24-341_4-Casting

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Metal Casting Processes and Equipment; Heat Treatment : 

Metal Casting Processes and Equipment; Heat Treatment

Casting Definition : 

2 Casting Definition Conversion of a Liquid Material into a Solid of Some Useful Shape Through the Interaction with a Mold

Issues in Casting : 

3 Issues in Casting Solidification and Shrinkage Fluid Flow of the Liquid Material into the Mold Heat Transfer Mold Material

Solidification Curves : 

4 Solidification Curves Figure 5.1 Alloy Cooling Curve Time Temp

Solidification Curves : 

5 Solidification Curves Thermoset Plastics Time Temp

Solutions : 

6 Solutions Solute Minor Component in a Solution Solvent Majority Component in a Solution Liquids - Solvent Water - Sugar Acetone - Oil Solid Solvent Iron - Carbon Aluminum - Manganese

Solutions : 

7 Solutions Types of Solid Solution Substitutional Solvent and Solute Similar in Size Solute Atoms Displace Solvent Atoms on Lattice Sites Interstitial Solute Atoms Smaller than Solvent Solute Atoms Sit in the Open Spaces in the Lattice Between Solvent Atoms

Intermetallic Compounds : 

8 Intermetallic Compounds Complex Combinations of Solute in Solvent at Local Specific Proportions Generally Hard, Strong and Brittle 2 Phase Alloys If Solid Solubility at a Given Temperature is Exceeded get a 2 Phase Mixture Minor Component will Precipitate In the Solid

2 Phase Mixtures : 

9 2 Phase Mixtures Phase Precipitation is a Major Strengthening Mechanism for Metals Figure 5.2

Equilibrium Phase Diagrams : 

10 Equilibrium Phase Diagrams Figure 5.3 Simple Eutectic Shows that Phase Compositions and Precipitation can be Complex Percentage of Phases Present at a Given Temperature Can be Calculated from the Lever Rule

Lever Rule : 

11 Lever Rule Lever Rule States a Relation Between the Weight Percentage of 2 Phases A and B Where CA is Weight Percent Phase A, CB is Weight Percent Phase B and C0 is Alloy Composition

Fe - C Phase Diagram : 

12 Fe - C Phase Diagram Figure 5.4 a Ferrite BCC max C is 0.022% at 727C From 912C to 1394C get Austenite FCC Holds 2.11% C at 1148C At 6.67% C get all Fe3C

Fe Fe3C : 

13 Fe Fe3C Figure 5.6 If Slow Cool Austenite At 0.77% C get Pearlite If Less than 0.77% C get some Proeutectoid Ferrite and Remaining Pearlite If More than 0.77% C get some Proeutectoid Cementite and Pearlite

Cast Iron : 

14 Cast Iron Cast Iron is Iron with from 2.11% C to 4.5% C Several Types Grey Iron Nodular Iron White Iron Malleable Iron Compacted Graphite Iron Because of Higher C Content Cast Iron Stays Liquid at a lower Temperature than Steels

As Cast Structure : 

15 As Cast Structure Structure Depends on Composition Cooling Rates Metal Flow Casting Process The Structure Significantly Affects the Properties of the Casting

Cast Structure Pure Metals : 

16 Cast Structure Pure Metals Quick Chill Zone at the Wall of the Mold causes Equiaxed Grains As Cooling Continues Grains Oriented Favorably Grow Resulting in Columnar Grains Toward the Center Can Modify this Structure by Addition of Nucleation Agents Figure 5.8a

Cast Structures Solid Solution Alloys : 

17 Cast Structures Solid Solution Alloys Solute is Added to Enhance Properties in the Final Alloy Solidification will Occur over a range of Temperature as Described by the Phase Diagram Alloy Solidifies in a Dendrite or Tree Like Pattern Liquid will Flow into space between Dendrite Arms as They Shrink

Cast Structures : 

18 Cast Structures Difference between Tl and Ts is Called Freezing Range Short Freezing Range is More Castable Alloy Liquid is Less Likely to get Locked off by Partially Frozen Metal so can Fill Solidification Shrinkage Grain Size and Shape can be Controlled by Cooling Rate

