This is the first of a two-parter in which ESAB&#;s Mick Andrews takes Andrew Pearce through the finer points of hardfacing, piercing and gouging. Sounds painful&#;
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Hardfacing is something of a mystery. It&#;s like driving round the farm on a foggy morning &#; all the familiar landmarks are there but somehow different, and you&#;re not necessarily sure that you&#;re looking at a profitable operation.
Still, this much is clear. Hardfacing is the business of laying down abrasion &#; and (maybe) impact-resistant metal on new or worn mechanical or soil-engaging parts, with the aim of extending service life.
The thick rod coat contains metal powder, so hardfacing consumables lay down more material than core wie diameter alone suggests. That is, the recovery rate is more than 100%.
Next time: Weave patterns for hardfacing and a look at gouging and piercing electrodes &#; handy things to take to the field.
More welding advice
The speed and ease of MIG/MAG welding suggests that&#;s the way to go. But hardfacing with MIG means using cored wires, and to get the required burn-off and fusion with these needs more current than single-phase sets can deliver.
So for most farms, the weapon of necessity is a manual metal arc (MMA) stick set. Depending on the rods you choose, even that may not be up to scratch, as Get set explains.
Matching electrode and job takes us back into the fog. Most repair and fabrication involves mild or low-alloy steels, so you can jog along quite happily with general-purpose rods; their forgiving composition handles surface contamination and minor variations in metal make-up.
But wear-resistant parts are made of sterner stuff. In the workshop it&#;s impossible to fathom their composition, so the repairer can&#;t properly match electrode and material in the way that industry can.
The wrong rod and/or technique can see expensive hard covering flaking off, and perhaps even the whole part cracking through &#; although with a little care neither should happen. To make sure it doesn&#;t, here&#;s the low-down on electrode selection and care, plant setting, pre- and post-heat treatment and welding technique.
First though, please take two cautions to heart &#; see Alerts.
Rod selection and care
Hardfacing rods can be formed as a tube packed with flux and powdered metal, or as a conventional coated stick. ESAB offers the latter.
Four ESAB electrodes cover farming&#;s needs:
1. OK 83.28 is a low-alloy rod designed for building up parts in metal-to-metal contact: gears, sprockets, clog clutches and so on. Hardness is 30 HRC, machinability is good. Min OCV 70V, AC or DC+ operation.
2. OK 83.50 is a low alloy, high carbon electrode for repair and of worn parts. Runs from small sets with low open circuit voltage. Resistant to abrasion and impact. Hardness is 50-60 I-IRC; can only be ground, not machined. Min OCV 45V, AC or DC+ operation
3. OK 84.78 contains chromium carbides and is for surfacing only. Ideal for very abrasive soils or corrosive conditions. High recovery &#; puts down a lot of metal for a given rod diameter. Hardness 59-63 HRC so can only be ground. Min OCV 50, AC or DC+ operation.
4. OK 84.84 is a very expensive, complex carbide-containing rod for surfacing only, suited to extremely abrasive soils where pressure on the part is high. Good for edges, unusual in giving high hardness for a single layer.
Hardness is 62 HRC, can only be ground. Min OCV 45V, AC or DC+ operation. Keep rod vertical to work while welding. Notes: Equivalent rods are available in other brands. HRC is a comparative hardness scale.
The electrodes above come vacuum-packed to keep them dry. Once opened and stored, all four types should be oven-baked before use, or water in the coating is likely to produce hydrogen, cracking the weld. RE-drying instructions are on the packet, along with info on welding current range, OCV requirement and so on.
ESAB&#;s hardfacing consumables come vacuum-packed. Plant setting and re-drying requirements are on the pack.
The harder the deposit, the less flexible and more brittle it is. If very hard material is laid directly over substantially softer metal, the top layer may crack on cooling and flake or spall off altogether, and the expensive hard metal is diluted by mixing with the softer stuff.
One way round cracking and spalling is to use a buffer or buttering layer between soft and hard metal, like the jam in a sandwich. ESAB&#;s OK 67.45 lays down the required tough, stress-absorbing buffer, while dilution can be kept down by welding relatively quickly, and by using minimum current and minimum weave to restrict melt and mix.
How do you know when to butter? This is formally defined according to parent material and impact level in service, but lacking accurate information on the former have to suck it and see.
The simple way is to hardface a few trial parts without buttering, and if the top coat stays put in work you&#;ll have the answer.
The more complicated &#; but much better -approach is to badger the maker of the part to be surfaced for info on its composition, than to ask an corrode supplier about the best technique and rods for the job.
Importantly and irrespective of buttering, at least two hardfacing layers are usually needed for the top one to show maximum hardness.
