May 8, 2012
Equipment wears as it’s used, particularly in heavy applications such as extracting natural resources from the earth. Surface welding used to repair and strengthen equipment helps prevent costly downtime.
In my part of the country—the Appalachians—we constantly battle equipment wear problems. Our major industries are involved in extracting natural resources from the earth. The land is mountainous and heavily laden with limestone and other hard rock material. To get to the coal in surface mining, we must remove the overburden (usually limestone).
The industries in this region also are involved in several types of drilling—core drilling, oil and gas well drilling, and just plain water well drilling—with various types of bits.
The wear surface of all the equipment must be enhanced to the point that production can be achieved without excessive downtime. I have heard estimates of $500,000 a day when a large coal/rock shovel is down for repair. Some of these shovels have buckets big enough to hold a very large off-road rock/coal truck.
I recently heard that losing a drill bit in an oil or gas well can cost as much as $2 million a day. The tool must be retrieved before a new one can be installed, and this sometimes is very difficult. One company in our state, West Virginia, makes equipment for “fishing” various drilling tools out of a hole. The drill rod must be made secure at each threaded joint. The equipment used in the relatively new horizontal drilling method is more likely to wear than plain vertical drilling equipment.
The method for securing the joints in the drill rods is called hard banding (Figure 1), which often is performed in the field with portable equipment. Submerged arc (SAW) and flux-cored welding (FCAW) are the most common processes used for this purpose, although in an emergency, shielded metal arc welding (SMAW) can be used.
In some instances, too much hardness can be detrimental. If extreme impact is involved, material that is too hard will crack.
Many materials on the market today offer multiple ranges of impact and hardness. The most common identifier for hardness in the metal-to-earth industry is the Rockwell C scale. Other scales are involved in measuring hardness, such as, Brinell, Vickers, Moh, and subletters and numbers for each of these. A simple but not totally accurate method of comparing the Brinell numbers with the Rockwell is that the Rockwell C is roughly 10 percent of the Brinell; for example, 500 Brinell = 50 Rockwell C.
For extracting coal and other minerals from the earth, core drilling bits (for soil testing and determining the depth of the overburden) can be very hard because there isn’t much impact on the rotating drill. Ideally, hardness in the range of 54 to 58 Rockwell C is used. These are simple, one-piece, fluted bits that can be hardened by flame hardening, flame spraying, or plasma spraying or by simply overlaying them with an arc welding process (Figure 2).
Rotary bits are much more complicated. These bits have rotating cutters with small buttons of tungsten or chromium carbide inserted in each rotary cutter segment. Because of the inserts, hardsurfacing the cutters is not necessary, but some hard weld material often is placed on the outer edges of the bit’s main frame (Figure 3). This ensures that the pin holding the bearings that allow the cutters to rotate is not exposed and doesn’t fall out, which would cause the rotary cutters to self-destruct. These bits are also connected to a hollow shaft that allows fluid or air to be shot into the hole to clean the dirt or rock away from the cutting area.
The largest bucket in the world at one time (1960s) was located in our area. It was on the “Big Bertha Shovel” used in the surface mining field for Ohio Power, just across the Ohio River from West Virginia. The bucket is still displayed at a roadside park near the mining area.
A monster bucket like this can move several tons of dirt, rock, or coal with just one dip. Because of its size, several weeks are needed to rebuild a bucket. This means a spare must be on hand to work while repairs are being made.
Many components on a dragline shovel (Figure 4) wear excessively. A dragline shovel can strike a powerful blow and often is used improperly. It is intended to dig, not hammer. For this reason, the bucket cannot be hardened excessively. Its parts—bucket lip, sides, and teeth—would simply break apart if its hardness is great than about 56 Rockwell C.
Also, the parts that allow the bottom of the bucket to open and close receive quite a bit of punishment if the operator chooses to swing the bucket while it is still buried in the dirt, rock, or coal.
The large cable sheaves wear because rock dust is very abrasive. Rollers that the tracks run on usually are buried in the abrasive material. The grouser bars on the tracks must be welded up or replaced with “wing dings.” A wing ding, the quickest, least costly method for repairing the grouser bars, often is used by small companies. Large companies remove the tracks from the machine and sub-arc-surface them.
The reason for the cross-hatching pattern shown in Figure 4 is that it minimizes a straight wear path. It deflects the wear in several directions, thereby lessening the depth of the wear.
The hardsurface material on these items usually is applied by FCAW. If the weather conditions are unsuitable, SMAW can be used.
The metal spray process can be used on the rollers and shafts. It is an economical process, if fixturing is available to rotate the parts. The torch is equipped with a hopper for the powder and a large tip for heating and spraying (Figure 5).
If the part is very large, another heat source can be used, such as a multiflame heating torch. The temperature for spraying with the oxyacetylene process can be about 2,000 degrees F, depending on the type of powder and its matrix. Though it is not an operator-friendly process, extremely hard metal may be sprayed with this method as well as with the high-velocity plasma spray process.
When an attempt is made to conserve cost by placing a very hard hard-surfacing material on low-carbon plate, such as ASTM A36, it is advisable to prepare the much softer plate with what is known as buildup material (Figure 6). Several companies provide special electrodes for this purpose. In my experience, an electrode or wire such as E9018 or ER90S-6 also does the trick. If the mid-surface (buildup) material is not applied, the harder material will have a tendency to pull out of the substrate (the A36).
Several hard surfacing materials are designed to check crack, the tendency for the material to stress-relieve itself by cracking on the surface. This is not a problem unless it also cracks through to the substrate (the material being surfaced). If the overlayment is tightly adhered to the base material and there is no likelihood of chunks of hard weld metal falling into a machine, the check cracks can be advantageous in alleviating stress in all directions.
Some of the most severe wear problems are found inside power plants. A coal pulverizer undergoes extreme wear and moderate to extreme impact. This calls for hardsurface material that has some ductility, yet is hard enough to withstand the abrasion that is involved in pulverizing coal. These units rotate at a fairly high speed, and if a part breaks off, the whole unit must be shut down for an undesirable amount of time. This is another situation that involves millions of dollars if the shutdown lasts too long.
For these reasons, power companies and others that furnish the rebuilt pulverizer parts, must maintain a suitable stock of replacement parts. As one pulverizer is rebuilt and put into use, the worn one that is taken out has to be rebuilt for stock.
Nearly all the parts are similar except for size, so it is relatively easy to maintain and record the stocked parts. The parts must be built-up with the relatively soft material (40 to 42 HRC) in some areas and then machined back to the original size. Parts that encounter severe wear are built-up and left as welded. These rebuilds are repeated time after time. Some rebuilt pulverizers still in use originally were built in the 1940s, like the pulverizer yoke in Figure 7.
There is no doubt that hardsurface welding saves millions of dollars in various industries around the world. If the correct material is applied, and applied by the most efficient and highest-quality methods, the rebuild industry will flourish for many years.