Standard bolts are used in large quantities and in a wide range of engineering and mechanical fields. Deformation-strengthened non-tempered steels are made. 8.8 high-strength bolts can eliminate the quenching and tempering process, reduce heat treatment processes and equipment, reduce energy consumption, and shorten production cycle. At the same time, it avoids the waste caused by deformation or quenching crack during heat treatment, and has broad application prospects [1 - 4]. The deformation hardening index n is usually used to characterize the deformation strengthening ability of the material. The larger the n value, the larger the uniform deformation amount, the better the cold forming performance and the stronger the strengthening effect. The main production process of non-quenched and tempered steel bolts is: hot-rolled wire → pickling → cold drawing → rolling wire → aging, deformation and strengthening of raw materials after cold drawing, and its mechanical properties, whether it can replace high-strength bolts made of quenched and tempered steel, Whether the deformation-enhanced margin after aging can ensure the safety of the bolts requires experimental research.
1 Experimental materials and methods
The raw material of MFT8 steel is <9mm hot rolled wire, the chemical composition (mass fraction / %) is: C 0. 21 , Si 0. 13 , Mn 1. 37 , P 01015 , S 0. 005 , Nb 0. 04 , Al 0. 047. The experimental materials were cold-drawn and strengthened after pickling and surface pretreatment, and the reduction ratios were 25% and 30%, respectively. The tensile test was carried out on the CSS244300 electronic universal testing Machine for the raw materials, cold drawn materials and aging samples, and the strain hardening index was calculated at any number of points in the uniform deformation interval, and 5 times according to the GB/T 228-2002 standard. Samples; microstructures were observed on a J SM26360LV scanning electron microscope. Longitudinal wire cutting to obtain a transmission electron microscope sample,
The microstructure was analyzed by H2800 transmission electron microscopy (TEM) after double jet electrolysis thinning.
Experimental scheme: (1) Study on the effect of deformation strengthening of raw materials, and judge the effects of cold forming properties and deformation strengthening; (2) Compare the deformation strengthening effects of cold drawing materials with two different reduction ratios, and preferably one of them to treat aging; 3) Analyze whether the final mechanical properties of the bolt product meet the technical requirements after aging, test the final deformation strengthening index, and predict the reliability of its use.
2 Experimental results
2. 1 Material deformation strengthening effect and performance under various conditions
8. Grade 8 (bolt diameter M is not more than 16mm) Bolt technical requirements [5]: tensile strength greater than 800MPa; yield strength greater than 640MPa; elongation after fracture is 12%; HRC23~32. The tensile test was used to test the n value and performance of raw materials and cold drawn materials. After the cold drawing of the bolt, due to the effect of the Bowinger effect, the forming resistance will decrease with the increase of the cold drawing rate. At 30%, the Bauschinger effect is the largest and the compressive true stress is the smallest [6]. It is preferred to treat the bar with a reduction rate of 30% at 300 °C for 2 h, and test the n value and performance index. The performance is shown in Table 1.
2. 2 organization of materials in various states
Microalloying technology is used to add a small amount of alloying element Nb to low carbon steel. Nb dissolved in austenite improves the stability of supercooled austenite, lowers the temperature of pearlite transformation, and changes the size of pearlite. Small, pearlite tablets are shredded. During the hot rolling process, as the temperature decreases, carbides or nitrides precipitate from the austenite, preventing the austenite from recrystallizing, and then the ferrite nucleus forms on the deformation zone of the austenite grains during cooling. Finally, very fine ferrite grains are obtained [7]. After testing, the matrix of the raw material was fragmented pearlite + ferrite with a grain size of 5 μm, and there were obvious precipitates in the enlarged ferrite grains (see Figures 1 and 2).
In order to improve the strength of the material, the raw material is cold-deformed. After cold drawing, the metallographic structure and grain size change little, but a large number of dislocations are formed inside the ferrite, and the dislocation entanglement forms a cellular substructure and the cell wall position. The error density is higher than intracellular, as shown in Figure 3 (a). After aging treatment at 300 °C / 2h, the dislocations in the ferrite recovered, resulting in a distinct subgrain boundary, as shown in Figure 3 (b). The recovery of dislocations and the formation of subgrain boundaries will reduce the dislocation density, eliminate internal stress, and improve the stability of the bolt size.
3 Analysis and discussion
During the entire deformation process of the metal, after the external force exceeds the yield strength, the plastic deformation does not continuously flow like the yielding platform, and it is necessary to continuously increase the external force to continue. This indicates that the metal material has an ability to prevent further plastic deformation, which is the deformation strengthening property. The strain hardening index n reflects the ability of the metal material to resist deformation and is a performance index for characterizing the strain hardening behavior of metal materials. The true stress S2 true strain e curve is a straight line in double logarithmic coordinates, so it can be represented by S = Ken, where K is the true stress at e = 1. In the extreme case, n = 1 means that the material is a perfectly ideal elastomer, S is proportional to e; when n = 0, S = K = constant, indicating that the material has no strain hardening ability, such as soft recrystallization at room temperature. Metal and materials that have been strongly strain hardened.
The experimental results show that the raw material has a high n value and good cold deformation ability. With the increase of the reduction ratio, the amount of plastic deformation increases, the strengthening effect increases, and the mechanical properties of the bolts all meet the technical requirements of 8.8 bolts. Plastic deformation is the cause of strengthening, and strengthening is the result of plastic deformation. As the amount of deformation increases, the value of n decreases, and the ability of the material to continue to strengthen is weakened. The n-value of the deformed material after 30% reduction is 0. 235, which means that it still has a certain ability to resist overload failure. The tensile fractures of the above three states all show that the material is ductile fracture, which indicates that the strength of the material after deformation is improved, but it still maintains a tough state.
The type and nature of the lattice of the metal determine its deformation strengthening effect [8], the slippage of the metal, the difficulty of forming the cell structure, and whether the slip of the screw dislocation can proceed smoothly and other factors have a significant effect on the value of n. Raw materials below
Its characteristics make it have a very high deformation strengthening ability: fine grains; easy to cross-slip ferrite structure; effect of interstitial carbon on deformation strengthening; carbonized material points formed by Nb in material; The dislocation proliferation caused by the deformation process, the combination of the above various factors makes the experimental material have a good deformation strengthening potential, so the test results show a higher n value.
As shown in Fig. 3, after cold drawing, a large number of dislocations are formed inside the ferrite, and the dislocation entanglement forms a cellular substructure [9, 10], and the n value decreases compared with the raw material, and the residual plasticity decreases. Non-tempered steel bolts used in the cold-strengthened state have high-density movable dislocations and other defects in the deformed structure, making them unstable. When the bolts are loaded, these movable dislocations will climb, causing a slight yield and affecting their performance. The aging treatment stabilizes the movable dislocations and also enables the non-tempered bolts to have good overall performance. Due to the aging treatment of the edge dislocation, sufficient energy can be obtained to make the climb, so that the irregular dislocations on the slip surface are redistributed and arranged into walls to form sub-crystals. Therefore, after the aging, the n value is restored, and the plasticity is basically maintained. The higher residual n value guarantees the ability of the bolt to resist accidental overload during use.
4 Conclusion
(1) MF T8 non-quenched and tempered steel has good deformation strengthening potential and is suitable for cold work strengthening.
(2) After the deformation strengthening, the mechanical properties of the experimental materials reach the technical requirements of 8.8 bolts, which can replace the quenched and tempered steel.
(3) After the aging treatment, the n value is restored and the safety of the bolt is ensured.
references
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