CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Annealing Temperature on Microstructure and Hardness of Ni60/WC Cladding Layer on H13 Steel by Plasma Transfer Arc Welding
Research Background and Motivation
The H13 hot work tool steel, designated as 4Cr5MoSiV1 in the Chinese standard system, is widely used for hot forging dies, extrusion dies, and casting molds where resistance to thermal fatigue, wear, and high-temperature strength are critical. However, the base H13 steel often suffers from insufficient surface hardness and wear resistance under aggressive working conditions, necessitating surface hardening through weld overlay cladding. The Ni60/WC composite cladding system, applied via plasma transferred arc (PTA) welding, has emerged as a promising solution because it combines the excellent bonding strength and thermal shock resistance of the Ni-based Ni60 alloy with the extreme hardness and low-friction coefficient of tungsten carbide particles. The annealing temperature of the cladding layer is a critical process variable that governs the final microstructure, hardness profile, and functional performance of the overlay.
Microstructural Evolution with Annealing Temperature
The PTA Ni60/WC cladding layer as-welded typically exhibits a dendritic microstructure consisting of an austenitic Ni-Fe matrix with eutectic carbides formed by the interaction between WC particles and the Ni-Cr matrix. During PTA deposition, the rapid solidification rate produces a fine, non-equilibrium microstructure with dissolved carbon and chromium in the matrix and partially decomposed WC particles that react with nickel to form Ni3W, Ni4W, and Cr7C3 phases. The as-welded hardness of such cladding layers typically ranges from 700 to 850 HV, depending on the WC particle size, volume fraction, and deposition parameters.
Post-weld annealing treatment is essential to relieve residual stresses, homogenize the microstructure, and optimize the carbide morphology and distribution. As the annealing temperature increases, several metallurgical transformations occur sequentially:
| Annealing Temperature (degrees C) | Primary Microstructural Changes | Typical Hardness (HV) | Carbide Morphology |
|---|---|---|---|
| 700 | Stress relief, minimal carbide change | 780-830 | Retained as-welded morphology |
| 750 | Onset of carbide coarsening, matrix softening | 740-790 | Slight rounding of carbide edges |
| 800 | Significant carbide coarsening, partial dissolution | 680-740 | Larger, more isolated carbide islands |
| 850 | Extensive carbide coarsening, matrix austenitization | 600-680 | Coarse, irregular carbide networks |
| 900 | Severe carbide dissolution, matrix homogenization | 500-600 | Sparse, large carbide particles |
The optimal annealing temperature for most engineering applications falls in the range of 750 to 800 degrees Celsius, where a favorable balance between hardness retention and residual stress relief is achieved. Temperatures above 850 degrees Celsius lead to excessive carbide coarsening and dissolution, resulting in significant hardness loss that compromises the wear resistance of the cladding layer.
Hardness-Structure Relationship and Engineering Trade-offs
The hardness of the Ni60/WC cladding layer is primarily governed by the volume fraction, size, and distribution of hard carbide phases within the Ni-Fe matrix. WC particles, with an intrinsic hardness exceeding 2400 HV, serve as the primary reinforcement. However, during PTA deposition, WC particles undergo partial chemical reaction with the molten Ni60 alloy, forming intermediate carbides and dissolving partially into the matrix. The extent of this reaction is influenced by the WC particle size, where smaller particles (below 45 micrometers) react more completely, while larger particles (above 150 micrometers) retain more of their original structure but may develop microcracks at the particle-matrix interface.
The annealing temperature directly affects the carbide-matrix interface integrity. At moderate annealing temperatures, the diffusion-driven homogenization reduces interface stresses and improves the bonding strength between carbide particles and the matrix, which can enhance the effective hardness contribution of the reinforcement phase. However, excessive annealing temperatures promote carbide coarsening through Ostwald ripening, reducing the total interfacial area and diminishing the dispersion strengthening effect. The resulting hardness profile after annealing typically shows a gradient from the cladding surface toward the fusion line, with the highest hardness near the surface where the WC particle concentration is greatest and the lowest hardness near the fusion zone where dilution by the H13 base metal occurs.
Process Optimization Recommendations
Based on the findings of this study and general engineering practice, the following process parameters are recommended for PTA Ni60/WC cladding on H13 steel:
- PTA deposition current: 180 to 250 amperes, with an arc voltage of 22 to 28 volts.
- Travel speed: 150 to 300 millimeters per minute, depending on the desired cladding thickness.
- Powder feed rate: 1.5 to 2.5 kilograms per hour for Ni60/WC composite powder with 5 to 15 weight percent WC content.
- Preheat temperature of H13 substrate: 200 to 300 degrees Celsius to minimize thermal cracking and control the cooling rate.
- Post-weld annealing: 750 to 800 degrees Celsius for 2 to 4 hours in air or controlled atmosphere, followed by furnace cooling.
The interpass temperature during multi-pass cladding should be maintained below 400 degrees Celsius to prevent excessive grain growth in previously deposited layers, which would adversely affect the final hardness and toughness of the cladding.
Critical Reflections and Practical Considerations
A key insight from this research is that the annealing temperature must be selected not only to optimize hardness but also to ensure adequate bonding strength between the cladding layer and the H13 substrate. Over-annealing, while it may produce a desirable carbide morphology in the bulk of the cladding, can also soften the fusion zone and reduce the overall joint strength. In applications where the cladding is subjected to cyclic thermal and mechanical loading, such as in hot forging dies, the toughness of the fusion zone is as important as the surface hardness. Therefore, the annealing temperature should be validated through combined hardness and impact testing, rather than hardness measurements alone.
Another practical consideration is the effect of annealing on the residual stress distribution. PTA cladding typically introduces significant tensile residual stresses in the cladding layer and compressive stresses in the substrate near the fusion line. Proper annealing treatment can reduce these stresses by 50 to 70 percent, significantly improving the fatigue life and resistance to spalling of the cladding layer. However, the degree of stress relief is temperature-dependent, and temperatures below 700 degrees Celsius provide only marginal stress reduction while temperatures above 850 degrees Celsius risk excessive microstructural degradation.
Summary
The study of annealing temperature effects on Ni60/WC PTA cladding on H13 steel demonstrates that post-weld heat treatment is a critical process variable that must be carefully optimized to achieve the desired balance of surface hardness, bonding strength, and residual stress relief. The optimal annealing window of 750 to 800 degrees Celsius provides the best compromise between hardness retention and microstructural stability, while temperatures outside this range lead to either insufficient stress relief or excessive carbide degradation. Engineers fabricating hot work tool claddings should adopt a systematic approach to annealing parameter selection, supported by microstructural characterization and mechanical property testing, to ensure reliable long-term performance of the cladded components.