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Research on the best heat treatment process of 30CrMoA steel bolt fasteners in Harbin Engineering University Chang Tiejun Yang Shiwei 1 Yin Shutong Gong Zhengchun 2 (1. School of Mechanical and Electrical Engineering, Harbin Engineering University, Harbin 150001, China; 2. Harbin Boiler Factory, Harbin 150040, Heilongjiang) The quality problem was carried out to study the influence of quenching and tempering temperature on microstructure and mechanical properties of 30CrMoA steel, and the influence of heat treatment process on microstructure was studied by electron microscope. The 940 1.5h air heat treatment process optimized on the basis of the experiment not only meets the requirements of the test standard, but also has a satisfactory microstructure.
CrMoA steel is mainly used in the manufacture of steam turbines and boilers for the manufacture of bolt fasteners, flanges and nuts with operating temperatures below 500 C. Bolt fasteners are one of the key components in boiler production. 30CrMoA steel is a high-strength and high-toughness low-alloy structural steel, which is usually quenched and tempered. Harbin Boiler Factory uses 30CrMoA steel as the boiler fastening bolt. The temperature is 480C. The factory uses the traditional 880C water quenching 620C tempering 1.5h heat treatment process. The performance after heat treatment meets the factory inspection standard 3084015 (see Table 1). However, the fastening bolts produced by the factory have been subject to serious products that do not meet the inspection standards for several years. The factory performed performance statistics on the bolts with diameter less than 40 CrMoA produced between January 1985 and August 1989 (see Table 2). The main factors for the product failure were the strength index ab and the hardness value HB. HB out of the standard range of unqualified rate accounted for 28%.
The hardness value HB is the most sensitive indicator in the heat treatment process, and the strength indicators a and a have a direct relationship with HB. Using the least square method to perform a linear regression on the statistical 173 data on the computer, the relationship between a, a and HB is obtained as follows: time specification quantity mechanical property fluctuation range fluctuation difference unqualified rate a/MPa54092738713. Office 14 199 (often M ((9C4lcTe) male mountain 螨 知 哈尔 ¥ ¥ 工 w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
1.2 Experimental methods Table 3 is the chemical composition of the experimental steel, heat treatment using H15 box type resistance furnace, static tensile test according to GB6397-86 standard Warranty 0 2 Experimental Results and Analysis 2.1 Mechanical Properties Determination and Analysis Table 4 shows the mechanical properties test results. (a) For the same quenching temperature (940C), different tempering temperatures for as, a! , and the effect of HB; (b) the effect of different tempering temperatures on the quenching temperature (940 C), 4 and Ak values. Approximate map for various heat treatment processes and inspection standards 3084015. It can be seen that as the quenching temperature is constant, with the increase of the tempering temperature, as and ab decrease obviously, and the hardness value HB decreases sharply when the tempering temperature exceeds 640C. Therefore, the tempering temperature of 640 C is a very sensitive temperature of HB. In order to obtain a homogeneous product of HB, the temperature accuracy of the tempering furnace must be strictly controlled. From the various heat treatment processes and the approximation chart of the inspection standard 3084015, it can be seen that the hardness value HB of the 5h air heat treatment process is located at the upper limit of the inspection standard, so it is difficult to control the pass rate of the HB value in actual production, and it is obvious The tempering temperature of 620C makes the HB value higher. At the matching temperature (620S, the higher the h quenching temperature is, the higher the quenching temperature is, the more the unreasonable product is caused by the above statistical analysis. To explore the optimal heat treatment process for 30CrMoA steel bolt fasteners, so that the product can ensure that the hardness value HB can be in the median area of ​​the inspection range is the key to solve the problem under the premise of ensuring the strength index and not lower than the inspection standard. . 1 Experimental scheme and experimental method 1.1 Experimental scheme design The experimental scheme design mainly examines the influence of quenching temperature and tempering temperature on the structure and properties. For this purpose, 850, 880, 910, a quenching temperature is designed; 620, 640, 650 is selected. 660C4 tempering temperatures were used to investigate the effects of different quenching temperatures on mechanical properties and the same quenching temperature on the mechanical properties of the same tempering temperature. The nbli hardness value HB from which the optimum quenching temperature and the optimum tempering temperature are preferred is also higher. Table 4 Mechanical properties under different heat treatment conditions No. Heat treatment process Less mechanical properties 逼 Heat treatment process and inspection standard 3084015 Approximation chart Naturally preferred from the above heat treatment process 112 The hardness value of the process is just in the median area of ​​the test standard (HB268), and the strength index ah also has a certain margin (ah = optical microscopy and scanning electron microscopy analysis of the microstructure of different quenching temperatures, analysis found that with the increase of quenching temperature, Markov in the tissue The body content is increased and the martensite lath is refined as the quenching temperature is increased, but the martensite lath becomes thicker when the quenching temperature exceeds 970C. The optical metallographic structure obtained by quenching, (c) its corresponding scanning electron microscope microstructure, (b) the effect of different tempering temperatures of 940C water quenching on the mechanical properties of 30CrMoA steel. It can be seen that the microstructure obtained by water quenching at 880C is quenched Markov and can be clearly seen. The martensite content (M) + retained austenite (A) + a small amount of ferrite (F) in the 940C water quenched structure, 940 The martensite content is higher than that of the 880C water quenching structure and the martensite water quenching structure is quenched martensite (M) + austenite (A) and the lath bundle is relatively fine. The morphology of the carbides in the tissue. It can be clearly seen that the former organized the metallographic microstructure of the cold obtained from 71994-2015 ChinaAcademicournal. It can be clearly seen that the microstructure obtained in the two heat treatment processes is very different in morphology. Although both structures are tempered sorbite (S), 880C/30' water + 620C/1.5h air heat treatment The soxite obtained by the process is much coarser than the sorbate obtained by the 940C/30' water + 650 ° C / 1.5 h air heat treatment process, and the orientation relationship of the original martensite (M) is clearly retained (see (a) (b)). In addition, in the transmission electron microscopy analysis of the microstructures obtained from the above two heat treatment processes, it was found that the carbide morphology in the two tissues also had a large carbide morphology in the microstructure obtained by the process, (d) 940C/30' The water +650 5h air heat treatment process is mainly composed of undissolved carbides, while the latter obviously has carbide precipitation. It can be seen from the above analysis of the organization that the quenching temperature of the original process using 880 C is significantly lower, so that the carbide is not completely dissolved, resulting in imperfect tissue transformation, which will inevitably affect its mechanical properties. The 940 650C/1.5h air heat treatment process, which is preferred from many processes in the process test, not only has satisfactory mechanical properties, but also satisfactory structural transformation products. 3 Conclusion The quenching temperature and tempering temperature of the 1.5h empty are both low, so the obtained structure is relatively coarse. The air heat treatment process makes the strength index and the larger margin and the hardness value HB is located in the median area of ​​the inspection standard. The microstructure obtained by the heat treatment process is ideal. Strict control of the tempering temperature and the uniformity and accuracy of the tempering furnace are the key to ensuring that the hardness value HB is qualified.
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