Metallographic Analysis of Stainless Steel Overlay Layers

Literature Overview

This 2003 publication in Petrochemical Equipment by Wang Huibin from Lanzhou Petrochemical Machinery Equipment Group Co., Ltd. provides a focused study on the microstructural characteristics of stainless steel overlay layers. While relatively early in the timeline of Chinese overlay welding literature, this work addresses fundamental metallurgical questions that remain relevant to current engineering practice.

Microstructural Fundamentals

The microstructure of a stainless steel overlay layer is governed by several interdependent factors: base material composition, overlay consumable chemistry, welding process heat input, interpass temperature, and cooling rate. Understanding these relationships is essential for predicting overlay performance.

Typical Microstructural Features

Feature	Description	Engineering Significance
Columnar grains	Elongated grains growing from the fusion line	Directional properties; potential crack propagation path
Dendritic structure	Branch-like solidification pattern	Interdendritic segregation; potential for hot cracking
Ferrite-austenite balance	In duplex or austenitic-ferritic structures	Controls corrosion resistance and mechanical properties
Dilution zone	Transition region between overlay and base	Critical for bond strength and corrosion performance
Grain boundary precipitates	Sigma phase, chromium carbides	Sensitization; intergranular corrosion susceptibility

Heat Input Effects on Microstructure

The welding heat input directly controls the cooling rate and thus the resulting microstructure:

Low heat input (<10 kJ/mm): Rapid cooling produces fine grain structure; higher hardness; increased residual stress; potential for microcracking
Medium heat input (10–20 kJ/mm): Moderate cooling rate; balanced microstructure; acceptable mechanical properties
High heat input (>20 kJ/mm): Slow cooling; coarse grains; potential for sigma phase formation; reduced corrosion resistance

For austenitic stainless steel overlays (304, 316), the ferrite content in the weld metal should be maintained between 5–15% delta ferrite to prevent hot cracking and maintain adequate mechanical properties. This is typically achieved through proper selection of consumable chemistry (molybdenum and nitrogen content) and welding process parameters.

Metallographic Examination Methodology

Sample Preparation

Proper metallographic sample preparation is critical for accurate microstructural assessment:

Sectioning: Cut perpendicular to the overlay surface; avoid excessive grinding heat
Mounting: Epoxy mounting with controlled pressure; avoid orientation artifacts
Grinding: Progress from 120-grit to 1000-grit SiC papers; use water as coolant
Polishing: Diamond paste (6μm → 1μm → 0.25μm); final alcohol polish
Etching:

For austenitic SS: 5% HF + 5% HNO₃ for 5–10 seconds
For ferritic/duplex SS: 10% NaOH + 10% H₂O₂ for 15–30 seconds
For carbide visualization: 10% HF + 10% HCl for 30–60 seconds

Key Observations

The metallographic examination should focus on:

Overlay-base interface: Check for full fusion; identify any unmelted base material inclusions
Dilution gradient: Map the compositional transition from overlay to base
Grain morphology: Assess grain size, shape, and orientation
Phase distribution: Identify ferrite, austenite, carbides, and intermetallic phases
Defects: Porosity, cracks, inclusions, and lack of fusion

Process Qualification Implications

The metallographic findings directly inform welding procedure qualification decisions:

Qualification Parameter	Metallographic Target	Verification Method
Heat input range	Appropriate grain size	Measure heat input; correlate with grain size
Interpass temperature	Prevent excessive grain growth	Thermocouple monitoring; grain size measurement
Preheat temperature	Control cooling rate	Surface temperature measurement
Layer thickness	Limit dilution	Chemical analysis across dilution zone
Weld geometry	Uniform coverage	Macro section examination

Engineering Practice Considerations

For petrochemical equipment overlays, the following practical guidelines emerge from metallographic analysis:

Multi-pass overlay: Each subsequent pass should be deposited with a heat input that refines rather than coarsens the previous pass microstructure. A common strategy is to use lower heat input for the first pass (to control dilution) and progressively increase heat input for subsequent passes (to refine the overall microstructure).
Directional control: For large surface areas, the welding direction should be planned to avoid grain growth in any single direction. Cross-hatch patterns are recommended for large flat surfaces.
Post-weld treatment: When sigma phase or excessive sensitization is detected, a solution treatment at 1050–1100°C with rapid water quench may be required. However, this is often impractical for large equipment, making preventive process control more important.
Base material influence: The carbon and alloy content of the base material significantly affects dilution. High-carbon base materials require additional attention to dilution control and may necessitate a transition layer.

Study Reflections

This work underscores the fundamental importance of microstructural understanding in overlay welding quality assurance. While modern non-destructive testing methods can detect gross defects, only metallographic examination reveals the microstructural features that govern long-term corrosion resistance and mechanical performance. Engineers should insist on representative metallographic examination as part of welding procedure qualification, not merely as a research tool. The correlation between process parameters and microstructural outcomes, once established through qualified procedures, provides a powerful tool for predicting overlay performance in service.