Sprockets are teeth like projections arranged on a wheel rim to engage the links of a chain. They engage chains in many different power transmission and conveyor systems. Inserted sprockets are also designed to reduce noise and operation. They are often an economic, reliable drive system for long running, continuous drive applications with maximum absorption of shock and minimum torque loads.
Optical Metallography Steps
I. Goal Setting
know the microstructure of sprocket gear, and determine what kind of steel to use for sprocket gear.
- using a metal wire or cable for cutting.
- the wire can have one strand or many strands braided together. The wire saw uses abrasion to cut. Depending on the application, diamond material may or may not be used as an abrasive. A single-strand saw can be roughened to be abrasive, abrasive compounds can be bonded to the cable, or diamond-impregnated beads (and spacers) can be threaded on the cable. Wire saws are often cooled and lubricated by water or oil.
- used cold resin
- for handling irregular form shape.
- for certain shape, like 20 mm diameter steel rod, mounting is not necessary.
IV. Grinding V. Polishing
- used rotary grinding machine
- used coarse sand paper (150 mesh grit size) up to fine sand paper (1500 mesh grit size)
- must cut the sand paper into circular shape
- sample must be move back and forth from and center and the edge.(using up all area of the sand paper surface).
- to minimize heat generated damage, and to maximize grinding-paper life, wet grinding should be used.
- wet grinding minimizes metal abrasive particles (clogging), and thus the abrasive is more fully exposed to the sample, which promotes cutting rather than smearing or burnishing.
- to produce a flat, reasonably scratch-free surface with highly reflectivity.
- used a diamond paste
- used canvas as a polishing cloths.
- the most common method: chemical etching –> dipping or swabbing technique.
- used Nital 2% (98 cc alkohol and 2 ml HNO3/ Nitrid acid) to etch ferrite grain boundary.
VII. Optical Microscope
used optical microscope, with general components:
- light source
- light tunnel
- lenses (objective and occulair)
- filter (colour metallography, light and dark)
- diaphragms (aperture and field)
The quality of the image depends on the components that will modify the beam along the light path from the light source to the eyes/camera
The magnification will depend on the components that modify the magnification, e.g. objective lense, occulair lense, special magnification knob
Magnification of Digital Pictures
- using scale bar
- advantages :
+ will not be affected by picture enlargement or shrinkage
+ Able to know the size, e.g. coating thickness, phase, etc.
and we got the microstructure of sprocket gear:
Heat treating process, which allows the surface of a low carbon steel, so the sprocket gear will be hardened to high hardness but only a shallow depth. This process usually applied to all over the surface. Below the carburized area, there will be proeutectoid cementite and pearlite structure.and below of this structure contain a ferrite structure. Which in each area, have a different hardness number.
literature: vander voort, METALLOGRAPHY: Principles and Practice, McGraw-Hill,Inc.