Sofia Martinez
Cryogenic treatment is a process where workpieces are treated at extremely low temperatures, typically around -300 °F / -184 °C, to remove residual stresses and improve wear resistance in metals and alloys. This process involves cryorolling, which enhances tensile strength and ductility by creating a higher fraction of twin boundaries in the material's microstructure. Plastic deformation is an essential aspect of metal processing, where external stress induces a shape change and alters mechanical properties. When metals are cooled, certain slip planes become deactivated, leading to a ductile-to-brittle transition. At cryogenic temperatures, unusual plastic deformation localization processes occur due to the competing activation of multiple deformation mechanisms. This phenomenon has been observed in alloys like Cu–Zn, nickel-based superalloys, and stainless steels, enhancing their mechanical strength and phase transformation.
| Characteristics | Values |
|---|---|
| Definition | Cryogenic dynamic plastic deformation refers to the dynamic strain ageing (DSA) and transformation from the parent phase (γ) to the secondary phases (α′ and ε) at extremely low temperatures. |
| Materials Used | Copper, copper alloys, stainless steel, aluminium alloys, Cu-Ni-Co-Si alloy, Cu–Zn alloy, Cu–Fe in situ composites, Cu–Ni–Si–Cr alloy, Cu–Fe–Ag in situ composite, Cu–Zn alloy |
| Temperatures | Very low temperatures, close to absolute zero (0 K) |
| Microstructural Changes | Deformation twinning, shear banding, formation of nano-sized grains, ultrafine grains with high-angle boundaries |
| Mechanical Properties | High strength, limited tensile ductility, increased plasticity, stable tensile strength, uniform microstructure, refined grain size |
| Testing Methods | Tensile tests, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD) |
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What You'll Learn
Cryorolling at liquid nitrogen temperature increases tensile strength
Cryorolling is an emerging severe plastic deformation method that involves rolling metals at cryogenic temperatures to obtain a bulk nanostructure-grained microstructure. It is a potential technique to produce nanostructured bulk materials from their bulk counterparts at extremely low temperatures.
Plastic deformation is an intrinsic part of the processing of most metals, where the aim is to achieve a shape change by means of externally applied stress while causing a controlled alteration in the material's mechanical properties. Plastic deformation processes are traditionally divided into hot and cold deformation methods. Cryorolling, as a cold deformation method, effectively suppresses the dynamic recovery of metals, leading to a microstructure containing several dislocation sites. This results in extreme strain hardening, increasing the strength and decreasing the ductility of metals like aluminium.
The use of liquid nitrogen in rolling aluminium is likely to obtain work-hardened aluminium with a higher dislocation density than aluminium rolled at room temperature. This rolled aluminium is expected to have higher strength and lower ductility. Cryorolling at liquid nitrogen temperatures has been applied to aluminium alloys and copper-zinc alloys, resulting in enhanced tensile strength and ductility.
Cryogenic rolling is a technique used to produce nanostructured materials by introducing high internal strains while retaining the original sample dimensions. By carefully controlling the subsequent annealing process, a high-strength and high-ductility material can be produced. Cryogenic hardening and rolling engineer nano-sized twin boundaries embedded in the metal, decreasing the grain boundary size and enhancing grain boundary strengthening. This process increases the ability of the grain boundary to accommodate more dislocations, leading to improved ductility.
Cryogenic tempering, another cryogenic treatment process, involves slowly cooling materials to ultra-low temperatures, typically around -300 °F / -184 °C, and then slowly reheating them. This process compresses the molecular structures of materials tightly in uniformity, often using liquid nitrogen to descend temperatures. Cryogenic treatment was invented by Ed Busch (CryoTech) in 1966 and has since been used to enhance the mechanical properties of various metals and alloys.
