Cryogenic treatment

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A cryogenic treatment is the process of treating workpieces to cryogenic temperatures (typically around -300°F / -184°C, or as low as −190 °C (−310 °F)) in order to remove residual stresses and improve wear resistance in steels and other metal alloys, such as aluminum. In addition to seeking enhanced stress relief and stabilization, or wear resistance, cryogenic treatment is also sought for its ability to improve corrosion resistance by precipitating micro-fine eta carbides, which can be measured before and after in a part using a quantimet.[ clarification needed ]

Contents

The process has a wide range of applications from industrial tooling to the improvement of musical signal transmission. Some of the benefits of cryogenic treatment include longer part life, less failure due to cracking, improved thermal properties, better electrical properties including less electrical resistance, reduced coefficient of friction, less creep and walk, improved flatness, and easier machining. [1]

Processes

Cryogenic tempering

Cryogenic tempering is two phase metal treatment that involves a descent and ascent phase, including a cryogenic treatment process (known as "cryogenic processing") where the material is slowly cooled to ultra low temperatures (typically around -300°F / -184°C), which is then optionally reheated slowly (typically up to +325°F / 162°C). Materials do not "harden" during the temperature descent or ascent, rather their molecular structures are compressed together tightly in uniformity through a computer controlled process that typically uses liquid nitrogen to slowly descend temperatures.

Invention History of Cryogenic Processing & Cryogenic Tempering

The cryogenic treatment process was invented by Ed Busch (CryoTech) in Detroit, Michigan in 1966, inspired by NASA research, which later merged with 300 Below, Inc. in 2000 to become the world's largest and oldest commercial cryogenic processing company after Peter Paulin of Decatur, IL collaborated with process control engineers to invent the world's first computer-controlled "dry" cryogenic processor in 1992 (where he was featured on the Discovery Channel's Next Step TV Show for his invention). Whereas the industry initially submerged metal parts in liquid nitrogen by dunking them or pouring liquid nitrogen over the parts, the earliest results proved inconsistent, which led Mr. Paulin to develop 300 Below's "dry" computer-controlled cryogenic processing equipment to ensure consistent and accurate treatment results across every processing run by introducing liquid nitrogen into a chamber above its boiling point, in a "dry" gaseous state, to ensure that parts in a chamber are not thermally shocked from being exposed to direct liquid contact of ultra low temperatures. A "dry" cryogenic process does not submerge parts in liquid, but rather ensures that temperatures are slowly descended at less than one degree per minute using short bursts of cold gas being introduced via a solenoid-metered pipe, which is controlled by a computer equipment paired with highly accurate RTD (Resistance Temperature Detector) sensors.

Science Behind Dry Cryogenic Processing & Cryogenic Tempering

Because all changes to metals take place on the quench, the first phase of the initial descent is called cryogenic processing, and by adding a second phase to heat the molecular structure of materials after an initial molecular re-alignment, both processes together are called cryogenic tempering. By using liquid nitrogen, the temperature can go as low as −196 °C, though the typical dwell temperature of cryogenic processing equipment is slightly above the boiling point of liquid nitrogen (closer to -300°F / -184°C) due to being injected into the processing chamber as a gaseous state and making every attempt not to introduce liquid into the chamber that could cause parts to become thermally shocked. Cryogenic processing (and especially cryogenic tempering) can have a profound effect on the mechanical properties of certain materials, such as steels or tungsten carbide, but the heating phase in cryogenic tempering is typically omitted for softer metals like brass in musical instruments, for piano strings, in certain aerospace applications, or for sensitive electronic components like vacuum tubes and transistors in high-end audio equipment. In tungsten carbide (WC-Co), the crystal structure of cobalt is transformed from softer FCC to harder HCP phase whereas the hard tungsten carbide particle is unaffected by the treatment. [2]

Applications of cryogenic processing

  • Aerospace & Defense: communication, optical housings, satellites, weapons platforms, guidance systems, landing systems.
  • Automotive: brake rotors, transmissions, clutches, brake parts, rods, crank shafts, camshafts axles, bearings, ring and pinion, heads, valve trains, differentials, springs, nuts, bolts, washers.
  • Cutting tools: cutters, knives, blades, drill bits, end mills, turning or milling [3] inserts. Cryogenic treatments of cutting tools can be classified as Deep Cryogenic Treatments (around -196 °C) or Shallow Cryogenic Treatments (around -80 °C).
  • Forming tools: roll form dies, progressive dies, stamping dies.
  • Mechanical industry: pumps, motors, nuts, bolts, washers.
  • Medical: tooling, scalpels.
  • Motorsports and Fleet Vehicles: See Automotive for brake rotors and other automotive components.
  • Musical: Vacuum tubes, Audio cables, brass instruments, guitar strings [4] and fret wire, piano wire, amplifiers, magnetic pickups, [5] cables, connectors.
  • Sports: Firearms, knives, fishing equipment, auto racing, tennis rackets, golf clubs, mountain climbing gear, archery, skiing, aircraft parts, high pressure lines, bicycles, motor cycles.

