Why ISO Calls Welding a Special Process

ISO standards classify welding as a special process. In practice, it means that once a weld is made, you cannot fully inspect quality into existence. If the metallurgy is wrong, the damage is already locked into the component.

To produce a welded joint you can guarantee for safety, the welder must understand not just technique, but the metallurgy of the material being joined.

Now, let’s consider what the welder did during a qualification test. In the attached video, the welder strikes the electrode on the parent material away from the joint before moving into the groove.

From a metallurgical standpoint, a serious defect has been introduced before the weld even began. ISO 6520-1 classifies this as an arc strike under Group 6 Miscellaneous Imperfections.

An arc strike, also called a stray arc, is defined as local surface damage caused when an arc is initiated outside the weld joint. The standard explicitly warns that arc strikes can have serious metallurgical and structural consequences, even though they occur in an instant.

Why arc strikes are dangerous

When the electrode was struck outside the joint, an electric arc with a plasma core exceeding 5000 °C was generated. This caused extremely rapid local heating, melting, and subsequent cooling by the surrounding cold base metal. Metallurgically, this is worse than normal welding. The resulting effects include:

1. Microstructure alteration and local hardening – here the rapid heating followed by severe cooling produces quench hardening. Martensite can form, which is extremely hard, brittle, and crack prone. This is particularly dangerous in carbon steels and low alloy steels.
2. Microcracking can occur. The severe local thermal stresses from rapid expansion and contraction can produce surface and subsurface cracks. These may remain undetected initially and propagate later under fatigue loading or pressure.
3. Surface damage – Arc strikes leave small craters, burn marks, and pitted areas. These reduce corrosion resistance, especially in stainless steels and coated steels.
4. Stress concentration – The damaged area behaves like a notch. Even when visually small, it can drastically reduce fatigue life.

Why some steels are more vulnerable

The severity of arc strike damage depends mainly on:

1. Carbon content
2. Alloying elements
3. Cooling rate
4. Hydrogen presence

These factors control hardenability and cracking susceptibility.

Steels most sensitive to arc strikes

1. High carbon steels above 0.3 wt% C: Extremely prone to martensite formation and cracking. Even a tiny arc strike can cause failure. Examples include tool steels, spring steels, and rails.
2. Low alloy high strength steels: Alloying increases hardenability. Arc strikes can trigger delayed hydrogen cracking. Examples include high strength structural steels and pipeline grades.
3. Quenched and tempered steels: Local arc strikes destroy the controlled heat treatment, creating brittle zones adjacent to tough base metal. Examples include pressure vessel steels, crane booms, and armor plate.
4. Chromium molybdenum steels such as P11 and P22: Highly crack sensitive materials used in high temperature service. Arc strikes often require full repair and post weld heat treatment.

Steels less sensitive but still unacceptable

1. Mild and low carbon steels below 0.2 wt% C: Reduced martensite risk, but arc strikes still initiate cracks and accelerate corrosion. Repair is still required by codes.
2. Austenitic stainless steels: No martensite formation, but arc strikes can cause hot cracking, oxidation, and loss of corrosion resistance.

What standards require

ISO and ASME codes do not permit arc strikes outside the weld zone. If an arc strike occurs:

1. Do not ignore it
2. Grind the area to sound metal
3. Blend smoothly into the surrounding surface
4. Inspect for cracking using VT, MT, or PT
5. Repair if cracks are found or if material removal exceeds allowable limits
6. Apply post weld heat treatment where required by the procedure or component criticality

Key takeaway

Even a brief accidental arc strike can alter steel microstructure, create brittle zones, initiate cracks, and significantly reduce component life.

Frequently Asked Questions

1. Why does ISO describe welding as a special process
   ISO classifies welding as a special process because weld quality cannot be fully verified by inspection alone. If metallurgical damage occurs during welding, it is already embedded in the component and cannot be corrected or inspected into compliance afterward.
2. What does “special process” mean in practical terms
   It means welding quality must be built in from the start. Correct technique, qualified procedures, and metallurgical understanding are essential because some defects are irreversible once introduced.
3. What exactly is an arc strike
   An arc strike, also known as a stray arc, is local surface damage caused when a welding arc is initiated outside the intended weld joint. It is classified in ISO 6520-1 under Group 6 Miscellaneous Imperfections.
4. Why is striking the arc outside the joint a problem
   Because it causes extremely rapid local heating and cooling of the base metal, creating severe metallurgical conditions that are often worse than those experienced during controlled welding in the joint.
5. How hot does an arc strike get
   The plasma core of a welding arc can exceed 5000 °C. Even though the duration is very short, the heating and cooling rates are extreme.
6. What metallurgical damage can an arc strike cause
   Arc strikes can cause local hardening, formation of brittle microstructures such as martensite, microcracking, surface damage, and stress concentration, all of which reduce structural integrity.
7. Why is martensite formation a concern
   Martensite is extremely hard and brittle. It is highly susceptible to cracking, particularly in the presence of hydrogen or under cyclic loading conditions.
8. Can cracks from arc strikes appear later in service
   Yes. Microcracks caused by arc strikes may not be immediately visible and can propagate over time under fatigue loading, vibration, or pressure, leading to delayed failure.
9. Do arc strikes affect corrosion resistance
   Yes. Arc strikes damage the surface condition and protective layers, creating localised corrosion sites, especially in stainless steels and coated steels.
10. Are all steels equally affected by arc strikes
    No. The severity of damage depends on carbon content, alloying elements, cooling rate, and hydrogen presence, all of which influence hardenability and cracking susceptibility.
11. Which steels are most sensitive to arc strikes
    High carbon steels, low alloy high strength steels, quenched and tempered steels, and chromium molybdenum steels are particularly vulnerable, even to very small arc strikes.
12. Are mild and low carbon steels safe from arc strike damage
    No. Although less sensitive, mild and low carbon steels can still suffer cracking, reduced fatigue life, and accelerated corrosion. Welding codes still require repair.
13. What about austenitic stainless steels
    Austenitic stainless steels do not form martensite, but arc strikes can still cause hot cracking, oxidation, and loss of corrosion resistance. They are not exempt from arc strike control requirements.
14. Do ISO and ASME standards allow arc strikes outside the weld zone
    No. ISO and ASME standards do not permit arc strikes outside the weld zone under any circumstances.
15. What should be done if an arc strike occurs
    The affected area must be ground back to sound metal, smoothly blended into the surrounding surface, inspected for cracking using visual or surface NDT methods, and repaired if required.
16. Is post weld heat treatment required after an arc strike
    Post weld heat treatment may be required depending on material type, thickness, hydrogen risk, and component criticality, as defined by the applicable welding procedure or code.
17. Can an arc strike ever be ignored
    No. Even a brief accidental arc strike can permanently alter the steel microstructure and significantly reduce component life.
18. What is the main takeaway for welders and inspectors
    Small actions have big consequences. A momentary arc strike outside the weld joint can introduce serious metallurgical damage. That is why welders are trained to strike arcs only in the joint or on run off tabs.

Read more…

Welding Imperfections and Classification – Part 6: Miscellaneous imperfections