The reduced toughness of metal hoisting wire in sub-zero temperatures does significantly increase the risk of brittle fracture. The core of this issue lies in the altered properties of the steel wire material due to low temperatures, as well as the combined effects of the stress characteristics of lifting operations and environmental factors. This requires a step-by-step analysis from various perspectives, including the material's nature, stress state, and environmental impact. As a core load-bearing component in lifting operations, the toughness of metal hoisting wire is directly related to its ability to absorb shock and disperse stress. Low temperatures fundamentally weaken this ability, creating a potential for brittle fracture.
From a microscopic perspective, metal hoisting wire is primarily made of high-carbon steel. At room temperature, high-carbon steel, due to its optimal crystal arrangement and internal structure, maintains both high tensile strength and a certain degree of toughness. When subjected to stress, it can buffer external forces through slight plastic deformation, preventing stress concentration that can directly lead to fracture. However, in sub-zero temperatures, the thermal motion of atoms within the steel slows significantly, hindering the movement of dislocations between crystals. This compresses the channels that could otherwise release stress through deformation, leading to a significant decrease in toughness indicators (such as impact toughness and fracture toughness). At this point, the metal hoisting wire no longer possesses the "elastic cushioning" capacity it possesses at room temperature and instead exhibits a more brittle nature. This means it struggles to undergo significant plastic deformation when subjected to stress. Once local stress reaches the material's brittle fracture threshold, it may suddenly break without apparent warning.
The risk of brittle fracture caused by this reduced toughness is further amplified when combined with the actual loads encountered during lifting operations. During operation, metal hoisting wire not only bears the static load of heavy objects, but also dynamic and impact loads generated by operations such as lifting start-up, braking, deceleration, and luffing adjustments. These include load fluctuations during lifting and the inertial impact of braking. At room temperature, these dynamic loads are absorbed by the wire's toughness and converted into minor deformations. However, at low temperatures, the lack of toughness prevents the impact energy from being effectively dissipated, directly impacting weak points in the wire. These weak points may be minor scratches left over from the production process, internal non-metallic inclusions, or microcracks formed during twisting. At low temperatures, these defects act as stress concentration points, causing cracks to propagate at a much faster rate than at room temperature, ultimately leading to brittle fracture of the entire wire. Other factors in low temperatures can also create a synergistic effect with the reduced toughness, further exacerbating the risk of brittle fracture. For example, sub-zero temperatures can cause the viscosity of the lubricant on the surface of metal hoisting wire to increase dramatically, even solidifying into a hard crust. On the one hand, the solidified lubricant loses its lubricating effect, increasing friction between the wires and between the wires and the pulleys/drums, leading to more uneven local stress distribution when loaded. On the other hand, the hardened lubricant exerts additional compressive stress on the wire surface, especially during bending and winding. This compressive stress exacerbates the growth of surface defects, indirectly accelerating the brittle fracture process caused by the reduced toughness. Ice and snow in the operating environment, which adhere to the wire surface or accumulate in the rope grooves, can add additional load and potentially cause the wire to slip. The sudden force generated by slipping can create a greater impact, and wires with insufficient toughness simply cannot withstand this sudden stress.
Long-term use of metal hoisting wire in sub-zero temperatures can further deteriorate its toughness due to the "cold work hardening" effect. During lifting operations, steel wires are repeatedly subjected to cyclic loads such as bending, stretching, and compression. Steel's plastic deformation capacity is inherently weak at low temperatures, and cyclic stresses cause accumulated plastic deformation within the wire, gradually forming a cold-work hardened layer. This hardened layer significantly increases its hardness, but further reduces its toughness, resulting in an imbalanced mechanical property of the hoisting wire: "hard yet brittle." This hardened area becomes a high-risk location for brittle fracture. Subsequent stresses, even the slightest impact, can cause fractures there, often rapidly and suddenly, posing a significant threat to lifting safety.
In practical applications, this risk is particularly pronounced during outdoor lifting operations in extremely cold regions, such as construction lifting in northern China during winter and heavy loading and unloading in cold chain logistics. In these scenarios, hoisting wires not only have to withstand continuous low-temperature erosion but also face the combined effects of wind, snow, freeze-thaw cycles, and other complex environmental conditions. This accelerates the decline in toughness and increases the risk of brittle fracture. If special protection measures are not taken for low-temperature environments and steel wire is used only at room temperature, the danger of decreased toughness can be ignored, leading to sudden rope breakage during lifting operations, resulting in equipment damage or even casualties.
However, this risk of brittle fracture is not insurmountable. Measures can be taken to mitigate this risk. For example, special metal hoisting wire with low-temperature toughness can be selected (for example, by optimizing the material formula by adding alloying elements such as nickel and manganese). Preheating the wire moderately before operation to activate atomic motion and restore some toughness. Regularly replace lubricants suitable for low temperatures to maintain lubrication. Clear ice and snow from the surface to avoid excessive loads and slippage. Furthermore, routine inspections should be strengthened, focusing on defects such as cracks and broken wires. Any abnormalities should be immediately removed and replaced. These targeted measures can effectively reduce the risk of brittle fracture caused by decreased toughness in low-temperature environments and ensure the safe use of metal hoisting wire in severe cold conditions.