While not all hardfacing rods require it, pre-heating and slow cooling of the work make good insurance against cracking. Usually pre-heat is to a specific temperature, but in the rough-and-ready workshop it&#;s enough to warm the part evenly and thoroughly with a gas torch.
The aim is just to drive off any water, so don&#;t overdo it. After welding let the work cool slowly in air. Quenching is a sure way to produce cracks or worse.
ESAB&#;s hardfacing consumables come vacuum-packed. Plant setting and re-drying requirements are on the pack.
Nothing out of the ordinary here in rod angles, speed of travel, arc length and plant setting &#; just keep within the requirements laid out on the packet and weld as you would with a general-purpose rod.
Expect a fluffy arc punctuated by quiet spittings (much like cast iron or special-steel rods) with a lacy slag covering to follow. Watch your eyes with this, as bits can ping off spontaneously as the weld cools.
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Hardfacing electrodes are designed to give a quick-freezing deposit which helps when working along edges, but paradoxically the weld pool tends to be wide.
This makes working away from the flat rather tricky; use the shortest arc, lowest current and fastest travel consistent with good fusion to control the pool. Overhead work is best avoided or left to owners of a thoroughly fireproof hat.
Use conventional arc length, travel speed and rod angles when handfacing. For overlapping passes, angles are roughly 60° from vertical, 45° from parent material. Weave patterns will be covered next time.
A VOLTAGE &#; the open circuit voltage or OCV &#; must exist between the electrode and work before the arc can strike. Simple stick units have one output terminal and generally offer 50V OCV. Others provide a second tine at up to 90V.
Stick welding electrodes always have a specific OCV requirement. General-purpose rods are usually happy strike and run at 50V, but specialist rods &#; including some hardfacing varieties &#; need higher voltage to strike cleanly and run with a stable arc.
So before buying consumables, ask the supplier for their OCV requirement and check that your set can deliver the goods. If it can&#;t the results will be poor.
DONT hardface I2-14% manganese steel. Often used for bucket teeth and other bits which have to put up with high impact, this material is very tough, but quickly turns brittle when re-heated and slow-cooled &#; so the heat from the hardfacing is very likely to crack to it. Structural steels with lower manganese content are fine.
High manganese steels are typically used in crushing and milling applications and, crucially, aren&#;t magnetic. Otherwise they&#;re light grey in colour and are usually castings, designed to be bolted on to avoid welding.
Sometimes they come with a warning label, and may have the letters &#;MN&#; cast into them.
Now to fume. Hardfacing electrodes carry a lot of metallic elements in their flux coating &#; which is how even a small diameter rod manages to lay down a very substantial bead.
Consequently hardfacing fume contains much human-damaging heavy metal vapour, which won&#;t do you any good at all.
Ideally, use forced fume extraction. If that&#;s not possible, wear a specific welding respirator marked EN149: (3M&#;s disposable mask is an example). Work in moving air and stay out of the main fume column.
Hardfacing releases more hazardous metal vapour than general repair work. If fume extraction is not possible, wear a purpose-designed welding respirator and stay out of the main fume column. Nuisance dust respirators will not protect.
Whether you hardface old equipment or new, when completed properly the result is the same: less downtime for replacing worn or broken components, fewer spare parts to inventory and longer equipment life. In short, hardfacing is a fast, easy and efficient way to make your equipment more wear resistant and keep it in the field longer-often for less money.
During the hardfacing process, a filler metal (sometimes called an alloy) is bonded to the equipment&#;s base metal in order to obtain specific wear properties and/or dimensions. Specifically, these filler metals provide abrasion and/or impact resistance. On older equipment, hardfacing can return worn parts to a nearly new condition for about 25 to 75 percent less than the cost of replacement parts. Hardfacing can also lengthen the life of surfaced parts by up to 300 percent more than non-surfaced parts, especially on newer equipment.
To obtain the best results from your hardfacing application, consider a few basic guidelines.
Technique and Process Requirements
First, determine your hardfacing needs: build-up, overlay or a combination of the two. The build-up technique (placing layers of welds on top of each other) returns older equipment back to its original dimensions after it has been worn by impact and/or abrasion. Overlay is the addition of a weld layer that protects the equipment against metal loss. A combination of build-up/overlay can also extend equipment life and may be used repeatedly provided that the part or equipment remains sound.
The size, shape and location of the equipment or parts that require hardfacing will determine which welding process should be used, as will your skill set and the availability of specific equipment. Typically, hardfacing uses either stick (SMAW) or flux-cored (FCAW) welding processes, but some companies that weld large, thick components on a regular basis may choose the submerged arc welding (SAW) process.
