Hydrogen-Induced Cracking and Stress Corrosion Cracking of 33CrNiMoA
Abstract
Hydrogen induced cracking (HIC) and stress corrosion cracking (SCC) are two different types of cracking, but they have very similar implications due to the similar origins of their materials. The 33CrNiMoA is a special metallurgy of the alloy which has seen fairly extensive use in numerous application such as, aviation and defense applications. In this paper we review the different ways that hydrogen and stress corrosion can manifest in the 33CrNiMoA and the effects that these manifest in the performance of components. We consider the implications of HIC and SCC for component performance and also for component selection in design.
Introduction
Hydrogen-induced cracking (HIC) and stress corrosion cracking (SCC) form two different types of material deformation. HIC is mainly caused by the diffusion of hydrogen into the steel of a component and is often associated with electrode welding. SCC is the result of intergranular cracking due to a combination of stress and corrosive environment. Both HIC and SCC can have similar negative effects on component performance. This paper discusses the implications of HIC and SCC on the 33CrNiMoA and its performance in applications.
Hydrogen Embrittlement of 33CrNiMoA
Hydrogen-induced cracking is a form of internal corrosion caused when hydrogen diffuses into susceptibility steels. Diffusion is primarily due to hydrogen charged with the micro-structure of the steel and the access of external hydrogen sources. The presence of hydrogen can weaken the material, leading to brittle behavior and ultimately, hydrogen cracks. The hydrogen is on the surface and in the center of the material. The outer surface layer can be protected from corrosion leading to reduced hydrogen embrittlement, but hydrogen can still form in the core of the material.
The 33CrNiMoA is a special metallurgical alloy designed for high-performance applications such as aviation and defense. Due to its high strength and toughness, the 33CrNiMoA is an ideal choice for components exposed to dynamic loadings and severe environments. However, due to its high carbon content, hydrogen-embrittlement may be present. In order to understand the implications of hydrogen embrittlement on the 33CrNiMoA, it is necessary to consider the amount of hydrogen in the environment and its interaction with the alloy.
Surface finish plays an important role in hydrogen embrittlement. Poor surface finishes with large number of protrusions, such as machined surfaces, are more susceptible to hydrogen embrittlement. Pitting corrosion can increase the surface roughness and promote hydrogen embrittlement by providing additional paths for hydrogen to diffuse. Phosphating and passivation, which provide a smooth and corrosion-resistant protective layer, can prevent hydrogen entry and reduce the degree of hydrogen embrittlement.
Stress Corrosion Cracking of 33CrNiMoA
Stress corrosion cracking (SCC) is a different form of corrosion than hydrogen embrittlement. It is the result of intergranular cracking due to stress combined with a corrosive environment. The stress component is typically caused by cyclic or static loading. SCC often appears in the form of intergranular cracks, which can be detected by metallographic analysis.
Stress corrosion cracking of the 33CrNiMoA can occur in certain aggressive environments. The most common aggressive environment is a chlorinated hydrocarbon mixture that contains important molecules such as chlorine and sulfur. This environment may cause the following temperatures in the component:
•High temperatures up to 250°F.
•Low temperatures down to 0°F.
•Accelerated temperature cycling.
These temperatures may be combined with high chloride levels in order to enhance the likelihood of SCC.
In order to detect SCC in the 33CrNiMoA, it is necessary to look for signs of intergranular cracking. Visible cracks can be detected with a magnifying glass, or with an optical or electron microscope. The grain boundaries may be stained with etchant to understand the nature and extent of the cracking.
Conclusion
The 33CrNiMoA is an incredibly important metallurgy that has seen intensive use in many applications. It is an alloy designed for high performance, so it must be studied carefully to keep it performing at a high standard. The two cracking types discussed in this paper, hydrogen-induced cracking and stress corrosion cracking, are two forms of cracking that can degrade a component’s performance. The implications of each of these cracking types must be understood in order to properly maintain and design a component. Knowing the microstructure of the material, the propensity to hydrogen-induced cracking, the extent of stress corrosion cracking, and the protective coatings will all have an impact on the performance of the part.
