New Materials and Processes for Bimetal Stamping Die Edge Weld Overlay

Literature Overview

This 1996 publication by Jiang Lexin, Wang Yuncheng, and Du Yining from the Hebei Institute of Mechanical and Electrical Engineering investigates new materials and processes for the weld overlay of bimetal stamping die edges used in railway vehicle manufacturing. The document addresses the critical challenge of balancing hardness, toughness, and wear resistance in stamping dies that must withstand millions of forming cycles while maintaining dimensional accuracy and surface quality.

Application Context and Performance Requirements

Stamping dies used in railway vehicle manufacturing are subjected to extreme forming conditions:

Repeated impact loading at temperatures up to 100-200°C during hot stamping
Severe abrasive wear from contact with sheet metal and lubricants
Plastic deformation from repeated high-pressure contact
Thermal fatigue from cyclic heating and cooling
Corrosive attack from lubricants and coolants

The die edge is the most critical region, experiencing the highest contact stresses and wear rates. Conventional die materials such as cold work tool steels (e.g., Cr12MoV, D2) typically achieve hardness of HRC 58-62 but suffer from inadequate toughness and wear resistance under severe forming conditions.

Bimetal Die Design Concept

The bimetal die concept combines a tough base material with a hard, wear-resistant overlay layer:

Layer	Material	Hardness	Function
Base	45 steel or 50CrV	HRC 28-35	Toughness; impact resistance; dimensional stability
Transition	Gradient alloy or intermetallic layer	HRC 40-50	Stress distribution; crack arrest
Overlay	Hardfacing alloy	HRC 60-70	Wear resistance; hardness retention

This layered approach allows each material to perform its intended function while mitigating the limitations of any single material. The base provides toughness to resist cracking and deformation, while the overlay provides the hardness and wear resistance required for extended die life.

Overlay Material Selection

The study evaluates several overlay material systems for stamping die applications:

Material System	Composition	Hardness (as-welded)	Hardness (after heat treatment)	Advantages	Limitations
High-carbon chromium steel	Cr12MoV equivalent	HRC 55-60	HRC 60-65	Good toughness; readily available	Moderate wear resistance
Cemented carbide composite	WC-Co or WC-Co-Cr	HRC 65-70	HRC 65-70	Excellent wear resistance	Brittle; poor toughness
Cobalt-based alloy	Stellite-type	HRC 45-55	HRC 55-60	Excellent red hardness; good toughness	High cost
High-speed steel	M2 or M36	HRC 55-60	HRC 62-67	Good combination of hardness and toughness	Moderate wear resistance
Ceramic composite	TiC or TiN reinforced	HRC 70-80	HRC 70-80	Superior wear resistance	Very brittle; difficult to weld

For railway stamping die applications, the optimal material selection depends on the specific forming conditions:

For cold stamping of mild steel sheets, high-carbon chromium steel overlays are cost-effective and provide adequate performance
For hot stamping of high-strength steels, cobalt-based alloys offer superior red hardness and wear resistance
For applications requiring maximum life, cemented carbide or ceramic composite overlays may be justified despite higher cost

Weld Overlay Process Development

The development of an effective weld overlay process for bimetal stamping dies requires careful optimization of welding parameters to achieve the desired microstructure and properties:

Submerged Arc Welding (SAW) Process

SAW is the preferred process for production-scale die overlay due to its high deposition rate and consistent quality:

Parameter	Typical Value	Optimization Target
Current	300-500 A	Maximize deposition rate while controlling dilution
Voltage	30-38 V	Control bead width and profile
Travel speed	100-200 mm/min	Balance deposition rate and cooling rate
Flux	Low-hydrogen flux	Minimize hydrogen-induced cracking
Wire diameter	2.5-4.0 mm	Match to current and voltage
Preheat	200-300°C	Reduce thermal gradient; prevent base metal cracking
Interpass temperature	≤300°C	Control cooling rate; prevent cracking
Post-weld treatment	Stress relief at 600-650°C	Reduce residual stress; improve toughness

