Failure Analysis and Hardfacing Repair of Large Crankshaft Forging Dies

Failure Investigation and Root Cause Analysis

Large crankshaft forging dies are critical tooling components in automotive and heavy-duty engine manufacturing. These dies are typically made from hot work die steels such as H13 (4Cr5MoSiV1) or H21 (3Cr2W8V) and are subjected to extreme conditions including temperatures above 800 °C, high compressive loads exceeding 3000 MPa, and repeated thermal cycling. The study documented a failure investigation of a large crankshaft forging die that experienced premature failure after approximately 6,500 strokes, well below the expected design life of 15,000 strokes.

The failure analysis followed a systematic approach combining visual examination, macroscopic and microscopic metallography, hardness mapping, and scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS). The following failure modes were identified:

Failure Mode	Location	Extent	Root Cause
Surface spalling	Die cavity surface	40% of surface area	Thermal fatigue + compressive stress
Cracking (longitudinal)	Near surface, 2–5 mm depth	Multiple cracks, up to 15 mm long	Thermal cycling + residual stress
Wear (abrasive)	High-contact areas	3–5 mm depth loss	Oxide scale abrasion
Galling/scuffing	Contact zones	Localized, up to 2 mm	Metal-to-metal adhesion
Chipping	Die edges and corners	Several locations	Stress concentration + thermal shock

The root cause analysis, structured using a 5W2H framework, identified the following contributing factors:

What: Multi-mode failure involving thermal fatigue, abrasive wear, galling, and stress-related cracking
Why: Insufficient thermal fatigue resistance, inadequate lubrication, and excessive impact loading
Where: Primarily at the die cavity surface and high-stress corners
When: Progressive degradation over 6,500 strokes, with rapid acceleration in the last 1,500 strokes
Who: Manufacturing process (heat treatment, surface preparation) and maintenance practices
How: Combined action of thermal cycling, mechanical loading, and chemical attack
Which: H13 die steel with standard heat treatment (1050 °C austenitize, oil quench, 560 °C × 3 temper)

Repair Strategy and Hardfacing Procedure

The repair strategy involved a comprehensive approach combining material removal of damaged areas, surface preparation, and multi-layer hardfacing. The following table outlines the repair procedure:

Step	Operation	Specification	Quality Check
1	Remove damaged material	CNC milling to remove 8–12 mm from cavity surface	Visual + UT for subsurface cracks
2	Surface preparation	Grind to Ra 3.2 μm; degrease	Visual; contact angle test
3	Preheat	Induction heating to 350 °C	Infrared thermometry
4	Interlayer deposit	3 mm Ni60 (Stellite 6) via SAW	Visual; bond strength test
5	Wear layer deposit	8 mm Cr₃C₂-NiCr via SAW	Hardness mapping; UT
6	Post-weld heat treatment	600 °C × 2 h in furnace	Hardness verification
7	Final machining	CNC grind to final dimensions	CMM dimensional check
8	Final inspection	PT + MT + UT	Acceptance per NB/T 47013

The selection of a two-layer structure (Ni60 interlayer + Cr₃C₂-NiCr wear layer) was based on the following considerations:

The Ni60 interlayer provides excellent ductility and thermal fatigue resistance, absorbing residual stresses from the hardfacing process and providing a tough transition between the brittle wear layer and the substrate.
The Cr₃C₂-NiCr wear layer offers exceptional hardness (1200–1500 HV) and wear resistance due to the presence of hard chromium carbide particles.
The combination addresses both the thermal fatigue and abrasive wear failure modes identified in the root cause analysis.

Performance Evaluation of Repaired Die

The repaired die was returned to service and monitored over a period of 12 months. The following performance data were collected:

Parameter	Original Die	Repaired Die	Improvement
Service life (strokes)	6,500	28,000+	4.3×
Surface hardness (HV)	420–460	1250–1400	2.9×
Surface roughness (Ra, μm)	3.2	1.6	Improved
Number of rework cycles	0	1 (at 15,000 strokes)	Acceptable
Product quality (dimensional accuracy)	±0.15 mm	±0.12 mm	Improved

The significant improvement in service life was attributed to the combined effect of the hardfacing layers and the post-weld heat treatment. The Ni60 interlayer effectively prevented crack propagation from the wear layer into the substrate, while the Cr₃C₂-NiCr layer provided the necessary hardness to resist abrasive wear from oxide scale.

Key Lessons and Engineering Recommendations

The failure analysis and repair study yielded several important lessons for engineering practice:

First, die failure is rarely caused by a single mechanism but rather by the synergistic action of multiple degradation modes. A comprehensive failure analysis must consider thermal, mechanical, and chemical factors simultaneously. The use of structured analysis tools such as FMEA and 5W2H is recommended for systematic root cause identification.

Second, the repair strategy must address all identified failure modes, not just the most visible one. In this case, focusing solely on wear resistance without addressing thermal fatigue would have resulted in premature failure of the repair. The multi-layer approach with functionally graded properties is the most effective strategy for multi-mode failure scenarios.

Third, the quality of the repair is heavily dependent on the surface preparation and preheating steps, which are often overlooked in time-pressed production environments. A disciplined approach to these steps, even if it adds 2–4 hours to the repair cycle, pays for itself many times over in extended die life.

Fourth, post-weld heat treatment is essential for relieving residual stresses and stabilizing the microstructure of the hardfacing layers. Skipping this step may result in cracking during the first few hundred strokes of service, negating all the benefits of the hardfacing.