Galled threads on an NPT fitting.An electron microscope image shows transferred sheet-material accumulated on a tool surface during sliding contact under controlled laboratory conditions. The outgrowth of material or localized, roughening and creation of protrusions on the tool surface is commonly referred to as a lump.The damage on the metal sheet, wear mode, or characteristic pattern shows no breakthrough of the oxide surface layer, which indicates a small amount of adhesive material transfer and flattening damage of the sheet's surface. This is the first stage of material transfer and galling build-up.The damage on the metal sheet illustrates continuous lines or stripes, indicating a breakthrough of the oxide surface-layer.The damage on the metal sheet or characteristic pattern illustrates an "uneven surface," a change in the sheet material's plastic behavior and involves a larger deformed volume compared to mere flattening of the surface oxides.
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.[1] Galling is caused by a combination of friction and adhesion between the surfaces, followed by slipping and tearing of crystal structure beneath the surface.[2] 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.
Galling is most commonly found in metal surfaces that are in sliding contact with each other. It is especially common where there is inadequate lubrication between the surfaces. However, certain metals will generally be more prone to galling, due to the atomic structure of their crystals. For example, aluminium is a metal that will gall very easily, whereas annealed (softened) steel is slightly more resistant to galling. Steel that is fully hardened is very resistant to galling.
Galling is a common problem in most applications where metals slide in contact with other metals. This can happen regardless of whether the metals are the same or different. Alloys such as brass and bronze are often chosen for bearings, bushings, and other sliding applications because of their resistance to galling, as well as other forms of mechanical abrasion.
Introduction
Galling is adhesive wear that is caused by the microscopic transfer of material between metallic surfaces during transverse motion (sliding). It occurs frequently whenever metal surfaces are in contact, sliding against each other, especially with poor lubrication. It often occurs in high-load, low-speed applications, although it also can occur in high-speed applications with very little load. Galling is a common problem in sheet metal forming, bearings and pistons in engines, hydraulic cylinders, air motors, and many other industrial operations. Galling is distinct from gouging or scratching in that it involves the visible transfer of material as it is adhesively pulled (mechanically spalled) from one surface, leaving it stuck to the other in the form of a raised lump (gall). Unlike other forms of wear, galling is usually not a gradual process but occurs quickly and spreads rapidly as the raised lumps induce more galling. It can often occur in screws and bolts, causing the threads to seize and tear free from the fastener or the hole. In extreme cases, the bolt may seize without stripping the threads, which can lead to breakage of the fastener, the tool, or both. Threaded inserts of hardened steel are often used in metals like aluminium or stainless steel that can gall easily.[3]
Galling requires two properties common to most metals, cohesion through metallic-bonding attractions and plasticity (the ability to deform without breaking). The tendency of a material to gall is affected by the ductility of the material. Typically, hardened materials are more resistant to galling, whereas softer materials of the same type will gall more readily. The propensity of a material to gall is also affected by the specific arrangement of the atoms, because crystals arranged in a face-centered cubic (FCC) lattice will usually allow material-transfer to a greater degree than a body-centered cubic (BCC). This is because a face-centered cubic has a greater tendency to produce dislocations in the crystal lattice, which are defects that allow the lattice to shift, or "cross-slip," making the metal more prone to galling. However, if the metal has a high number of stacking faults (a difference in stacking sequence between atomic planes), it will be less apt to cross-slip at the dislocations. Therefore, a material's resistance to galling is primarily determined by its stacking-fault energy. A material with high stacking-fault energy, such as aluminium or titanium, will be far more susceptible to galling than materials with low stacking-fault energy, like copper, bronze, or gold. Conversely, materials with a hexagonal close packed (HCP) structure and a high c/a ratio, such as cobalt-based alloys, are extremely resistant to galling.[4]
Galling occurs initially with material transfer from individual grains on a microscopic scale, which become stuck or even diffusion welded to the adjacent surface. This transfer can be enhanced if one or both metals form a thin layer of hard oxides with high coefficients of friction, such as those found on aluminum or stainless steel. As the lump grows, it pushes against the adjacent material, forcing them apart and concentrating most of the friction heat energy into a very small area. This, in turn, causes more adhesion and material build-up. The localized heat increases the plasticity of the galled surface, deforming the metal until the lump breaks through the surface and begins plowing up large amounts of material from the galled surface. Methods of preventing galling include the use of lubricants like grease and oil, low-friction coatings and thin-film deposits like molybdenum disulfide or titanium nitride, and increasing the surface hardness of the metals using processes such as case hardening and induction hardening.