Microstructural Control

The microstructure of the overlay layer directly influences its mechanical properties and wear resistance. Key microstructural features include:

Carbide morphology: Fine, uniformly distributed carbides provide the best wear resistance. Coarse carbides or carbide networks reduce toughness and can lead to spalling.
Grain size: Fine grains improve both hardness and toughness. Excessive grain growth during welding reduces toughness.
Phase composition: The balance between matrix and carbide phases determines the wear mechanism. A high volume fraction of hard carbides provides abrasive wear resistance, while a tough matrix provides impact resistance.

Defect Prevention

Common defects in bimetal die overlay and their prevention include:

Defect	Cause	Prevention
Cracking (hot)	Sulfur/phosphorus in base metal; excessive thermal gradient	Clean base metal; preheat; use low-sulfur filler
Cracking (cold)	High carbon content; high residual stress	Post-weld stress relief; control carbon content
Poor bond	Incomplete melting; surface contamination	Thorough surface preparation; adequate heat input
Excessive dilution	High heat input; thin base metal	Reduce current; increase travel speed; multiple thin passes
Carbide network	Excessive carbon; slow cooling	Adjust composition; rapid quench if needed
Surface porosity	Gas entrapment; contamination	Ensure clean surfaces; adequate shielding

Heat Treatment and Final Properties

The overlay layer typically requires post-weld heat treatment to achieve optimal properties:

Stress relief: 600-650°C for 2-4 hours to reduce residual stress and improve toughness
Hardening and tempering: 1000-1050°C austenitizing followed by double tempering at 540-580°C to achieve HRC 60-65
Aging (for cobalt-based alloys): 840-900°C for 2-4 hours to precipitate fine carbides

The final hardness and wear resistance depend on the specific material system and heat treatment cycle:

Material System	Hardness after heat treatment	Wear life (relative)
Cr12MoV overlay	HRC 60-65	1.0 (baseline)
Cobalt-based overlay	HRC 55-60	2-3×
Cemented carbide overlay	HRC 65-70	5-10×
High-speed steel overlay	HRC 62-67	2-4×

Performance Evaluation and Economic Analysis

The economic benefit of bimetal stamping dies with hardfaced edges can be quantified:

Metric	Conventional die	Bimetal die (Cr12MoV overlay)	Bimetal die (Cobalt overlay)
Die life (strokes)	50,000-100,000	200,000-300,000	500,000-800,000
Replacement frequency	Monthly	Quarterly	Annually
Downtime per year	~200 hours	~50 hours	~10 hours
Cost per stroke	Low	Moderate	High (initial) / Low (operating)
ROI	Baseline	3-6 months	6-12 months

For high-volume railway vehicle manufacturing, where stamping dies are critical production assets, the economic case for bimetal dies with hardfaced edges is compelling. The extended die life reduces replacement frequency, downtime, and quality issues associated with worn dies.

Practical Recommendations

Based on the findings of this study and subsequent industrial experience, the following recommendations are offered for engineers implementing bimetal stamping die overlay:

Select the overlay material based on the specific forming conditions, balancing hardness, toughness, and cost
Use SAW for production-scale overlay to ensure consistent quality and high deposition rate
Apply preheat to reduce thermal gradient and prevent base metal cracking
Use multiple thin passes rather than a single thick deposit to control dilution and microstructure
Perform post-weld heat treatment to optimize carbide distribution and achieve target hardness
Conduct regular inspection of the overlay surface for signs of wear or damage
Rebuild worn surfaces by overlaying additional material rather than replacing the entire die
Maintain detailed records of die life and performance to optimize material selection and process parameters for future applications

This literature provides a valuable reference for engineers working on bimetal stamping die applications, demonstrating the practical application of weld overlay technology for extending die life and reducing manufacturing costs in railway vehicle production.