Thermal Processing of Pump Materials Effects of Welding Speed and Heat Treatment on Plasma Surfacing Nickel-Based Alloys

Literature Overview

This paper by Tian Xin and colleagues from Dalian University of Technology and Shenyang Blower Group Co., Ltd., published in "Pump Technology" (2014, Issue 1, pages 42-46), investigates the effects of welding speed and post-weld heat treatment on the microstructure and mechanical properties of plasma arc surfacing (PAS) deposits of nickel-based alloys on pump components. The study systematically varies welding parameters and thermal post-treatment conditions to establish process-property relationships that enable rational optimization of the surfacing process for pump material applications.

Core Technical Content

Pump impellers, casing linings, and wear rings operate in aggressive chemical environments where corrosion resistance and mechanical durability are paramount. Nickel-based alloys such as Alloy 625, Alloy 617, and Hastelloy C-276 are commonly selected for surfacing due to their excellent resistance to pitting, crevice corrosion, and stress corrosion cracking. However, achieving optimal performance requires careful control of the surfacing process parameters and post-weld thermal treatment.

Welding Speed Effects on Microstructure

The study demonstrates that welding speed is a dominant parameter controlling solidification microstructure in plasma arc surfacing deposits:

As welding speed increases, the cooling rate increases proportionally, resulting in finer dendritic and cellular structures. The reduced solidification time suppresses the growth of interdendritic precipitates and maintains a more homogeneous solid solution microstructure. The dilution rate decreases with increasing speed because less base metal is melted per unit length of weld, which is directly related to the observed inverse proportionality between dilution and hardness.

Dilution-Hardness Relationship

The inverse relationship between dilution rate and overlay hardness is a critical finding for process optimization. High dilution introduces base metal elements (typically carbon, manganese, and silicon from carbon steel substrates) into the overlay, which can either increase or decrease hardness depending on the specific alloy system. For nickel-based alloys on carbon steel substrates, increased dilution generally introduces carbon that forms carbides, potentially increasing hardness but at the expense of corrosion resistance due to chromium carbide precipitation at grain boundaries.

The study establishes that maintaining dilution below 15% is essential for preserving the corrosion resistance characteristics of nickel-based overlay alloys while achieving acceptable hardness levels. This requires careful selection of welding speed, powder feed rate, and layer thickness.

Heat Treatment Effects

Post-weld heat treatment at 450 °C was found to improve the hardness of Alloy B and Alloy C deposits while simultaneously reducing welding residual stresses. This temperature range corresponds to solution treatment conditions that dissolve deleterious intermetallic phases (such as μ-phase and σ-phase) that may have formed during the rapid solidification of the surfacing process.

The differential response among alloys indicates that the precipitation strengthening behavior is composition-dependent. Alloys B and C, likely containing higher concentrations of niobium, titanium, or aluminum, form strengthening precipitates during the 450 °C treatment that enhance hardness. Alloy A, possibly a solid-solution-strengthened composition, shows minimal response.

Process Window Optimization

The combined analysis of welding speed and heat treatment effects enables the definition of an optimal process window for pump material surfacing:

1. Welding speed: 400-600 mm/min provides a balance between fine microstructure and acceptable deposition rate.
2. Powder feed rate: 1.5-2.5 kg/h to maintain layer thickness of 0.3-0.5 mm per pass.
3. Plasma current: 150-250 A depending on powder particle size and composition.
4. Shielding gas flow: 15-20 L/min argon to prevent oxidation of the melt pool.
5. Post-weld treatment: 450 °C for 2 hours in still air or argon atmosphere.

Engineering Practice Integration

For pump manufacturers specifying nickel-based alloy surfacing, this paper provides quantitative guidance for establishing welding procedure specifications (WPS). The key insight is that process parameters cannot be optimized in isolation—welding speed affects microstructure, which in turn determines the response to heat treatment. A systematic approach using Design of Experiments (DOE) methodology is recommended to establish the full parameter-interaction matrix.

In practice, the plasma arc surfacing process for pump components requires attention to geometric challenges such as concave impeller passages and thin-walled casing sections. The high energy density of the plasma arc (typically 50-100 kW/cm²) provides good penetration control but requires precise torch positioning to avoid undercutting on thin sections.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation. First, the study does not address the long-term corrosion resistance of the treated deposits in actual pump service environments containing chlorides, sulfides, or organic acids. Second, the effect of multiple overlapping passes on the cumulative thermal cycle and resulting microstructure evolution is not fully explored. Third, the transition from laboratory-scale specimens to production-scale components may introduce additional variables related to thermal mass and cooling conditions.

The finding that 450 °C treatment can simultaneously improve hardness and reduce residual stress is particularly valuable for engineering practice, as it suggests that a single thermal cycle can address two critical quality requirements. This simplifies the manufacturing process and reduces production costs compared to separate stress relief and solution treatment operations.

Summary

This paper provides a systematic investigation of the two most influential parameters in plasma arc surfacing of nickel-based alloys for pump applications—welding speed and post-weld heat treatment. The established relationships between these parameters, microstructure, dilution rate, and hardness provide a solid foundation for process optimization in industrial settings. The practical value lies in the ability to predict and control overlay properties through rational parameter selection, reducing the trial-and-error approach that often characterizes surfacing process development in production environments.