Plasma Arc Overlay of Nickel-Based Alloy on Control Valve Components

Literature Overview

Published in "Welding" (1994, Issue 5, pp. 20-21), this paper by Xu Hongbo from Nanchang Valve Factory presents a practical application of plasma arc overlay welding (PAW) for depositing nickel-based alloy on control valve sealing surfaces. The work is classified under TG455 and addresses a specific industrial requirement: producing liquid-level control valves with 2Cr13 stainless steel valve plates and nickel-based alloy sealing faces.

Technical Requirements and Specifications

The paper provides clear performance specifications for the overlay deposit, which serve as acceptance criteria for the welding process:

The abstract explicitly states that the sealing face must be machined after overlay welding, with the final thickness not less than 2.5 mm. This requirement implies that the initial overlay deposit thickness must be significantly greater (typically 4-6 mm) to allow for post-weld machining.

Weldability Analysis

The abstract notes that 2Cr13 is classified as a martensitic steel, which carries important implications for welding behavior. Martensitic stainless steels are known for their susceptibility to cold cracking during welding due to the formation of hard, brittle martensite in the HAZ. The high carbon equivalent and the tendency toward martensitic transformation during rapid cooling create significant residual stresses that can initiate cracks.

Key weldability concerns include:

1. Cold cracking susceptibility: The martensitic structure of 2Cr13 is prone to hydrogen-induced cracking, especially in the HAZ where rapid cooling occurs
2. Heat input sensitivity: Excessive heat input can lead to grain coarsening and reduced toughness in the HAZ
3. Residual stress: The high thermal expansion mismatch between the nickel-based overlay and the steel base generates significant interfacial stresses
4. Dilution: The chemical composition of the overlay deposit is affected by dilution from the base metal, which can reduce the corrosion resistance and hardness of the final deposit

Plasma Arc Welding Process Parameters

Plasma arc welding offers several advantages for this application compared to conventional arc welding methods:

• Concentrated heat source: The plasma arc provides a high energy density (up to 10^8 W/m²), resulting in a narrow, deep weld pool with minimal dilution of the base metal
• Precise control: Arc current, gas flow rate, and nozzle-to-workpiece distance can be precisely controlled to optimize deposit composition
• Reduced HAZ: The concentrated heat input minimizes the heat-affected zone, reducing the risk of cracking in the martensitic base material
• Single-pass capability: For thin deposits, a single pass may suffice, reducing thermal cycling

Typical PAW parameters for nickel-based overlay on steel substrates include:

Engineering Practice and Quality Control

The requirement for HRC ≥ 38 hardness in the overlay layer indicates that the nickel-based alloy must maintain its as-welded hardness without extensive post-weld heat treatment. Most Stellite-type alloys (e.g., Stellite 6, Stellite 21) achieve this hardness range in the as-welded condition due to the formation of hard carbide phases (Cr7C3, Mo2C) during solidification.

The absence of cracks, porosity, and slag inclusions is a critical quality requirement. In practice, achieving this level of quality requires:

• Careful control of powder feed rate and arc stability
• Proper shielding gas coverage to prevent oxidation
• Clean base metal surface preparation (grinding or machining to remove scale and contaminants)
• Interpass temperature monitoring to prevent overheating

The post-weld machining requirement means that the overlay surface must be flat and free of significant undercut or overlap. This imposes additional constraints on the welding parameters and may require the use of a backing plate or a specific welding sequence to ensure uniform deposit thickness.

Study Insights

This paper represents a well-defined industrial application where the welding process must meet specific mechanical and metallurgical requirements. The choice of PAW over conventional SMAW or GTAW is justified by the need for low dilution and precise control over the deposit composition. The 2Cr13 base material presents a challenging weldability situation, but the plasma arc process mitigates many of these concerns through its concentrated heat input and controlled deposition rate.

From a broader perspective, this work illustrates the importance of matching the welding process to the specific application requirements. The valve sealing surface demands both hardness and corrosion resistance, which the nickel-based alloy provides, but only if the welding process preserves the intended composition through minimal dilution.