|
Introduction
Much of the recent literature on defects that occur in jewellery
manufacture is focused on those occurring in lost wax
(investment) casting, for example reference (1). However, defects
can also occur during casting of ingot and its fabrication into
sheet, tube, wire and rod as well as their onward fabrication into
jewellery components by processes such as stamping and
forging. These defects, along with their causes and prevention,
are briefly reviewed in this article, although those defects
occurring in the die-striking of findings are described elsewhere in
this issue (2). Examples of many defects can be found in the
Handbook of Casting and Other Defects (1), and are referenced in
the text where appropriate.
Defects arising from the cast ingot or
continuously cast stock Piping and shrinkage porosity
A pipe is the funnel-like depression at the top of a cast ingot that
results from shrinkage during solidification. When the ingot is
subsequently worked (forged and/or rolled), this will give a centre-
line defect along sheet, strip or wire, particularly if the pipe
surface is oxidised. This can lead to a splitting apart of strip and
sheet during subsequent rolling, a defect known as 'alligatoring',
Figure 1. An example of delamination due to a pipe is given in the
Handbook of Casting and Other Defects (1), Case 16 on page 47.
The region containing the pipe should be cropped off before
working and recycled as scrap of known quality. Internal shrinkage
porosity should weld up and disappear provided the inner
surfaces are clean and free from oxide.
 |
| Figure 1 - Example of 'Alligatoring' (splitting) of a gold alloy rod
during first rolling pass (courtesy Engelherd-CLAL, U.K.). This hard
alloy is known to be difficult to fabricate. |
Blistering
Blistering on the surface of sheet and strip may be caused by gas
porosity trapped in the cast stock or arising from reactions with
the atmosphere during annealing treatments. An example of
blistering and internal porosity in 18 ct yellow gold due to the
'steam' reaction during annealing is given in reference (1), Case
26 on page 67.
The problem can be avoided by control of casting or annealing
conditions, including factors such as the dissolved gas or base
metal oxide content in feedstock materials through good de-
oxidation procedures in melting, annealing temperature and
avoiding the use of hydrogen-rich annealing atmospheres.
Inclusions
Inclusions in the cast stock are insoluble particles such as oxides
and silicides. These can lead to problems of cracking during
working or to the formation of hard spots that can affect the
quality of final polished surfaces ('comet tail' effect). Inclusions
may be pieces of crucible or furnace lining which have fallen into
the metal or they may be formed by chemical reaction, e.g.
between absorbed gas and an alloy constituent. Regular
inspection of crucibles and furnace linings, cleanliness of working
surroundings and a consideration of possible reactions is
important if inclusions are to be kept to a minimum. Many
examples of defects due to inclusions are given in reference (1).
Contamination
Contamination of the melt may cause embrittlement and
catastrophic cracking during working. The most notable example
is the presence of very small amounts of lead that usually has
been accidentally introduced as soft solder traces in recycled
scrap. Other embrittling contaminants include silicon, sulphur
and other low melting metals such as bismuth. Do not recycle
scrap of unknown quality; such scrap should be analysed and
checked for such impurities. Many examples of defects due to
contamination of the alloy are given in reference (1).
Surface quality
Surface quality of the final product may depend on the surface
quality of the initial cast stock. Remove surface oxide by acid
pickling prior to working since it is more difficult (and costly) to
remove the oxide after it has been ground into the surface of
sheet or rod. Excessive amounts of a mould dressing such as
machine oil or trapped flux can give large depressions in the ingot
surface. A thin continuous film of oil applied to the mould wall is
recommended and excess flux should be removed before pouring
the melt. Splashes, slivers and spills are caused by molten metal
splashing on the mould wall and solidifying with an oxide surface
before being covered as the melt fills the mould. These can
separate and peel away at the oxide interface during working,
resulting in an uneven surface. Inspect ingot surfaces so that they
can be trimmed and filed, where necessary, to smooth out
depressions, remove splashes and spills, and gouge out particles
embedded into the surface
Defects arising as a result of the working
process
1. Rolling of flat products – sheet, strip and foil
Finishing rolling
Poor surface quality can arise from use of poor quality rolls with
scratched or damaged surfaces. Finishing rolling should be done
using small diameter rolls with highly polished or chromium plated
surfaces to achieve a mirror-bright finish. Roll surfaces should be
continuously wiped to keep dust and other particles from scoring
or marking the rolls or being rolled into the strip surface. Cover the
rolling mill when not in use to protect roll surfaces
Roll misalignment
Misalignment of the rolls leads either to curvature of the strip to
one side as it comes out of the roll gap, if it is relatively thick, or to a wavy edge on one side if it is thin strip, Figure 2. Adjust the
roll screws to give a parallel roll gap.
 |
| Figure 2 - Defects due to misalignment of the rolls |
 |
| Figure 3 - Defects due to roll bending |
 |
| Figure 4 - Minimising roll bending using a 4 high mill |
 |
| Figure 5 - Edge cracking |
Roll bending
Roll bending under the action of the rolling force required to
reduce strip thickness can lead to uneven thickness across the
width of the strip or to wavy edges on both sides of the strip,
Figure 3. Either decrease the reduction per pass, together with
more frequent interstage annealing, to reduce the rolling force
or, preferably, use a four-high rolling mill where small diameter
work rolls are backed by larger rolls to prevent the work rolls
bending under load, Figure 4.
Edge cracking
Edge cracking is commonly caused by overworking between
anneals. It is important to trim edges at the time it occurs as
further rolling will increase the danger of some cracks suddenly
running into the centre of the strip and turning through right
angles, greatly increasing the amount which has to be scrapped,
Figure 5.
