WEAR CHARACTERISTICS FOR THE SELECTION OF HARD-FACING ALLOYS

A general classification of different forms of material degradation is necessary in order to assist in the selection of a hard-facing alloy as seen in Table 1.

Table 1

Classification of Hard-Facing Alloys

Serial Numbers	Hard-Facing Alloys
1.	High Chromium Irons
2.	Martensitic Alloy Irons
3.	Austenic Alloy Irons
4.	Martensitic Steels
5.	Semi-Austensitic Steels
6.	Pearlitic Steels
7.	Austenitic Steels
8.	Cobalt-Base Alloys
9.	Nickel-Base Alloys
10.	Tunsten Carbide Composites

Selection is based upon the type of wear anticipated and the environmental conditions. The types of wear which can be encountered are listed below. The types of materials which should be used for each type of wear are listed in Table2.

Table 2

Selection of Hard-Facing Alloys – A General Guide

Service Conditions	Preferred Materials
Adhesive a.       Oxidative or Mild Wear b.      Metallic or Severe Wear	Iron-base alloys (6 – 16% Cr); Co or Nickel-base alloys (15 – 30% Cr.) Cobalt or Nickel-base alloys.
Abrasive Wear a.       Low stress scratching abrasion b.      High stress grinding abrasion c.       Gouging abrasion (plus impact)	Carbide grains in alloy iron, Cobalt or Nickel. High chromium irons (3.5 – 4.5% carbon). High chromium martensitic irons (2.0 – 3.5%C.), Martensitic alloy steels (0.4 – 1.5%C.) Austenitic manganese steels with high chromium irons (2.0 – 3.5%C.)
Erosion a.       Low angle impingement b.      High angle impingement c.       Cavitation erosion	Carbide grains in alloy iron, Cobalt or Nickel. High chromium irons (3.5 – 4.5%C.), Hyper-eutectic cobalt or Nickel-base alloys. High chromium irons (2.0 – 3.5%C.), Hypo-eutectic cobalt-base alloys. Hypo-eutectic cobalt-base alloys
Fretting Corrosion	Cobalt or Nickel-base alloys
Corrosion - Erosion	Cobalt or Nickel-base alloy, High chromium steels (12 – 18%Cr, 0.2 – 1.2%C.)
Oxidation – Hot Corrosion	Laves phase Ni or Co-base alloy, Carbide containing Ni or Co-base alloys.
High Temperature Wear	Cobalt-base alloys, Laves phase cobalt or Nickel-base alloys.

The finishes on even the most highly polished metal surfaces are not completely flat; microscopic asperities and depressions exist. When two surfaces are brought into contact under a load normal to the planes of the surfaces, the asperities come into contact and deforms elastically or plastically until the contact area can no longer carry the load. At this point, a bond can occur between the two surfaces, which are stronger than the weaker of the two materials. When relative motion between the two surfaces occurs, the weaker material tears, and the material is transferred to the contacting surfaces. This process is known as adhesive. The two other primary wear types are abrasive and erosive. Abrasive wear is the displacement of materials from a surface by hard particles or protuberance sliding along the surface.

Erosive wear (erosion) – the loss of surface material due to relative motion in contact with a fluid containing solid particles – occurs in the form of different mechanisms, which depends on the nature of the eroding particles, their velocity and angle of impact and the composition and microstructure of the surface being eroded. Other types of wear are not considered primary, but are sometimes afforded separate status. These include surface fatigue, fretting and cavitations erosion. Since more than one wear mechanism often is operating simultaneously, it is difficult to separate the effects of one from the other. Hence the need to consider multiple wear mechanism at the same time with technological inheritance technique, which can be used to determine the different wear mechanism and other competing failure modes like vibration, temperature, etc as well as the single integrated reliability of the component/system.http://www.integrated-consultancy.com/

SELECTION OF MATERIALS FOR HARD-SURFACING AND ITS APPLICATIONS

A wide range of materials are available for hard-surfacing and these are discussed in manufacturer’s literature as well as in several handbooks. Conceptually, any worn part can be rebuilt with an alloy of identical composition by coating techniques. Accordingly, all existing coating alloys can be called hard-facing alloys. These can be divided into five different categories:

Iron-Base Alloys range from low carbon steels to highly alloyed chromium irons. The iron-base alloy system is generally characterized by excellent abrasive wear resistance and impact resistance depending on the composition. In general, iron-base alloys lack the heat and corrosion resistance of their nickel or cobalt-base counterparts.