Cast Structures : 

19 Cast Structures 102 K per Second Coarse Grains 104 K per Second Fine Grains 106 K per Second Amorphous As Grain Size Decreases Dendrite Spacing Decreases Microporosity in Between Dendrites is Reduced Tendency to Hot Crack is Reduced

Structure and Properties : 

20 Structure and Properties Cooled Slowly Dendrites are Uniform in Composition at Equilibrium Fast Cool Dendrite Composition Changes from Inside to Outside Referred to as Cored Dendrites Happens Because Time for Diffusion in the Dendrite is Insufficient for Equilibrium Composition of the Liquid is Typically Enriched in Solute

Segregation : 

21 Segregation Coring Results in Segregation or Local Variations in the Alloy Composition Macrosegregation can Result When Planar Solidification Front Rejects Solute of Lower Melt Point Towards Center of Casting Gravity Can Create Concentration Gradients Due to Density Differences

Segregation : 

22 Segregation Figure 5.12 Can Add a Nucleation Agent to Create Fine Equiaxed Grains Gravity Can Also Break-up Dendrites and Give a More Equiaxed Structure Vibration or Stirring can also Break up Dendrites

Impact of Metal Flow : 

23 Impact of Metal Flow Figure 5.14 Metal Must Fill Complete Mold Before Solidification Mold Design can Help Trap Inclusions and Oxides Need to Make Metal Available to Fill in for Metal Shrinkage on Solidification

Fluid Flow : 

24 Fluid Flow Governed by 2 Simple Theorems Bernoulli’s Principle Continuity Q = A1V1 = A2V2 This Implies that Sprues Must Taper to Keep Metal from Leaving the Walls of the Mold and Possible Entraining Gas

Flow : 

25 Flow Turbulence of Metal Flow Important Turbulent Flow Likely to Entrain Air and Cause Dross or Oxide Inclusion Formation Reynolds Number is Measure of Turbulence Re = vDr/h Re < 20000 Considered ok for Casting

Fluidity of Metal : 

26 Fluidity of Metal Fluidity is Ability of Metal to Fill a Casting Function Viscosity Surface Tension Inclusions Solidification Pattern Long Freezing Range Bad for Fluidity Dendrites Extend and Trap Liquid Slowing Flow

Fluidity and Casting Design : 

27 Fluidity and Casting Design Mold Design Mold Material and Surface Finish Metal Superheat Pouring Rate Heat Transfer Castability Generally Refers to Ease of Turning a Particular Molten Metal into a Casting

Casting Solidification : 

28 Casting Solidification Figure 5.15 Chvorinov’s Rule Solidification Time is Function of Casting Volume External Corners Will Freeze Faster

Defects In Castings : 

29 Defects In Castings Shrinkage Contraction of Liquid on Cooling Contraction Due to Phase Transformation Contraction of Solid On Cooling Largest Amount Occurs During Cooling of the Solidified Casting Shrinkage Can Result in Porosity of Cracking of Castings

Defects in Castings : 

30 Defects in Castings Projections Fins, Flash, Rough Surfaces Cavities Discontinuities Cracks Tears Defects of Surface Incomplete Casting Dimension Inclusions

Defects : 

31 Defects Porosity Shrinkage Can Be Controlled by Chills Can be Corrected by HIP Trapped Gas Results from Difference in Solubility of Gas in Molten and Solid Metal Can be Controlled by Degassing Molten Metal

Furnaces : 

32 Furnaces Electric Arc High Rate Melting Works with Scrap Can Hold Molten Metal Induction Metal is Melted by Eddy Currents Coils Typically Water Cooled Copper Electrical Load Matching is Required to as a Function of Metal Charge

Furnaces : 

33 Furnaces Crucible Fuel Gas or Fossil Fuel Fired Cupolas Refractory Lined Steel Vessels High Capital Cost Have High Melt Rates Levitation Melting Magnetic Materials Only Material is Suspended by a Magnetic Field No Contact with Vessel

Foundries : 