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Cryogenic treatment removes residual stress and improves wear resistance
Cryogenic treatment is a process of treating workpieces to extremely low temperatures, typically around −300°F (−184°C or lower), to improve their properties. It is a supplementary process to conventional heat treatment, enhancing the tribological properties of steels and other metal alloys, such as aluminium. Cryogenic treatment is particularly effective in reducing residual stress and improving wear resistance.
Residual stress is common in materials like aluminium and steel due to the introduction of stresses during machining operations. These stresses can affect the performance and properties of the material. Cryogenic treatment helps relieve these residual stresses by causing the material to undergo a phase transformation. During this process, the material becomes more brittle and susceptible to cracking or breaking along stress concentration points, thereby relieving some of the residual stresses. This process is especially useful for parts that move during machining, improving their wear resistance and fatigue strength.
The effectiveness of cryogenic treatment in improving wear resistance has been observed in various experiments. One study found that cryogenic treatment improved the wear resistance of En 31 steel by up to 75%, depending on the service conditions. Another experiment by Zhang et al. demonstrated that cryorolling at liquid nitrogen temperature significantly enhanced the tensile strength of dynamically plastic-deformed copper while maintaining high ductility. This was achieved by engineering nano-sized twin boundaries embedded in the copper.
The cryogenic process is believed to promote the complete transformation of retained austenite into martensite, enhancing wear resistance. Additionally, it encourages the formation of fine carbides in the martensite matrix, increasing toughness and wear resistance. Longer cryogenic treatment times result in a finer microstructure and higher surface hardness, improving galling properties. The specific temperature and duration of the treatment depend on the type and severity of the stresses present in the material and the desired properties.
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Plastic deformation at cryogenic temperatures
Plastic deformation is an intrinsic part of the processing of most metals. It involves achieving a shape change by applying external stress while controlling alterations in the material's mechanical properties. Plastic deformation is the primary source of Acoustic Emission (AE) in loaded metallic materials. The AE behaviour differs across metals and alloys during plastic deformation. Typically, the initiation of plasticity around yielding contributes to the highest level of AE.
At room temperature, plastic deformation is time-independent and occurs within a short period. At higher temperatures, atom and vacancy migration occurs extensively, and time-dependent plastic deformation (creep) continues until fracture. Several deformation mechanisms, such as diffusion creep and dislocation creep, appear based on testing stress and temperature.
At very low temperatures, these materials undergo dynamic strain ageing (DSA) and transformation from the parent phase (γ) to secondary phases (α' and ε). This behaviour is associated with material instabilities and an oscillatory mode of plastic flow. Discontinuous (serrated) yielding is observed at extremely low temperatures, below a specific temperature threshold.
The deformation behaviour of alloys, such as Ti-36Nb-2Ta-3Zr-0.35O, has been studied at cryogenic temperatures. The results indicate that the plastic deformation behaviour is sensitive to test temperature and the alloy's cold deformation history. At cryogenic temperatures, the tensile strength and ductility of the alloy without cold deformation are significantly enhanced due to the increased density of mechanical twinning.
Additionally, the plastic deformation behaviour of a Cu-Zn alloy subjected to dynamic plastic deformation (DPD) at liquid nitrogen temperature (77 K) has been investigated. Deformation twinning and shear banding dominate the plastic deformation of samples with different strains. Deformation twins contribute to strengthening, while the appearance of shear bands reduces the ductility of the material.
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Dynamic recrystallization and post-dynamic recrystallization
Cryogenic dynamic plastic deformation involves the study of plastic deformation at extremely low temperatures, often in metallic materials. Plastic deformation is an intrinsic part of metal processing, where an external force is applied to change the shape of a metal, while also altering its mechanical properties. Plastic deformation is the primary source of Acoustic Emission (AE) in loaded metallic materials.
Dynamic recrystallization (DRX) is a type of recrystallization process observed in the fields of metallurgy and geology. It occurs when the nucleation and growth of new grains happen during deformation, as opposed to static recrystallization, where this occurs afterward as a separate heat treatment. DRX can be further classified into discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX), and geometric dynamic recrystallization (GDRX). The onset of DRX can be identified by a distinct peak in the flow stress in hot working data, or from an inflection point in plots of the strain-hardening rate against stress.
