Effect of Welding Current on Microstructure and Cracking of Plasma Cladding H13 Steel

Literature Overview and Research Background

H13 (equivalent to 4Cr5MoSiV1 in Chinese standards) is a hot work die steel renowned for its excellent hot hardness, wear resistance, and thermal fatigue performance. It is extensively used in hot forging dies, extrusion dies, and continuous casting nozzles. When H13 is applied as a cladding layer on carbon steel or low-alloy steel substrates, it provides a wear-resistant and thermally stable surface while utilizing the structural strength of the base material.

Plasma transferred arc (PTA) cladding is the preferred method for depositing H13 layers due to its high deposition efficiency and relatively low dilution compared to other arc welding processes. However, H13 is highly susceptible to cracking during cladding due to its high carbon and alloy content, which creates a wide solidification range and promotes both hot cracking and cold cracking. This study investigates the influence of welding current on the microstructure, cracking tendency, and mechanical properties of H13 plasma cladding layers.

Core Technical Points and Microstructural Analysis

Solidification Behavior and Microstructure

H13 steel solidifies through a complex sequence involving austenite, ledeburite, and martensite formation. The microstructure of the as-cladded layer is predominantly martensitic with retained austenite and carbide precipitates (MC, M7C3, and M23C6 types). The morphology and distribution of these carbides are strongly influenced by cooling rate, which is directly related to welding current.

At low welding currents (100-130 A), the cooling rate is high, resulting in fine martensite with dispersed carbides. The retained austenite content is relatively low (5-8%), and the microstructure is characterized by acicular martensite with fine carbide particles.

At medium currents (150-180 A), the cooling rate decreases, leading to coarser martensite with larger carbide aggregates. Retained austenite increases to 10-15%, and the microstructure shows a mix of acicular and plate-like martensite.

At high currents (200-250 A), the cooling rate is low, resulting in coarse martensite with significant carbide coarsening. Retained austenite reaches 18-25%, and the microstructure exhibits blocky austenite and coarse carbide networks.

Cracking Mechanisms

The study identifies two primary cracking modes in H13 plasma cladding layers:

1. Hot Cracking (Solidification Cracking): Occurs during solidification due to the wide solidification range of H13 and the restraint from the substrate. Liquid films at dendrite boundaries are susceptible to cracking under tensile stresses. Hot cracks are typically intergranular and appear as straight or slightly curved cracks along the dendrite boundaries.
2. Cold Cracking (Hydrogen-Induced Cracking): Occurs after solidification, typically within the first few hours, due to hydrogen diffusion and the high hardness of the martensitic structure. Cold cracks are transgranular and appear as short, branched cracks in the HAZ or at the cladding-substrate interface.

Effect of Welding Current on Cracking

The data reveals a non-monotonic relationship between welding current and cracking. At low currents, hot cracking is prevalent due to high thermal gradients. At medium currents, cold cracking dominates due to the high hardness and hydrogen susceptibility of the martensitic structure. At very high currents, both types of cracking increase due to excessive dilution and thermal distortion.

Optimal Process Parameters and Crack Mitigation

Recommended Process Window

Based on the study findings, the optimal welding current for H13 plasma cladding is 150-170 A, which provides the lowest cracking index while maintaining adequate hardness. The following process parameters are recommended:

Crack Mitigation Strategies

The study proposes several strategies to minimize cracking in H13 plasma cladding:

1. Substrate Preheating: Preheating to 200-300 °C reduces thermal gradients and decreases the cooling rate, which helps prevent hot cracking. However, excessive preheating can promote carbide coarsening and reduce hardness.
2. Multi-Layer Deposition: Depositing multiple thin layers (1-2 mm each) with controlled interpass temperatures allows for stress relief between layers and reduces the overall thermal gradient.
3. Post-Weld Heat Treatment: Stress relief annealing at 650-700 °C for 2-4 hours reduces residual stresses and transforms retained austenite to martensite, improving dimensional stability. However, this must be followed by re-hardening to restore the martensitic structure.
4. Hydrogen Control: Using low-hydrogen flux, dry electrode storage, and proper gas shielding minimizes hydrogen absorption, reducing cold cracking susceptibility.
5. Substrate Selection: Using a low-carbon, low-alloy steel substrate with good weldability reduces the overall cracking tendency of the cladding system.

Engineering Practice Implications

For hot work die repair and cladding applications, this study provides critical guidance:

• H13 cladding on carbon steel substrates requires careful control of welding current to balance hardness and crack resistance
• Preheating and controlled interpass temperatures are essential for multi-layer cladding
• Post-weld stress relief is mandatory to prevent delayed cracking during service
• The cladding-substrate interface is the most critical region for crack initiation and must be inspected thoroughly using magnetic particle testing (MT) or dye penetrant testing (PT)
• Dilution should be monitored through hardness profiling and metallographic examination of the interface

The study also emphasizes the importance of substrate preparation. Surface roughness, contamination, and oxide layers can significantly affect the metallurgical bond and crack initiation at the interface. Mechanical grinding or shot peening is recommended before cladding.

Key Questions and Reflections

A significant question that arises is the long-term thermal fatigue performance of H13 cladding layers under cyclic thermal loading. While the as-deposited microstructure shows good crack resistance at optimal currents, repeated thermal cycling can cause retained austenite transformation, carbide coarsening, and microcrack initiation. The thermal fatigue life of H13 cladding under conditions typical of hot forging dies (500-800 °C cyclic loading) warrants further investigation.

Another reflection is the role of retained austenite in the cracking behavior. While retained austenite can act as a crack arrestor due to its lower modulus, excessive retained austenite (above 20%) can lead to dimensional instability and delayed cracking during cooling. The optimal retained austenite content for H13 cladding appears to be in the range of 10-15%, which provides a balance between toughness and dimensional stability.

Summary and Study Insights

This literature establishes that welding current is a critical parameter in H13 plasma cladding, directly influencing microstructure, cracking tendency, and mechanical properties. The optimal current range of 150-170 A provides the lowest cracking index while maintaining adequate hardness for hot work die applications. However, the cracking behavior of H13 cladding is complex, involving both hot and cold cracking mechanisms that are influenced by thermal gradients, cooling rates, and hydrogen content. For engineers designing cladding processes for hot work dies, the key insight is that welding current must be selected not only to achieve desired hardness but also to minimize cracking susceptibility through careful control of thermal input and post-weld heat treatment. The interplay between microstructure, residual stresses, and thermal history demands a holistic approach to process design that considers both as-deposited and in-service performance.