34 Foundries Foundries have 2 Separate Activities Mold Making Metal Melting Mold and Pattern Making are Traditionally Manual Skilled Operations Patterns Now Made using CAD and CAM Techniques Mold Making Can be Highly Automated

Foundary Safety : 

35 Foundary Safety Dangers Include Dusts Metal Fumes Molten Metal Furnace Fuels Water in Molds Metal Fluxes

Cast Irons : 

36 Cast Irons Gray Iron sometimes Called Flake Iron Can be Three Types Ferritic Graphite in Ferrite Matrix Pearlitic Graphite in Pearlite Matrix Martensitic Graphite in Martensite Low Ductility in Tension Good Vibration Damping due to the Graphite Shape Often Used for Machine Bases, Engine Blocks, Pipe

Cast Irons : 

37 Cast Irons Ductile Iron or Nodular Iron Graphite is Spheroidized by Additions Mg or Ce Ferritic Pearlitic Tempered Martensitic Ductile and Shock Resistant Used for Crankshafts, Pipe, and Machine Parts

Cast Irons : 

38 Cast Irons White Iron Very Hard Wear Resistant and Brittle Rapid Cooling of Gray Iron Carbon is Tied up in Fe3C Fracture Surface has a White Appearance Used for Rolling Mill Rolls and Liner for Abrasive Transport

Cast Irons : 

39 Cast Irons Malleable Iron Annealing White Iron in CO and CO2 Cementite Decomposes into Graphite and Iron Graphite Clusters Ductile, Strong and Shock Resistant Used for Railroad Equipment and Hardware

Cast Irons : 

40 Cast Irons Compacted Graphite Short thick Interconnected Flakes Machinablity is Good Used for Crankcases, Cylinder Heads, Brake Disks Cast Steels Hard to Cast Because of High Temperatures Properties more Isotropic Can be Welded but Must be Post Weld Heat Treated

Cast Irons : 

41 Cast Irons Stainless Steels Problems Similar to Cast Steel Have Long Freezing Ranges

Non Ferrous Alloys : 

42 Non Ferrous Alloys Aluminum Base Wide Range of Properties Fluidity Depends on Alloying and Inclusion Content Used for Architectural, Aerospace, Ground Transportation, and Electrical Copper Base Zinc Base Common Die Cast Alloys Good Fluidity and Medium Strength

Non Ferrous Alloys : 

43 Non Ferrous Alloys High Temperature Alloys Temperatures of 3000F to Cast Ti and Superalloys Special Techniques are Required for Items like Turbine Blades

Ingot Casting : 

44 Ingot Casting Ingot Large Block of Metal Strictly for Use in Further Processing Always has a Defined Start and Stop Point Steel forms CO in Solid as it Solidifies 3 Types of Ferrous Ingots Killed Semi-Killed Rimmed

Ingot Casting : 

45 Ingot Casting Killed Steel Fully Deoxidized O2 is Reacted with Aluminum in Molten State Free from Gas Porosity Has a Large Shrinkage Cavity Called a Pipe Semi-Killed Steel Partially Deoxidized Some Porosity Virtually No Pipe

Ingot Casting : 

46 Ingot Casting Rimmed Low C Content Gasses form Porosity near Outside or Rim of Ingot These are ok As Long as They Do not Break Surface Making Shaping and Treating of Steel

Continuous Casting : 

47 Continuous Casting No Defined Start and Stop Point Minimize Waste No Pipe Steel is Cast in a Strand and Cut to Length after Solidification Metal is Frozen in Mold by Water Stream Typically 0.5” Skin of Solid at Exit of Mold Strand is Typically 10” Thick

Expendable Mold Casting : 

48 Expendable Mold Casting Sand Casting Sand Packed Around a Pattern Which is Removed and Reused Gates and Runners Etc. are added Molten Metal Poured Sand is Broken Up and Casting Removed Sand is Recycled Types of Sand Natural Synthetic

Expendable Mold Casting : 

49 Expendable Mold Casting Synthetic Sand is Preferred because It can be Controlled Fine Sand Forms Good Surface Features but has Lower Gas Permeability Permeability is Important to Allow Gases and Steam to Escape Cores and Molds Must be Able to Collapse on Cooling to Avoid Tearing

