OCTOBER 19-20, 2012 - YMCA University of Science & Technology

Proceedings of the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
4. METALLOGRAPHY AND CALCULATION
Metallography
Metallography is used to reveal the microstructure of metals, which is affected by alloy composition and
processing conditions including cold working, heat treatment and welding. Metallography surface should be flat
and free from scratches, stains, and other imperfections which tend to mar the surface, contain all non-metallic
inclusions intact, show no chipping or galling of hard and brittle intermetallic compounds and be free from all
traces of disturbed metal. A metallographic specimen reveals inclusions, porosity, cracks, intergranular
corrosion, surface conditions, etc. After metallography, chemical etching is performed with an acid or basic
chemical solution on specimen.
This process is performed to get information on variations in structure, such as grain size, flow lines, columnar
structure, dendrites, etc., variations in chemical composition as evidenced by segregation, carbide and ferrite
banding, coring, inclusions, and depth of carburization or decarburization also show the presence of
discontinuities and voids, such as seams, laps, porosity, flakes, bursts, extrusion rupture, cracks.
Metallography and etching process also used to study the weld structure, definition of weld penetration, dilution
of filler metal by base metals, entrapment of flux, porosity, and cracks in weld and heat-affected zones, etc.
In welding process, weldments cut perpendicular to the direction of welding to measure and study weld
penetration, heat affected zone, structure, etc. Careful preparation is usually rewarded with highly detailed
structure giving a large amount of information. Welds involving dissimilar metals will produce problems in
etching. The best method is to etch the least corrosion resistant portion first, and the more resistant portion
afterward. Occasionally an intermediary etchant may be required. The boundaries between etched and unetched
portions will give an idea of weld penetration and dilution.
In this work, low alloy steel pipes are used for welding. For measuring the depth of penetration of weldments,
etchant is prepared by solution of hydrochloric acid (HCL) and distilled water (HCL-50ml and H 2 O-50ml). After
preparing solution different specimens are dipped in it at 60 0 to 80 0 C for 30 to 45 minutes. [9]
5. Calculations
In arc welding process, welding current is the most influential variable which is used to control the heat intensity
of the electrode. Welding voltage is the potential difference between electrode tip and the surface of the
specimen. Welding voltage is very much responsible for the depth of the penetration. Welding speed is defined
as the rate of travel of the electrode along the seam. Varying welding speed, maintaining constant current and
constant voltage results in variation in the depth of penetration.
Average Terminal Voltage = 20 V (constant)
Average Welding Current = 100 A (constant)
Speed of Welding =
Heat Input Rate or Arc Energy =
V
I
v
Arc Voltage in Volts
Welding Current in Amperes
Speed of Welding in mm/min
joules/mm
Only arc time varied to get the variation in heat input rate which directly affects the depth of penetration. [8]
Different specimens of same dimension welded at different weld speed and cut perpendicular to the direction of
welding. Macroetching has been used for revealing the large-scale structure of a welded specimen, that is,
structure visible with the unaided eye, by etching an appropriately prepared surface. All cut sections of specimen
dipped in etchant for 45 minutes for revealing the microstructure of metal. Readings has been measured using
venire caliper by measuring the dilution of filler metal by base metal. Macroetching reveals the macrostructure
for the examination with the unaided eye or at a magnification of 50× or less, for more detailed study of the
specimen, microetching has to be used in which magnification of 50x or higher is used.
468