Cracking of Flattening Tests of Welded Pipes

Sep 24, 2022

Abstract: When the flattening of welded steel pipes was tested, cracking happened. Different tests were carried out to find out the reasons.

Welded steel pipes for low-pressure fluid transportation are used for transporting general low-pressure fluids such as water, gas, oil and heating steam. The steel pipes must meet the requirements of GB/T 3091-2008. According to the requirements of GB/T 3091-2008, the electric resistance welded steel pipe with an outer diameter greater than 60.3 should be subjected to a flattening test. The length of the flattened sample should not be less than 64 mm. The welding seam of each specimen shall be located at the place where it forms 90° and 0° with the direction of the force application. During the test, when the distance between the two flat plates is 2/3 of the outer diameter of the steel pipe, no cracks are allowed in the welding seam. A welded steel pipe with a diameter of 88.9 mm produced by a factory was subjected to a flattening test. When the distance between the two flat plates was 2/3 of the outer diameter of the steel pipe, cracks occurred at the welding seam position, as shown in Figure 1. Through the material and mechanical inspection of the steel pipe, as well as the analysis of the microstructure, inclusions and fractures, the reasons for the cracking of the welded steel pipe during the flattening test are discussed.

Figure 1 Flattening and cracking of steel pipes

1. Physical and chemical tests and results
1.1 The analysis of materials of steel pipes
The sample was taken from the cracked steel pipe, and the chemical composition was analyzed with a vacuum direct-reading emission spectrometer GS1000. The analysis results are shown in Table 1. From the test results in Table 1, it can be concluded that the material of the steel pipe conforms to GB/T 700-2006 "Requirements for the Q235B grade in Carbon Structural Steel.

Table 1 Analysis of results of the chemical composition of materials w/%

	C	Si	Mn	P	S	Cr	Ni	Cu
Samples of steel pipes	0.11	0.093	0.38	0.032	0.021	0.046	0.014	0.010
GB/T 700-2006	Less than and equal to 0.20	Less than and equal to 0.35	Less than and equal to 1.40	Less than and equal to 0.045	Less than and equal to 0.045	Less than and equal to 0.30	Less than and equal to 0.30	Less than and equal to 0.30

1.2 Mechanical properties tests of steel pipes
Use this steel pipe to prepare mechanical samples and perform mechanical testing according to methods of GB/T 228.1-2010; the test results are shown in Table 2. It can be seen that the mechanical properties meet the requirements of the mechanical properties of the Q235B grade in GB/T 3091-2008.

Table 2 Mechanical properties of the samples

	ReL/MPa	Rm/MPa	A/%
Samples of steel pipes	320	460	23
GB/T 3091-2008	Greater than and equal to 235	Greater than and equal to 370	Greater than and equal to 15

1.3 The analysis of the fracture
Samples were prepared at the cracked part of the steel pipe, and the fracture was analyzed with a scanning electron microscope EVO18, as shown in Figure 2. It was found that the morphology of the fracture was a woodgrain layered fracture with many longitudinally arranged cracks on the surface.

The longitudinally arranged cracks are further magnified and observed, and it is found that the inclusions in the cracks were longitudinally distributed. The energy spectrum analysis of the inclusions showed that the inclusions were sulfide inclusions. The energy spectrum analysis is shown in Figure 3 and Table 3.

Figure 2 The morphology of the fracture morphology

Figure 3 The analysis of the energy spectrum

Table 3 Analysis of results of the energy spectrum

Elements	S	Fe	O	Ca	Cr
The percentage of weights	20.21	46.17	31.73	0.92	0.97

1. 4 Metallographic inspections
From the above fracture samples, the metallographic samples on the fracture surface were taken, and inclusions were analyzed with a Leica metallographic microscope. Non-metallic inclusions were graded according to the standard GB/T 10561-2005 "Standard Grading Chart Micro-Test Methods for Determination of Non-metallic Inclusion Content in Steel", and the grades were A greater than 3. 0, B 0. 5, C 0. 5, D 1.5, DS 0. Type A is Sulfide, type B Alumina, type C Silicate, type D spherical oxide, type DS single-particle spherical, ss shown in Figure 4. Among them, the sulfide inclusions were distributed in strips, arranged longitudinally, and there were many. The sulfide inclusions were very serious, and the rating result was greater than 3. The longitudinal cracks on the fracture surface were caused by the sulfide inclusions.

Figure 4 Inclusions on the surface of the fracture

From the above samples, metallographic samples with a section perpendicular to the fracture surface were prepared, and microstructure analysis was carried out with a metallographic microscope, as shown in Figure 5. In Figure 5(a), zone 1 is the welding seam, zone 2 the heat-affected zone, and zone 3 the base metal near the heat-affected zone; the fracture occurred in zone 3. Figure 5(b) is a partially enlarged view of area 3 in Figure 5(a). The microstructure of the fracture edge was ferrite plus pearlite; the microstructure was normal, and the fracture was not caused by overheating or over burning.