Hardfacing Cladding Process and Application of Cone Bodies

Component Description and Service Requirements

Cone bodies, also known as conical reducers or conical transition pieces, are integral components in piping systems, pressure vessels, and process equipment that connect pipes or vessels of different diameters. In aggressive service environments such as chemical processing, pulp and paper, mining, and power generation, cone bodies are subjected to severe abrasive and corrosive wear, particularly at the internal surface where high-velocity fluid flow impinges on the conical geometry. The converging geometry of the cone creates localized high-velocity zones that accelerate wear, often resulting in premature failure within a fraction of the expected service life.

This study note examines the hardfacing cladding process for cone bodies, addressing the unique geometric challenges of welding on conical surfaces, the selection of hardfacing materials for different service conditions, and the quality control measures required to ensure reliable performance in aggressive environments.

Geometric Challenges and Welding Strategy

The conical geometry of these components presents significant challenges for hardfacing cladding. The varying circumference along the cone axis means that the welding travel speed and bead overlap must be continuously adjusted to maintain uniform coverage. Additionally, the internal surface of the cone is often difficult to access, requiring specialized welding positions and torch configurations.

The welding strategy for cone body hardfacing typically involves the following approach:

Surface Preparation: The base metal surface is ground to remove scale, rust, and contaminants. For internal surfaces, a grinding wheel or wire brush is used to expose clean metal to a depth of at least 2 mm.
Groove Preparation: A shallow U-groove with a depth of 1–2 mm and an included angle of 60–75° is machined or ground along the cone surface to promote mechanical bonding and reduce dilution.
Welding Sequence: The hardfacing is applied in a spiral or circumferential pattern, starting from the small end of the cone and progressing toward the large end. This sequence ensures that each pass is deposited on a previously solidified layer, minimizing dilution and promoting uniform coverage.
Multi-Pass Build-Up: For thick overlay requirements (3–5 mm), multiple passes are applied with a 50–70% overlap between adjacent passes. The first pass is deposited with a low-dilution wire or powder, and subsequent passes use the full-strength hardfacing material.

Hardfacing Material Selection

The selection of hardfacing material for cone bodies is governed by the dominant wear mechanism and the chemical environment. The following table summarizes the most commonly used hardfacing materials and their typical applications.

Material	Hardness (HRC)	Wear Mechanism	Typical Application
Stellite 6 (Co-Cr-W)	40–45	Abrasive, high-temp	Hot gas ducts, high-temp cones
Stellite 21 (Co-Cr)	45–50	Abrasive, moderate-temp	Pulp and paper, moderate-temp
56% Cr-Mo	50–55	Abrasive, moderate-temp	Mining, slurry service
60% Cr-Mo	55–60	Abrasive, high-temp	Cement, high-temp abrasion
D2 Tool Steel	58–62	Abrasive, impact	High-impact abrasion
Carbide-Bearing (WC)	60–65	Severe abrasion	Slurry, sand-laden fluids
Austenitic 310	25–30	Corrosive + moderate wear	Acidic environments

For cone bodies in chemical processing service where both corrosion and abrasion are present, a duplex strategy is often employed: a corrosion-resistant underlayer of 310 or 316L is deposited first, followed by a wear-resistant overlay of Stellite 6 or 56% Cr-Mo. This approach provides the best combination of corrosion resistance and wear resistance.

Process Parameters for Cone Body Hardfacing

The welding process parameters for cone body hardfacing depend on the cone diameter, wall thickness, and the selected welding process. For a typical cone body with a diameter range of 100–500 mm and a wall thickness of 10–20 mm, the following parameters are recommended:

Parameter	Small Cone (<200mm)	Medium Cone (200–400mm)	Large Cone (>400mm)
Process	GTAW or GMAW	SAW or FCAW	SAW or FCAW
Current (A)	150–250	400–600	500–800
Voltage (V)	18–22	28–32	30–35
Travel Speed (mm/min)	150–300	200–400	250–500
Preheat (°C)	150–200	200–250	250–300
Interpass Temp (°C)	≤250	≤250	≤250
Overlay Thickness (mm)	2–3	3–4	4–5

The preheat temperature is particularly important for cone bodies because the varying wall thickness along the cone axis creates non-uniform thermal conditions. The small end of the cone, with its smaller circumference and often thinner wall, cools faster than the large end. Preheating ensures that the cooling rate is uniform enough to prevent cracking in the hardfacing overlay.

Defect Analysis and Quality Control

The most common defects in cone body hardfacing include:

Defect	Location	Root Cause	Countermeasure
Cracking	Overlay surface	High carbon, rapid cooling	Preheat, reduce cooling rate, use low-C filler
Poor Bond	Overlay/substrate interface	Inadequate surface prep, high dilution	Improve surface prep, reduce dilution
Uneven Coverage	Along cone axis	Inconsistent travel speed	Use automated welding, monitor speed
Porosity	Overlay interior	Contaminated surface, inadequate shielding	Clean surface, increase gas flow
Excessive Dilution	First pass	High heat input, deep groove	Reduce groove depth, use lower current

Post-weld inspection includes visual examination, magnetic particle testing for surface cracks, and ultrasonic testing for bond integrity. Hardness testing across the overlay cross-section verifies that the target hardness has been achieved uniformly. For critical applications, a bond strength test per ASTM A263 or API 934 is performed to confirm that the overlay is metallurgically bonded to the substrate.

Application Case Study

A representative application involves the hardfacing of a 400 mm diameter cone body in a pulp and paper mill, where the cone is exposed to a slurry of wood fibers and chemicals at temperatures up to 120°C. The original cone body, fabricated from carbon steel, failed within 6 months due to severe abrasive wear at the internal surface. The repair strategy involved the following steps:

Removal of the worn cone body and surface preparation of the replacement cone.
Application of a 2 mm corrosion-resistant underlayer of 310L stainless steel using SAW with a 310L flux-cored wire.
Application of a 3 mm wear-resistant overlay of Stellite 21 using SAW with a Stellite 21 flux-cored wire.
Post-weld machining of the internal surface to restore the original conical geometry and achieve a surface roughness of Ra ≤ 3.2 μm.
Stress relief heat treatment at 540°C for 2 hours to eliminate residual stresses.

The repaired cone body achieved a service life exceeding 24 months, representing a fourfold improvement over the original carbon steel cone. The hardness profile measured across the overlay cross-section showed a uniform hardness of 42–46 HRC in the Stellite 21 layer, with a smooth transition to 28–32 HRC in the 310L underlayer and 25–28 HRC in the carbon steel substrate.

Study Insights and Engineering Recommendations

The literature reinforces that the successful hardfacing of cone bodies requires a holistic approach that integrates material selection, process optimization, geometric considerations, and quality control. The conical geometry demands a welding strategy that accounts for the continuously varying circumference and wall thickness, and the hardfacing material must be selected to address the specific combination of abrasive and corrosive wear mechanisms present in the service environment. The case study demonstrates that a well-designed hardfacing strategy can extend component life by a factor of three to five, providing substantial economic and operational benefits. The key to success lies in the careful selection of overlay materials, the disciplined execution of the welding process, and the rigorous verification of overlay quality through non-destructive and destructive testing.