Gauge (thickness) control
Modern large mills usually have sophisticated automatic gauge
control systems but for the jewellery manufacturer this may not
be possible. Care should be taken to ensure uniform thickness
along the strip length and across its width. Variations in thickness
will give variations in forces required in subsequent sheet metal
forming operations and this may lead to higher reject rates and
risk of increased tool wear and breakage. If strip is sold to a
specified minimum thickness, any additional thickness over
specification has to be paid for by the strip manufacturer and for
carat gold strip this will be expensive. Finish rolling with light
reductions as sizing passes before final annealing will help to
control gauge.
2. Rod rolling
Fins and laps
Fins are caused by trying to push too much metal into the rolling groove, i.e. attempting too large a reduction, so that the rolls are
forced apart and the excess metal is squeezed out sideways,
Figure 6. If the fins are subsequently rolled into the rod, they
become laps that form planes of weakness and they can open up
at later stages, particularly under torsion or a twisting motion.
Such defects can be prevented by avoiding excessively large
reductions and by rotating the rod through 90. between each
successive pass.
 |
| Figure 6 - Formation of fins and laps |
 |
| Figure 7 - Cuppy wire |
3. Drawing
Cuppy wire
The most common defect in drawing is breakage or necking
down as the wire emerges from the die, Figure 7. There are four
possible causes.
- a) The wire is overworked and requires annealing.
- b) The presence of inclusions can give rise to weak spots in the wire.
- c) Too large a reduction per draw is being attempted. For large diameter wires, the reduction may be 25-45% depending on the workability of the particular alloy but, as the diameter is decreased, the reductions may be down to 15-20%.
- d) A breakdown in lubrication gives an increase in friction between the wire and die surfaces and this lowers the reduction that can be given.
4. Sheet metal forming
The occurrence of defects in sheet metal forming and their
prevention is a complex subject. Fracture during forming will take
place at the weakest or thinnest point in the part being formed.
This is most likely to be where the sheet has been bent under
tension round an angle, as extra thinning will occur there. There
is a maximum or limiting size of blank that can be successfully
formed without failure occurring at this point. It may be
necessary to partly form in one punch-die set and then further
form in other punch-die sets. The subject has been reviewed (3)
and this should be published in the near future.
5. Orange peel effect
This is the formation of a rumpled surface on deformed sheet or
wire, visible to the naked eye, and is due to a large grain size that
results from over-annealing. An example is shown in Figure 11 of
reference (2) on page 19 in this issue. Some 14 ct alloys are
particularly prone to this effect. The formation of large grain sizes
is sometimes compounded by annealing material with low
amounts of deformation and/or by the presence of small
additions such as silicon in the alloy (4). Orange peel may show up
on chain link surfaces or on the surface of die stampings. The
effect is minimised or prevented by control of working and
annealing schedules such that coarse grain growth does not
occur. High annealing temperatures, such as occur with torch
annealing, and low amounts of deformation favour large grain
sizes.
6. Stress corrosion cracking This is failure by spontaneous cracking of jewellery, due to either
internal residual stresses or stresses applied in service, in the
presence of a corrosive atmosphere. This latter can include acid
fumes or gaseous chlorides (e.g. from domestic cleaners and
chlorinated swimming pools). An example in 14 ct nickel white
gold is shown in Figures 5 & 6 in reference (5), page 28 in this
issue. Fracture can occur in service (i.e. jewellery is being worn or
stored), long after the jewellery has been manufactured and
tends to prevail in low carat jewellery (8-14 carat).
It is important to emphasise that low carat jewellery, if left in
the worked condition, will contain residual stresses and that this
leads to an increased risk of stress corrosion cracking. Residual
stresses can be removed or reduced in magnitude by a stress
relief anneal, typically 30 minutes at 250.C. This reduces the risk
of stress corrosion cracking. Stress relief should always be done
after jewellery repair or re-sizing of rings.
7. Fire cracking in nickel white golds Fire cracking is the sudden massive cracking of nickel-containing
white golds during annealing, following cold working, and is caused
by large internal stresses due to the metallurgy of these alloys. It
occurs with fast cooling (water quenching) after annealing. However,
slow cooling in air can lead to phase separation and colour changes.
Often, it can be alleviated by an intermediate rate of cooling such as
placing the item on an iron plate to cool or forced air cooling. The
topic has been discussed by Rapson (6) and reviewed more recently
by Normandeau (7) - see reference (5) in this issue, where an
example of fire cracking is shown in Figure 7 on page 28.
8. Cracking due to alloy embrittlement by
impurities This has been briefly discussed under the section on
Contamination above. Small amounts of impurities such as lead
and silicon segregate to grain boundaries and form low melting
point phases. Ott has reviewed this problem in reference (8).
Some examples are shown in reference (1), Cases 17 and 18.
References:
- D. Ott, "Handbook of Casting and Other Defects in gold
jewellery manufacture", publ. World Gold Council, 1997.
- F. Klotz and S. Grice, "Live and Let Die (Struck)", Gold
Technology, No. 36, Winter 2002, p 16-22
- M.F. Grimwade, "Sheet metal forming operations", Proc.
Santa Fe Symposium, 1992, Met-Chem Research Inc
(awaiting publication)
- D. Ott, "Optimising carat gold alloys for the manufacturing
process", Gold Technology, No. 34, Spring 2002, 37-44
- M.F. Grimwade, "The 16th Santa Fe Symposium on jewellery
manufacturing technology", Gold Technology No. 36,
Winter 2002, p 24-32
- W. Rapson & T. Groenewald, "Gold Usage", Academic press,
1978
- G. Normandeau, "Fire cracking in white gold jewelry
materials", Proc Santa Fe Symposium, 2002, Met-Chem
Research Inc, p429-450
- D. Ott, "The effect of small additions and impurities on
properties of carat golds", Gold Technology No. 22, July
1997, p31-38
|
