Effect of Magnetic Field Configuration on Microstructure and Properties of Plasma Arc Cladding Layers

Literature Overview and Research Background

Plasma transferred arc (PTA) cladding is one of the most widely employed thermal spray and overlay techniques in the fabrication of bimetallic components, particularly for hydrogenation reactors, heat exchangers, and columns requiring corrosion- or wear-resistant surface layers. The process relies on a high-temperature plasma arc to melt both the substrate surface and the fed alloy powder, producing a dilution-controlled, metallurgically bonded overlay. However, the rapid solidification rates inherent to PTA cladding (typically 10^3 to 10^4 K/s) often lead to columnar dendritic microstructures, hot cracking susceptibility, and residual stress concentrations. The referenced study investigates how externally applied magnetic fields — in various configurations including axial, transverse, and rotating fields — influence the microstructure, phase composition, hardness distribution, and bonding characteristics of PTA-clad layers deposited on carbon steel substrates using nickel-based alloy powders.

This research is of significant practical importance because residual stress and microstructural anisotropy in PTA overlays are primary drivers of delamination, intergranular cracking, and post-weld distortion in thick-section bimetallic pressure vessels governed by ASME VIII Div.1 and NB/T 47002. Understanding magnetic field effects provides a non-contact, real-time process parameter that can be adjusted without altering consumable selection or arc geometry.

Core Technical Findings

Magnetic Field Configuration and Solidification Behavior

The study examined three primary magnetic field configurations applied during PTA cladding of Inconel 625 powder onto SA-106 Gr. C carbon steel substrates:

The axial field configuration primarily acts through the magnetohydrodynamic (MHD) effect on the molten pool, inducing a downward flow that promotes epitaxial grain growth from the substrate. This can increase columnar grain fraction but simultaneously reduces the interdendritic spacing from approximately 25 μm (no field) to 12–15 μm at 30 mT. The transverse field, by contrast, generates a rotational Lorentz force within the melt pool that enhances convective mixing, promotes equiaxed grain nucleation, and reduces the columnar-to-equiaxed transition (CET) undercooling requirement by 15–20%.

Microstructural Evolution

Metallographic analysis revealed that under a 30 mT transverse field, the overlay microstructure transitioned from predominantly columnar dendrites (no field) to a mixed columnar-equiaxed structure with approximately 60% equiaxed grain fraction. The secondary dendrite arm spacing (SDAS) decreased from 18 μm to 9 μm, consistent with the Hurley-Hunt correlation for increased local cooling rates induced by enhanced convective transport. Phase composition analysis by X-ray diffraction confirmed that the gamma (γ) matrix with gamma-prime (γ′) precipitates in the Inconel 625 overlay remained unchanged in type, but the volume fraction of γ′ increased from 12% to 18% under the rotating field condition, attributed to more uniform solute redistribution during solidification.

Mechanical Property and Bonding Performance

The most significant finding was the reduction in residual stress under transverse field conditions — from 420 MPa to 295 MPa — which represents a 30% decrease. This reduction is attributed to the enhanced fluid stirring effect that promotes more uniform thermal distribution and reduces thermal gradient-driven stress accumulation. The bond strength improvement from 185 MPa to 215 MPa is clinically relevant for applications governed by API 934, where minimum bond strength requirements for weld overlay cladding must be verified.

Process Parameters and Engineering Implications

The study maintained a constant plasma arc current of 180 A, arc voltage of 25 V, travel speed of 250 mm/min, and powder feed rate of 250 g/min across all magnetic field conditions. The substrate was preheated to 150°C to minimize hydrogen-induced cracking (HIC) susceptibility in the carbon steel base metal. The magnetic field was generated using a permanent magnet assembly positioned below the substrate, avoiding interference with the plasma arc stability.

From an engineering practice perspective, the findings suggest that integrating a permanent magnet fixture into the PTA cladding setup is feasible without major equipment modification. For large-diameter pressure vessel shells where PTA cladding is performed in multiple passes (as specified in NB/T 47014 for weld procedure qualification), the transverse field configuration offers the most beneficial combination of reduced residual stress, refined microstructure, and improved bond strength. However, the field must be carefully positioned to avoid distortion of the plasma arc column, which could compromise dilution control and overlay geometry.

FMEA Considerations for Magnetic Field Application

Key Questions and Reflections

The study raises several important questions for further investigation. First, the effect of magnetic field strength above 50 mT was not explored — would higher fields provide additional benefits, or would they risk arc instability and powder trajectory disruption? Second, the interaction between the magnetic field and multi-pass cladding sequences was not addressed, yet in practical bimetal pressure vessel fabrication, 3–5 overlay passes are typical for achieving the specified overlay thickness (typically 3–6 mm for NB/T 47002 compliance). Third, the study focused on Inconel 625 on carbon steel, but the field effects on dissimilar systems such as Hastelloy C276 on stainless steel or titanium on steel would likely differ due to varying thermal conductivity, solidification range, and magnetic permeability of the substrate.

I find the residual stress reduction result particularly compelling because it addresses one of the most persistent challenges in overlay cladding — the mismatch between the thermal expansion coefficients of the overlay and substrate. In bimetallic pressure vessels operating under cyclic thermal loading, residual stresses superimposed on operating stresses can initiate fatigue cracks at the cladding-to-base interface. A 30% reduction in residual stress could translate to a meaningful extension of fatigue life, though quantifying this requires dedicated fatigue testing under representative service conditions.

Study Insights and Practical Recommendations

The integration of external magnetic fields into PTA cladding represents a promising process intensification strategy that does not require changes to consumables, base metal selection, or equipment capital investment beyond the magnet fixture. For engineers involved in the fabrication of clad-plate pressure vessels under ASME VIII Div.1 or GB/T 150, the key takeaway is that the transverse field configuration at 20–30 mT offers the optimal balance of microstructural refinement, residual stress reduction, and bond strength improvement. This should be considered during weld procedure qualification (WPQ) per NB/T 47014 or ASME IX, where the magnetic field parameter would be classified as a supplementary essential variable. Future work should focus on multi-pass studies, field-substrate material interactions, and full-scale validation on pressure vessel components subjected to hydrostatic testing per GB/T 150 Section 8.