Cause Analysis of Cracks in Heat Affected Zone of X52 High Frequency Longitudinal Welded Pipe

pick Summary : Use a metallographic microscope and a scanning electron microscope to inspect the crack fracture and inclusion morphology of the X52 test steel welded pipe, and analyze the micro-area composition of the inclusions , and confirm that there are a large number of densely distributed rare earth sulfide and rare earth sulfur oxide layers in the steel Banded inclusions are the main reason for the formation of cracks , and layered banded inclusions are separated internally under the action of high temperature and welding extrusion force to generate cracks .
Key words : pipeline steel ; high frequency welded pipe ; crack ; rare earth inclusions

, especially the implementation of the " West- to-East Gas Pipeline " project in the Western Development , a large number of low-alloy high-strength steel plates are used to weld and produce large-diameter oil or gas pipelines. The high shear fracture resistance requires the steel plate to have high transverse tear resistance . How to improve the purity of steel and obtain non-deformed metamorphic sulfides have become the core technical issues of various iron and steel production enterprises . X52 pipeline steel is a commonly used steel for welded steel pipes for oil and gas transportation in China. Generally , calcium treatment is adopted to reduce the content of impurities such as sulfur and phosphorus in the steel , and reduce the total amount of inclusions in the steel ; or add rare earth to the steel to achieve The purpose of spheroidizing sulfide is to prevent the formation of long strips of MnS inclusions . Both production processes have achieved good results .
factory produced X52 high-frequency welded pipes of 0323.9 mm X8.0 mm X 12 000 mm , it chose X52 test steel coils treated with rare earth. There are also longitudinal cracks in the welding heat-affected zone of the outer wall of a very small number of welded pipes . In order to analyze the reasons for the cracks in X52 welded pipes , a comprehensive inspection and analysis was carried out on the X52 test steel coils and welded pipe cracks and inclusions .
1 Analysis of test results
1-1 Chemical composition and mechanical properties test
The chemical composition and mechanical properties of the X52 test steel coils of 3 furnace numbers were inspected , and the inspection results are shown in tables 1 and 2 .
It can be seen from Table 1 that the sulfur and phosphorus impurities in the X52 test coil are low , the smelting chemical composition is relatively stable , the RE / S value in the finished steel is about 6 , and the total oxygen in the steel is only ( 18 ~ 32) X10 — 6 . It can be seen from Table 2 that the test steel

Table 1 Chemical composition of X52 test steel coil

 

		Table1		Chemical composition of X52 experimental coils							%
Coil No.	C	Si	Mr	Cr	Ni	V	Q	S	Als	O	RE
1 #	0.11	0.24	1.14	0.02	0.04	0.06	0.010	0.004	0.025	0.0021	0.025
2 #	0.12	0.21	1.25	0.01	0.03	0.06	0.015	0.005	0.004	0.0032	0.024
3 #	0.09	0.15	1.19	0.01	0.04	0.07	0.016	0.004	0.038	0.0018	0.024

Note: ①The tensile sample is a plate drawing sample with a gauge length of 508mm and a width of 381mm within the gauge length, and the test temperature is 25 ° C .
②The size of the transverse impact specimen is 5 mm X 10 mm X 55 mm, and the test temperature is 0° C .
The mechanical properties meet the X52 technical index , and the 0 C transverse impact energy is relatively high . The test steel adopts the wire feeding method in the continuous casting crystallizer , and 300 g of rare earth is added per ton of steel to achieve the purpose of spheroidizing the sulfide .
12 Test coil flaw detection inspection
Double-sided magnetic particle inspection was carried out on the edges of 8 X52 test coils along the rolling direction , and no alarm was found in the results . It shows that the steel coil has no defects such as cracks before high-frequency welding , and the cracks in the heat-affected zone of the welded pipe are generated during the pipe welding process , which excludes the possibility that the cracks come from the X52 test steel coil .
13 metallographic inspection
of the X52 test steel welded pipe was horizontally sampled and made into a metallographic sample, and the microstructure and crack morphology were observed . The X52 welded pipe

Take samples longitudinally from steel coils , and grind inclusion samples for inclusion rating .
The inspection results of microstructure and crack morphology are shown in Figures 1 to 7 .
be seen from the metallographic examination results that the X52 test steel structure is all F + P , and the grain size is 110 ~ 11.5 . According to GB/ T10561-1989

The crack expands outwards and breaks, and two obvious areas can be seen on the fracture, the old and oxidized and blackened crack source area, and the tearing fibrous expansion area during sample preparation . The depth of the crack source area is generally about 10 ~ 20 mm, and some of the length reaches 1 . . mm , some are only 3. 0 ~ 5 . 0 mm , and the length is different . The morphology of the crack fracture and inclusions are shown in Figures 8 to 12 , and the chemical composition of the micro-area is shown in Table 3 .

