Analysis of Fusion Zone Microstructure and Hardness Gradient in Gradient Surfacing on 45 Steel Substrate

Literature Overview

This 2015 paper by Zhu Chengjun and Li Sicheng from Henan Polytechnic Institute examines the fusion zone microstructure and hardness gradient in a two-step gradient surfacing process on 45 steel substrate. Two filler materials are employed: Material 1 (C-Cr-Mo-Ni system) as a transition layer and Material 2 (Cr-Mo-Ni-W-V system) as the wear-resistant layer. The study compares three configurations—substrate + Material 1, substrate + Material 2, and substrate + Material 1 + Material 2—to evaluate the effectiveness of gradient design in achieving smooth microstructural and hardness transitions.

Core Technical Content

Gradient Surfacing Concept

Gradient surfacing is a technique designed to address the fundamental challenge of joining dissimilar materials with significantly different properties. When a high-hardness wear-resistant overlay is directly deposited on a moderate-strength substrate, several problems arise:

• Large hardness mismatch at the fusion boundary causes stress concentration
• Differential thermal expansion between layers promotes delamination during thermal cycling
• Carbon redistribution at the fusion zone creates decarbonized or hypercarbonized zones
• Microstructural incompatibility leads to poor interfacial bonding

The gradient approach introduces an intermediate layer with properties between the substrate and the final wear layer, creating a smooth transition.

Material Composition and Properties

Microstructural Analysis of Fusion Zones

Substrate + Material 1 Configuration:

• Fusion zone exhibits a gradient from pearlite/ferrite (substrate) to martensite with dispersed carbides (surfacing layer)
• Carbon redistribution is moderate, with a narrow decarbonized zone (0.05–0.1 mm) on the substrate side
• Grain size transition is gradual, with no sharp grain boundary discontinuity
• Hardness gradient: 240 HV → 350 HV → 480 HV over approximately 1.5 mm

Substrate + Material 2 Configuration:

• Fusion zone shows abrupt microstructural change from pearlite/ferrite to high-carbon martensite with dense carbide network
• Significant carbon redistribution creates a wide decarbonized zone (0.2–0.4 mm) and a hypercarbonized zone
• Coarse grain formation in the heat-affected zone (HAZ) on the substrate side
• Hardness gradient: 240 HV → 380 HV → 680 HV over approximately 1.0 mm (steep transition)

Substrate + Material 1 + Material 2 Configuration (Optimal):

• Two-step gradient provides smooth transition through three microstructural zones
• First fusion zone: pearlite/ferrite → fine martensite + dispersed carbides
• Second fusion zone: fine martensite → coarse martensite + dense carbides
• Carbon redistribution is distributed across both fusion zones, reducing local concentration
• Hardness gradient: 240 HV → 350 HV → 480 HV → 600 HV → 700 HV over approximately 3.0 mm

Process Parameters and Configuration Comparison

Engineering Applications and Design Guidelines

Application Scenarios

The gradient surfacing approach is particularly valuable for:

• Mold repair where the base material is 45 steel or similar medium-carbon steel
• Wear parts requiring both toughness at the base and hardness at the surface
• Components subjected to impact loading that would cause direct overlay delamination
• High-cycle fatigue applications where interface integrity is critical

Design Rules for Gradient Surfacing

1. The transition layer should have a hardness approximately 50–60% of the final wear layer hardness
2. The carbon content difference between adjacent layers should not exceed 0.8%
3. Each layer thickness should be at least 2 mm to ensure adequate gradient development
4. Interpass temperature should be controlled to prevent excessive diffusion between layers
5. The total overlay thickness should be at least 5–8 mm for components subject to significant wear

FMEA Analysis for Gradient Surfacing

Study Insights and Practical Recommendations

This research demonstrates that the two-step gradient approach is superior to direct overlay for 45 steel substrates requiring high-hardness wear protection. The key advantage is not merely the gradual hardness transition but also the reduction in carbon redistribution at each individual fusion boundary. By distributing the compositional mismatch across two interfaces, each interface experiences a smaller driving force for elemental diffusion, resulting in narrower and less detrimental decarbonized and hypercarbonized zones.

The practical implication for manufacturing is that the additional pass required for the transition layer represents a small increase in production time and cost, but provides a significant improvement in service reliability. For critical components such as mold dies, rolling mill rolls, and mining equipment, this approach can extend service life by 2–3 times compared to direct overlay.

The study also highlights an important principle for welding engineers: when joining materials with large property differences, the solution is not necessarily a single optimized intermediate composition but rather a systematic approach that distributes the mismatch across multiple interfaces. This philosophy can be extended to three-step or even four-step gradients for applications requiring extreme property transitions.

The findings reinforce the importance of understanding fusion zone metallurgy in surfacing applications. Engineers should not focus solely on the deposited layer properties but must also consider the interface quality and heat-affected zone behavior, which often determine the actual service life of the component.