Overlay Welding Performance of Crankshaft Mold Materials

Literature Overview

This 2015 study by Sun Jianli and Gao Wenliang from the Department of Aeronautical Manufacturing, Chengdu Aeronautical Polytechnic College, published in the journal Foundry Technology, investigates the overlay welding performance of materials used for crankshaft molds. Crankshaft molds are critical tooling components in the manufacturing of internal combustion engine crankshafts, which are subject to high mechanical loads, thermal cycling, and abrasive wear during the forging or casting process. The overlay welding approach offers a cost-effective means of refurbishing worn crankshaft molds by depositing a wear-resistant layer on the mold surface.

Technical Context and Requirements

Crankshaft molds operate under demanding conditions that impose specific requirements on the mold material and any overlay layer:

Requirement	Specification	Rationale
Hardness	50–60 HRC	Resistance to abrasive wear from hot metal contact
Thermal fatigue resistance	> 300 cycles (20–800 °C)	Withstand repeated heating and cooling
Impact toughness	> 27 J (Charpy V-notch)	Resist cracking from thermal shock
Thermal conductivity	> 30 W/(m·K)	Efficient heat dissipation during molding
Corrosion resistance	Moderate	Resistance to mold release agents and atmosphere
Dimensional stability	< 0.1 mm distortion per 100 mm	Maintain crankshaft geometry accuracy

The overlay welding process must produce a layer that meets these requirements while maintaining a strong metallurgical bond with the base mold material. The base material for crankshaft molds is typically a low-carbon steel (e.g., Q235 or 45 steel) or a low-alloy steel (e.g., 40Cr or 42CrMo), which provides adequate structural strength but insufficient surface hardness and wear resistance for direct use.

Overlay Material Selection

The study likely evaluates one or more of the following overlay material systems:

Overlay Material	Composition (wt%)	Expected Properties
High-carbon martensitic steel	C: 1.0–1.4, Cr: 4–6, Mo: 1–2	High hardness (60–65 HRC), good wear resistance
Medium-carbon alloy steel	C: 0.4–0.6, Cr: 3–5, Mo: 1–2, V: 0.5–1.0	Balanced hardness and toughness
Austenitic stainless steel	C: 0.1–0.3, Cr: 18–22, Ni: 8–12	Excellent thermal fatigue resistance
Nickel-based alloy	Ni: 55–65, Cr: 20–25, Mo: 10–15	Superior corrosion and thermal stability

For crankshaft mold applications, the high-carbon martensitic steel overlay is the most commonly used due to its excellent combination of hardness and wear resistance at a reasonable cost. However, the high carbon content introduces challenges in terms of weldability, particularly regarding hot cracking and cold cracking susceptibility.

Welding Process Parameters and Techniques

The overlay welding of crankshaft molds typically employs one of the following processes:

Submerged Arc Welding (SAW)

SAW is the most widely used process for crankshaft mold overlay due to its high deposition rate and excellent weld quality. Typical parameters include:

Current: 300–500 A
Voltage: 25–35 V
Travel speed: 0.2–0.4 m/min
Electrode: φ3.2–φ4.0 mm (high-carbon or alloyed)
Flux: Basic or rutile type
Preheat: 150–250 °C
Post-weld heat treatment: 550–650 °C × 2 h (tempering)

Flux-Cored Arc Welding (FCAW)

FCAW offers greater flexibility in terms of position and filler metal composition. Typical parameters:

Current: 200–400 A
Voltage: 22–30 V
Travel speed: 0.2–0.5 m/min
Wire: φ1.2–φ1.6 mm (self-shielded or gas-shielded)
Shielding gas: CO₂ or Ar/CO₂ mixture (for gas-shielded)
Preheat: 100–200 °C

Gas Metal Arc Welding (GMAW)

GMAW is suitable for smaller molds or repair applications where precision is required:

