What causes weld hot and cold cracking and how to avoid it?

Question:What causes weld hot and cold cracking and how to avoid it?

Author: Professor Adrian Gerlich

Answer: The phenomenon of cold cracking has been a source of confusion and frustration in many industries when welding carbon steels. The main issue is that this type of cracking, often called hydrogen cracking or delayed cracking, because it often does not occur until days afterwards. Many quality standards in fact specify that inspection of welds is not performed until at least 48 hours after the weld is performed. Cold cracking is typically driven by the presence of hydrogen in the weld metal and heat affected zone (HAZ), but can occur at underbead locations or even in the weld metal.

Cold cracking ultimately occurs when three factors coincide: 

• there is some residual tensile stress, 

• ii) the steel has transformed to susceptible martensite or bainitic microstructures

• iii) there is a supply of hydrogen in the steel. 

The first factor is hard to avoid, as tensile residual stresses normally accumulate near the HAZ and weld toe, due to thermal contraction on cooling. 

The second issue is dependant on the type of steel, especially the carbon equivalent, as this will determine how likely it is to form brittle martensite. Although a general guideline is to avoid welding steels with a carbon equivalent of more than 0.4%, tools such as the Graville diagram also consider the actual carbon consent to identify when to apply control of hardness through preheating and post-weld heat treatment. 

The third factor is the overall diffusible hydrogen content in the weld, which ideally be maintained to a value of [H] less than 5 ppm. 

Practices such as adequate drying of stick electrodes and storing them in heated ovens after opening is essential. However, hydrogen may find a way into the weld from other external sources such as contaminants on the joint prep, such as oil or grease, and rust (which is actually composed of a mixture of iron hydroxides). The absorbed hydrogen can then accumulate in the HAZ by a 3 step process shown in Figure 1, where it is first dissolved and dispersed as diffusible (atomic hydrogen) by the arc, and trapped in the solidified austenite at (1). As the weld metal cools, the carbon steel will transform in to ferrite + carbide at (2), where the solubility of hydrogen suddenly decreases, and the hydrogen begins to move to the austenite in the HAZ. There the austenite is more soluble, and the transformation may be slower if the base metal has a higher carbon content than the weld metal. Then the diffusible hydrogen is accumulated in the HAZ at (3), and will lead to the potential crack if the steel transforms to brittle martensite and tensile residual stresses arise. The eventual cracking occurs when the diffusible atomic hydrogen recombines into molecular hydrogen at the microscopic interfaces in the steel microstructure, which may take several hours. 

Figure 1: Mechanism of hydrogen diffusion into the heat affected zone of steel.