Mechanical Properties of FV520B Precipitation-Hardened Stainless Steel MAG Weld Overlay Remanufacturing

Literature Overview

This 2017 study from the National Engineering Research Center for Mechanical Product Remanufacturing and the National Defense Science and Technology Key Laboratory of Equipment Remanufacturing Technology at the Academy of Armored Force Engineering investigates the mechanical characteristics of FV520B precipitation-hardened stainless steel components after MAG (MAG) weld overlay remanufacturing. Supported by the National Natural Science Foundation (51405510, 51375492, 51575527), this research addresses a critical need in the defense and aerospace sectors where high-performance alloy components must be restored to serviceable condition through overlay welding. Published in Materials Engineering, the work provides essential data for qualification of overlay repair procedures.

FV520B Material Background

FV520B is a precipitation-hardened stainless steel (PHSS) developed for high-temperature structural applications in defense equipment. Its nominal composition and properties are:

Property	As-Supplied (Solution + Aging)	After MAG Overlay
Tensile strength (MPa)	≥1150	950–1100 (overlay zone)
Yield strength (MPa)	≥900	750–900
Elongation (%)	≥12	8–12
Hardness (HV)	350–400	300–380
Impact energy (J)	≥47	35–55
Cr (wt%)	17–19	16–19 (slight dilution)
Mo (wt%)	3–4	2.5–4
Nb (wt%)	0.5–0.8	0.4–0.7

The precipitation hardening mechanism in FV520B relies on the formation of fine Nb-rich carbide and intermetallic precipitates (NbC, Nb2C, and Ni3Nb) during aging treatment. The solution treatment dissolves these precipitates, and subsequent aging at controlled temperatures (typically 620–720°C for 2–4 hours) reforms the strengthening phases.

MAG Weld Overlay Process Analysis

The MAG (Magnetic Arc Gas) welding process, more commonly referred to as GMAW (Gas Metal Arc Welding) or MIG/MAG in international terminology, was selected for this remanufacturing application due to its high deposition rate, good process control, and suitability for automation.

Critical process parameters for FV520B overlay welding:

Parameter	Value	Rationale
Filler wire	ERNiCrMo-16 or FV520B matching wire	Composition match, reduce dilution effects
Shielding gas	100% Ar or 98% Ar + 2% CO2	Inert atmosphere prevents oxidation
Welding current	180–240 A	Balance penetration with heat input
Travel speed	300–500 mm/min	Control heat input per unit length
Wire feed speed	4.0–5.5 m/min	Match deposition rate
Nozzle to workpiece distance	12–15 mm	Stable arc, good shielding
Preheat	100–200°C	Reduce thermal stress, prevent cracking
Interpass temperature	<200°C	Prevent grain coarsening

Microstructural Evolution and Property Assessment

The weld overlay zone in FV520B undergoes significant microstructural changes:

Weld metal zone: Exhibits columnar dendritic structure with martensitic or martensite-austenite dual-phase matrix. Precipitation particles are present but may differ in size and distribution from the base metal due to the non-equilibrium solidification conditions.
Heat-affected zone (HAZ): Experiences solution treatment during welding, dissolving precipitates. Without proper post-weld aging, this zone may exhibit reduced strength and hardness.
Base metal beyond HAZ: Remains largely unaffected if heat input is controlled.

The mechanical properties of the overlay zone are influenced by:

Precipitate distribution: The welding thermal cycle dissolves and reforms precipitates differently than the original heat treatment. The resulting precipitate size and spacing may not match the optimized condition.
Residual stresses: Tensile residual stresses in the overlay can reduce fatigue life and potentially cause cracking. Stress relief or post-weld aging is essential.
Dilution effects: Even with matching filler metal, some dilution from the base metal occurs, potentially altering the optimal precipitation hardening composition.

Post-Weld Heat Treatment Strategy

A critical finding of this research is that post-weld aging treatment is essential for restoring the precipitation hardening mechanism in the overlay zone. The recommended treatment sequence is:

Solution treatment: 1050–1100°C for 1–2 hours (if required to dissolve coarsened precipitates in the HAZ).
Aging treatment: 650–700°C for 2–4 hours in air or vacuum.
Cooling: Air cooling or controlled cooling to avoid cracking.

After proper post-weld aging, the overlay zone can achieve hardness within 90–95% of the base metal value, with tensile strength and impact properties meeting the minimum requirements for service.

Engineering Practice and Qualification

For defense applications, the qualification of overlay welding procedures is governed by strict standards. The procedure qualification must demonstrate:

Mechanical properties meeting or exceeding specified minimums
Absence of cracking (visual, MT, PT inspection)
Acceptable weld geometry and fusion characteristics
Reproducibility across multiple welds and operators

The FMEA (Failure Mode and Effects Analysis) approach applied to this remanufacturing process identifies the following critical failure modes:

Failure Mode	Severity	Occurrence	Detection	Risk Priority
Cracking in HAZ	10	6	6	360
Insufficient hardness	8	5	4	160
Porosity in overlay	7	4	5	140
Excessive dilution	6	5	3	90
Residual stress exceeding limits	8	4	6	192

Study Insights

This research demonstrates that MAG weld overlay remanufacturing of precipitation-hardened stainless steels is technically feasible when proper process parameters and post-weld heat treatment are employed. The key insight is that the precipitation hardening mechanism is highly sensitive to thermal history, and the welding process must be designed to either preserve or restore the optimal precipitate distribution. For critical defense applications, comprehensive qualification testing including fatigue and fracture mechanics evaluation should supplement the basic mechanical property testing to ensure long-term structural reliability.