Multi-Pass Submerged Arc Surfacing Microstructure and Property Analysis on 16Mn Steel

Literature Overview

This study, published in 2015 in the Journal of Shanghai University of Engineering Science (Vol. 29, No. 3, pp. 258–261), was conducted by He Xiaofeng, Lu Qinghua, Peng Birong, Chen Xuan, and Zhang Jing from the School of Materials Engineering at Shanghai University of Engineering Science, supported by the National Natural Science Foundation of China (Grant No. 51305253). The research investigates the microstructural evolution and mechanical properties of multi-pass submerged arc surfacing (SAS) deposits on thick 16Mn steel plates, with particular focus on how inter-pass thermal interactions influence the heat-affected zone (HAZ), weld metal microstructure, and microhardness distribution.

Core Technical Findings

The study systematically compares single-pass and multi-pass SAS deposits, revealing several critical metallurgical observations that carry direct implications for engineering practice.

HAZ Microstructural Zones

Both single-pass and multi-pass SAS produce a HAZ consisting of three distinct zones: incomplete normalizing zone, normalizing zone, and overheated zone. This is consistent with the thermal cycle experienced by 16Mn steel (a low-carbon low-alloy steel with 0.12–0.20% C, 1.20–1.60% Mn, and trace amounts of Ti and Nb), where peak temperatures exceeding 900°C produce austenitization followed by air cooling.

Weld Metal Microstructure

The weld metal is characterized by columnar dendritic crystals with proeutectoid ferrite forming along grain boundaries. In multi-pass welding, two distinct phenomena were observed:

• Post-heating effect: Previously deposited passes that are subsequently reheated by subsequent passes exhibit grain coarsening due to prolonged time at elevated temperatures.
• Pre-heating effect: Subsequent passes deposited onto previously deposited, still-warm passes experience hindered columnar crystal growth, with equiaxed grains nucleating at the interface. This is attributed to the higher initial temperature reducing the thermal gradient at the solidification front.

Microhardness Distribution

The microhardness of the weld metal is governed by the interplay between inter-pass thermal interactions and local cooling rates. Passes subjected to post-heating exhibit lower hardness due to grain coarsening and possible tempering of hard phases. Passes deposited with pre-heating may show modified hardness profiles due to the altered solidification morphology.

Engineering Practice Implications

Process Parameter Considerations

Practical Considerations for Surfacing Applications

In industrial surfacing operations on low-alloy steel components, the multi-pass thermal cycling effect must be carefully managed. The grain coarsening observed in post-heated passes can significantly reduce transverse impact toughness, which is critical for components subjected to cyclic loading or impact. Engineers should consider the following:

1. Limiting the number of passes in a single welding sequence to minimize cumulative thermal exposure.
2. Implementing inter-pass temperature control to prevent excessive reheating of earlier passes.
3. Performing metallographic examination of the interface between passes to verify equiaxed grain formation.
4. Conducting microhardness mapping across the full weld cross-section to identify soft zones that may be susceptible to wear or fatigue.

Connection to Pipe Fitting and Piping Applications

In the context of pipe fitting manufacturing and repair, multi-pass SAS is commonly employed for building up wear-resistant or corrosion-resistant surfaces on carbon steel and low-alloy steel substrates. The findings of this study directly inform process design for:

• Thick-wall pipe repair: Where multiple surfacing passes are required to build up specified overlay thickness.
• Valve seat surfacing: Where hardness uniformity is critical for sealing performance.
• Pipe end preparation for welding: Where the HAZ properties of the base material must be maintained after surfacing operations.

Key Questions and Reflections

The study raises an important question regarding the optimal balance between deposition efficiency and microstructural quality. In production environments, the tendency is to maximize deposition rate, which often means using higher heat inputs and more passes. However, this study demonstrates that each additional pass introduces thermal effects that can degrade the metallurgical quality of previously deposited material.

A further consideration is the applicability of these findings to thicker deposits. The study focuses on a single surfacing layer, but in practice, multiple layers may be required. The cumulative thermal cycling effect across multiple layers would be more severe than the two-pass comparison examined here.

Study Insights and Implications

This research provides valuable quantitative insight into the metallurgical behavior of multi-pass SAS on 16Mn steel. The observation that inter-pass thermal interactions fundamentally alter the weld metal microstructure—transforming columnar to equiaxed grains in pre-heated passes while coarsening grains in post-heated passes—has significant implications for process optimization. Engineers should adopt a systematic approach to surfacing process design, incorporating thermal cycle monitoring, inter-pass temperature control, and post-deposit microstructural verification as standard practice. The fundamental principle that cooling rate and thermal history govern microstructure and properties in surfacing operations is reaffirmed, reinforcing the need for rigorous process qualification and in-service inspection protocols.