Petrochemical forging, as an important part of key equipment, plays an important role in the petrochemical production process. However, due to the presence of corrosive media and alternating stresses in the petrochemical environment, forgings are prone to stress corrosion cracking (SCC), leading to equipment failure and safety accidents. In this paper, the causes and protective measures of stress corrosion cracking of petrochemical forgings will be discussed in order to provide reference for related research and application.
Stress corrosion cracking is a phenomenon of metal cracking caused by tensile stress and corrosive media. Its mechanism mainly includes the following three stages:
Crack initiation: Under the action of corrosive media and tensile stress, the passivation film on the metal surface is destroyed, forming the anode and cathode regions. The dissolution reaction in the anode region leads to the preferential dissolution of local metals and the formation of crack sources.
Crack propagation: The crack gradually expands under the action of tensile stress. During the process of crack propagation, the crack tip is in a state of high stress, which accelerates the crack propagation speed. At the same time, the corrosive medium continues to enter the crack, promoting the further expansion of the crack.
Instability fracture: When the crack extends to a certain extent, the metal cannot withstand the tensile stress and a transient fracture occurs. This process is sudden and catastrophic, often leading to the complete failure of the equipment.
Factors influencing stress corrosion cracking
Metal materials: The composition, structure and mechanical properties of metal materials can affect their stress corrosion cracking sensitivity. For example, high-strength steels and low-alloy steels are prone to stress corrosion cracking.
Environmental factors: the type, concentration and temperature of corrosive media will affect the stress corrosion cracking behavior of metals. For example, H2S/CO2 environment at high temperature and pressure is one of the common stress corrosion cracking environments for petrochemical forgings.
Stress state: The type and magnitude of tensile stress will affect the stress corrosion cracking behavior of a metal. Both alternating stress and residual stress accelerate crack initiation and propagation.
Forging and heat treatment processes: Forging and heat treatment processes can affect the microstructure and mechanical properties of metals, which can affect their stress corrosion cracking behavior. For example, residual stress during forging and structural changes during heat treatment can both increase the stress corrosion cracking sensitivity of a metal.
Protective measure
Reasonable selection of materials: Select metal materials with good resistance to stress corrosion cracking, such as stainless steel and duplex steel. At the same time, considering the material strength, toughness and other mechanical properties requirements to ensure the safety and reliability of the equipment.
Optimization of forging and heat treatment process: by controlling parameters such as forging temperature and cooling rate, the ideal microstructure and mechanical properties are obtained. Heat treatment processes such as solution treatment and aging treatment can further improve the corrosion resistance and stress corrosion cracking resistance of the metal.
Reduce tensile stress: By optimizing equipment design, reducing residual stress and avoiding alternating stress, reduce the tensile stress level of the metal, thereby delaying the initiation and propagation of cracks.
Protective coating: The protective coating is applied on the surface of the metal to isolate the direct contact between the corrosive medium and the metal, thereby reducing the sensitivity of the metal to stress corrosion cracking. Choosing the right coating material and process is the key.
Regular testing and maintenance: Regular non-destructive testing and metallographic inspection of petrochemical forgings, timely detection and treatment of potential cracks and corrosion problems to ensure the safe operation of equipment.
Establish a strict management system: Establish a sound management system and operating procedures to ensure the correct use and maintenance of equipment. Strengthen employee training, improve employee safety awareness and operation skills, and reduce the risk of equipment failure caused by human factors.
Stress corrosion cracking of petrochemical forgings is one of the important reasons for equipment failure and safety accidents. The mechanism and influencing factors of stress corrosion cracking of petrochemical forgings can be studied deeply, and effective protective measures can be taken to reduce the failure risk and improve the safety performance of equipment. In the future, with the continuous emergence of new materials and new technologies, the stress corrosion cracking protection of petrochemical forgings will face more challenges and opportunities. Relevant enterprises and researchers need to constantly explore and innovate to adapt to changes in market demand and contribute to the sustainable development of the petrochemical industry.
