The successful utilization of a vacuum heat furnace hangs on several important factors, most importantly of which would be the integrity of this vacant space. Contingent on the purity of the vacuum, the efficacy of the equipment seal, and the condition of the integrated quenching tank, furnace downtime is a manageable productivity obstacle. With that said, let’s sound out the quenching section. Is that a crack?
Quench Tank Micro-Fractures
Heavy heat loads torture this fluid station. Oil or water, that liquid absorbs thermal energy. The metal assembly containing the quench medium then acts as a heat sink. It distributes this energy, but material conduction effects are combining with the circulating hotness in the quench pool. The result is component expansion, plus a parts-fatiguing contraction effect. Micro-fractures form in the vacuum heat furnace. They propagate, then water seeps into the cracks, widening them. Seal continuity fails as component corrosion impacts the area.
Seal Continuity Defects
This complex piece of equipment operates most effectively when its vacuum is fully intact. There are no unpredictable air currents disturbing the heat treatment cycle, so the treated workpiece is processed as a distortion-free, contaminant-less product, one that satisfies the most stringent processing guidelines. However, seal flaws allow air and moisture into the void. Hot spots form, the operation does not yield a high-quality resolution, and the equipment pumps fail to pump down. If the leak continues, the vacuum won’t form, so the pumping stage operates continually. Like falling dominoes, that one near invisible leak undermines the void, allows water into the heat treatment enclosure, and causes the pumps to age then fail. Furnace downtime is an inevitability when a damaged seal fails. Check that seal with a helium leak detector.
Weighing Extraneous Causative Factors
Depending on unique design influences, certain vacuum heat furnaces can incur undesirable periods of system downtime when a subsystem component fails. For example, jacketed cooling systems are often employed in these equipment stations. They add an element of augmented thermal control to the intricate architecture of a vacuum heat treatment furnace. That cooling jacket must be maintained. If it’s compromised, perhaps by dirt or corrosion, the process is jeopardized. Gasket damage, thermocouple failure, instrumentation miscalibration, any one of these peripheral system flaws can and will incur equipment downtime.
It’s a fast-paced site, this vacuum-enabled heat treatment facility. Downtime is the one event that simply cannot be allowed here, not when workpiece integrity is the most important attribute invested by the vacuum heat furnace. Helium leak detectors are all very efficient, but this test must be conducted cold, which means more lost processing time. Check for seal nicks and gasket tears before they leak, and avoid that expensive work suspension period.
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.