Table 2 Mechanical properties and hardness of SS400 Tensile strength after fracture Average elongation of ferrite size 2 Ultrafine grain steel HAZ grain growth trend and structure, performance To study the effect of welding heat cycle on ultrafine grain steel f Use Gleeble "1500 welding heat simulation test machine pair. (a) is the influence of welding heat input on the original austenite grain size in the coarse grain area at a peak temperature of 1350 * C. The grain size of the original austenite increases continuously. When t8 / 5 is 20S, the grain size of austenite reaches 170 pts. This shows that the grain growth tendency of the welding heat affected zone of ultra-fine grain steel is serious.
(B) The effect of the peak temperature on the original austenite grain size at t8 / 5S production. When Tp is between 1100 * CT1200 * C, the austenite grain size begins to coarsen obviously. This temperature range can be regarded as SS400 The effect of the steel roughening temperature Tp on the original austenite grain size. The results of the thermal simulation test of 400MPa grade steel welding (c) and (d) are the results of microhardness measurement. It can be drawn from the data in the above chart: when TpWSO-C, with the increase of / 5, the hardness of the coarse-grained area of ​​the heat-affected zone gradually decreases and tends to be stable: when t8 / 5 = 5S, with the peak temperature As it rises, its microhardness gradually increases. After ta / s = 5S and welding heat simulation at different peak temperatures, the hardness of the entire heat-affected zone of SS400 steel is not lower than that of the base material, so it can be predicted that when t8 / 5 = 5S, the heat-affected zone of SS400 steel is not Applicability study of pulse MAG welding of ultra-fine grain steel of 3400MPa grade will appear softening phenomenon. The test plates are butted and pressed on the worktable inlaid with shaped copper bars, and 95% Ar + 5% C02 gas protection is used for small line energy Pulse MAG welding SS400 steel pulse MAG welding specifications and the corresponding tensile properties of the welding head are listed in Table 3. Table 3 SS400 pulse MAG welding and joint tensile test results (all broken in the base material) No. current voltage welding speed test temperature. c Tensile strength MPa elongation Hardness distribution in the middle of No. 5 joint Because ultrafine grain steel is mainly obtained under deformation conditions and cannot be recovered by heat treatment, HAZ may appear to soften after welding, especially when high heat It's more obvious when you type. However, the influence of this peripheral softening on the overall strength of the joint is controlled by other factors, such as the width of the local softening zone, the plate thickness, and the strength of the weld seam. The tensile of SS400 joints under the three specifications are all broken in the base material, indicating that at least when tSA grows seriously in the coarse grain area of ​​the welding heat affected zone, its structure is mainly U bainite, along the original austenite grain boundary , There are more side lath ferrite, this kind of structure toughness level is often lower. In order to evaluate the performance level of the heat affected zone, a V-notch impact test was taken from the welding test plate, the sample size was 2.5X10X55, the position of the notch is as follows, the test results are listed in Table 4 Table 4 Pulse MAG welded joint heat affected zone impact test Results (J / cm2) The sample crystalline ratio of the base material notch impact sample notch location schematic 0 * indicates t8 / 5 = lS (1 single from the impact energy test results, the value of the heat affected zone is compared with the base material There is no obvious difference, but the crystalline proportion of the fracture in the heat affected zone of the low temperature test is lower than that of the base material. Position I The crack growth front is composed of the weld seam + coarse grain area + fine grain area + base material, and the crystalline fracture ratio is the most; the position is The coarse-grained area + fine-grained area + base metal is composed of the second highest proportion of crystalline fractures; the position I is composed of the fine-grained area + the base material, and the proportion of crystalline fractures is the same as the base material. These results are obviously the same as the coarse-grained area at the crack front The proportion of coarse grains in position I is the highest, and the proportion of crystalline fractures is also the highest. The proportion of coarse grains in position m is zero, and the proportion of crystalline fractures is also zero as the base material. The toughness of the crystal region is lower than that of the base material. However, due to The welded joint, especially near the fusion line, is a composite structure, and the coarse grain area with poor toughness only accounts for a small proportion of the crack propagation front, so the crack resistance of the welded joint does not necessarily have a significant gap compared with the base material. This has been reflected in the impact value. However, due to the small size of the test piece and the thickness of only 2.5, the conclusion of the test must be further confirmed by the fracture toughness test.
4 Conclusion The ultrafine grain steel welding heat affected zone has a serious tendency to grow grains, and the growth rate grows rapidly with the increase of welding heat input. Although the 400MPa grade ultrafine grain steel welding heat affected zone grains are severely coarsened, But there is no softening.
There is obvious brittle structure in the welding heat affected zone, but it is difficult to see the embrittlement phenomenon on the notched impact energy. Judging from the proportion of crystalline fractures, the more coarse grains in the front of the notch, the greater the proportion of crystalline fractures. This indicates that the toughness of the coarse grains is lower than that of the base metal, but how much is the local brittleness of the weld joint , Still needs further research to confirm.
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