Cobalt-Base Alloys are characterized by excellent metal-to-metal wear resistance, galling resistance and corrosion and heat resistance.

Nickel-Base Alloys are characterized by extremely high abrasion resistance, combined with corrosion resistance and good metal-to-metal wear resistance.

Copper-Base Alloys are primarily bronzes which resist metal-to-metal wear, corrosion and cavitations-erosion.

Carbide Composites are usually composed of extremely hard carbides of tungsten, titanium or vanadium in a relatively softer matrix of an iron, nickel or cobalt-base alloy.

Hard-facing applications for wear control vary widely. On the one hand, we have very severe abrasive wear service such as rock crushing and pulverizing where a few kilograms of material are quickly worn away. On the other hand, we may need to minimize metal-to-metal wear in control valve, where even a few thousands of a millimeter of wear is intolerable. Hard-facing is used for controlling abrasive wear, such as encountered by mill harmers, digging tools, extrusion screws, cutting shears, parts of earthmoving equipments, ball mills and crusher parts. It is also used to control the wear of un-lubricated or poorly lubricated metal-to-metal sliding contacts such as occur in control valves, undercarriage parts of tractors, shovels and high performance bearings. In addition, hard facing is applied to control combinations of wear and corrosion as encountered by seals, plows in roasting ovens, knives in the food processing industry, valve and pumps handling corrosive liquids or slurries.

In most instances, parts are typically made of either plain carbon steel or stainless steel which by themselves, do not provide desirable wear life. The hard-surfacing alloys are applied in critical wear areas either of original equipment or during the reclaiming of worn parts. Selection of the right hard-facing alloy for a given application is best achieved through careful analysis of the service conditions and matching alloy properties to those conditions. Quite often, a balance between wear properties, environmental resistance and coating ability can be achieved for maximum reliability at minimum cost with technological inheritance technique.

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SELECTION OF WEAR COATINGS AND TREATMENTS FOR MAXIMUM ACHIEVABLE RELIABILITY OF COMPONENTS

The selection of the proper coatings and treatments to be used for a wear resistant job is the most important step in doing the job successfully. Each wear coating has different characteristics of strength, shrinkage, hardness etc such that there is a material for every wear resistant category. There are two general material approaches to wear resistance. Firstly, wear resistant material can be used. Such a material resists wear by virtue of its composition or its properties. Second, a structural material can be used and its surface modified by treatments and coatings to provide any desired level of wear protection.

A wide variety of treatments and coatings have been developed using different coating techniques and methods of application. These coatings and treatments fall into three categories:

-          Soft Coatings or Solid Film Lubricants. These coatings provide protection by preventing adhesion between the substrates. Since they have low shear strength they shear in preference to the substrate giving a low friction coefficient. The coating will also flow to better distribute the contact area. This promotes increased load and temperature capacity.

-          Surface Treatments. Surface treatments modify the surface either to make it harder or to provide a more wear resistant alloy at the surface. Generally, these treatments increase the surface hardness and provide wear resistance by virtue of that increase. Nitriding is an example of this approach.

-          Hard Surface Replacement Coatings (Hard Surfacing). Such coatings do not modify the existing surface but replace it with another surface. The wear behavior is only a function of the coating and not the base material. Examples of such coatings are chrome plate and weld overlays. Sprayed carbide and ceramics are also included in this category.

Hard Surfacing Category can easily be controlled by an engineering technique to achieve a maximum reliability, since a desired controlled surface can be derived mechanically. Actually, hard-surfacing is a process by which an alloy coating is welded, fused, or sprayed onto a critical area of a metal part. Although the name implies a wear resistant coating, hard-surfacing is undertaken to provide a variety of desired properties including wear resistance, corrosion resistance, oxidation resistance and strength. At this point, it is worth mentioning that, there are two kinds of hard-surfacing: Hard-facing and spray coatings. Hard-facing is applied using welding techniques which assures a metallurgical bond; spraying is accomplished by impinging the substrate with high velocity powder. Bonding in this case is usually mechanical. Spraying applies a coating which is up to 0.050 cm.

There are two general uses for hard-surfacing. It can be applied to repair worn surfaces, and it can be applied to obtain more desirable surface properties. Hard-surfacing competes with surface treatments and soft coatings and should be given equal consideration in the design of mechanical components. Irrespective of the range of coating properties, the advantages of hard-surfacing are the greater thickness available for maximum achievable reliability at minimum cost with technological inheritance technique.

