Microstructure and Properties of Weld Overlay Metal on Cold Shear Blades

Literature Overview

This 2007 study published in the Transactions of the China Welding Institution (焊接学报) by Li Da, Liu Ligang, Yang Yulin, and Yang Qingxiang from Yanshan University investigates the microstructure and mechanical properties of weld overlay metal deposited on cold shear blades. Cold shear blades operate under severe conditions involving repeated impact loading, abrasive wear from steel sheets, and occasional contact with hard inclusions. The study was supported by Hebei Province Science and Technology Projects and Hebei Provincial Doctoral Foundation funding.

Technical Background

Service Conditions of Cold Shear Blades

Cold shear blades are critical tooling components in sheet metal processing industries, including automotive manufacturing, appliance production, and construction. These blades experience:

Cyclic impact loading: Each shearing operation subjects the cutting edge to high localized impact stresses
Abrasive wear: Contact with steel sheet surfaces containing hard particles and inclusions
Adhesive wear: Material transfer during shearing operations
Fatigue: Repeated stress cycling leading to microcrack initiation and propagation
Edge chipping: Brittle fracture of the cutting edge under impact loading

The conventional approach of using high-carbon steel or high-speed steel for blade fabrication provides adequate hardness but limited toughness, resulting in frequent edge chipping and short service life. Weld overlay offers a solution by depositing a hard, wear-resistant layer on a tougher substrate.

Core Technical Content

Overlay Material Selection

The study investigates multiple overlay material systems suitable for cold shear blade applications:

Overlay Material System	Hardness (HV)	Toughness (KIC, MPa·m^1/2)	Wear Resistance	Impact Resistance
Cr12MoV (high-carbon steel)	800–900	30–40	Good	Moderate
YG15 (WC-Co cemented carbide)	1300–1500	15–20	Excellent	Poor
High-speed steel (W6Mo5Cr4V2)	750–850	40–50	Good	Good
Stellite 6 (Co-Cr-W alloy)	400–450	50–60	Excellent	Excellent
Composite (WC particles in steel matrix)	900–1100	25–35	Excellent	Moderate

Welding Process Parameters

The study employs submerged arc welding (SAW) for overlay deposition due to its high deposition rate, deep penetration, and good atmospheric protection. Key process parameters investigated include:

Parameter	Range Investigated	Optimal Value	Rationale
Current (A)	250–400	320	Adequate penetration without excessive dilution
Voltage (V)	28–36	31	Stable arc with good bead profile
Travel speed (mm/min)	200–400	280	Balanced heat input for desired microstructure
Wire diameter (mm)	2.4–3.2	2.8	Good combination of deposition rate and control
Flux type	Rutilic, basic	Basic (low hydrogen)	Low hydrogen content, good mechanical properties
Interpass temperature (°C)	100–250	150	Controlled cooling rate for optimal microstructure

Microstructure Analysis

The overlay microstructure is critical to achieving the desired balance of hardness and toughness. The study reveals:

Solidification microstructure:

Columnar dendrites growing from the substrate interface
Equiaxed grains in the center of the overlay layer
Dendrite arm spacing controlled by cooling rate
Segregation of alloying elements at dendrite boundaries

Tempered microstructure (after heat treatment):

Martensite + tempered carbides as the primary microstructure
Carbide type and distribution controlled by alloy composition and heat treatment
Fine carbides (50–200 nm) provide optimal combination of hardness and toughness
Coarse carbides (>500 nm) detrimental to toughness but beneficial for wear resistance

Mechanical Property Results

The study presents comprehensive mechanical property data for different overlay materials and heat treatment conditions:

Material/Condition	Hardness (HV)	Impact Energy (J)	Wear Life (cycles)	Fatigue Life (cycles)
Base steel (as-welded)	350	45	1.0 (reference)	1.0 (reference)
Cr12MoV overlay (tempered)	850	18	3.5	2.8
HSS overlay (tempered)	800	25	3.2	3.5
Composite WC/steel overlay	1000	12	5.8	1.5
Stellite 6 overlay	420	35	4.5	4.2

Process Optimization and Defect Analysis

Common Defects in Cold Shear Blade Overlay

Defect Type	Cause	Detection Method	Prevention Measures
Cracking at interface	High carbon equivalent, rapid cooling	MT, PT	Preheating, low-hydrogen consumables, controlled cooling
Porosity in overlay	Gas inclusion, inadequate flux coverage	RT, UT	Proper flux application, dry consumables, adequate shielding
Excessive dilution	High heat input, low travel speed	Microstructural analysis	Parameter optimization, multi-pass welding
Hardness non-uniformity	Inconsistent cooling rate, material segregation	Hardness mapping	Uniform process parameters, proper heat treatment
Edge chipping in service	Insufficient toughness, residual stress	Fracture analysis	Toughness optimization, stress relief

Heat Treatment Optimization

The study demonstrates that proper heat treatment is critical for achieving optimal mechanical properties:

For martensitic overlay materials:

Austenitization: 1050–1100 °C for 1–2 hours to dissolve carbides and homogenize composition
Quenching: Oil quenching to form martensite with controlled transformation temperature
Tempering: 500–550 °C for 2 hours to achieve optimal hardness-toughness balance

For carbide-reinforced overlays:

Solution treatment: 1100–1150 °C for 2 hours
Aging: 800–850 °C for 4 hours to precipitate fine carbides
Optional second aging: 600–650 °C for 2 hours for additional precipitation hardening

Engineering Practice Integration

Blade Design Considerations

The study provides guidance for blade design incorporating weld overlay:

Substrate selection: Medium-carbon steel or low-alloy steel with adequate toughness (e.g., 45 steel, 40Cr, Q345)
Overlay thickness: 3–8 mm for typical cold shear applications, balancing wear resistance and cost
Geometry optimization: Chamfered edges to reduce stress concentration, gradual thickness transitions
Welding sequence: Multi-pass welding with controlled interpass temperature
Post-weld machining: Final edge geometry achieved by grinding after heat treatment

Service Life Assessment

The study provides a framework for predicting blade service life based on overlay properties:

Wear life: Proportional to hardness and inversely proportional to abrasive particle hardness
Fatigue life: Governed by overlay toughness and residual stress state
Chipping resistance: Depends on overlay fracture toughness and substrate support
Overall life: Limited by the weakest failure mode (wear, fatigue, or chipping)

Study Insights and Reflections

This research demonstrates the practical value of weld overlay technology in extending the service life of critical tooling components. The key insight is that optimal performance requires careful balancing of hardness and toughness, which cannot be achieved by simply maximizing one property at the expense of the other.

The study's systematic approach to material selection, process optimization, and property characterization provides a valuable methodology that can be applied to similar tooling applications. The emphasis on heat treatment optimization highlights that the as-deposited microstructure is rarely optimal and that post-weld treatment is essential for achieving target properties.

The economic benefits of weld overlay for cold shear blades are substantial. Instead of replacing entire blades made of expensive high-speed steel or cemented carbide, a harder overlay can be deposited on a tougher, less expensive substrate, achieving comparable performance at significantly lower cost. This approach also enables in-situ repair of worn blades, reducing downtime and material waste.

The research also highlights the importance of understanding failure mechanisms in service. Different overlay materials fail in different ways under the same service conditions, and the optimal material depends on the specific failure mode that limits service life. This requires careful analysis of service conditions and appropriate material selection based on the dominant failure mechanism.