Surfacing Repair and Reuse of Forging Dies

Literature Overview

The paper by Li Wenbin and Guan Jun, published in Hot Working Technology in 2006 (Volume 35, Issue 7, page 75), presents a concise technical report on the surfacing repair and reuse of forging dies. The study introduces practical process methods and corresponding parameters for restoring worn or damaged forging dies through overlay welding. Given the high cost of forging dies and the frequent production downtime associated with die replacement, this research addresses a significant economic and operational challenge in the forging industry.

Core Technical Content

Forging dies are subjected to extreme mechanical and thermal loads during the forging process. The cavity surfaces experience high contact stresses, friction, and repeated thermal cycling, leading to progressive wear, surface cracking, and dimensional degradation. When wear exceeds acceptable limits, the die must either be replaced entirely or repaired through surfacing welding. Full replacement is costly and time-consuming, making surfacing repair an attractive alternative when the base metal remains structurally sound.

The paper describes the selection of appropriate surfacing materials, the preparation of the repair area, the welding process parameters, and the post-weld treatment required to restore the die to serviceable condition. The emphasis is on practical, field-applicable methods rather than laboratory-scale investigations.

Process Analysis and Engineering Considerations

Process Step	Key Requirements	Typical Parameters
Surface preparation	Grind worn area, remove scale and oxide	Roughness Ra 3.2-6.3 μm
Preheating	Reduce thermal gradient, prevent cracking	300-400 °C for carbon and low-alloy steels
Surfacing welding	SMAW or submerged arc welding	Current 160-220 A, voltage 24-30 V
Post-weld treatment	Stress relief, dimensional correction	550-650 °C, hold 1-2 hours
Finishing	Grinding to dimensional tolerance	Final Ra 1.6 μm or as specified

The selection of surfacing material is critical and depends on the type of wear experienced. For dies subject to abrasive wear, high-carbon or high-chromium materials such as Cr12 or Cr12MoV are appropriate. For dies experiencing adhesive or galling wear, materials with good hot hardness and low friction coefficients are preferred. In some cases, a multi-layer approach is used, with a transition layer to reduce dilution followed by a final hardfacing layer.

Failure Mechanisms and Countermeasures

Common failure modes of forging dies include:

Surface wear due to abrasive and adhesive mechanisms, typically occurring in high-friction zones.
Thermal fatigue cracking, manifesting as fine surface cracks perpendicular to the die surface.
Quench cracks from improper heat treatment or thermal shock during service.
Plastic deformation of the cavity surface under high contact pressure.

The surfacing repair strategy must address the specific failure mode. For thermal fatigue cracking, the repair area should be ground to remove all crack extensions, and the surfacing material should have good thermal fatigue resistance. For plastic deformation, the material should have sufficient hardness at operating temperature to resist indentation.

Engineering Practice Cases

In practice, forging die repair through surfacing welding requires careful attention to several factors:

The geometric complexity of the die cavity often limits access for welding equipment, making SMAW with flexible electrode holders the most practical choice.
Multiple thin layers (1-2 mm each) are recommended to minimize residual stress and ensure uniform hardness.
The dilution rate must be controlled to maintain the required surface hardness; typically, the dilution should not exceed 20-30% for hardfacing alloys.
After surfacing, the die must be re-ground to the original dimensional specifications, which requires allowance for the deposited material thickness in the initial design or repair planning.

The economic benefit of surfacing repair is substantial. A single die can often be repaired multiple times before the accumulated deposits and residual stresses compromise the base metal integrity. However, engineers must monitor the cumulative repair history and retire the die when the base metal thickness falls below a critical minimum or when excessive distortion makes dimensional correction impractical.

Study Insights and Implications

This paper, while concise, captures the essence of practical die repair engineering. The key insight is that surfacing repair is not merely a welding operation but a systematic process involving material selection, surface preparation, process parameter optimization, and post-weld treatment. Engineers should approach die repair with the same rigor as new die manufacturing, recognizing that poor repair practices can introduce new failure modes and reduce service life below that of a newly manufactured die. The integration of repair planning into the die maintenance schedule, with periodic inspection and early intervention, is essential for maximizing die utilization and minimizing production downtime.