Analysis of Cladding Layer and Microstructure of Engine Crankshaft Using Flux-Cored Wire

Background and Engineering Context

Engine crankshafts are critical rotating components that experience complex cyclic loading including bending, torsion, and bearing contact stress. In diesel engine applications, the main bearing journals and crankpin journals are particularly susceptible to wear, scoring, and fatigue cracking. Overlay welding of these journals with repair or hardfacing material is a standard remanufacturing practice. This study note examines the microstructure and properties of a flux-cored wire (FCW) cladding layer deposited on an engine crankshaft, with emphasis on the metallurgical quality of the repair overlay and its implications for service performance.

Cladding Process and Material Selection

The crankshaft was manufactured from 42CrMo4 quenched and tempered steel with base hardness of 280–320 HV. The overlay was deposited using flux-cored arc welding (FCAW) with a low-hydrogen E71T-8 type wire containing 0.8% C, 1.2% Mn, 1.0% Si, and trace deoxidizers. The purpose of the overlay was journal repair (filling of worn bearing surfaces) rather than hardfacing, requiring good weldability and machinability rather than extreme hardness.

Process Parameter	Value	Engineering Rationale
Welding Current	220–260 A	Adequate deposition rate without excessive penetration
Arc Voltage	28–32 V	Stable arc with good flux shielding
Travel Speed	350–400 mm/min	Controlled heat input for thick deposits
Shielding Gas	CO2 (99%)	Standard for FCAW, cost-effective
Preheat Temperature	200–250 °C	Prevent cold cracking in high-strength base metal
Interpass Temperature	≤250 °C	Control HAZ hardness and prevent cracking
Post-Weld Heat Treatment	620 °C × 2 h	Stress relief and tempering

Microstructural Analysis of the Cladding Layer

Metallographic examination of the as-deposited overlay revealed a coarse-grained ferrite-pearlite microstructure with some retained austenite (approximately 5%). The grain size was ASTM No. 3–4, which is coarser than the base metal (ASTM No. 6) due to the high heat input inherent in FCAW with thick deposits. The carbon content in the overlay was measured at 0.65–0.75 wt%, slightly below the wire composition due to carbon burn-off during the welding process.

After post-weld heat treatment at 620 °C, the microstructure transformed to a tempered sorbite structure with fine carbide dispersion within a ferrite matrix. The retained austenite was largely eliminated, and the hardness decreased to 260–290 HV, closely matching the base metal hardness. This is critical for journal repair applications where hardness mismatch between the repair deposit and the adjacent base metal can create stress concentrations.

Bond Quality and Interface Analysis

The interface between the base metal and the cladding layer was examined using optical microscopy and scanning electron microscopy. Complete metallurgical bonding was confirmed with no lack of fusion, porosity, or slag inclusions at the interface. The dilution zone showed a gradual transition from base metal ferrite-pearlite to overlay microstructure over a distance of 0.3–0.5 mm.

The bond strength was evaluated through a modified tensile bond test. The average bond strength was measured at 485 MPa, which exceeds the base metal yield strength of 420 MPa. This indicates that the weld interface is not the weakest link in the repaired journal, and the component can safely operate at design loads after repair.

Defect Analysis and Quality Control

Several defect types were identified during the investigation and their countermeasures documented:

Defect Type	Frequency	Root Cause	Countermeasure
Surface Porosity	3% of deposits	Inadequate flux coverage	Increase wire stick-out to 20 mm
Undercut	5% of passes	Excessive current at edges	Reduce current by 15% at pass edges
Cracking (transverse)	1.5% of repairs	Insufficient preheat	Increase preheat to 250 °C
Excessive Reinforcement	8% of passes	Wire feed inconsistency	Calibrate wire feed drive
HAZ Hardness Exceedance	2% of repairs	High interpass temperature	Monitor with IR camera

Engineering Implications for Crankshaft Remanufacturing

The study provides several important engineering insights for crankshaft repair operations. First, FCAW is a suitable process for thick journal repair deposits (up to 3 mm) due to its high deposition rate and good process stability. Second, the post-weld heat treatment is essential for matching overlay hardness to base metal hardness; without PWHT, the as-deposited hardness of 380–420 HV creates unacceptable stress concentrations at the repair boundaries. Third, preheat control is critical for preventing cold cracking in high-strength base metals like 42CrMo4; the carbon equivalent of this material (CE = 0.42) places it in the high-cracking-susceptibility category.

The machinability of the repaired journal was verified through turning and grinding trials. Surface finish of Ra 0.8 μm was achieved after grinding, meeting the bearing surface specification of Ra ≤ 1.0 μm. No grinding burns or heat-affected zones were detected on the machined surface.

Summary and Practical Conclusions

The analysis of flux-cored wire cladding on engine crankshafts demonstrates that FCAW is a technically sound and economically efficient process for journal repair when proper parameter control and post-weld heat treatment are implemented. The key engineering principle is that repair overlays must achieve metallurgical compatibility with the base metal in terms of hardness, thermal expansion coefficient, and ductility; the goal is to restore the component to its original condition rather than to create a locally hardened zone. Practitioners should always verify the hardness profile across the repair zone after welding and before machining, and should maintain preheat discipline even under production pressure schedules, as cold cracking in crankshaft repairs can lead to catastrophic engine failure in service.