CLADDING TECHNOLOGY SHANXI CO., LTDSurface Strengthening of 45 Steel Mold by Tungsten Carbide Cladding
Literature Overview
Published in 2003 in the journal Die and Mould Industry, this study by Li Ke, Hu Zihua, and Pan Renjian from Xiangtan University investigates the use of tungsten carbide (WC) cladding to strengthen the surface of 45 steel molds. The research addresses a fundamental challenge in mold engineering: enhancing the wear resistance and surface durability of medium-carbon steel molds through the application of hard-facing cladding layers containing carbide-forming elements.
Core Technical Content
Background and Motivation
45 steel (equivalent to AISI 1045) is one of the most commonly used steels for mold manufacturing due to its favorable combination of strength, toughness, and machinability. However, its surface hardness and wear resistance are inherently limited, particularly when used in applications involving abrasive contact with workpieces or tooling. The study investigates the feasibility and effectiveness of depositing WC-based cladding layers on 45 steel surfaces to create a composite structure that combines the toughness of the base metal with the extreme hardness and wear resistance of the overlay.
The inclusion of WC in the cladding layer introduces a powerful hardening mechanism through the formation of secondary carbides during the welding process. When WC particles are melted during arc welding, the tungsten and carbon atoms react with iron and other alloying elements to form a complex network of M7C3, M2C, and M6C carbides (where M represents W, Cr, V, or Fe). These carbides provide exceptional hardness and wear resistance, making WC-based cladding layers highly effective for surface strengthening applications.
Cladding Process and Parameters
The study employed submerged arc welding (SAW) as the primary cladding method, which offers advantages of high deposition rate, deep penetration, and low spatter compared to other arc welding processes. The following parameters were optimized:
| Parameter | Value |
|---|---|
| Welding current | 300–400 A |
| Arc voltage | 28–35 V |
| Travel speed | 8–12 cm/min |
| Flux type | Low-alloy flux with WC addition |
| Wire composition | Low-carbon steel wire with alloying elements |
| Preheating temperature | 150–200°C |
| Cladding layer thickness | 3–5 mm |
| Number of passes | 2–3 |
The use of SAW with WC-containing flux or wire allows for controlled dilution and uniform distribution of carbide-forming elements throughout the cladding layer. The flux serves a dual purpose: it protects the molten weld pool from atmospheric contamination and acts as a carrier for the WC particles, ensuring their effective incorporation into the weld metal.
Microstructure Characterization
The microstructure of the WC-cladded layer exhibited a complex hierarchy of phases. The matrix consisted of martensite and bainite, depending on the cooling rate, while the carbide phase formed a network of angular and skeletal structures. The carbides were primarily composed of W2C, WC, and Fe3W6C, with additional Cr7C3 and Fe3C carbides formed from the interaction between tungsten and the iron matrix.
| Phase | Morphology | Hardness (HV) | Distribution |
|---|---|---|---|
| W2C | Angular, blocky | 2400–2800 | Dispersed in matrix |
| WC | Irregular | 2000–2500 | Network along grain boundaries |
| Fe3W6C | Skeletal | 1500–1800 | Interdendritic |
| Cr7C3 | Rod-like | 1200–1500 | Fine dispersion |
| Fe3C | Cementite | 800–1000 | Matrix constituent |
| Martensite | Lath structure | 550–650 | Matrix phase |
The study found that the cooling rate had a significant effect on the carbide morphology. Slower cooling rates promoted the formation of larger, coarser carbides, while faster cooling rates produced finer, more uniformly distributed carbide networks. The optimal cooling rate was determined to be in the range of 10–50°C/s, which produced a fine carbide network with good distribution and adequate toughness.
Mechanical and Wear Properties
| Property | Cladding Layer | Base Metal (45 Steel) | Improvement Factor |
|---|---|---|---|
| Hardness (HV) | 950–1100 | 220–280 | 3.5–5.0× |
| Abrasive wear resistance | 4.0–5.5× | 1.0 (reference) | 4.0–5.5× |
| Impact toughness | 8–12 J/cm² | 25–35 J/cm² | Reduced |
| Compressive strength | 3500–4200 MPa | 750–850 MPa | 4.0–5.0× |
The dramatic improvement in hardness and wear resistance achieved through WC cladding is attributed to the presence of extremely hard carbide particles (HV > 2000) embedded in a tough martensitic matrix. The wear resistance improvement of 4–5.5 times the base metal makes WC-cladded surfaces highly suitable for applications involving severe abrasive wear, such as punch dies, blanking dies, and forming tools.
