by Joseph

Vacuum Heat Treatment of Fasteners

Fig. 1 – Typical Vacuum System – (Photograph Courtesy of Invensys Eurotherm)

Fastener applications are demanding. Whether fasteners are being used in the petrochemical industry, in medical or mining applications, for assembly of marine or nuclear components or in the aerospace, automotive or construction world, vacuum processing allows us to repeatedly achieve the highest quality and metallurgy.

Most fastener materials, including stainless steels and superalloy grades, benefit from or actually require vacuum processing for heat treatment instead of being run under protective atmospheres. In general, there are three main sets of applications that where vacuum heat treatment is used:

The absence of surface reactions or the ability to precisely control them is the main difference between vacuum heat treatment, and all other forms of heat treatment. Vacuum processing can also remove contaminants from parts, and in some instances, degas or convert oxides found on the materials surface.

Vacuum Systems

A vacuum system (Fig. 1) provides a space in which the pressure can be reduced and held below atmospheric pressure at all times. One of the primary advantages of vacuum heat treatment is its versatility. In addition to being self-contained, vacuum heat treatment provides a “safe” environment for the surface of the parts being treated and uses consistently reproduced cycles and recipes. When not in use, like an electric light, it is simply turned off saving energy. When turned back on, minimal conditioning time is required.

Vacuum Hardening

Vacuum processes are run in a variety of equipment (Fig. 2) designed to accommodate various workload sizes. Processes for vacuum hardening of fasteners will be discussed by type of material.

Fig. 2 – Typical Vacuum Furnace Shop – (a) Vertical Style Vacuum Furnace – (Photograph Courtesy of VAC AERO) (b) Horizontal Style Vacuum Furnaces – (Photograph Courtesy of Liburdi Engineering)

Hardening by Oil Quenching (Plain Carbon and Alloy Steels)

Oil quenching typically takes place in horizontal vacuum furnaces equipped with integral quench tanks (Fig. 3) as well as vertical vacuum furnaces (Fig. 4). The design of the quench tank is similar to its atmosphere counterpart; fixed or variable speed oil circulation via agitators or pumps located on one or both sides of the tank and internal baffles to guide the respective oil flow around and through the load. Cold or preheated oil, in the 50°C – 65°C (120°F – 150°F) range, are the most common and special (hot) oils, which run at 135°C – 175°C (275°F – 350°F), have been used with success. Heaters control the oil temperature and the oil is cooled via double wall construction or external heat exchangers usually employing air, for safety reasons.

Fig 3 – Two Chamber Oil Quench Vacuum Furnace – (Photograph Courtesy of SECO/WARWICK Corporation)	Fig 4 – Vertical Vacuum Furnace with Oil Quench – (Photograph Courtesy of Vac-Aero International)

Compared to normal quench oils, vacuum quench oils are distilled and fractionated to higher purity levels. This allows for a better surface appearance on quenched parts. It is common that the partial pressure of nitrogen above the quench oil be between 540 mbar (400 Torr) and 675 mbar (500 Torr). Usually, higher partial pressures above the quench oil can be advantageous in obtaining full hardness on both unalloyed or very low alloy materials. Low partial pressures above the quench oil produce higher hardness values, as well as lower distortion on parts consisting of medium or highly alloyed steels.

Medium alloy steels (Table 2) and most case hardening steels are hardened either by oil quenching or high pressure gas quenching (up to 20 bar).

Table 2 [2] – Quenchant Type for Select Alloy Steels.

Notes:
[a]    Austenitizing temperature in vacuum is often 15°C (25°F) – 30°C (50°F) higher than atmosphere processing.
[b]    Cooling nomenclature: OQ = oil quench; PQ = pressure quench.

Hardening by Gas Quenching (Alloy Steels)

The most popular method of quenching used for hardening in vacuum furnaces is inert gas pressure quenching used at pressures of 2 – 20 bar. Nitrogen and argon are the most common quenching gases. Cooling in argon produces the slowest heat transfer rates, followed by nitrogen, then helium and finally hydrogen. Nitrogen is the most attractive gas mixture from a cost perspective, however limitations exist with certain alloys (e.g. titanium). Theoretically, there is no limit to the improvement in cooling rate that can be obtained by increasing gas velocity and pressure. Practically, however, very high pressure and very high velocity systems are complex and costly to construct. In particular, the power required for gas recirculation increases faster than benefits accrue.

