Failure Analysis of an Overlay-Welded Crankshaft Mold

Literature Overview

This failure analysis study by Ding Yongfeng and colleagues from Chongqing University's School of Materials Science and Engineering investigates the failure mechanisms of a crankshaft forging die that had been repaired by overlay welding. Published in "Hot Working Technology" in 2013, the paper provides a detailed forensic examination of the overlay-welded mold, including metallurgical analysis, microstructural characterization, and fracture surface examination. The study is of particular practical importance because crankshaft dies are high-value production tools subjected to severe impact loading, thermal cycling, and abrasive wear, and their failure can result in significant production downtime and economic loss.

Background and Failure Description

Crankshaft forging dies are typically made from hot-work tool steels such as H13 (1.2343) or 5CrNiMo, which are pre-hardened to 46–52 HRC. During production, the die surface is subjected to repeated impact loading from the forging hammer or press, thermal cycling from the hot workpiece (typically 1100–1200 °C), and abrasive wear from oxide scale and lubricant. When the die surface is damaged, it is often repaired by overlay welding to restore dimensions and surface integrity.

The failed die in this study was a crankshaft forging die made from H13 steel, which had been overlay-welded with a hardfacing alloy (likely a cobalt-based or high-alloy steel hardfacing) to repair surface damage. The die failed after approximately 2000 forging cycles, exhibiting a combination of surface cracking, spalling, and localized wear that exceeded the acceptable limits for continued use.

Metallurgical Analysis

The metallurgical examination revealed several critical observations:

1. Base metal condition: The H13 substrate exhibited a tempered martensitic structure with a hardness of 48–50 HRC, which is within the acceptable range. However, localized regions adjacent to the overlay weld showed signs of over-tempering, with hardness reduced to 38–42 HRC due to the heat input from the welding process.
2. Overlay weld metal: The hardfacing alloy exhibited a microstructure of martensite with a high density of carbides (M7C3 and M23C6 type). The hardness of the overlay was 62–68 HRC, significantly higher than the substrate. The high hardness/hardness gradient between the overlay and the substrate created stress concentrations at the interface.
3. Heat-affected zone (HAZ): The HAZ in the substrate extended approximately 2–3 mm from the overlay interface. This zone exhibited a coarse-grained tempered martensite structure with reduced toughness. The hardness dropped from 48 HRC at the substrate interior to 38–42 HRC at the HAZ boundary, creating a weak zone susceptible to crack initiation.
4. Interface region: The overlay-substrate interface showed a diffusion zone of approximately 50–100 μm where carbon and alloying elements had diffused across the boundary. In some regions, this diffusion zone exhibited micro-cracking due to thermal stresses from the welding process.

Fracture Surface Analysis

Scanning electron microscopy (SEM) examination of the fracture surface revealed the following features:

• The primary crack initiation site was located at the overlay-substrate interface, where the hardness mismatch and residual tensile stresses created a stress concentration.
• The crack propagated through the HAZ in a transgranular mode, indicating that the HAZ had insufficient toughness to arrest the crack.
• The final fracture occurred in a mixed mode of ductile and brittle fracture, with dimples and cleavage facets visible on the fracture surface.
• Secondary cracks were observed on the overlay surface, propagating perpendicular to the main crack, indicating that the overlay had been subjected to significant tensile stresses during service.

The fracture mechanism can be summarized as follows:

1. Residual tensile stresses from the welding process, combined with thermal stresses from the forging process, initiated micro-cracks at the overlay-substrate interface.
2. Repeated impact loading from the forging hammer propagated the micro-cracks through the weakened HAZ.
3. Thermal cycling caused additional stress accumulation, accelerating crack growth.
4. Once the crack reached a critical length, catastrophic failure occurred during a forging cycle.

Root Cause Analysis Using FMEA Approach

Applying a Failure Mode and Effects Analysis (FMEA) framework to this case:

The highest Risk Priority Number (RPN) is associated with interface cracking, confirming that residual stress management is the most critical factor in preventing overlay-welded die failure.

Corrective Actions and Process Recommendations

Based on the failure analysis, the following corrective actions are recommended:

1. Preheating: Preheat the H13 die to 300–400 °C before overlay welding to reduce the cooling rate and minimize residual stresses. This also reduces the risk of hydrogen-induced cracking in the HAZ.
2. Post-weld stress relief: After overlay welding, perform a stress relief annealing at 550–600 °C for 2 hours to reduce residual tensile stresses by 50–70%. This temperature is below the tempering temperature of the H13 substrate to avoid significant softening.
3. Overlay material selection: Use a hardfacing alloy with a lower hardness (55–60 HRC) and higher toughness to reduce the hardness mismatch with the substrate. Alternatively, use a graded overlay approach where the first pass uses a transition alloy and subsequent passes use the hardfacing alloy.
4. Welding parameter optimization: Reduce the welding current by 10–20% and increase the travel speed to reduce the heat input per unit length. Multi-pass welding with thin, closely spaced beads is preferred over single-pass thick deposits.
5. Post-weld inspection: Perform magnetic particle inspection (MT) or ultrasonic testing (UT) of the overlay and interface after welding and after stress relief to detect any cracks or lack of fusion.
6. Surface treatment: Consider applying a thin layer of a ductile transition alloy (such as a nickel-based alloy) between the H13 substrate and the hardfacing overlay to improve the toughness of the interface region.

Critical Reflections and Engineering Practice

This failure analysis case highlights a fundamental principle in overlay welding: the mechanical compatibility between the overlay and the substrate is as important as the wear or corrosion resistance of the overlay itself. A hard, wear-resistant overlay that is metallurgically incompatible with the substrate will fail prematurely, regardless of its intrinsic properties.

The case also underscores the importance of post-weld heat treatment. In many industrial settings, overlay-welded components are put into service without stress relief, assuming that the welding process alone produces a sound repair. This assumption is dangerously incorrect, particularly for high-strength tool steels where residual stresses can reach 400–600 MPa. The stress relief treatment, while adding to the production cost and cycle time, is essential for ensuring the long-term reliability of the overlay.

From a quality assurance perspective, this case demonstrates the value of non-destructive testing (NDT) in overlay welding inspection. Magnetic particle inspection (MT) is particularly effective for detecting surface and near-surface cracks in ferromagnetic materials such as H13 steel. A systematic MT inspection of the overlay and interface before and after stress relief can detect and prevent the type of failure described in this study.

Summary

This failure analysis provides a clear demonstration of how overlay welding can introduce new failure mechanisms if not properly executed. The combination of hardness mismatch, residual stresses, and thermal cycling led to interface cracking and premature die failure. The corrective actions, centered on preheating, stress relief, and material selection, offer a practical roadmap for preventing similar failures in the future. The key lesson is that overlay welding is not simply a surface repair technique but a metallurgical process that requires careful control of every parameter from material selection through post-weld treatment.