defects in ductile iron

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Metallurgical Defects in Ductile Iron Some Problem Elements

Common Metallurgical Defects in Ductile IronCompacted Graphite Low Nodule Count Chunky graphite Exploded graphite Nodule Alignment Carbides Graphite flotation Ferrite/Pearlite - ratio Slag inclusions Shrinkage porosity Gas porosity

Compacted Graphite in Ductile IronPOSSIBLE CURE POSSIBLE CUREincrease Mg and/or RE increase Mg and/or RE reduce S in base iron reduce S in base iron reduce fade time reduce fade time better inoculation better inoculation possibly reduce Ti possibly reduce Ti contamination from CGI contamination from CGI returns returns

Titanium in Ductile Ironcauses compacted causes compacted graphite structures graphite structures combines with and combines with and removesnitrogen removes nitrogen leads to poor nodule leads to poor nodule shape,with flakes shape, with flakes branching out from branching out from nodules nodules causes a need for more causes a need for more magnesiumto counteract magnesium to counteract this this

Low Nodule CountPOSSIBLE CURE POSSIBLE CUREimprove inoculation improve inoculation or reduce fade time or reduce fade time avoid superheating avoid superheating and excessive hold and excessive hold times times improve base iron improve base iron response to response to inoculation inoculation (preconditioning) (preconditioning) avoid excessive Mg avoid excessive Mg151 Nodules/mm2 712 Nodules/mm2

Chunky GraphitePOSSIBLE CURE POSSIBLE CUREreduce RE content in reduce RE content in nodulariser or reduce nodulariser or reduce addition rate of addition rate of nodulariser nodulariser reduce eventual other reduce eventual other additions of RE additions of RE add S or use a Sadd S or use a Scontaining containing recarburiser to recarburiser to neutralise excess RE neutralise excess RE improve inoculation improve inoculation to reduce to reduce segregation effects of segregation effects of REs REs

Exploded GraphitePOSSIBLE CURE POSSIBLE CUREreduce carbon reduce carbon equivalent equivalent increase cooling rate increase cooling rate reduce RE content in reduce RE content in nodulariser or reduce nodulariser or reduce addition rate of addition rate of nodulariser nodulariser reduce RE vs. reduce RE vs. sulphur ratio sulphur ratio

Spiky GraphitePOSSIBLE CURE POSSIBLE CUREreduce amount of reduce amount of tramp elements such tramp elements such as Pb, Bi and Sb as Pb, Bi and Sb watch for Pbwatch for Pbcontaining scrap or containing scrap or Pb-paints Pb-paints avoid Bi-containing avoid Bi-containing inoculant inoculant increase RE addition increase RE addition

Lead in Ductile Ironwill segregate to grain will segregate to grain boundariesand cause boundaries and cause intercellular flakes at intercellular flakes at levels above 0.002% levels above 0.002% this effect is countered this effect is countered bythe presence of rare by the presence of rare earths in the treatment earths in the treatment alloy (0.005 wt%) alloy (0.005 wt%) harmful to mechanical harmful to mechanical properties properties

Irregular Graphite Irregular GraphitePOSSIBLE CURE POSSIBLE CURE

Improve inoculation Improve inoculation Avoid long hold Avoid long hold times after times after inoculation inoculation Reduce magnesium Reduce magnesium addition addition

Nodule AlignmentPOSSIBLE CURE POSSIBLE CUREincrease carbon increase carbon equivalent equivalent use better use better inoculation inoculation

Intercellular Iron Carbides (Chill)POSSIBLE CURE POSSIBLE CUREincrease carbon increase carbon equivalent and/or silicon equivalent and/or silicon content content reduce Mg and/or RE reduce Mg and/or RE content content reduce content of carbide reduce content of carbide promoting traces (Mn, Cr, promoting traces (Mn, Cr, V, Mo, etc.) V, Mo, etc.) better inoculation better inoculation avoid tellurium cups in avoid tellurium cups in charge charge increase pouring increase pouring temperature temperature

Segregation Carbides (Me-C)POSSIBLE CURE POSSIBLE CUREreduce content of reduce content of carbide forming traces carbide forming traces (Cr, V, Mo, etc.) (Cr, V, Mo, etc.) better inoculation to better inoculation to minimise segregation minimise segregation change nodulariser to change nodulariser to improve nucleation improve nucleation

Chromium in Ductile IronCase Study: Sample shown had a chromium content of .42 wt%. This caused large carbides in addition to excess pearlite.

