Erosion phenomena in sand moulds

P u M i M qlrarterty as Ihe organ d the rounav Conmidm dthe Polish Amdemy dSeiencea
Erosion phenomena in sand moulds
A. ~hojccki'.,J. Mocck '**
'Faculty of Foundry Engineering, University of Scence, and Technology AGH
Reymonta 23,30-059 Krak6w.
Email address: 'ach@ agh.edo.pl, *'jrn~cek@a~h.edu.~l
Received 15.02.2008;accepted in revised form 22.03.2008
Abstract
Authors studicd the erosion phcnorncna in sand moulds pured with cast iron. Thc study comprises an evaluation of erosion
resistance of thc three sands: grccn sand. sand bondcd with inorganic or organic bindcr. It was concluded that thc most resistant is [he
classic green sand with thc addition of 5 B coal dust. Resistance of the sand with organic binder is generally weak and dcvnds on
kind of used raisin. Spccinl nztcntion was paid to the sands with no organic bindcr watcr glass and phospha~c.It was Sound that thcir
rcsistance depends on dehydratation conditions. When the mould is stored in law humidity of atmosphcrc the very strong crosion can
be expected. It rcsul ts h r n thc micro fractures in the bridges of binders, joining the grains of the sable. This phcnomcna facilitates the
tearing away of fragments of sand [tom the surface
Keywodst Sand rnuld; Erosion.
Thc crosion phenomena is directly rclatcd to thc kinetic energy
of thc jct of liquid metal. Thc cornpnent OF momcntum normal to
thc surface causes thc formation of comprcssivc strength in a
mould material. The metal pcnctntcs thc spaccs betwoen the
grains, the bonds suffer locat destruction. and as the result, some
of grains are detached from thc subslralc and pushed out by the
forcc of hydrostatic lift. Thc dcmsition of croded material in
other region of thc mould cavity forms [he macro inclusions in the
casting or rhc slag inclitsions at the raw casting surracc. Thc
removing of thc dcfcca From casting surface is moreover, one of
thc most tedious operations. Hcnce. numerous studics havc bccn
carried oul during thc past few ycars on the methods to rcstria the
crosirln phcnorncna [ 1. 2. 3. and 41. In farrncr papers the authors
prcscntccl thc mcrhod of quantitative dctcrminalicrn or the extent
of crosion 15. 61. The aim of prescnted papcr is to compare the
crosion resistance of thc t h m typical foundry sand:
Grccn sand. bwndcd with hcntonitc
Sand with organic bindcrs
Sand with inorganic hinder
Special attcnt ion was paid on ~ h hydrophilic
c
sand with inorganic
binder. The sclf bnrdcning sablcs arc till now bounded with
synthetic raisins mainty furfuran, but in the lulure they will bc
replaced by inorganic bindcrs. Sodium silicate, phosphatc.
mineral salu or biocomponcnts arc safe for cnvimnment and thc
hearth of lround~staff [71. Such tendency is supported by
numcrous sludics in 'UEand USA conccrning thc propcnics and
mcrhod of the rcgcncmtion of the sand boundcd with watcr glass.
Thc ncw hardcncrs for watcr glass wcre tcstcd to climinatc from
the sand thc sodium acetate [7. 8.91. new kinds of sodium silicate
have bccn intmduccd. the new phosphates and bioc~mponcnts
tcstcd in foundry industry [7,101.
2. Method of investigation
Thrce kinds of foundry sands wcrc chosen [a thc investigation
oherosion:
Green sand with R Z hcntonile GEKO and 5% of the
coal dust.
Sclf hatdcning sands with inorganic binder
I.
with 3.5% of sodium silicate R145 hardencd with
0.358 cstcr of acetate acid FLODUR 5
with 4.5% orthophosphoric acid and 0.5% MgO as the
11.
hardener.
Sclf hardening sands with organic bindcr:
Furan sand with 1% of furfuran raisin FURTOLIT 1031
I.
hardcncd with 0.5 8 of rhc RS-20 hardcncr.
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Ail the foundry sands havc been preparcd with silica sand
from Jawrzno mine. The diameter of grains was 0.3 15-0.4 mm.
Self hardening sabIcs havc bccn prcparcd in mixer and the test
inserts wcrc then executed. A r m thc 24 hours of hadcning in air
atrnosphcrc (20-25"C,
humidity 28-326) there werc installed in
the test mould. Thc construction of ihc mould and the method of
evaluation of thc sable rcsistancc wcrc descrikd in thc fermcr
papers 15, 61. Thc insert of grccn sand has bcen prcparcd just
before the test. All the inserts in the moulds were inclined 60"to
the vertical orientation of the metal jet, because in such a position
the erosion is largest. Moulds wcrc pourcd with gray cast iron
grade EN-GJL-200s (nominal composition 3.2-3.4%C, 1.9-2.0%
Si, 0.654.75SbMn. 0.154.2%P, 0.084.18S). The effccts of
erosion has h e n evaluated by volumetric measure [5, 6, 1 El.
In the second stage the effect of thc humidity of the atmosphcre,
in which the insert bounded with sodium silicate was stored, has
been tested. The inserts wcrc dehydrated 24 hours in air washcr in
14-16, 28-32 or 50-606 o f humidity. In thc same chambcr the
samples of the sand for cvaluazion of thc bcnding strength were
storcd.
