Microstructure and Mechanical Properties of Weld Overlay Layers on Hot Shearing Blades

Literature Overview and Context

The research by Wang Jianguo, Wang Gui, and Liu Xiaogang, published in 1999 in "Mechanical Engineering Materials" and supported by the Inner Mongolia Autonomous Region Science and Technology Key Project, investigates the microstructure and mechanical properties of weld overlay layers applied to hot shearing blades. This work was conducted at Baotou Steel Institute (now Baotou University), a leading institution in metallurgical research in northern China. The study addresses a critical industrial need: extending the service life of hot shearing blades used in steel rolling mills, which operate under extreme conditions of elevated temperature, high stress, and severe abrasive and adhesive wear.

Hot shearing blades are used in the finishing section of hot rolling mills to trim strip edges and cut coils to length. They operate at temperatures ranging from 600 to 1000 °C, in contact with hot steel strips that are continuously being sheared. The combination of thermal cycling, mechanical impact, and material transfer from the hot steel to the blade surface creates a highly aggressive wear environment. Without surface hardening or overlay protection, blade life is extremely short, leading to frequent blade changes, production interruptions, and increased costs.

Material Selection and Overlay Design

Base Blade Material

Hot shearing blades are typically manufactured from high-speed steel (HSS) grades such as:

Steel Grade	Composition Highlights	Hardness (as-received)	Hot Hardness at 600 °C
W6Mo5Cr4V2	W 6%, Mo 5%, Cr 4%, V 2%	62–65 HRC	~55 HRC
W18Cr4V	W 18%, Cr 4%, V 1%	63–66 HRC	~50 HRC
M2	W 6%, Mo 5%, Cr 4%, V 2%	62–65 HRC	~55 HRC
Cr12MoV	Cr 12%, Mo 1%, V 1%	58–62 HRC	~45 HRC

These high-speed steels provide excellent hot hardness and wear resistance but are expensive and can be further enhanced through surface overlay.

Overlay Material Selection

The overlay material selection for hot shearing blades must address:

High-temperature hardness retention: The overlay must maintain hardness at operating temperatures of 600–1000 °C.
Wear resistance: Resistance to abrasive, adhesive, and erosive wear mechanisms.
Thermal shock resistance: The overlay must withstand rapid temperature changes without cracking.
Bond strength: Adequate metallurgical bonding to the HSS base material.
Weldability: Compatibility with the base material to avoid cracking during welding.

Common overlay materials for hot shearing applications include:

Overlay Material Type	Typical Composition	Hardness at 600 °C	Key Advantage
High-carbon martensitic	C > 2%, Cr 5–10%	55–60 HRC	High hardness, good toughness
High-chromium alloy	Cr 20–30%, C 1–3%	50–58 HRC	Excellent corrosion and wear resistance
Carbide-reinforced	Cr 5–10%, WC/Co additive	60–65 HRC	Superior abrasive wear resistance
Nickel-based	Ni-Cr-Mo alloy	45–55 HRC	Excellent thermal shock resistance
Cobalt-based	Co-Cr-W alloy	50–60 HRC	Best hot hardness retention

Microstructural Analysis

Microstructural Evolution in the Overlay Layer

The microstructure of the overlay weld layer is critical to its performance and is influenced by:

Cooling rate: Determines grain size and phase composition
Dilution rate: Affects the final composition and phase balance
Heat input: Influences the heat-affected zone (HAZ) and the overlay microstructure
Number of passes: Multiple passes create a complex microstructure with varying cooling rates

Key Microstructural Features

The overlay layer on hot shearing blades typically exhibits the following microstructural features:

Matrix phase: Martensite, austenite, or a combination thereof, depending on the composition and cooling rate. In high-carbon, high-chromium overlays, retained austenite is common and contributes to toughness.
Carbide phases: Chromium carbides (Cr₇C₃, Cr₂₃C₆, Cr₃C), vanadium carbides (VC, V₂C), and tungsten carbides (WC) provide hardening and wear resistance. The morphology, size, and distribution of carbides are critical:

Fine, uniformly distributed carbides provide the best combination of hardness and toughness.
Large, coarse carbides can act as crack initiation sites.
Carbide networks at grain boundaries reduce toughness.

Bond line microstructure: The transition zone between the base HSS and the overlay layer is the most critical region. A gradual transition in composition and microstructure is desirable, achieved through:

Multi-pass welding with progressive composition changes.
Controlled dilution rates (target: 15–25% for the first pass, 5–10% for subsequent passes).
Appropriate heat input to avoid excessive grain growth.

