First, the influencing factors
There are many factors that affect the process of stress corrosion cracking, and clarify their influence law. It is very meaningful for the stress corrosion failure analysis and the practice of preventing stress corrosion cracking. The following summarizes the research conclusions of several main influencing factors.1. The effect of strengthMaterial strength is the most influential parameter for stress corrosion cracking sensitivity. The higher the strength, the greater the stress corrosion sensitivity, the lower the K1c and K1scc, the faster the crack propagation speed. The stress corrosion cracking tendency of steel increases sharply with the increase of α0.2, but there are exceptions for some high-strength aluminum alloys.2. The influence of microstructureThe effect of microstructure on K1scc is also obvious. Low-alloy steel has high sensitivity to stress corrosion cracking in the low temperature temper brittleness range, while isothermally treated materials are not.The study of delayed fracture behavior of 20MnvB steel in different aqueous states in 0.1N HCl solution proves that the fracture resistance increases in the following order: quenched martensite, F/M dual phase structure of martensite matrix, pearlite + ferrite , F/M dual phase structure of ferrite matrix, isothermal bainite.The original austenite grain size of low alloy steel has little effect on K1scc, but the grain size has an influence on the sensitivity of Cr-Ni stainless steel stress corrosion cracking. The larger the grain size, the shorter the breaking time.The grain orientation has a great influence on K1scc, especially when the grain is relatively coarse, the press-formed fibrous structure has higher stress corrosion sensitivity than the recrystallized structure. The stress corrosion cracking tendency when loaded along the transverse direction of the fibrous structure is always high when loaded in the longitudinal direction.3. The effect of heat treatmentSince heat treatment can change the microstructure of steel, it usually has a strong influence on da/dt and Klscc. So far, various toughening treatment processes have been used to improve the stress corrosion resistance of steel.In general, the following heat treatment process can improve the material Klscc, improve the tempering temperature, ultra-high temperature quenching, sub-temperature quenching, deformation and deformation heat treatment, these processes can also effectively reduce da / dt, although austempering can not improve Klscc But it can make the da/dt drop significantly. The above process has different effects on Klscc and da/dt due to different materials. There are many research results in this area, which can be used for reference.4. The influence of alloying elementsIn general, all the gold elements and processes that can affect the flexural strength of steel can significantly affect the stress corrosion sensitivity.In low alloy steels, Cr, Mo, S, P, Co, Cu, and Si have little effect on Klscc, and C and Mn are harmful to Klscc. Generally, it is below 0.4% carbon content, and its stress corrosion sensitivity increases with increasing carbon content, as shown in Figure 8-29. There is also a lack of systematic research on steels with carbon sequestration above 0.4%.
Figure 8-29 Effect of different carbon contents on stress corrosion properties of 30CrMnSiNi2AThe manganese content is less than 30%, which has an effect on the stress corrosion resistance of steel. As the manganese content increases, Klscc will decrease.The effect of phosphorus on the stress corrosion properties of high-strength steel is similar to that of carbon, see Figure 8-30.
Figure 8-30 Effect of phosphorus content on stress corrosion cracking of 30CrMnSiNi2A steelSi can significantly improve the stress corrosion resistance of Cr-Ni stainless steel in concentrated chloride, especially in 42% boiling MgCl2. Most experiments show that C is beneficial to improve the stress corrosion resistance of Cr-Ni stainless steel. Mo can improve the stress corrosion cracking resistance of stainless steel resistant to organic acids such as H2SO4 and acetic acid. However, the presence of a small amount of Mo in the high concentration of chloride is disadvantageous in preventing stress corrosion cracking. The addition of Mn is generally detrimental and will accelerate the stress corrosion cracking of Cr-Ni stainless steel. It is generally believed that N, P, and S have an adverse effect on the stress corrosion cracking of Cr-Ni stainless steel. Cu has no or little effect on the stress corrosion cracking of Cr-Ni stainless steel in chloride.Increasing the content of alloying elements Cr, Ni, and Mo can slow the crack growth rate of steel. The addition of Si to the low alloy steel can significantly slow the crack growth rate. C and P may accelerate crack propagation in maraging steel.5. Impact of environmental mediaFor low-alloy steels, as the environment changes from hydrogen-free to