In engineering science and other technical aspects, the term galling is widespread. The influence of acceleration in the contact zone between materials has been mathematically described and correlated to the exhibited friction mechanism found in the tracks during empiric observations of the galling phenomenon. Due to problems with previous incompatible definitions and test methods, better means of measurements in coordination with a greater understanding of the involved frictional mechanisms have led to the attempt to standardize or redefine the term galling to enable a more generalized use. ASTM International has formulated and established a common definition for the technical aspect of the galling phenomenon in the ASTM G40 standard: "Galling is a form of surface damage arising between sliding solids, distinguished by microscopic, usually localized, roughening and creation of protrusions (e.g., lumps) above the original surface".[5]
When two metallic surfaces are pressed against each other, the initial interaction and the mating points are the asperities, or high points, found on each surface. An asperity may penetrate the opposing surface if there is a converging contact and relative movement. The contact between the surfaces initiates friction or plastic deformation and induces pressure and energy in a small area called the contact zone.
The elevation in pressure increases the energy density and heat level within the deformed area. This leads to greater adhesion between the surfaces, which initiates the material transfer, galling build-up, lump growth, and creation of protrusions above the original surface.
If the lump (or protrusion of transferred material to one surface) grows to a height of several micrometers, it may penetrate the opposing surface oxide-layer and cause damage to the underlying material. Damage in the bulk material is a prerequisite for plastic flow found in the deformed volume surrounding the lump. The geometry and speed of the lump define how the flowing material will be transported, accelerated, and decelerated around the lump. This material flow is critical when defining the contact pressure, energy density, and developed temperature during sliding. The mathematical function describing acceleration and deceleration of flowing material is thereby defined by the geometrical constraints, deduced or given by the lump's surface contour.
If the right conditions are met, such as geometric constraints of the lump, an accumulation of energy can cause a clear change in the material's contact and plastic behavior, increasing the friction force required for adhesion and further movement.
In sliding friction, increased compressive stress is proportionally equal to a rise in potential energy and temperature within the contact zone. The energy accumulation during sliding can reduce energy loss from the contact zone due to a small surface area on the surface boundary, thus, low heat conductivity. Another reason is the energy continuously forced into the metals, which is a product of acceleration and pressure. In cooperation, these mechanisms allow constant energy accumulation, causing increased energy density and temperature in the contact zone during sliding.
The process and contact can be compared to cold welding or friction welding because cold welding is not truly cold, and the fusing points exhibit an increase in temperature and energy density derived from applied pressure and plastic deformation in the contact zone.
Incidence and location
Galling is often found between metallic surfaces where direct contact and relative motion have occurred. Sheet metal forming, thread manufacturing, and other industrial operations may include moving parts, or contact surfaces made of stainless steel, aluminium, titanium, and other metals whose natural development of an external oxide layer through passivation increases their corrosion resistance but renders them particularly susceptible to galling.[6]
In metalworking that involves cutting (primarily turning and milling), galling is often used to describe a wear phenomenon that occurs when cutting soft metal. The work material is transferred to the cutter and develops a "lump." The developed lump changes the contact behavior between the two surfaces, which usually increases adhesion, and resistance to further cutting, and, due to created vibrations, can be heard as a distinct sound.
Galling often occurs with aluminium compounds and is a common cause of tool breakdown. Aluminium is a ductile metal, which means it possesses the ability for plastic flow with relative ease, presupposing a relatively consistent and significant plastic zone.
High ductility and flowing material can be considered a general prerequisite for excessive material transfer and galling because frictional heating is closely linked to the structure of plastic zones around penetrating objects.
Galling can occur even at relatively low loads and velocities because it is the real energy density in the system that induces a phase transition, which often leads to an increase in material transfer and higher friction.
Generally, two major frictional systems affect adhesive wear or galling: solid surface contact and lubricated contact. In terms of prevention, they work in dissimilar ways and set different demands on the surface structure, alloys, and crystal matrix used in the materials.
In solid surface contact or unlubricated conditions, the initial contact is characterized by the interaction between asperities and the exhibition of two different sorts of attraction: cohesive surface-energy or the molecules connect and adhere the two surfaces together, notably even if a measurable distance separates them. Direct contact and plastic deformation generate another type of attraction through the constitution of a plastic zone with flowing material where induced energy, pressure, and temperature allow bonding between the surfaces on a much larger scale than cohesive surface energy.