Table 3 Micro-domain chemical composition
Table 3 Composition of crack source %

drawing number	microzone number	o	S	al	Ca	mn	Fe	La	Ce
Figure 8	1	20.14					76.48	2.33	1.05
	1		0.64			1.29	97.13
	2					1.18	98.82
Figure 9	3						100.0
	4			1.0		1.39	92.56	2.82	2.23
	1	11.29	1.09				70.55	12.36	4.71
Figure 10	2	6.44	4.46				27.35	37.35	24.40
	3	3.08	2.46				58.57	21.67	14.42
	1	16.62	6.95				14.80	13.01	48.62
Figure 11	2	14.08	3.83		0.45		35.48	26.36	19.80
	3	16.57	1.12		0.97	0.88	76.0		4.36
Figure 12	1	4.46				1.71	93.83
	2			2.21		2.37	83.65	7.10	4.67

There are rare earth sulfide and rare earth sulfur oxide inclusions distributed in layered bands in the grain tearing area. Although the size of a single inclusion is less than 10 RD, the distribution is concentrated and the density is relatively high . It can be seen from Figure 8 that the crack depth is about 1-0 mm. Because the oxidation of the crack source in this sample is relatively light , it can be clearly seen that the inclusion particles are embedded in the entire metal matrix of the crack source . Micro-area energy spectrum analysis shows that the crack source area The content of rare earth sulfur oxides is relatively high . It can be seen from Fig. 9 and Fig. 12 that there are layer-like distribution of rare earth sulfide and rare earth sulfur oxide inclusions in the crack root and tear area of the sample . Reaching 02 ~ 1.0 mm, the rare earth local micro-area composition reached about 3% . It can be seen from Figure 11 that in the middle crack sample of 1767 welded pipe, the composition analysis of the micro-area shows that there are rare earth sulfur oxide inclusions in the crack , and the re/s reaches 8.8-12.1 times . The above electron microscope inspection results show that cracks are closely related to rare earth inclusions , and rare earth sulfide and rare earth sulfur oxide inclusions can be found in cracks , and the RE/S value in the inclusions is relatively high .
2 Analysis and Discussion
2-1 Causes of Welding Crack Defects From the results of metallographic and electron microscopy inspections , it can be known that there are a large number of densely distributed , layered and banded rare earth sulfur oxides or rare earth sulfide inclusions at the source and root of welding cracks . Rare earth sulfides or rare earth sulfur oxides have a higher melting point of 1 690 ~ 2 450 °C 1 ], which are brittle inclusions . The reason for the formation of cracks is due to the strong extrusion of the hot metal , which makes the original longitudinally distributed layered inclusions bend outwards too much and cause the cracking phenomenon ［ 2,3 ］ . Analyzing the internal causes of cracks from a microscopic point of view , because the layered inclusions are in a discontinuous state inside the metal , under the action of welding extrusion force , the internal separation of rare earth sulfide or rare earth oxysulfide interlayers results in cracks , which can be It is confirmed by the inspection results of inclusions in the corresponding parts of the sample on both sides of the crack .
22 Causes of rare earth inclusions in steel
When rare earth is added to the treatment, only the size and distribution of inclusions after deoxidation and desulfurization and the influence on the mechanical properties of steel are considered , and the influence of inclusions on welding performance is rarely involved , especially for special high-frequency longitudinal seam welded pipes. affect . Regarding the calculation of the amount of rare earth added in steel, Lu et al. proposed an approximate calculation formula for the minimum amount of rare earth added when considering the effects of rare earth deoxidation and desulfurization :
RE= 5.85 (［O］ Original- ［ 0 ］ Residue + 2. 93 ［ S ］ Original- ［S ］ Residue ) + R EResidue
the average atomic weight of mixed rare earths is 140.5 , and based on the chemical equivalents of RE2O3 and RE2S3, 5.85X10-6 rare earths are used to remove 1X1CT6 oxygen ; 2.93X10-6 are used to remove 1X10-6 sulfur Rare earth [4] . It can be seen from the calculation formula that the main function of rare earth is deoxidation , followed by desulfurization .
In modern steelmaking production technology , molten steel is generally pre-deoxidized by deoxidizers such as Al or Si - A1 - Ba , so that the [0] activity in molten steel reaches ( 20 ~ 30) X10-6, and then powder spraying is carried out on this basis. Calcium treatment to purify steel or add rare earth spheroidization to modify and treat sulfide . At present, rare earth wires are widely used in continuous casting crystallizers in steel mills , and the rare earth yield is as high as 70 % to 90 % [5] . At this time, the rare earth added should only consider the effect of desulfurization , generally calculated according to the final sulfur content, Rare earths are added at a ratio of RE/S = 3 . For example, when WISCO produces X60 and X65 pipeline steel , rare earth filaments are fed into the slab continuous casting crystallizer . Theoretically, the amount of rare earth required to turn all the sulfur into RE2S3 inclusions is 2.94XS%, but the actual RE/ The ratio of S = 2.5 is added in [5] .
