Analysis of the Causes for Transversal Crack of 4130X Steel Pipe for Making Large-volume Gas Cylinder

Analysis of the Causes for Transversal Crack of 4130X Steel Pipe for Making Large-volume Gas Cylinder

Abstract: In order to find out the causes for the transversal crack of the 4130X steel pipe for making of the large volume gas cylinder, the fracture surface is analyzed by means of both the macro and micro analyses, while the specimen is tested in terms of chemical composition, mechanical property, and metallurgical structure. As a result, it is identified that the crack is a quenching-crack which is caused by the residual stress during quenching of the cylinder. When the pipe is being quenched, the milling scratch of it gets the stress concentrated, which acts as the incentive for the quench crack. Accordingly, the steel pipe manufacture process is optimized with measures such as descaling the pipe with the pickling process and reduction drawing so as to avoid the re-occurrence of the problem.

Key words: gas cylinder; 4130X steel pipe; large volume; milling scratch; quench crack

Large-volume seamless steel gas cylinders first appeared in the 1960s in the United States, Europe, and other industrially developed countries and regions, and are commonly used to assemble long tube trailers for transporting compressed natural gas, hydrogen, and helium, and other high-pressure gases. Large-volume seamless steel gas cylinders typically refer to seamless steel cylinders with an outer diameter greater than 559 mm and a water volume of 150 to 3000 L. Sulfides residual in compressed natural gas have a stress corrosion effect on high-pressure gas cylinders in a humid environment. High-pressure hydrogen can cause hydrogen embrittlement in high-strength steel, and high-pressure gas cylinders also have to withstand fatigue loads, impact loads, extreme environmental temperatures, and stress loads caused by temperature changes during the charging and discharging process. Therefore, large-volume gas cylinders have very strict requirements for the manufacturing of raw material seamless steel pipes, and the material is generally mainly chrome-molybdenum steel, with strict control of sulfur, phosphorus, and gas content in the steel. Since 2008, major domestic steel pipe manufacturers have successively developed seamless steel pipes for large-volume gas cylinders, with a main process flow of continuous rolling pipe (periodic rolling pipe) + heat expansion + cold drawing, and an annual output of more than 80,000 tons.

A company in China supplied a customer with a specification of 711 mm in diameter, 21.5 mm in wall thickness, and 11,150 mm in length, and the grade is 4130X gas cylinder tube. After processing into a gas cylinder and undergoing a hydrostatic test, one of the gas cylinders did not show an upward trend in pressure value during the slow pressurization process after being filled with water in a vertical shaft. The gas cylinder was pulled out for inspection, and a transverse crack was found about 3 meters away from the shoulder of the bottle, so a crack sample was taken for analysis.

1. Macro analysis
The macro appearance of the crack is shown in Figure 1. The crack is approximately a straight line expanding along the cross-section of the pipe wall, showing through cracking. The crack is about 200 mm long on the outer surface of the pipe wall, as shown in Figure 1(a), and about 190 mm long on the inner surface of the pipe wall, as shown in Figure 1(b). No obvious scratches, pits, or other defects were found on the surface at the crack. After the crack fracture was cut open, a layer of brittle oxide was found on the surface, which was cleaned with water and alcohol, and the fracture appearance is shown in Figure 2. According to the chevron pattern on the fracture surface and the shear lip on the inner surface, it can be considered that the crack originated from the outer surface of the pipe wall, expanded radially towards the inner surface and both sides of the pipe wall, the fracture is relatively flat, and no obvious macro plastic deformation was seen, which is a one-time brittle fracture.

2. Microscopic analysis
A sample was taken from the source of the crack shown in Figure 2, and the sample was placed in a JSM-6490 type scanning electron microscope for microscopic observation. The morphology of the crack source area is shown in Figure 3, and the entire fracture surface shows a molten oxidation form (Figure 4). A depression about 200 μm deep was found on the edge of the crack, that is, the outer surface of the pipe, and the crack expanded along the bottom of the depression. The section of the shear lip area shows a fibrous pattern, which is a tough fracture, and the fracture appearance is shown in Figure 5. No obvious abnormal inclusions or slag were found on the section.

