Hammer time: several factors can influence the life expectancy of cast auto shredder parts.

Anyone who operates a shredding plant has experienced casting failure or shorter than expected life of a casting at some time.

Most often, when this situation is investigated, the manufacturer of the casting will report that the problem was not the fault of the casting itself. It is frustrating for the operator to hear this, but for much of the time, it is probably true.

Following is our attempt to list as many of the factors as possible that affect hammer, grate and liner casting life. We hope that this list can help operators identify the cause or causes of the failure or changes in wear life that they may experience.

CHEMISTRY CONCERNS. Knowing the chemical specifications and the heat treatment of castings is critical to trying to understand casting life. When a casting is sent to us for analysis, we check the chemical analysis with our spectrograph and carbon determinator. Examining the micro-structure of the casting through a high-power microscope checks the effectiveness of heat treatment.

To safeguard our own castings from having the wrong chemistry, we charge the furnaces with known materials of known chemistry. During the melting cycle of each heat, a thorough analysis is made before the molten material is poured from the furnace, allowing corrections to be made as required. Another analysis, known as the final analysis, is made when the material goes from the furnace to a ladle.

The heat treatment cycle is governed by programmable digital controllers, eliminating some of the human error element. We heat treat castings of various thicknesses according to proven schedules of heat soaking at various temperatures.

Transferring castings from the furnace to an agitated water quenching tank must take place in about 45 to 660 seconds. When this is done properly, the austenitic grain structure is fixed, the casting becomes tough and is then capable of being work hardened.

Even though such an occurrence is rare, an error is possible. On a random basis, we select a casting from the finished product, cut it apart and subject it to a rigorous examination of its microstructure.

MATERIAL ISSUES. If there is a problem with a complete set of hammers, grates or liner plates, rather than being a specification problem, the most likely answer can be found in one of the other two variables--the type of material being shredded or the mechanical questions of shape and size of all the parts involved.

The material being processed is the biggest variable, with the most dramatic event being the inclusion of a massive unshreddable item in the material fed into the shredder. This can cause breakage and shorter life to any of the castings in the machine. It should be noted that damage caused by an unshreddable does not always show up immediately--sometimes it appears several days after the event.

In the past few years, the types of material being shredded have chanted significantly. While automobiles and appliances are still a significant part of the stream, a full range of heavier scrap is being added. This heavier scrap contains a higher percentage of unshreddables and requires tougher shredding mechanics and tougher and better designed castings.

Another variable concerns the instances when scrap and dirt accumulation from the bottom of a stockpile is processed. When this happens, it can affect the life span of one set of hammers, although it probably does not explain a long-term variation.

When long-term trends point to shorter casting life spans, a significant increase in the amount of material such is upholstery, dirt and rubber making up the stream could be to blame. For example, if die weight loss from unprocessed to marketable shredded material goes from 20 percent to 33 percent, a significant amount of extra material has been shredded, changing the equation when measuring casting life against the number of ferrous tons produced.

THE DENSITY TEST. Almost everything else that affects casting wear life is related to the density of the finished shredded steel scrap product.

Density is dependent on the type of material being processed and the number of hits the scrap takes, both from hammers and from the other castings in the shredder. It should be noted that not all hits are of the same efficiency, with some hits causing more wear than others.

Ultimately, efficiency can be measured in how many kilowatt hours (KWH) of electrical consumption is used to shred one ton. Some of the factors affecting density and the efficiency with which the density is obtained (and thus affecting casting wear), include:

* Internal clearance between the hammers and the wearing castings. The standard minimal internal clearance in most steel shredders is 1 inch in top discharge shredders, 1.5 inches in top and bottom shredders, and 2 inches in super heavy duty shredders.

* Grate sizes. The size of the grate openings and the percentage of open spaces in the grate compared to its total area affects the efficiency of the shredding operation.

* Reject door. The operation of the reject door and the amount of open space averaged can raise or lower density and casting consumption.

* Hammer design. We believe the bell-shaped hammer has more casting weight in the hitting area. Consequently, the ratio of the amount of casting purchased to the amount of casting discarded is improved, resulting in a lower cost per ton for casting usage and replacement.

* Rotor rpm. During the last few years, we have found that in general, a slower rpm yields a longer casting life. There is a trade-off, however, in productivity. Higher rpms create more striking force and better shredding characteristics.

* Length of usage of castings. Some operators will use castings longer than other operators based on the operator's perception of how well the shredder is running. Some operators value long casting life more than tons-per-hour output. In normal situations, we believe the hammers can be worn so they have less than one-half their original weight remaining before being discarded.

Deciding on how long to use a set of hammers entails considering many factors. Running automobiles under normal conditions, hammer life varies according to the size of the shredder and the size of the hammers.

In disc-type battle rotors with 10-hammer arrangements, 80/104 SHD hammers should last 1,200 to 1,500 tons per side, making for a total life per set of 2,400 to 3,000 tons. In the 98/104 SHD, hammers will last about 2,400 to 3,000 tons per side, for a total life per set of around 5,000 to 6,000 tons. And in the 120/104 super-sized shredder, hammers will last about 5,000 tons per side for a total life span of around 10,000 tons.