Expendable Mold Casting : 

50 Expendable Mold Casting Sand is Conditioned before Use Mixed Uniformly with Binders to Hold It Together in the Mold Sand Molds Green Sand Most Common Metal is Poured While the Sand is Still Damp Lowest Cost Produces Recyclable Sand Modification Involves Curing the Surface to Improve Strength

Expendable Mold Casting : 

51 Expendable Mold Casting Cold Box Organic and In-organic Binders are Added to the Sand Binders React Chemically to Strengthen the Sand Better Dimensional Accuracy but Higher Cost Sand is Harder to Recycle No-Bake Molds Liquid Resin Mixed with Sand Reacts at Room Temperature to Bind the Mold

Expendable Mold Casting : 

52 Expendable Mold Casting Major Parts of a Mold Figure 5.14 Flask Surrounding Container for Mold Cope is top Half Drag is Bottom Half Seam is the Parting Line Additional Segments Called Cheeks Pouring Basin or Cup

Expendable Mold Casting : 

53 Expendable Mold Casting Sprue Downward Passage for Molten Metal Gate Passage to Control Metal Flow Rate Runner Passage to Deliver Molten Metal From Gate to Mold Cavity Riser Reservoir of Molten Metal to Supply Casting During Solidification Shrinking

Expendable Mold Casting : 

54 Expendable Mold Casting Cores Sand Inserts Used to Defined Internal Hollow Passages in a Casting Vents Passages to Allow Escape of Gasses Evolved During Casting

Expendable Mold Casting : 

55 Expendable Mold Casting Patterns Shape of the Casting Includes an Allowance for Shrinkage of the Metal During Solidification and Cooling Made From Wood Metal Plastics Rapid Prototyping

Expendable Mold Casting : 

56 Expendable Mold Casting Patterns are Coated for Protection Patterns are Coated to Release them From the Mold Surface Without Damage Patterns are Reused to Manufacture a large Number of Molds Pattern Material is Often Dictated by Size of Production Run

Expendable Mold Casting : 

57 Expendable Mold Casting Pattern Types 1 Piece Simple Shapes Low Volume 2 Piece One Piece for the Cope and One for the Drag Shapes Can be More Complex Match Plate Similar to 2 Piece but Both Halves are Mounted on a Plate Improves Productivity

Expendable Mold Casting : 

58 Expendable Mold Casting Cores Needed for Hollows Core Patterns Manufactured by the Inverse of the Pattern Process Sand is Compacted into a Core Box Supported on Projections Called Core Pins Cores are Made by Processes Similar to Molds

Expendable Mold Casting : 

59 Expendable Mold Casting Sand Molding Processes Simplest is Hand Ramming Sand is Compacted in Layers by Use of Pneumatic or Hydraulic Tampers Sand can be Compacted by Placing Layers and Jolting the Flask Sand Can be Slung at High Velocity into Mold Resulting in Uniform Compaction Can be Compacted by Impulse Loads (Explosions or Compressed Gas) Vacuum Molding Sand is Placed and Compacted by Applied Vacuum

Expendable Mold Casting : 

60 Expendable Mold Casting Casting Process Cope and Drag Assembled Assembly is Clamped Together or Weighted Down Metal Is Poured Carefully to Minimize Turbulence Casting Allowed To Solidify Sand is Shaken Off and Out and Casting is Inspected Riser, Runner, and Gate Designs are Critical to Minimize Turbulence and Get Consistent Complete Fills Riser Placement is Critical to Allow Shrinkage Filling

Expendable Mold Casting : 

61 Expendable Mold Casting Shell Molding Pattern is Always Metal Pattern is Heated and a Sand / Resin Mixture is Applied Evenly Resin is Heat Cured to Create a Uniform Shell Shells are Thin and Gas Porous Shells Have Very Smooth Surface Shell Walls Promote Good Uniform Metal Flow 2 Mold Halves Joined Together

Expendable Mold Casting : 

62 Expendable Mold Casting Thin Shell Walls Require Support Usually Steel Shot in a Container Can Produce Thinner Sections, Sharper Corners, and Smaller Features than Standard Sand Molds are More Expensive Can be Automated and Requires Fewer Secondary Operations

