Welding - Carbon Cracking

Introduction

When welding low carbon steels, there is virtually no structural transformation in the heat affected zone (HAZ), however as the carbon content within the steel increases, the risk of undesirable structures in the HAZ also increases.

These undesirable structures can often result in weld cracking; there are many types of cracking, the most common being;

• Solidification cracking (weld metal hot cracking)

• Lamellar tearing

• Heat affected zone burning and hot tearing (HAZ liquation cracking)

• Hydrogen induced heat affected zone cracking (HAZ cracking, cold cracking, hydrogen cracking)

This assignment will examine welding procedures to avoid hydrogen cracking when butt welding two carbon manganese steel plates using the manual metal arc (MMA) process.

Hydrogen Cracking

Hydrogen cracking has the effect of significantly reducing strength properties of the; weld metal, parent metal or both. It is therefore highly important that the causes and effects are known and welding procedures are designed so that the risk of cracking is kept to a minimum.

This cracking is caused by the diffusion of hydrogen to the highly stressed and hardened heat affected zone of the weld. Hydrogen cracking is mainly associated with high strength steel i.e. steel with high carbon content. For cracking to occur a susceptible microstructure such as martensite must exist as well as the presence of hydrogen within the microstructure.

If steel is cooled down rapidly from its austenitic condition, the iron will undergo an atomic change, but the dissolved carbon will not have enough time to separate out. This causes instability within the steel structure and gives rise to a hard, brittle constituent known as matensite. It is for this reason that susceptibility to hydrogen cracking increases with carbon content of the steel.

Hydrogen may enter the microstructure of the heat affected zone in several ways; the most common being from the moisture contained within the coating or...