Sealing porous leaking castings by pressure impregnation involves the same process as pressure testing, but it uses a sealant as the media rather than compressed air or water, with the benefit that the leak is not just identified but also can be sealed at the same time. Pressure impregnation gives engineers an alternative to vacuum batch impregnation processes by being faster and achieving a higher success rate, and at a lower cost per part.
Pressure impregnation is not a new process and is detailed as an approved method in US MIL-STD-276 for the Impregnation of Porous Metal Castings and Powdered Metal Components, first published in 1956. The standard was updated in 1992 and is still current today, with thousands of castings having been pressure impregnated: in some cases, it is the only possible method for sealing a casting, for example when the part is too large to fit into an autoclave; or if it is in-situ, for example on an engine in a ship.
In contrast to vacuum batch processing, pressure impregnation requires identification of the leak area and the leak rate, which is vital information for the casting manufacturer too, and allows the impregnation process to be customized by applying a sealant of the right viscosity as well as the appropriate pressure to seal the porosity.
Consequently, a wider leak rate can be sealed by pressure impregnation than by other techniques. It is a single-stage process and has no cold-water wash or hot-water curing stages, both of which contribute to the loss of sealant from the porosity and a higher failure rate.
Pressure impregnation is carried out by applying the sealant directly to the area of the casting where it leaks and then applying pressure over the sealant to push it into the porosity. The process time is dependent on the size of the porosity, the type of sealant used, and the level of pressure applied. For example, a US-MIL-1-17563 porous test ring, which is the recognized test piece for qualifying impregnation sealants, can be fully penetrated in just a few seconds using modern low-viscosity sealants and a pressure of just 2 bar. Most castings can be sealed in under a minute. Pressure cycling can be used to achieve penetration of the sealant into very fine porosity.
The benefits of pressure impregnation are:
• Speed. Castings can be impregnated in <1 minute and pressure tested after 30 minutes.
• Simplicity. It can be a single-stage process with no need for cold-water washing and hot-water curing.
• Low CapEx. It requires minimal equipment and investment.
• Low OpEx. Significant savings derive from very low consumption rate for sealant.
• Reliability. There is no risk of sealant contamination in machined holes, oil ways, and water jackets.
• Adaptability. Partly assembled castings or in-situ castings can be impregnated without disassembly.
Contamination such as machining lubricants and dye penetrants can be purged from the porosity during pressure impregnation to ensure a high-quality seal is achieved; contaminants go unseen within the porosity, but can be the cause of poor impregnation and potential failure in service.
Room temperature-curing anaerobic sealants are the best option for pressure impregnation as they react with the metal, bond to the walls of the porosity, and will not exude out during curing. Also, any surface residue can be wiped off as the presence of air on the surface will prevent the sealant from curing.
Pressure impregnation can be carried out using a simple impregnation gun, which is mobile and allows the process to be taken to the part; or, the process can be automated and the equipment remain ready near a leak test machine for use during operation. There is no need to factor in the availability of a water supply, connection to the drain, steam extraction, additional process chemicals, or liquid waste.
Paul Young is the director of Impregnation Solutions Ltd. Contact him at [email protected], or visit www.impregnationsolutions.co.uk