How to Check MIG Welding Penetration: A Comprehensive Guide for DIY Enthusiasts

Ensuring proper weld penetration is crucial for the structural integrity and longevity of any metal fabrication project. As a DIY enthusiast, understanding the factors that affect MIG welding penetration and how to effectively measure it can make the difference between a successful and a failed weld. In this comprehensive guide, we’ll delve into the advanced techniques and critical factors that you can use to check the penetration of your MIG welds.

Factors Affecting MIG Welding Penetration

1. Polarity

The type of welding polarity used has a significant impact on the level of weld penetration. Direct current positive (DC+) polarity produces more weld penetration, while direct current negative (DC-) polarity results in less weld penetration. This is because with DC+ polarity, more arc energy is focused into the base plate, while with DC- polarity, more arc energy is directed into the electrode.

2. Welding Process

The various arc welding processes, such as Submerged Arc Welding (SAW), Flux-Cored Arc Welding (FCAW), and Gas Metal Arc Welding (GMAW), have distinct weld penetration characteristics. The SAW, FCAW, and GMAW (in a globular, spray, or pulse spray arc metal transfer mode) processes are known for their higher levels of weld penetration. In contrast, the Gas Tungsten Arc Welding (GTAW), GMAW-C, and GMAW (in a short circuit metal transfer mode) processes typically have lower levels of weld penetration.

3. Type of Electrode

Even within the same welding process, electrodes of different classifications can exhibit varying penetration characteristics. For example, with the Shielded Metal Arc Welding (SMAW) process, an E6010 electrode typically has deeper penetration, while an E7024 electrode typically has shallower penetration.

4. Travel Angle

The degree of the travel angle, which is the angle between the electrode and the base plate, affects the amount of arc force directed into the base metal. A travel angle of 0° to 10° (i.e., the electrode perpendicular or mostly perpendicular to the plate) will result in more weld penetration, as the arc force is more focused on the base plate.

5. Shielding Gas Type

The type of shielding gas used can also influence the weld penetration profile. Shielding gases with a higher rate of thermal conductivity, such as 100% carbon dioxide (CO2) or 100% helium (He), will produce welds with a broader and deeper penetration profile.

6. Electrode Diameter

When welding with two different diameters of the same electrode and at the same current level, generally more penetration is achieved with the smaller diameter electrode than with the larger diameter electrode.

7. Travel Speed

The speed at which the electrode travels down the joint affects the amount of time the arc energy has to transfer into the base metal. As travel speed increases, the resulting level of weld penetration decreases, and as travel speed decreases, the resulting level of weld penetration increases.

8. CTWD Variations

In the GMAW, FCAW, and SAW processes, where a constant voltage (CV) power source is used and the wire feed speed and voltage are set, the contact tip to work distance (CTWD) can also impact the level of weld penetration. As the CTWD is increased, the level of weld penetration decreases.

Checking MIG Welding Penetration

how to check mig welding penetrationImage source: Mig weld example

To assess the weld penetration, you can use either destructive or non-destructive testing methods. Destructive testing methods involve cross-sectioning and macroetching the weld, which allows for a visual inspection of the weld’s internal structure. Non-destructive testing methods include X-ray radiography, ultrasonic testing, and liquid penetrant testing.

Destructive Testing Methods

  1. Cross-Sectioning: Cut a sample of the welded joint perpendicular to the weld bead and polish the cross-section. This will reveal the internal structure of the weld, including the depth of penetration, the shape of the weld profile, and any potential defects.

  2. Macroetching: Etch the polished cross-section of the weld with a suitable chemical solution, such as a mixture of nitric acid and water. This will reveal the weld’s microstructure, including the fusion zone, heat-affected zone, and the depth of penetration.

Non-Destructive Testing Methods

  1. X-Ray Radiography: Use X-ray imaging to visualize the internal structure of the weld without damaging the sample. This method can detect any internal defects, such as porosity, lack of fusion, or incomplete penetration.

  2. Ultrasonic Testing: Utilize high-frequency sound waves to detect any discontinuities or lack of fusion within the weld. This method can provide information about the depth of penetration and the overall quality of the weld.

  3. Liquid Penetrant Testing: Apply a colored or fluorescent dye-based liquid to the weld surface, which will penetrate any surface-breaking defects. This method can help identify any cracks, porosity, or lack of fusion on the weld surface.

By understanding the factors that affect MIG welding penetration and utilizing the appropriate testing methods, you can ensure the quality and integrity of your welded joints, making your DIY projects more successful and long-lasting.


  1. Variables that Affect Weld Penetration – Lincoln Electric
  2. [How Strong Are Your Welds] Testing Your MIG WELDS at Home (Will …](
  3. MIG Welding Wire Speed and Penetration – YouTube