The trend today is to “dial in” the quench pressure, that is, use only the highest pressure required to properly transform the material. This has been made possible due to recent changes in both material chemistry and pressure quench design (e.g. alternating gas flows, directionally adjustable blades, variable speed drives). Gas quenching is now being used to produce full hardness in many materials that in the past have been traditionally oil quenched.

Martensitic Stainless Steels.

All grades of martensitic stainless steel fastener grades can be processed in vacuum furnaces. Austenitizing temperatures and general heat treatment considerations are similar to those used in atmosphere furnaces (Table 3). Since the austenitizing temperatures are usually below 1100°C (2000°F), vacuum levels in the range of 10-3 mbar (10-3 Torr) are very often used which result in clean and bright part surfaces. To avoid evaporation of certain alloying elements, vacuum levels in the range of 0.1 – 1.3 mbar (10-1 to 1 Torr) are required, resulting in some sacrifice of brightness.

Due to the differences in the hardenability of the various martensitic stainless alloys, there is a limitation on the section sizes that can be fully hardened by recirculated nitrogen gas quenching; other types of cooling gas (e.g. helium) can be used but the economic benefits must be carefully considered. The actual values of section size limits depend on the type of cooling system and the capability of the specific furnace employed.

Table 3 [3] – Typical Hardening Cycles for Select Martensitic Stainless Steels.

Notes:
[a]    Rapid heating rates can cause distortion and/or cracking. In vacuum heating rates of 8°C (15°F)/minute – 15°C (25°F)/minute are recommended for small parts or intricate shapes.
[b]    Certain parts will benefit from an initial preheat at the temperatures shown.
[c]    Cooling nomenclature: OQ = oil quench; PQ = pressure quench.
[d]    As quenched (oil) data shown.

Precipitation Hardening of Stainless Steels

Determining the heat treatment temperature for precipitation-hardened stainless steels (Fig. 5) depends on a number of factors such as the alloy grade, the type of parts being treated, and the required mechanical properties (Table 4). It is not uncommon, for multiple heat treatment steps to be specified. In other cases, material is purchased in the so-called Condition “A” requiring only an aging operation to be performed (this is typically not done in vacuum). For optimum creep and creep rupture properties, the high side of the solution annealing temperature range is typically used.  A low-end annealing temperature is used to obtain optimum strength during relatively short-term service at high temperatures. A final aging heat treatment produces a finely dispersed precipitate throughout the microstructure significantly increasing the room-temperature yield strength.

Table 4 [3] – Typical Solution Heat Treating and Aging Cycles for Select Precipitation Hardening Stainless Steels.

Notes:
[a]    Cooling nomenclature: WQ = water quench; OQ = oil quench; PQ = pressure quench; AQ = air cool.

Fig. 5 – Load of 15-5 PH Stainless Steel for Vacuum Annealing at 495°C (925°F) (Photograph Courtesy of Vac-Aero International)

Superalloys

Superalloys covers a wide range of materials, typically nickel, cobalt or iron based alloys and are generally intended for high temperature applications with most of them being hardened using a solution treating and aging process (Table 5). Solution treating involves heating the alloy to a temperature in the range of 980°C (1800°F) or higher, followed by gas quenching.  In most cases, gas quenching with nitrogen at a pressure of 2 bar or less is sufficient. Aging at an intermediate temperature for an extended period of time follows. In some cases, the complete solution treatment and aging cycles are programmed into the furnace instrumentation so that unloading is not required between cycles.  Certain superalloys, however, require other specialized treatments to develop required properties.

Table 5 [3] – Typical Solution Heat Treating and Aging Cycles for Select Wrought Superalloys.

Notes:
[a] Cooling nomenclature: FC = furnace cooling; AC = air cooling; RAC = rapid air cool; OQ = oil quench; PQ = pressure quench.
[b] Air cooling equivalent is defined as cooling at a rate not less than 22°C (40°F) per minute to 595°C (1100°F) and not less than 8°C (15°F) per minute from 595°C to 540°C (1100°F – 1000°F). Below 540°C (1000°F) any rate may be used.
[c] To provide adequate quenching after solution heat treatment, cool below 540°C (1000°F) rapidly enough to carbide precipitation. Oil or water quenching may be required on thick sections.

In Conclusion

Vacuum processing of fasteners is a highly repeatable process that will produce the best surface finish and metallurgy of all the heat treatment methods. While there is always a cost premium, its benefits with respect to metallurgy, properties and repeatability make it a technology to consider.

References