What is De-Embrittlement in Heat Treatment?17 January 2018
Metal embrittlement occurs during numerous heat treatment processes. Furthermore, certain electrochemical techniques are known to promote this undesirable effect. The result is a finished metal product that’s drastically weakened. Fracture-prone and flawed, the embedded microcrystalline defects cast a blemish-ridden shadow on what should be a universally toughened alloy part. If there’s an increased likelihood of embrittlement failure, what de-embrittlement operations can we call upon from our heat treatment arsenal?
Favoured De-Embrittlement Procedures
A steel core that’s laden with fracturable microcrystalline intersections equals a potential disaster. Brittle fasteners snap when this issue is neglected. The same goes for a structural steel beam, although the chance of a catastrophe is greatly increased at this point. There must be a way to counteract these tiny structural flaws, one that addresses the fault by calling upon an existing equipment station. Known as de-embrittlement, the process is applied as a carefully controlled bake operation. In effect, the flawed metal part is heated in a furnace. Raised to between 200°C and 220°C, this thermal peak is maintained for approximately 8 hours. Of course, dependent on the type of hardened steel held in the stress relief bake, that interval can increase significantly. Bake relief cycles can last as long as twenty-two hours because of this metallurgically-sensitive proviso.
Why Is The Steel Part Brittle?
Electroplating processes cause a phenomenon known as hydrogen embrittlement. The simple molecules accumulate at the plating terminal, they’re diffused into the metal, and the formerly hardened metal is transformed into a rigid but non-deformable mass. If stress is applied to that rigid workpiece, it fractures. Acid cleaning procedures are also known hydrogen generators, as are several other cleaning and electrochemical pre-treatment operations. If that acid pickling or electroplating stage must be applied, it should be intelligently managed so that the embrittlement effect is minimized. For example, a mechanically induced cleaning agent, such as sand, won’t produce a hydrogen byproduct, which is a smart move because the excess hydrogen facilitates material embrittlement.
In conclusion, if a de-embrittlement phase is added to the heat treatment operation, more time in the furnace is on the books. That time is spent subjecting the steel workpiece to a stress relieving bake, a thermally active cycle that skirts between 200°C and 220°C. Held at this temperature for a predetermined length of time, the mandatory bake ejects the fracturable structure. Incidentally, this is a time-sensitive process. It must be conducted within 3 hours of the original causative incident, be it a coating stage or a chemical cleaning procedure.
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