Комаров Артём о диагностике и предотвращении растрескивания сварного шва (eng)
Комаров Артём о диагностике и предотвращении растрескивания сварного шва(eng)

Комаров Артём о диагностике и предотвращении растрескивания сварного шва(eng)

Artem Komarov clarified that weld cracking is a complex and serious problem that can happen to anyone. First, determine what is causing the cracking problem. It sounds simple enough, but it’s not always the case.

The first thing to realize is that 98% of all weld cracks are either cold or hot cracks. The remaining 2% is caused by factors such as arc strike, material contamination (usually oil-based), plate delamination, fatigue, base metal heat-affected zone, or metal cracking when the weld solidifies. Some of these other cracks are usually the result of improper welding technology or filler metal.

A cold crack forms after the weld metal has cooled below 200 degrees Fahrenheit. This type of crack formation is delayed and can take up to 72 hours. This is usually due to hydrogen embrittlement or residual stresses that exceed the strength of the filler metal or base materials.

A hot crack begins to form during the solidification of the weld metal, typically at a weld temperature of 1000 to 1300 degrees Fahrenheit. Once formed, these cracks in the weld are known to propagate, i.e., grow, until conditions are improved. This type of cracking mechanism is a major problem.

Once you have determined the presence of a crack by visual inspection or other non-destructive method, the next thing you need to do is look at the orientation of the crack in the weld metal. Are the cracks transverse (across the weld) or longitudinal (in the center of the weld)? The crack orientation speaks for itself.

Растрескивание сварного шва

Crack from cold/stress. To form a cold crack, a weld must have three characteristics:

— Residual stress in the weld

— Source of hydrogen/moisture

— Presence of martensite (formation of martensite can occur when the base material has sensitive microstructure, chemical composition, and accelerated cooling rates)

Looking at these three points, you should immediately understand that some residual stress in the weld is inherent in it and therefore cannot be eliminated. However, you can solve the problem of cracking by referring to two other points.

The first is hydrogen. Hydrogen and oxygen atoms in water decay due to the strong heating of the arc, which can lead to hydrogen retention in the steel microstructures. You can solve this problem by using low hydrogen electrodes or flux and making sure these materials are properly handled according to the manufacturer’s recommendations.

Then make sure the surface of the material is dry before welding. Due to the characteristics of dew points and ambient temperature, the easiest way to ensure that the material is dry is to simply hold the flame close to the surface and wait for the moisture that is collected from the flame to evaporate. A common mistake is to use the first pass of a multi-pass weld to preheat the steel. This is the worst thing you can do, as this method simply locks the hydrogen in the weld metal structure, distributing it throughout the weld.

Consumables Corner

Another way to prevent cracking is to stop the formation of martensite. Martensite forms from the rapid cooling and solidification of the weld metal, so you must apply enough preheat to the adjacent base metal to slow down the cooling rate.

For example, if we are welding 1½» thick carbon steel, we need to ensure that the adjacent base material is sufficiently preheated to 200 degrees Fahrenheit or more, depending on the carbon equivalent of the base material. This preheating prevents the adjacent material from dissipating heat from the weld metal, resulting in accelerated cooling of the weld.

Hot crack. When the weld metal solidifies, a hot crack is formed, which always runs longitudinally to the weld itself. The weld metal does not have sufficient strength to overcome shrinkage due to cooling in the temperature range described earlier. This type of cracking is commonly associated with fillet welds, especially when welding the second side of a T-joint configuration. This also happens on grooved joints, usually on the first root pass.

To form a hot crack, a weld must have one or more of the following properties:

— Low melting compounds or contaminants in the weld metal

— Excessively stressed welded joint (too strong hold) and/or poor fitting of the joint

— Filler metal with incorrect strength, insufficient weld size and inappropriate shielding gas for FCAW

— Incorrect welding technology or poor welding parameters

— Insufficient or missing preheating

Low melting point compounds are not very common because the filler metals meet the strict standards of American manufacturers. Core materials can be problematic as some suppliers in the global supply chain have poor quality control standards.

Excessively stressed connections are a common occurrence. It is important to have sufficient fastening to maintain dimensional tolerances. Poor fitting of the joint, creating excessive gaps, will result in a high shrinkage stress of the weld.

Proper filler metal strength and appropriate weld size are required to overcome residual stresses due to weld shrinkage. If you are using FCAW, make sure your shielding gas meets AWS classification and manufacturer’s recommendations. The use of a high argon shielding gas with wire designed for carbon dioxide will result in excessive alloy extraction in multi-run welds, which can lead to cracking.

Incorrect welding parameters and incorrect technique can lead to cracking. The technology that produces a concave weld does not provide sufficient reinforcement of the weld at the neck of the weld to overcome shrinkage stresses.

Use preheating for heavier base material areas or where there is a large heat sink to slow down the cooling rate, reduce weld shrinkage stress and prevent cracking, Artem Komarov summed up.