In parts one and two of this series, we considered distortion control principles that can be implemented on the drawing board. In this final installment, we’ll examine shop practices that will limit distortion.
For general overall weld economy as well as to control distortion, the shop should concentrate on depositing welds of the required size: No more and no less. This is a particular challenge for small welds, for which a slight change in weld size results in a significant change in the volume of shrinking metal. The first chart in part one of this series can be used to estimate the effect of depositing welds larger than those specified on the drawings.
For example, when a 3/16 in. fillet weld is required, but the shop produces a 1/4 in. fillet instead, 78 percent more weld metal is deposited. Because small welds typically are associated with relatively thin metal that is flexible, seemingly minor increases in weld size can have significant – and significantly negative – effects.
Limit weld reinforcement
All hot metal, whether weld metal or base metal, can contribute to distortion when it cools and shrinks. Furthermore, all portions of the weld can contribute to the distortion, and this includes the weld reinforcement. While slightly convex welds are helpful in avoiding some types of solidification cracking, an excessively convex weld profile only adds to costs and distortion.
To quantify the impact, consider the weld illustrated in Figure 1. Moving from a reasonable value of 1/8 in. reinforcement to 3/8 in. (which, if you look at actual welds, doesn’t necessarily look “wrong”) results in a 28 percent increase in weld metal volume.
Unless there is a specific reason to do otherwise, limit weld penetration
While all welds are required to achieve fusion, only certain welds are required to have defined penetration levels.
Unless there is a specific penetration requirement, providing additional penetration simply adds to the volume of molten metal that will eventually cool, shrink and contribute to distortion. Practically speaking, control of weld penetration may require changes to welding variables, such as the polarity, current and electrode size that are used.
Limit the amount of backgouging on two-sided groove welds
Before making the second side of a double-sided, complete joint penetration (CJP) groove weld, you may need to backgouge the weld root of the first pass. This is required, for example, by AWS D1.1 for prequalified welding procedure specifications (WPSs) that use two-sided groove welds. Backgouging to remove any unfused metal ensures complete through-cross-section fusion when the opposite side is welded. However, excessive backgouging creates a larger cavity which, in turn, must be filled with more weld metal that will contribute to additional distortion.
Welded construction offers the advantage that welds can accommodate variations in fitup. Bolted construction, for example, requires more precise alignment before mechanical fasteners can be inserted. But with welding’s built-in advantage comes a disadvantage: It can encourage sloppiness in the shop (in terms of controlling fitup), resulting in larger-than-required gaps that then have to be filled by welding.
To quantify the effect of fitup variations, we’ll look at two factors, starting with the root opening. For the 1 in. bevel groove weld shown in Figure 2 and with an ideal root opening of 1/4 in., 3.08 pounds of shrinking weld metal are applied for each foot. Increase the root opening to 1/2 in., and the weld metal volume is increased 31 percent.
The bevel angle (Figure 3) is the other factor. Experience has shown that this is difficult to detect without gauges — small variations can be quite misleading to the human eye. However, a 10 degree increase in the bevel angle from 45 degrees to 55 degrees results in a 29 percent increase in weld volume.
To further understand the significance of fitup, consider the table shown in Figure 4, which illustrates combinations of root openings and included angles. Just to keep things interesting, we’ll also examine the influence of the previously mentioned weld reinforcement.
The table lists the increases due to combinations of factors. It is easy to see how increases of 50 percent and more in weld metal can easily occur with even seemingly minor variations in these factors. If distortion is a problem, check out variations in the fitup of your weldments.
Make welds of the required size in the fewest number of passes
Limiting the number of heating and cooling cycles to which you submit the weldment will reduce distortion. Making a weld of a given size in three passes versus six, for example, will reduce distortion. While this principle may seem to conflict with those that were previously mentioned, the subtle difference is this: All of the preceding distortion control principles involved limiting the amount of weld metal deposited. Here, the total amount of weld metal is unchanged. For a given weld size, the number of passes used should be minimized to limit distortion.
Use welding processes and procedures that limit heat input
H = 60EI/1000S
E = arc volts
I = welding current
S = welding travel speed.
When Imperial units are used, the resultant value is measured in kilojoules per inch (KJ/in.).
Heat input also is related to the size of the weld bead. Every welder knows that in order to make a larger weld in one pass, either the amperage must go up, or the travel speed go down, or both. Notice that both of these changes result in increased heat input.
Again, we must consider this principle in context of the overall picture. It would be wrong, for example, to reduce heat input by simply increasing the travel speed since this would require more weld passes. This distortion control principle assumes that the weld volume per length remain unchanged. When this is the case, lower heat input levels will reduce distortion.
Here is where some of today’s latest technologies offer advantages. Pulsed spray GMAW, squarewave AC SAW and other such developments reduce heat input levels and correspondingly reduce distortion. Of course, the old standby methods still work too: polarity changes, process changes, longer electrical stickout levels, etc., all can be used to limit heat input as well.
In one way or another, I’ve dealt with distortion for nearly 80 years, starting with my early days as a welder in my dad’s shop. While distortion can never be eliminated, the application of these design and fabrication principles can keep distortion under control and within acceptable limits.
|Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Co., struck his first arc on his grandfather’s welder at the age of ten. He is the author of Design of Welded Structures and Design of Weldments, and an internationally recognized expert in the field of weld design. In 1999, Blodgett was named one of the “Top 125 People of the Past 125 Years” by Engineering News Record. Blodgett may be reached at (216) 383-2225.|