In metallic compounds and sheet metal forming, the asperities are usually oxides, and the plastic deformation primarily consists of brittle fracture, which presupposes a very small plastic zone. The accumulation of energy and temperature is low due to the discontinuity in the fracture mechanism. However, during the initial asperity/asperity contact, wear debris or bits and pieces from the asperities adhere to the opposing surface, creating microscopic, usually localized, roughening and creation of protrusions (in effect lumps) above the original surface. The transferred wear debris and lumps penetrate the opposing oxide surface layer and cause damage to the underlying bulk material, plowing it forward. This allows continuous plastic deformation, plastic flow, and accumulation of energy and temperature. The prevention of adhesive material transfer is accomplished by the following or similar approaches:
Low-temperature carburizing treatments such as Kolsterising can eliminate galling in austenitic stainless steels by increasing surface hardness up to 1200 HV0.05 (depending on the base material and surface conditions).[7]
Less cohesive or chemical attraction between surface atoms or molecules.
Avoid continuous plastic deformation and plastic flow, for example, through a thicker oxide layer on the subject material in sheet-metal forming (SMF).
Coatings deposited on the SMF work tool, such as chemical vapor deposition (CVD) or physical vapor deposition (PVD) and titanium nitride (TiN) or diamond-like carbon coatings exhibit low chemical reactivity even in high energy frictional contact, where the subject material's protective oxide layer is breached, and the frictional contact is distinguished by continuous plastic deformation and plastic flow.
Lubricated contact places other demands on the surface structure of the materials involved, and the main issue is to retain the protective lubrication thickness and avoid plastic deformation. This is important because plastic deformation raises the temperature of the oil or lubrication fluid and changes the viscosity. Any eventual material transfer or creation of protrusions above the original surface will also reduce the ability to retain a protective lubrication thickness. A proper protective lubrication thickness can be assisted or retained by:
Surface cavities or small holes can create a favorable geometric situation for the oil to retain a protective lubrication thickness in the contact zone.
Cohesive forces on the surface can increase the chemical attraction between the surface and lubricants and enhance the lubrication thickness.
Oil additives may reduce the tendency for galling or adhesive wear.
See also
Tribology– Science and engineering of interacting surfaces in relative motion
Rheology– Study of the flow of matter, primarily in a fluid state
Pin on disc tribometer– Instrument that measures friction and wear between surfacesPages displaying short descriptions of redirect targets
Related Research Articles
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
A lubricant is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces. The property of reducing friction is known as lubricity.
Stainless steel, also known as inox, corrosion-resistant steel (CRES), or Rustless steel, is an alloy of iron that is resistant to rusting and corrosion. It contains at least 10.5% chromium and usually nickel, and may also contain other elements, such as carbon, to obtain the desired properties. Stainless steel's resistance to corrosion results from the chromium, which forms a passive film that can protect the material and self-heal in the presence of oxygen.
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials by chemical or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.
Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical or chemical. The study of wear and related processes is referred to as tribology.
Tribology is the science and engineering of understanding friction, lubrication and wear phenomena for interacting surfaces in relative motion. It is highly interdisciplinary, drawing on many academic fields, including physics, chemistry, materials science, mathematics, biology and engineering. The fundamental objects of study in tribology are tribosystems, which are physical systems of contacting surfaces. Subfields of tribology include biotribology, nanotribology and space tribology. It is also related to other areas such as the coupling of corrosion and tribology in tribocorrosion and the contact mechanics of how surfaces in contact deform. Approximately 20% of the total energy expenditure of the world is due to the impact of friction and wear in the transportation, manufacturing, power generation, and residential sectors.
Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex cross-sections; and to work materials that are brittle, because the material encounters only compressive and shear stresses. It also creates excellent surface finish and gives considerable freedom of form in the design process.
Friction welding (FWR) is a solid-state welding and bonding process that generates heat through mechanical friction between workpieces in relative motion to one another. This process is used with the addition of a lateral force called "upset" to plastically displace and fuse the materials. No melting occurs, friction welding is not a fusion welding process, but a solid-state welding technique more like forge welding. Friction welding is used with metals and thermoplastics in a wide variety of aviation and automotive applications.
