Research on Stainless Steel Strip Electrode Surfacing Process

Literature Overview

This paper, authored by Huang Siluo from Maoming Petrochemical Machinery Factory and He Xiang from Guangdong Yuehai Cylinder Factory, was published in the journal Welding in 1999 (Issue 2, pp. 12-16). The study investigates the stainless steel strip electrode slag surfacing (SES) process, focusing on the influence of process parameters on the dilution rate of the surfacing layer and the effect of an external magnetic field on welding quality. The authors established critical thresholds for dilution rate and process parameters for a 0.4 mm x 50 mm stainless steel strip electrode, and conducted metallographic analysis to identify the microstructural features governing resistance to hydrogen-induced blistering.

Process Description and Parameter Analysis

Strip electrode slag surfacing is a specialized welding process in which a continuous strip of filler metal, rather than a wire or powder, is fed into the arc as the electrode. The process is characterized by the formation of a slag pool that encapsulates the molten metal, providing a protective atmosphere and influencing the solidification behavior. The 0.4 mm x 50 mm strip electrode used in this study represents a relatively thin and narrow configuration, which offers the advantage of low heat input and minimal dilution but requires careful parameter control to maintain arc stability.

The dilution rate, defined as the proportion of base metal incorporated into the surfacing layer, is a critical quality indicator in surfacing applications. Excessive dilution introduces unwanted alloying elements from the base metal into the overlay, degrading the corrosion resistance, mechanical properties, and microstructural integrity of the surfacing layer. For stainless steel surfacing onto carbon steel substrates, dilution is particularly problematic because the introduction of carbon and manganese from the base metal can promote the formation of intermetallic phases and reduce the chromium content below the threshold required for passivity.

The authors identified two critical thresholds: a dilution threshold of 50 kJ/cm and a process threshold of 90 kJ/cm. These energy-based thresholds provide a more fundamental basis for process control than conventional parameters such as current and speed alone, because they directly correlate with the thermal input into the weld system. The dilution threshold represents the maximum linear energy input at which the dilution rate remains within acceptable limits, while the process threshold defines the boundary beyond which process instability, such as arc blow or slag entrainment, becomes likely.

Microstructural Analysis and Hydrogen-Induced Blistering

The metallographic analysis conducted by the authors revealed that the coarse austenite microstructure in the fusion zone and the width of the carbon-enriched layer are the dominant factors influencing the surfacing layer's resistance to hydrogen-induced blistering. Hydrogen-induced blistering is a degradation mechanism particularly relevant in hydrogen-containing environments, such as those encountered in petroleum refining, chemical processing, and high-pressure hydrogen service. When atomic hydrogen diffuses into the microstructure of the surfacing layer, it accumulates at microvoids, grain boundaries, and inclusions, eventually generating sufficient pressure to cause blister formation and spalling.

The coarse austenite grains in the fusion zone provide fewer grain boundaries per unit area, which paradoxically can either facilitate or impede hydrogen diffusion depending on the grain boundary character distribution. However, the more critical factor identified by the authors is the carbon-enriched layer at the interface between the surfacing layer and the base metal. This layer forms due to the redistribution of carbon during solidification, where carbon is rejected from the growing austenite dendrites into the interdendritic liquid. The resulting carbon-enriched zone is susceptible to hydrogen attack because it contains a high density of carbide precipitates that act as hydrogen trapping sites and nucleation points for blister formation.

External Magnetic Field Effects on Welding Quality

The investigation of external magnetic field effects on the strip electrode surfacing process adds another dimension to the study. The application of a magnetic field during slag surfacing can influence the arc stability, slag pool dynamics, and solidification behavior. The authors found that a properly applied magnetic field improved the homogeneity of the surfacing layer microstructure and reduced the width of the carbon-enriched zone, thereby enhancing the resistance to hydrogen-induced blistering. This finding is consistent with the general principle that electromagnetic stirring promotes more uniform solidification and reduces compositional segregation.

Study Insights and Practical Recommendations

This research provides valuable engineering guidance for the application of strip electrode slag surfacing in hydrogen-containing service environments. The identification of energy-based thresholds for process control represents a methodological advance over traditional parameter-based approaches, as it accounts for the coupled effects of multiple variables on the thermal cycle. For practitioners in the petroleum and chemical industries, where stainless steel surfacing is frequently employed to upgrade carbon steel equipment for corrosion resistance in hydrogen-rich streams, the findings of this study have direct relevance.

The emphasis on the carbon-enriched layer as a critical vulnerability is particularly important for quality assurance protocols. Standard non-destructive testing methods such as ultrasonic testing and radiographic testing may not reliably detect the presence or extent of a carbon-enriched zone. Therefore, destructive testing, including metallographic examination of cross-sections and hydrogen permeation testing, should be incorporated into the qualification procedure for surfacing systems intended for hydrogen service. The lesson from this study is that the microstructural details of the fusion zone and the metallurgical interface are not merely academic concerns but are directly linked to the long-term integrity and safety of pressure-containing equipment.