Some basic knowledge on strain strengthening of austenitic stainless steel pressure vessels

Thread Content

Austenitic stainless steels have a low yield-to-tensile strength ratio; their allowable stress is determined by the yield strength, resulting in an excessive safety margin that prevents the full utilization of their load-carrying capacity, leading to material waste and increased equipment weight. Increasing the yield strength of austenitic stainless steels is key to raising their allowable stress and achieving lightweight design for austenitic stainless steel cryogenic containers. I. Strain strengthening techniques There are two forms of strain strengthening techniques, namely room-temperature strain strengthening and low-temperature strain strengthening. Room-temperature strain strengthening involves subjecting austenitic stainless steel containers to overpressure at room temperature in order to increase their strength. Low-temperature strain strengthening involves placing the molded container in a mold filled with liquid nitrogen; once the container’s temperature reaches -196°C, pressure is applied to strengthen it. However, the high technical difficulties and costs limit its development. Austenitic stainless steel is stretched to plastic deformation at room temperature and then unloaded; when loaded again, the yield strength of the material increases while its plasticity decreases. Room-temperature strain strengthening technology utilizes this principle: at room temperature, clean water is used to subject the inner container of an austenitic stainless steel cryogenic vessel to overpressure, causing it to undergo a certain amount of plastic deformation; this in turn increases the yield strength of the austenitic stainless steel and thus raises the allowable stress. Countries such as China, the United States, Germany, and Australia **have used this technology to manufacture inner containers for austenitic stainless steel cryogenic vessels. Currently, the standards for strain strengthening include AS 1210, EN 13458-2 and EN13530-2, as well as ASME Ⅷ-1 Appendix 44, among others. II. Phase transformation during strain strengthening: Among the changes in the mechanical properties of austenitic stainless steels after strain strengthening, the increase in \"yield strength\" is the most noticeable. The strain-induced martensite formed during strengthening has higher strength and hardness compared to the original austenitic structure. Moreover, the martensite phase is dispersed throughout the austenitic matrix, exerting a pinning effect that thereby increases the yield strength of the stainless steel. Furthermore, the increase in dislocation density during stretching, as well as dislocation pile-ups, can also enhance the strength of stainless steel. It is worth noting that nickel, as an austenite stabilizing element, can effectively suppress the martensitic transformation of austenite. Austenitic stainless steel in the 316 series has a higher nickel content than that in the 304 series; its microstructure remains more stable during cold deformation, making it less prone to deformation-induced martensitic transformation. III. Strain rate and strain: Experiments were conducted on 304 stainless steel under room temperature conditions, with the strain increasing from 0 to 40%. The yield strength rose from 300 MPa to 676 MPa, an increase of more than twice. The pre-stretch amount has little effect on tensile strength; at a pre-stretch amount of 9%, the yield strength ratio of the material is 0.62, indicating relatively good plasticity. Some foreign standards and practices also require that the residual strain at the strain concentration areas after strain strengthening shall not exceed 9% or 10%. Other studies have shown that an appropriate strain rate can significantly increase the strength of austenitic stainless steels while maintaining a certain degree of ductility and toughness; however, too slow a strain rate can cause the material to exhibit serrated yielding behavior, which may lead to plastic instability. IV. Effects on fatigue life and stress corrosion resistance: Strain strengthening can improve the fatigue life of austenitic stainless steels, as the strain-induced martensite formed during this process acts as a pinning mechanism at dislocation boundaries. However, the pinning force is very weak; if the stress amplitude is high, the dislocations will overcome this pinning force and continue to move, resulting in a decrease in fatigue performance. Studies have found that austenitic stainless steel exhibits increased stress corrosion sensitivity in 42% MgCl2 after strain strengthening at room temperature. Strain-induced martensitic transformation is the main reason. After strain strengthening, austenitic stainless steel can achieve an increase in yield strength by controlling the strain level, while still retaining a certain degree of plasticity. At certain stress amplitudes, strain strengthening is beneficial for the fatigue properties of austenitic stainless steel; however, it increases the stress corrosion sensitivity of this material after strain strengthening.