Friction stir welding (FSW) is a solid-state joining process that uses a non-consumable tool to join two facing workpieces without melting the workpiece material. Heat is generated by friction between the rotating tool and the workpiece material, which leads to a softened region near the FSW tool. While the tool is traversed along the joint line, it mechanically intermixes the two pieces of metal, and forges the hot and softened metal by the mechanical pressure, which is applied by the tool, much like joining clay, or dough. It is primarily used on wrought or extruded aluminium and particularly for structures which need very high weld strength. FSW is capable of joining aluminium alloys, copper alloys, titanium alloys, mild steel, stainless steel and magnesium alloys. More recently, it was successfully used in welding of polymers. In addition, joining of dissimilar metals, such as aluminium to magnesium alloys, has been recently achieved by FSW. Application of FSW can be found in modern shipbuilding, trains, and aerospace applications.
Nanotribology is the branch of tribology that studies friction, wear, adhesion and lubrication phenomena at the nanoscale, where atomic interactions and quantum effects are not negligible. The aim of this discipline is characterizing and modifying surfaces for both scientific and technological purposes.
Fretting refers to wear and sometimes corrosion damage of loaded surfaces in contact while they encounter small oscillatory movements tangential to the surface. Fretting is caused by adhesion of contact surface asperities, which are subsequently broken again by the small movement. This breaking causes wear debris to be formed.
In materials science, asperity, defined as "unevenness of surface, roughness, ruggedness", has implications in physics and seismology. Smooth surfaces, even those polished to a mirror finish, are not truly smooth on a microscopic scale. They are rough, with sharp, rough or rugged projections, termed "asperities". Surface asperities exist across multiple scales, often in a self affine or fractal geometry. The fractal dimension of these structures has been correlated with the contact mechanics exhibited at an interface in terms of friction and contact stiffness.
The stick–slip phenomenon, also known as the slip–stick phenomenon or simply stick–slip, is a type of motion exhibited by objects in contact sliding over one another. The motion of these objects is usually not perfectly smooth, but rather irregular, with brief accelerations (slips) interrupted by stops (sticks). Stick-slip motion is normally connected to friction, and may generate vibration (noise) or be associated with mechanical wear of the moving objects, and is thus often undesirable in mechanical devices. On the other hand, stick-slip motion can be useful in some situations, such as the movement of a bow across a string to create musical tones in a bowed string instrument.
Dry lubricants or solid lubricants are materials that, despite being in the solid phase, are able to reduce friction between two surfaces sliding against each other without the need for a liquid oil medium.
Machinability is the ease with which a metal can be cut (machined) permitting the removal of the material with a satisfactory finish at low cost. Materials with good machinability require little power to cut, can be cut quickly, easily obtain a good finish, and do not cause significant wear on the tooling. Factors that typically improve a material's performance often degrade its machinability, presenting a significant engineering challenge.
Burnishing is the plastic deformation of a surface due to sliding contact with another object. It smooths the surface and makes it shinier. Burnishing may occur on any sliding surface if the contact stress locally exceeds the yield strength of the material. The phenomenon can occur both unintentionally as a failure mode, and intentionally as part of a metalworking or manufacturing process. It is a squeezing operation under cold working.
Tribocorrosion is a material degradation process due to the combined effect of corrosion and wear. The name tribocorrosion expresses the underlying disciplines of tribology and corrosion. Tribology is concerned with the study of friction, lubrication and wear and corrosion is concerned with the chemical and electrochemical interactions between a material, normally a metal, and its environment. As a field of research tribocorrosion is relatively new, but tribocorrosion phenomena have been around ever since machines and installations are being used.
Friction stir processing (FSP) is a method of changing the properties of a metal through intense, localized plastic deformation. This deformation is produced by forcibly inserting a non-consumable tool into the workpiece, and revolving the tool in a stirring motion as it is pushed laterally through the workpiece. The precursor of this technique, friction stir welding, is used to join multiple pieces of metal without creating the heat affected zone typical of fusion welding.
Aluminium alloys are often used due to their high strength-to-weight ratio, corrosion resistance, low cost, high thermal and electrical conductivity. There are a variety of techniques to join aluminium including mechanical fasteners, welding, adhesive bonding, brazing, soldering and friction stir welding (FSW), etc. Various techniques are used based on the cost and strength required for the joint. In addition, process combinations can be performed to provide means for difficult-to-join assemblies and to reduce certain process limitations.
Extreme tribology refers to tribological situations under extreme operating conditions which can be related to high loads and/or temperatures, or severe environments. Also, they may be related to high transitory contact conditions, or to situations with near-impossible monitoring and maintenance opportunities. In general, extreme conditions can typically be categorized as involving abnormally high or excessive exposure to e.g. cold, heat, pressure, vacuum, voltage, corrosive chemicals, vibration, or dust. The extreme conditions should include any device or system requiring a lubricant operating under any of the following conditions:
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