FOCUS ON PROPER WELD SIZE WHEN DEVELOPING WPS PARAMETER RANGES
The fabricator and the erector must determine the most appropriate range of travel speeds for a particular application, rather than relying blindly on published product certs. However, following the methodology presented here provides another possible blueprint on how this may be achieved.
A structural steel fabrication company is preparing for a project that is to be welded in accordance with the American Welding Society D1.8 Seismic Supplement and would like to use a .045 in E71T-1 gas-shielded flux-cored (FCAW-G) wire. This fabricator has the luxury of being able to position the steel so that they can weld in the downhand position.
They would like to use a particular manufacturer’s wire, but the travel speed at which the high/low heat input envelope testing was conducted is not listed on their D1.8 certs. They need this to show a travel speed range on their Welding Procedure Specification (WPS). Is there a reason that the travel speeds are not listed?
Although it would be much easier to transfer these travel speeds directly to the WPS, it will be shown shortly that this may not be such a good idea. Granted, a few straightforward and systematic calculations are required, but there is no reason to be intimidated by numbers or equations in the welding field.
The most recent D1.8 cert for the E71T-1 electrode is shown below in Table 1. Although not displayed here, both the high heat input and low heat input deposits met the mechanical property requirements of AWS D1.8, including tensile strength, yield strength, elongation and Charpy V-notch impact toughness at 70 deg F. If so desired, the travel speed could be easily determined by rearranging the welding heat input formula presented in Figure 1. For illustration purposes, the resulting travel speeds for both the high heat input and low heat input tests are shown in Figure 2.
Based on the question posed above, one would assume that it is now appropriate to use the calculations from Figure 2, and hence display the “travel speed range” on their WPS as 2.9 ipm to 10.8 ipm. Furthermore, a fabricator or erector may also want to derive a current range of 160 amps to 215 amps from the D1.8 cert. We will now discuss why caution must be used here.
If a welder or welding operator wished (or was incented) to maximize his productivity, he may want to decrease his travel speed. This would be logical from the welder’s perspective as it would decrease the number of passes required to fill up the joint. And therefore, he assumes, it would save the company money (or make him more money) as cleaning and set up time would be reduced. So, according to the travel speed range listed on his WPS, he can use a travel speed as low as 2.9 ipm.
Finally, as long as he stayed within the current range of 160 amps to 215 amps and the voltage range of 24V to 26V listed on the WPS, he would then appear to have some justification for using this travel speed. However, a problem arises with this logic when the uppermost values of 215 amps and 26 volts are used. Again, a higher wire feed speed would result in higher productivity.
Figure 3 shows the resulting welding heat input in this scenario. Obviously 116 kJ/in is far too high of a heat input to be used with a cored wire. Some problems that may arise by using a 116 kJ/in heat input include reduction of the Charpy V-notch impact properties in the weld, softening of the base material in the heat-affected zone (HAZ), or lack of penetration and/or fusion (because the arc is riding on the puddle). Also, this heat input exceeds the maximum heat input of the “envelope” established on the D1.8 cert.
Consider the opposite situation. What if the fastest travel speed (10.8 ipm) and the lowest current (160 amps) and voltage (24V) settings are used during fabrication or erection? Obviously this will result in a lower welding heat input than the 30 kJ/in value shown on the D1.8 cert. But why is this important?
The answer to this question boils down to ensuring that the weld size is large enough (and hence the welding heat input is high enough) relative to the thickness of the plate being welded. Welds that are too small can contribute to HAZ or underbead cracking. It would then be a worthwhile exercise to see just exactly how large the welds would be with these ultra-low heat input procedures.
The process of analyzing the weld size begins with determining the deposition rate. A quick peek at the literature on this E71T-1 wire reveals that at a wire feed speed of 275 ipm, a deposition rate of 5.5 lb per hour is yielded. This, combined with the 10.8 ipm travel speed, yields a “weld nugget” of 0.102 lb per foot as demonstrated in Figure 4.
