Failure Analysis of a Crankshaft Surfacing Die

Literature Overview

This case study by Ding Yongfeng, Xu Wujiao, Lei Fan, and Tan Jiangli from Chongqing University, published in Hot Working Technology (2013, Vol. 42, No. 23, pp. 207-208), presents a systematic failure analysis of a surfacing die used in crankshaft manufacturing. The investigation employed chemical composition testing, microhardness profiling, metallographic examination, and SEM scanning electron microscopy to identify the root cause of premature die failure.

Failure Investigation Methodology

The investigation followed a structured approach consistent with FMEA principles. Visual inspection identified surface damage patterns and crack morphology, chemical analysis verified surfacing layer composition against nominal specifications, microhardness testing assessed hardness distribution across the cross-section, metallographic examination evaluated microstructure and phase distribution, and SEM analysis provided detailed examination of crack initiation sites and propagation paths.

Root Cause Analysis

The primary failure mode was identified as fatigue wear, characterized by extensive fatigue cracking on the die cavity surface. The investigation identified two critical contributing factors: elevated sulfur content in the surfacing material and inadequate heat treatment. Excess sulfur promotes the formation of MnS inclusions, which act as crack initiation sites under cyclic loading. In surfacing applications where materials undergo repeated thermal cycling and mechanical loading, even small amounts of harmful inclusions can dramatically reduce fatigue life.

The tempering time during post-weld heat treatment was not properly controlled. Insufficient tempering leads to retained austenite and untempered martensite in the surfacing layer, both of which are susceptible to cracking under thermal and mechanical stress. This combination of material quality issues and process control failures created a scenario ripe for premature fatigue failure under the severe cyclic loading conditions of crankshaft forging.

Engineering Implications and Recommendations

For engineers involved in die manufacturing and maintenance, this case study underscores several critical quality control measures. Incoming material certification must include verification of sulfur and phosphorus levels, heat treatment parameters particularly tempering time and temperature must be documented and verified, regular inspection of die surfaces for early signs of fatigue cracking should be implemented, and consideration of alternative surfacing materials with inherently lower sulfur content should be given for fatigue-critical applications.

This failure analysis demonstrates the importance of integrating materials science knowledge with process engineering in surfacing applications. For crankshaft manufacturing operations where dies must withstand millions of forging cycles, the selection of surfacing materials and the control of post-weld heat treatment are paramount. Engineers should adopt similar systematic approaches when investigating surfacing-related failures, ensuring that both material specifications and process parameters are verified at each stage of the manufacturing chain to prevent recurrence.