Cryogenic machining

Cryogenic machining is a machining process where the traditional flood lubro-cooling liquid (an emulsion of oil into water) is replaced by a jet of either liquid nitrogen (LN2) or pre-compressed carbon dioxide (CO2). Cryogenic machining is useful in rough machining operations, in order to increase the tool life. It can also be useful to preserve the integrity and quality of the machined surfaces in finish machining operations. Cryogenic machining tests have been performed by researchers for several decades, [6] but the actual commercial applications are still limited to very few companies. [7] Both cryogenic machining by turning [8] and milling [9] are possible. Cryogenic machining is a relatively new technique in machining. This concept was applied on various machining processes such as turning, milling, drilling etc. Cryogenic turning technique is generally applied on three major groups of workpiece materials—superalloys, ferrous metals, and viscoelastic polymers/elastomers. The roles of cryogen in machining different materials are unique. [10]

Cryogenic deflashing

Cryogenic deburring

Cryogenic rolling

Cryogenic rolling or cryorolling, is one of the potential techniques to produce nanostructured bulk materials from its bulk counterpart at cryogenic temperatures. It can be defined as rolling that is carried out at cryogenic temperatures. Nanostructured materials are produced chiefly by severe plastic deformation processes. The majority of these methods require large plastic deformations (strains much larger than unity). In case of cryorolling, the deformation in the strain hardened metals is preserved as a result of the suppression of the dynamic recovery. Hence large strains can be maintained and after subsequent annealing, ultra-fine-grained structure can be produced.

Advantages

Comparison of cryorolling and rolling at room temperature:

  • In cryorolling, the strain hardening is retained up to the extent to which rolling is carried out. This implies that there will be no dislocation annihilation and dynamic recovery. Where as in rolling at room temperature, dynamic recovery is inevitable and softening takes place.
  • The flow stress of the material differs for the sample which is subjected to cryorolling. A cryorolled sample has a higher flow stress compared to a sample subjected to rolling at room temperature.
  • Cross slip and climb of dislocations are effectively suppressed during cryorolling leading to high dislocation density which is not the case for room temperature rolling.
  • The corrosion resistance of the cryorolled sample comparatively decreases due to the high residual stress involved.
  • The number of electron scattering centres increases for the cryorolled sample and hence the electrical conductivity decreases significantly.
  • The cryorolled sample shows a high dissolution rate.
  • Ultra-fine-grained structures can be produced from cryorolled samples after subsequent annealing.

Cryogenic treatment in specific materials

Stainless steel

The torsional and tensional deformation under cryogenic temperature of stainless steel is found to be significantly enhance the mechanical strength while incorporating the gradual phase transformation inside the steel. [11] This strength improvement is the result of following phenomenon.

Copper

Zhang et al. exploited the cryorolling to the dynamic plastic deformed copper at liquid nitrogen temperature (LNT-DPD) to greatly enhance tensile strength with high ductility. [12] The key of this combined approach (Cryogenic hardening and Cryogenic rolling) is to engineer the nano-sized twin boundary embedded in the copper. Processing with the plastic deformation of grained bulk metal decreases the size of the grain boundary and enhances the grain boundary strengthening. However, as the grain gets smaller, the interaction between grain and the dislocation inside impedes further process of grains. Among the grain boundaries, it is known that the twin boundaries, a special type of low-energy grain boundary has lower interaction energy with dislocation leading to much smaller saturation size of the grain. [13] The cryogenic dynamic plastic deformation creates higher fraction of the twin boundaries compared to the severe plastic deformation. Following cryorolling further reduces the grain boundary energy with relieving the twin boundary leading to higher Hall-Petch strengthening effect. In addition, this increases the ability of grain boundary to accommodate more dislocation leading to the improvement in ductility from cryorolling.

