By Dan Herring

Low temperature vacuum heat treatment offers unique advantages to a variety of industries including Aerospace, Automotive, Electronics, Household Appliances, Machine Tools and Tool and Die as well as Commercial Heat Treaters who must serve all of these customer.

This article will discuss the key factors required to optimize processing parameters in low temperature heat treatments (e.g. tempering, stress relief) by focusing on the following topics:

Figure 1 – Typical Vacuum Tempering Furnace (Photograph Courtesy of C. I. Hayes)

How They Operate

Low temperature processing can be batch or continuous, either as stand-alone units or “modules” incorporated into a continuous vacuum furnace system. The following is a basic description of the operation of a typical batch vacuum furnace (Fig. 1).

Once a workload has been positioned into the unit, an outer door is closed and the vacuum process can commence. A mechanical vacuum pump, optionally equipped with a blower, produces a vacuum level as low as 1.3 x 10^-3 mbar (1 x 10^-3 torr), a common vacuum level being under 1.3 x 10^-1 mbar (1 x 10^-1 torr). This is normally achieved in 10 – 20 minutes depending on the size of the pumping system and the nature of any contamination present on the workload. In some instances a double pumpdown sequence is initiated once an initial vacuum level lower than 6.7 x 10^-1 mbar (5 x 10^-1 torr) is reached.

Once the desired final vacuum level is reached, the unit is backfilled in the range of 667 mbar (500 torr) to 3.4 x 10^-2 bar (0.5 psig) positive pressure with an inert gas such as nitrogen, argon or nitrogen/hydrogen (3% maximum) and heating begins. After reaching setpoint and soaking at temperature, a cooling cycle is initiated, typically with hot gases circulated through an internal or external heat exchanger to accelerate the process.

Design Features and Performance

Table 1 – Field Data on Temperature Uniformity in a Vacuum Tempering Furnace

Heating elements are typically positioned either on the sides of the workload or in a 360º pattern around the workload and designed to allow for rapid heating (Fig. 2) and to achieve tight temperature uniformity (Table 1). A circulating fan is provided to aid in uniformity and provide for even distribution of process gas through the workload.

The materials of construction of the heating chamber are typically such that the furnace can be opened and unloaded hot at any required temperature. In most cases however, surface condition is important and the workload must be cooled to at least 150°C (300°F) before unloading and more commonly to below 65°C (150°F) as measured by a thermocouple positioned in the workload itself. This makes the speed of cooling (Fig. 3) equality important.

Common workload sizes range from 0.05 m3 – 0.35 m³ (2 – 12 ft³) for smaller units with payloads up to 550 kg (1200 lbs.) to larger units with workload sizes in the 1.0 – 1.5 m³ (36 – 54ft³) and capacities in the 450 – 2500 kg (1000 – 5500 lbs.) range. Typical workloads for these sizes are between 150 – 400 kg (300 – 900 lbs.) and 680 – 1360 kg (1500 – 3000 lbs.).

Positive pressure capability, up to 1.37 x 10^-1 bar (2 psig) during both heating and cooling is possible with 3.4 x 10^-2 bar (0.5 psig) being the most common positive pressure used.

Figure 2 – Typical Heating Rate Performance

Applications

Applications vary widely as do materials treated and process temperatures used. A survey of vacuum tempering users provided the following insights.

The most common processing temperatures can be expressed as

Minimum and Maximum Temperature. Most units being manufactured today have operational ranges from 120°C (250°F) to 800°C (1450°F). Special applications extended this temperature range to as high as 925°C (1700°F), but this was unusual.

Most Common Processing Temperatures. Most processing temperatures were reported to be in the range of 175°C – 730°C (350°F – 1350°F). with the most common processing range being from 315°C (600°F) to 650°C (1200°F).

The most common low temperature vacuum processes include:

Figure 3 – Typical Cooling Rate Performance

Typical materials may be categorized as, but not limited to:

Typical markets served included:

Parts processed varied widely and included such items as:

Why Vacuum?

