Choose Your Weapon (Against Wear)
Geronimo Alloys specializes in producing replacement wear parts for industrial equipment. A layman’s opinion of combating wear follows. There are hundreds of thousands of alloys. Since the Bronze Age, man has been combining different elements in an attempt to eliminate wear. Metals wear, deteriorate or fail from abrasion, corrosion, temperature or a combination of these. The most critical element in combating wear is determining exactly what is causing the wear. Sand blasting is an example of dry abrasion. If the sand is mixed with water, then we have a combination of fluid and particle abrasion. If the combined sand and water are pumped at a rate to create turbulent flow, then a third factor has been introduced. The wear rate will be increased or decreased by changing the flow rate or the percentage of sand concentration. If we lower the PH of the water by introducing an acid, we have now added corrosion to the mix of wear agents. Temperature can greatly affect the wear rate of an alloy. In general, the higher the temperature the greater the effect of the wear agent. High temperature itself is a deteriorating factor. Extreme cold may change the mechanical qualities of an alloy to the extent it will break from any shock. Chemical elements such as chlorides in a liquid can cause accelerated corrosive wear. There are so many factors that affect wear; it is often difficult to determine exactly what alloy will provide the longest life at the lowest cost.
Hardness is often considered an indicator of wear life. That may or may not be correct, but the majority of the time, hardness is not the determining factor in prolonging the life of an alloy. The best example of this can be seen by examining the walls of a sand blast room. Steel walls will have a very limited life while rubber matting on the walls will last indefinitely. A recent abrasive test of 3 alloys at different hardness’s gave some interesting results. An alloy with a hardness of a 41RC had wear life 166% longer than an alloy with a hardness of 62RC. Another alloy with an added chemical element and a hardness of 58RC last 500% longer than the 62RC alloy and 300% longer than the 41RC alloy. However, when abrasion is not very present, the 62RC maintained a sharp edge much longer than the other alloys. A very soft nickel based alloy has been found to be very effective in combating fluid erosion. One of the most wear resistant alloys has a hardness range of 38RC to 53RC. This family of alloys gets it’s wear resistance from a combination of a relatively low coefficient of friction and the formation of carbides when the alloy goes from liquid to solid. We replaced a pump that had been made with cast iron with Cobalt 6. Not only did the wear life greatly increase, but the output of the pump went up 15% because of the lower co-efficient of friction. The cobalt family of alloys do not gall when moving against each other. This allows them to act as bearings.
The nickel based alloys, in general, have the ability to withstand more corrosion and higher temperatures. Nickel based alloys have the ability to work harden which helps them withstand some types of abrasive wear. Work hardening is the ability of an alloy to continually reharden lower layers of the alloy as the upper layer wears away. The ability to work harden is determined by the elements of the alloy and the degree that it is worked. There is a nickel and cobalt based alloy that does work harden, but when we used it in an application that did not work it hard enough, it would not work harden.
The cost of various alloys can range from as little as .30$ to as high as hundreds of dollars a pound. The cost of forming the alloy into a useable shape can also vary greatly. Machining can add cents a pound to hundreds of dollars. It is always wise to calculate the cost of replacement. Cost of replacement should include the loss of production caused by the need to shut down while the replacement is in process. We provide replacement parts for a machine that produces 16,000 pounds of product per hour. The product sells for about $1.00 per pound. The replacement time is 1 to 2 days. This means that the replacement cost is $40,000.00 for the parts, labor cost of $3,600.00 (1.5 days labor at $75.00 per hour for 4 men), and lost production of $576,000.00. The total cost would be $619,600.00. This justifies virtually any cost for the replacement parts if they will last two days longer. The other example is the changing of residential hose bib. A plumber would charge about $100.00 for the job and the hose bib cost $5.00. There is no lost production. The hose bib may last 10 years. It is obvious that making the hose bib from materials that would extend the life to 30 years but cost $1,000.00 is not very economical. However, if the valve, and that is what a hose bib is, controls the flow of a liquid that is critical to the production of a plastic that sells for $110.00 a pound, then the life of the valve becomes extremely important.
Remember that the only reason the alloy exists is that someone was trying to find the answer to a problem. The Bronze Age changed to the Iron Age because a better material was needed for swords. The smallest change in the elements or their percentage can greatly affect its ability to full fill the purpose for which it was invented. Inconel 718 has a specified percentage of aluminum and titanium, both of these elements are very susceptible to transforming themselves into oxides in the presence of oxygen. This means that this alloy is most successfully melted in a vacuum. We have melted Inconel 718 in our non-atmospheric furnaces by simulating a vacuum by attempting to enclose the whole process in an inert gas. Although we were able to meet the chemical and physical properties required by the ASTM specification, the presence of oxides made the product unsellable. The oxides produced black specs in the alloy that ruined the appearance of the finished product. It is critical that nothing contaminated the alloy so that the various elements don’t get outside the specification. We often find parts that are returned to us for repair by customers that have been made of an alloy that is outside the specified range. This is especially true for 316 and the cobalt alloys. Where 316L (ASTM CF-3M) has been specified by the customer because of a desire to prevent intergranular corrosion, we often find that the alloy is really 316 (ASTM CF-8M). The higher carbon allowable in CF-8M defeats the purpose of the specification. The cobalt alloys are often found to have iron at twice the allowed amount. This reduces the alloys ability to withstand abrasion. There is a specified heat treat for each alloy. The heat treat can make the alloy more wear resistant if done correctly. It can also make the alloy less wear resistant if done incorrectly. An example of this is the heat treatment of the 300 series stainless alloys. The heat treat chart can show that the alloy was heat treated correctly, but there is no chart showing the amount of time between when the alloy leaves the correct temperature in the oven and the quench point. These alloys are water quenched to quickly get the temperature below 900 F so that carbon chromium does not depletion does not occur in the intergranular areas. You must depend on the reputation of the provider.
It is wise, when in doubt, to seek the advice of an expert. We have been using one of the best metallurgists in the U.S. for the last 30 years. He teaches ASTM courses. Anyone wanting to combat a wear problem should determine at least the following things:
- PH of the environment
- Sources of abrasion
- Degree of shock both thermal and physical
- Cost to replace:
- Cost of part
- Labor cost
- Lost production
- Degree of difficulty
- Special corrosive agents such as chlorides
- Any other factors affecting wear.
No expert can provide meaningful advice without being informed of at least the above factors. Do your research before choosing your weapon. You don’t want to take a knife to a gun fight.