Case Studies
Diesel Engine Inlet Valves
Background
An oil & gas company operated a generator fleet of around 60 6-cylinder, 12.5 litre, turbocharged and aftercooled diesel engines. The fuel used in the engines was crude oil. Numerous inlet valves were reported to have failed by losing their ability to seal the combustion chamber.
Initial Hypothesis
There was some synergistic combination of thermal, mechanical, and chemical conditions at the inlet valves that caused accelerated material loss.
Data Collection
Visual comparison of the used and unused inlet valves showed clear evidence of wear at the valve seat surface on the used valve.
Chemical analysis revealed that the inlet valve material was X85CrMoV18-2 (W.Nr. 1.4748) martensitic stainless steel. The exhaust valve material was a nickel-based superalloy similar to INCONEL® alloy 718.
Metallographic examination of an unused inlet valve showed globular carbides in a tempered martensite matrix. The microstructure in the used inlet valve was similar to the unused valve, with two notable differences. First, there was material displacement at the valve seat face, with around 0.1 mm of material overhanging at the outer extremity of the seat face surface (Fig. 1). Second, bleedout encountered during etching revealed that crevices had developed between carbide particles and the martensite matrix at the seat face surface (Fig. 2).
SEM/EDS analysis on the used inlet valve showed significant levels of sulfur, chlorine, phosphorus, and fluorine on its seat face and fillet radius. All these elements are potentially corrosive to stainless steel.
Analysis
Metallographic examination showed that the interface between the carbides and the martensite matrix in the inlet valve material was subject to localised attack. SEM/EDS analysis results showed significant levels of corrosive elements sulfur, chlorine, phosphorus, and fluorine on the inlet valve surfaces.
The influence of corrosive elements, coupled with the galvanic difference between chromium-rich carbide particles and the martensite matrix, resulted in corrosion at the interface between the carbides and the matrix. This corrosion was accelerated by the elevated service temperature at the valve face.
Under repeated mechanical stress from closing of the valve, carbides were removed from the seat face surface. Because the valve material is essentially a metal matrix composite (hard carbide particles in a softer metal matrix), removal of carbides would have sharply lowered the surface hardness at the seat face. This hardness change was responsible for the material displacement that was observed at the outer edges of the valve seat.
Loss of surface carbides and matrix deformation exposed previously subsurface carbides to the corrosive conditions, and the material loss continued by the same mechanisms until failure.
Conclusion
The hypothesis is accepted — a combination of thermal, mechanical, and chemical conditions at the inlet valves caused accelerated material loss. The root cause of failure was the use of crude oil rather than refined diesel fuel in the engines. The harsh chemical conditions that this created degraded the material in a way that made it vulnerable to otherwise benign mechanical loading.
Recommendation
Although running the engines on refined diesel fuel would eliminate the root cause of the valve failures, the client stated that this would represent an unacceptable, ongoing increase in operating costs.
Given that the exhaust valves in the engines showed no signs of degradation while exposed to similar chemical and mechanical conditions (and considerably harsher thermal conditions) compared to the inlet valves, it is recommended that inlet valves made from a similar superalloy material be installed. The cost increase associated with a valve material change would be significantly lower than the cost of changing the type of fuel used, but the efficacy in improving engine uptime is anticipated to be similar.
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