Zhuojin Pipe Fitting Co., LtdMulti-Pass Submerged Arc Surfacing Microstructure and Properties
Literature Overview
The paper by He Xiaofeng et al. (2015), published in the Journal of Shanghai University of Engineering Science, investigates the microstructural evolution and mechanical behavior of multi-pass submerged arc surfacing (SAW) deposited on 16Mn thick steel plates. Funded by the National Natural Science Foundation of China (Grant No. 51305253), the study systematically examines how the thermal interaction between successive weld passes affects the heat-affected zone (HAZ), weld metal microstructure, and microhardness distribution. This work is particularly relevant to engineers working on heavy plate repair, pipe spool cladding, and thick-section component hardfacing where multi-pass surfacing is routinely employed.
Core Technical Findings
The authors identified that both single-pass and multi-pass SAW produce a HAZ consisting of three distinct sub-zones: the incomplete normalized zone (INZ), the fully normalized zone (NZ), and the overheated zone (OZ). In the weld metal itself, the microstructure is dominated by columnar grains with acicular proeutectoid ferrite forming along grain boundaries. The critical observation lies in the differential thermal history experienced by adjacent passes.
Thermal Interaction Mechanisms
The multi-pass SAW process introduces two competing thermal phenomena:
| Thermal Effect | Description | Microstructural Consequence |
|---|---|---|
| Post-heating (subsequent pass heats previous pass) | Later pass acts as a post-weld heat treatment on earlier passes | Grain coarsening in previously deposited weld metal |
| Pre-heating (previous pass pre-heats base metal for subsequent pass) | Earlier pass raises local temperature before the next pass is deposited | Columnar grain growth is inhibited; equiaxed grains nucleate |
This dual thermal interaction creates a complex microstructural gradient within the multi-pass weld deposit. Passes that receive post-heating from subsequent passes exhibit grain coarsening, which generally reduces strength and toughness. Conversely, passes that benefit from pre-heating by previously deposited metal show restricted columnar grain growth and the formation of equiaxed grains, which typically improve transverse toughness.
Microhardness Distribution
The microhardness values in the weld metal are governed by two primary factors: the mutual thermal interaction between passes and the local cooling rate. Slower cooling rates (associated with higher thermal input from inter-pass heating) promote softer microstructures with coarser grains, while faster cooling rates (associated with isolated or first-pass deposits) yield harder but more brittle microstructures. The authors emphasize that the microhardness profile is not uniform across the multi-pass deposit but rather exhibits a characteristic variation that must be considered when evaluating the functional performance of the surfacing layer.
Engineering Practice Implications
From a practical standpoint, this study reinforces several process control principles that engineers should apply when specifying multi-pass surfacing operations on thick sections:
- Inter-pass temperature control must be carefully managed. Excessive inter-pass temperatures promote grain coarsening and softening of previously deposited passes, while insufficient inter-pass temperatures may lead to cracking susceptibility in low-alloy steels such as 16Mn.
- Weld sequence optimization is critical. The order in which passes are deposited determines which passes receive post-heating versus pre-heating, and this directly influences the final microstructural uniformity.
- Post-weld heat treatment (PWHT) may be necessary to homogenize the microstructure across all passes, particularly when the surfacing layer is intended for critical service applications.
- Process parameter selection (current, voltage, travel speed, wire feed rate) must be tailored to balance deposition efficiency against microstructural quality.
In pipe repair and heavy plate cladding applications, the findings of this study are directly applicable. For instance, when performing multi-pass hardfacing on the interior of a large-diameter pipe spool, the thermal interaction between passes will inevitably create microstructural non-uniformity. Engineers should anticipate this and design inspection protocols that account for variable hardness profiles across the surfacing layer.
Key Reflections and Study Insights
This paper provides a clear demonstration of how the fundamental principles of welding metallurgy manifest in multi-pass surfacing operations. The concept of thermal interaction between passes is well established in welding science, but its specific effects on surfacing microstructures deserve greater attention in engineering practice. Many field welders and inspectors are aware of inter-pass temperature requirements but may not fully appreciate how the thermal history of each individual pass influences the final microstructure and, consequently, the service performance.
The study also highlights an important limitation: the research was conducted on 16Mn steel, which is a relatively low-carbon, low-alloy structural steel. The thermal interaction effects observed here may differ significantly for higher-alloy steels, stainless steels, or nickel-based alloys where phase transformations and precipitation hardening play more dominant roles. Engineers extrapolating these findings to other material systems should exercise caution and conduct their own validation studies.
The work is a valuable reference for welding engineers involved in specifying multi-pass surfacing procedures, particularly for thick-section components where thermal interaction effects are most pronounced. The systematic approach to characterizing the HAZ and weld microstructure provides a methodological template that can be adapted to other surfacing applications.