Titanium

Cryogenic hardening of Titanium is hard to manipulate compare to other face centered cubic (fcc) metals because these hexagonal close packed (hcp) metals has less symmetry and slip systems to exploit. Recently Zhao et al. introduced the efficient method to manipulate nanotwinned titanium which has higher strength, ductility and thermal stability. [14] By cryoforging repetitively along the three principal axes in liquid nitrogen and following annealing process, pure Titanium can possess hierarchical twin boundary network structure which suppresses the motion of dislocation and significantly enhances its mechanical property. The microstructure analysis found that the repeated twinning and de-twinning keep increasing the fraction of nanosized twin boundaries and refining the grains to render much higher Hall-Petch strengthening effect even after the saturation of microscale twin boundary at high flow stress. Especially, the strength and ductility of nanotwinned titanium at 77 K, reaches about 2 GPa, and ~100% which far outweighs those of conventional cryogenic steels even without any inclusion of alloying.

Related Research Articles

<span class="mw-page-title-main">Ductility</span> Degree to which a material under stress irreversibly deforms before failure

Ductility is a mechanical property commonly described as a material's amenability to drawing. In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stress before failure. Ductility is an important consideration in engineering and manufacturing. It defines a material's suitability for certain manufacturing operations and its capacity to absorb mechanical overload. Some metals that are generally described as ductile include gold and copper, while platinum is the most ductile of all metals in pure form. However, not all metals experience ductile failure as some can be characterized with brittle failure like cast iron. Polymers generally can be viewed as ductile materials as they typically allow for plastic deformation.

<span class="mw-page-title-main">Heat treating</span> Process of heating something to alter it

Heat treating is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

<span class="mw-page-title-main">Martensite</span> Type of steel crystalline structure

Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation.

<span class="mw-page-title-main">Plasticity (physics)</span> Non-reversible deformation of a solid material in response to applied forces

In physics and materials science, plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the material itself. In engineering, the transition from elastic behavior to plastic behavior is known as yielding.

<span class="mw-page-title-main">High-strength low-alloy steel</span> Type of alloy steel

High-strength low-alloy steel (HSLA) is a type of alloy steel that provides better mechanical properties or greater resistance to corrosion than carbon steel. HSLA steels vary from other steels in that they are not made to meet a specific chemical composition but rather specific mechanical properties. They have a carbon content between 0.05 and 0.25% to retain formability and weldability. Other alloying elements include up to 2.0% manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare-earth elements, or zirconium. Copper, titanium, vanadium, and niobium are added for strengthening purposes. These elements are intended to alter the microstructure of carbon steels, which is usually a ferrite-pearlite aggregate, to produce a very fine dispersion of alloy carbides in an almost pure ferrite matrix. This eliminates the toughness-reducing effect of a pearlitic volume fraction yet maintains and increases the material's strength by refining the grain size, which in the case of ferrite increases yield strength by 50% for every halving of the mean grain diameter. Precipitation strengthening plays a minor role, too. Their yield strengths can be anywhere between 250–590 megapascals (36,000–86,000 psi). Because of their higher strength and toughness HSLA steels usually require 25 to 30% more power to form, as compared to carbon steels.

In metallurgy, a shape-memory alloy (SMA) is an alloy that can be deformed when cold but returns to its pre-deformed ("remembered") shape when heated. It is also known in other names such as memory metal, memory alloy, smart metal, smart alloy, and muscle wire. The "memorized geometry" can be modified by fixating the desired geometry and subjecting it to a thermal treatment, for example a wire can be taught to memorize the shape of a coil spring.

<span class="mw-page-title-main">Brittleness</span> Liability of breakage from stress without significant plastic deformation

A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a sharp snapping sound.

<span class="mw-page-title-main">Maraging steel</span> Steel known for strength and toughness

Maraging steels are steels that are known for possessing superior strength and toughness without losing ductility. Aging refers to the extended heat-treatment process. These steels are a special class of very-low-carbon ultra-high-strength steels that derive their strength not from carbon, but from precipitation of intermetallic compounds. The principal alloying element is 15 to 25 wt% nickel. Secondary alloying elements, which include cobalt, molybdenum and titanium, are added to produce intermetallic precipitates. Original development was carried out on 20 and 25 wt% Ni steels to which small additions of aluminium, titanium, and niobium were made; a rise in the price of cobalt in the late 1970s led to the development of cobalt-free maraging steels.

<span class="mw-page-title-main">Galling</span> Form of wear caused by adhesion between sliding surfaces

Galling is a form of wear caused by adhesion between sliding surfaces. When a material galls, some of it is pulled with the contacting surface, especially if there is a large amount of force compressing the surfaces together. Galling is caused by a combination of friction and adhesion between the surfaces, followed by slipping and tearing of crystal structure beneath the surface. This will generally leave some material stuck or even friction welded to the adjacent surface, whereas the galled material may appear gouged with balled-up or torn lumps of material stuck to its surface.

<span class="mw-page-title-main">Work hardening</span> Strengthening a material through plastic deformation

In materials science, work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. Work hardening may be desirable, undesirable, or inconsequential, depending on the context.

