Ferrography

ferrography

Wear particle analysis, or Ferrography, is a technique that is growing every day. With industrial needs for reliability and higher productivity to compete in the world market, techniques such as wear particle analysis become vital in making diagnostic and specific recommendations regarding potential machine problems.

Having wear particles provide evidence of machine problems depends on the industrial application from a clean environment to a dirty one. However, showing photographic proof and describing the surface texture, size, shape, and morphology of the particles gives us evidence that something is potentially wrong with the machine prior to major damage.

Three of the major types of equipment used in wear particle analysis are the Direct-Reading (DR) Ferrograph, the Analytical Ferrograph (FM Ferrograph) used to make the ferrograms, and the Ferroscope used to examine the ferrograms optically.

Quantitative Measurements Using the DR-7 Ferrograph

The DR Ferrograph Monitor is a trending tool that permits condition monitoring through examination of fluid samples on a scheduled periodic basis. DR Ferrograph quantitatively measures the concentration of ferrous wear particles in lubricating or hydraulic oil. The DR Ferrograph separates out particles having positive magnetic susceptibility by means of a high-gradient magnetic field.

Machines starting service go through a wearing-in process, during which the quantity of large particles quickly increases and then settles to an equilibrium concentration during normal running conditions. A key aspect of ferrography is that machines wearing abnormally will produce unusually large amounts of wear particles, indicating excessive wear condition by the DR Ferrograph in WPC readings. If WPC readings are beyond the normal trend, a ferrogram sample slide is made with the fluid for examination by optical microscopy.

The Analytical Ferrograph FM-6 Ferrogram Maker: Additional information about a wear sample can be obtained with the FM-6 instrument, which can provide a permanent record of the sample, as well as analytical information for viewing. The FM-6 is used to prepare a ferrogram, which is a fixed slide of wear particles for microscopic examination and photographic documentation. The ferrogram is an important predictive tool, since it provides an identification of the characteristic wear pattern of specific pieces of equipment. After the particles have deposited on the ferrogram, a wash is used to flush away the oil or water-based lubricant. After the wash fluid evaporates, the wear particles remain permanently attached to the glass substrate and are ready for microscopic examination using the ferroscope.

The Microscope: Ferrograms are typically examined under a microscope that combines the features of a biological and metallurgical microscope. Such equipment utilizes both reflected and transmitted light sources, which may be used simultaneously. Green, red, and polarized filters are also used to distinguish the size, composition, shape, and texture of both metallic and non-metallic particles. A new feature of the ferroscope is the ability to measure particles within the imagery software and provide a split-screen image to compare before and after photomicrographs once the ferrogram is heat treated.

Types of Wear Particles: Recently the ASTM organization standardized the naming description for all wear particles in ferrography, patch testing, and any other media. The standard is ASTM D7690.

With industry needing to operate plants reliably, techniques of this nature are a must to pinpoint where potential problems may lie. Equipment life expectancies, safety factors, performance ratings, and maintenance recommendations are predicated on normally occurring wear. However, using wear particle analysis/ferrography pinpoints the problem without taking the equipment out of service.


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