Heat-affected zone (HAZ): The HAZ in the HSS base material experiences:

Grain growth near the fusion boundary.
Carbide dissolution and possible softening.
Potential formation of brittle phases if cooling is too rapid.
The HAZ hardness typically drops by 2–5 HRC compared to the base material.

Microstructural Characterization Methods

Method	Information Provided	Application
Optical microscopy	Grain size, phase distribution, carbide morphology	Qualitative assessment
SEM (Scanning Electron Microscopy)	Detailed microstructure, elemental mapping	Carbide identification, inclusion analysis
XRD (X-ray Diffraction)	Phase identification, retained austenite content	Quantitative phase analysis
EBSD (Electron Backscatter Diffraction)	Crystallographic orientation, texture	Grain boundary character
Microhardness testing	Hardness profile across the overlay and HAZ	Dilution assessment, hardness uniformity

Mechanical Property Evaluation

Hardness Distribution

The hardness profile across the overlay layer and into the base material is a key performance indicator:

Region	Typical Hardness	Significance
Overlay surface	60–68 HRC	Wear resistance
Overlay mid-thickness	58–65 HRC	Uniformity assessment
Bond line	55–62 HRC	Dilution indicator
HAZ (near fusion line)	55–60 HRC	Softening assessment
Base material	62–65 HRC	Reference value

A well-designed overlay should maintain hardness within 5 HRC of the base material hardness throughout the overlay thickness, indicating good dilution control.

Wear Resistance Testing

Wear resistance is evaluated through:

Pin-on-disk testing: Simulates abrasive wear against hot steel strips.
Reciprocating sliding wear: Evaluates adhesive and abrasive wear mechanisms.
Hot wear testing: Conducted at elevated temperatures (600–800 °C) to simulate service conditions.
Field testing: Comparison of blade life in actual mill service.

Impact Toughness

The Charpy impact test at room temperature and elevated temperatures (200 °C, 400 °C, 600 °C) provides critical toughness data. The overlay layer must maintain adequate toughness to resist crack initiation and propagation during impact loading from shearing operations.

Engineering Practice and Performance Outcomes

Service Performance Comparison

Parameter	Uncoated HSS Blade	Overlay-Coated Blade	Improvement
Blade life (strips per blade)	500–1,000	2,000–5,000	3–5×
Edge wear rate (mm/1000 strips)	0.5–1.0	0.15–0.3	3–5× reduction
Re-sharpening frequency	Every 500 strips	Every 2,000 strips	4× reduction
Cost per strip (blading)	Baseline	30–50% of baseline	50–70% savings

Common Defects and Countermeasures

Defect Type	Cause	Countermeasure
Surface cracks	High carbon content, rapid cooling, thermal stress	Reduce carbon content, increase preheat, control cooling rate
Bond line cracking	Dilution mismatch, hydrogen embrittlement	Low-hydrogen consumables, proper preheat, post-weld heat treatment
Excessive dilution	High heat input, wrong welding sequence	Reduce heat input, use transition layer, multi-pass technique
Incomplete fusion	Poor surface preparation, low current	Thorough cleaning, adequate current and travel speed
Porosity	Moisture in flux, contamination	Dry flux storage, proper joint preparation

Study Insights and Reflections

This research contributes significantly to the understanding of overlay weld microstructure-property relationships in hot working tool applications. Several key insights emerge:

Microstructure dictates performance: The wear resistance and toughness of the overlay layer are fundamentally determined by the microstructure, particularly the type, size, and distribution of carbide phases. Optimizing the carbide morphology is as important as selecting the correct composition.
The bond line is the critical region: The metallurgical transition between the HSS base and the overlay layer must be carefully managed. A gradual compositional and microstructural transition, achieved through multi-pass welding with progressive composition changes, is essential for long-term reliability.
Hot hardness is the key metric: At operating temperatures of 600–1000 °C, room-temperature hardness is irrelevant. The overlay material must be selected and processed to maintain adequate hardness at elevated temperatures, which requires high-temperature testing and validation.
Process control is essential: The overlay welding process must be tightly controlled to ensure consistent dilution rates, uniform microstructure, and adequate bond strength. Welding procedure qualification and periodic process verification are mandatory.
Field validation is irreplaceable: Laboratory testing provides valuable data, but the ultimate validation of overlay performance must come from field service trials in actual mill conditions. The correlation between laboratory results and field performance should be documented and used to refine future designs.

The work by Wang Jianguo and colleagues exemplifies the integration of materials science fundamentals with practical engineering requirements. The systematic approach to microstructure-property-performance relationships established in this study provides a foundation for the continued development of advanced overlay materials and processes for hot working tools, a field that remains actively researched and developed today.