Surface Hardening of 45 Steel Molds by Tungsten Carbide Cladding

Introduction and Motivation

45 steel (a medium-carbon steel with approximately 0.42–0.50% carbon) is one of the most widely used materials for mold fabrication due to its favorable combination of strength, toughness, and machinability. However, 45 steel molds suffer from limited wear resistance, particularly in applications involving abrasive contact, such as forging dies, extrusion dies, and forming tools. The surface hardening of 45 steel molds through tungsten carbide (WC) cladding offers a promising solution to extend mold life while retaining the toughness and formability of the base material.

This study note examines the metallurgical aspects, process parameters, and practical considerations of WC cladding on 45 steel molds, drawing on both published research and engineering experience.

Metallurgical Behavior of WC Cladding on 45 Steel

The cladding of WC-based hardfacing alloys on 45 steel involves complex metallurgical transformations at the interface. During the cladding process, the high heat input causes the base metal to partially melt and mix with the molten WC-based filler material, creating a transition zone with a composition gradient.

The microstructure of a typical WC-Co based hardfacing deposit on 45 steel consists of several distinct zones:

Base metal zone: The unaltered 45 steel substrate, which may show a heat-affected zone (HAZ) with modified grain structure and hardness.
Diffusion zone: A thin layer (typically 50–200 μm) where carbon and alloy elements have diffused into the base metal, creating a hardened layer.
Transition zone: A region where the composition transitions from base metal to overlay, characterized by a mixture of WC, Fe3C, and possibly W2C phases.
Overlay zone: The primary cladding layer consisting of WC particles dispersed in a cobalt or iron-nickel binder matrix.

The presence of W2C in the transition zone is a common concern because W2C is a brittle, low-hardness phase that can significantly reduce the wear resistance of the cladding. The formation of W2C is promoted by high carbon activity at the interface and is particularly likely when the WC content in the filler material is high (>70 wt%).

Microstructural Phase	Hardness (HV)	Brittleness	Formation Condition
WC	2400–2800	High	Stable at all temperatures
W2C	1000–1200	Very High	High carbon activity, >1000°C
Fe3C	800–1100	Moderate	Carbon enrichment at interface
Co solid solution	200–400	Low	Binder phase
Martensite (in HAZ)	500–700	Moderate	Rapid cooling of base metal

Process Selection and Parameter Optimization

Several cladding processes are suitable for WC-based hardfacing of 45 steel molds, each with distinct advantages:

Submerged Arc Welding (SAW) overlay: Offers high deposition rates and good metallurgical quality. Suitable for thick overlays (up to 10–15 mm). The flux provides excellent protection and deoxidation. Typical parameters: current 400–600 A, voltage 30–40 V, travel speed 200–400 mm/min.
Gas Metal Arc Welding (GMAW) overlay: Offers good process control and flexibility. Suitable for medium-thickness overlays (2–8 mm). Typical parameters: current 200–350 A, voltage 25–35 V, travel speed 300–600 mm/min.
Flame spraying (oxy-fuel): Suitable for thin overlays (1–5 mm) and complex geometries. Lower deposition rate but excellent process control. Typical parameters: heat input 1.5–3.0 kW, travel speed 50–150 mm/min.
Plasma Transferred Arc (PTA) cladding: Offers excellent metallurgical quality and the ability to deposit high-performance alloys. Suitable for medium-thickness overlays (2–10 mm). Typical parameters: current 200–400 A, powder feed rate 200–500 g/min, travel speed 200–500 mm/min.

The selection of the cladding process depends on several factors:

Required overlay thickness: Thick overlays favor SAW or multi-pass GMAW, while thin overlays favor flame spraying or PTA.
Geometry complexity: Complex geometries favor processes with good access and flexibility, such as GMAW or flame spraying.
Production volume: High-volume production favors automated processes such as SAW or PTA with mechanized travel.
Required metallurgical quality: Critical applications favor PTA or SAW, while less demanding applications may use GMAW or flame spraying.

Performance Evaluation and Testing

The performance of WC-cladded 45 steel molds is evaluated through a combination of laboratory testing and field trials:

Hardness testing: The hardness profile through the overlay thickness is measured using Vickers or Knoop indentation. A typical target is a surface hardness of 1200–1500 HV for WC-Co based overlays, decreasing to 400–500 HV at the interface.

Wear testing: Pin-on-disk or block-on-ring wear tests are conducted under controlled conditions to compare the wear resistance of the cladded surface against untreated 45 steel and alternative hardfacing materials. Typical results show 3–8 times improvement in wear life for WC-cladded surfaces compared to untreated 45 steel.

Impact testing: Charpy V-notch impact tests on the cladded samples verify that the cladding does not excessively reduce the toughness of the base metal. A minimum impact energy of 20–30 J at room temperature is typically required for mold applications.

Crack resistance: The resistance to thermal shock and mechanical impact is evaluated through cyclic thermal testing and impact testing. The presence of W2C in the transition zone can significantly reduce crack resistance, and its formation must be minimized through process optimization.

Practical Considerations and Countermeasures

Several practical considerations must be addressed when implementing WC cladding on 45 steel molds:

Preheating: Preheating the base metal to 150–250°C reduces the thermal gradient and minimizes cracking in the HAZ. For thick sections or high-alloy overlays, preheating to 300–400°C may be required.
Interpass temperature control: Maintaining the interpass temperature between 150–250°C prevents excessive hardness in the HAZ and reduces residual stress.
Post-weld heat treatment: A tempering treatment at 500–600°C for 1–2 hours can reduce residual stress and improve toughness without significantly reducing the overlay hardness.
Overlay thickness control: The overlay thickness must be sufficient to provide the required wear life but not so thick as to cause excessive distortion or cracking. Typical thicknesses range from 3–8 mm for forging dies to 5–15 mm for heavy-duty applications.
Surface preparation: The base metal surface must be clean and free of scale, oil, and contamination. Machining or grinding to a smooth finish improves the bond quality and reduces the risk of inclusions.

Engineering Case Study

A typical application involves the cladding of a 45 steel forging die used for producing automotive components. The die experiences severe abrasive wear from the hot metal flow and contact with abrasive scales. After several thousand shots, the die surface develops excessive wear, requiring regrinding or replacement.

By applying a 5 mm thick WC-Co based hardfacing overlay to the die surface, the service life is extended from approximately 2000 shots to 8000–12000 shots, representing a 4–6 times improvement. The overlay is applied using multi-pass SAW with a WC-Co filler wire, followed by a tempering treatment at 550°C for 2 hours. The resulting hardness profile shows a surface hardness of 1350 HV, decreasing to 550 HV at the interface, with no evidence of W2C formation in the transition zone.

Study Insights

The surface hardening of 45 steel molds by WC cladding is a well-established technology that offers significant productivity benefits when properly implemented. The key to success lies in the careful selection of the cladding process, the optimization of process parameters to minimize W2C formation, and the implementation of appropriate post-weld heat treatment. The metallurgical complexity of the interface between the WC-based overlay and the 45 steel substrate requires a thorough understanding of phase transformations and diffusion behavior to achieve the desired performance.