3. Physical and chemical property analysis
3.1 Chemical composition analysis
Sampling near the crack of the sample, the chemical composition was detected by a QSN750 type direct reading spectrometer, and the results are shown in Table 1. The chemical composition meets the technical agreement requirements.

3.2 Mechanical property testing
Tensile samples were taken along the longitudinal direction of the steel pipe near the crack, using a 10 mm round bar sample with a gauge length of 50 mm. The room temperature tensile test was carried out according to the GB/T 228.1—2010 “Metallic Materials Tensile Test Part 1: Room Temperature Test Method” standard. The test equipment was a WAW-300 microcomputer-controlled electro-hydraulic servo universal testing machine. The hardness of the pipe section was tested with a BRIN 200B-T Brinell hardness tester.

Impact testing at -40°C low temperature was performed on the processed samples with a JB-500S digital display impact testing machine, and the sample size was 10 mm × 10 mm × 55 mm. The mechanical property test results of the sample are shown in Table 2, and the mechanical properties meet the customer’s agreement requirements.

3.3 Metallographic analysis
According to the GB/T 10561—2005 “Steel Non-metallic Inclusion Content Determination Standard Rating Diagram Microscopic Test Method”, the crack sample was tested for non-metallic inclusions, and the result was A0.5, B0, C0.5, D1.0. The non-metallic inclusions of the sample meet the technical agreement requirements. A fracture section was cut from the source of the crack sample, ground and polished into a metallographic sample, and observed under an Axio Imager M1m type optical microscope. The microstructure is tempered sorbite + a small amount of ferrite, and no obvious decarburization is seen on the surface. There is a V-shaped depression about 200 μm from the outer surface, and the metallographic structure of the sample is shown in Figure 6.

4. Comprehensive analysis
4.1 Gas cylinder processing
It was learned from the user that the gas cylinder processing procedure is: spinning bottom closing → tempering → external sandblasting → cutting end and machine processing thread → magnetic particle inspection → hydrostatic pressure → gas tightness inspection → internal and external sandblasting → ultrasonic inspection → painting → packaging → storage. After the gas cylinder is formed, it is heated to 880°C and then quenched. The typical chevron pattern of the fracture and the results of the microscopic analysis show that the crack penetrates the pipe body in a horizontal straight line, the fracture is brittle, and no obvious decarburization is seen at the crack edge, which basically excludes the existence of original cracks in the pipe body before quenching, and belongs to quenching cracks.

4.2 Steel pipe forming process
The production process of this large-volume gas cylinder tube is: pipe blank heating → piercing → pipe rolling → annealing → internal and external grinding → cold drawing first time → cold drawing second time → flaw detection, thickness measurement → storage. After pipe rolling, it is necessary to perform annealing treatment on the rough pipe to reduce hardness for subsequent cold drawing processing. Long-time annealing of steel pipes will inevitably produce a lot of scale. In actual production, the process of internal and external grinding was used to remove it, and grinding is horizontal coarse grinding with an abrasive wheel. If the grit number of the abrasive wheel is selected unreasonably or operated improperly, it will produce uneven grinding marks. The V-shaped depression shown in Figure 6 is the residual grinding mark on the outer surface of the steel pipe. Deep grinding marks are difficult to eliminate through a limited wall reduction amount during the subsequent cold drawing process, and the residual grinding depth does not reach the alarm threshold of ultrasonic flaw detection, eventually forming a lot of stress concentration on the surface of the steel pipe. By searching, some residual grinding marks were found on the outer surface of some steel pipes, as shown in Figure 7.

