A 3g weld test welded with 7018 is the most common and is often given along with a 4G test.
Scroll down for Dual Shield and GMAW-s 3g tests
A 3G weld test with 7018 stick electrodes is commonly 3/8" thickness with 22.5 degree beveled plates and a 1/4" thick backing strap. Thickness of test plate can also be 1 inch or greater depending on scope of job.
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Another Common 3G weld test is Dual shield flux core also known as gas shielded flux core
A 3G weld test using Dual shield flux core also known as gas shielded flux core is also commonly 3/8" thickness with 22.5 degree beveled plates and a 1/4" thick backing strap
You might also need to know how to pass a 3G mig test
A 3G test using MIG aka Gas Metal Arc Welding could be short circuit mig, or pulse spray mig and is often 3/8" thick plate with 37.5 degree bevels with an open root.
The root might be downhill or uphill and is usually gapped at around 1/8" root opening.
3g weld tests are sometimes x rayed but are often bend tested by cutting one root bend strap and one face bend strap.
For a bend test, you don't want restarts right on a bend strap
Pro tip: plan ahead and make any restarts in middle area so that restarts wont be on bend strap areas
Some best practices for any welding test
Review welding codes and standards: Familiarize yourself with the relevant welding codes and standards that apply to the test. These may include codes such as AWS D1.1 (Structural Welding Code - Steel) or ASME Section IX (Welding and Brazing Qualifications).
Read and comply with the WPS ( welding procedure specification)
Ask the test supervisor "are there any additional requirements I should know before I start". Welding inspectors and welding test administrators all have their personal preferences. Asking this question might give you a little insight into some pet peeve or preference they have.
Clean mill scale back at least 1/2" from weld area ..that will help you avoid undercut.
Take extra care on the fit up. Structural welding plate tests per D1.1 usually have a gap requirement of 1/4" +- 1/16". As far as ensuring penetration on the root pass goes, a wider gap usually helps so I recommend making sure to have a full 1/4" gap that is even from one end to the other.
Sometimes there are time limits so plan your work and work methodically. But don't rush. let the piece cool a bit before the cover pass.