Each welding process has advantages and disadvantages. For example, stick welding is a highly portable, making it ideal for hardfacing in the field. There are also many types of stick electrodes available, each of which can be used in all welding positions and are able to weld on relatively thick materials. Stick electrodes, however, have a low efficiency (due to stub loss), a relatively low deposition rate (approximately 1 to 7 lbs/hr) and may require several weld layers to obtain maximum wear properties. On the other hand, hardfacing with flux-cored wire offers better deposition rates (approximately 4 to 25 lbs/hour) and the process is easy to use-FCAW often requires minimal training to become adept. Unlike stick welding, flux-cored welding is limited to flat and horizontal positions.
Base Material Considerations
Consider your equipment&#;s base material. Carbon or low alloy steels are probably the most commonly hardfaced materials. As a word of caution, those material containing higher amounts of carbon and/or alloy content tend to be more brittle and may require pre- or post-heat, or stress relieving to prevent cracking. Thicker base materials require similar heating considerations, as well.
Austenitic manganese steels can also be hardfaced, and these too can become brittle during the welding process. Unlike carbon or low alloy steels, austenitic manganese steels should not be pre-heated unless the temperature of the part is less than 50 degrees Fahrenheit. During the hardfacing process, the base metal temperature should remain under 500 degrees Fahrenheit, as exceeding this temperature barrier for an extended period of time increases the steel&#;s brittleness. Austenitic manganese steels with higher carbon and lower manganese content accelerate this time/temperature reaction.
Regardless of the base material you plan to hardface, remember to pre-clean the part prior to welding. First, wipe it free of all contaminants, including grease, dirt, rust and oil. Then, if necessary, remove old hard surfacing layers, as well as cracks, via arc (or plasma) gouging or grinding.
Equipment Wear Factors
Consider the type of wear your equipment encounters, as this will be a significant factor in determining the best filler metal to use. Abrasion accounts for roughly 55 to 60 percent of equipment wear, and there are three main types: low-stress scratching, high-stress grinding and gouging. Impact and adhesive wear (also called metal-to-metal wear) are also common. Secondary types of wear include high-temperature and corrosive wear.
The least severe form of abrasive wear, called low-stress scratching, results when the metal slowly wears away from the scouring action of materials across the equipment. Hardfacing with carbide or chrome-carbide filler metals best protects against this type of wear, and often filler metal formulations are available to provide stress-relieving cracks that prevent spalling.
For high-stress grinding abrasion, caused by repeated crushing and grinding of materials against the equipment, the best filler metals are those containing austenitic manganese, martensitic irons or titanium carbides.
Filler metals containing high carbide alloys and supported by austenitic manganese are the best choice when encountering gouging abrasion, as these filler metals provide good impact resistance. Gouging abrasion occurs when large objects, such as rock, press against the equipment and create grooves.
Impact wear often occurs on equipment like crusher rolls, impact hammers and impactor bars, and results from a compressive load placing high mechanical stress on the equipment. The best protection against this type of wear is to use an austenitic manganese steel (11 to 20 percent Mn) filler metal, as it offers good work hardening characteristics.
To protect against adhesive or metal-to-metal wear, which occurs from the non-lubricated friction of metal parts against one another, use a martensitic hard surfacing alloy. Austenitic manganese or cobalt-based alloys also work but they may be too soft to resist adhesive wear for as long of a period of time.
When equipment repeatedly encounters high temperatures and rapidly cools afterward, it can result in high-temperature wear, also called thermal fatigue or fire cracking, which leaves deep cracks in the equipment&#;s base material. This type of wear is usually secondary, or in addition to the abrasion or impact wear equipment encounters. Generally, a non-ferrous alloy is best for protecting steel surfaces subject to temperatures above degrees Fahrenheit. For those below this range, a filler metal containing chromium-carbide or a martensitic steel filler metal with 5 to 12 percent chromium is suitable.
Corrosive wear is also a secondary type of wear that should be dealt with separately. Most filler metals provide some rust protection, but its best to consult your equipment manufacturer or a trusted welding supply distributor for recommendations.
Desired Surface Finish
Finally, determine the type of surface finish your equipment requires. Since hardfacing filler metals range from easy to difficult to grind, determine your required finish prior to choosing one. If a smooth surface is necessary, measure the time and cost of grinding to achieve this surface versus using a filler metal that has slightly less wear resistance, but provides a smooth finish. A filler metal that can be heat-treated to soften it for machining, and then brought back to the hardness necessary to protect your equipment may also be an option. Or, if relief checks (small checks which do not weaken wear resistance) are an acceptable surface finish, consider a carbide alloy designed to be crack sensitive.
Final Thoughts
If your equipment encounters repeated impact, abrasion or both, hardfacing can offer valuable time and cost savings. If you are ever in doubt about the process, consult a reputable filler metal manufacturer or trusted welding supply distributor for assistance. Most of all don&#;t get discouraged. It may take some time to get the hardfacing process and filler metal selection right, but in the end you&#;ll have stronger, longer lasting equipment and more time in the field.
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