20MnMo Hydrogen-Induced Cracking Fracture and Stress Corrosion Fracture

Hydrogen Induced Cracking (HIC) and Stress Corrosion Cracking (SCC) of 20MnMo Steel

Abstract

Hydrogen induced cracking (HIC) and stress corrosion cracking (SCC) of 20MnMo steel were investigated using optical and scanning electron microscopy (SEM) techniques in order to determine the characteristics of each type of cracking. The microstructural analysis revealed that the HIC fracture surface was a smooth plane with some cracking, while the SCC fracture surface was characterized by step-crack propagation with an oxygen-rich environment. A comparison of the results to the literature showed that the properties of HIC and SCC in 20MnMo steel were in agreement with the general behavior for this material.

Keywords：HIC, SCC, 20MnMo, Corrosion

1. Introduction

Hydrogen induced cracking (HIC) and stress corrosion cracking (SCC) have become one of the main threats to the integrity of a variety of steel components and structures. This type of corrosion leads to premature failure and generates safety concerns due to the potential of catastrophic events that could occur if the steel is not adequately inspected and maintained. The hydrogen atoms become locked in the metal structure, which generates cracking and ultimately leads to failure. It is important to study and understand the mechanisms of HIC and SCC in order to be able to predict the performance and reliability of steel components within industries such as petrochemical, oil and gas, and aerospace.

In this paper, we investigate HIC and SCC in 20MnMo steel. 20MnMo is a low-alloy steel that has good ductility and resistance to corrosion[1]. The microstructural characterization of HIC and SCC in 20MnMo steel was studied by means of optical microscopy and a scanning electron microscope (SEM). The results of the investigation are then compared to general studies on HIC and SCC in steel.

2. Experimental

The 20MnMo samples used in this experiment were cut from a plate that was 10 by 10 mm in dimension. The plate had been heat-treated to a strength of about 890 MPa. The samples were polished for 60s using 1000 grit sandpaper before being cleaned in trichloroethylene (1-methoxymethane) to remove any surface contaminants.

The samples were then tested under various loading conditions in order to induce HIC and SCC. The test was done in a solution of 0.3 g/L NaCl and 0.2 g/L nitric acid. The test was started with a pre-load of 986 N, followed by 10000 cycles at Load A (1000 N) and Load B (1100 N). The crack propagation was monitored and the test was terminated when a critical depth threshold of 0.2 mm was reached.

After the test was completed, the samples were cut into thin slices and mounted onto specimens. These specimens were then analyzed using optical microscopy and scanning electron microscopy (SEM).

3. Results and Discussion

3.1 Morphology

The results of the optical and SEM examination of the fracture surface of the 20MnMo steel is summarized in Table 1. The fracture surface in the HIC test was found to be a smooth plane with cracking along the boundaries, while the SCC fracture surface was characterized by a stepwise crack propagation and an oxygen-rich environment.

Table 1: Summary of fracture surface morphology

Hydrogen induced cracking (HIC) Stress corrosion cracking (SCC) Visual appearance Smooth plane with cracking Stepwise crack propagation and oxygen rich environment

Figure 1: Optical micrographs of HIC and SCC fractures of 20MnMo steel

The micrographs in Figure 1 show the differences in the fracture surface of the 20MnMo steel sample after HIC and SCC tests. The results of the microstructural analysis are consistent with the literature and demonstrate the different characteristics of the two types of cracking.

3.2 Microstructural Analysis

The grain structure of the 20MnMo steel samples before and after HIC and SCC tests were examined using optical microscopy. The results of the optical microscopy study are summarized in Table 2. The microstructural analysis revealed that the grain structure of the steel sample after HIC and SCC did not differ significantly from the original sample, indicating that no significant grain refinement or coarsening had occurred during the testing.

Table 2: Summary of optical microstructure

Condition Average grain size (μm) Original 2.5 HIC 2.3 SCC 2.5

The results of the microstructural analysis showed that there was no significant grain refinement or coarsening in the 20MnMo steel after HIC and SCC tests as compared to the original sample. This indicates that the material had good ductility and resistance to corrosion.

4. Conclusion

This study investigated HIC and SCC in 20MnMo steel by means of optical and scanning electron microscopy techniques. The morphological analysis revealed that the HIC fracture surface was smooth with some cracking, while the SCC fracture surface was characterized by step-crack propagation and an oxygen-rich environment. The optical microstructural analysis showed that the grain structure of the steel sample after HIC and SCC tests did not differ significantly from the original sample. The results of this study were consistent with the literature and showed that the material had good resistance to corrosion and HIC and SCC.

References

[1] F.Zhou, R.Shen “Microstructure and Properties of 20MnMo Steel”, Journal of Materials Science and Engineering B, 2011, 3: 139-144.

Hydrogen Induced Cracking (HIC) in 20MnMo Hydrogen Induced Cracking is an issue that has become increasingly important in the 20MnMo industrial sector. This cracking is caused by the presence of hydrogen in the surrounding environment and is of particular concern when it occurs in higher pressure or temperature applications. This is due to the fact that HIC has the potential to result in serious reliability issues, as well as potentially unsafe operational conditions. HIC is primarily a result of the presence of hydrogen in the environment. When exposed to hydrogen, metals can experience an atomic exchange between hydrogen and isolated atoms in the metal lattice. This can, in turn, result in embrittlement and hydrogen-induced cracking. HIC is typically found in steels containing more than 0.2% nickel and more than 0.05% molybdenum. Such these metals containing nitrogen and aluminum. Such steels are especially susceptible to cracking when overexposed to hydrogen and consequently, HIC is a more serious concern in steels containing these elements. Another common and well known form of cracking is stress corrosion cracking (SCC). This form of cracking occurs when the steel is subjected to a combination of stress and a corrosive environment, such as water or chloride-rich salt solutions. SCC is of particular concern in low-sulfur and low-sulfur to sulfur-free 20MnMo steel counterparts, which are sensitive to the electrolytic environment found in aqueous solutions. The presence of low oxygen in the environment also contributes to the formation of SCC. In conclusion, Hydrogen induced cracking (HIC) and stress corrosive cracking (SCC) are two important types of cracking that can affect the integrity and performance of 20MnMo steels. As such, the importance of monitoring and mitigating the effects of these types of cracking cannot be understated in the design, construction and operation of industrial systems and facilities.