When is helium and hydrogen used as a partial-pressure atmosphere, and can they be used for quenching? Will using these gases result in cycle-time savings?
The above question can logically be broken into two parts:
1. Hydrogen/Helium as a process gas
2. Hydrogen/Helium as a quenchant gas
From my experience:
… with respect to partial pressure
The addition of small quantities of gas into a vacuum furnace to create a partial pressure is fairly common. During this process, the furnace is still being pumped typically through a bypass circuit while a controlled flow of gas into the furnace holds the pressure relatively constant – the range of 500–5,000 microns (0.67-6.67 mbar) being common for most applications. Partial pressure is used to prevent elements from vaporization at elevated temperature.
1a) Due to cost, helium is only used as a partial-pressure gas in specialized applications, such as those in the electronics industry. Argon, which is also totally inert, is often substituted.
1b) Hydrogen, on the other hand, is commonly used as a partial-pressure gas in many applications, including brazing (oxide reduction), hydriding (titanium, tantalum), sintering and even during the heat-up portion of the cycle in low-pressure (vacuum) carburizing. Another common use of hydrogen is for clean-up cycles involving either the furnace or the baskets/grids/fixtures. The principal limitation is safety and a number of modifications to the equipment are required to meet (or exceed) NFPA requirements (c.f. Part Two). Running a partial pressure during a heat-treating process has little effect on the overall cycle time.
1c) Several companies have used/are using hydrogen in an over-pressure (+0.5 psig) situation, not for heating but for processing (powder metal), primarily to aid in lubricant removal. This is not very common, however, and requires all the proper safeties to be in place. Convection heating using nitrogen is extremely effective in reducing heat-up/cycle time as well as being highly cost effective, negating any advantages that hydrogen might bring.
… with respect to quenching
2a) Helium as a quenching gas is used by a number of companies throughout the industry in the range of 2-20 bar to increase the cooling rate of a load during quenching. In some steels, this allows proper transformation to take place. In others, such as very large, thick parts or very dense loads, it allows for shorter cycle times. Typically, a recycling system is required, and this is one of the factors that causes the cost of helium quenching to rise.
2b) Hydrogen quenching has been investigated by a number of R&D groups over the years and is an active development topic. This is especially true today as equipment design has become a limiting factor in quenching. Three companies I know of that have/had programs in this field are ALD Vacuum Technologies GmbH (Germany), Solar Atmospheres (U.S.) and SECO WARWICK, S.A. (Poland). It has been reported that a 25 psig hydrogen-quench furnace in a single-chamber vacuum furnace is capable of producing results similar to those obtained by water quenching. Of course, your results should be checked by actual testing.