. In the process of heat treatment and quenching, thermal stress and tissue stress are inevitably generated. At the moment when the workpiece is quenched, the external ash of the workpiece is first cooled and the temperature at the core is still high, thus creating a temperature difference in the heat. Under the action of the stress, the tensile stress is generated and the compressive stress is generated in the core. As the cooling time goes on, the temperature inside and outside the workpiece tends to be the same. At this time, the core begins to shrink, but it is cooled, and the surface is restrained. Tensile stress is generated in the core, and compressive stress is generated on the surface. When the solid body is transformed into martensite upon cooling, its volume increases due to changes in the structure of the structure, and the stress distribution is now opposite to that when the thermal stress is cooled. From the shape, we can also see that the wall thickness difference of the piston rod is very large, which causes the cooling speed and the phase change to be very large, mainly to become the phase transformation stress of the foot. Now the force is mainly the compressive stress, and the rod is Both sides have a slower cooling rate due to the thicker wall thickness. Under the dual action of thermal stress and tissue stress, the tensile stress is mainly present. In this way, in the case of machine stress and welding stress and heat treatment quenching stress multiple stresses, if the stress exceeds the yield strength, plastic deformation will occur. If the stress exceeds the tensile strength, cracks will occur, so that on both sides of the piston rod weld It is easy to produce longitudinal cracks.
3 In the heat treatment process, 45 steels are more sensitive to quenching cracks. As long as the shape of the workpiece is complicated, the difference in wall thickness is large, and cracks are easily generated during quenching. The heating temperature of steel-like austenitization is 20, 86, and then Quenched green sodium solution. In order to prevent deformation and cracking of the workpiece, it is preferable to have such a quenching medium at a slower cooling rate of 820,700, and rapid cooling at 7, 3, 550 to avoid the formation of pearlite, and finally to cool slowly at 300,200. . This ideal quenching medium is not currently available, and fractional quenching is used in the practice of heat treatment. However, it is difficult to carry out the cooling solution after entering the inner cavity of the piston rod, and it is easy to cause corrosion to the workpiece. In the production practice, we tried the following method to quench the heating and heat preservation, and then the air was cooled to 750, and the water was quenched and oiled.
After the heating and heat preservation of 781, the direct quenching and quenching of the industry is not ideal. The pass rate of the magnetic powder flaw detection shield product is only about 5. 6 We are still making a fuss about sub-temperature quenching.
82,1 heating and holding for 40 minutes, cooling to 730 for 30 minutes, water and oil quenching and water cooling for 3,5 seconds. With this treatment method, the median ratio is increased to 70, 80, but the product quality is still å«+ stable.
4 The choice of the final heat treatment process The final heat selection of the art is only slightly adjusted on the basis of the above-mentioned first process, the water quenching is changed to water air quenching, and the other small changes are as follows: heating and holding for 40 minutes to cool down to 730 heat preservation for 30 minutes water air quenching water cooling 8,12 seconds, after quenching and fire, first in 200 heat for 30 minutes and then rise to 4801 for high temperature tempering and heat preservation for 90 minutes.
A total of 97 apricots were examined for magnetic particle inspection, and 4 cracks were found.
5 process analysis and 40 minutes of thermal insulation carbon to dissolve the austenite, with the extension of the holding time, the carbon iron is widely diffused, the austenite composition has become uniform, and the workpiece begins to slowly cool down in the furnace. From 82,1 to 73,1 takes about 30,40 minutes. In this process, ferrite is precipitated from the austenite. As the ferrite enthalpy increases, the carbonaceous strontium in the austenite also increases. . After the 730 is kept for 30 minutes, the overall temperature of the product is uniform. At this time, the workpiece is taken out from the furnace and quenched directly. During the quenching process, the austenite transforms into martensite and the least retained austenite, so that after quenching The structure is ferritic martensite + retained austenite, and the rest of the austenite to martensite workpiece is swollen. Since the presence of retained austenite offsets the expansion of the partial volume, the air cooling is taken after 8 and 12 seconds, and the temperature of the workpiece is about 150 250, and the martensite transformation temperature of the steel is 340. The tissue transformation is completed during the slow cooling process in the air. During tempering, it is kept at 200 1 for 30 minutes to evaporate and discharge the inner part of the piston rod, so as to avoid the high gas pressure caused by the gas not being discharged inside the workpiece at high temperature.
From the above analysis, we can see that the quenching temperature has dropped from 820 to 730. This greatly reduces the thermal stress of quenching. Similarly, quenching in the sub-temperature zone reduces the microstructure stress due to the presence of retained austenite. Finally, Slowly cooling at 150, 25, 0 to complete the tissue transition, which again reduces tissue stress. In this way, the thermal stress and the structural force of quenching are sufficiently reduced during the heat treatment, which greatly reduces the possibility of cracking of the workpiece during quenching.
6 Summary air compressor grade piston rod 45 steel has been treated 2, 3 batches with the above process, the total number of workpieces is more than 200, the qualified rate of magnetic particle inspection products is stable at 95, which better solves the problem of 45 steel quenching crack . Using this method of sub-temperature quenching, we believe that this can not only solve the quenching crack problem of the 01-stage live-cold rod, but also solve the problem that some products made of 15 steel with complicated shape or large wall thickness difference and hard to crack occur. .
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