Hydrogen-Induced Delayed Cracking in TRIP-Aided Lean-Alloyed Ferritic-Austenitic Stainless Steels
Susceptibility of three lean-alloyed ferritic-austenitic stainless steels to hydrogen-induced delayed cracking was examined, concentrating on internal hydrogen contained in the materials after production operations. The aim was to study the role of strain-induced austenite to martensite transformation in the delayed cracking susceptibility. According to the conducted deep drawing tests and constant load tensile testing, the studied materials seem not to be particularly susceptible to delayed cracking. Delayed cracks were only occasionally initiated in two of the materials at high local stress levels. However, if a delayed crack initiated in a highly stressed location, strain-induced martensite transformation decreased the crack arrest tendency of the austenite phase in a duplex microstructure. According to electron microscopy examination and electron backscattering diffraction analysis, the fracture mode was predominantly cleavage, and cracks propagated along the body-centered cubic (BCC) phases ferrite and α’-martensite. The BCC crystal structure enables fast diffusion of hydrogen to the crack tip area. No delayed cracking was observed in the stainless steel that had high austenite stability. Thus, it can be concluded that the presence of α’-martensite increases the hydrogen-induced cracking susceptibility.
Hydrogen-assisted cracking
Abstract This article reviews a number of case studies involving hydrogen-assisted cracking in steels. Before the case studies are presented, the current state of understanding of hydrogen-assisted cracking is briefly reviewed. At present there are two leading approaches to the development of a mechanism of hydrogen-assisted cracking, one based upon bond weakening, and the other upon strain localization. As the case studies illustrate, hydrogen-assisted cracking can lead to failures in perhaps unexpected circumstances. The need for the wider dissemination of information about hydrogen cracking to design engineers is indicated.
Hydrogen embrittlement of stainless steels by lithium hydride
Tests were made on 304L and 17-7 PH stainless steels in contact with LiH powder. Reduction in area relative to ductility in air decreased for both alloys. It was essential that the LiH be baked in contact with the alloys for the ductility loss to be observed; thermodynamic and kinetic evidence indicated that the LiH was reacting with surface oxides to furnish [H] to the steel. The depth to which hydrogen affected fracture surface morphology in 304L was greater than could be accounted for by diffusion, and it was concluded that dislocation transport accounted for the difference. Although direct evidence was not available, the locale of hydrogen damage in 304L was suggested to be the interface between the matrix and nonmetallic inclusions. A rationale based on this possibility was shown to be selfconsistent. The generality of the importance of [H] furnished by LiH reaction was illustrated by the results on 17-7 PH.