Metallography and Microhardness

Question 1

Metallography

Answer 1

Study of the structural characteristics or constitution of a metal or an alloy in relation to its physical and mechanical properties
Art of preparing a metal surface for analysis by grinding, polishing and etching to reveal microstructural constituents

Question 2

Macroscopy and microscopy

Answer 2

Macroscopy: study of metal either with bare eyes or low powered microscope (60X) showing grain size, shape and distribution of secondary phases and non-metallic inclusions

Question 3

Stages of Preparation for microscopy

Answer 3

Sectioning -> mounting -> coarse grinding -> find grinding -> rough polishing -> fine polishing -> etching

Question 4

Mounting

Answer 4

samples mounted in a phenolic, bakelite or epoxy resin providing a safe way to hold the small sample during grinding and polishing operations

Question 5

Grinding

Answer 5

removes epoxy layer and sharp or jagged edges; surface kept cool due to possible heat generation; avoid applying excessive pressure on belt grinder which might cause deep scratches

Question 6

Rough Polishing

Answer 6

remove surface deposits or level irregular surfaces and prepares it for further polishing; surfaces are flattened by polishing with emery paper

Question 7

Fine Polishing

Answer 7

abrading specimen with a series of grits of decreasing coarseness to produce a smooth lustrous surface required for microscopic examination; distilled water + abrasive powder poured onto cloth for a scratch free surface

Question 8

Etching

Answer 8

Microstructural constituents of sample are revealed using a suitable chemical or electrolytic etchant. Essential step when observing microstructure and grain size

Question 9

Microhardness

Answer 9

Vickers or Knoop diamond pyramid indenters with loads

Question 10

Microhardness Test

Answer 10

Pyramidal diamond inderter impressed into material at loads from 15 - 1000 gf; microscope measures indentation which gives hardness measurement; a smaller load used requires a higher surface finish

Question 11

Vickers hardness

Answer 11

Load of 10 - 1000 gf applied for 10-15 seconds and diagonals of indentation are measured
HV = 2Fsin(angle/2)/d^2

Question 12

Disadvantages of Vickers

Answer 12

Variability in measurements based on need to optically measure the indentation size
Time consuming due to sample preparation time