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Maximum Achievable Reliability of Parts and Equipment with Minimum Maintenance Cost

The responsibility of agricultural enterprises for providing the population with foodstuffs and natural industry with raw materials is great. The farms and agro-allied industries therefore have, at their disposal, large amounts of machinery and equipment that are characterized by great diversity and complexity. These machinery and equipment require cost effective maintenance strategies for higher productivity and reliability. The requirements to be met by agro-allied industries and maintenance organizations of machinery and equipment are stringent. Correct operation, conscientious care and maintenance, correct procedures for long term failure prevention and repair work are factors that contribute to ensuring that the machinery belonging to farms and related industries:

-          Are available when needed and give rise to little down-time that may complicate the process of agricultural production, part development and equipment operation;

-          Will consume a decreasing proportion of costs to minimum value for necessary maintenance.

Farms and agro-allied industries employ a variety of maintenance methods in accordance with the different failure modes of equipment, its assemblies and components, agro-allied service conditions and maintenance facilities available. These methods include day-to-day and routine maintenance and rust prevention, long-term parking, operational repairs during service or during breaks between shifts (mainly during harvest operation), overhauls, preventive maintenance, predictive maintenance, proactive maintenance and reconditioning of parts as permitted by the maintenance facilities available at the farms and related industries. The use of failure resistant (surface-hardened and reconditioned) parts plays the vital role, as the only means of fully satisfying the demands for spare parts used for high quality repairs and maintenance as well as attaining maximum achievable reliability.

The main trend of industrial development in maintenance and repairs is the use of hard-surface and reconditioned parts, the setting up of specialized industries, factories and workshops equipped with high quality and efficient facilities using effective methods of depositing coatings and surface finish. These methods, often based on the use of wear, corrosion and temperature resistant metallic powder materials, include gas-thermal, gas-flame, plasma spraying and others. Such techniques have been adopted in many repair and maintenance factories in the different countries like United States, Russia, Germany, Poland and others and have proved highly effective in extending the service life and reliability of machinery and equipments.

At this juncture, interest will be concentrated on carrying out up-to-date highlights of hard coatings suitable for wear, corrosion and temperature resistance in agricultural and industrial applications. The wide range of applications in hardening and reconditioning various items machinery and equipment using gas flame and plasma spraying of powder materials has created an awareness of the need to select suitable wear, corrosion and temperature resistant coating materials, surface finish and maintenance methods.

Technological Inheritance Technique can now be used for optimum selection of the process conditions for the attainment of maximum achievable reliability at minimum maintenance cost.

     

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Determination of System and Component Reliability Parameters

Component and System Requirements are specified using reliability parameters. Although, the most common existing reliability parameter is the mean-time-between-failure, (MTBF), which can also be specified as the failure rate or the number of failures during a given period. These parameters are very useful for systems that are operating on a regular basis, such as most vehicles, machinery, and electronic equipment. Reliability increases as MTBF increases. The MTBF is usually specified in hours, but can be used as the probability of mission success. For example, reliability of a scheduled aircraft can be specified as a dimensionless probability or percentage. In the so-called zero defect experiments, only limited information about the failure distribution is acquired. Here the stress, stress time, or the sample size is so low that not a single failure occurs. Due to insufficient sample size, only an upper limit of the early failure rate can be determined. At any rate, it looks good for the customer if there are no failures. In a study of intrinsic failure distribution, which is often a material property, higher stresses are necessary to get failure in a reasonable period of time. Several degree of stress has to be applied to determine an acceleration model. The empirical failure distribution is often parameterized with a Weibull or a log-normal model. It is generally done to model, where the early failure rate has an exponential distribution. This less complex type of model for the failure distribution has only one parameter - the constant failure rate. Due to the fact that, reliability is a probability, even highly reliable systems have some chance of failure. However, existing methods of testing reliability requirements are problematic for several reasons. A single test is insufficient to generate enough statistical data. Multiple tests or long-duration tests are usually very expensive. Some tests are simply impractical. Reliability engineering is used to design a realistic and affordable test program that provides enough evidence that the system meets its requirement. Statistical confidence levels are used to address some of these concerns. Care is therefore needed to select the best combination of requirements. An integrated reliability model testing that can perform at various levels, such as component, subsystem, and system as well as such that can address many factors during testing, such as extreme temperature and humidity, shock, vibration, and heat is highly essential for the different industries. Integrated reliability model with technological inheritance coefficients can be applied for effective test strategy at all levels of design, development and operations so that all parts are exercised in relevant environments. The benefits of integrated reliability model testing for design, operations and maintenance of components and systems in the different environments with the help of technological inheritance coefficients will be discussed in the subsequent blogs of this site.

