Plasma Arc Overlay Welding for Projectile Driving Band Manufacturing

Literature Overview

This paper by Wang Long, published in Welding (1997, Issue 7), describes an innovative application of plasma arc overlay welding to manufacture driving bands for military projectiles. Driving bands serve the critical function of sealing the propellant gas behind the projectile and imparting spin stabilization through engagement with the rifling grooves of the gun barrel. The conventional manufacturing method produced driving bands that failed to meet national military standards during projectile strength testing, specifically because the pre-stress from driving band engagement caused excessive dimensional changes in the already thin projectile body.

Technical Challenge and Plasma Arc Solution

The fundamental challenge is achieving a driving band with precise dimensional tolerance, controlled hardness profile, and sufficient bond strength to the projectile body, all while minimizing the thermal effect on the thin projectile wall. Conventional methods such as brazing or press-fitting introduce either excessive heat or mechanical distortion that compromises the projectile's structural integrity.

Plasma arc overlay welding offers several advantages for this application:

Advantage	Mechanism	Benefit to Driving Band
High energy density	Plasma jet temperature 10,000–30,000 K	Deep, narrow penetration with minimal lateral heat spread
Precise parameter control	Arc current, gas flow, and travel speed are independently adjustable	Tight control over dilution and microstructure
Low dilution	Plasma arc transfers wire with minimal base metal melting	Preserves projectile body material properties
Fast deposition rate	Compared to TIG welding	Suitable for production-scale manufacturing
Uniform bead geometry	Stable plasma arc produces consistent bead profile	Critical for driving band dimensional accuracy

Process Parameters and Microstructure

The plasma arc overlay welding process for driving bands typically employs the following parameters:

Parameter	Range	Effect on Microstructure
Arc current	100–200 A	Controls heat input and penetration depth
Transfer gas flow	8–15 L/min (Ar)	Shields weld pool, stabilizes plasma jet
Sheath gas flow	2–4 L/min (Ar)	Protects torch nozzle
Wire feed speed	200–400 mm/min	Controls deposition rate and dilution
Travel speed	300–600 mm/min	Controls bead width and height
Standoff distance	5–10 mm	Maintains arc stability and energy density

Microstructural Analysis

The resulting driving band microstructure exhibits a gradient from the projectile body through the overlay layers:

Projectile body zone: Unaffected or minimally affected base material with original microstructure retained. The heat-affected zone is typically limited to 0.5–1.0 mm due to the high energy density of the plasma arc.
Dilution zone: A narrow transition region (0.1–0.3 mm) where base metal and filler metal are mixed. The composition and microstructure here depend on the dilution ratio, which plasma arc welding can control to 5–15%.
Overlay zone: The functional driving band material with the desired hardness and wear resistance. For military driving bands, typical compositions include high-strength steel with controlled carbon content (0.3–0.6%) and alloying elements such as chromium, molybdenum, and vanadium to achieve hardness of 35–45 HRC.

Dimensional Accuracy and Quality Control

The driving band must meet stringent dimensional requirements:

Dimension	Tolerance	Measurement Method
Outer diameter	±0.05 mm	Coordinate measuring machine
Width	±0.03 mm	Micrometer or optical comparator
Hardness	35–45 HRC ±2	Rockwell hardness tester
Bond strength	≥ 150 MPa	Peel test or shear test
Surface roughness	Ra ≤ 0.8 μm	Surface profilometer

The plasma arc process achieves these tolerances through precise control of the arc parameters and travel speed. The narrow bead profile produced by the plasma arc allows for building up the driving band in multiple passes, each adding a controlled increment of material.

Comparison with Alternative Processes

Process	Dilution (%)	HAZ Width (mm)	Productivity	Dimensional Control	Cost
Plasma arc overlay	5–15	0.5–1.0	High	Excellent	Moderate
TIG welding	15–30	1.0–2.0	Low	Good	Low
Brazing	0 (no melting)	N/A	High	Good	Moderate
Press-fitting	N/A	N/A	Very high	Excellent	Low

The plasma arc method offers the best balance of dilution control, productivity, and dimensional accuracy for this application. Brazing, while offering zero dilution, requires careful joint design and may not provide sufficient bond strength for the cyclic loading experienced in artillery firing.

Engineering Significance

This application demonstrates the versatility of plasma arc technology beyond conventional welding. The same principles apply to overlay welding in other high-precision applications such as hardfacing of cutting tools, repairing aerospace components, and manufacturing specialized mechanical parts where both material properties and dimensional accuracy are critical. The key insight is that the plasma arc's high energy density and precise controllability make it suitable for applications where conventional welding processes introduce unacceptable thermal distortion or dilution.

For engineers considering plasma arc applications in their own work, the lesson is that process selection should be driven by the specific requirements of the application rather than conventional wisdom. In this case, the military specification for projectile driving bands could not be met by conventional methods, and the solution required a deliberate evaluation of alternative processes and their metallurgical consequences.