Expendable Mold Casting : 

63 Expendable Mold Casting Other Particulate Processes Composite Molds Molds Constructed of Different Materials Generally More Costly Can Improve Quality Can Reduce Secondary Operations Sodium Silicate Process Sand and Water Glass Binder Cured By CO2 Exposure Cores can be More Compliant and Reduce Hot Tearing Tendency Rammed Graphite Used Instead of sand for Reactive Metals Humidity Control Is Critical

Expendable Mold - Lost Foam : 

64 Expendable Mold - Lost Foam Also Called Expendable Pattern Casting, Evaporative Pattern, and Lost Pattern Casting Pattern is Made from Molded Polystyrene Polystyrene Pattern is Left in the Mold after Sand Packing Pattern Vaporizes on Contact with the Molten Metal Polystyrene is Molded in a Female Pattern

Expendable Mold - Lost Foam : 

65 Expendable Mold - Lost Foam Polystyrene is Expanded and Cured in the Metal Pattern Mold Complex Polystyrene Patterns can be Made by Gluing up Simpler to Mold Shapes Raw Pattern is Coated with a Refractory Slurry Sand is Compacted Into and Around the Pattern Molten Metal is Poured

Expendable Mold - Lost Foam : 

66 Expendable Mold - Lost Foam Polystyrene Breakdown Products and other Gasses vent through the Sand Metal Flow Velocity is Slower than in Standard Sand Casting and Can be Controlled Cooling Rates are Typically Higher due to Energy to Breakdown Polystyrene Fluidity is Less than in Sand Casting

Expendable Mold - Lost Foam : 

67 Expendable Mold - Lost Foam Advantages No Parting Line in the Mold Flasks are Simple Patterns are Cheap Excellent Surface Closer to Net Shape High Volume Process Easily Automated New Developments Incorporation of Reinforcement Fibers in Pattern to Create MMC Composites Incorporation of Grain Refiners in Pattern to Improve Structure as Cast

Expendable Mold - Plaster : 

68 Expendable Mold - Plaster Molds are Made from Plaster of Paris Molds Must be Dried Molds have Low Permeability Patterns Must be Metal or Plastic Wood Swells in Water and Iron Rusts Can not Cast Steel Plaster Would Break Down Low Heat Transfer Slow Cooling Less Warping

Expendable Mold - Ceramic Molds : 

69 Expendable Mold - Ceramic Molds Similar to Plaster Except Ceramic Replaces Plaster Due to Refractory can Cast Steels Good Surface Finish Good Dimensional Accuracy Process is Expensive

Expendable Mold - Investment Casting : 

70 Expendable Mold - Investment Casting Also Called Lost Wax Pattern is Molded Wax Wax Pattern is Coated By Dipping into a Refractory Slurry Allow to Dry and Repeat Until Appropriate Thickness is Achieved Heat to Cure Refractory and Melt Out the Wax

Expendable Mold - Investment Casting : 

71 Expendable Mold - Investment Casting Pour Metal Break Up the Refractory to Remove the Casting Produces Parts With Very Good Surface Dimensional Accuracy is Good Tendency for Alloy Being Cast to Pickup C from Residual Wax

Expendable Mold - Vacuum Casting : 

72 Expendable Mold - Vacuum Casting Porous Mold Is Made By any Technique Mold Gate Held Below Surface of Molten Metal Vacuum is Drawn on the Back Side of the Mold Metal is Drawn into the Mold Cavity by the Vacuum Molten Metal is Held at Close to the Liquidus Temperature

Permanent Mold Processes : 

73 Permanent Mold Processes Casting is Removed without Destroying the Mold Mold is Typically Metallic Gates, Risers, Etc. are an Integral Part of the Mold Cores are Still Required for Hollows Can be Sand or Metal Mold Release Agents are Requried

Permanent Mold Processes : 

74 Permanent Mold Processes Ejector Pins are Often Required Dies are Clamped Together Mechanically Dies Heated to Reduce Thermal Shock and Promote Metal Flow Molds Can be Force Cooled Steel Casting Requires Graphite Molds Good Surface Finish Uniform and Acceptable Properties

