Failure Analysis of Automotive Valves

Engineering 45 Semester Project Dr Younes Ataiiyan December 2, 2003

Sadie McEvoy Kevin Nolen Darryll Fletcher The objective of this project was to analyze a variety of failed automotive valves. After gathering a selection of valves from various sources, we completed an external failure analysis for each, using the methods and information we found during our research. Next, we prepared samples for viewing under the microscope, by cutting small cross section views of damaged areas, putting them in molds, and then polishing and etching them. We took photographs of both the damaged valves and the microscope images for documentation.

Valve 1

A typical valve stem is subjected to less stress than the valve head, but it still experiences stress levels of about 2700 psi, or 300 lb. over a .110 square inch area. However, the stem is also affected by the seating load, which is generally less than 20,000 psi. Valve 1 was in very good condition, with little wear or corrosion. However, the stem was bent almost 30 degrees. This failure is most likely not of a gradual or progressive nature, and was probably caused by a collision within the cylinder. Some engines have tight enough tolerances where the valves actually protrude far enough into the combustion space that they can collide with the piston. These engines are referred to as "interference engines" which we believe this valve came out of. This type of failure usually occurs when an engines timing belt fails allowing the valves to be open when the piston is on its compression stroke. Though in this case the damage was mild, it can usually render the engine unsalvageable. We viewed two samples from valve 1 under the microscope; a control section and a cross section from the point of failure. Although the grains were unmistakably elongated in the bent sample, it was difficult to make conclusions from the images. Without any knowledge of the production or treatment of the valve, it was impossible to distinguish what effects were products of the failure itself.

Valve 2

Valve heads are typically subjected to loads of approximately 3500 lb. On top of which they endure extreme thermal stresses, due to temperature differentials of up to 300 F across a 2.5” diameter face. Obviously, only the smallest shift in seating could quickly cause failure. Valve 2 appeared to be an older, well used valve, although there was very little build up or corrosion on the valve. The only damage was a small chip out of lip of the valve head. This chip was hard to analyze, because it appeared to be the product of several things. First, the placement and size of the chip suggested that it was probably caused by fatigue. However, the concentric rings that show the propagation of such a break were almost imperceptible. The second possible mode of failure is adhesive wear. Adhesive wear is micro-welding caused by excessive temperatures that are the product of a contact point between the valve and another metal. The heat, extra load and welding work together to weaken that region of the valve. The chipped section of valve 2 showed the smooth, almost glossy surface that is exemplary of adhesive wear. Most likely, the broken section was bearing a disproportionate amount of the load, which created both the heat necessary for adhesive wear and the stress to cause fatigue. For Valve 2, we cut directly into the crack, so that the face we viewed was from the crack surface, to the base of the stem. We hoped that this would allow us to see a change in the grains, from one end to the other. Unfortunately magnification of 100 to 1000X is necessary to view the effects of micro-welding and we were not able to see any change in micro structure from our images.

Valve 3

Valve 3 had several points of damage, so on this valve not only the mode of failure had to be determined, but also, what actually failed and what was simply damaged in the process. The three main points of damage were; a jagged chunk cut out of the head, a lip that was bent upward and pushed toward the stem, and finally the absence of the stem itself. There were also several small cuts in the sides of the valve, which we attributed to damage and not a cause of failure. The upturned lip had a ragged edge and a very shiny surface that suggested it had reached a high enough temperature to actually melt. The large chunk cut out of the far side of the face also had very sharp angles, but the glossy surface. These two points of damage lead us to believe that the failure did not progress slowly, but was sudden and violent. Finally, the base of the stem appeared almost like a spiraling staircase with very large cracking planes, pointing radially outward from the center. It was clear to us that the stem had been twisted off, causing injury to the rest of the valve. Most likely, there was some torsional fatigue at the base of the stem, which invariably resulted in breakage. The damage to the face was probably caused by a collision similar to that which was described above. Other possibilities include a valve retainer failing. In the event of a failure the valve would no longer be held closed by its respective spring allowing it to fall freely into the combustion chamber. This type of failure is also extremely detrimental to the engine possibly damaging the cylinder walls, piston, and combustion bowl of the head. In order to view the different regions of valve 3, we had to cut it into several pieces. We cut directly down the center of the valve, through both damaged edges and the stem base. Next, we cut the sections we wanted down to small separate pieces. Unfortunately we could not see any evidence to support our hypothesis or changes in the microstructures, due to the equipment we used.

Valves 4 and 5

Valves 4 and 5 both failed due to corrosion. We performed a quick experiment to find out what caused the corrosion. We scraped some of the deposits on to a piece of thin paper and then held the paper over a magnet. If deposits are magnetic, the source of corrosion is from the combustion process (deposits from any iron based alloy, even austenitic steel, will be magnetic). Our samples were magnetic, so we concluded that the corrosion was caused by combustion.

Valve 6

Valve 6 was fairly easy to analyze, since the valve was not from a diesel, the corrosion could not be from sulfur, and we knew that it was not from the combustion process, so we were left with oxidation corrosion, the most common form. Sources

1) http://www.maneyperformance.com/tech-valve-stress.html 2) http://www.maneyperformance.com/tech-failure-1.html 3) http://www.manyperformance.com/tech-breakage-2.html 4) http://www.maneyperformance.com/tech-corrosion-3.html 5) http://www.maneyperformance.com/tech-wear-4.html

Note: There are frequently problems accessing these sites directly, in this event go to http://www.maneyperformance.com and follow the “Tech Report” links.