Effect of chemical composition of X80 steel φ1422 mm×35.2mm bend on the performance of induction heating bend
In order to study the effects of different chemical components on the properties of hot simmer pipe bends, the mechanical properties and microstructure of two kinds of chemical components of X80 steel φ1422 mm× 35.2mm pipe were compared after different bending processes. The results show that the chemical composition of the main pipe has a great influence on the mechanical properties of the pipe bend. When the chemical composition of the main pipe is different, the optimum heat treatment process range is obviously different, and the tissue heredity and the strength and toughness match obtained by quenching and tempering treatment are completely different. When the chemical composition of the main pipe deviates greatly, it is easy to cause problems such as poor matching performance of the pipe bend, small adjustment interval of the process and difficult production.
Bend is an important part of long-distance pipeline [1-3]. With the continuous development of pipeline construction, the economic demand for the construction of large-diameter, high-pressure and long-distance oil and gas transmission pipeline is increasing, and the production and supply of large-diameter thick wall induction heating bend has become the norm. There are many factors affecting the process performance of bend products, and the chemical composition of raw materials is one of the main factors . Reasonable chemical composition of the main pipe can provide a matrix with stable performance for the manufacture of bend, and expand the scope and space for subsequent process adjustment. If the chemical composition of the main pipe is improper, it is easy to cause poor performance matching between the pipe body and the weld, resulting in failure of bend process adjustment. Therefore, the chemical composition of the main pipe is very important for induction heating bend. However, it is worth paying attention to how it affects the performance of bend and whether the main pipe with different chemical composition can be optimized by adjusting the bending process. In this study, the bend performance of two 35.2mm thick main pipes with chemical composition after hot bending by different bending processes is compared, and the law is studied, in order to provide technical support for the production and manufacture of thick wall induction heating bend.
Test materials and methods
In this study, two kinds of A and B produced from the same steel plant were used Φ 1422mm × 35.2mm longitudinal submerged arc welded pipe is used as the main pipe. The chemical composition of the main pipe body and weld is shown in Table 1 and table 2 respectively.
Table 1 main chemical composition of main pipe
Table 2 main chemical composition of main pipe weld
For the plate for the parent pipe of X80 steel grade 35.2mm large wall thickness bend, the influence of hot bending and heat treatment process on the performance of steel pipe, such as hardenability, reduction of steel pipe strength and tempering stability, that is, through Cr, Mo and Ni solid solution strengthening, while controlling the upper limit of C and Mn content to reduce the tendency of tempering brittleness .
It can be seen from table 1 and table 2 that parent pipes A and B belong to low C and Mn NB Mo alloy steels. Comparing the two chemical compositions, on the basis of controlling the upper limit of C content, parent pipe a improves the content of C, Mo and Ni elements, improves the strength of pipeline steel, improves the hardenability of pipeline steel, and ensures that the pipe can obtain more bainite structure with excellent performance after austenitizing heating and rapid cooling [6-7]. The improvement of Ni element in the weld optimizes the impact toughness of the weld and lays a composition foundation for good matching of overall properties.
According to formula (1), the complete austenitizing temperature of steel is:
The austenitizing temperature of steel A was determined to be 924℃ and that of steel B to be 939℃.
At the temperature above austenitizing, two kinds of pipe were bent by induction heating. A and B pipe were simmered at different heating temperatures. The quenching temperature of 940℃, 970℃, 1000℃ and the same tempering process were used for quenching and tempering, and six kinds of samples with different heat treatment conditions were obtained.
In accordance with the standard requirements of the bending induction heating bend sampling, tensile, charpy impact and hardness test respectively. The mechanical tensile is the round bar sample, the diameter of the standard distance is 12.7mm, the standard distance is 50mm, the test equipment is UTM5305 material testing machine, the test standard is ASTM A370-18; Charpy V-notch impact test, the test temperature is -10℃, sample size is 10mm×10mm×55mm, the test equipment is ZBC2752-B impact testing machine, the test standard is ASTM A370-18; Metallohardness test was carried out using a 10kg load Vickers hardness tester (KB30BVZ-FA, test standard ASTM192-17). The inner and outer surfaces of the base metal were 1.5mm each and the center of wall thickness was evenly taken 9 points for the test.
Physical and chemical properties were tested and analyzed in the neutral zone with unchanged wall thickness and weld at the bending parts of a and B bending pipes. Charpy impact temperature was -10℃. The test results of mechanical properties were shown in Table 3, and the curve of mechanical properties changing with temperature was shown in Figure 1.
Tab.3 Test results of mechanical properties of two bend pipes
FIG.1 Mechanical properties of two kinds of bend with temperature change curve
It can be seen from the test values and data distribution that, under the three heating temperatures, the strength of base metal of bend A increases first and then decreases with the increase of quenching temperature, and reaches the peak value at 970℃. The yield strength of base metal is 640MPa and the tensile strength is 729MPa, and the numerical distribution is optimal. The tensile strength of the weld shows a decreasing trend with the increase of temperature, and the hardness values meet the requirements at all heating temperatures. The test values have enough allowance and are relatively stable.
