A nanocrystalline layer was prepared on the surface of 34CrMo4 steel by time controlling shot peening (SP, i.e., 1, 5, 10, and 20 minutes). Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) analysis, and transmission electron microscope (TEM) were applied to analyze the surface, cross-sections, and grain size of the specimens before and after SP. The electrochemical corrosion behavior was used to simulate a liquid under the oil and gas wells environment. It was characterized by the potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS). The analysis results show that the surfaces of the SP samples were very rough and had numerous cracks. A passive film on SP surface was formed by nanocrystalline grains. However, the passive film formed in the initial stage was not dense or uniform, and cracks occurred in the passive film during peening, resulting in a decrease in corrosion resistance.
The Cr-Mo series steels are widely used because of their excellent properties by reasonable heat treatment process, such as favorable hardenability and shock absorption, less tendency of temper brittleness, and good corrosion resistance [
This study aims to improve the corrosion properties of 34CrMo4 steel upon SP; the effect of peening time on corrosion resistance was investigated in the simulated environment of liquid under oil wells. In addition, a variety of methods were used to characterize and analyze the mechanism of corrosion resistance.
Chemical composition of the 34CrMo4 steels used in the test is shown in Table
Chemical composition of the 34CrMo4 steel (wt.%).
Elem. | C | Si | P | S | Cr | Mn | Mo | Cu | Ni | Fe |
---|---|---|---|---|---|---|---|---|---|---|
Standard | 0.30~0.37 | 0.10~0.40 | ≤0.035 | ≤0.035 | 0.90~1.20 | 0.60~0.90 | 0.15~0.30 | ≤0.030 | ≤0.030 | Bal. |
Measured | 0.34 | 0.22 | 0.026 | 0.02 | 1.11 | 0.68 | 0.15 | - | - | 97.4 |
The microstructure morphology of untreated and SP samples was etched in a 3% HNO3 + 97% alcohol solution, followed by optical microscope (OLYMPUS, GX71) characterization. The surface before and after shot peening and the electron back-scattered diffraction (EBSD) result of raw sample were observed by scanning electron microscope (FESEM, JSM-7800F). The different depth microstructure of peening samples was researched by transmission electron microscope (TEM, JEM-2100F operating at 200 kV). Phase composition and crystallite size measurements of shot peening samples were studied by X-ray diffraction (XRD, SmartLab-9) with a CuK
The microstructure of 34CrMo4 steel matrix was investigated by FESEM and can be observed in Figure
Matrix of 34CrMo4 steel (a) FESEM image and (b) EBSD IPF mapping.
Figure
Cross-sectional OM observation of test specimens: (a) untreated, (b) 1 min peened, (c) 5 min peened, (d) 10 min peened, and (e) 20 min peened. Dashed arrows show the affected depth by shot peening.
The morphology of the surface and cross-sectional layers by SP was studied. Figure
FESEM images of surface and cross-sections of the untreated (a, a′, and a′′), 5 min shot peened (b, b′, and b′′), and 20 min shot peened specimens (c, c′, and c′′).
Figure
X-ray diffraction patterns of untreated and peened samples with different peening times.
Figure
TEM images of (a) untreated and (b) 20 min SP specimens at ~10
The potentiodynamic polarization curves of the 34CrMo4 steel before and after shot peening in a simulated liquid environment of the oil well are measured. The curves do not discernibly alter them for 1 min peening when compared with the untreated specimen. It is important to highlight that the anode curve exhibits a sharp slope (i.e., passivation zone) when the peening time is over 5 min, as can be observed in Figure
The polarization curves of untreated and SP specimens.
The values of
Figures
The Nyquist plots of untreated and SP specimens.
Bode plots of untreated and SP specimens.
According to the electrochemical experimental data shown in Figure
Fitting results of equivalent circuit.
Peening time (min) | Rs |
C (10−4) |
Rct |
Q (10−3) |
Nn | Rf |
---|---|---|---|---|---|---|
0 | 59.24 | 3.921 | 1048 | - | - | - |
1 | 15.81 | 2052 | 264.6 | 16.97 | 0.5594 | 216 |
5 | 17.58 | 56.84 | 418.5 | 4.208 | 0.6695 | 312.1 |
10 | 11.85 | 0.000426 | 10.28 | 6.388 | 0.7357 | 56.2 |
20 | 19.26 | 0.01354 | 53.28 | 13.08 | 0.7213 | 20.86 |
Equivalent circuits of (a) untreated and (b) SP specimens.
Based on the experimental results, the effect reason and mechanism of corrosion resistance of the 34CrMo4 steel by shot peening had been demonstrated. Firstly, the corrosion resistance property is a clear illustration of how the untreated and peened samples have changed. Corrosion resistance property decreased the steel shot with high energy to hit the specimen surface, bringing about numerous defects including cracks, roughness, loose lamellar structures, and others as can be seen in Figure
The 34CrMo4 steel surface was modified by SP. The following use of XRD diffraction and TEM results observed and confirmed the existence of a nanolayer, with the grain size at the top of the surface being approximately ten nanometers.
The surface of SP shows many defects, including roughness and cracks, leading to an obvious decrease in corrosion resistance of 34CrMo4 steel. The electrochemical behavior results indicated that the peened materials show greater passivation compared with the untreated material on account of crystal refinement. It can then be seen that the resistance of passive film increases at first and then decreases by the fitting results of equivalent circuits, which is attributable to the passive film at the beginning not being compact or uniform during short peening time. However, film fracture occurred after increasing peening time.
The authors declare that they have no conflicts of interest.
This work was financially supported by Graduate School of Science and Technology Innovation Project of Chongqing University of Science and Technology (YKJCX1620205, YKJCX1620209, YKJCX1620203, and CK2015Z12), Chongqing Research Program of Basic Research and Frontier Technology (cstc2015jcyjA50004, cstc2016jcyjA0232), and the Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1501303, KJ1709202).