The high temperature mechanical properties of non-quenched and tempered steel MFT8 were studied by high temperature tensile and long-term experiments. The fracture morphology and microstructure evolution of high temperature tensile and long-term experimental samples were observed by scanning electron microscopy. The results show that the grain size and precipitate morphology of MFT8 steel have not changed significantly from room temperature to 500 °C. The decrease of high temperature strength is mainly due to the weakening of dislocation strengthening. At 450 °C, MFT8 steel has to wear. The low temperature creep characterized by crystal fracture and high plasticity, although some creep pores appear inside the material, the main cause of creep rupture is wedge cracking caused by stress concentration at the precipitated phase particles; linear extrapolation of persistent data The endurance strength of the MFT8 steel at 450 ° C at 100,000 hours is 184.4 MPa.
Non-quenched and tempered steel has similar mechanical properties to quenched and tempered steel, and at the same time eliminates high energy consumption processes such as quenching and tempering, and has gained extensive attention and application in recent years. As a typical cold-worked non-tempered steel, the MFT8 adopts a combination of cold-rolling and aging treatment, and uses fine grain strengthening, dislocation strengthening, dispersion strengthening, etc. to make the strength satisfy 8. 8 The national standard for fastening bolts has been fully applied in the field of automobile manufacturing. At present, the research on non-steel steel mainly focuses on the analysis of strengthening mechanism and the improvement of processing and production process, that is, the system theory of the relationship between the organization, performance and production process of such materials at room temperature has been established. It is worth noting that high-temperature fasteners are also in large demand in the fields of energy and power. If non-tempered steel can be used in these fields, it will be beneficial to the realization of China's energy-saving emission reduction targets. However, according to the classical theory, various strengthening mechanisms of non-steel steel have the possibility of degradation at high temperatures, such as grain coarsening, dislocation cancellation, and precipitation phase dissolution.
At present, there are few studies on the high temperature mechanical properties of non-steering steel, and it is mainly reported for the transient high temperature tensile properties, lacking the research on the thermal strength of the material, that is, the long-term performance of high temperature creep. In view of the above problems, this paper analyzes the high temperature tensile and long-term properties of non-quenched and tempered steel MFT8, studies the weakening degree of different strengthening mechanisms at high temperature, analyzes the high temperature tensile and long-lasting fracture behavior, and provides a theory for expanding the application range of the material. And experimental basis.
1. Experimental materials and methods
The nominal composition of the non-quenched and tempered steel for the experimental use of MFT8 is shown in Table 1. The diameter of the hot rolled MFT8 steel is 11.2 mm, and after cold drawing to a diameter of 9. 0 mm, it is kept at 250 °C for 2 h. For optimum mechanical properties at room temperature. After the standard production process, the microstructure of MFT8 steel is shown in Figure 1. The basic structure is ferrite + shredded pearlite, and the grain size is 12 (determined according to GBT 6394 -2002).
Table 1 Nominal chemical composition of non-quenched and tempered steel MFT8 (mass fraction, %)
Nominal chemical composition of non-tempered steel MFT8
Original structure of MFT8 steel
Figure 1 Original structure of a MFT8 steel (a) and precipitated phase morphology (b)
The high temperature tensile test temperature was selected to be 100, 150, 200, 250, 300, 400 and 500 °C, and the experiment was carried out in accordance with GB/T 4338-1995. The long-term experiment was carried out on the GNCJ-100 creep durability tester. The experimental conditions were analyzed by the high temperature tensile test results. The specific temperature was 450 °C and the stress range was 680-290 MPa. All samples were fractured by JSM-6360LV scanning electron microscopy after fracture. At the same time, in order to analyze the microstructure evolution during high temperature stretching and long-lasting process, the morphology was analyzed by longitudinal and horizontal sampling in parallel areas behind the fracture. The samples used for tissue observation were treated with a standard metallographic preparation method (sandpaper sanding + mechanical polishing) and then etched with a 4% nitric acid solution.
2, conclusion
1) There is no significant change in grain size and precipitation phase morphology of MFT8 steel from room temperature to 500 °C. The decrease of yield strength and tensile strength is mainly due to the weakening of dislocation strengthening mechanism;
2) At 450 °C, MFT8 steel exhibits typical low-temperature creep characteristics. The material is transgranular fracture and has high plasticity. Although some creep pores appear inside the sample, the cause of creep fracture is precipitation. Wedge cracking caused by stress concentration at phase particles;
3) The linear extrapolation of the persistent data shows that the permanent strength of MFT8 steel at 450 °C for 100,000 hours is 184.4 MPa, which has the potential to replace the same grade of quenched and tempered steel in thermal power plants.
relevant information:
Effect of Stabilization Treatment on Performance of 8.8 Bolts in MFT8 Steel Production
Deformation strengthening effect of MFT8 bolt
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