Weldment Reliability of High Strength Steel

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Weldment Reliability of High Strength Steel

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ABSTRACT

The development of methods to control the diffusible hydrogen content during high strength steel welding was performed. The perfection of the use of irreversible weld metal hydrogen traps was demonstrated. Efforts have effectively alleviated the available diffusible hydrogen, and thus the susceptibility of hydrogen assisted cracking, by the use of ferro-yttrium additions to the welding consumable. The range of welding parameter for the most effective use of yttrium as a hydrogen getter was determined.

The investigation of the influence of retained austenite on cracking susceptibility was also investigated. Issues regarding the amount of acceptable retained austenite and its tendency to release hydrogen upon changes of service temperature and stress were evaluated.

The development of an advanced measuring apparatus for diffusible hydrogen content based on electronic, optical, and magnetic property measurements was investigated. The effort is searching for a rapid and accurate determination for both diffusible hydrogen content and distribution. This investigation used the instrumentation acquired during the DURIP grant program. Optoelectronic property measurements of color shifts in W0 3 , which was attached to the steel in the form of a thin film detector, have successfully measured the diffusible hydrogen content of higher strength steel hydrogen contents. Preliminary results have also illustrated that both magnetic property and thermo electric (Seebeck) coefficient measurements can also be used to assess hydrogen issues in steel. The Seebeck coefficient appears to offer the most convenient and rapid determinations of diffusible hydrogen content.

1.0 ABSTRACT. 1
2.0 TABLE OF CONTENTS. 2
3.0 INTRODUCTION. 3
4.0 METHODS TO MINIMIZE HYDROGEN DAMAGE BY HYDROGEN MANAGEMENT. 3
4.1 Fundamental Aspects of Hydrogen Trapping in Steel Weld Metal. 3
4.2 Rare Earth Metal Additions to the Weld Metal. 14
4.3 Influence of Welding Parameters on the Hydrogen Trapping Effectiveness of Rare Earth Containing Welding Consumables (In Progress). 22
4.4 Role of Retained Austenite in Hydrogen Management of High Strength Steel . 23
4.5 Accomplishments .27
5.0 ADVANCED METHOD TO MEASURE DIFFUSIBLE HYDROGEN AND HYDROGEN DISTRIBUTIONS . 28
5.1 Present Welding Industrial Practice. 28
5.2 Present Investigation and Accomplishments. 28
5.3 Design Requirements for a Rapid Weld Hydrogen Analysis. 38
5.4 Assessment of the State of Hydrogen Development . 39
6.0 REFERENCES. 40
7.0 PUBLICATIONS, THESIS, PATENTS, AND HONORS RESULTING FOR THIS ARO CONTRACT. 43
8.0 LIST OF PARTICAPATING SCIENTIFIC PERSONNEL. 44

High strength low alloy (HSLA) steels are known to be susceptible to hydrogen cracking. As the strength increases, so does the risk of hydrogen assisted cracking (HAC) after welding. The hydrogen assisted cracking in HSLA steel welds is considered to take place when all the necessary conditions for cracking are satisfied simultaneously. These conditions include the combination of unacceptable diffusible hydrogen content, high restraint tensile stress, high hardness or a susceptible microstructure, and a temperature ranging between -50 and 100 °C (1, 2, 3). The main goal to make a HAC resisting high strength steel weldment is to reduce the amount of diffusible hydrogen. Common practice to reduce hydrogen cracking in high strength steel welding is the pre- or post- weld heat treatment. This practice is cost intensive and, in some critical cases, not effective. Alternative methods based on metallurgical principles have been studied both for technological and economic merits.

Irreversible weld metal hydrogen traps have been demonstrated to be effective in managing diffusible hydrogen content, and thus the susceptibility for hydrogen-assisted cracking. Ferroyttrium additions to the weld pool have significantly reduced the diffusible hydrogen content. The effort to fully characterize the influence of the welding parameters on yttrium transferability across the welding arc and on hydrogen trapping behavior in the weld deposition was performed. This information is necessary for a promising technological transfer from this R&D approach to the practical development of welding consumables.

The efforts to characterize and model the role of retained austenite in the higher strength steel weld metal in storing and supplying diffusible hydrogen, as well as to assess the associated issue of mechanical integrity was performed.

The successful efforts in developing a diffusible hydrogen sensor that increased the measuring efficiency by reducing test time and by allowing diffusible hydrogen testing were performed on the steel welds. Electronic property measurement techniques have shown that even further advances can be made for both rapid diffusible hydrogen content and distribution determinations associated with steel weldments. The Seebeck coefficient has been correlated to the diffusible hydrogen content in materials, allowing for very portable, but accurate, hydrogen measurements to be made on fabricated steel structures. The electronic property measurement equipment is now in place, resulting from our recent DURIP instrumentation grant received from the U.S. Department of Defense.