Vehicle engine designers are beset by conflicting demands. On the one hand, they must reduce costs as intense international competition is driving the need to cut product prices so that companies can maintain their market positions. On the other hand, they must also deliver more powerful, more efficient engines, as there is an equally intense customer demand for engines that offer improved performance and longer warranties while emitting fewer pollutants. The two demands are often mutually exclusive, but both now promise to be met thanks to a new advanced alloy developed for the automotive industry.
It's the demand for lower pollutant emissions that has given engine designers a major headache. Redesigned engines and cleaner burning fuels are forcing the specification of higher cost raw materials for engine components to meet tough new standards. For valve train components, particularly valves and valve seat inserts (VSIs), the problem is a simple one. Established materials for these applications wear out faster in engines redesigned to produce fewer pollutants - in other words, the old valve and VSI materials cannot meet wear resistance targets in the new engines.
Wear-resistance, always a primary concern for engine designers. It has become even more important in this era of redesigned engines, completely new engine designs and cleaner burning fuels. The operating environment within an engine valve train has always involved intense thermal and mechanical stress, especially for the small but vital VSIs. Positioned at the intake and exhaust ports of the combustion cylinder, VSIs must withstand seating pressures of up to 20.7MN.m-2 and operating temperatures reaching 650°C. These intense operating conditions have become even more challenging as a result of increasingly stringent clean-air regulations. The new regulations call for the use of fuels that burn hotter and cleaner, so leaving fewer combustion deposits on the valve and seat interface. When established VSI materials are used in new engines, valve-train components no longer benefit from the layer of coating protection deposited via combustion by-products. These parts are therefore more vulnerable to the effects of frictional sliding and, as a result of metal-to-metal contact, wear out at a faster rate. To deal with these more demanding operating environments, engine designers found themselves confronted with the task of finding new VSI materials that could measure up to more demanding performance requirements.
Materials to Suit New Fuels
It is an accepted rule of thumb that a broad range of performance improvements such as those outlined above are rarely combined with lower costs. That certainly appeared to be the case when engine manufacturers turned to cobalt-base materials to help guard against the increased wear that occurred when new designs and new fuels rendered traditional materials obsolete.
Cobalt-base alloys were the obvious first choice because of their ability to resist wear in the most severe engine applications. Cobalt’s unique material structure provides superior resistance to abrasion, corrosion, oxidation, and sulfidation. Cobalt usually solves the problem of wear in new and developmental engine programmes, which use the newer generation of cleaner-burning fuels. But despite cobalt’s attractive wear-resistant properties, its relatively high cost and price instability make it less-than-ideal for valve train components.
Clearly, selecting cobalt-base alloys is an expensive solution. A number of diesel engine manufacturers made known their need for new valve train component materials that would provide acceptable wear-resistance properties, but would do so at lower cost and without cobalt’s pricing unpredictability. Winsert Inc, a manufacturer of cast and machined VSIs, was one company able to respond. The company had recognised the need for new VSI materials to reduce wear and replace cobalt and saw an opportunity to use its alloy design capabilities to develop alternative materials.
In 1992, Winsert began work with a team of engineers at a major diesel engine OEM that had recently changed the design of its engines. The higher cylinder pressures and temperatures of these engines created the need for an affordable, wear-resistant alloy. After a five-year R&D effort, W77T6 was introduced. It is an iron-base alloy with advanced heat- and wear-resistance properties.
The W77T6 alloy is designed to replace cobalt-base materials traditionally used in demanding engine applications. The alloy is characterised by a blend of hard M6C, M2C and MC type carbides unique to iron-base VSI materials. These carbides are distributed uniformly throughout the material and provide the alloy with strong wear-resistance capabilities.
The superior wear-resistance properties of the W77T6 alloy have been demonstrated in a number of wear tests, each of which has shown the level of wear on W77T6 to be equivalent to, and sometimes better than, that on Stellite 3. In tests using the high performance diesel engine of a major OEM, the W77T6 alloy exhibited an average recession wear of 4.7 x 10-4cm per 1000 hours. When tested in another series of natural gas fuel engines run at 1200 rpm, the alloy demonstrated an average recession of 2.4 x 10-3cm of wear after more than 5500 hours. In both of these tests, the performance of the W77T6 alloy exceeded the objective of 7.8 x 10-4cm of wear per 1000 hours.
Currently Winsert's foundry is supplying four high-volume diesel engine programmes with VSIs made from the patented W77T6 alloy (figure 1). The primary application for these engines is class 8, over-the highway trucks. The rapid acceptance of this material by truck engine OEMs has in part been aided by the company’s wear-testing capability. They have designed a wear-tester that can simulate both sliding and adhesive wear in VSI materials. The benefit of this capability is that tests of a variety of materials and surface treatments can be conducted quickly and at relatively low cost.
Figure 1. To meet blue print requirements for hardness and wear resistance, VSIs are heat treated to temperatures above 1010°C.
Tests conducted in their laboratory have been validated by field tests in routine driving conditions. The materials tested in the field showed similar wear levels, confirming the accuracy of the laboratory wear tester.
At present, eight engine programmes are testing the W77T6 alloy. If the material demonstrates acceptable levels of wear in these OEM laboratory and field tests, it is likely to replace cobalt-base VSIs in every one of these programmes for one all-important reason - VSIs manufactured with W77T6 cost less than half as much as VSIs made with cobalt-base materials. It seems that, thanks to new materials from alloying technology, performance and environmental improvements do not necessarily come at a price.