Metallurgy and Welding 101: The Basics for Doing Hardfacing and Other Welding for Construction-Equipment Repair – Part 3

April 5, 2015

Continuation of an interviewe with Thomas J. Black, manager of hardfacing and high-alloy products for The Lincoln Electric Company in Cleveland, OH

What is hardfacing?

Black: Hardfacing is the application of a wear-resistant layer onto a surface that is to be exposed to wear in order to extend the life of that surface. It is the process of depositing that wear-resistant layer or pattern of ridges by one of various welding techniques.

In hardfacing, say, the inside or outside surfaces of a loader or excavator bucket, why is it not common practice to coat the entire surface with the weld material? Why is it that welders generally create some sort of grid pattern composed of 0.125-inch-high “ridges”?

Black: The approach you suggest, laying down a complete coating of hardfacing material over the bucket surface, would be the most effective surface protector. But that is rarely done because of cost and time required. Rather than laying down a continuous coating, it is much more economical of both hardfacing material and of a welder’s time to apply a grid of elevated ridges or other geometrical pattern. Using a grid pattern rather than a continuous layer also reduces weight. A typical hardfacing grid line that a welder would lay down would be two beads (0.25 in.) wide and one bead (0.125 in.) high.

If working in a softer material, such as a loamy soil, the aim should be to lay down a grid pattern of hardfacing ridges that will tend to trap the soil on the steel surface, forming a layer of clinging earth that will protect the steel surface from further abrasion. This is best done by laying down a series of parallel ridges, perhaps 2 in. or less apart, that are 90º to each other.

In other cases, say when operating in more rocky soils, the aim is not to trap soil on the surface – the rockiness of the soil would make that difficult – but to protect the underlying steel from abrasion caused by the movement of the rocky soil directly over the steel surface. This can be done by laying down a pattern of hardfacing ridges in the direction of flow, like rails, or even a pattern of hardfacing “dots” – anything that will prevent the rocky soil from coming into direct contact with the steel surface; the soil in effect slides over the tops of the ridges without coming into direct contact with the steel surface.

Many construction-equipment welders seem to use a welding rod called E7018 for a wide range of welding applications. Is E7018 a sort of cure-all for a wide range of welding tasks?

Black: Definitely not! Welding rods of the E7018 class are not a cure-all for all sorts of welding repair applications. Many welders are using E7018 not because it is the appropriate alloy for a particular task but because it is cheap. And that is a serious mistake because it means that the welding workmanship will not last very long. Consequently, the construction equipment will suffer further downtime, which can be very costly.

E7018 is a welding alloy suitable for joining certain metals together; for example, welding a brace onto an excavator boom. But welders are misusing this material because it is cheap. Often they are using it for the buildup material that serves as the base for the hardfacing alloy that is subsequently laid on top of it. And sometimes they are using the E7018 for the hardfacing material itself.

Such weldings won’t last and won’t have anywhere near the durability that they should have. This is because E7018 is a relatively soft material, unsuitable for applications calling for high abrasion resistance and high resistance to compressive loadings.

In doing the hardfacing of an excavator bucket, a dozer blade, or a cable pulley, many welders often select both the wrong buildup material and the wrong hardfacing material. Is that your contention?

Black: Yes! E7018 should not be used in hardfacing applications as a buildup material since it has a tensile strength of only 70,000 psi and a yield strength of only 58,000 psi, whereas the softest legitimate buildup alloys have tensile strengths of 100,000 psi and, more typically, 150,000 psi and a yield strength of 98,000 psi.

In sum, at least half of the welders in the construction industry are incorrectly doing the hardfacing of loader and excavator buckets, bulldozer blades, pulleys, and other construction-equipment components. They are using the E7018-type welding rod or wire to construct the buildup layer, when that material is too soft and too weak for the application.

And even if they do select an appropriate buildup material, they often select a hardfacing material that is not suitable. Sometimes they are using E7018 for the hardfacing layer, which is far too soft and weak. This is because they fail to consider what the specific wear environment is for that construction-equipment component and pick the product by hardness.

Please provide some guidance as to how welders should go about selecting the appropriate rod or wire for hardfacing various construction-equipment components.

Black: It is most important that the maintenance repairer ask himself, “What is the wear environment for the particular construction-equipment component I am about to hardface?” Is it metal-to-metal wear; for example, a cable winding over a pulley or onto a metal drum? Metal-to-metal wear plus impact? Severe impact, such as a crushing hammer? Metal-to-earth abrasion plus impact, such as dump-truck body surfaces, an excavator bucket and bucket teeth, a dozer blade? Or severe metal-to-earth abrasion plus impact, such as an excavator bucket and bucket teeth, a dozer blade, or a scraper used in especially abrasive soils, one requiring a tungsten carbide hardfacing material?

What particular buildup material to use for what application? In some cases, the same material can be used for both buildup and hardfacing. In most cases, a buildup material with a Rockwell hardness of 20-35 Rc is needed. And that will also be the minimum hardness range needed for any hardfacing material.

The hardfacing material in a metal-to-metal application needs to have a Rockwell hardness in the range of 38-58 Rc. If, for instance, this involved a small metal gear meshing against a large one, one should make the surface of the larger, more expensive gear harder than that of the smaller gear; that way, all the wear is on the smaller, less expensive gear.

In hardfacing applications where there is moderate metal-to-earth abrasion and impact, the hardfacing materials would often be austenite and chrome carbide alloys with a hardness range of 28-53 Rc. For more severe metal-to-earth abrasion and impact, one would use austenite and chrome carbide alloys with a higher hardness range (49-59 Rc). And for severe-abrasion applications with little impact, one would use chrome carbide alloys containing as much as 5.5% carbon and 30% chrome, with hardness ranging from 55 to 70 Rc. The most severe-abrasion applications – the hardfacing of bucket teeth, blades, and scrapers in abrasive soils – would use a tungsten carbide alloy for a hardfacing material.

In many cases, welders use a metal-to-metal welding material for applications that demand a material with good abrasion resistance – and the results don’t last because the abrasion resistance of metal-to-metal materials is poor.

In applications where there is impact – such as rock crushers and rolls used in crushers – the hardfacing material needs to be made of Hadfield manganese, a steel alloy containing 14% manganese.

In hardfacing a loader or an excavator bucket, a relatively soft alloy will work fine when handling soft, loamy soils. On the other hand, sandy soils demand the use of a hardfacing material with high abrasion resistance.

To reiterate: Welders often select the wrong hardfacing material. For instance, they might select a material that has been designed for metal-to-metal wear when they should be selecting a material that has excellent abrasion resistance in a metal-to-earth environment. They are doing this because they fail to realize that there is a wide range of welding materials and that each is designed for use in a particular wear environment. They must: (1) identify the wear environment and (2) select the most appropriate welding rod or wire for that wear environment.

In rebuilding construction equipment or in applying hardfacing to critical construction-equipment components, the overall goal is to minimize future equipment downtime. Too many maintenance people focus on the cost of the welding rods or welding wire, selecting alloy materials more for their low cost than for their suitability. Such an approach constitutes false economy. The cost of the welding consumables is not all that important. But what can be very costly is equipment downtime; a single piece of equipment being out of service frequently costs a contractor thousands of dollars per day. And when hardfacing is done using cheaper and inappropriate welding alloys, the certain result will be that the welding repair will fail sooner than if it had been done correctly, leading to costly downtime.