Spectrum 1: Cr-C Spectrum 2: Pearlite

Graphite FlotationPOSSIBLE CURE POSSIBLE CUREreduce Carbon reduce Carbon equivalent, especially equivalent, especially C-content C-content reduce pouring reduce pouring temperature temperature increase cooling rate increase cooling rate better inoculation better inoculation

Low Ferrite/Pearlite RatioPOSSIBLE CURE POSSIBLE CUREreduce content of reduce content of pearlite stabilising pearlite stabilising elements (Mn, Cu, Sn, elements (Mn, Cu, Sn, Sb....) Sb....) use more high purity use more high purity charge materials (pig charge materials (pig iron) iron) better inoculation better inoculation increase Silicon increase Silicon content content avoid hot shake out avoid hot shake out

Low Pearlite / Ferrite RatioPOSSIBLE CURE POSSIBLE CUREincrease content of increase content of pearlite stabilising pearlite stabilising elements (Mn, Cu, Sn, elements (Mn, Cu, Sn, Sb....) Sb....) reduce Silicon content reduce Silicon content Watch out for Boron Watch out for Boron contamination contamination

Slag InclusionsPOSSIBLE CURE POSSIBLE CUREimprove deslagging improve deslagging practice practice empty ladle between empty ladle between treatments treatments improve pouring practice improve pouring practice improve gating design to improve gating design to reduce turbulence reduce turbulence use filter or slag traps use filter or slag traps reduce Mg/nodulariser reduce Mg/nodulariser addition addition

Flake Graphite SurfacePOSSIBLE CURE POSSIBLE CUREreduce sulphur in the reduce sulphur in the sand sand use special mould use special mould coatings coatings increase Mg or Mg/S increase Mg or Mg/S ratio ratio avoid use of long avoid use of long runner-systems runner-systems reduce pouring reduce pouring temperature temperature increase pouring rate increase pouring rate use RE-containing use RE-containing inoculant inoculant

Shrinkage PorosityPOSSIBLE CURE POSSIBLE CUREincrease mould strength increase mould strength improve gating and improve gating and feeding design feeding design increase carbon content increase carbon content up to 3.8 up to 3.8 reduce Magnesium reduce Magnesium addition addition reduce alloying reduce alloying contents contents use better inoculant use better inoculant reduce inoculation fade reduce inoculation fade time time

Hydrogen PinholesPOSSIBLE CURE POSSIBLE CUREreduce moisture in reduce moisture in sand sand ensure that tools, ensure that tools, refractory and refractory and coatings are dry coatings are dry use clean and dry use clean and dry cover material cover material reduce Al and Ti reduce Al and Ti content in iron content in iron improve venting improve venting

Hydrogen in Ductile IronCase Study: Shows the two types of porosities mentioned above. To the left is the gas related pore with the black inner surface. And to the right is the shrinkage with a metallic surface and dendrites.

Hydrogen in Ductile Iron

Hydrogen pinhole: carbon layer, nodule depleted zone

Magnesium gas defect: no carbon layer

Hydrogen pinholing:H2O + C Followed by: 2 CO

H2 + CO C + CO2

This is the reason for the nodule depleted zone

Conclusion/SummaryTo avoid metallurgical defects in ductile iron To avoid metallurgical defects in ductile iron the following must be emphasised: the following must be emphasised: use good quality raw materials with low use good quality raw materials with low contents of trace elements (Mn, S, V, Mo, Ti, contents of trace elements (Mn, S, V, Mo, Ti, Cr.. Cr.. use a consistent treatment process giving use a consistent treatment process giving reproducible analyses, high recoveries and reproducible analyses, high recoveries and good temperature efficiency good temperature efficiency use high quality nodularisers with consistent use high quality nodularisers with consistent analyses and sizing analyses and sizing use high quality inoculants specific to the use high quality inoculants specific to the requirements requirements

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