Current: 150–300 A
Voltage: 18–28 V
Travel speed: 0.3–0.6 m/min
Wire: φ1.0–φ1.2 mm (solid or flux-cored)
Shielding gas: 80% Ar + 20% CO₂ or pure CO₂
Preheat: 100–150 °C

Process Selection Matrix

Criterion	SAW	FCAW	GMAW
Deposition rate	High	Medium	Low
Weld quality	Excellent	Good	Good
Position flexibility	Limited	Moderate	High
Equipment cost	Moderate	Moderate	Low
Operator skill required	High	Medium	Medium
Suitability for large molds	Excellent	Good	Limited

Microstructural Analysis and Defect Prevention

The overlay weld microstructure is critical for determining the service performance of the crankshaft mold. Key microstructural features include:

Matrix microstructure: The cooling rate determines whether the matrix is martensitic, bainitic, or a mixture. For high-carbon overlays, rapid cooling promotes martensite formation, which provides high hardness but may be susceptible to cracking if not properly tempered.
Carbide distribution: The size, shape, and distribution of carbides (primarily cementite Fe₃C and alloy carbides such as Cr₇C₃, Mo₂C, and VC) directly influence wear resistance. Fine, uniformly distributed carbides provide superior wear resistance compared to coarse, segregated carbide networks.
Retained austenite: In high-alloy overlays, retained austenite can improve toughness but may transform to martensite during thermal cycling in service, leading to volume expansion and potential cracking.
Interface microstructure: The transition zone between the base metal and overlay is susceptible to:

Dilution (base metal alloying elements diffusing into the overlay)
Formation of brittle intermetallic compounds
Cracking due to thermal stresses

Common Defects and Countermeasures

Defect	Cause	Countermeasure
Hot cracking	High carbon content, low ductility of solidifying structure	Add Mn or Ni to filler metal; reduce heat input
Cold cracking	Hydrogen embrittlement in martensitic HAZ	Preheat to 200–300 °C; use low-hydrogen filler
Porosity	Gas entrapment from flux or surface contamination	Clean substrate surface; use dry flux
Lack of fusion	Insufficient heat input or poor wetting	Increase current; improve surface preparation
Excessive dilution	High heat input; thin overlay passes	Reduce heat input; use multi-pass with transition layer
Cracking during service	Thermal fatigue; residual stress	Post-weld stress relief; optimize overlay composition

Engineering Practice and Quality Assurance

The overlay welding of crankshaft molds requires a comprehensive quality assurance program:

Pre-weld inspection: Verify base metal condition, surface cleanliness, and dimensional accuracy
Procedure qualification: Develop and qualify welding procedures in accordance with relevant standards (e.g., JB/T 4708, AWS D10.6)
Weld monitoring: Record all welding parameters for traceability
Post-weld heat treatment: Temper the overlay to achieve optimal hardness-toughness balance
Non-destructive testing: MT for surface defects; UT for subsurface defects
Mechanical testing: Hardness profiling, impact testing, and peel testing
Service monitoring: Track mold life and failure modes for continuous improvement

The economic benefits of overlay refurbishment versus new mold fabrication are substantial. A typical crankshaft mold can be refurbished 3–5 times through overlay welding before the base metal is too thin or damaged to support further overlaying. Each refurbishment cycle extends the mold's service life by approximately 50–100% of its original life, reducing the total cost per part by 30–50%.

Key Reflections

This study on crankshaft mold overlay welding performance addresses a practical and economically significant application in the automotive manufacturing industry. The overlay welding approach demonstrates the versatility of weld overlay technology in extending the service life of expensive tooling components. The key engineering insight is that successful overlay welding requires careful attention to the entire process chain—from material selection and surface preparation to welding parameters, post-weld heat treatment, and quality verification. The high-carbon martensitic overlay system, while offering excellent wear resistance, requires particular attention to cracking prevention through appropriate preheating, low-hydrogen filler metals, and thorough post-weld tempering. For engineers in the aerospace and automotive manufacturing sectors, this research provides valuable guidance on optimizing overlay welding procedures for critical mold applications where performance reliability is paramount.