Gondon et al. show that when the sulfur content in steel is extremely low (W0.003%), the harm of sulfide has been greatly weakened, the transverse performance has been significantly improved , and the metamorphic effect of rare earth is not obvious at this time ［ 4] . It is generally believed that for high-quality pipeline steel , if it is not treated with rare earth , its sulfur content should be at least less than 0 005% . When the sulfur content is very low, its performance is very sensitive to the change of sulfur content . In order to stabilize the performance , it is also beneficial to perform rare earth treatment at the same time as low sulfur treatment .
can be seen from Table 1 , the content of [0] in the X52 test steel is relatively low , and when the sulfur content is only 0.004 % to 0.005 % , according to the theory of Gondon et al., it is not necessary to add rare earth , or strictly control the amount of rare earth added , which is enough to ensure The performance of X52 test steel . However, it can be seen from Table 1 that the RE/S value in the tested X52 test steel is about 6 , which is much higher than 3. In addition, because the slab continuous casting mold in the steel plant does not have an electromagnetic stirring device , this leads to The rare earth sulfides or rare earth sulfur oxides are segregated in a large number of layers and bands .
23 The reason why there are more cracks in the inner pipe wall than in the outer pipe wall
The size specification of the high frequency welded pipe is 0323.9 mm X 8.0 mm X 12000mm. After the hot-rolled sheet is coiled and formed , there is always a V-shaped opening in the thickness direction of the hot - rolled sheet . During the welding process , the welding extrusion force deforms the hot metal . Due to the existence of the V- shaped opening , the deformation rate of the inner wall of the pipe is always greater than that of the outer wall . When there are layered non-deformable brittle inclusions in the plate , the possibility of cracks will increase . The thicker the steel pipe , the more obvious this tendency .
24 Measures to improve the qualification rate of welded pipe flaw detection
According to the test statistics , the cracks of the X52 test steel welded pipe are located in the welding heat-affected zone of the inner wall of the pipe , which is consistent with the characteristics of the high-frequency welded pipe production process. The streamline angle of the inner wall of the pipe is larger than that of the outer wall . The larger the streamline angle , the larger the deformation and the easier it is to crack . From the formation of welding cracks, it can be seen that there are two basic conditions for cracks in X52 high-frequency welded pipes . One is the existence of inclusions inside the steel , and the other is relatively large welding extrusion force . According to the information [2,3] , the measures to avoid such defects are firstly to ensure the quality of the base metal and limit the content of impurities , and secondly to adjust the welding parameters so that the extrusion force should not be too large . During the welding process test , through the metallographic inspection and the method of controlling the streamline angle , under the premise of ensuring the extrusion of oxides and ensuring an appropriate amount of deformation , the welding extrusion force is slightly reduced , and the pass rate of welded pipe flaw detection is significantly improved , but there are still a few The pipe was found to have crack defects during flaw detection .
in conclusion

1. The occurrence of welding cracks is the result of the joint action of layered rare earth sulfide and rare earth sulfur oxide inclusions in the X52 test steel coil and welding extrusion force , which improves the purity of the steel , reduces the content and size of inclusions in the steel , and avoids The segregation of inclusions is the fundamental measure to eliminate cracks in the heat-affected zone of welding .
2. X52 test steel coil are formed by the relatively high amount of rare earth fed during continuous casting , resulting in an excessively high RE/S value .
3. In terms of welding process, the process parameters suitable for the material are adopted , and the welding extrusion force is properly reduced , which can reduce the probability of cracks and improve the pass rate of flaw detection .

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