Low temperature vacuum processing is often referred to generically as “vacuum tempering”, places the component parts in a controlled heat treat environment designed to optimize part quality. Surface condition, productivity, choice of atmosphere, and absolute control of process and equipment variability are superior to alternative technologies. Cost, both initial and operational, is often offset by reduction in scrap rates, elimination of work in process inventories and lower energy consumption.

Low temperature vacuum processing is synonymous with these key features:

Quality Assurance. Vacuum processing guarantees quality standards will be achieved, especially as they relate to surface condition; process repeatability; control, the ability to predict results, monitoring of key variables in real time, and automated cycles. For example, vacuum heat treatment of tool steels provides positive process adherence, a “two plus two” temper achieves part cooling below 65ºC (150ºF) before initiation of the second heating cycle.

“Clean and Bright” Work. In all cases, the surface condition of a component part is improved. Since “clean” and “bright” are subjective terms and difficult to define, we say instead that the part surface will not be metallurgically damaged and if a change occurs it is, generally speaking, a positive one.
Often parts are contaminated with oils from prior operations, such as stamping, or have water and/or alkaline wash compound residues, including marking dyes, shop dirt and other prior processing related contaminants. By the very nature of heat-treating in vacuum, these surface contaminants are removed. In some instances, oxide reduction can take place but in general vacuum levels are too high and temperature too low for this to be effective.

Environmentally Friendly Processing. Done properly, vacuum processing should have no impact on the environment. The only source of pollutants is the discharge from the vacuum pumping system, which meets current OSHA and EPA air quality and environmental standards. The byproducts should be water vapor and carbon dioxide.  It is often recommended that the vacuum pump exhaust be piped outside the plant, although in some instances this is not necessary. If discharge inside the factory is undesirable, special filters called oil-mist eliminators can be provided – these are always a good idea to include on any pumping system.

The problem with most non-vacuum processes related to the contaminants present on the parts. Burning oils off in a furnace or oven running in air or under an inert atmosphere and letting these fumes discharge either inside or outside the plant is no longer acceptable. Filters or afterburners to handle these discharges add cost to these systems. Thus cleaning prior to low temperature processing is mandatory for both vacuum and air or atmosphere systems.

Safety. Vacuum systems are self-contained and present minimal safety hazards to operators or plant personnel. There are seldom concerns over noise levels, pinch points, electrical hazards, discharge of heat into the room, and there is no “burnoff” or flames as with non-vacuum equipment.

Lean Manufacturing. Vacuum furnaces are ideal for manufacturing cells and fit well into today’s lean manufacturing environments. Vacuum temper furnaces can be provided either with hot or cold wall construction and combined with their quiet and pollution free operation makes incorporation onto the shop floor and integration into a manufacturing cell environment both practical and realistic.

Figure 4 – Typical Energy Consumption

Summary

Low temperature vacuum tempering is becoming increasingly popular. The desires to avoid “post-mortem” quality methods as well as to assure process repeatability are key motivations. Designs capable of meeting these needs are available and perform well.

The key considerations in Low temperature vacuum processing can be prioritized as follows:

1. Vacuum heat-treating is considered mandatory for parts that must be processed without surface damage such as oxidation. Parts at all stages of manufacturing, not just finished parts will benefit from this treatment.
2. High productivity and energy efficiency (Fig. 4) requirements are best achieved by vacuum processing. The ability to heat slightly more rapidly and especially cool much more rapidly in a positive pressure greatly reduces cycle time. Uniformity of heating and cooling add significant benefits.
3. Minimum atmosphere consumption is achieved during vacuum processing. Once a backfill gas has been introduced into the chamber only small amounts of makeup gas is needed (typically during cooling as the gas contracts), Even in cases where gas flushing is required, far less atmosphere is used. Gas savings of as much as 10:1 have been reported.
4. Process control is absolute including the ability to upload and download information in real time, including in many cased both process and equipment variables. This allows a more complete history to be maintained either for quality assurance or to allow planned preventative maintenance to occur.

Daniel H. Herring / Tel: (630) 834-3017) /E-mail: dherring@heat-treat-doctor.com

Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.