TIPS FOR OVERLAYING JOURNALS AND CASTER ROLLS

One reason for this can be attributed to whether or not the welding process used incorporated flux (such as the submerged arc welding process and the flux-core arc welding process) and thus the slag that was produced helped to facilitate puddle control. Another case where this rule-of-thumb was “violated” was when the Power Wave^® AC/DC 1000 SD power source and AC welding polarity was used to reduce heat input (compared to DC+ submerged arc welding at equivalent deposition rates) and hence an improvement in puddle fluidity was realized.

Now that we have documented the deposition rate and puddle fluidity as important factors in roll overlay, we can drill down even further into the specifics of how the weld metal is balanced on the top of the roll. One such consideration is the distance that the torch is positioned from top-dead-center (TDC) of the roll, i.e., clock position as displayed in the end view of the roll shown in Figure 1.

The larger the roll diameter, the farther that the torch can be positioned from TDC as can be seen in the table in Figure 2. The welding operator must be careful not to position the torch too far from TDC or else the weld puddle can roll underneath the welding arc and cause instability, or worse yet it can trap slag.

However, we want to take full advantage of the ranges listed in Figure 2. The reason is that puddle fluidity, combined with the effect of gravity, will cause the weld puddle to pull to the middle of the weld before it has completely solidified. This phenomenon results in an inconsistent bead height and a relatively tall line of weld metal straight down the middle of the bead. This line has been frequently called “dragonback” by many welding practitioners.

One can take confidence in the utilization of the uppermost end of the range of distances from TDC for a given roll diameter, particularly when using the submerged arc welding process. The reason that welders can take heart is because experience has shown that when conducting SAW roll rebuilding, it is part of best practice to tilt the welding head so that the wire points directly into the center-point of the roll. The effect of this torch angle is to help maintain the highest level of puddle stability. One rationale for this effect is that the arc force keeps the weld puddle at-bay and prevents it from rolling into the arc.

That being said, the torch angle that I just recommended that is applicable for the SAW process may possibly be thrown out the window when using an open-arc process such as FCAW or GMAW. This is because, in my personal experience, a slight “push angle” has actually been shown to be the most effective in promoting stability and in inhibiting weld metal porosity.

So now that we have established the deposition rate and torch angle for overlay applications, the next variable is how fast should the roll be rotated during welding?  In “Focus On Proper Weld Size When Developing WPS Parameter Ranges” (Welding Tips, February 2011), I mentioned that the most favorable weld “nugget” size is equivalent to a 5/16 in fillet weld (10 percent over-welded). This, by and large, holds true for overlay applications as well.

With the help of a little geometry (or if you refer to Table 2 of my February 2011column), it can be determined that this weld nugget corresponds to a weld metal weight of 0.201 pounds per linear foot of weld. A simple calculation then provides a good starting point for “surface travel speed” (see Equation 1).

Again, this 12 ipm surface travel speed is a starting point. For a 12 in outer diameter roll and a 12 ipm welding travel speed, this means that the roll will make one complete revolution in about three minutes and eight seconds (or π = 3.14 minutes).

So now we have just two final pieces of the puzzle that remain to be solved. Specifically, in what manner should the longitudinal travel of the welding head take place and how quickly should it occur?

Regarding the manner of travel, the schematic in Figure 3 shows the method in which there is continuous travel of the welding head. Essentially, the torch must travel (extremely slowly) in the longitudinal direction by the amount of the “stepover” distance in the time that it takes to rotate the roll one full revolution. This, however, is not always easy to carry out, as a welding travel carriage with a special gear ratio is required in order to accomplish this extremely slow travel speed.

Alternatively, this stepover distance can also occur in a “step-change” fashion, where the longitudinal position on the roll is the same during one entire revolution. Although this may very well be the most economical method, it requires the welding operator to stop paying attention to everything else he or she is doing in order to make the stepover step-change.

These important functions that the welder must forego could be (to name a few):

1) observation of the bead shape,

2) adjusting roll rotational speed,

3) removing slag,

4) adding flux to the hopper, or

5) changing welding parameters on the fly.

As far as “how quickly” the longitudinal travel should occur, a good starting point for the stepover distance is such that the welding electrode should be just inside the toe of the previous weld bead. This will help to ensure that the beads tie-in smoothly with each other. For the SAW process, the most common starting point for this stepover distance is about ½ in. For open arc processes, this value may be closer to 5/16 in.

However, this stepover distance can be altered by increasing or decreasing the welding arc voltage to obtain a specific bead width and hence a specific layer height. After all, thicker (not to be confused with “wider”) weld layers equate to fewer weld layers needed to achieve a required deposit thickness. Furthermore, when fewer layers are deposited, it is possible that fewer PWHT cycles may be required.

Let’s look at an example where we want to calculate what our stepover distance should be if we want each weld layer to be 3/16 in thick. Using the same weld nugget size of 0.201 lb per linear foot of weld that we stated above (in the present case, think of the “weld nugget” in terms of 0.201 lb of weld metal deposited in every 0.32 revolutions of the roll), we can estimate what the weld bead width – or stepover distance – must be.

If we solve for d in the equation below, using the parameters that we specified above, we can determine that the stepover distance should be approximately ½ in (see Equation 2).

Granted, it may take some manipulation of the arc voltage to achieve a ½ in bead width. But once that appropriate voltage setting is found, the weld layer height will be very close to the required 3/16 in.

Following the rules-of-thumb and “best practice” recommendations listed above, roll overlay will surely become less of a guessing-game.