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

Literature Overview

This 2023 study published in Applied Laser (Vol. 43, No. 3, pp. 33–41) investigates plasma arc surfacing of H13 hot work die steel powder onto 5CrNiMo base material, with particular focus on how welding current influences weld geometry, microstructure evolution, and crack susceptibility. The research was funded by the Regional Science Fund Project (Grant 51964030) and conducted collaboratively between Kunming University of Technology and Chongqing Dajiang Jiexin Forging Co., Ltd.

Core Technical Findings

Weld Geometry Response to Current Variation

The study systematically varied welding current and measured resulting weld bead dimensions:

Welding Current (A)	Weld Depth (mm)	Weld Width (mm)	Dilution Trend
100	Shallow	Narrow	Low
120	Moderate	Medium	Moderate
150	Deeper	Wider	Higher
180	Deep	Wide	High

Both weld depth and width increase monotonically with increasing welding current, as expected from the fundamental relationship between arc energy input and melt pool dimensions. However, the rate of increase is not linear; at higher currents, the proportional increase in dimensions diminishes slightly due to enhanced convection-driven heat dissipation from the expanding melt pool.

Microstructure Evolution

The surfacing layer microstructure consists primarily of martensite and retained austenite, consistent with the high-carbon, high-alloy composition of H13 powder:

Single-pass surfacing:

Martensite morphology transitions from fine lath martensite at low currents to coarser dendritic martensite at high currents.
Retained austenite content increases with current due to higher cooling rates at the weld center being partially offset by increased thermal input.

Multi-pass surfacing:

A notable asymmetry exists at interpass interfaces: the austenite content is significantly higher above the interface than below it.
This asymmetry is attributed to the reheat cycle: the previously solidified layer below the interface experiences austenitization during subsequent passes, while the newly deposited material above retains higher austenite due to slower cooling from the elevated starting temperature.

Crack Sensitivity Analysis

The research identifies two primary crack formation mechanisms:

Crack Type	Formation Mechanism	Primary Driving Factor	Mitigation Strategy
Stress-induced cracks	Residual stress exceeding material ductility	High cooling rate, high restraint	Preheating, reduced current, interpass temperature control
Defect-induced cracks	Initiation at pre-existing porosity or inclusions	Poor powder feeding, gas contamination	Improved shielding, powder quality control

The optimal welding current of 150 A produces the lowest crack density, representing a balance between sufficient heat input for adequate melting and bonding, and avoidance of excessive thermal gradients that promote cracking.

Process Analysis and Engineering Considerations

Comparison with GMAW Surfacing

The study explicitly addresses the motivation for adopting plasma surfacing over conventional GMAW (MIG) surfacing:

Parameter	GMAW Surfacing	Plasma Surfacing
Energy density	Moderate (diffuse arc)	High (constricted arc)
Deposition efficiency	Lower (more dilution)	Higher (less dilution)
Heat-affected zone	Wider	Narrower
Labor cost	Higher (slower deposition)	Lower (faster deposition)
Equipment cost	Lower	Higher
Process flexibility	High	Moderate

The plasma arc's concentrated energy density enables faster deposition rates while maintaining lower dilution levels, which is particularly advantageous for H13 surfacing where maintaining the precise alloy composition is critical for achieving the target hardness and hot work resistance.

Crack Prevention Strategy Development

Based on the findings, the following process recommendations emerge for H13 plasma surfacing operations:

Preheating: Base material preheating to 200–300°C reduces thermal gradients and residual stresses.
Current selection: Maintain welding current at 150 A for optimal crack resistance.
Interpass temperature control: For multi-pass builds, maintain interpass temperature between 250–350°C to balance austenitization benefits against stress accumulation.
Shielding gas: Use high-purity argon with helium addition (Ar/He 80:20) to improve arc stability and heat input.
Powder feed consistency: Ensure uniform powder feed rate to avoid porosity formation that initiates defect-induced cracks.

Key Questions and Reflections

The research raises important questions regarding the practical implementation of plasma surfacing for hot work die repair and rebuilding:

The optimal current of 150 A is specific to the powder feed rate, travel speed, and nozzle geometry used in the study; systematic parameter interaction studies are needed for robust process qualification.
The retained austenite content in the surfacing layer, while beneficial for toughness, may transform to brittle martensite during subsequent service at elevated temperatures, potentially leading to delayed cracking.
The dilution level at 150 A represents a trade-off: higher dilution improves bond strength but compromises the hardenability of the H13 composition.

Study Insights and Engineering Implications

This research provides practical guidance for engineers tasked with plasma surfacing of hot work die components. The identification of 150 A as the optimal current for minimizing cracking offers a concrete starting point for process development, though site-specific optimization remains necessary. The distinction between stress-induced and defect-induced cracking mechanisms enables targeted quality control measures: process parameter control addresses the former, while material and shielding quality control addresses the latter. The multi-pass interpass asymmetry in austenite distribution highlights the importance of build sequence planning in thick surfacing applications, where thermal history accumulation can significantly alter final microstructure and properties.