Blog / Cast Iron
26 July 2024 | Cast Iron

Effective Brazing Cast Iron: Wetting Ability and Surface Preparation Techniques

Brazing cast iron, especially white and gray iron, can be challenging due to their reduced plasticity. Brazing, however, exposes the metal to significantly lower temperatures compared to welding, minimizing the risk of structural transformation. By using filler metals with a melting point lower than the pearlite-austenite critical transformation temperature of cast iron (700-730°C / 1300-1350°F), the base metal's integrity is preserved.

Factors Affecting Cast Iron Brazing

Surface Condition and Carbon Form

The effectiveness of brazing depends on the cast iron type, carbon form, surface preparation, and the choice of filler metal. Cast iron pieces typically have an oxide layer, and sand particles may be embedded in the surface of sand-cast components. Additionally, graphite, being non-metallic, resists wetting by molten metals, especially when present as flakes or elongated particles.

Types of Cast Iron and Wetting Ability

  1. Ductile Iron (Spheroidal Graphite Iron) :
    Ductile iron is more brazeable due to its spherical graphite form, created through the addition of magnesium (Mg) or cerium (Ce) during casting. This shape minimizes the graphite surface area and enhances wetting.
  2. Malleable Iron (White Core and Black Core) :
    • White Core Malleable Iron: Derived from white cast iron via a decarburizing anneal at 900°C, it has a carbon-free surface, resembling low-carbon steel. This increases its wetting ability.
    • Black Core Malleable Iron: Also annealed at high temperatures, the graphite forms irregular nodules. Its wetting ability is moderate due to the larger graphite surface area.
  3. Gray Cast Iron (Lamellar Graphite Iron) :
    Graphite appears as flakes, leading to extensive surface exposure and poor wetting. This type is prone to thermal cracking and exhibits low ductility.
  4. White Cast Iron :
    White cast iron has all its carbon in carbide form, making it prone to thermal cracking. Its surface can be wetted easily if low-melting brazing alloys with high fluidity are used.

Brazing Process Steps

  1. Surface Cleaning
    • Grind off oxide layers and sand residues.
    • Degrease to remove oil and grease, as these can worsen wetting issues.
  2. Surface Decarbonization
    • Thermal Method: Use an oxidizing oxyacetylene flame.
    • Mechanical Method: Blast with steel or glass shot to remove graphite.
    • Chemical Method: Immerse in a heated salt solution (KNO3 and NaNO3) to dissolve graphite.
  3. Surface Roughening : Shot blasting with angular particles creates rough surfaces, enhancing capillary action and bond strength.
  4. Assembly and Gap Adjustment : Maintain a tight gap (0.015” max) to improve joint strength.
  5. Uniform Heating
    • Use a furnace for even heating, especially for large parts, or a wide-flame propane torch for smaller pieces.
    • Ensure temperatures stay below 700-730°C to prevent unwanted structural changes.

Selecting Filler Metals

Silver-rich, low-melting-point alloys are ideal for brazing cast iron without altering its microstructure. Alloys with a narrow melting range improve fluidity and reduce the risk of overheating. Sodel 472 and Sodel 477
are recommended for various applications, as shown in the table below.

Sodel
Sodel

Figure 1 Wetting aptitude of cast irons

For personalized guidance on brazing various types of cast iron and selecting the most suitable filler metals for your applications, don’t hesitate to contact us. Our experts are here to help you choose the most effective solutions to preserve the integrity of your cast iron parts.