The strength and hardness values of base metal of B bend increase obviously with the increase of temperature, and meet the standard requirements at all temperatures. The tensile strength of weld seam decreases slightly at first and then increases, showing an overall upward trend. When the temperature increases from 940℃ to 970℃, the strength value does not change significantly with the temperature and is relatively low on the whole. When the temperature reaches 1000℃, the strength value increases significantly.
There is no obvious linear relationship between weld impact energy and heating temperature of bend A and B. With the increase of heating temperature, weld impact energy shows a trend of rising first and then decreasing. Under the three heating temperatures, weld impact energy reaches the maximum value at 970℃. The impact energy of base metal shows a decreasing trend with the increase of heating temperature. Among them, the impact energy of base metal of B bend changes obviously with the increase of heating temperature. When the heating temperature is 1000℃, the impact energy is 41J, which is lower than the standard requirements .
Based on the above experimental values, the mechanical properties of bend A meet the standard requirements at three heating temperatures, and the overall performance index of bend A is better than that of bend B. However, when the heating temperature of B bend is 940℃ and 970℃, although the mechanical properties can meet the requirements, the strength value is low, the base metal is sensitive to impact temperature, and the safety allowance is low.
The microstructure at 400 times of the wall thickness center of the neutral region of bend A and ELBOW B is shown in FIG. 2 and FIG. 3 respectively. After quenching and tempering treatment at different temperatures, the microstructure of the body of bend A and bend B is granular bainite with uniform distribution, and the bainite grains gradually grow with the increase of quenching heating temperature. The dotted or linear black matrix distributed between the ferrite matrix is m-A island structure [9-10]. As can be seen from the figure, the M-A island structure of A bend is distributed in dotted or linear dispersion, uneven and even adjacent to each other. However, the m-A islands in B bend were mostly point-like, and the number of lines was relatively small. Under the same process conditions, the grain size of bend A is relatively coarse and the local lath martensite morphology is more obvious at 970℃ and 1000℃.
When the heating temperature is 970℃, the microstructure of the weld at the bending section of bend A and B is shown in Figure 4. The microstructure of the weld at the bending section of bend A is dense, mainly consisting of acicular ferrite + granular bainite. The microstructure of the weld at the bending section of the B bend is granular bainite + partial polygonal ferrite, and the ferrite is distributed at the austenite grain boundary in a long strip or block.
FIG.2 Metallographic structure of the middle wall thickness center of bend A
FIG.3 Metallographic structure of the center of wall thickness in the neutral zone of bend B
FIG.4 Metallographic structure of weld center of bending section of pipe A and B at 970℃
Analysis and discussion
The content of C, Mn, Ni and other austenite-forming elements in A and B parent tubes is high, and these elements expand the stable presence region of austenite during induction heating, which is conducive to austenite nucleation and growth [11-12]. At the same time, with the increase of heating temperature, alloying elements such as Cr and Mn are constantly dissolved, and the strengthening effect of solid solution is continuously enhanced, so the strength of both A and B bend gradually increases. In the process of austenite grain coarsening, the content of carbide in the matrix is increasing, and the surface hardness of the material increases with the increase of carbon content.
Due to differences in chemical composition, A main pipe austenitizing temperature is relatively low, as the dissolution of alloying element in the process of heating and austenitic grain size grew up, the number of the grain boundary to reduce gradually, the role of material resists the pay slip is abate, weakening dislocation strengthening effect is obvious, reached A critical state, early strength and toughness. However, the austenitizing temperature of main pipe B is relatively high, and the alloying elements dissolution is relatively slow. Although the M-A islands in the ferrite matrix in the pipe body of bend A and B are distributed diffused, compared with bend A, the number of M-A islands in the pipe body of bend B is small and mostly distributed in isolation, which can not only improve the toughness of the material, but also reduce the toughness [13-15]. Therefore, with the increase of heating temperature, the impact toughness of b bend decreases gradually and is lower than that of A bend.
A main pipe C, Mn, Ni elements in the weld were higher than B main pipe, and C, Mn, Ni austenitic stabilizing element, can reduce the phase transition temperature of the γ→α transformation, inhibit the proeutectoid ferrite phase change at high temperatures, so in the process of A weld metal forming proeutectoid ferrite microstructure transformation was suppressed, and weld metal were formed part B proeutectoid ferrite, They are long strips or lumps distributed at grain boundaries. Granular bainite has good strength and toughness, while polygonal ferrite has poor strength. Therefore, under the same process, the weld strength of bend A is much higher than that of bend B.
(1) The mechanical properties and microstructure of the bend obtained by the same process are obviously different due to the difference in the chemical composition of the Main pipe, but the bend properties can be optimized by adjusting the heat treatment process.
(2) The chemical composition of the main pipe determines the austenitizing temperature during the heat treatment process, which has a great influence on the temperature range of the heat treatment process adjustment of the bend. Under the same heat treatment process, the mechanical properties of the bend obtained by the Main pipe with different chemical composition are greatly different.
(3) For the φ1422mm×35.2mm pipe of X80 steel, reasonable chemical composition can expand the range of bend process, the performance of the bend is relatively stable, if the deviation of chemical composition is large, the problem of small process adjustment window is easy to occur, and there is a certain process control risk in batch production.
Author: GUO Baoli, LI Zhongcheng, LIU Bin, HUANGFU Xudan
Source: China Pipe Bend Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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