docDefect formation in cast iron
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Defect formation in cast iron
Defect formation in cast iron Lennart Elmquist School of Engineering Jönköping University Sweden Tammerfors, Finland, November 8, 2012 Introduction Casting • • • • • • Mould cavity Melting process Pouring Solidification Mould removal Cleaning and finishing Lennart Elmquist Tammerfors 2012-11-08 Advantages • • • • • Complex shapes Holes and inner cavities Wide range of alloys Melt treatment Economical 2 Introduction DEFECTS! Common defects: • • • • • • Blowholes Pinholes Shrinkage Penetration Inclusions etc… Slag inclusion defect Shrinkage porosity Carbon monoxide blowhole Lennart Elmquist Tammerfors 2012-11-08 Metal expansion penetration 3 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary Lennart Elmquist Tammerfors 2012-11-08 4 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary Lennart Elmquist Tammerfors 2012-11-08 5 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary (Source: Torsten Sjögren) Lennart Elmquist Tammerfors 2012-11-08 6 Gas defects – Introduction Source: Diószegi, 2005 Lennart Elmquist Tammerfors 2012-11-08 Source: UAB Casting Engineering Laboratory (CEL), 2005 7 Gas defects – Introduction • Degradation of binder material • Gas precipitate during solidification due to decreased solubility • Gas may be picked up by the melt during mould filling • Slag constituents can react with alloying additions in the metal and form gaseous reaction products Lennart Elmquist Tammerfors 2012-11-08 8 Gas defects – Introduction • • • • • • Hydrogen pinholes Nitrogen pinholes Blowholes Fissures Carbon monoxide Slag blows Source: www.ikominerals.de Lennart Elmquist Tammerfors 2012-11-08 9 Pinholes • • • • • Hydrogen, nitrogen, CO Revealed after machining Usually small Round shape Graphite lining Remedies • Low levels of Al and Ti • Control of steel scrap • Cores and water‐based coatings fully cured and dried Source: www.ikominerals.de Lennart Elmquist Tammerfors 2012-11-08 10 Pinholes Hydrogen pinholes • • • • • • Water and organic materials Air humidity Rusty charge material Damp refractory material Moisture in mold and core Core binder Nitrogen pinholes • Resin‐bonded cores and molds • Sand additives • Coatings and glues • Charge materials Source: http://www.objet.sk/content/products/sand_casting_a4.pdf Lennart Elmquist Tammerfors 2012-11-08 11 Pinholes Carbon monoxide pinholes • Reaction of carbon and oxygen – Liquid slag rich in iron oxide – CO2 react with iron – Carbon react with water • No graphite lining • MnS and slag inclusions Lennart Elmquist Tammerfors 2012-11-08 Source: www.ikominerals.de 12 Blowholes Large cavities Normally irregular in shape Smooth walls Graphite lining High turbulence during mold filling • High gas pressure in the mold • • • • • Source: www.ikominerals.de Lennart Elmquist Tammerfors 2012-11-08 13 Blowholes • • • • • • • • • • Casting design Gating Sand permeability Core print area Type of resin system Extent of cure and drying Pouring temperature Rusty chaplets Clogged vents ……etc Lennart Elmquist Tammerfors 2012-11-08 Source: www.ikominerals.de 14 Fissures • Narrow crack‐like cavities • Frequently have dendritic structures • Excessive nitrogen main cause • Hydrogen may intensify the defect Lennart Elmquist Tammerfors 2012-11-08 Source: www.ikominerals.de Sources • Nitrogen content high • High nitrogen content in core binder • Poor core venting 15 Solubility of gases 1. Diffusion of gas to the surface 2. Chemical reaction on the surface 3. Diffusion of one or more reaction products away from the surface H 2 g 2 H H K H P H 2 Source: Svensson et al, 1980 Lennart Elmquist Tammerfors 2012-11-08 16 Solubility of gases Influence of alloying elements on solubility Hydrogen Nitrogen Source: Weinstein et al, 1963 Lennart Elmquist Tammerfors 2012-11-08 Source: Dawson et al, 1965 17 Solubility of gases N K N PN 2 • The solubility of hydrogen and nitrogen affect each other PTOT PH 2 PN 2 • Cumulative effect that increase the risk for porosity Lennart Elmquist Tammerfors 2012-11-08 Source: Svensson et al, 1980 18 Gas levels in production • • • • Melting methods Different foundries Different furnaces Long period of time Lennart Elmquist Tammerfors 2012-11-08 19 Gas levels in