Cryogenic Treatment of Stainless Steels

Understanding Cryogenic Treatment Fundamentals

The term "cryogenics" derives from two Greek words: "kryos," meaning frost or freezing, and "genic," meaning to produce or generate. Technologically, cryogenic treatment involves the study and application of materials at extremely low temperatures. Deep subzero treatment of metals and alloys functions as an advanced stress-relieving technology that addresses fundamental material science challenges.

During manufacturing operations, materials inevitably experience various stresses that manifest as defects within the crystal structure. The most commonly observed defects include vacancies, dislocations, and stacking faults. As stress levels increase, the density of these defects grows, leading to increased interatomic spacing. When the distance between atoms exceeds a critical threshold, cracks develop and material failure occurs.

Thermodynamic Principles in Cryogenic Processing

The third law of thermodynamics states that entropy reaches zero at absolute zero temperature. Deep subzero treatment leverages this principle to relieve material stresses effectively. Materials subjected to extremely low temperatures for extended periods develop equilibrium conditions, which eliminates structural defects and achieves the minimum entropy state.

According to thermodynamic laws, absolute zero represents the lowest achievable temperature limit. At absolute zero, molecules exist in their lowest finite energy state. Absolute zero equals -273.15°C or -459.67°F. The cryogenic region typically encompasses temperatures below approximately 120 K (-153°C) on the Kelvin scale.

Common permanent gases transition from gas to liquid at atmospheric pressure at specific temperatures called the normal boiling point (NBP). These liquids are classified as cryogenic liquids or cryogens.

Table 1. Normal boiling points of common cryogenic fluids

Material Behavior Under Cryogenic Conditions

Temperatures below 273K are generally considered low temperatures, requiring special consideration in construction, transportation, and power sectors, particularly for pipeline applications. Steels designed for cryogenic conditions were specifically developed for transporting flowing gases. Additionally, material superconductivity occurs under temperature conditions below 273K.

In materials deformed at temperatures below 273K, dynamic recovery processes become fully inhibited, leading to dislocation accumulation. As dislocation density increases, their movement slows and becomes concentrated. This transforms the plastic deformation mechanism from conventional dislocation slip to slip and mechanical twinning. With further tension increases, grain size refinement to nanometer scale becomes possible. Since grain size directly relates to mechanical properties, cryogenic forming can significantly enhance the mechanical properties of selected metallic materials.

Heat Treatment Sequence for Optimal Results

The complete steel treatment process consists of hardening through austenitizing and quenching, followed by cryogenic treatment or deep cryogenic treatment (DCT), and finally tempering. To achieve optimal steel microstructure and desired properties, most researchers recommend executing DCT after quenching completion but before tempering in the conventional heat treatment cycle.

Figure 1: Heat treatment sequence for maximizing martensite transformations

Experimental Research on Austenitic Stainless Steel 316LN

Research conducted by A. Fedorikova investigated the effects of plastic deformation on mechanical properties under cryogenic conditions using nitrogen-enhanced austenitic stainless steel 316LN. The steel underwent hot forging followed by heat treatment at 1050°C for 60 minutes, then rapid cooling to room temperature. Test samples were extracted from the slab's middle section.

Table 2. Chemical composition of experimental material (wt. %)

Comparative Rolling Experiments and Testing Methods

Two experimental rolling types were performed for comparison: room temperature rolling and cryogenic condition rolling. Specimens underwent approximately 10% reduction per pass, achieving total deformations of 10%, 30%, and 50%. Cryogenic rolling conditions were maintained by immersing specimens in liquid nitrogen for 30 minutes before rolling and 10 minutes after each pass. Specimen dimensions were: h₀ = 11mm, b₀ = 41mm, l₀ = 80mm.

Static tensile testing utilized a universal testing machine (ATLAS, 650kN load capacity) at three temperature ranges from 4.2K to 293K. Cryogenic temperatures were maintained using a cryostat system integrated into the Atlas testing machine, filled with liquid helium (4.2K) and liquid nitrogen (77K).

Mechanical Properties and Performance Results

The mechanical characteristics obtained from all tests demonstrate that yield strength and ultimate tensile strength of modified 316LN increase with deformation, and rolling temperature significantly impacts final mechanical properties. However, total elongation decreases substantially with deformation due to plasticity exhaustion and increased deformation resistance during experimental rolling. Therefore, post-rolling heat treatment becomes necessary to restore ductility.

Table 3. Static tensile test results

Key Findings and Applications

Based on experimental results, several important conclusions emerge regarding cryogenic treatment of stainless steels. Higher deformation levels combined with decreasing test temperatures increase both yield strength and ultimate tensile strength, while plasticity decreases significantly. Austenitic stainless steel can achieve substantial strengthening through plastic deformation under cryogenic conditions.

To preserve final material plasticity, additional heat treatment is required in most applications. However, specific applications requiring high strength without plastic properties can utilize materials prepared through cryogenic rolling without subsequent heat treatment. This flexibility makes cryogenic treatment particularly valuable for specialized engineering applications where strength takes precedence over ductility.

The research demonstrates that cryogenic treatment represents a viable method for enhancing stainless steel mechanical properties while providing options for tailoring material characteristics to specific application requirements. The process offers significant advantages in developing high-performance materials for demanding industrial applications.