Microstructure and Properties of Weld Overlay Layer on Hot Shearing Blades

Literature Overview

This 1999 study by Wang Jianguo, Wang Gui, and Liu Xiaogang from Baotou Steel Institute (now Inner Mongolia University of Science and Technology), funded by the Inner Mongolia Autonomous Region Science and Technology Project, investigates the microstructure and mechanical properties of weld overlay layers applied to hot shearing blades in steel mill operations. Hot shearing blades operate under extreme conditions involving high temperatures (600–900°C), severe abrasive and adhesive wear, cyclic thermal loading, and high shear stresses. The weld overlay approach provides a cost-effective method for extending blade life by depositing a hardfacing layer with superior hot hardness and wear resistance compared to the base tool steel.

Technical Background

Hot shearing blades in steel mills are used to cut red-hot steel slabs, billets, or strips at temperatures ranging from 600°C to 900°C. The cutting operation subjects the blade edge to a combination of:

Shear stress: Up to 800–1200 MPa during the cutting stroke
Abrasive wear: From hard inclusions and scale particles in the hot steel
Adhesive wear: From metal-to-metal contact at high temperature
Thermal fatigue: Cyclic heating and cooling with each cutting operation
Oxidation: Accelerated at elevated temperatures, leading to surface degradation

The base material of hot shearing blades is typically a high-speed steel or hot work tool steel such as H13 (4Cr5MoSiV1), which provides adequate hot hardness but insufficient wear resistance for prolonged service. Weld overlay with a hardfacing alloy provides a surface layer with significantly improved properties while maintaining the toughness of the base material.

Microstructural Analysis of Overlay Deposits

Common Overlay Alloy Systems for Hot Shearing Blades

Overlay System	Base Alloy	Hardness (RT, HRC)	Hot Hardness (800°C, HV)	Key Microstructural Features
High-carbon Cr alloy	Cr 15–20%, C 2–4%	58–65	200–300	Martensite + Cr7C3 carbides
Ni-Cr-C type	Ni 15–20%, Cr 8–15%, C 3–5%	50–58	250–350	Austenite + M7C3/M23C6 carbides
Fe-WC type	Fe base, WC 30–50%	65–72	300–400	Martensite + WC particles
Co-based (Stellite)	Co base, Cr 25–30%, W 15–20%	40–50	350–450	Austenite/FCC + M6C carbides
High-speed steel type	W 6–8%, Mo 5–6%, V 4–5%	62–68	280–380	Martensite + MC/M2C carbides

Microstructural Evolution During Solidification

The microstructure of the weld overlay layer is determined by the solidification behavior of the molten weld pool, which is influenced by cooling rate, alloy composition, and welding process parameters:

Columnar dendritic structure: Forms when cooling rates are relatively high (typical of single-pass overlay), with primary dendrites growing perpendicular to the fusion boundary. This structure provides good directional strength but can be susceptible to transverse cracking.
Equiaxed grain structure: Achieved through higher heat input, multiple passes, or grain refiners in the consumable. Provides more isotropic properties and better crack resistance.
Carbide morphology: The type, size, and distribution of carbides are critical for wear resistance. Fine, uniformly distributed carbides provide superior abrasion resistance compared to coarse, clustered carbides.
Transition zone microstructure: The interface between the base steel and overlay layer develops a diffusion zone with intermediate composition and properties. This zone is critical for bond strength and crack resistance.

Effect of Heat Input on Microstructure

Heat Input (kJ/mm)	Cooling Rate	Grain Size	Carbide Size	Hardness (HRC)
5 – 8	High (>50°C/s)	Fine columnar	Small (1–3 μm)	62–68
10 – 15	Medium (20–50°C/s)	Medium columnar	Medium (3–8 μm)	58–65
18 – 25	Low (<20°C/s)	Coarse/equiaxed	Large (8–20 μm)	52–60

Mechanical Properties and Performance

Hardness Distribution

The hardness profile from the base metal through the transition zone to the overlay surface is a critical performance indicator:

Base metal (H13): 45–52 HRC
Transition zone: 50–58 HRC (gradual increase)
Overlay layer: 58–72 HRC (depending on alloy system)

A steep hardness gradient at the transition zone can promote cracking under cyclic loading, while a gradual transition provides better stress distribution and fatigue resistance.

Hot Hardness and Wear Resistance

The primary performance metric for hot shearing blade overlays is hot hardness at operating temperature. The overlay alloy must maintain sufficient hardness at 600–900°C to resist plastic deformation and abrasive wear. Key factors influencing hot hardness include:

Carbide stability: Refractory carbides (WC, Mo2C, VC) maintain hardness at elevated temperatures better than iron carbides.
Matrix strength: The base matrix must retain strength through solid solution strengthening and precipitation hardening.
Oxidation resistance: Chromium content above 12% provides adequate oxidation resistance at 800°C, reducing surface degradation.

Thermal Fatigue Resistance

Hot shearing blades experience thermal cycling with each cutting operation, creating thermal stresses that can cause fatigue cracking. The overlay layer must have:

Adequate thermal conductivity to minimize thermal gradients
Sufficient ductility to accommodate thermal strain
Good bond strength to the base metal to prevent delamination
Appropriate coefficient of thermal expansion match with the base metal

Process Parameters and Quality Control

Recommended Welding Parameters for Hot Shearing Blade Overlay

Process	Current (A)	Voltage (V)	Speed (mm/min)	Interpass Temp (°C)
GTAW (TIG)	120–200	12–18	30–80	≤ 200
GMAW (MIG)	180–300	20–28	50–150	≤ 250
SAW	300–500	25–35	80–200	≤ 200
Oxy-fuel	—	—	30–60	≤ 150

Defect Prevention Measures

Preheating: 200–300°C for H13 base steel to reduce cracking susceptibility
Interpass temperature control: Maximum 200–250°C to maintain overlay microstructure
Post-weld heat treatment: Solution treatment and tempering to relieve residual stresses and optimize carbide distribution
Multi-pass technique: Multiple thin passes rather than single thick deposits to reduce cracking and improve microstructure
Surface preparation: Grinding to remove scale and oxidation, ensuring clean fusion

Engineering Practice and Application Insights

The research demonstrates that weld overlay is a highly effective method for extending the service life of hot shearing blades in steel mill operations. Typical service life improvements range from 2–5 times that of uncoated blades, depending on the overlay alloy selection and process control quality. The economic benefit is substantial given the high cost of blade replacement and the production downtime associated with blade changes.

The key engineering insight from this research is that overlay performance is not solely determined by the overlay alloy composition but is critically dependent on the microstructural quality of the deposit, which in turn is governed by welding process parameters. Optimal results require careful attention to heat input control, interpass temperature management, and post-weld heat treatment to achieve the desired balance of hot hardness, wear resistance, and toughness.

Study Conclusions

The investigation of weld overlay layer microstructure and properties on hot shearing blades provides valuable guidance for the selection and application of hardfacing overlays in severe hot wear environments. The research establishes clear relationships between welding parameters, microstructural development, and service performance, enabling engineers to design overlay systems that meet specific operational requirements. For steel mill maintenance engineers, this work demonstrates that systematic approach to overlay design — encompassing material selection, process optimization, and quality control — can significantly extend component life and reduce maintenance costs in demanding hot shearing applications.