Failure Analysis of a Crankshaft Surfacing Mold

Literature Overview

This technical paper by Ding Yongfeng, Xu Wujiao, Lei Fan, and Tan Jiangli, published in Hot Working Technology (2013, Vol. 42, No. 23, pp. 207–208), presents a failure analysis of a crankshaft surfacing mold. The authors are affiliated with the College of Materials Science and Engineering at Chongqing University. The analysis employed chemical composition testing, microhardness measurement, metallographic examination, and Scanning Electron Microscopy (SEM) to identify the failure mechanisms and contributing factors.

Failure Analysis Methodology

The investigation followed a systematic approach combining multiple analytical techniques:

Analytical Methods Employed

The failure analysis followed the standard PDCA (Plan-Do-Check-Act) framework, with each analytical technique contributing specific information to the overall failure mechanism determination.

Failure Mechanism Identification

Primary Failure Mode: Fatigue Wear

The investigation confirmed that the primary failure mode of the forging die was fatigue wear, characterized by the presence of extensive fatigue cracks on the cavity surface. This failure mode is consistent with the cyclic loading conditions experienced during crankshaft forging operations, where repeated impact and pressure cycles lead to progressive crack initiation and propagation.

Contributing Factor 1: Excessive Sulfur Content

A critical finding was that the sulfur content in the surfacing layer material exceeded the nominal specification value. Sulfur is a well-known detrimental element in steels and surfacing alloys, as it:

• Forms manganese sulfide (MnS) inclusions that act as crack initiation sites
• Reduces hot workability and increases susceptibility to hot shortness
• Weakens the intergranular bonding in the microstructure
• Promotes intergranular fracture under cyclic loading

The elevated sulfur content likely originated from either the base metal composition or the filler material used during surfacing. This highlights the importance of incoming material inspection and specification verification in surfacing operations.

Contributing Factor 2: Improper Heat Treatment

The analysis indicated that the tempering time during heat treatment was not adequately controlled. In surfacing mold applications, the heat treatment process is critical for:

• Relieving welding residual stresses that can promote crack initiation
• Achieving the desired hardness and toughness balance
• Stabilizing the microstructure against temper embrittlement
• Ensuring uniform hardness distribution across the surfacing layer

Insufficient tempering time can result in retained austenite and high residual stresses, while excessive tempering can lead to over-softening and reduced wear resistance. The optimal tempering parameters must be carefully selected based on the specific surfacing alloy composition and the service conditions.

Microstructural Characterization

Surface Crack Morphology

SEM analysis revealed typical fatigue crack morphology on the cavity surface, including:

• Initiation sites associated with material inclusions (particularly MnS)
• Fatigue striations indicating progressive crack growth
• Secondary cracking patterns consistent with multiple loading cycles
• Surface roughness evolution from initiation to final fracture

Hardness Distribution

Microhardness testing revealed non-uniform hardness distribution across the surfacing layer, which is a common issue in surfacing operations due to:

• Thermal cycles during multi-pass surfacing
• Differential cooling rates at different depths
• Incomplete homogenization during heat treatment
• Variations in local composition due to dilution effects

Engineering Practice Recommendations

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

Material Specification Control

• Implement strict incoming inspection of surfacing filler materials with verified chemical composition
• Establish sulfur content limits not exceeding 0.02% for high-cycle fatigue applications
• Require mill certificates and independent laboratory verification for critical components

Process Optimization

• Develop a standardized heat treatment procedure with controlled tempering time and temperature
• Implement preheat and interpass temperature control during surfacing to minimize residual stresses
• Consider post-weld stress relief treatment (typically 550–650°C for 2–4 hours) for critical mold applications
• Establish a regular inspection and maintenance schedule for surfacing molds

Quality Assurance Measures

• Conduct hardness testing at multiple locations on the surfacing layer after heat treatment
• Perform non-destructive testing (NDT) including magnetic particle testing (MT) to detect surface cracks
• Implement a failure analysis protocol for early detection of developing cracks
• Maintain records of mold usage cycles and implement preventive replacement schedules

Study Reflections and Implications

This failure analysis case study provides valuable lessons for engineers working with surfacing molds in forging and forming applications. The identification of sulfur content as a contributing factor underscores the importance of material specification control throughout the supply chain. The emphasis on heat treatment optimization highlights the critical role of post-weld processing in achieving the desired service life. For the steel pipe and fitting industry, similar failure analysis approaches can be applied to surfacing repairs on critical components such as valve seats, pump shafts, and tooling used in pipe manufacturing processes. The systematic use of multiple analytical techniques demonstrates the value of comprehensive investigation in identifying root causes rather than addressing only symptoms.