Microstructure and Mechanical Properties of Multi-Pass Submerged Arc Weld Overlay

Literature Overview

This study investigates the microstructural evolution and mechanical property development in multi-pass submerged arc weld (SAW) overlay layers, which are commonly applied to carbon steel and low-alloy steel base metals to provide corrosion resistance, wear resistance, or both. Multi-pass overlay is the standard approach for achieving the required overlay thickness in industrial applications, typically ranging from 3 to 25 mm depending on the service requirements. Understanding how the microstructure and properties vary through the thickness of a multi-pass overlay, and how they are influenced by welding parameters, is essential for predicting service performance and optimizing the overlay design.

Core Technical Points

Microstructural Evolution Through Multi-Pass Build

The microstructure of a multi-pass SAW overlay exhibits significant variation from the base metal interface to the surface, driven by differences in thermal history experienced by each pass:

Pass Location	Thermal History	Dominant Microstructure	Typical Hardness (HV)
Base metal	Original or preheated	Original BM structure	As-received (e.g., 150-250 HV for CS)
First pass (root)	Highest heat input per unit volume	Coarse martensite, retained austenite, carbides	400-600 HV (depending on alloy)
Intermediate passes	Moderate heat input, reheating by subsequent passes	Refined martensite, mixed carbides	350-550 HV
Top pass (cap)	Lowest heat input per unit volume, rapid cooling	Fine martensite, higher retained austenite	450-650 HV

The first pass experiences the most severe thermal cycle with the highest peak temperature and slowest cooling rate relative to the total heat input, often resulting in coarser microstructures and higher retained austenite content. Subsequent passes reheat the previous layers, partially tempering the hard microstructures and promoting carbide coarsening. The top pass, cooled by air or water, experiences the fastest cooling rate and typically exhibits the finest microstructure with the highest hardness.

Influence of Welding Parameters on Microstructure

Heat input: Higher heat input increases the peak temperature and reduces the cooling rate, promoting retained austenite formation and carbide coarsening. The literature reports that for a typical stainless steel overlay on carbon steel, increasing heat input from 15 to 30 kJ/mm can increase retained austenite content from 15% to 40% and reduce hardness from 550 HV to 400 HV.

Interpass temperature: Higher interpass temperatures reduce the thermal gradient between passes, resulting in more uniform microstructure through the overlay thickness. However, excessive interpass temperatures (>300 °C) can promote carbide coarsening and reduce overall hardness.

Welding speed: Faster welding speeds reduce heat input and increase cooling rates, producing finer microstructures but potentially increasing the risk of hot cracking and hydrogen-induced cracking.

Shielding flux composition: Basic fluxes (CaO-SiO2-Al2O3 system) promote grain refinement and reduce hydrogen pickup, while acidic fluxes (SiO2-TiO2 system) can increase carbon pickup and affect carbide formation.

Mechanical Property Development

The mechanical properties of multi-pass overlays exhibit characteristic through-thickness variations:

Hardness: Typically exhibits a "U-shaped" or "inverted U-shaped" profile through the thickness, depending on the alloy system and welding parameters. The base metal interface often shows lower hardness due to dilution with the carbon steel base metal, while the top surface shows higher hardness due to rapid cooling.
Tensile strength: Generally decreases with increasing dilution at the base metal interface and increases toward the top surface. The literature reports tensile strengths ranging from 400 MPa (high dilution zone) to 800 MPa (low dilution zone) for stainless steel overlays on carbon steel.
Impact toughness: The base metal interface zone typically exhibits the lowest impact toughness due to the combination of dilution and potential microstructural brittleness. The top surface zone, with finer microstructure, generally shows better toughness.
Corrosion resistance: The dilution zone at the base metal interface is the most vulnerable to corrosion due to reduced chromium and alloying element content. The literature recommends a minimum dilution control of 15% for the first pass to ensure adequate corrosion resistance in the overlay-base metal transition zone.

Dilution Control Strategies

Dilution is a critical factor governing the metallurgical properties of the overlay-base metal interface. The literature discusses several strategies for dilution control:

Root pass technique: Using a narrower electrode or lower heat input for the first pass to minimize base metal melting.
Consumable selection: Using consumables with higher alloy content than the target overlay composition to compensate for dilution.
Pre-weld groove preparation: Machining a shallow groove in the base metal surface to increase the local heat capacity and reduce dilution.
Multi-step approach: Using a transition layer with intermediate composition between the base metal and the final overlay alloy.

Engineering Practice Implications

The literature emphasizes several practical implications for engineers designing and specifying multi-pass overlay operations:

Thickness optimization: The required overlay thickness should be determined based on the expected service life and wear rate, not simply maximized. Excessive thickness increases cost and introduces additional thermal stresses without proportional benefit.
Inspection strategy: The base metal interface zone should receive the most rigorous inspection, as this is where dilution-related property degradation and fusion defects are most likely to occur.
Post-weld heat treatment: For overlays requiring improved toughness, a post-weld tempering treatment can homogenize the microstructure and reduce hardness variation through the thickness. The literature recommends tempering at 550-650 °C for 1-2 hours for martensitic stainless steel overlays.
Service life prediction: Engineers should account for the through-thickness property variation when predicting service life, particularly for applications involving wear, corrosion, or fatigue.

Key Reflections and Study Insights

A particularly important insight from this literature is the recognition that multi-pass overlay is not simply a repetition of single-pass welding but involves complex thermal-metallurgical interactions between passes that significantly influence the final product quality. Each pass modifies the thermal history of all previously deposited material, creating a cumulative effect that is difficult to predict without detailed process modeling. The literature advocates for a systematic approach to overlay design that considers the entire build sequence as an integrated process, rather than optimizing individual passes in isolation. This holistic perspective requires engineers to develop a deep understanding of welding metallurgy, thermal analysis, and mechanical property development, and to leverage process simulation tools where available to predict and optimize the final overlay quality. The practical value of this study lies in its ability to bridge the gap between fundamental metallurgical understanding and practical engineering decisions, providing actionable guidance for improving the quality and reliability of multi-pass overlay operations in industrial applications.