aqueous media to H2s, the sensitivity of stress corrosion cracking increases, and da/dt increases.The temperature of the medium is a very important factor. Generally speaking, as the temperature increases, the stress corrosion of the material increases. The sensitivity of Cr-Ni stainless steel to stress corrosion in hot water and high temperature water increases, but many studies are different. The material has a temperature range which is most susceptible to stress corrosion. For example, Cr-Ni stainless steel is about 180-280 ° C, austenitic stainless steel hydrogen embrittlement occurs in the range of 50-300 ° C, and low carbon steel is close to solution temperature. At the boiling point, it is most likely to cause alkali brittle fracture, alkali brittle fracture caused by slightly alkaline high temperature and high pressure water, usually occurring in the temperature range of 150 to 300 °C.The effect of medium concentration on stress corrosion is significant. In general, the higher the concentration of alkali, the more easily the alkali brittle of low carbon steel occurs. Only when the alkali concentration is more than 5% to 15%, alkali brittle fracture can occur. When the alkali concentration reaches 50%, the sensitivity of alkali brittleness will be greatly increased. The Klscc of titanium alloy in hydrochloric acid decreases with increasing HCl concentration.The electrode potential, the type of chemical medium, and the pH of the solution are also factors that have a significant influence on the stress corrosion sensitivity of the material. In general, different metals have a potential range that is more susceptible to stress corrosion under different environmental conditions. For example, the fracture potential of low carbon steel in 25 ° C 35% NaOH solution is about -1.1 ~ 0.9V, while the low carbon steel nitrate salt rupture potential is between -300 ~ +1300mv, usually, the electrode When the potential is about -0.6v~-0.9v, the stress corrosion sensitivity of most high-strength steels is small, and both the anodic polarization and the cathodic polarization will accelerate the hydration process of the high-strength steel environment. Therefore, the cathode Protection is not applicable to high strength steel. There is a critical rupture potential for austenitic stainless steels, such as a critical rupture potential of about -0.13 V (SHE) in a boiling 42% MgCl2 solution. The potential plays an important role in the stress corrosion cracking of titanium alloys in aqueous solutions containing Cl-, Br-, I-. In an aqueous solution containing Cl- and Br-, the sensitive potential of Ti-2Al-1Mo-lv is about -500 mv to 600 mv, while the above-mentioned Omv is the most sensitive potential in an aqueous solution containing I-.It is generally believed that as the pH of the solution increases, the stress corrosion cracking sensitivity of the steel decreases. When the pH of the solution is <4 to 5 (acidic), the smaller the pH value, the more likely the stress corrosion cracking occurs. In the case of a large pH and a strong alkalinity, it can slow down or even prevent stress corrosion cracking, and when the pH is in the middle range of 3 to 10, it is not sensitive to stress corrosion. For Cr-Ni stainless steel, only general corrosion occurs when the pH is low, and 18-8Cr-Ni stainless steel is most sensitive to stress corrosion when the pH is 6-7.6. Influence of surface stateSteel has different surface treatment methods, different surface states, and different stress corrosion sensitivities.It is generally believed that ordinary mechanical polishing of stainless steel in boiling MgCl2 is more sensitive than vacuum annealing and electrolytic polishing stress corrosion cracking. Different mechanical polishing methods have different surface roughness, and their stress corrosion cracking tendency is also very different. Surface roughness will increase the tendency of stress corrosion. The higher the surface strength and hardness, the greater the tendency of delayed fracture of steel. The heat treatment or shot peening, sand blasting, hammering, rolling and other methods can reduce the tendency of stress corrosion cracking. Chemical heat treatment (such as seepage) N) or the coating can also induce compressive stress, producing the same effect.7. Influence of the degree of notch stress concentrationFor structural parts such as bolts, the stress concentration of the notch significantly affects its ability to resist stress corrosion. As the radius of curvature of the notch increases, Klscc increases sharply.Table 8-4 shows the Klscc values ​​for 30CrMnsiNi2A steel constant displacement specimens (B=20mm, a/w=0.3) with different curvature radius for σb=1568MPa. Table 8-5 shows the Klscc value of the WOL constant displacement specimen with different notch curvature radius in water, and formula (8-1) is the Klscc experimental result of the ultrahigh strength steel 30CrMnsiNi2A notch specimen measured in aqueous medium.
