Types of Cast Iron Problems & Welding Procedures by Mr. Mohan at Trinity NDT

Types of Cast Iron Problems & Welding Procedures

  • Machine Beds 
  • Automotive Engine Cylinder Blocks  
  • Crank Shafts
  • Forming Dies 
  • Impellar Blades  
  • Rollers  
  • Pump, Turbine, Compressor Casings

Characteristics of Cast Iron

  • Carbon Range  –  1.7 – 4.5 %
  • Castings – Intricate Shape  & Size
  • Damping  Capacity
  • Excellent  Wear Resistance
  • Self  Lubricating  Properties
  • Poor Elongation & Low Ductility
  • Graphite Formation (Si, Al, Ti, Cu)

Types Of Cast Iron

  • White Cast Iron
  • Gray Cast Iron
  • Malleable Cast Iron
  • Ductile Cast Iron (Sg Iron)
  • Ni Hard Cast Iron
  • Ni Resist Cast Iron
  • Mehanite Cast Iron

Chemical Composition of Gray  Cast Iron ASTM A48, A159

  • Carbon – 2. 50 – 4.00
  • Manganese    –  0. 25 – 1.00
  • Phosphorus – 0. 05 – 1.00
  • Sulphur – 0.02 – 0.25
  • Silicon – 1.00 – 3.00

Alloying Elements

  • Manganese         
  • Magnesium
  • Cerium
  • Chromium
  • Nickel                    
  • Titanium
  • Copper
  • Silicon & Aluminium – To Promote Graphite Formation

Effects of elements on Cast iron

  • Phosphorous: Solution Harden, Corrosion Resistance, Better Machining
  • Silicon: Removes Oxygen, Increases Oxidation Resistance
  • Titanium: Forms Carbides, Age Hardening Possible
  • Chromium: Corrosion Resistance, Does Not Promote Graphite Formation
  • Nickel : Used With Cr,  Low Temperature Strength, Toughness
  • Manganese:  along with S forms as MnS, improves machining
  • Molybdenum: MoC provide abrasion resistance, improves corrosion resistance, improves hot strength
  • Tungsten: Similar to Mo, delays tempering to higher temperatures

Microstructure of Cast Iron

  • Ferrite – BCC, low carbon solubility
  • Graphite – Most stable form of carbon in iron, more carbon means more Graphite  
  • Pearlite – Mixture of ferrite and cementite, more carbon means less Pearlite
  • Austenite – FCC
  • Cementite – Iron carbide with good toughness
  • Martensite – Rapidly cooled by quenching
  • Retained Austenite

Welding Procedures 

Weldable or non weldable and Repair weld

  • Skip weld technique
  • Preheating
  • Inter pass Temperature
  • Suitable electrode selection
  • Machineable or Non machineable
  • Depositing short weld length
  • Use stringer beads
  • Use very low heat input
  • Buttering on crack edges Steel filler is not recommended for machineable repair weld deposits

Welding Precautions

Cracking tendency is very high

Higher Preheat is applied for

  • Allowing more time for H2 escape
  • Reduces the cooling rate
  • Reduction of hardness in HAZ
  • Avoid the risk of martensite formation

Cast Iron Welding Precautions

PWHT / Stress Relieving

     Furnace cooling    Reduce residual stresses

          – Block welding

          – Depositing short weld lengths

Selection of filler metal for Cast Iron Welding

  • Based on strength
    •  Hardness
    •  Abrasion resistance
  • Special properties

      – ductility

     –  impact strength

     –  wear resistance

Weld metal / Electrode for Welding Cast iron

  • Steel, Stainless steels
  • Nickel base alloys, Cast iron
  •  AWS A5.15 Standard 
  • The mechanical properties of weld metal are affected by
    • Weld metal Chemistry (C,Ni, Si)
    • Thermal cycle experienced (CR)
  • Nickel Iron Electrodes: Nickel – Iron electrodes compared to Nickel are of Lower cost and Stronger & More Ductile

Advantages of Nickel filler for Welds

  • Low risk of cracking
  • High ductile weld
  • Minimum contraction stresses
  • Minimum Hardness

Trinity NDT – Centre of Excellence Provides Nondestructive testing-NDT lab services, WPS services, welder qualification, welders training and certification, Consultation on welding. Contact or visit: Trinity NDT – Welding Website

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