Question

"Cold cracking" in the heat affected zone of a high strength steel weld can take place because of

• A. martensite formation
• B. retained austenite
• C. sufficiently hydrogen present in the welding arc
• D. relatively high sulfur content in the base metal

Hint:

Remember the "Triangle of Cold Cracking":

• Stress (Residual)

• Hydrogen (Diffusible)

• Susceptible Microstructure (Hard Martensite)
All three must be present. The question asks for a cause, and the formation of the susceptible microstructure is a fundamental cause.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Cold cracking, also known as hydrogen-induced cracking or delayed cracking, is a type of weld defect that occurs at or near room temperature after the weld has cooled. It requires the simultaneous presence of three factors:

• A susceptible, brittle microstructure.

• The presence of diffusible hydrogen.

• High residual tensile stresses.
The question asks for a cause of this phenomenon.

Step 2: Detailed Explanation:
Let's analyze the factors involved, particularly in high-strength steels:

• (A) Martensite formation: High-strength steels have high hardenability. The rapid heating and cooling cycle of welding in the Heat Affected Zone (HAZ) acts like a quench. This causes the austenite formed during heating to transform into martensite, which is a very hard, strong, and brittle microstructure. This brittle martensite is highly susceptible to cracking and is the primary microstructural requirement for cold cracking to occur.

• (C) Sufficiently hydrogen present: Hydrogen is the second critical ingredient. It can be introduced from moisture in the flux, the atmosphere, or contaminants. Hydrogen diffuses into the brittle martensitic structure and causes embrittlement, significantly reducing the stress required to initiate a crack.

• While hydrogen is a necessary condition, the crack cannot form without the susceptible microstructure. The formation of martensite creates this vulnerable state. Therefore, in the context of high-strength steels, the transformation to martensite is the key metallurgical event that makes the material prone to this type of cracking.

• (B) Retained austenite: This is a softer, tougher phase. Its presence can sometimes be beneficial as it can transform to martensite under stress, absorbing energy and potentially arresting cracks. It is not a primary cause of cracking.

• (D) High sulfur content: This is associated with "hot cracking" or solidification cracking, which occurs at high temperatures, not cold cracking.

Step 3: Final Answer:
Cold cracking occurs when hydrogen embrittles a susceptible microstructure under stress. In high-strength steels, the rapid cooling of welding leads to the formation of hard, brittle martensite in the HAZ, which is the susceptible microstructure required for this type of cracking.