Permanent Mold Processes : 

75 Permanent Mold Processes High Production Rates are Possible High Cost for Mold Production Difficult to Cast Intricate Shapes

Slush Casting : 

76 Slush Casting Good for Open Hollow Parts No Cores are Required for The Hollows Mold is Filled Solidification is Allowed to Proceed until Desired Wall Thickness is Reached Excess Molten Metal is Poured Out Low Volume Production Not Very Accurate on Thickness

Pressure and Vacuum Casting : 

77 Pressure and Vacuum Casting Molten Metal Supplied From Mold Bottom Helps Fill Shrinkage Porosity Pressure or Vacuum is Maintained Until Casting is Solidified Vacuum will Evacuated any Gasses in Mold Advantage is Reduction of Metal Required by Eliminating Risers

Die Casting : 

78 Die Casting Pressure Ranges from 0.1 to 100 ksi Hot Chamber Process Metal is Injected from a Heated Injection Chamber Die Usually Cooled To Increase Cycle Time Cold Chamber Process Molten Metal is Injected from the Unheated Shot Cylinder Typically Higher Pressure Process

Die Casting : 

79 Die Casting Dies Single Cavity One Part Per Shot or Cycle Multi-Cavity Multiple Parts per Shot or Cycle Combination Cavity Several Different Parts Per Shot or Cycle Unit Dies Several Small Dies that can be Ganged Together

Die Casting : 

80 Die Casting Die Wear Will Increase with Increasing Operation Temperature Die can Last upwards of 500,000 shots Dies Can be Attacked Chemically by the Molten Metal Release Agents are Required Parts will Generally require Ejector Pins

Die Casting : 

81 Die Casting Can Mold in Fasteners, Collars, Etc. Insert Molding Die Castings have Better Properties and Thinner Walls Than Most Other Processes Large Die Castings With Intricate Features Require Large High Tonnage Presses Process is Only Economical for High Volumes

Centrifugal Casting : 

82 Centrifugal Casting Figure 5.33 True Centrifugal Casting has no Inner Mandrel Semi-Centrifugal Casting Uses Centrifugal Force to Distribute Metal Outward Centrifuging Metal is Thrown into Peripheral Dies

Squeeze Casting : 

83 Squeeze Casting Mold Is Closed on Molten Metal and High Pressure is Maintained Until Solidified Keeps Porosity Low Excellent Heat Transfer due to Metal Mold and High Pressure Gasses are Forced to Stay in Solution by the Pressure and Allowed to Diffuse Out Later

Semi-Solid Casting : 

84 Semi-Solid Casting Also Called Semi-solid Forging, Semi Solid Forming, Rheocasting, Thixocasting, Thixoforming Metal Viscosity Decreases as Deformation Rate Increases Feedstock is Hard to Make Molten Material is Stirred to Break up Dendrites and Cause them to Spheroidize

Single Crystal Casting : 

85 Single Crystal Casting Highly Specialized Process Desirable Because Grain Boundaries are Weak Areas Directional Solidification can Help by Eliminating Transverse Grain Boundaries Figure 5.36

Rapid Solidification : 

86 Rapid Solidification By Cooling Fast no Time For Atoms to Arrange in Crystal Easiest to do with Eutectic Alloys Good Corrosion Resistance Strength Approaches Theoretical Low Magnetic Hysteresis Excellent For Transformer Cores Will Revert to Crystal if Temperature Gets too High

Heat Treating : 

87 Heat Treating Heat Treating is Controlled by TTT (Time-Temperature-Transformation) Diagrams Figure 5.39 Pearlite Alternating Plates of Ferrite and Cementite

Heat Treating : 

88 Heat Treating Spheroidite Pearlite Held at Elevated Temperature for a Long Time Cementite becomes Spherical Strucure is Tougher and More Ductile Bainite Fine Distribution of Cementite in Ferrite Requires Higher Cooling Rates Stronger and More Ductile Than Pearlite

Heat Treating : 

