316L stainless steel bolt corrosion fracture analysis and protection recommendations

2021-04-23 12:46:51

A total of 6 independent production lines after the butadiene rubber device of a petrochemical company, the process flow of each production line is basically similar, that is, the pellets entering the production line passes through the vibrating screen, dehydrated extruder, dryer, hot tank, vertical improvement Dehydration, drying and drying, and finally producing polybenzo rubber. During the operation, the dryer fastening bolt frequently occurring corrosion breakage, in order to ensure safety production, the company must replace the dryer bolt. Bolt corrosion breaks, which seriously affects the safety and stable operation of each production line of the post-treatment unit. In order to find the causing cleavage caused by the dryer bolt, the test method of macro-examination, chemical composition analysis, hardness detection, metallographic examination, scanning electron microscopy and X-ray diffraction (XRD) is used.

First, corrosion analysis

(1) Macro check

When the bolt corrosion break is detected: the breaking portion of the bolt is in the vicinity of the bolt head, the surface of the bolt head, the screw, the thread, and the other surfaces have a layer of red corrosion products. The corrosion of the medium bolt is most severe, followed by the screw tank portion, and the threaded portion is slightly corroded, and the thread is partially still metal gloss. The bolt fracture site is concentrated in the root of the bolt, and the fracture is non-developable, which is brittle. The screw tight rod surface shows a large amount of ring-to-crack, and there is a small amount of axial crack, from the bolt head to the threaded portion crack depth, the width is gradually smaller. This can be concluded that the crack originated from the bolt head and expanded toward the screw rod and the threaded direction, and a large amount of sheet-like corrosion product (as shown in Figure 1) is expanded to the screw rod and the threaded direction. The surface of the screw is polished, and it is visible after the grinding: the surface is covered with a thin loop crack, and the crack has a significant branch, which is a dendritic, and has a typical stress corrosion cracking feature.

(2) Analysis of material chemical composition

The spectrometer is used to analyze the bolt, and the results are shown in Table 1. It can be seen from Table 1 that the bolt chemical composition meets the standard requirements of 316L stainless steel chemical components. 1. 3 Hardness analysis is taken in the bolt fracture site and the threaded site sampling microscopic crack test, and the test results are shown in Table 2. It can be seen from Table 2 that the hardness value of the bolt fracture site and the threaded portion is higher, and the hardness average value of the two positions is 298, respectively. 2 HV and 300. 3 HV, higher than the 316L steel maximum hardness value of 235 HV in the correlation standard. The bolt hardness value is high, which will greatly increase the sensitivity of bolt stress corrosion cracking.

(3) Jin phase organization analysis

Sampling in the bolt fracture site, the bolt cross section is observed, and the results are shown in Figure 2. As can be seen from Fig. 2, the metal phase tissue of the bolt is austenite tissue, and there is a large amount of twins in the tissue, resulting in an increase in bolt tensile strength, and thus hardness is also high, and the sensitivity of bolt stress corrosion crack is added. There is a large number of through-through cracks on the metallographic tissue, and the crack originated from the outer surface of the bolt, and the extended mode is a mixed type along the crystalline.