The Role of Technological Inheritance Technique for Maximum Achievable Component Reliability

It is indeed quite interesting to note that, it is just of recent that manufacturers resolved an aspect of quality control that highlights the fact that, the outcome of a manufactured machine part is dependent mainly on the technological process conditions and the type of the manufacturing surface-finish methods.

Researchers have shown that many of the surface quality parameters of machine parts are formed not only during the surface finish operations of a manufacturing process but also during its initial and throughout the development operations, thereby creating a sustainable opportunity for maximum achievable reliability of component. The desired surface quality parameters that are formed early in the process needs to be sustained till the final surface finish, as the development operations help to transfer the quality parameters from one process to another in such a way that, the undesired negative traits are eliminated, while the positive desired quality characteristics are sustained to achieve maximum reliability. The transference and sustenance of the positive desired qualities of components and the elimination of the undesired qualities (defects) is possible with the use of technological inheritance technique.

Technological Inheritance is defined as the transference of component properties (e.g quality, defect and failure characteristics) from its initial operation to the final operation in a technological process, which in turns influences the service operation (plants, processes, parts, equipments and instruments) and the lifetime of components/systems.

At this point, it is worthy to note that inheritance is the process by which one object can acquire the properties of another object. This is important because it supports the concept of hierarchical classification. If you think about it, most knowledge is made manageable by hierarchical (that is, top-down) classifications. Without the use of hierarchies, each object would have to explicitly define all of its characteristics. Using technological inheritance technique, an object needs only to define those properties that make it unique within its class (e.g the machine part reliability, quality, defect, failure mode, and others). It can inherit its general attributes from its parent part and base material. Thus, it is the inheritance mechanism (e.g reliability inheritance mechanism or distribution) that makes it possible for one object to be a specific instance of a more general case and structure. Technological Inheritance Technique will therefore be considered in this blog to measure component/system reliability growth, degradation, life cycle cost and life-time

Estimate Component Reliability Growth, Degradation and Cost with Technological Inheritance Technique

Industry and agricultural consolidation with worldwide competition are putting today’s plants, processes, parts,  equipments, instruments and monitoring/maintenance strategies under intense financial pressure, which makes operations and maintenance budgets to be among the first to be cut. Fewer personnel working fewer hours are expected to operate and maintain more equipment at lower cost, while also delivering higher throughput, higher availability, and higher profits with aging assets.

It’s a trend that shows no sign of changing. Agriculture and Industrial Organizations must therefore increase the productivity of their existing maintenance and operations teams, while continuing to look for ways to reduce costs, waste, energy and even more.

Fortunately, there are still opportunities and potentials for improvement in almost every operation. Technological Inheritance Technique can help to estimate the potentials in plants, processes, parts and equipments in agriculture and industrial organizations. The technique can be used to measure component/system reliability growth, degradation and cost.

Reliability of Parts and Equipment

As the industry and technology continues to grow, the decision to invest in reliability of parts and equipments program becomes more valuable. The success of this blog also increases with the needs to have the right training, books and tools geared towards maximum achievable life-time reliability of outdoor power engine parts and equipments. There are over 45 million engines and equipments in service today, and new engines, parts and equipments are being built at a rate in excess of 1,000,000 per month. The OPE engine, parts and equipments industry is one of the fastest growing industries in the country today. In the past 22 years it has literally grown by over 2,400 percent. Almost all of this vast number of engines are less than 20 horsepower and are used to power a wide range of products such as lawnmowers, tractors, snow-blowers, edgers, roto-tillers, golf carts, generators, go-karts, motorcycles and other special appliances. By far the largest number of OPE engines produced each year are in the 4H.P to 16H.P range used on most rotary lawnmowers. Such growths have created a tremendous demand for training, repairs, reliability maintenance service business, e-books writings, and marketing and selling of OPE engines, parts and equipments. OPE engine service and marketing business is recession prone and has yet to reach its full potential. In order to exploit the full potential and benefits of this viable business, it is therefore necessary to blog on any of its related subject as well as have a technique that can be used to measure the reliability maintenance of OPE engines, parts and equipment that classifies the types of maintenance, marketing and select the best technique for maximum profit and benefits. Technological inheritance coefficient as shown in a white paper from www.integrated-consultancy.com provides a long lasting solution to the problems of OPE engines, parts and equipments towards a sustainable development in the maintenance and marketing organization.