Crucible Cleaning and Preheating Practice

Charging a new crucible straight after it arrives is a problem that has occurred at many sites. Machining fluid, packaging debris and adsorbed moisture left on the blank surface release gas during heat-up, which at best affects vacuum quality and at worst produces bubbles or inclusions in the first melt.

Cleaning and preheating look simple, but they need fixed media, a fixed sequence and acceptance criteria; otherwise the results from different shifts vary widely.

How to Select the Cleaning Medium

After machining, the crucible surface may carry cutting fluid and metal chips. Remove visible residue with soft tools first, then scrub with an organic solvent, and finally rinse with deionized water or anhydrous ethanol. Chlorinated solvents should be avoided because chloride ions accelerate the corrosion of molybdenum at high temperature.

Long soaking in strong acids is not recommended. Pickling removes oxide tint but changes the surface morphology, and residual acid that is not rinsed away reacts with molybdenum during heating. If pickling is genuinely required, the concentration and time should be controlled, followed by neutralization and rinsing.

Drying and Checking for Residual Liquid

• After rinsing, dry with a lint-free cloth to avoid fiber residue;
• Dry in an oven at low temperature; the temperature should not be too high;
• After drying, visually inspect the inner wall for water marks and stains;
• Handle with clean gloves before assembly to reduce fingerprint contamination.

Water marks usually mean that residual liquid in that area has not fully evaporated. In a vacuum furnace this moisture is released slowly during pump-down and extends the pumping time. The criteria for judging residue are given in the cleanliness requirements of quality inspection.

Setting the Preheating Schedule

The purpose of preheating is to let the crucible complete its initial thermal expansion and surface gas desorption under low load. The usual practice is slow heating under vacuum or inert atmosphere, with a hold at an intermediate temperature, and continued heating to the working temperature once the vacuum level has stabilized.

The heating rate depends on the crucible wall thickness; thick-wall parts need a longer soak time. Preheating should not be carried out at full load: an empty or lightly loaded crucible releases gas more readily. If the crucible cannot be used immediately after preheating, it should be stored clean to avoid reabsorbing moisture. Preheating requirements for different sizes can be checked on the crucible specification page.

Handling Residual Material After Use

After a melt, residue and wall buildup should be removed promptly once the crucible has cooled to a safe temperature. Scraping with hard tools easily leaves scratches on the inner wall, and those scratches later become starting points for melt wetting and sticking. The residue can first be softened with a solvent compatible with the melt and then removed with a soft scraper. On furnaces running continuously, the cleaning time has to be coordinated with the shutdown rhythm so that the crucible is not left covered with residue for long periods.

After cleaning, check the inner wall for reaction layers, pitting and discoloration. Areas with a thick reaction layer can be locally dressed, but the wall thickness after dressing must be measured again and confirmed to be still within the permitted range before the crucible is returned to service.

Points to Watch on the First Melt

For the first run, a reduced charge and a shorter hold at high temperature are recommended, with observation of how the melt contacts the inner wall. After the first run, check the inner wall for abnormal discoloration, pitting or local sticking; this information reflects whether cleaning and preheating were adequate better than dimensional measurement alone.

Record the cleaning media, drying temperature, preheating curve and first-run results in the usage file so that they can be compared when the batch is changed.