This study, conducted by D. R. G. Achar et al., examines the effects of dissolved nitrogen in ferritic steel weld metals on toughness and strain aging embrittlement. The study evaluated nitrogen content ranging from 80 to 210 ppm (wt) in C-Mn steel weld metals produced using the manual metal arc (MMA) method.

Research Methods:

Samples were tested under four different post-weld conditions:

As-welded (untreated),

Stress-relieved,

Artificially strain-aged,

Both artificially strain-aged and stress-relieved.

Microstructure and Microhardness: Quantitative metallography and low-load microhardness analyses were applied to assess microphases.

Findings:

Detrimental Effect of Nitrogen: High nitrogen content significantly reduced the toughness of C-Mn steel weld metals, especially under strain-aging conditions.

Microstructural Changes: A reduction in the amount of acicular ferrite was observed alongside an increase in primary ferrite and secondary phase ferrite content. A notable increase in the microhardness of acicular ferrite was also recorded.

Stress Relief Heat Treatment: For weld metals with low nitrogen content, stress-relieving only provided limited improvement, changing fracture behavior from brittle to ductile or semi-ductile.

Comparison of Test Results: Crack tip opening displacement (CTOD) test and Charpy test results showed similar trends in nitrogen's impact on toughness. However, the fracture mechanics approach yielded more conservative results.

This study highlights the detrimental effects of nitrogen content on the microstructure and mechanical properties of weld metals, emphasizing the importance of controlling nitrogen levels.

#ferriticsteel | #weldtoughness | #strainageing | #fracturemechanics | #nitrogeneffects

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