It behooves us at this point to harken back to an earlier column (“Pre-Calculating Wire-Feed Speed, Travel Speed and Voltage,” Welding Tips, November-December 2010) where my colleague Kevin Beardsley discussed pre-calculating a travel speed when the deposition rate and desired weld size are known. It can be seen from Table 2 (excerpted from that column) that a weld that weighs 0.102 lb per foot is equivalent to a fillet weld with less than a ¼ in leg size.
Moreover, let’s assume that you are welding on mild steel plate with a thickness of ¾ in. According to Table 5.8 in the AWS D1.1-2010 Structural Welding Code – Steel, the minimum fillet weld leg size required is ¼ in. Even if you are not making fillet welds, but rather complete-joint penetration (CJP) groove welds, the concept is still the same – the “weld nugget” should be equivalent to that of a ¼ in fillet weld in order to ensure a cooling rate that is sufficiently slow to prevent underbead cracking or other weld defects such as lack of fusion. Therefore, according to Table 2, a ¼ in fillet weld with a flat face (and 10 percent overwelded as is customary in shop practice) will have a weld nugget of 0.128 lb per foot.
So, if we decide that we actually like how the E71T-1 electrode performs at 275 ipm wire feed speed, 160 amps and 24V, what is the limit on how fast we can travel and still have a weld pass larger than an equivalent ¼ in fillet? This is a matter of simply re-arranging the formula shown in Figure 4. In order to ensure that the equivalent of a minimum ¼ in fillet weld is deposited, the travel speed can be no faster than the 8.6 ipm as shown in Figure 5. Compare this result with the 10.8 ipm that was extracted from the D1.8 certification testing.
Now that we have determined an upper limit on travel speed, what is the thought process for the lower limit on travel speed? How slow is too slow? Mentioned above was the unacceptable scenario where a heat input of 116 kJ/in was obtained when the highest current and voltage settings and the slowest travel speed were utilized based on the “ranges” extracted directly from the D1.8 certs.
For guidance on the largest weld that can be made, one may look to the AWS D1.1 code, which never requires a single pass fillet weld leg size greater than 5/16 in. From a practical standpoint, it is quite difficult to deposit (with a single electrode) a single pass fillet weld in the downhand position greater than 5/16 in without encountering defects such as trapped slag, overlap, or lack of penetration and/or fusion (because the arc is riding on the puddle). Based on this, we will begin the minimum travel speed calculation with a 5/16 in fillet weld as the target. Once again referring to Table 2 above, a flat 5/16 in fillet weld will require a weld nugget of 0.201 lb per foot.
Also, a welder may prefer to weld it hot – using the 325 ipm wire feed speed (which translates to a deposition rate of 6.5 lb per hour) and 26V setting listed on the D1.8 cert. In order to ensure that the equivalent of a maximum 5/16 in fillet weld is deposited, the travel speed can be no slower than the 6.5 ipm as shown in Figure 6. Compare this result with the 2.9 ipm that was extracted from the D1.8 certification testing.
We have achieved our ultimate goal of constructing a “conservative” travel speed range of 6.5 ipm to 8.6 ipm that increases the chances of depositing a sound weld. The range of 2.9 ipm to 10.8 ipm that could be extracted from the D1.8 certs may misapplied in a worst case scenario. This is the primary reason that the travel speeds are not listed on this manufacturer’s D1.8 certs.
In the end, it is up to the fabricator and the erector to determine the most appropriate range of travel speeds for a particular application, rather than relying blindly on published product certs. Following the methodology presented above provides one possible blueprint on how this may be achieved.
About the Author: Regis Geisler III is a registered Professional Engineer, Specials Certification Engineer and AWS Certified Welding Inspector who has worked the last eleven years at The Lincoln Electric Company, 22800 Saint Clair Avenue, Cleveland, OH 44117-8542, 216-481-8100, Fax: 216-486-1751, www.lincolnelectric.com. For questions or comments on this column, contact Regis at firstname.lastname@example.org.