Microstructure and Properties of H1Cr24Ni13 Surfacing on Q235A Steel Fusion Zone

Literature Overview

This study by Wang Nengli, Zhang Xiyan, Pan Xide, and Xue Jin, published in Hot Working Technology (2007, Vol. 36, No. 7), examines the microstructure and properties of the fusion zone in a surfacing weld joint where H1Cr24Ni13 austenitic stainless steel welding wire is used to deposit an overlay on Q235A carbon steel substrate using tungsten inert gas (TIG) welding. The research was conducted collaboratively between Changchun University of Science and Technology and Xi'an Jiaotong University. The authors employed metallography, SEM, EDS, and microhardness testing to characterize the fusion zone.

Core Technical Findings

The primary finding of this study is that welding current variations affect the fusion zone microstructure and joint properties, but the microhardness of the fusion zone is relatively insensitive to changes in process parameters. This observation has important practical implications, as it suggests that the fusion zone hardness is largely governed by the alloy chemistry and solidification behavior rather than by the specific thermal cycle parameters within a reasonable process window.

The H1Cr24Ni13 welding wire (equivalent to ER308/ER316 class austenitic stainless steel) is specifically designed for surfacing applications on carbon steel substrates where corrosion resistance of the overlay is required. The high Cr (24 percent) and Ni (13 percent) content ensures that the overlay solidifies as a fully austenitic structure, providing excellent corrosion resistance. However, the fusion zone—where the overlay alloy mixes with the substrate iron—presents a metallurgical challenge due to the large compositional gradient.

Fusion Zone Metallurgy

The fusion zone in this type of dissimilar material surfacing joint is a region of significant compositional and microstructural gradient. As the molten pool solidifies, the local composition varies from pure overlay alloy at the top to pure substrate at the bottom, creating a complex solidification sequence. The Schaeffler diagram (iron-chromium-nickel ternary phase diagram) is the standard tool for predicting the solidification mode and final microstructure in such joints.

For H1Cr24Ni13 on Q235A, the dilution by substrate iron shifts the effective composition toward the ferrite-forming region of the Schaeffler diagram. This means that the fusion zone may contain delta ferrite, which is generally undesirable in austenitic stainless steel welds due to its susceptibility to intergranular corrosion and its brittleness at low temperatures. However, a small amount of ferrite (1-5 percent) can be beneficial in preventing hot cracking.

Process Parameter Effects

The finding that microhardness is relatively insensitive to welding current variations is consistent with the understanding that the fusion zone composition is determined primarily by the dilution ratio (the fraction of substrate material melted into the weld), which is itself governed by the geometry of the weld pool rather than by the absolute heat input. Within a reasonable range of current values, the dilution ratio may not change significantly, leading to similar fusion zone compositions and hence similar hardness values.

The slight increase in hardness with increasing current is likely due to increased dilution by the harder substrate iron, which introduces more ferrite into the fusion zone. However, the magnitude of this change is small, indicating that the process is relatively insensitive to current variations within the tested range.

Engineering Practice Considerations

For engineers applying H1Cr24Ni13 surfacing to carbon steel components in practice—such as pump casings, chemical processing equipment, and marine applications—the following points are critical:

1. Dilution control: The dilution ratio should be minimized to preserve the corrosion resistance of the overlay. Single-pass, low-dilution techniques are preferred.
2. Fusion zone inspection: The fusion zone should be examined for delta ferrite content using ferrite gauges. Values above 10 percent may indicate excessive dilution and potential corrosion issues.
3. Post-weld inspection: The fusion zone is the most vulnerable region in terms of both corrosion resistance and mechanical properties. Non-destructive testing (NDT) of the fusion zone should be included in the quality assurance plan.
4. Welding procedure: TIG welding provides good control over dilution due to the non-consumable tungsten electrode, making it suitable for this application. However, GMAW with pulsed current can also achieve low dilution with higher productivity.

Key Questions and Reflections

The study's finding that microhardness is insensitive to process parameters raises an important question about the sensitivity of other properties, such as corrosion resistance and fracture toughness, to process variations. While hardness may be stable, the microstructure of the fusion zone—particularly the ferrite content and grain structure—may still vary with process parameters, which could significantly affect corrosion performance. This suggests that hardness alone is not a reliable indicator of fusion zone quality in dissimilar material surfacing joints.

Another consideration is the long-term corrosion performance of the fusion zone in aggressive environments. The compositional gradient in the fusion zone creates a heterogeneous microstructure that may be susceptible to selective corrosion, particularly in chloride-containing environments where pitting and intergranular corrosion are concerns. Accelerated corrosion testing of the fusion zone under simulated service conditions would provide valuable data for engineers making material selection decisions.

Study Insights and Outlook

This paper provides a useful baseline understanding of the fusion zone metallurgy in H1Cr24Ni13-on-Q235A surfacing joints. The insensitivity of microhardness to process parameters is a practical finding that simplifies welding procedure development, as it suggests that a wide range of welding parameters can produce acceptable hardness in the fusion zone. However, engineers should not interpret this as license to neglect process control, as other critical properties such as corrosion resistance, toughness, and crack resistance may be more sensitive to process variations than hardness alone.