Engineering Practice Integration
Process Optimization Using PDCA Cycle
The development of a reliable WC cladding process for 45 steel molds can be systematically approached using the PDCA (Plan-Do-Check-Act) cycle:
- Plan: Define the target hardness (HV 950–1100), cladding thickness (3–5 mm), and wear resistance improvement (≥4×). Select appropriate wire composition and flux with controlled WC content (typically 15–30% by weight).
- Do: Execute the cladding process with optimized parameters, maintaining preheating at 150–200°C and interpass temperature below 250°C. Ensure proper surface preparation and flux coverage.
- Check: Perform hardness profiling, metallographic examination, and wear testing to verify that the cladding layer meets the specified requirements. Check for defects such as porosity, cracks, and lack of fusion.
- Act: Adjust welding parameters based on test results. If hardness is insufficient, increase WC content or adjust cooling rate. If cracking is observed, increase preheating temperature or modify the filler composition.
Defect Analysis and Prevention
| Defect | Root Cause | Prevention Strategy |
|---|---|---|
| Tungsten carbide segregation | Uneven distribution of WC in flux or wire | Pre-mix WC into flux with uniform particle size distribution |
| Cracking at interface | Thermal mismatch and residual stress | Control preheating and interpass temperature; use transition layer |
| Excessive porosity | Inadequate flux coverage or high travel speed | Ensure continuous flux coverage; optimize travel speed |
| Dilution softening | Excessive base metal melting | Use lower current, higher travel speed, or multi-pass with thinner layers |
| Carbide coarsening | Slow cooling rate | Increase cooling rate through water quenching or reduced layer thickness |
Application Scenarios
WC cladding on 45 steel molds is particularly effective in the following industrial applications:
- Punch and die sets: Blanking, piercing, and forming operations where abrasive wear from sheet metal contact is the primary failure mode.
- Cold heading dies: High-cycle applications involving repeated contact with steel wire or bar stock.
- Wire drawing dies: Where the die bore surface is subject to severe abrasive wear from high-speed wire passage.
- Roller surfaces: Contact surfaces in rolling mills or calendering equipment that experience continuous abrasive contact.
Key Questions and Reflections
A critical question arising from this study is the long-term stability of WC-based cladding layers under thermal cycling conditions. While WC carbides provide exceptional hardness and wear resistance at room temperature, their effectiveness may be compromised at elevated temperatures where diffusion-driven coarsening and phase transformations can occur. The study does not extensively address the high-temperature performance of WC cladding layers, which limits its applicability to hot-work applications. Engineers should be aware of this limitation and consider alternative cladding compositions for high-temperature service.
Another important consideration is the machinability of the cladding layer. The extremely hard WC carbides make subsequent machining of the cladded surface extremely difficult, requiring specialized tooling such as diamond or CBN inserts. This poses a challenge for applications where the cladded surface requires precise dimensional finishing. The study suggests that cladding should be applied as a final operation, with dimensional accuracy achieved through the welding process itself rather than post-weld machining.
Study Insights and Implications
This research demonstrates that WC cladding is a highly effective method for surface strengthening of 45 steel molds, achieving 3.5–5 times the hardness and 4–5.5 times the wear resistance of the base metal. The key to success lies in optimizing the welding parameters to achieve fine, uniformly distributed carbides while maintaining adequate toughness in the cladding layer. The study provides a solid foundation for the industrial application of WC-based cladding in mold manufacturing.
From a practical standpoint, the economic benefits of WC cladding are significant. The ability to extend the service life of 45 steel molds by 4–5 times through surface cladding can substantially reduce tooling costs and production downtime. However, engineers must carefully evaluate the cost-benefit ratio, considering the additional time and expense of the cladding process, the specialized equipment required, and the challenges associated with post-weld finishing.
Summary
The study on WC cladding of 45 steel molds provides compelling evidence that tungsten carbide-based overlay layers can dramatically enhance the surface properties of medium-carbon steel molds. The cladding layer achieves hardness levels of 950–1100 HV with 4–5.5 times the wear resistance of the base metal, making it an attractive solution for applications involving severe abrasive wear. The success of the cladding process depends on careful control of welding parameters, particularly the cooling rate and WC distribution, to achieve fine carbide networks with good mechanical properties. Engineers should adopt a systematic approach to process optimization, combining metallographic analysis with mechanical testing to ensure reliable performance in service.