3. ResuIts
The comparison of the resistance of diffcrcnt sands is
presented in (hc Figure I. Thc ctfccl of the crosion is calculated as
the volume of the sand detached by 1 kg of poured metal. Figure 1
Figure 2. Surfacc the casting pourcd in the grccn sand rnotrld.
presents also the total effect of erosion and penerration, mcasurcd
on the reference insert.
04-'
Bwtn sand
wlth sohum
sulitbtr
with phosyhorrc
scldc
with fursn
rtsln
Figure 1. Erosive effects in molding sands made with different
binders.
In the figures 2-5 the photos of the destruction of the surfaces
are illustrated. They confirm the general opinion that the most
resistant i s the green sand mould. Only the IittIe traces of erosion
are visible along thc routc or mctal, flowing on thc surface of
insert. Total volume of detached material is vanishing. Also the
penetration measured on the reference insen is very limited.
Excellent resistance results from the stability of the bonds
between the sand grains, little penetration is the effect of the lack
of wettability causcd by the formation of pyrolytic carbon Euszrous and arnotphc. Particles of the arnorphe carbon plugs the
voids between the sand grains.
50
Figure 3. Si~rfrlccthc casting. poi~rctlIn thc si~nrlhoundcd with I
sodium sil icatc.
~ C
The erosion of the sand with thc illran raisin is particuIarly
intense. It occurs in the zone whcn thc sand is dircc~ly hit by thc
metal jet and also along the routc of thc flowing rnctsl. EfCccl o f
erosion is similar For the sand bounded with or~hophosphoricacid.
The behavior of thc furan h n d s may bc cxplaincd by burning of
the organic bridges joining thc sand grains. It is fnvorahlc hccn~isc
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inRucnce o f setting atmosphcrc. Thc rcst~ltsarc illustrated in thc
Figurc 6. The inscrts dchydratcd in dry air is casily destroyed bc
the rnctal jet. Efrccccls o f crosion arc similar as in c x c o fiiran sand.
That results may hc confirm4 by thc nnalysis of fmclurc o f t hc
samples o f sand. prcsentcd in Ihc Figurc 7. Quick rlchydra~ion
causcs thc apparition of cracks o f thc hridpcs oi hindcrjoining thc
grains. Then the jet of rnctal casily dctachcs Ihc Iiiycr of snnd.
Figurc 4. S11rf:icc thc cnsting, pnurcd in ~ h csand mould bounded
with phosphoric arid.
Figurc 6. Effcct OF rbc humidily of ~ h atmosphcrc
c
on crosion o r
sand mould with sodium silicnlc.
Figurc 8 prcscnts thc rcsults of thc cxarninaiion of the hcnding
strcss. In thc dry air thc sand altains vcry high bcnding strcss hut
i t dccrcnscs quickly with iirnc.
F i y r c 5. Si~rSnccrhc casting. pourcd In thc sand houndcd with
Cur;tn raisin.
rhc sand c;wily losf thc initial strcngth and doesn't hlock the
casting shrinki~gc. Lilt lc rcsistancc o f phosphate bonds rcsults
lrnm vcry low strcngth cvcn at the ambient remperature. The
cavitics o f moultls houndcd with luran raisins and the
ortliophosph(~ricacirl inilsl hy covcrcd hy prorecrive coatings. For
thc si~ndshnntlctl by warcr glass thc grenl scattcr of rcsults has
bccn ohscrvctl. 'Thcrcfc~rc rhc iu~thors dccidcd lo study the
Figurc 7. Crackings OF lhc hridges of hindcr joining rhc grains o f
sand.
Aftcr thc 24 hours only 30R>o f the initial valuc is ohscn~d.
For thc hurnidi~yof 28% ~ h csnnd is wcakcr- thc hcnding strcss
ncvcr attain 70% OF rhc valuc ohscrvcd in fosmcr case, but this
valuc rcsts constant during thc 20 hours. In thc wct air (5M070)
only 46% of thc mmirntlrn is nchicvcd aftcr thc 4 hotrrs but, as i t
can bc scen in rhc liyurc 6 i t docsn'r rcsults in increasing crosion
becausc of high pjasticity of thc hontls.
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Conclusions
Thc cxpcrimcnts proved that ~ h grccn
c
sand is most resistant to
corrosion causcd by the jet of rhc tiquid mtal. Thc bonds bctween
thc silica grains arc vcry strong, cvcn in high lcmpcnrurc. No
crosivc cffcct h4asbccn obscrvcd in thc ronc whcrc thc mould is
dircctly hit by thc m#al jct.
Thc dcfccu of thc cavity of thc mould boundcd with ~ h sodium
c
silicatc am also small and thc rcsistancc of ~ h csand is satished.
but thc moulds h a w to bc storcd in thc middtc humidity. Whcn
thc dehydration is quick the brittlc iracturc of the bridges joining
thc grains can bc obscrvcd, This rcsults thc easy dctachmcnt of
the fragment of sand laycr when thc metal stream hits thc surfacc
of thc mould. In to high humidity thc sand ncvcr achievcs ~ h c
cxpcctcd strcngth.
The sands with Ihc organic bindcrs, nowadays gaining widespread
popularity in foundries bccausc of abcir numerous advantages. nrc
characterized by some significant drawbacks. They rcquirc a vcry
carcfuIly designed gating systcm to slow downs Ihe dircct impact
of metal against thc mould wall. The application of high quality
protcctivc coatings may be ncccssary.
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