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