Brittle Fracture of Welded Joints of Ferritic Stainless Steel
Ferritic stainless steel is a type of stainless steel which contains a high percentage of iron and chromium. It is also known as ‘409’ grade stainless steel and has a unique set of mechanical and physical properties. Ferritic stainless steel is used in many industrial applications and is becoming increasingly popular due to its excellent corrosion resistance properties and potentially low cost.
Due to the high iron content, ferritic stainless steel has the tendency to be susceptible to brittle fracture failure under certain conditions, especially when welded. Such failure can sometimes occur at a lower stress level than what is generally required for ductile fracture and can sometimes look like a fatigue crack. The likely causes of brittle fracture are associated with the welded joint. These include the high temperature of welding process, increase of the residual stress in the welded joint and decrease of toughness at the weld joint due to the decreases of carbon and other alloying elements.
At temperatures below 0 °C, ferritic stainless steel weld joint are particularly vulnerable to brittle fracture due to a decrease in toughness of the weld joint at these temperatures. Low toughness will result in drops of the fracture toughness, especially in the region near the welded joint, resulting in a decreased resistance to brittle fracture. The stress concentration or any other form of stress concentration, such as weld root or shape and size of the weld joint, could also cause brittle fracture.
Welding with ferritic stainless steel requires careful attention to the welding process parameters and joint design. Some of the necessary measures include the use of proper post weld heat treatment and the reduction of weld joint stress by minimizing peak weld temperatures and the use of pre-heat tempering to reduce the hardness on the beam of the weld joint.
When welding with ferritic stainless steel and working at temperatures lower than 0 °C, attention to joint design is also important. This includes adequate weld preparation, correctly sized and shaped welds and eliminating any potential stress raisers, such as sharp corners, bead size, and weld root and neck.
To minimize the risk of brittle fracture failure in ferritic stainless steel weld joint, it is important to follow the appropriate manufacturing and welding techniques and to use the right type of filler materials. Proper preheat, interpass temperature control and post weld heat treatment should be employed to reduce the weld joint stress. It is also important to avoid any form of overloading, use of incorrect filler materials, or neglecting proper weld preparation.
In conclusion, ferritic stainless steel welded joints are highly susceptible to brittle fracture failure due to its high iron content. At temperatures below 0°C, the weld joint is further weakened by the decrease in toughness of the weld joint. Welding with ferritic stainless steel requires special attention to correct welding and joint design procedures to prevent such failure. Fabricators and engineers should use proper welding process parameters, proper preheat and interpass temperature control, and post welding heat treatment for ferritic stainless steel weld joints. This will ensure that the weld joint is strong enough and resistant to brittle fracture failure.
