Fricton And Wear Properties Of The Bucket Tappet Material Of The Valve Train Of A High Speed And Heavy Load Diesel Engine

As an important part of the diesel engine, valve train controls the opening and closing of the intake and exhaust valves of each cylinder according to the requirements of the working cycle and the ignition sequence. Based on this, the quality of ventilatio n can be ensured and the burning and working of the engine can be improved. Bucket tappet is a key component of the valve train, which transmits the force from the cam to the valve and is also used to adjust the valve clearance, making the valve train operate normally. However, due to suffering a heavy, alternating contact stress, the working surface of the present bucket tappet made of the 30 CrMoVM carburized steel appeared abnormal wear. Therefore, in this paper the high-strength carburized tungsten alloy and the nitrided steel were expected to replace the present material. Sliding wear behavior and rolling contact fatigue(RCF) performance of the different materials, and the reliability of the bucket tappets were investigated, which demonstrated that the bucket tappet made of the nitrided steel had the best anti-wear and RCF performance. Hence, this work has important academic value and practical significance.The dynamic model of the value train was established using AVL Excite Timing Drive software for the kinetic analysis. Results showed the separation of the cam and bucket tappet appeared in some stage, causing a great impact to the interface. The interface between the cam and bucket tappet worked in instant elastohydrodynamic lubrication(EHL) state at the flying-off stage. The lubricating mechanism was mixed lubrication(ML) state at the beginning and end of the lift stage, and the friction pair worked in boundary lubrication(BL) state for a long time. Furthermore, the movement rules, contact stress, and lubrication condition of the parts in each cylinder were similar.Dry sliding wear properties of the carburized 30 CrMoVM steel used in the diesel were finished under the line-contact condition using a SRV IV friction and wear test rig. The friction coefficient and wear loss at different loads were obtained to be a reference for the evaluation of friction and wear properties for the new materials. RCF properties of the 30 CrMoVM carburized steel were studied at elevated temperature. The result showed that decarbonization of the 30 CrMo VM steel was serious in the post-treatment after the carburizing process, attributing to a low RCF performance and earlier failure of the original bucket tappet. The friction pair worked in ML state at a low temperature, changing into BL state as the temperature increased, resulting in the RCF life of the 30 CrMo VM carburized steel severely decreasing. Moreover, the formation of the synergistic tribofilm around the center of the contact area effectively prevented initiation and propagation of RCF crack under boundary lubrication state, resulting in the obvious variation of the failure mode of the carburized layer. As a result, the probability of pitting was higher than that of delamination.As for the tungsten heavy alloy, the depth of the carburized layer increased as the carburizing time increased. The carburized layer of the alloys with and without cobalt contained a porous, outer WC layer and a modified subsurface where W particles were surrounded by a shell. However, the depth of the WC layer of the tungsten alloy containing cobalt was over 50 μm, which was less than 10 μm for the tungsten alloy without cobalt. The structure phase of the subsurface shell of the alloy with cobalt was ternary carbide M6W6C(M referred to Fe and Co), whereas it was WC for the alloy without cobalt. The carburized layer was damaged in the porous WC layer in the form of the spalling of WC particles where there were some microcracks and micropores, accompanied with the peeling of WC layer due to the solid tribofilm being pushed away. Spalling was the main RCF failure mode of the carburized alloys, and the spalled-off WC particles acted as a third-body and aggravated severe abrasion of the rolling pair. In view of the above analysis, the WC layer was porous and discontinuous, causing a weak adhesion between the WC particles and the WC particles were easy to spall from each other. The anti- wear and RCF lifetime of the carburized tungsten alloy were inferior. At the same time, the hard WC layer caused serious wear loss of the matched part. Thus, the carburized tungsten alloy was not able to be used as the anti- wear shim for the bucket tappet part.A thin compound layer and a thick diffusion layer were generated on the surface 38 CrMoAl steel after nitriding. The thickness of the nitrided layer was about 230 μm, and the thickness of the white layer was 4~8 μm. The maximum of the surface microhardness of the nitrided steel reached to 1060 HV0.2. Compared with the carburized 30 CrSiMoVM steel, the friction coefficient and the wear volume of the 38 CrMoAl nitrided steel decreased by 7.1% to 11.5% and 11.3% to 23.1%, respectively, under the same working conditions. The surface of 38 CrMoAl nitrided steel was damaged by oxidation wear, abrasive wear and fatigue peeling of the solid oxide film. Under dry sliding, wear debris was a mixture of Fe2O3, Fe3O4 and Cr2O3. The debris could not escape immediately from the friction interface, but was repeatedly rolled, grinded and oxidized, causing the appearance of the amorphous structure. Under alternating contact stress, the white layer of the nitrided layer easily spalled, resulting in the ring-type crack initiation and expansion prematurely below the shallow-spalling. Thereby the RCF performance of the 38 CrMo Al nitrided steel was diminished. The rated life(L10), the median life(L50) and characteristic life(L63.2) of the 38 CrMo Al nitrided steel without the white layer increased by 157.9%, 57.9%, 54.0%, respectively, compared with those of the 38 CrMo Al nitrided steel with the white layer. And they were 105.2 times, 24.8 times and 20.1 times, respectively, as those of the 30 CrSiMo VM carburized steel. Both the sliding wear and the RCF properties of the 38 Cr MoAl nitrided steel were better than the used 30 CrSiMoVM carburized steel.The durability of the bucket tappets made of various materials was investigated using the valve train testing bench. Results showed that under great, alternating contact stress associated with variours impact forces, wear loss of the bucket tappet made of 30 CrMnSiVM carburized steel was obvious at the center of its working surface, and severe deformation occured at the central region of the bucket tappet working surface made of 12 CrN i3A carburized steel. Further, there were some notable spallings on the working surface of the bucket tappet made of 38 CrMo Al nitrided steel with the white layer. The working surface profile of the bucket tappet made of 38 CrMo Al nitrided steel without the white layer scarcely changed, which indicated it had the best anti-wear and RCF performance. Therefore, 38 CrMoAl nitrided steel without the white layer was a new choice for the bucket tappet material of the valve train of the high speed and heavy load diesel engine.