A102D Welding Rod Root Pass to Solve Alloy Overlay Cracking

Literature Overview

This technical paper, published in Welding (1997) by Wang Hengxian and Pan Limin from Dalian Rubber and Plastic Machinery Factory, addresses a persistent and costly problem in alloy overlay welding: hot cracking and cold cracking in the root pass and subsequent overlay layers. The authors propose the use of a specific nickel-iron austenitic welding electrode, designated A102D (equivalent to AWS A5.4 E309L or a low-carbon variant), as the root pass to eliminate cracking in alloy overlay deposits on carbon steel substrates. This is a highly practical contribution to the overlay welding technology community, as cracking remains one of the most common and frustrating defects in field overlay applications.

Cracking Mechanisms in Alloy Overlay Welding

Hot Cracking

Hot cracking in alloy overlay welding occurs during solidification or in the high-temperature range (above 600 °C) and is primarily caused by the following factors:

• Sulfur and phosphorus segregation: Sulfur forms low-melting FeS phases at grain boundaries, and phosphorus promotes grain boundary embrittlement. In high-carbon or high-alloy overlay consumables, these elements tend to segregate to the last-solidifying interdendritic regions, creating crack-prone zones.
• Restricted solidification cracking: The high thermal conductivity of the carbon steel substrate rapidly draws heat away from the weld pool, causing rapid solidification. This restricts the ability of liquid metal to feed shrinkage cavities, leading to interdendritic cracking.
• Columnar grain structure: The steep thermal gradient at the weld pool boundary promotes columnar grain growth, which provides continuous paths for crack propagation.

Cold Cracking

Cold cracking, or hydrogen-induced cracking, occurs at lower temperatures (below 200 °C) and is driven by:

• Diffusible hydrogen: Hydrogen from moisture in the flux or electrode coating diffuses into the weld metal and accumulates at the heat-affected zone (HAZ) of the carbon steel substrate, where it is most susceptible due to the presence of hard martensitic microstructures.
• High carbon equivalent: The carbon steel substrate (e.g., Q345 or 16Mn) may have a carbon equivalent (CE) above 0.4%, making the HAZ susceptible to hydrogen embrittlement.
• High residual stress: The thermal contraction of the overlay deposit generates high tensile residual stresses at the weld root, which, combined with hydrogen, initiate and propagate cracks.

The A102D Root Pass Strategy

Material Properties of A102D

The A102D electrode is a low-carbon nickel-iron austenitic welding consumable with the following approximate composition:

The key features of A102D are:

• Low carbon content (≤ 0.04 wt.%): This eliminates the risk of carbide precipitation at grain boundaries, which is a major contributor to hot cracking in austenitic welds.
• High nickel content (28–34 wt.%): Nickel stabilizes the austenitic structure, provides ductility, and accommodates thermal strain without cracking.
• Austenitic structure: The fully austenitic weld metal has a low thermal expansion coefficient mismatch with the carbon steel substrate and does not undergo martensitic transformation, which eliminates the risk of cold cracking due to hydrogen embrittlement.
• Low diffusible hydrogen: The low-carbon austenitic structure has a lower hydrogen solubility and diffusivity compared to martensitic or ferritic structures, reducing the risk of hydrogen-induced cracking.

Process Design

The A102D root pass strategy involves the following sequence:

1. Surface preparation: The joint is beveled to a 60–75° V-groove and cleaned to bare metal. Any rust, scale, or oil is removed.
2. Preheating: The base metal is preheated to 150–250 °C to reduce the cooling rate and minimize the formation of hard martensite in the HAZ.
3. Root pass with A102D: A single pass of A102D electrode is deposited in the groove, ensuring complete root fusion and a sound metallurgical bond. The deposition rate is kept moderate (60–80 mm/min) to avoid excessive dilution.
4. Intermediate pass (optional): If the groove is deep, a second pass of A102D or a similar nickel-iron alloy may be deposited to fill the groove partially.
5. Overlay passes with alloy consumable: The target alloy overlay (e.g., high-carbon martensitic stainless steel, cobalt-based alloy, or high-chromium cast iron) is deposited in subsequent passes. The dilution from the A102D root pass is now much lower than if the alloy consumable were used directly on the carbon steel substrate.
6. Post-weld heat treatment: A stress relief treatment at 600–650 °C for 1–2 hours is recommended to reduce residual stresses.

Effectiveness of the Strategy

The use of A102D as the root pass addresses both hot and cold cracking simultaneously:

• Hot cracking mitigation: The low-carbon austenitic root pass eliminates the risk of interdendritic cracking due to sulfur and phosphorus segregation. The high ductility of the austenitic weld metal accommodates solidification shrinkage without cracking.
• Cold cracking mitigation: The austenitic structure of the root pass does not undergo martensitic transformation, eliminating the hard and crack-prone microstructure that is susceptible to hydrogen embrittlement. The low carbon content also reduces the carbon equivalent of the weld metal.
• Dilution control: The A102D root pass acts as a metallurgical buffer between the carbon steel substrate and the high-alloy overlay. The subsequent overlay passes experience significantly lower dilution from the substrate, ensuring that the overlay composition and properties are closer to the target specification.

Comparative Analysis

Engineering Application Case

The A102D root pass strategy was successfully applied to the overlay repair of rubber mixing mill rolls at the Dalian Rubber and Plastic Machinery Factory. The rolls, made of Q345 steel, were overlaid with a high-carbon martensitic stainless steel (modified 410) to improve abrasion resistance against the rubber compound. Without the A102D root pass, the initial attempts resulted in hot cracks in 30–40% of the welds. After implementing the A102D root pass strategy, the crack rate dropped to less than 2%, and the overlay hardness achieved the target value of 48–52 HRC.

Key Technical Insights

The success of the A102D root pass strategy can be attributed to three fundamental metallurgical principles:

1. Structure control: By depositing a fully austenitic root pass, the metallurgist eliminates the formation of hard, brittle, and crack-prone microstructures at the critical weld root interface.
2. Composition buffering: The nickel-iron alloy root pass acts as a composition buffer, reducing the dilution effect of the carbon steel substrate on the subsequent alloy overlay passes.
3. Hydrogen management: The austenitic structure has lower hydrogen diffusivity and solubility, reducing the risk of hydrogen-induced cracking in the root region.

From a process engineering perspective, the strategy also demonstrates the power of a systematic approach to welding problem-solving. Rather than trying to modify the base metal or the overlay consumable, the solution introduces a third material—the A102D root pass—that addresses the root cause of the problem at the interface. This is analogous to the concept of a "transition layer" in brazing and soldering, where a third material is used to bridge the metallurgical gap between two dissimilar metals.

Summary and Study Insights

This paper presents a simple yet highly effective solution to the cracking problem in alloy overlay welding: using a low-carbon nickel-iron austenitic electrode (A102D) as the root pass. The strategy simultaneously addresses hot cracking, cold cracking, and dilution issues by providing a ductile, crack-resistant, and compositionally buffered interface between the carbon steel substrate and the high-alloy overlay. The engineering case study demonstrates a dramatic reduction in crack rate from 30–40% to less than 2%, validating the effectiveness of the approach. For practitioners in the overlay welding field, this paper reinforces the importance of interface engineering in dissimilar metal welding and demonstrates that sometimes the most elegant solution is not to change the consumable but to change the sequence.