Repair of S195 Diesel Engine Crankshaft Journals Using Vibration Surfacing and Arc Spray Methods

Literature Overview

This paper by Hao Jianjun, Shen Yuzeng, and Ma Yuejin from Hebei Agricultural University reports on the repair of four S195-type diesel engine crankshafts that had exceeded the maximum allowable oversize limit for conventional grinding restoration. The authors employed two distinct surface engineering techniques—vibration surfacing (vibration arc welding) and arc spray coating—on the main journals and connecting rod journals, respectively, to restore the critical dimensions before re-machining and reinstallation. The study was published in the journal Agricultural Machinery in 2005, and it represents an early but practical contribution to the field of in-situ repair of heavy-duty engine components using advanced surfacing technologies.

Core Technical Approach

The fundamental challenge addressed in this work is the end-of-life condition of crankshafts where the journal diameters have worn beyond the last permissible oversize grade. In standard maintenance practice, a crankshaft journal can typically be ground down through several oversize grades (commonly +0.25 mm, +0.50 mm, +0.75 mm, and +1.00 mm increments), but once the minimum remaining material thickness is breached, conventional grinding repair becomes impossible. The authors' solution was to add material back to the worn journals using two complementary techniques and then machine the restored surfaces to the required dimensional tolerances.

Vibration surfacing, also known as vibration arc welding or pulsed arc welding with vibration, operates by superimposing a high-frequency vibration on the welding arc current or on the electrode oscillation. This technique produces a significantly reduced heat input compared to conventional SMAW or GTAW surfacing, resulting in a narrower heat-affected zone (HAZ) and finer microstructure in the deposited layer. The vibration action also promotes better fluidity of the molten pool and reduces spatter, yielding a smoother as-deposited surface. For crankshaft journal repair, this is particularly advantageous because it minimizes the risk of distortion in the slender crank web geometry and reduces the likelihood of microcracking in the high-carbon alloy steel substrate.

Arc spray, on the other hand, is a thermal spray process in which wire electrodes are melted by an electric arc, and the molten metal droplets are propelled onto the substrate by a stream of compressed air or nitrogen gas. The resulting coating is characterized by a lamellar structure with excellent metallurgical bonding to the substrate. Arc spray offers higher deposition rates than vibration surfacing and can be used to build up substantial material thickness in a single pass. However, the as-sprayed surface roughness is considerably higher than that of vibration surfacing, necessitating more extensive post-machining.

Comparison of the Two Techniques

Process Parameters and Machining Strategy

The paper describes two distinct machining strategies applied after surfacing: eccentric grinding and normal grinding. Eccentric grinding involves offsetting the workpiece center relative to the grinding wheel axis, which allows for the removal of a larger amount of material from the high spots of the deposited layer while preserving the underlying substrate geometry. This technique is particularly useful when the surfacing deposit is thicker than necessary, as it permits material removal without requiring the grinding wheel to contact the base metal excessively. Normal grinding, by contrast, is a conventional concentric grinding operation that provides the final surface finish and dimensional accuracy.

The choice between eccentric and normal grinding depends on the thickness of the deposited layer and the geometric constraints of the journal. For vibration surfacing deposits, which tend to be thinner and more uniform, eccentric grinding may be sufficient for initial stock removal followed by normal grinding for finishing. For arc spray deposits, which are thicker and more uneven, a combination of eccentric grinding and normal grinding is typically required to achieve the final Ra value of less than 0.4 μm specified for crankshaft journals.

Test Results and Performance Evaluation

The authors report that the coating-substrate bonding was excellent, with no delamination, spalling, or peeling observed during eccentric grinding or subsequent normal grinding operations. This is a critical finding because many surfacing repair attempts fail at the machining stage due to poor interfacial adhesion. The successful machining performance indicates that the vibration surfacing and arc spray processes were well-parameterized to ensure adequate metallurgical or mechanical bonding.

After machining, the restored crankshafts were reinstalled and subjected to bench testing, including run-in (break-in) and load operation. The run-in test is essential for validating the repair because it subjects the journal-bearing interface to the most severe conditions of the engine's operating envelope. The fact that no failures occurred during bench testing demonstrates that the repaired journals maintained adequate load-carrying capacity, surface durability, and thermal stability under real operating conditions.

Engineering Practice Implications

This work has significant practical value for the agricultural machinery maintenance industry, where diesel engines of the S195 type were widely deployed for tractor and generator applications in rural China during the early 2000s. The ability to restore crankshafts that had exceeded conventional oversize limits extends the service life of expensive engine assemblies and reduces the need for complete engine replacement.

However, several considerations must be noted for engineers applying these techniques in modern practice. First, the metallurgical compatibility between the surfacing material and the crankshaft steel must be carefully evaluated. Crankshafts are typically made from medium-carbon alloy steels (such as 42CrMo or 40Cr) with hardness in the range of 28–32 HRC after induction hardening of the journals. The surfacing material must be selected to match or slightly exceed the hardness of the base metal to ensure wear resistance without introducing excessive hardness mismatch that could lead to cracking.

Second, the residual stress state after surfacing is critical. Vibration surfacing typically introduces compressive residual stresses due to the rapid cooling of the thin deposited layer, which is beneficial for fatigue resistance. Arc spray coatings also tend to develop compressive stresses due to the quenching effect of the substrate on the rapidly deposited molten droplets. However, if the residual stress is too high or if tensile stresses develop due to thermal cycling during machining, the risk of cracking increases significantly.

Third, the heat input management during vibration surfacing must be tightly controlled. The vibration frequency and amplitude directly influence the arc stability and heat distribution. A typical vibration frequency range of 50–200 Hz with an amplitude of 1–3 mm is used in industrial practice. Excessive vibration can lead to arc instability and porosity, while insufficient vibration reduces the benefits of the technique over conventional surfacing.

Key Reflections

The study by Hao et al. demonstrates a practical and cost-effective approach to crankshaft repair that leverages two well-established surface engineering technologies. The successful combination of vibration surfacing and arc spray for different journal locations reflects an understanding of the trade-offs between deposition quality and deposition rate. From a metallurgical perspective, the absence of cracking or delamination during machining and subsequent operation confirms that the process parameters were well-optimized for the specific substrate material and geometry.

One limitation of the study is the lack of detailed metallurgical characterization of the deposited layers, such as microhardness profiles, microstructure analysis, or residual stress measurements. Modern repair practice would typically include these analyses to provide a more comprehensive understanding of the repair quality. Nevertheless, the functional validation through bench testing provides strong evidence that the repair technique is effective and reliable for its intended application.

This work remains relevant today as a foundational reference for surface engineering repair of rotating machinery components, and its principles can be extended to modern applications involving laser cladding, cold spray, and hybrid welding processes for crankshaft and shaft repair in heavy-duty industrial equipment.