Microstructure and Properties of Fusion Zone in H1Cr24Ni13 Surfacing on Q235A Steel

Literature Overview

The paper by Wang Nengli, Zhang Xiyan, Pan Xide, and Xue Jin (2007), published in Hot Working Technology (Vol. 36, No. 7, pp. 7–9), investigates the microstructure and properties of the fusion zone formed during TIG surfacing of H1Cr24Ni13 austenitic stainless steel wire onto Q235A carbon steel substrates. The study was conducted jointly by Changchun University of Science and Technology and Xi'an Jiaotong University, combining expertise in welding technology and materials science.

Core Technical Content

The combination of austenitic stainless steel surfacing (H1Cr24Ni13, equivalent to AWS A5.9 ER308) on carbon steel (Q235A, equivalent to ASTM A36) creates a significant metallurgical challenge due to the large compositional and thermal property mismatch between the two materials. The fusion zone, where the surfacing metal and base metal intimately mix, is the critical region governing joint performance.

Welding Process and Parameters

The study employed TIG (GTAW) surfacing with H1Cr24Ni13 wire, varying welding current to examine its effect on fusion zone characteristics. The TIG process was selected for its precision control over heat input, which is essential for managing dilution in dissimilar material surfacing applications.

Key Findings

Microstructure variation: Changes in welding current significantly affect the fusion zone microstructure, including grain morphology, phase distribution, and elemental segregation.
Hardness stability: Despite microstructural variations, the microhardness of the fusion zone showed relatively insensitivity to welding parameter changes.
Phase evolution: The fusion zone exhibits a gradient of phases from the austenitic surfacing metal through mixed austenite-ferrite in the fusion zone to the ferritic-pearlitic base metal.

Welding Parameter	Effect on Fusion Zone Microstructure	Effect on Microhardness
Low current	Coarse grains, higher dilution	Relatively constant
Medium current	Moderate grain size, controlled dilution	Relatively constant
High current	Fine grains, lower dilution, possible defects	Relatively constant

Interpretation of Technical Points

The insensitivity of microhardness to welding parameters, despite significant microstructural changes, is an important finding. This suggests that the hardness in the fusion zone is primarily governed by the overall alloy composition (dilution ratio) rather than by the specific microstructural morphology. The austenitic matrix provides a relatively uniform hardness regardless of grain size or phase fraction variations within a certain range.

The microstructural variations with current are attributable to changes in:

Cooling rate: Higher currents produce larger weld pools but also higher cooling rates due to greater thermal mass removal.
Dilution ratio: Higher currents increase base metal melting, increasing dilution and altering the local composition.
Solidification mode: The balance between cellular and dendritic solidification changes with thermal gradient and growth rate.

The dilution control is critical in this dissimilar material application. Excessive dilution introduces carbon from Q235A into the austenitic matrix, potentially forming chromium carbides (M₂₃C₆, Cr₇C₃) at grain boundaries, which can cause intergranular corrosion susceptibility. Insufficient dilution may result in incomplete fusion and poor mechanical bonding.

Process and Standards Analysis

For TIG surfacing of austenitic stainless steel on carbon steel, the following process considerations apply:

Parameter	Recommended Range	Rationale
Welding current	80–150 A	Control dilution and heat input
Wire feed speed	1.5–3.0 m/min	Maintain arc stability
Shielding gas	Ar (99.99%) or Ar+2% O₂	Protect molten pool
Travel speed	100–250 mm/min	Balance penetration and deposition
Wire diameter	1.6–2.4 mm	Match heat input
Preheat temperature	0–150°C	Minimize cracking risk

Relevant standards include AWS D10.9 (procedure qualification for surfacing), ASME B31.3 (process piping), and GB/T 985 for weld preparation. The dissimilar material nature of this application requires attention to the weld metal specification—typically ER308 or ER309L for carbon steel to austenitic stainless steel transitions.

Integration with Engineering Practice

This type of surfacing application is common in:

Pipeline repair: Restoring worn valve bodies, flange faces, and pump housings made of carbon steel with stainless steel overlays for corrosion resistance.
Fitting fabrication: Creating corrosion-resistant inner surfaces on carbon steel elbows, tees, and reducers.
Equipment refurbishment: Adding wear and corrosion resistant surfaces to existing carbon steel components without complete replacement.

The finding that microhardness is relatively stable across parameter variations provides confidence that the fusion zone will maintain acceptable mechanical properties across a reasonable range of field welding conditions. However, corrosion resistance in the fusion zone remains sensitive to dilution and must be verified through corrosion testing (e.g., ASTM G48 for intergranular corrosion).

Key Questions and Reflections

The study's focus on microhardness as the primary performance metric is somewhat limited for a dissimilar material surfacing application. Corrosion resistance, particularly intergranular corrosion susceptibility, is often the critical property in austenitic stainless steel surfacing on carbon steel. The absence of corrosion testing data is a notable gap.

Additionally, the study does not address the effect of welding sequence and interpass temperature on multi-pass builds, which are common in practical surfacing operations. The residual stress state of the fusion zone, which can significantly affect fatigue performance and distortion, is also not examined.

Study Insights and Implications

This work provides valuable baseline data on the metallurgical behavior of austenitic stainless steel surfacing on carbon steel substrates. The observation that microhardness is relatively insensitive to parameter variations while microstructure is highly sensitive suggests that engineers can use hardness as a quick field screening tool, but must rely on more comprehensive testing (metallography, corrosion testing) for critical applications. The study reinforces the importance of dilution control in dissimilar material surfacing and highlights the need for systematic parameter qualification before production deployment. For pipeline and fitting manufacturers, this type of research supports the development of qualified welding procedures (WPS) for repair and refurbishment operations where cost-effective surface upgrades are preferred over complete component replacement.