production Hydrogen Nitrogen • Preheating • Holding‐time • Melting method • Charge material 160 3.5 Cup. melt, sampling 1 Cup. melt, sampling 2 Ind. melt, sampling 1 Ind. melt, sampling 2 Nitrogen concentration [ppm] Hydrogen concentration [ppm] 3 2.5 2 1.5 1 Cup. melt, sampling 1 Cup. melt, sampling 2 Ind. melt, sampling 1 Ind. melt, sampling 2 0.5 0 120 80 40 1300 1350 1400 1450 Temperature [C] Lennart Elmquist Tammerfors 2012-11-08 1500 1550 1250 1300 1350 1400 1450 Temperature [ ] C 1250 1500 1550 Source: Orlenius, Elmquist, Diószegi, 2007 20 Gas levels in production Oxygen Total amount of oxygen • Dissolved oxygen • Oxygen found in oxides • Difference between processes 4 12 3 Total oxygen [ppm] Dissolved oxygen [ppm] 3.5 2.5 2 1.5 8 4 1 0.5 0 1250 Cupola furnace Induction furnace 0 1300 1350 1400 1450 Temperature [°C] Lennart Elmquist Tammerfors 2012-11-08 1500 1550 1280 1320 1360 1400 1440 1480 Temperature [°C] Source: Elmquist, Orlenius, Diószegi, 2007 21 Gas levels in production After mold filling 140 Before casting In the mold Nitrogen concentration [ppm] 120 100 80 60 40 20 0 1 2 Sample No Lennart Elmquist Tammerfors 2012-11-08 Source: Orlenius, Elmquist, Diószegi, 2007 22 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary Lennart Elmquist Tammerfors 2012-11-08 23 Gas absorption Source: Orlenius et al, 2008 Lennart Elmquist Tammerfors 2012-11-08 24 Gas absorption • Investigate relationship gas absorption – mold filling • Compare simulation – reality – Turbulence during filling – Permeability – Binders Source: Orlenius et al, 2008 Lennart Elmquist Tammerfors 2012-11-08 25 Gas absorption Source: Orlenius et al, 2008 Lennart Elmquist Tammerfors 2012-11-08 26 Gas absorption Filling velocity Source: Orlenius et al, 2008 Lennart Elmquist Tammerfors 2012-11-08 27 Gas absorption • Turbulent mould filling Source: UAB Casting Engineering Laboratory (CEL), 2005 Lennart Elmquist Tammerfors 2012-11-08 28 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary Lennart Elmquist Tammerfors 2012-11-08 29 Mould and core gas evolution Volume and rate of gas evolution Source: Orlenius et al, 2008 Lennart Elmquist Tammerfors 2012-11-08 30 Gas evolution Core geometry • Volume equal • Amount of binder equal • Highest rate for parallelepiped Source: Orlenius et al, 2008 Lennart Elmquist Tammerfors 2012-11-08 31 Gas evolution Binder type • Inorganic binder • Organic binder • Amount of binder • Storage time • Temperature Source: Orlenius et al, 2008 Lennart Elmquist Tammerfors 2012-11-08 32 Gas evolution Coatings • Improve surface • Aluminium silicate • Zircon • Drying process Source: Orlenius et al, 2008 Lennart Elmquist Tammerfors 2012-11-08 33 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary Lennart Elmquist Tammerfors 2012-11-08 34 Inclusions – sand • Occur at widely varying positions • Sand torn away by metal stream • In association with CO blowholes and slag • Carefully blow out mold cavities • Avoid high pouring rates • Ensure uniform mold compaction Lennart Elmquist Tammerfors 2012-11-08 • Edge disintegration • Sand crust formation • Erosion Source: www.ikominerals.de 35 Inclusions – slag • • • • Irregular‐shape Non‐metallic inclusions Upper casting surfaces Formation of CO Possible causes • Oxide content of the charge too high • Poor dissolution of inoculants • Poor slag practice Source: www.ikominerals.de Lennart Elmquist Tammerfors 2012-11-08 36 Inclusions – slag • Filter • Design of the gating system – slag traps Lennart Elmquist Tammerfors 2012-11-08 37 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary Lennart Elmquist Tammerfors 2012-11-08 38 Introduction Source: Elmquist et al, 2008 Lennart Elmquist Tammerfors 2012-11-08 39 Shrinkage – Introduction Shrinkage defects Generally • Open shrinkage defects (macroshrinkage) • Closed shrinkage defects (shrinkage porosity) • Liquid contraction • Liquid‐to‐solid contraction • Solid contraction Cast iron – graphite expansion Source: Stefanescu, 2005 Lennart Elmquist Tammerfors 2012-11-08 40 Shrinkage – Introduction Factors promoting shrinkage porosity • • • • • Nature and properties of the sand mould Metal composition Pouring temperature Degree of eutectic nucleation of the metal Solidification mode Compensates for the shrinkage • Feeding of metal • Graphite