4.3 Cause of crack formation
The material of the large-volume gas cylinder is medium carbon chromium molybdenum steel, similar to 30CrMo. The quenching temperature of the gas cylinder is about 880°C, and the quenching medium is generally water-based polymer, requiring control of the cooling speed not to exceed 80% of the water cooling speed. Kunitake and Susigawa proposed a relationship formula to express the combined influence of carbon content and other elemental components on the quenching crack tendency of steel, represented by carbon equivalent Ceq, and the formula is as follows: Ceq=C+Mn/5+Cr/4+Mo/3+Ni/10+V/5+(Si-0.5)/5+Ti/5+W/10+Al/10, where the elemental symbols represent the mass fraction of the element, and the larger the Ceq value, the higher the sensitivity of the steel to cracking. Ceq<0.4%: the steel is not easy to crack; 0.4%≤Ceq<0.7%: the steel is more prone to cracking; Ceq>0.7%: the steel is prone to cracking. According to the composition in Table 1, the calculated Ceq is 0.7%, which belongs to the range of easy cracking composition.

Generally speaking, there are two types of internal stresses generated in the steel during the quenching process: one is the thermal stress caused by the temperature gradient between the surface and the interior of the workpiece; the other is the phase change residual stress caused by the volume expansion when austenite transforms into martensite or other phase change products, also known as organizational stress. The action of thermal stress causes residual compressive stress on the surface of the workpiece, and organizational stress causes the surface of the workpiece to be under tensile stress. When the residual organizational stress during quenching exceeds the fracture stress of the steel, the workpiece will produce cracks or fractures locally.

During the rapid cooling of the gas cylinder quenching, only the outer surface of the cylinder comes into contact with the cooling medium, and the uneven cooling formed by the heat stress and the organizational stress caused by the martensitic phase change of the cylinder interact to produce a huge tensile stress on the outer surface. The horizontal grinding marks are prone to cause organizational stress concentration during the quenching process. When the residual tensile stress exceeds the tensile strength of the material, it causes the pipe body to crack from the outer surface during quenching, and then the crack expands from the outer to the inner wall of the pipe, eventually penetrating the entire pipe wall.

5. Improvement measures
Through the above analysis, the steel pipe manufacturing process is optimized in a targeted manner:
(1) Cancel the internal and external grinding process and change to the method of pickling + manual grinding to remove scale and local defects on the surface of the steel pipe. The pickling process can remove the scale on the surface of the steel pipe without leaving the sharp grinding marks caused by the abrasive wheel grinding. The outer surface of the steel pipe after pickling is shown in Figure 8. Local small defects can be removed by manual grinding with an abrasive wheel, and the grinding area requires a smooth transition.

(2) Improve the cold drawing process and adjust the cold drawing deformation, changing from the original equal diameter drawing to reduced diameter drawing. During equal diameter drawing, the radial uneven plastic deformation of the pipe wall is more serious, with greater deformation on the inner surface and smaller or almost no deformation on the outer surface, resulting in a large axial residual tensile stress on the outer surface of the pipe due to uneven plastic deformation after the drawing is completed. This makes it easy to produce transverse crack defects during subsequent heat treatment and other process operations. After changing to reduced diameter drawing process, the outer surface of the pipe is in a state of compressive stress, while also improving the surface quality, and greatly reducing the tendency of the outer surface of the pipe to crack.

(3) Improve the sensitivity of non-destructive testing, strengthen manual inspection of suspected defects found by flaw detection, and eliminate them in time. After using the optimized process measures, the produced cylinders have not had similar cracking, proving that the process improvement is relatively reasonable.

6. Conclusion
(1) The cracking of the large-volume gas cylinder is due to the existence of horizontal grinding marks on the surface of the cylinder body, which cause organizational stress concentration during the quenching process, resulting in quenching cracks on the outer surface of the cylinder body. The cracks expand from the outer to the inner wall of the cylinder, eventually penetrating the entire cylinder wall.

(2) In response to the cause of cracking, process improvement measures have been proposed to effectively solve the problem of quenching cracking of the steel cylinder.