1. Residual stress and deformation:
When steel is heated to about 450℃, it changes from an elastic body to a plastic body, and plastic deformation is likely to occur at this time. On the other hand, when heated to about above 450℃, the stress disappears due to the residual recrystallization. Therefore, as long as it is not heated quickly, even if there is residual stress, it will not deform due to heating. However, if it is heated quickly, the part heated to above 450℃ will become a plastic body, and the plastic body will deform due to the residual stress of the internal low-temperature part. After these parts cool down, the deformation will be shown. Because the vacuum furnace heats slowly, there is less heating deformation. However, even if the parts heated in the vacuum furnace are quenched in oil, the same quenching strain as ordinary electric furnace heating will occur. Uniform and slow heating is difficult to achieve at the heat treatment site. Therefore, the parts to avoid quenching strain should be annealed to eliminate stress first. In addition to eliminating stress by heat treatment, for large parts, the method of eliminating stress by mechanical vibration is also effective.
2. How to prevent deformation.
Deformation is the result of the combined effect of phase change stress and thermal stress. To prevent deformation, the most important thing is to reduce these stresses as much as possible.
(1) Before quenching, the processing stress (internal stress) should be eliminated in advance, and annealing should be done carefully, at least stress elimination annealing (450~600℃) should be performed.
(2) Heating should be carried out slowly and evenly. It is best to preheat the steel parts. The first stage of preheating is 400℃, and the second stage of preheating is preferably a temperature close to the critical temperature.
(3) To prevent sagging caused by deadweight, iron plates should be laid on the heating furnace bed, or supports should be used. The spacing of the supports is: 1≤3d (d is the workpiece diameter). It is best to use a heating method that hangs the parts.
(4) Austenite isothermal quenching graded quenching, or hot bath quenching should be performed.
(5) Press quenching (press quenching) or die quenching (die quench) should be performed.
(6) Anti-warping measures should be taken before quenching.
(7) Reinforcement ribs should be tied to the side where the quenching bend occurs before quenching.
(8) The wall thickness of the parts should be uniform, process holes and removable holes should be opened, or asbestos, clay, etc. should be filled in the thin wall areas.
In addition, to prevent the deformation of non-ferrous metals, die quenching has been popular for a long time (that is, the quenched parts are placed in the mold for cooling during quenching). This quenching method has the least deformation and is often used to obtain the specified shape. The commonly used Gleason gear-forming quenching machine is an ordinary die-quenching machine. Aluminum alloy, titanium alloy plate, 17–7pH stainless steel This die quenching method is widely used in precision quenching (shaping quenching) of (dispersion-hardened stainless steel). The following three die-quenching methods have entered the practical stage.
① Die casting die quenching method (sliot dlie quenching). Only the male die is used, and hard steel pellets are used on the female die, so the processing model only takes half the time, which not only reduces costs but also is suitable for parts with very precise shape requirements. This method has now been applied to aluminum alloys.
② Prestress forming method (pre-stress forming, another name is lizard forming method). This method is to clamp the parts with a clamp at room temperature, clamp them with a force less than the yield point of the material, and then heat them to the artificial aging temperature. If the temperature rises, the yield point decreases, and the external force will naturally exceed the yield point, so The parts are permanently deformed and formed, that is, the method of using scorpion forming.
③Hot forming or hotsizing. The method of pre-rough forming the parts and then putting them into a hot mold (450~750℃) for precision molding under pressure is called hot mold sizing. Among the methods to prevent quenching deformation, there is also a method called blank treatment. This method is to pre-apply a treatment similar to the formal heat treatment.
3. How to correct deformation
(1) Press tempering should be performed. Press tempering uses the plastic flow caused by structural changes. Therefore, the more thoroughly quenched the parts are, the more effective the press tempering is, and it only needs to be performed once. If there is no structural change, even if the press tempering is repeated, It is also in vain. The pressure applied by the pressure tempering method is large and the pressure application time is long, so it is an effective method to correct deformation.
(2) Apply external force during tempering heating. This method is often used to correct bending, but the pressure applied is small and the time is short. Therefore, it is difficult to say that it is an effective and appropriate method.
(3) Use the shot peening hardening method. This method is used to correct the shape of thin-walled parts. Using this method, the side that is shot peened will bulge out. Taking advantage of this feature, the side that is pre-convex can be shot peened.
4. Test method for quenching deformation
The specimen usually used is half-moon-shaped. This specimen is put into the specified quenching liquid from the quenching temperature for quenching, and then the following five-dimensional changes of the specimen are measured: ① outer diameter; ② inner diameter; ③ opening of the opening; ④ wall thickness; ⑥ thickness. The measured results are compared and studied to obtain the difficulty of quenching the deformation of various specimens. Some test specimens make a notch in the wall thickness as shown in Figure 3-10, which can determine the difficulty of quenching deformation. There are two methods to test quenching deformation. One is to keep the material (steel) of the test piece fixed and test the difficulty of quenching deformation by changing the quenching method (cooling method); the other is to keep the cooling method fixed and test the difficulty of quenching deformation by changing the material (steel) of the test piece. After the test, the results are analyzed and compared. The following is a simple method for testing quenching deformation: open a keyway with a width of 4 mm and a depth of 4 mm on one side of a round bar with a diameter of 15 mm and a length of 100 mm, quench it according to the specified process, and determine the difficulty of quenching deformation according to the degree of bending.
Post time: Nov-05-2024