expansion Lennart Elmquist Tammerfors 2012-11-08 • Atmospheric gas • Dissolved gas (hydrogen, nitrogen) 41 Shrinkage – Introduction • Different types of shrinkage • Related to cooling curve Source: Soivio, Elmquist, 2012 Lennart Elmquist Tammerfors 2012-11-08 42 Shrinkage – Introduction Source: Elmquist, Adolfsson, Diószegi, 2008 Lennart Elmquist Tammerfors 2012-11-08 43 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary Lennart Elmquist Tammerfors 2012-11-08 44 Defect characterization Case: Cylinder Heads Economy Environment • • • • • Shrinkage porosity • Characteristic features • Knowledge about mechanisms Two foundries Two designs Production line With and without porosity Lennart Elmquist Tammerfors 2012-11-08 45 Defect characterization Similarities • Position on casting • Surface defect • Connection surface – porosity • Penetrating through the casting • Enclosing certain units • Primary crystals? 10mm Source: Elmquist, Adolfsson, Diószegi, 2008 Lennart Elmquist Tammerfors 2012-11-08 46 Defect characterization • Border between surface and porosity • Oxide layer – connection to atmosphere • No graphite layer Source: Elmquist, Adolfsson, Diószegi, 2008 Lennart Elmquist Tammerfors 2012-11-08 Shrinkage Porosity vs. Penetration 47 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary Lennart Elmquist Tammerfors 2012-11-08 48 Migrating hot spot • • • • Heat is removed during solidification Different cooling conditions in different parts Initial temperatures in casting/mold Part of the casting that solidifies last • Migrates during solidification • Depends on geometry • Heat removal L T Y • Global/Local hot spots Lennart Elmquist Tammerfors 2012-11-08 49 Migrating hot spot CH1 CH2 Source: Elmquist, Adolfsson, Diószegi, 2008 • Thermal centre at mold/metal interface • Migrating hot spot Lennart Elmquist Tammerfors 2012-11-08 50 Migrating hot spot SOLTIME [s] 1200 1100 1000 900 800 700 Hot spot migrates • Casting/mold interface • Shrinkage porosity • Metal expansion penetration 600 500 • Geometry developed by simulations • Final position of hot spot Source: Elmquist, Adolfsson, Diószegi, 2008 Lennart Elmquist Tammerfors 2012-11-08 51 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary Lennart Elmquist Tammerfors 2012-11-08 52 Shrinkage Porosity Develop a casting that generates shrinkage porosity Source: Elmquist, Diószegi, 2008 Lennart Elmquist Tammerfors 2012-11-08 53 Shrinkage Porosity Geometry generating porosity Migrating hot spot Lennart Elmquist Tammerfors 2012-11-08 Source: Elmquist, Diószegi, 2008 54 Solidification Solidification of grey cast iron • Nucleation • Growth • Primary austenite • Eutectic cells Source: Elmquist et al, 2008 Lennart Elmquist Tammerfors 2012-11-08 55 Solidification Macrostructure Columnar Equiaxed zone zone • Columnar dendrites • Equiaxed dendrites • CET Microstructure • Dendrite Arm Spacing (SDAS) • Fraction Austenite • Eutectic cell size Source: Elmquist et al, 2008 Lennart Elmquist Tammerfors 2012-11-08 56 Shrinkage and the macrostructure Similarities • • • • Shrinkage porosity Surface defects Penetrating network Contact with atmosphere Source: Elmquist, Diószegi, 2008 Lennart Elmquist Tammerfors 2012-11-08 57 Shrinkage and the macrostructure DAAS (Direct Austempering After Solidification Macrostructure • Equiaxed dendrites • Columnar dendrites • CET Columnar zone Abnormal columnar zone Similar CET Source: Elmquist, Diószegi, 2008 Lennart Elmquist Tammerfors 2012-11-08 58 Shrinkage and the macrostructure Porosity between primary crystals Source: Elmquist, Diószegi, 2008 Lennart Elmquist Tammerfors 2012-11-08 59 Shrinkage and the macrostructure Formation of shrinkage porosity • • • • • • • Hot spot Migration Last area of solidification Weak columnar zone Atmospheric gas Shrinkage porosity Surface defect Source: Elmquist, Diószegi, 2008 Lennart Elmquist Tammerfors 2012-11-08 60 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary Lennart Elmquist Tammerfors 2012-11-08 61 Penetration – Introduction • Metal penetration is a casting condition resulting from reactions at the mould‐metal interface – Physical – Mechanical – Thermo‐chemical “Metal penetration is defined as the condition in which cast metal has entered into the pore spaces of the mold and core beyond the mid‐point of the