In this article, we are talking about some ways of failure of carbide wear components.
Carbide wear components could fail in a number of different ways. Unfortunately, there is no “a best” grade of carbide for all wear applications. Replacing one grade with another may reduce the probability of one type of failure from occurring, but generally increases the chances that a different failure occurs. Carbide grades are normally selected so that the service life of the wear part and the type of failure are quite predictable. So unscheduled or disruptive outages of the assemblies which include the carbide component are avoided and the scheduled replacement of parts prior to failure is an option. Please find the brief description of each of the common types of failure as following.
Abrasive Wear/Erosion Abrasive or erosion wear represents a form of mechanical attack at the working surface of the carbide component. It is the progressive loss of material at the carbide surface due to a high velocity stream of liquid or gas containing solid particles directed into or flowing across the surface of the carbide. It may also result from the sliding or rubbing of a mating component or work material over the carbide surface. The rate of wear depends in part on the force or pressure with which the contact medium impinges on the carbide surface. The progressive wear caused by abrasion or erosion is a preferred mode of failure since it does not result in a total breakdown or stoppage of the operation dependent on the failing component.
Corrosion/Leaching Corrosion wear is the result of a chemical or electrochemical reaction between carbide surfaces and the environment. These reactions generally involve the metallic binder matrix (cobalt or nickel) rather than the tungsten carbide grains and, with the loss of the binder metal at exposed surfaces, the mechanical bond holding the carbide grains in place is also lost. Acidic liquids are particularly corrosive. A cobalt binder is far more susceptible to chemical attack or “leaching” than nickel and should be avoided in moderate to highly corrosive environments. Corrosion rate data for specific situations are seldom available and are typically determined empirically.
Chipping/Fracturing Except at relatively high temperatures, carbides undergo little plastic deformation prior to failing by brittle fracture. Carbide grades vary widely in tensile strength and toughness and premature failure by chipping or fracture occurs whenever the carbide component lacks the toughness to withstand service conditions that include mechanical shock or impact. Sharp, unprotected edges are particularly vulnerable to chipping. Cracks generally initiate in areas associated with high stress concentrations and, once moving, propagate rapidly through the carbide component to produce failure by fracture. Carbides have relatively high compressive strengths, however, and any change in either part geometry or operating conditions that converts tensile stresses to compressive stresses reduces the probability of failure by fracture. Design considerations that reduce the risk of failure by chipping or fracture include the use of protective edge chamfers and radii, avoiding sharp inside edges and other stress-inducing features, increasing the wall thickness of tubes and wear sleeves, and improving surface finishes. In certain instances chipping or fracture occurs as a result of the increased stresses associated with a badly worn carbide component. When this is the case, wear is the primary cause of failure even though catastrophic failure by fracture or breakage may mask it.
Thermal Cracking Cemented carbides have relatively poor thermal shock resistance and sudden changes in temperature often produce failure by thermal cracking. Thermal cracks can result from thermal stresses developed in a single heating-cooling cycle if the temperature change is rapid and significant. These cracks often take the form of a network of fine cracks occurring throughout the bulk of the carbide component and normally result in catastrophic failure. Thermal cracks may also develop gradually over thousands of thermal cycles involving very small changes in temperature. The latter are more localized and typically occur as closely-spaced, radial cracks that extend from edges of the part. As the process progresses these cracks deepen and widen resulting in the spalling away of the carbide located between them.
Thermal Deformation Cemented carbides retain their hardness and strength to relatively high temperatures, but will deform plastically under load as service temperatures approach the softening point of the binder metal. Although deformation can occur with no loss of material at the carbide surface, even small changes in the size and shape of the carbide component may result in the failure of the assembly that contains it.
Cratering Cratering Failure known as “cratering” is a localized form of erosion in which a depression develops at the point where a continuous stream of metal or particulate-bearing liquid or gas strikes the working surface of the carbide component. At operating temperatures high enough to soften the binder metal (those exceeding about 900° C.) cratering can take place rapidly. Very fine-grained carbides are particularly susceptible to cratering since smaller grains are locked into the carbide surface by a relatively thin layer of binder metal. As the crater increases in size and depth the carbide is weakened to the point where normal operating stresses produce mechanical failure in the area of the crater.
Adhesion Wear/Galling Adhesion wear occurs when a carbide surface and that of a mating part or the work material rub together or slide past each other with sufficient force to create excessive friction and pressure at high points of contact between the materials. The high temperatures and pressures generated in these contact zones result in plastic deformation at the material surfaces, penetration of one surface by the high points on the other, and finally the transfer of material (welding) of one material to the other. As material builds up on one of the surfaces, the still higher points of contact create more heat, leading to more deformation, penetration, and material transfer. This process is often referred to as “galling”. Failure by adhesion wear can take several forms, but typically the buildup reaches a stage where it eventually is dislodged pulling chunks of the carbide surface with it.
Carbide wear parts include carbide dies, die punch, carbide ejector pin, and punches and dies. The useful life of a carbide wear part is determined by many factors. These include the composition and properties of the carbide, the geometry of the carbide component, the design of the assembly, and the chemical, physical, and mechanical environments in which the carbide operates. Changes in any of these factors can produce an associated increase or decrease in service life. The chart below considers only one of these elements – the carbide grade.
|If the problem is…||Switch to a grade….|
|abrasive wear or erosion,||…of lower cobalt or nickel content and/or one with finer WC grains (i.e. a harder, more wear resistant grade). This will result in some reduction in strength and toughness, however.|
|corrosion or leaching,||…with nickel rather than cobalt as the metallic binder and/or one that contains other corrosion-inhibiting additives such as chromium or molybdenum.|
|cracking or fracturing in metalworking applications where high compressive stresses or impact are present,||…of higher cobalt or nickel content and/or one with coarser WC grains (i.e. a grade of greater toughness). This will result in reduced abrasive wear resistance, however.|
|chipping or fracturing of edges in wear or metalcutting applications,||…of higher cobalt content and/or one with very fine WC grains (known as micrograin or submicron grades). Micrograin grades often possess superior combinations of tensile strength and hardness.|
|thermal cracking,||…of higher cobalt or nickel content and/or coarser WC grains (i.e. a more thermal shock resistant grade), or one that contains additives such as TaC or NbC that improve the thermal properties of the metallic binder.|
|thermal deformation or
adhesion wear (galling),
|…of lower cobalt or nickel content (i.e. a harder, more deformation-resistant grade), or one that contains additives that increase the softening temperature of the binder such as TaC or NbC.|
|cratering,||…of lower binder content and/or coarser WC grains, or one that contains TaC or NbC.|