All of the above results indicate that the degree of stress concentration of the notch is highly sensitive to stress corrosion resistance.Second, measures to prevent stress corrosion cracking
Preventing or slowing down the stress corrosion cracking phenomenon is not only necessary but also possible. As long as the basic conditions of stress corrosion and the influence of various influencing factors on K1scc and da/dt are known, we can try to slow down or even prevent stress corrosion cracking. . However, due to the complexity of the stress corrosion phenomenon, so far, there are still a lot of problems that have not been solved. Therefore, people can only take limited measures on the known laws, and more perfect methods need further exploration. The following measures are more generally indicative means.1. Control and reduce stressOne of the conditions for the occurrence of tensile stress and full stress corrosion is to effectively control and reduce the tensile stress, which is an effective protection measure;On the one hand, in the setting of the zero component, the working stress is not only lower than the yield strength of the material, but also far lower than the critical stress of the material stress corrosion, considering the microscopic crack of the material. For the existence of defects, the fracture mechanics method should be used to determine the load stress intensity factor of the crack tip under the conditions of use and the critical crack size allowed by the component according to the parameters of K1scc and da/dt measured in the corrosive environment. For the necessary notch, a larger radius of curvature should be used to avoid the sharpness of sharp corners, corners and structures. The closed symmetrical structure should be used to avoid gaps and dead corners that may cause residual corrosion and prevent the concentration of harmful ions. On the other hand, it is necessary to minimize the processing stress, heat treatment stress, assembly stress and other residual stresses from the material processing, manufacturing process and structural design. The use of annealing and other means to eliminate residual stress, rolling, shot peening, ultrasonic, vibration and other methods can also reduce residual stress or compressive stress on the surface of the material, which is also an effective method to improve the stress corrosion resistance of the material.2. Improve corrosive environment and take necessary protective measures(l) Change the media conditions and, where possible, avoid active corrosive media or reduce and control the amount of hazardous media. Try to eliminate or reduce harmful chemical ions that cause corrosion cracking;(2) Change the temperature, concentration, impurity content and pH of the solution during the production process. According to the experimental results and empirical data, the above parameters are properly regulated so that they are in the water half which is most unfavorable for the occurrence of stress corrosion. Of course, not necessarily several parameters reach the ideal value at the same time, and it is necessary to grasp the main contradiction according to actual needs and possibilities;(3) Correctly use corrosion inhibitors to change the nature of the corrosive environment. According to the actual situation, the proper choice of corrosion inhibitor can significantly slow down the stress corrosion process. Any corrosion inhibitor can change the pH of the medium; promote the polarization of the cathode or anode, and prevent the intrusion of oxygen or the adsorption of harmful substances. The selection of corrosion inhibitors can be found in the information;(4) Adopting the method of external potential to make the potential of the metal in the dielectric towel away from the stress corrosion cracking sensitive potential area, such as the more commonly used cathodic protection and anode protection method, etc. The choice of the specific method should be determined according to the actual material and the medium.3. Reasonable material selection and appropriate surface treatment and heat treatment process(1) Since the stress corrosion process depends on the special combination of sensitive metals and specific corrosive environments, rational material selection is the most basic work to prevent or mitigate stress corrosion. Materials that have not been found to have stress corrosion cracking or are less sensitive in the medium used, K1scc should be selected as much as possible. Generally, the gold-plated materials produced by vacuum melting, vacuum remelting, vacuum casting, etc. should be used to ensure high purity and prevent excessive non-metallic inclusions. It should also be noted that precipitation hardening of many alloys at normal temperature or higher can greatly improve the stress corrosion sensitivity, while overaging, graded aging or deformation aging can significantly improve the stress corrosion resistance of high strength aluminum alloy gold;(2) The phase composition, phase morphology and distribution of the alloy phase are changed by using various new processes such as ultra-high temperature quenching and sub-temperature quenching. The surface composition changes the composition and structure of the alloy, eliminates the segregation of impurity elements, refines the grain, improves the uniformity of the composition and structure of the island, improves the toughness of the material, and improves the stress corrosion resistance of the alloy.(3) Using an organic coating, an inorganic coating or a metal coating, or covering the surface of the metal with an inert gas, and using a rubbing oil, a resisting agent, etc., prevents the metal from coming into direct contact with an etchant that may cause stress corrosion cracking. High-strength steel is very effective for epoxy paint polyurethane coating and polyhydrogen oxygen protection. The aluminum coating is also effective for the protection of Al-Taijin in high-strength steel and tyrosine-zinc primer in the upper atmosphere. Cadmium plating and galvanizing can also prevent stress corrosion cracking of various materials, but the use of cadmium plating, galvanizing or inorganic zinc coating may cause hydrogen embrittlement of the plated parts, and heat treatment to dehydrogenate after plating.
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