Hydrogen-Embrittlement,High Risk for High Strength Steels.SOLUTIONS with Effective, Practical Advice
In this presentation the treatment of the matter is limited to steel and only to two operations that should concern those involved in fabrication of steel parts or structures. (Sponsored Links) When working with steel at elevated hardness levels, one must avoid the presence of hydrogen during welding, and one must remove hydrogen absorbed during electrocleaning or electroplating operations, because of the dangers of Hydrogen-Embrittlement. This matter was briefly considered in our page on Alloy Steel Welding and also in several issues of Practical Welding Letter when appropriate. The old assumption held that hydrogen, present in solution in the metal in atomic form, would precipitate at internal locations as molecular hydrogen, building up pressure and producing cracks. Now it is agreed that this concept, while interpreting the general idea of Hydrogen-Embrittlement, cannot explain all the experimental facts. Hydrogen-Embrittlement is addressed here in more complete form but only from a practical standpoint. The reason is that, although having been researched for a very long time, there is no agreement about the mechanisms operating the damage, and the explanations are too complex to be summarized briefly. A few facts should be recalled. Hydrogen Induced Cracking (HIC) is associated with welds in low-alloy steels in presence of hydrogen, high tensile stress, susceptible microstructures and low temperatures. Hydrogen solubility in iron decreases drastically with temperature. It is much higher in molten than in solid iron. Absorbed hydrogen remains trapped during solidification, and its small atoms are highly mobile, drifting freely and concentrating where possible. In presence of minute amounts of hydrogen, in steels treated to a strength level up to 485 MPa or 75 ksi, longitudinal (underbead) cracks are observed in the heat affected zone (HAZ). At higher strength levels, above 830 MPa or 120 ksi, transverse cracks appear also in the weld metal. Hydrogen-Embrittlement is detected as a remarkable decrease in ductility by slow bend or by tensile tests of notched and unnotched specimens charged with hydrogen. Interestingly, however, impact tests on material removed between cracks would not reveal the ductility loss, the toughness remaining equivalent to that of specimens without hydrogen. By testing in tension the above specimens under a dead-weight loading, the time to fracture can be recorded, demonstrating the delayed action. The precautions to apply, to avoid the effects of Hydrogen-Embrittlement, require constant attention to clean and dry the steel before welding by removing rust, dirt, grease and moisture around the welding area, to use dry consumables and low hydrogen electrodes. For each high strength low alloy steel, preheat schedules and specified interpass temperature must be observed, and baking treatments should be used where applicable to let the absorbed hydrogen leak away. Limiting the amount of residual stresses will also improve the chances of avoiding cracking, as a minimum stress and longer times are required to initiate cracks. As for processes requiring the immersion of steels that are harder than about 35-40 RC, in electrolytes (for electrocleaning or electroplating), the standard procedures require definite recommendations. Before plating, depending on the steel and its hardness and on the plated material and thickness, mechanical cleaning is preferred, followed sometimes by stress relieving of surface stresses following machining or grinding. Hydrogen absorbed during pickling and cathodic cleaning and before plating must be removed by suitable baking. Parts subjected to fatigue shall be shot peened after baking and before plating of hard chrome. As soon as possible after plating one should start a new baking cycle at definite temperatures for sufficient time to allow for the diffusion of hydrogen out of the steel. Although a complete theory of Hydrogen-Embrittlement is still missing, as noticed above, it is believed that a few different mechanisms potentially describe some of the phenomena associated with the manifestation of hydrogen induced cracking. An Article on Preventing Consumables from being Hydrogen Sources was published (4) in Issue 78 of Practical Welding Letter for February 2010. Click on PWL#078 to see it. Readers wishing to delve deeper into the theory can see the publication: Any questions or comments or feedback? Write them down and send them to us by e-mail. Click on the Contact Us button in the NavBar at top left of every page.
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