The set of hammers should be changed when they have worn to the point that production falls or when the hammers are worn unevenly and the rotor becomes unbalanced. Some operators have found that if some of the hammers are changed every day, that the balance of the turning rotor and the hourly production of the shredder (as well as the density of the product) is more consistent.

Not all hammer designs will allow for a 50 percent usage. Some super heavy-duty hammers, where there is a large amount of casting around the pin shaft to provide enough strength in the hammer, will not achieve 50 percent "throw away" weight.

Hammers are designed so that the maximum amount of material can be worn away before the hammer must be removed from the shredder. The main constraint on lower "throw away" weight is the amount of casting that is required around the pin hole to keep the hammer in the shredder as long as possible. When shredding heavy scrap and when shredding bales, we have decided that a good compromise is to increase the amount of metal around the pin hole, even though this worsens the "throw away" ratio.

Some hammers may have to be replaced on a random, as needed basis because of irregular wear, cracking or for other reasons. Such a condition or situation may dictate that the opposite hammer also must be replaced to maintain the rotor's proper balance. Replacing worn hammers as needed, and the replacement of all hammers as scheduled for rotation, will generally help maintain the rotor and shredder operation at their peak efficiency.

ANALYZE THIS. If the answer to a casting failure has not been found after checking mechanical and material feed variables, then it can probably be found in the analysis of the casting itself.

The chemistry of the manganese alloy used in most shredder applications is a variation of Hadfield manganese steel alloy, which was invented in the 1800s by Sir Robert Hadfield of Sheffield, England. This type of manganese alloy has proven to be a wonderful steel for impact and abrasion types of applications, with characteristics that provide for maximum forgiveness for the user of the casting.

In general, this is a steel alloy with 12 percent to 14 percent manganese added. The carbon content is usually within a range of 1 percent to 1.2 percent, and various other carbide-producing elements, such as vanadium, chrome, nickel and molybdenum, are added in small amounts, according to the recipe of the specific foundry making the casting.

As noted earlier, the foundry will take samples to analyze the chemistry at various points of the production process. But an important addition at some foundries, including our own, is the addition of powerful microscopes to the available laboratory equipment. Through proper microscopic analysis, the entire history of a particular casting can be learned. In combination with the chemical analysis, it is possible to determine if the heat treatment procedure was carried out properly and whether the casting has been work-hardened.

The most important thing that is checked with a micrgraph test is the position of the carbides in the alloy. Ideally, the carbides will be incorporated into the grains of the casting with only small amounts of those carbides remaining in the grain boundaries. When excessive carbides are in the grain boundary, the casting will not have as much strength and is much more likely to break.

Even by studying shredder mechanics, in-feed materials and micrograph analyses, it is possible that no explanation can be found for casting failure, and that the failure is the result of an accident or random occurrence, such as an inclusion. This seems to be the situation in about one casting out of every 10,000 that are made. Normally, this kind of failure rate would be impressive, but our goal is to have zero unexplained casting questions.

Changes to composition or of heat treatment procedures are done very slowly and with great care, so that only one change is made at a time. Through our fortunate connection with so many good shredding operations, it is possible for us to monitor the progress of a specific experiment.

CASTING CALL

Making a sound decision as to when to change hammers and other castings involves a variety of factors, many of which are variable depending on the material being shredded and the shredding plant's settings.

In addition to the size of the shredder and the hammers, following are some additional items to consider when setting up a schedule for casting change procedures.

* Liners should be replaced when they have worn so thin as to expose the bolt head or are in danger of breaking.

* The anvil is a replaceable item that should be changed when the cutting edge wears far enough away from the hammers to significantly change production rates and the wear begins to affect the density of the finished product.

* The reject door is subject to wear across its bottom edge, resulting in a bowed shape. It must be built up or replaced when it has worn to the point that its effectiveness is reduced, such as when a door has worn off 4 inches to 8 inches or is bowed beyond its ability to function properly.

* Pin protectors will need to be replaced when their teeth have worn off and they no longer afford protection to the disk, when they have worn enough to be in danger of breaking or when they do not properly protect the pin they surround.

* When "saddles" have worn into hammer pins where the hammers have swung, the pin may be switched to a location where the hammers fall on an unworn portion of the pin. This will let the pins be used for a longer period of time. Hammer pins should be replaced when they have had a hammer run in each available location.

* The grates can be replaced as necessary. This means that the grates should be checked regularly to ascertain when one or more have worn to the point that there is danger of breakage. Typically, grates used with grate supports will need to be replaced when 40 percent to 50 percent of their original mass has worn away. The appearance of large cracks in the grates provides an obvious indication of the need for a grate change. Grate wear that causes excessive gaps between the hammer and grate surface will harm shredder efficiency, affecting both product density and production rates.

The authors are officers of The Shredder Company LLC Canutillo, Texas, a provider of shredding plants and components. President Scott Newell Jr. can be contacted at [e-mail].