Weld Overlay Repair of Fiber Carbon Forming Machine Screw Shafts

Background and Component Criticality

Screw shafts in fiber carbon forming machines operate under severe tribological conditions, combining high axial loads, cyclic torsional stresses, and abrasive contact with carbon fiber composite materials. The working surfaces of these shafts are subject to rapid wear, often within thousands of hours of continuous operation, necessitating periodic weld overlay repair to restore dimensional tolerances and surface hardness. This study note addresses the metallurgical challenges, process selection, and quality assurance protocols associated with the weld overlay repair of such critical rotating components.

Material Selection and Overlay Strategy

The base material of fiber carbon forming machine screw shafts is typically a medium-carbon alloy steel such as 42CrMo or 40CrNiMoA, quenched and tempered to a hardness of 28–32 HRC. The overlay material must provide wear resistance significantly exceeding the base hardness while maintaining adequate toughness to resist impact loading. Common overlay material selections include:

Overlay Material	Hardness (HRC)	Dilution Tolerance	Typical Application
D2 Tool Steel	58–62	Moderate	General wear surfaces
Stellite 6	40–45	Low	High-temperature wear
Hardfacing Flux-Cored Wire	55–60	High	Heavy abrasive wear
56% Cr Alloy	50–55	Moderate	Moderate abrasion

The selection of overlay material must account for the dilution that inevitably occurs during welding. When using submerged arc welding (SAW) or gas metal arc welding (GMAW) on the 42CrMo substrate, dilution rates of 10–25% are typical. This means that the as-deposited overlay hardness will be lower than the pure material hardness, and the overlay composition must be selected accordingly. For example, a Stellite 6 overlay with 10–15% dilution from 42CrMo will yield an as-welded hardness of approximately 38–42 HRC, which may be insufficient for the application. In such cases, a multi-pass approach with a transition layer of 56% Cr followed by a final Stellite 6 pass is recommended to achieve the target hardness.

Welding Process Selection and Parameters

The geometry of screw shafts presents unique challenges for weld overlay: the helical thread profile requires careful groove preparation, and the rotational symmetry demands uniform heat input around the circumference. The following process options are commonly employed:

Submerged Arc Welding (SAW): Suitable for large-diameter shafts where a continuous overlay bead is required. The flux provides excellent protection and allows high deposition rates. Typical parameters include 500–700 A, 28–32 V, and a travel speed of 200–350 mm/min.
Gas Metal Arc Welding (GMAW): Preferred for smaller diameters and where precise bead placement is needed. Parameters typically range from 150–250 A, 22–26 V, with a self-shielded or gas-shielded hardfacing wire.
Plasma Transferred Arc (PTA): Offers the lowest dilution (5–10%) and excellent compositional control, making it ideal for critical applications where overlay purity is paramount. Parameters include 100–200 A, with powder feed rates of 100–300 g/min.

The groove geometry for screw shaft overlay is critical. A U-groove with a root radius of 1.5–2.0 mm and an included angle of 60–75° provides adequate access for the welding torch while minimizing dilution. The groove depth should be limited to 2–3 mm per pass to control heat input and prevent excessive softening of the substrate heat-affected zone.

Heat Treatment and Post-Weld Stress Relief

Following weld overlay, the screw shaft must undergo stress relief to eliminate residual stresses that could lead to distortion or fatigue failure during service. For 42CrMo shafts with overlay hardness exceeding 45 HRC, a tempering cycle at 540–580°C for 2–4 hours is recommended. This treatment simultaneously relieves welding residual stresses and stabilizes the overlay microstructure without significantly reducing overlay hardness.

It is important to note that the overlay material's hardness may decrease by 5–10 HRC after tempering. Therefore, the overlay material selection must account for this reduction. For example, if the target service hardness is 55 HRC, the as-welded overlay should be selected to achieve 60–62 HRC before tempering.

Defect Analysis and Prevention

The most common defects in screw shaft weld overlay include:

Defect	Location	Root Cause	Prevention
Cracking	Overlay/substrate interface	High carbon in HAZ, insufficient preheat	Preheat to 200°C, use low-carbon filler
Porosity	Overlay surface	Inadequate shielding, flux contamination	Ensure proper flux drying, maintain gas flow
Excessive Dilution	First pass	High heat input, deep groove	Reduce groove depth, use PTA for first pass
Hardness Inconsistency	Along shaft length	Inconsistent travel speed, operator variation	Use automated welding, monitor parameters
Distortion	Shaft body	Uneven heat input, lack of clamping	Apply circumferential clamping, control interpass temp

Study Insights and Practical Recommendations

The literature underscores that the success of weld overlay repair on screw shafts depends not only on the welding process itself but also on the subsequent machining and heat treatment operations. The overlay must be deposited with sufficient build-up height to allow for post-weld machining to the final dimensional tolerance, typically 0.5–1.0 mm of excess build-up. Furthermore, the overlay surface must be machined smooth to achieve a surface roughness of Ra ≤ 1.6 μm to ensure proper contact with the carbon fiber composite material. The study reinforces the importance of integrating welding, machining, and heat treatment into a unified repair protocol rather than treating them as independent operations.