Cylinder liner with its inner peripheral surface formed with surface treatment layer, and method for machining to the surface treatment layer

#The present invention relates to a cylinder liner having an inner peripheral surface provided with a surface treatment layer by a water vapor treatment, etc, and a method for processing the cylinder liner for use in an internal combustion engine.

#In a cylinder liner for a conventional Diesel engine, phosphate treatment or nitriding treatment is performed to form a surface treatment layer on an inner peripheral surface of the liner in order to improve wear resistance. Then a honing is performed on the surface treatment layer on the inner peripheral surface of the cylinder liner for final machining to the inner surface. In the conventional final grinding to the inner peripheral surface, a resinoid bonded grindstone containing silicon carbide is used. The grain size of the silicon carbide is not more than 3000, and the silicon carbide is contained in an amount less than 50% in the resultant grindstone. As a result, the surface treatment layer has a cross-hatching pattern providing oil pockets which are advantageous for a lubricating property.

#Recently, the diesel engine is subjected to a severe working condition due to a severe requirements in exhaust gas. Therefore, corrosion resistance as well as wear resistance are required in the cylinder liner. In order to meet with the demand, a cylinder liner having a surface treatment layer provided by a water vapor treatment is described/in WO 01/33065A1.

#In the drawings;

#FIG. 1 is a cross-sectional elevation showing a cylinder liner according to one embodiment of the present invention;

#FIG. 2(a) is a specific roughness profile of an inner peripheral surface of the cylinder liner according to the embodiment;

#A cylinder liner and a method for machining to the inner peripheral surface of the cylinder liner according to one embodiment of the present invention will be described with reference to the drawings. As shown in FIG. 1, a cylinder liner 1 has an inner peripheral surface formed with a water vapor treatment layer 2. The cylinder liner 1 is made from a cast iron containing 3.15 mass % of C, 2.0 mass % of Si, 0.75 mass % of Mn, 0.35 mass % of P, 0.06 mass % of S, 0.4 mass % of Cu, 0.3 mass % of Mo, 0.09 mass % of B, and the balance Fe.

#A thickness of the water vapor treatment layer at the inner peripheral surface of the cylinder liner ranges from 10 to 30 μm. After machining to an outer peripheral surface of the cylinder liner 1, the inner peripheral surface of the cylinder liner is subjected to grinding by a honing machine which employs a grindstone containing diamond grains. The diamond grain has a grain size of 5000, an average grain diameter of 2.5 μm, grain diameter distribution ranging from 1.5 to 4.0 μm. These diamond grains are congregated together with binding agent (metal bond) of MH5 at a binding degree K. As a result of grinding, the inner peripheral surface is finished into plateau configuration providing a surface roughness Rz of 0.8 to 5.9 μm, and providing a reduced peak height Rpk of not more than 0.64 μm, a core roughness depth Rk of 0.05 to 1.8 μm, and a reduced valley depth Rvk of 0.15 to 3.3 μm, those Rpk, Rk and Rvk are defined in DIN4776.

#The iron oxide film formed upon water vapor treatment provides a coarse surface in which acute peaks protrude from the surface. However, in the illustrated embodiment, the reduced peak height Rpk is set not more than 0.64 μm in order to remove large peaks without destroying the iron oxide film to form the surface into plateau configuration. As a result, surface roughness Rz of 0.8 to 5.9 μm can be obtained which can avoid generation of thermal seizure as a result of sliding contact with a piston ring. Accordingly, sufficient lubrication oil retaining property at the time of initial break-in period can result thereby providing excellent initial break-in property.