surface layer of sand grains.” (Draper and Gaindhar, 1977) Lennart Elmquist Tammerfors 2012-11-08 62 Penetration – Introduction Four basic mechanisms: • • • • Mechanical (liquid‐state) Chemical Vapor‐state Explosion penetration Source: Draper and Gaindhar, 1977 The liquid metal exerts: • Static pressure • Dynamic pressure • Pressure because of expansion Lennart Elmquist Tammerfors 2012-11-08 Source: Stefanescu, 2008 63 Penetration – Introduction • Generally occurs at locations where the sand is the hottest (cores, concave sections) and is of lowest density • Low surface tension • High P, Si, or Mn content • Elevated static or dynamic metal pressure • High metal and sand temperature • Sand too coarse, or insufficiently rammed • Thermal conductivity of sand too low • Poor quality mold wash or blacking (AFS International atlas of casting defects) Lennart Elmquist Tammerfors 2012-11-08 64 Penetration – Introduction • Only removed by extensive chipping and grinding • Often so severe that castings are beyond the point of economical rework and must be scrapped • Tool wear Lennart Elmquist Tammerfors 2012-11-08 (Source: Levelink and Julien, 1973) 65 Penetration – Introduction Source: Diószegi and Dugic, 2007 Lennart Elmquist Tammerfors 2012-11-08 66 Metal Expansion Penetration CASE Study – Cylinder heads • Shrinkage – no Penetration • No shrinkage – Penetration Source: Elmquist et al, 2008 Shrinkage Porosity vs. Penetration Migrating hot spot is related also to penetration problems Lennart Elmquist Tammerfors 2012-11-08 67 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary Lennart Elmquist Tammerfors 2012-11-08 68 Metal Expansion Penetration Geometry generating metal penetration Source: Elmquist, Diószegi, 2008 Lennart Elmquist Tammerfors 2012-11-08 69 Metal Expansion Penetration Test Casting Source: Diószegi and Dugic, 2007 Lennart Elmquist Tammerfors 2012-11-08 Columnar zone Equiaxed zone 70 Metal Expansion Penetration Source: Diószegi and Dugic, 2007 The volumetric changes cause either a surplus or a deficit of melt Hot spot at casting/mold interface – the weakest part Lennart Elmquist Tammerfors 2012-11-08 71 Metal Expansion Penetration Equiaxed austenite grains. Liquid metal. Columnar austenite grains. Eutectic cells. Eutectic strip. Sand grains. Penetrated eutectic phase. Source: Diószegi and Dugic, 2007 Lennart Elmquist Tammerfors 2012-11-08 72 Metal Expansion Penetration Equiaxed austenite grains. Columnar to equiaxed transition. Columnar austenite grains. Eutectic cells. Sand grains Penetrated anomalous phase containing late nucleated austenite dendrites. Source: Diószegi and Dugic, 2007 Lennart Elmquist Tammerfors 2012-11-08 73 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary Lennart Elmquist Tammerfors 2012-11-08 74 Other Problems • • • • Residual stresses Prior history of the material Segregation Degenerated graphite Source: Elmquist Source: Soivio, Elmquist, 2011 Lennart Elmquist Tammerfors 2012-11-08 75 What About Future? • Inspection/detection – Most of the problems never reach the customer • • • • Research Process control/thermal analysis Casting simulation Optimization – Many of the problems can be solved or minimized with a combination of process control and optimal design of component as well as gating system Lennart Elmquist Tammerfors 2012-11-08 76 Future • Sustainability – Life cycle analysis – The influence of defects • Near‐net shape – Minimize the need for machining • High‐performing cast iron materials – Increased strength – Less variations in properties Lennart Elmquist Tammerfors 2012-11-08 77 Outline • Gas defects – Gas absorption – Gas evolution • Inclusions • Shrinkage porosity – Defect characterization – Migrating hot spot – Primary austenite • Metal expansion penetration – Primary austenite • Other quality problems • What about the future? • Summary Lennart Elmquist Tammerfors 2012-11-08 78 Summary Casting • • • • • • Mould cavity Melting process Pouring Solidification Mould removal Cleaning and finishing Lennart Elmquist Tammerfors 2012-11-08 Advantages • • • • • Complex shapes Holes and inner cavities Wide range of alloys Melt treatment Economical 79 Summary Why? Who cares? Lennart Elmquist Tammerfors 2012-11-08 80 Thank you for your attention! Lennart Elmquist Tammerfors 2012-11-08 81