89 Heat Treating Martensite FCC Rectangular Prism Created By very Rapid Cooling From Austenite Extremely Hard and Brittle Must be Further Heat Treated or Tempered to be Usable If Temperature of Quench is too High some Austenite is not Transformed Retained Austenite Causes Dimensional Problems

Heat Treating : 

90 Heat Treating Quenching Media Agitated Brine Still Water Still Oil Cold Gas Still Air

Heat Treating Non-Ferrous : 

91 Heat Treating Non-Ferrous Heat Treatment is Typically by Different Mechanisms than Steels Strength is Derived from Precipitation of Very Small Uniform Distribution of a Second Phase

Heat Treating Non-Ferrous : 

92 Heat Treating Non-Ferrous Steps to Solution Heat Treatment Hold Metal Above the Temperature Required for Total Solute Dissolution Quench Rapidly to Lower Temperature Age for Strength Natural Aging Strength Increase by Holding at Room Temperture

Heat Treating Non-Ferrous : 

93 Heat Treating Non-Ferrous Artificial Aging Increase in Strength Due to Short Time Exposure to Elevated Temperature (Much Lower than SHT) If Hold Too Long or at too High a Temperature can get Over Aging Precipitates get Large and Become Ineffective at Blocking Dislocation Motion

Surface Hardening : 

94 Surface Hardening Case Hardening Increase in Strength Confined to a Surface Layer Has the Advantage that Cracks can be Stopped in the Lower Strength Tougher Bulk Done by Adding C, N, or another Small Element to a Surface Layer If C Content is High Enough Can Flame or Induction Harden the Surface

Surface Hardening : 

95 Surface Hardening If Atmosphere During Heat Treatment is Oxidizing the Surface Layer can be De-Carburized Reducing Strength Decarburization Lowers Fatigue Life by Lowering Endurance Limit

Annealing : 

96 Annealing Thermal Process to Remove Effects of Cold Work or Precipitation Hardening Process Heat Soak Cool Slowly Recrystallization often Accompanies Annealing

Annealing : 

97 Annealing Ferrous Full Annealing Coarse Pearlite Structure Normalizing Cooling in Still Air Fine Pearlite Process Annealing Partial Anneal to Improve Ductility Without Recrystallization or Increasing Grain Size Stress Relief Anneal Hold Below Austenitizing Temperature to Improve Dimensional Stability

Tempering : 

98 Tempering Reduces Brittleness Increases Ductility and Toughness Temperatures and Times are Alloy Dependent Can get Embrittlement if Wrong Temperature is Used Due to Segregation of Impurities to Grain Boundaries Austempering Quench from Austenite Region to Intermediate Temperature Structure Becomes Bainite

Tempering : 

99 Tempering Martempering Part is Quenched into Oil Until Isothermal Slow Cooled In Still Air Reheated to Intermediate Temperature to Temper the Martensite and Improve Toughness

Design for Heat Treating : 

100 Design for Heat Treating Complex Parts Tend to Warp and Distort Especially on Quench Media and Temperatures Must be Carefully Chosen and Controlled Non-Uniform Quench Gives Distortion and Residual Stresses Thickness Need to Be Uniform Sharp Corners Should be Avoided Thin Cross-sections can Warp

Casting Quality : 

101 Casting Quality Castings Will Have Defects Core Shifts No Fills Trapped Sand Porosity Large Complex Castings Can be “Upgraded” or Repaired Large Iron Castings are Repaired with Thermite

Casting Design : 

102 Casting Design Riser Placement and Design is Based on Experience and Experiment is Being Guided By CFD With CFD can Get Insight into Solidification Pattern and Riser Placement and Effectiveness

Casting Design : 

103 Casting Design Figure 5.42 Flats Should be Avoided as They Warp Shrinkage Allowance Must be Provided Parting Lines Should Be Planar Parting Lines Should Be Chosen To Minimize Cores

Casting Design : 

104 Casting Design Parting Lines Should be Kept Off Flat Surfaces so Flash will be Less Objectionable Draft is Required so Molds or Castings can Be Removed Easily without Damaging the Mold Tolerances are Cumulative Between Different Mold Components Allowances for Machining must be Allowed on Critical Surfaces