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.
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.
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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.
At first glance, hardfacing can be confusing and troublesome; in reality, it isn't. Understanding some of the basics about hardfacing can go a long way toward instilling confidence in your hardfacing product selection.
The following 19 answers to frequently asked questions may help you select hardfacing products that are most appropriate for your application.
1. What is hardfacing?
Metal parts often fail their intended use not because they fracture, but because they wear, which causes them to lose dimension and functionality. Hardfacing, also known as hardsurfacing, is the application of buildup or wear-resistant weld metals to a part's surface by means of welding or joining.
2. What base metals can be hardfaced?
Carbon and low-alloy steels with carbon contents of less than 1 percent can be hardfaced. High-carbon alloys may require a special buffer layer.
The following base metals can be hardfaced:
3. What is the most popular procedure used to apply hardfacing?
In order of popularity, the following procedures can be used:
FCAW and GMAW may be interchangeable or the same in terms of popularity. However, the trend is toward use of semiautomatic and automatic welding procedures.
ProcedureDeposition Rate (lbs./hr.)FCAW8 to 25GMAW5 to 12SMAW3 to 5SAW8 to 25GTAW3 to 5OFW5 to 104. With so many welding processes available, which ones are the most economical?
Many factors affect the economics of hardfacing, but a major one is the deposition rate. Figure 1shows the estimated deposition rate for each process.
Yes. Many different categories of wear exist&#;too many to cover in one article&#;but the most typical modes of wear are as follows (percentages are estimates of total wear):
Most worn parts don't fail from a single mode of wear, such as impact, but from a combination of modes, such as abrasion and impact. For example, a mining bucket tooth usually is subjected to abrasion and impact, and depending on what type of material is mined (soft or hard rock), one mode may be more dominant than another. This will dictate the welding product used.
Determining the wear mode can be challenging and may require trial and error when you select hardfacing products.
Yes. Iron-base alloys can be divided into three main categories:
It depends on the hardfacing alloy. Many chromium carbide alloys check-crack when cooled to moderate temperatures; this is normal. Others, such as the austenitic and martensitic families, don't crack when applied with proper welding procedures.
Check-cracking, or checking as it's sometimes called, occurs in the metal carbide families and can be seen as cracks that are perpendicular to the bead length (see Figure 2). They generally occur from 3/8 to 2 inches apart and are the result of high stresses induced by the contraction of weld metal as it cools.
The cracks propagate through the thickness of the weld bead and stop at the parent metal, as long as it's not brittle. In cases in which the parent metal is hard or brittle, you should select a buffer layer of a softer, tougher weld metal. The austenitic family is a good choice for a buffer deposit.
Generally, these are iron-base alloys that contain high amounts of chromium (greater than 18 percent) and carbon (greater than 3 percent). These elements form hard carbides (chromium carbides) that resist abrasion. The deposits frequently check-crack about every 1/2 in., which helps relieve stress from welding. Their low friction coefficient also makes them desirable in applications that require material with good slip.
Generally speaking, the abrasion resistance increases as the amount of carbon and chromium increases, although carbon has the most influence. Hardness values range from 40 HRC to 65 HRC. They also can contain other elements that can form other carbides or borides that help increase wear resistance in high-temperature applications. These alloys are limited to two or three layers.
Complex carbides generally are associated with the chromium carbide deposits that have additions of columbium, molybdenum, tungsten, or vanadium. The addition of these elements and carbon form their own carbides and/or combine with the present chromium carbides to increase the alloy's overall abrasion resistance. They can have all of these elements or just one or two. They are used for severe-abrasion or high-heat applications.
No, this isn't a good idea. A martensitic alloy and a chromium carbide alloy can have the same hardness, let's say 58 HRC, and perform vastly different under the same abrasive conditions. The metallurgical microstructure is a better measuring stick, but that isn't always available.
The only time hardness can be used to predict wear is when the alloys being evaluated are within the same family. For example, in the martensitic family, a 55 HRC alloy will have better abrasion resistance than a 35 HRC alloy. This may or may not be the case in either the austenitic or metal carbide families. Again, you have to consider the microstructure. You should consult with the manufacturer for recommendations.
It depends on the type of wear involved, but in the case of abrasive wear&#;by far the most predominant wear mechanism&#;the ASTM Intl. G65 Dry Sand Rubber Wheel Test is used extensively. This essentially is a test in which the sample is weighed before and after the test, and the result usually is expressed in grams of weight loss or volume loss.
A sample is held against a spinning rubber wheel with a known force for a number of revolutions. A specific type of sand, which is sized carefully, is trickled down between the sample and rubber wheel. This simulates pure abrasion, and the numbers are used as guidelines in material selection (seeFigure 3).
Low penetration and dilution are the major objectives in hardfacing, so pure argon and mixtures of argon with oxygen or carbon dioxide generally will produce the desired result. You also can use pure carbon dioxide, but you'll get more spatter than you would with an argon mixture.
Welding wires produce either a spray transfer or a globular (ball) transfer of molten metal across the welding arc. Spray transfer is a dispersion of fine molten metal drops and can be characterized as a smooth-sounding transfer. These wires are desirable in joining applications in which you require good penetration.
Ball transfer wires disperse larger molten metal drops, or balls. This type of transfer promotes low penetration and dilution, suitable for hardfacing. It has a noisier arc that produces an audible crackling sound and generally has a higher spatter level than spray transfer wires. Welding parameters such as electrical stickout, gas (if any), amperage, and voltage can affect the size of the ball and its transfer. Gasless, or open arc, wires all have a globular or ball transfer.
As a rule, you should bring all parts at least to room temperature. You can select higher preheat and interpass temperatures based on the base metal chemistry and hardfacing product you're using.
Manganese and some stainless steels and similar hardfacing products require no preheating, and welding temperatures should be kept as low as possible. Other steels usually require proper preheat and interpass temperatures. You should consult the manufacturer for the best combination to prevent cracking and spalling.
Cobalt alloys contain many types of carbides and are good for severe abrasion at high temperatures. They also have good corrosion resistance for some applications. Deposit hardness ranges from 25 HRC to 55 HRC. Work-hardening alloys also are available.
Nickel-base alloys can contain chromium borides that resist abrasion. They can be good particularly in corrosive atmospheres and high temperatures when abrasion is a problem.
Limited-layer products usually are in the metal carbide families, such as chromium carbide and tungsten carbide. You can apply martensitic and austenitic products in unlimited layers unless the manufacturer specifies otherwise.
The brittle nature of the metal carbides leads to check-cracking, and as multiple layers are applied, stress continues to build, concentrating at the root of the check cracks, until separation or spalling occurs between the parent metal or buffer and the hardfacing deposit.
These alloys often resemble the parent metal alloy and are applied to severely worn parts to bring them back to dimension or act as a buffer for subsequent layers of a more wear-resistant hardfacing deposit. If the hardfacing produces check cracks, then it's wise to use a tough manganese product as the buffer to blunt and stop the check cracks from penetrating into the base metal.
Yes, but you must take preheat and interpass temperatures into account. Nickel and nickel-iron products usually are suitable for rebuilding cast iron. These products aren't affected by the carbon content of the parent metal and remain ductile. Multiple layers are possible. If further wear protection is required, metal carbide products can work well on top of the nickel or nickel-iron buildup.
These frequently asked questions only begin to address hardfacing. Hardfacing product manufacturers and specialists can contribute to a greater in-depth understanding of hardfacing and help assist you in product and process selection for your application
Bob Miller is a materials and applications engineer at Postle Industries Inc., P.O. Box , Cleveland, OH , 216-265-, www.postle.com.
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