Hardfacing of Conventional and Hollow-stem Augers -- Part 1
Other than critical maintenance on the rig itself, the most important cost-saving maintenance procedures for users of conventional flighted or hollow-stem augers are maintenance of the cutterhead and hardfacing on the auger flights.
Cutterhead maintenance can involve sharpening or replacing cutter teeth (as well as buttons and/or bullets on some heads). This article does not address those procedures.
Wear DeterminationA homemade ring gauge can help determine when hardfacing is required on the edges of the flights. The inside diameter of the gauge should be approximately equal to the original outside diameter of the new auger flights before hardfacing. This is not precision work, so gauges can be made with a fly cutter, from light sheet metal, heavy tin or any reasonably stiff material. The experienced operator usually can determine, by eye, when hardfacing on the flights should be renewed. Nevertheless, the use of a gauge is encouraged, particularly with personnel that lack experience. This basic ring gauge can be used to determine when hardfacing should be done and also as a try-gauge for overly worn flights that have to be built up prior to hardfacing. Also, it may also be useful to have a try-gauge that is 1/4-inch to 3/8-inch larger (on the diameter) than the basic ring gauge for use during the hard facing operation. To some extent, this is dependent on the thickness of the deposition of the electrodes used.
If you have new or slightly used augers, it is not hard to determine the proper size for the basic ring gauge. If the augers have been used substantially, you will have to verify the original major diameter of the unhardfaced flights, and make your ring gauge based on that diameter. With worn augers, determination of the original diameter can be more of a problem that it seems. Looking only at the nominal size of 6 inches, Acker makes a 51/2-inch and a 57/8-inch, and Foremost- Mobile makes a 6-inch, while others make other sizes just for variety. If there is any doubt, find out the original diameter of the flights from the manufacturer.
Some electrodes require that all previous hardfacing be removed before a new application is made. Read the product information for your selected hardfacing electrode carefully and completely understand this requirement, if it exists, as it can change the sizing of the ring gauge. Or, it may be better to consider a different electrode.
Base Metal CharacteristicsMost auger flights are manufactured from mild steel. For example, the flights on a typical auger from Foremost-Mobil are made of hot-rolled, flat, AISI 1020 steel with a thickness of about 1/4-inch. AISI 1020 is a general purpose, low-carbon (0.17 - 0.24%) mild steel. It is easily machinable and is weldable by any welding process. Some auger manufacturers use A-36 mild steel with a carbon content of about 0.29 percent or the very similar 44W mild steel with a carbon content of 0.22 percent to 0.23 percent. Most hardfacing electrodes will properly fuse and bond to these steels. The procedures reviewed below should result in success for most mild steels with carbon content of less than 0.5 percent. Knowing the composition of the auger's base metal is of critical importance in deciding which electrode to use.
Wear FactorsReduction of the diameter of auger flights can be attributed to more than one type of wear. Generally speaking, most wear on the flight edges (usually in excess of 60%) can be attributed to two types of abrasion -- high-stress grinding abrasion and gouging abrasion. Depending on the formations encountered, impact wear also can be a factor, although far less on the flights than on the cutter head. High temperature also can contribute to flight wear if the cutter head is worn to the point that it is cutting an under-gauge hole. Although not a major factor, the entire auger also is subjected to low-stress scratching abrasion.
Areas to be covered by the hardfacing material are, in order of importance: the edge of flight and the first half-inch of the upper face of the flight. The area to be hardfaced will vary considerably depending on hardness and material of typical formations in your area. Due to some of the difficulties encountered while hardfacing the upper face, many are content with doing only the edge of the flight. Others do the edge of the flight with an occasional touchup on the face of the flight. Some do the entire upper surface of the leading flight of the lead auger. The most important thing will be actual observation of primary wear points in your typical formations with your specific augers.
Electrode SelectionElectrode selection is extremely important. It is directly dependent on the base metal and the specific wear factors involved for the part being hardfaced. Everything here should be considered as general guidelines. In all cases, you must read the product information for the electrode selected.
If the amount of wear exceeds the amount that can be covered with two layers, a special buildup electrode (with malleable deposition) should be considered. Depending on the hardfacing electrode selected, two layers of hardfacing usually are adequate -- and usually are the maximum recommended. I am not familiar with any hardfacing electrode that the application of more than four layers is advised.