Cryogenic hardening is a cryogenic treatment process where the material is cooled to approximately −185 °C (−301 °F), usually using liquid nitrogen. It can have a profound effect on the mechanical properties of certain steels, provided their composition and prior heat treatment are such that they retain some austenite at room temperature. It is designed to increase the amount of martensite in the steel's crystal structure, increasing its strength and hardness, sometimes at the cost of toughness. Presently this treatment is being used on tool steels, high-carbon, high-chromium steels and in some cases to cemented carbide to obtain excellent wear resistance. Recent research shows that there is precipitation of fine carbides in the matrix during this treatment which imparts very high wear resistance to the steels.

Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. A harder metal will have a higher resistance to plastic deformation than a less hard metal.

In metallurgy and materials science, annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for an appropriate amount of time and then cooling.

In materials science, hardness is a measure of the resistance to localized plastic deformation, such as an indentation or a scratch (linear), induced mechanically either by pressing or abrasion. In general, different materials differ in their hardness; for example hard metals such as titanium and beryllium are harder than soft metals such as sodium and metallic tin, or wood and common plastics. Macroscopic hardness is generally characterized by strong intermolecular bonds, but the behavior of solid materials under force is complex; therefore, hardness can be measured in different ways, such as scratch hardness, indentation hardness, and rebound hardness. Hardness is dependent on ductility, elastic stiffness, plasticity, strain, strength, toughness, viscoelasticity, and viscosity. Common examples of hard matter are ceramics, concrete, certain metals, and superhard materials, which can be contrasted with soft matter.

<span class="mw-page-title-main">Embrittlement</span> Loss of ductility of a material, making it brittle

Embrittlement is a significant decrease of ductility of a material, which makes the material brittle. Embrittlement is used to describe any phenomena where the environment compromises a stressed material's mechanical performance, such as temperature or environmental composition. This is oftentimes undesirable as brittle fracture occurs quicker and can much more easily propagate than ductile fracture, leading to complete failure of the equipment. Various materials have different mechanisms of embrittlement, therefore it can manifest in a variety of ways, from slow crack growth to a reduction of tensile ductility and toughness.

<span class="mw-page-title-main">Nanocrystalline material</span>

A nanocrystalline (NC) material is a polycrystalline material with a crystallite size of only a few nanometers. These materials fill the gap between amorphous materials without any long range order and conventional coarse-grained materials. Definitions vary, but nanocrystalline material is commonly defined as a crystallite (grain) size below 100 nm. Grain sizes from 100 to 500 nm are typically considered "ultrafine" grains.

Methods have been devised to modify the yield strength, ductility, and toughness of both crystalline and amorphous materials. These strengthening mechanisms give engineers the ability to tailor the mechanical properties of materials to suit a variety of different applications. For example, the favorable properties of steel result from interstitial incorporation of carbon into the iron lattice. Brass, a binary alloy of copper and zinc, has superior mechanical properties compared to its constituent metals due to solution strengthening. Work hardening has also been used for centuries by blacksmiths to introduce dislocations into materials, increasing their yield strengths.

TRIP steel are a class of high-strength steel alloys typically used in naval and marine applications and in the automotive industry. TRIP stands for "Transformation induced plasticity," which implies a phase transformation in the material, typically when a stress is applied. These alloys are known to possess an outstanding combination of strength and ductility.

Severe plastic deformation (SPD) is a generic term describing a group of metalworking techniques involving very large strains typically involving a complex stress state or high shear, resulting in a high defect density and equiaxed "ultrafine" grain (UFG) size or nanocrystalline (NC) structure.

Twinning-Induced Plasticity steel which is also known as TWIP steel is a class of austenitic steels which can deform by both glide of individual dislocations and mechanical twinning on the {1 1 1}γ<1 1 >γ system. They have outstanding mechanical properties at room temperature combining high strength and ductility based on a high work-hardening capacity. TWIP steels have mostly high content in Mn and small additions of elements such C, Si, or Al. The steels have low stacking fault energy at room temperature. Although the details of the mechanisms controlling strain-hardening in TWIP steels are still unclear, the high strain-hardening is commonly attributed to the reduction of the dislocation mean free path with the increasing fraction of deformation twins as these are considered to be strong obstacles to dislocation glide. Therefore, a quantitative study of deformation twinning in TWIP steels is critical to understand their strain-hardening mechanisms and mechanical properties. Deformation twinning can be considered as a nucleation and growth process. Twin growth is assumed to proceed by co-operative movement of Shockley partials on subsequent {111} planes.

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