Discontinuous dynamic recrystallization (DDRX) involves the nucleation of new, dislocation-free grains, followed by their growth through the migration of high-angle grain boundaries (HAGBs). It is common in materials with low to medium stacking fault energy (SFE). During DDRX, the nucleation generally occurs along pre-existing grain boundaries, and the rate of recrystallization increases as the initial grain size decreases.
Geometric dynamic recrystallization (GDRX) occurs in grains with local serrations. Upon deformation, these grains elongate until their thickness falls below a threshold, causing the serration boundaries to intersect and small grains to pinch off into equiaxed grains. GDRX generally occurs during deformation at elevated temperatures and in materials with high stacking fault energy.
Continuous dynamic recrystallization (CDRX) is still not perfectly understood, as it lacks data regarding flow stress behavior. CDRX is also known as in situ or cDRX.
Post-dynamic recrystallization (PDRX) occurs when the annealing temperature does not drop sufficiently fast after the critical strain for DDRX, leading to the growth of recrystallization nuclei with no incubation time.
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Austenitic stainless steels at cryogenic temperatures
Cryogenic temperatures refer to temperatures that are extremely low, close to absolute zero. Austenitic stainless steels are Fe-Cr alloys with sufficient nickel and manganese (and sometimes nitrogen) to stabilize austenite, a face-centered cubic (FCC) phase. Austenitic stainless steels, particularly Types 304 and 316, retain their engineering properties at cryogenic temperatures and are commonly used in arctic locations and in the handling and storage of liquid gases such as liquid nitrogen and liquid helium. They are also used in the construction of pipes, pumps, and valves in petrochemical processes requiring low-temperature operation, such as fractional distillation in ethylene plants.
The exceptional performance of austenitic stainless steels at cryogenic temperatures is due to their face-centered cubic (FCC) atomic structure, achieved through careful nickel addition during manufacturing. This structure results in improved mechanical properties, particularly increased tensile strength and toughness, at decreasing temperatures. The cryogenic tempering process transforms the crystal lattice structure from body-centered cubic to face-centered cubic, reducing the space for interstitial defects and yielding a stronger and more durable material.
At very low temperatures, austenitic stainless steels exhibit discontinuous plastic flow (serrated yielding), which reflects the dynamic strain ageing effect. This behaviour is observed in materials with low stacking fault energy (LSFE), which undergo both dynamic strain ageing (DSA) and transformation from the parent phase (γ) to the secondary phases (α' and ε) at extremely low temperatures. The physical mechanism of the intermittent plastic flow (IPF) of austenitic steels at extremely low temperatures is explained by microstructure evolution and 3D deformation in the area of shear bands.
While austenitic stainless steels demonstrate impressive performance at cryogenic temperatures, they also have some disadvantages. They are more expensive than other alloys, have lower yield strength than ferritic Fe-Ni steels and aluminium alloys, and their machinability is poorer than that of aluminium alloys. Additionally, care must be taken when designing structures for cryogenic service to ensure that all components and accessories are suitable for the intended operating temperatures.
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Frequently asked questions
Cryogenic dynamic plastic deformation is a process that treats workpieces to cryogenic temperatures to remove residual stresses and improve wear resistance in steels and other metal alloys.
Plastic deformation is an intrinsic part of the processing of most metals. It aims to achieve shape change through externally applied stress, causing a controlled alteration in the material's mechanical properties.
Cryogenic treatment improves the mechanical strength of materials like stainless steel and enhances ductility.
Cryogenic treatment involves temperatures typically around -300 °F / -184 °C, or as low as −190 °C (−310 °F).
Cryogenic dynamic plastic deformation creates a higher fraction of twin boundaries compared to severe plastic deformation. Cryorolling further reduces grain boundary energy, leading to a higher strengthening effect.