High-carbon electrodes provide good protection against both abrasion and impact. Tungsten-carbide electrodes provide excellent protection against abrasive wear, but they lack resistance to impact and cost more than high-carbon electrodes. Chromium-carbide electrodes share approximately the same characteristics as tungsten-carbide electrodes but are slightly less abrasion resistant and resist impact slightly better. Depending on cost and specific characteristics, any of these probably are adequate for auger flights. Stainless steel electrodes provide the best resistance to impact, but have very little resistance to abrasion and should not be considered for this application.
It is important to bear in mind the degree of competence of the welder. In some cases, it may pay to select an electrode with inferior wear qualities to gain ease and correctness of application. As with most things, tradeoffs are involved. Also, consider your other hardfacing requirements to see if one electrode can be selected that will be adequate for all of your hardfacing requirements. Additionally, if you value your time, you will put ease of application ahead of electrode cost when you make your selection.
PreparationRemove all dirt, rust, paint, grease, oil or other contaminants before welding. Otherwise, it is an invitation to porosity and possible spalling. When using low-dilution, low-penetration hardfacing techniques, great care must be paid to cleanliness of the weld area to ensure proper fusion and bonding.
Prepare a sound foundation by removing fatigued and/or rolled-over metal. It should be removed by grinding and then built up to the proper profile, with a suitable buildup electrode if the profile was substantially altered during the grinding process.
Prior to hardfacing, repair any cracks or damage and buildup any edges that are more than 1/8-inch (on the diameter) smaller than the basic ring gauge diameter. If buildup or repair is necessary, the buildup electrode must be compatible with both the hardfacing electrode and the base metal. Be sure you know exactly what the auger's base metal is so that you can select a suitable buildup electrode.
With most suitable electrodes, preheating is not necessary for augers fabricated of mild steel. If the carbon content of the base metal exceeds 0.30 percent, then preheating of the auger might be considered. The higher the carbon content, the higher the required preheat temperature. This is true to a lesser degree for the total content of other alloys.
Application of the Surfacing ElectrodeThe recommendations that follow are a general guideline and may deviate from instructions found elsewhere. In all cases, know exactly what the auger material is and read the instructions from the supplier of the selected hardfacing electrode.
- Position the auger horizontally at a comfortable height to enable welding to be done with a downhand motion. If downhand welding is not practical, be sure the electrode selected is recommended for all-position welding.
- With most electrodes, to achieve maximum wear resistance, apply two layers of hardfacing. The first layer will produce an admixture with the base metal, and the second layer will produce a true wear-resistant surface. Depending on the hardfacing electrode selected, three layers are sometimes acceptable, but four layers are -- almost always -- too many. A deposit of hardfacing alloy that is too thick can give more problems than no deposit. Some of the harder alloys are too brittle for multiple layer buildup.
- Use a medium-long arc with either a straight pass or a weave. The arc should be somewhat longer than the diameter of the electrode. There must be no sticking to the puddle as this indicates a short arc. A hint of splattering should be all right. In many cases, a weave is preferable because, at low amperage, it causes less dilution of the deposit. With most electrodes, the weave deposits a flatter bead that allows subsequent layers to be more evenly deposited. Do not run any pass longer than six inches, or one-quarter of the flight's circumference, whichever is less. After the first short pass on the first flight (without rotating the auger), do a similar pass on the next flight, and so on, until each flight has one short pass on it. To help force checking, chip each short pass, while still hot. After all flights have one short pass, rotate the auger approximately 90 degrees and do the second short pass on each flight in the same manner. Make sure all subsequent passes are tied together properly. Repeat this until each flight has one full layer all around, and then start over again with the second layer. Thorough chipping and wire-brushing of each and every short pass is very important to minimize slag inclusion problems.
- Weld as cold (low heat) as possible to prevent dilution of the hardface deposit by the base metal. In other words, use just enough amperage to maintain an arc. Check the electrode manufacturer's instructions. There are many hardfacing electrodes that will leave an adequate deposit with settings as low as 80 amps to 120 amps. Higher amperage equals higher deposition; lower amperage equals lower dilution of weld metal.