Abstract: The stainless steel 304 check valve on the production equipment of a company broke during use. The reasons for the fracture of the check valve were analyzed by means of macro analysis, chemical composition analysis, fracture analysis, non-metallic inclusion detection, metallographic inspection, and microhardness tests. The results show that the fracture position of the check valve is located at the welding seam, which is a one-time overload fracture. There is an incomplete welding defect at the welding seam due to an improper welding process, which reduces the strength of the check valve tube. Under the action of great medium stress, a one-time overload fracture occurs.

A check valve is a valve that opens and closes the valve disc through the flow of the medium, thereby controlling the one-way flow of the medium, which is indispensable in increasingly developing industrial production. The check valve is automatic. Its main function is to prevent the backflow of the medium, the reverse rotation of the pump and the driving motor, and is used for the discharge of the container medium. The check valve on a company's production equipment was broken during use, causing great losses to production. The check valve is made of stainless steel 304, which has excellent heat resistance and corrosion resistance. A series of physical and chemical tests on the broken check valve were carried out, the reasons for the breakage were analyzed and suggestions were put forward to hope that such accidents will not happen again.

1. Physical and chemical tests
1. 1 Macro analyses
One end of the check valve is connected to a cover plate by a flange, and the fracture is on the other end of the valve tube, as shown in Figure 1. It can be seen that the outer wall of the valve tube is deformed due to being squeezed by the gas in the valve tube. In order to measure the deformation degree of the outer wall of the valve tube, starting from the flange end, select 6 measurement points from A to F with a gradient of 5cm, and measure the outer circumference at each point, as shown in Figure 1b. The test results are in Table 1. It can be seen from Table 1 that the outer diameter of the valve tube decreases one by one from the flange end to the fracture.

Table 1 Measurement results of the outer diameter of the valve tube

Positions	A	B	C	D	E	F	Fracture parts
Outer diameters	787	780	775	770	752	717	705

a) The outer wall of the valve tube b) The measurement point of the outer diameter
Figure 1 Macroscopic appearance of the fractured check valve

1.2 The analysis of the chemical composition
Samples were taken near the fracture on the fractured check valve, and the chemical composition was analyzed. The results are shown in Table 2. It can be seen that it meets the composition requirements of GB/T20878-2007 "Stainless Steel and Heat-resistant Steel Grades and Chemical Composition" for stainless steel 304.

1.31 The analysis of the macroscopic fracture
Use wire cutting to cut off the valve tube at the fractured part, as shown in Figure 2a. It can be seen that the cut valve tube has poor roundness and the tube body is deformed. The fracture was observed under a stereo microscope, as shown in Figure 2b. The color of the fracture near the inner wall of the valve tube is significantly different from that of the outer wall. The fracture near the inner wall of the valve tube is yellow-brown, which should be a welding defect; the fracture near the outer wall of the valve tube is silver-gray with a metallic luster, and has the characteristics of a shear lip, which should be a fresh fracture surface. It is preliminarily judged that the valve tube of the stainless steel 304 check valve has welding defects, which reduces the strength of the welded joint of the valve body; the valve tube deforms under medium stress and finally breaks.

Figure 2 The macroscopic appearance of the fracture

1.32 The analysis of the micro-fracture
Samples were taken from the fracture of the valve tube, and the fracture was fully cleaned with ultrasonic waves, and then observed with a scanning electron microscope (SEM), as shown in Figure 3. The fracture is divided into areas A, B, and C. The area A is close to the outer wall of the valve tube, and the shear lip feature can be seen. The area B is the fresh fracture surface, and the area C is the welding defect region. After measurement, the width of the fracture is about 2.75mm, and the width of the C area is about 1.08mm, which is about 39% of the total width.

a) The fracture at a low magnification b) The fracture of the area A at a high magnification
c) The fracture of the area B at a high magnification d) The fracture of the area C at a high magnification
Figure 3 The SEM morphology of the fracture

The SEM morphology of area A is shown in Figure 3b, showing obvious dimples; the SEM morphology of area B is shown in Figure 3c, and obvious dimples were also seen; the SEM morphology of area C is shown in Figure 3d. It can be seen that the surface was covered with oxidation corrosion products, and there were obvious processing marks, as shown in the parallel lines in Figure 3d. It can be further judged from the SEM morphology characteristics of the fracture and the macro-morphological characteristics of the fracture that the area C is a welding defect surface, and the fracture property of the valve tube is a one-time overload fracture.

Table 2 Chemical composition of fractured check valves

Items	C	S	Cr	Ni	Mn	Si	P
Actual values	0.03	0.003	18.19	8.22	1.16	0.45	0.030
Standard values	Less than and equal to 0.08	Less than and equal to 0.030	18.00 to 20.00	8.00 to 11.00	Less than and equal to 2.00	Less than and equal to 1.00	Less than and equal to 0.045

Sep 10,2022

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Next: Causes of Fracture of SS 304 Check Valves (Part Two)

Previous: The Sealing of Valves

Causes of Fracture of SS304 Check Valves (Part One)

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