Research on Hot Cracking Problems in Welding of Stainless Steel and Heat-Resistant Alloys

Stainless steel and heat-resistant, corrosion-resistant alloys are widely used in industries such as petroleum, kimia, aviation, aeroangkasa, nuclear energy, metallurgy, electricity, pengangkutan, textile, and electronics. These materials are prized for their anti-rust, tahan kakisan, oxidation-resistant, and high-temperature-resistant properties. Namun begitu, welding these materials presents significant challenges, particularly with hot cracking.

Types of Hot Cracks

Hot cracks in welding typically fall into three categories:

1. Solidification Cracks:
   • Occur in the weld metal.
   • Form in high-temperature areas where liquid and solid phases coexist.
2. Liquefaction Cracks:
   • Occur in the heat-affected zone or the reheating zone of multi-layer welds.
   • Also form in high-temperature areas with liquid and solid phases under thermal strain.
3. Low-Plasticity Cracks:
   • Occur at lower temperatures.
   • Appear in the coarse-grained area of the heat-affected zone and are not related to the liquid phase.

Sensitivity to Solidification Cracks

In stainless steel welding, solidification cracks are a major concern. The sensitivity of these cracks depends on the delta ferrite content. Cracks are least likely to occur when ferrite content is between 5% dan 20%. Namun begitu, when the content exceeds 40%, crack sensitivity increases significantly. To prevent solidification cracks, the chemical composition of the weld metal can be adjusted to maintain ferrite content within a few percent.

Nickel-Based Alloys

Nickel-based alloys, such as Connickel 625 atau 718, have a broader brittle temperature range compared to Hastelloy X or C-276, making them more sensitive to solidification cracks. These alloys contain more Nb, which forms NbC, gamma phase (Ni3Nb), and Laves phase. During welding, γ/NbC or γ/Laves phase eutectics form in the final solidification area, leading to solidification cracks due to their low melting points. Elements like C and Si also increase crack sensitivity. Selain itu, undissolved NbC in the parent material can lead to liquefaction cracks at the γ/NbC interface.

Factors Influencing Hot Cracks

The occurrence of hot cracks in stainless steel and heat-resistant, corrosion-resistant alloys is influenced by several factors:

• Alloy Composition and Structure:
  • The material’s inherent alloy composition and structure are significant factors.
• Inclusions and Solute Elements:
  • Elements such as P, S, Pb, Sn, and Zn create low-melting-point substances that cover grain boundaries, causing cracks under small thermal strains.
• Ferrite Content:
  • For austenite-ferrite stainless steel welds, ferrite content significantly affects crack sensitivity.
• Impurities:
  • In pure austenite stainless steel welds, lower P+S content reduces sensitivity to hot cracks.
• Eutectics Formation:
  • In nickel-based alloys, elements like Nb, C, and Si form low-melting-point eutectics, increasing crack sensitivity. Impurities like P and S at grain boundaries also contribute to hot cracks.

Secara ringkasnya, hot cracking in welding stainless steel and heat-resistant, corrosion-resistant alloys is influenced by alloy composition, structural factors, inclusions, solute elements, and impurities. Controlling these factors can help mitigate the risk of hot cracks.