Effect of Welding Current on Microstructure and Cracking in Plasma Surfacing of H13 Steel

Literature Overview

This study by Li Zhongcheng and colleagues from Kunming University of Science and Technology investigates the application of plasma arc surfacing (PAS) technology to deposit H13 hot work die steel powder onto a 5CrNiMo substrate. The research addresses a practical industrial pain point: the low efficiency and high labor cost associated with traditional GMAW surfacing for hot work die repair. The work was supported by a regional science fund project (51964030) and published in Applied Laser (Vol. 43, Issue 3, 2023, pp. 33–41). The central research question is straightforward yet critical for production: how does welding current influence penetration depth, weld width, microstructure evolution, and crack susceptibility in multi-layer H13 plasma surfacing deposits?

Core Technical Findings

The study systematically varied welding current and recorded the resulting geometric, metallurgical, and integrity characteristics of the surfacing layers. The key findings can be summarized as follows:

Interpretation of Microstructural Mechanisms

The microstructure of the H13 surfacing layer is dominated by martensite and retained austenite, which is consistent with the high carbon equivalent and high hardenability of H13 steel (approximately 1.2 wt% C, 5 wt% Cr, 1.5 wt% Mo, 1.5 wt% Ni). The plasma arc provides a highly concentrated heat source with a deep, narrow weld profile compared to GMAW. As welding current increases, the heat input per unit length rises, leading to higher peak temperatures and slower cooling rates in the weld center. This promotes the coarsening of martensite dendrites in the mid-section of the surfacing layer. The authors observed that at higher currents, the dendrite arm spacing increases noticeably, which has direct implications for toughness and crack resistance.

The interlayer interface phenomenon is particularly instructive. In multi-layer surfacing, the layer deposited above an interlayer interface contains more retained austenite than the layer below it. This is attributed to the thermal history: the underlying layer acts as a heat sink during the deposition of the upper layer, creating a temperature gradient across the interface. The upper portion of the new layer experiences a slower cooling rate due to the thermal mass of the previously deposited material, which stabilizes austenite and reduces the martensite start temperature threshold. This retained austenite, while potentially beneficial for toughness, may also be detrimental if it transforms during subsequent heat treatment or service, leading to volume expansion and additional residual stress.

Crack Formation Mechanism Analysis

The study identifies two primary crack formation mechanisms in the H13 surfacing deposits:

1. Stress-induced cracks: These are thermal-mechanical cracks caused by the high residual stresses developed during rapid solidification and cooling. H13 steel has a high carbon equivalent, which promotes martensitic transformation with associated volume expansion. The combination of thermal contraction and transformation strain generates tensile stresses that can exceed the local tensile strength, particularly at the weld toe and interlayer interfaces.
2. Defect-induced cracks: These cracks originate from metallurgical discontinuities such as inclusions, porosity, or unmelted powder particles. In plasma surfacing with powder feed, incomplete melting of individual powder particles can create weak points in the deposit. The thermal cycling during multi-layer deposition can then propagate cracks from these initiation sites.

The optimal current of 150 A represents a balance: sufficient heat input to ensure complete powder melting and good wetting, but not so much that excessive dilution, overheating, and thermal stress accumulation degrade the deposit integrity.

Engineering Practice Implications

For production engineers working on hot work die repair and remanufacturing, this study provides actionable guidance:

• Current selection: 150 A is identified as the sweet spot for H13 plasma surfacing on 5CrNiMo substrates. Deviation from this value, particularly in the higher direction, increases crack susceptibility.
• Multi-layer strategy: The interlayer interface is a critical zone. Engineers should consider reducing the number of layers where possible, or applying a controlled interpass temperature to minimize the thermal gradient across interfaces.
• Powder feed rate coordination: While not explicitly varied in this study, the powder feed rate must be synchronized with current to maintain a consistent dilution ratio and avoid unmelted powder inclusions.
• Post-deposition treatment: Given the high retained austenite content at interlayer interfaces, a controlled tempering or stress relief treatment may be necessary to stabilize the microstructure before the component enters service.

Key Questions and Reflections

Several questions arise from this study that deserve further investigation. First, the study does not report on the effect of scanning speed or powder feed rate independently, which are equally important process variables in plasma surfacing. Second, the mechanical properties (hardness, impact toughness, fatigue strength) of the surfacing layer at different currents are not discussed in the abstract, which limits the practical applicability of the crack-count-based recommendation. Third, the substrate preheating temperature is not mentioned, which is a well-known lever for reducing thermal stress in high-carbon steel surfacing. Future work should adopt a full factorial or response surface methodology to decouple the effects of current, scanning speed, powder feed rate, and preheat temperature.

Study Insights and Implications

This study demonstrates that plasma arc surfacing is a viable and efficient alternative to GMAW for H13 hot work die repair, with the potential for significant productivity gains. The identification of 150 A as the optimal current provides a clear starting point for process development. However, the crack sensitivity of H13 surfacing deposits remains a significant challenge, and the interlayer interface phenomenon highlights the importance of multi-layer deposition strategy in surfacing applications. For engineers in the die and mold industry, the key takeaway is that plasma surfacing can deliver high-quality H13 deposits, but only within a carefully controlled process window, and with attention to the unique microstructural challenges at interlayer boundaries.