My Facebook

Herdin Ekalaksana's Facebook profile

Wednesday, 9 April 2008

Welding_Etching

ETCHING — PROCESSES AND REAGENTS

General
The surfaces to be etched should be smoothed by filing,
machining, or grinding on metallographic papers. With
different alloys and tempers, the etching period will vary
from a few seconds to several minutes, and should be
continued until the desired contrast is obtained. As a
protection from the fumes liberated during the etching
process, this work should be done under a hood. After
etching, the specimens should be thoroughly rinsed and
then dried with a blast of warm air. Coating the surface
with a thin clear lacquer will preserve the appearance.

For Ferrous Metals
Etching solutions suitable for carbon and low alloy
steels, together with directions for their use, are suggested
in QW-472.1 through QW-472.4.

QW-472.1 Hydrochloric Acid. Hydrochloric (muriatic)
acid and water, equal parts, by volume. The solution
should be kept at or near the boiling temperature during
the etching process. The specimens are to be immersed
in the solution for a sufficient period of time to reveal all
lack of soundness that might exist at their cross-sectional
surfaces.

QW-472.2 Ammonium Persulfate. One part of
ammonium persulfate to nine parts of water, by weight.
The solution should be used at room temperature, and
should be applied by vigorously rubbing the surface to
be etched with a piece of cotton saturated with the solution.
The etching process should be continued until there
is a clear definition of the structure in the weld.

QW-472.3 Iodine and Potassium Iodide. One part
of powdered iodine (solid form), two parts of powdered
potassium iodide, and ten parts of water, all by weight.
The solution should be used at room temperature, and
brushed on the surface to be etched until there is a clear
definition or outline of the weld.

QW-472.4 Nitric Acid. One part of nitric acid and
three parts of water, by volume.
CAUTION: Always pour the acid into the water. Nitric acid causes
bad stains and severe burns.
The solution may be used at room temperature and
applied to the surface to be etched with a glass stirring rod.
The specimens may also be placed in a boiling solution of
the acid, but the work should be done in a well-ventilated
room. The etching process should be continued for a
sufficient period of time to reveal all lack of soundness
that might exist at the cross-sectional surfaces of the weld.

QW-473 For Nonferrous Metals
The following etching reagents and directions for their
use are suggested for revealing the macrostructure.

QW-473.1 Aluminum and Aluminum-Base Alloys
Hydrochloric acid (concentrated) 15 ml
Hydrofluoric acid (48%) 10 ml
Water 85 ml
This solution is to be used at room temperature, and
etching is accomplished by either swabbing or immersing
the specimen.

QW-473.2 For Copper and Copper-Base Alloys:
Cold Concentrated Nitric Acid. Etching is accomplished
by either flooding or immersing the specimen for
several seconds under a hood. After rinsing with a flood
of water, the process is repeated with a 50-50 solution
of concentrated nitric acid and water.
In the case of the silicon bronze alloys, it may be
necessary to swab the surface to remove a white (SiO2)
deposit.

QW-473.3 For Nickel and Nickel-Base Alloys
Material Formula
Nickel Nitric Acid or Lepito’s Etch
Low Carbon Nickel Nitric Acid or Lepito’s Etch
Nickel–Copper (400) Nitric Acid or Lepito’s Etch
Nickel–Chromium–Iron Aqua Regia or Lepito’s Etch
(600 and 800)

MAKEUP OF FORMULAS FOR AQUA REGIA AND
LEPITO’S ETCH
Aqua Lepito’s
Regia Etch
[(1), (3)] [(2), (3)]
Nitric Acid, Concentrated — HNO3 1 part 3 ml
Hydrochloric Acid, Concentrated —
HCL 2 parts 10 ml
Ammonium Sulfate —
(NH4)2(SO4) . . . 1.5 g
Ferric Chloride — FeCl3 . . . 2.5 g
Water . . . 7.5 ml

NOTES:
(1) Warm the parts for faster action.
(2) Mix solution as follows:
(a) Dissolve (NH4)2(SO4) in H2O.
(b) Dissolve powdered FeCl3 in warm HCl.
(c) Mix (a) and (b) above and add HNO3.
(3) Etching is accomplished by either swabbing or immersing the specimen.

QW-473.4 For Titanium
Kroll’s Etch Keller’s Etch
Hydrofluoric acid (48%) 1 to 3 ml 1⁄2 ml
Nitric acid (concentrated) 2 to 6 ml 21⁄2 ml
Hydrochloric Acid . . . 11⁄2 ml (concentrated)
Water To make 100 ml To make 100 ml
QW-473.5 For Zirconium
Hydrofluoric acid 3 ml
Nitric acid (concentrated) 22 ml
Water 22 ml
Apply by swab and rinse in cold water.
These are general purpose etchants which are applied
at room temperature by swabbing or immersion of the
specimen.

No comments: