Cracking the case

How to choose the right filler metals to weld AR plate

By Keith Packard

July 11, 2006

Before welding abrasion-resistant plate, it's important to take certain precautions and choose the most appropriate filler metal for your application and weld metal.

It's important to consider ways to make any welding process more efficient and effective. Welding abrasion-resistant plate, commonly known as AR plate, is no exception.

Taking certain precautions and choosing the most appropriate filler metal for your application—whether you're repairing service equipment or fabricating a new part or structure that requires the protection of AR plate—are vital to ensuring welding success.

AR Plate and Typical Applications

Typical AR plate products are assigned numbers ranging from 200 to 500 in their name; AR 200, AR 400, and AR 500 are examples. These numbers designate the hardness of the material in Brinell (BHN), which can be converted into alternative hardness scales such as Rockwell B (HRB) or Rockwell C (HRC). Typically, the higher the hardness of the material, the more resistant it will be to abrasive wear. Therefore, the harder grades of AR plate, such as AR 400 (about 42 HRC), AR 450 (about 46 HRC), and AR 500 (about 50 HRC), are the most common. Plates can range in thickness from 1/2 inch to 2 in. and thicker and come in a variety of lengths and widths depending on the manufacturer.

Because AR plate has such a high hardness, heavy equipment manufacturers and job shops rely on it to protect equipment from excessive wear. Typical applications include backhoe buckets and teeth, bulldozer blades, dump truck beds, ore and coal chutes, augers, and aggregate conveyors.

Keep in mind that AR plate is designed to protect equipment against wear; it shouldn't be used in a structural or load-bearing design. To ensure the best results, review your application with an AR plate manufacturer.

Beyond Filler Metal: What You Should Know Before Making a Selection

Welding AR plate to itself or any structure with dissimilar and softer or lower-strength steels poses particular challenges, the biggest of which is the potential for cracking in the weld metal or heat-affected zone (HAZ) of the AR plate. Many factors can contribute to cracking, but some common causes are rapid cooling, highly restrained joints, excessive hydrogen in the weld metal, and filler metals that have limited resistance to cracking.

Preheating the base metal before welding is an important defense against rapid cooling and can help reduce hydrogen levels; both are factors that can lead to cracking. The material to be preheated and its thickness determine the proper preheat temperature (see Figure 1).

Figure 1 Maximum interpass should not exceed 400oF to ensure hardness is maintained.

Joint design is another consideration. If possible, don't locate the weld joint in a highly restrained area. A highly restrained joint is defined by the inability of the base material, weld metal, or overall weldment to expand and contract freely. Welding short, small fillet welds also can reduce heat input and overall residual stresses on the AR plate to help minimize cracking.

A filler metal with the least amount of hydrogen content—as well as one that provides good toughness (high impact value)—also helps reduce cracking potential.

Selecting a Filler Metal

Choosing the right filler metal for welding AR plate to dissimilar steels is much like choosing filler metal for other welding applications. The base metal you are welding the AR plate to will determine which filler metal is most appropriate. Common ASTM-grade base metals joined to AR plate are A36, A572 GR50, A656 GR80, and A514 steels.

Your filler metal choice will depend on the tensile strength of the base material being welded to the AR plate and whether you're repairing the plate or fabricating a new weldment. As a rule, filler metals with low tensile strengths and low hydrogen will yield the best results with the least amount of potential for cracking (see Figure 2).

Figure 2 Filler Metal Recommendations for Abrasion-resistant Plate.

Remember, different AR plate manufacturers recommend different tensile strengths for welding AR plate to a particular grade of base metal. Always check those recommendations before making your final filler metal selection.

Filler Metals for Repair. Three basic types of filler metals can be used to repair AR plate:

1. Shielded Metal Arc Welding (SMAW) Electrodes. To repair or replace AR plate on existing equipment, using a low-hydrogen SMAW electrode with a basic slag system (an AWS H4R E7018 rod, for example) can provide distinct advantages: good as-welded mechanical properties, resistance to hydrogen pickup, and the ability to weld through contaminants (light mud, rust, or oil) that can accumulate on serviced heavy equipment. Low-hydrogen basic slag SMAW electrodes produce welds with good toughness and resistance to cracking.
   
   As for other welding applications, SMAW electrodes offer portability for field repairs—they're lightweight and don't require shielding gas—and many welding operators feel comfortable using them.
   
   However, SMAW electrodes do have disadvantages: slag removal; frequent electrode changes, which make them inefficient for large AR plate repairs; and the potential for low-quality bead appearance.
   
   
2. Self-shielded (Gasless) Flux-cored Wire. As with SMAW electrodes, self-shielded flux-cored wire is suitable for repairing AR plate outdoors. It doesn't require shielding gas, which eliminates the need to haul gas tanks into the field.
   
   The main disadvantage of self-shielded flux-cored wire is that many of the popular wires lack required impact properties (typical for classifications T-4, T-7, and T-11). This increases cracking potential.
   
   
3. Gas-shielded Flux-cored Wire. Gas-shielded flux-cored wire offers versatility for welding AR plate to heavy equipment because it can be used for fabrication as well as repairs. T-5 basic slag gas-shielded flux-cored wire is recommended because of its toughness, resistance to hydrogen pickup, and ability to weld through light rust and mill scale. These features improve the filler metal's crack resistance.
   
   Flux-cored wire does produce slag, which must be removed if you plan to weld multiple passes.

Filler Metals for Fabrication. As with repair, three main types of wire are suitable for fabricating AR plate:

1. Solid Wire. Solid wire, such as an ER70S-3 or ER70S-6 welded on A36 base metal, is best-suited for welding AR plate onto a new heavy equipment structure indoors on clean materials. Advantages of solid wire include a low purchase cost, no need for slag removal, and good bead appearance. Another advantage is its familiarity: It's a more common filler metal, which often makes operators feel more comfortable with using it.
   
   Solid wire does have disadvantages, however. Deoxidizers in this filler metal are limited; shielding gas is required; and heavy spatter can occur on dirty materials (with rust or mill scale, for example).
   
   
2. Metal-cored Wire. Metal-cored wire performs well on clean materials and material with light mill scale and rust, and it creates little or no spatter. In instances of poor fit-up between the AR plate and the base metal, metal-cored wire bridges gaps efficiently. It also offers high deposition rates to help increase production.
   
   Metal-cored wire also can increase travel speed, which minimizes heat input. Most metal-cored wires also offer good toughness properties.
   
   Disadvantages to metal-cored wire include the need for shielding gas, which can add to welding costs. These wires also can be difficult to use for out-of-position welding and require short-circuit or pulsed capability power sources to do so.
   
   
3. Gas-shielded Flux-cored Wire. For welding AR plate to a new piece of heavy equipment, T-5 basic slag gas-shielded flux-cored wire is recommended because of its toughness, ability to resist hydrogen pickup, and capacity to weld over light rust and mill scale.
   
   Flux-cored wire does produce slag that requires removal and can cause additional time and expenses for cleanup.

Piecing It All Together

In the end, choosing filler metals for welding AR plate is as much a matter of education as it is a matter of requirements. Considering your base metal and remembering to use low-tensile-strength filler metals with high toughness and low hydrogen content are important. Applying proper preheat for the given material thickness and choosing an appropriate joint design are equally important.

In addition to these factors, you must determine your requirements or goals: Do you simply want to get a piece of equipment back in service, or do you want to weld AR plate on a new weldment and make its appearance appealing?

Whichever the case, if you're armed with basic information, you can avoid cracks—as well as time and frustration—when welding AR plate.

Keith Packard is product manager, tubular wire, at Hobart Brothers, 400 Trade Square, Troy, OH 45373, 937-332-4000, www.hobartbrothers.com.