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Technology Trends in Vacuum Part Two: Processes and Applications

Daniel H. Herring – The HERRING GROUP, Inc., Elmhurst, Ill. Vacuum thermal processing is key for aerospace component manufacturers due to industry demands for the highest possible quality. and surface treatment are two process areas where vacuum technology is utilized by the aerospace industry.

art two of this series contin- ues our discussion of vacuum heat-treating processes and P applications. Brazing The volume of work produced by vacuum brazing far exceeds that of any other pro- cess in which vacuum furnaces are being utilized. The transportation (automotive and aerospace) industry has provided the impetus for the increasing use of vacuum furnaces for brazing, and the use of light- weight, high-strength materials have also contributed to the popularity of brazing. As with all brazing, variables that need to be controlled to produce mechanically sound braze joints include base- selec- Fig. 1a. Jet engine turbine brazed in a horizontal vacuum furnace tion and characteristics, fi ller-metal selection and characteristics, component design, joint design and clearance, surface preparation, fi ller-metal fl ow characteristics, temperature and time, and rate and source of heating. Some factors affect the ability to pro- duce a metallurgically sound braze joint by infl uencing the behavior of the brazed joint, while others affect the base-metal properties, while still others infl uence the interactions between the base metal and fi ller metal. Effects on the base metal include precipitation, embrittlement, the nature of the heat- affected zone, stability and embrittlement. Filler-metal effects include vapor pressure, alloying, phosphorous em- brittlement and stress cracking. Interac- tion effects include post-brazing thermal Fig. 1b. Various aerospace components brazed in a bottom-loading vacuum furnace

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Fig. 3. Family of aerospace, automotive and industrial components illustrating the Fig. 2. Typical honeycomb seals versatility of low-pressure vacuum treatments, corrosion resistance and dis- -base brazing alloys are used for the furnace is fi rst evacuated to a low pressure similar-metal combinations. fi ller metal. Generally, alloys that contain 10-2 to 10-4 mbar (10-2 to 10-4 torr) to re- Vacuum furnaces can be either hori- easily vaporized elements for lowering the move residual air. The temperature is then zontal or vertical in design (Figs. 1a, 1b) melting points are avoided. With respect raised to approximately 955°C (1750°F) to and have technical advantages including: to the heat treatment of , the allow outgassing and to remove any sur- • The process permits brazing of com- and the -base brazing alloys are the face contamination. Finally, the furnace plex, dense assemblies with blind pas- most widely used fi ller . is heated to brazing temperature normally sages that would be almost impossible 1100–1120°C (2000–2050°F) under a to braze and adequately clean using at- Aluminum and Aluminum Alloys partial pressure of inert up to 1 mbar mospheric fl ux-brazing techniques. In the brazing of aluminum components, it (7.5x10-1 torr) to inhibit evaporation of • Vacuum furnaces operating at 10-5 to 10-4 is important that vacuum levels be main- the copper. When brazing is completed, mbar (10-5 to 10-4 torr) remove essential- tained in the 10-5 mbar (10-5 torr) range usually within minutes after the set-point ly all that could inhibit the fl ow of or better. Parts are heated to 575–590°C temperature has been reached, the work brazing , prevent the development (1070–1100°F) depending on the alloy. is allowed to slow cool to approximately of tenacious oxide fi lms and promote the Temperature uniformity is critical, typi- 980°C (1800°F) so that the fi ller metal wetting and fl ow of the braze alloy over cally ±5.5°C (±10°F) or better, and multi- will solidify. Parts can then be rapidly the vacuum-conditioned surfaces. ple-zone temperature-controlled furnaces cooled by gas , typically in the • Properly processed parts are unloaded are common. Cycle times are dependent range of 2 bar. in a clean and bright condition, often on furnace type, part confi guration and avoiding additional processing. part fi xturing. Longer cycles are required Nickel-Base Alloys • A wide variety of materials ranging for large parts and very dense loads. Brazing with nickel-base alloys is usually from aluminum, cast , stainless done without any partial pressure at the steel, , alloys, nickel al- Copper and Copper Alloys vacuum levels in the range of 10-3 to 10-5 loys and -base superalloys are Copper fi ller metal applied to the base mbar (10-3 to 10-5 torr). Normally, a pre- brazed successfully in vacuum furnaces metal either as paste, foil, clad or solid heat soak at 920–980°C (1700–1800°F) without the use of any fl ux. copper can be vacuum brazed recognizing is used to ensure that large workloads that the high vapor pressure of copper at are uniformly heated. After brazing, the Many different types of nickel, nick- its causes some evaporation furnace temperature can be lowered for el-base, copper, copper-base, -base, and undesirable contamination of the fur- additional solution or heat -base, aluminum-base and some nace internals. To prevent this action, the treatments before gas cooling and un- loading. Table 1. Honeycomb Materials Component AMS Specifi cation Major Constituents Aerospace Brazing Process Example Honeycomb 5536 5878 Ni, Cr, Co, Mo A honeycomb seal (Fig. 2) is a jet engine component designed to in- Rings and Details 5662 5596 5706 Ni, Cr, Mo, Cb, Ti crease engine effi ciency by surround- 4777 4779 4782 Ni, Si, Cr, B ing the airfoil or turbine and

58 November 2008 - IndustrialHeating.com Vacuum/Surface Treating

Fig. 4. Heavy-duty truck shafts low-pressure vacuum carburized Fig. 5. Automotive transmission gears low-pressure vacuum and oil quenched carburized and high-pressure gas quenched prevent airfl ow around the blade tips. Honeycomb seals are timate contact and achieve a sound joint during brazing. All di- made from a variety of nickel and cobalt superalloys (Table 1) mensions are set during the tacking process. Braze tolerances of designed to withstand the harsh service environments of jet en- 0.25–0.50 mm (0.010–0.020 inch) are common. Proper cleaning gine applications. is another critical pre-braze step. Every effort should be made to and tacking are the two most important steps in the ensure the part is clean and free of all , contaminants and pre-braze assembly process. Firm tack is necessary to ensure in- oils prior to braze preparation.

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60 November 2008 - IndustrialHeating.com The furnace cycle is equally as critical as part preparation to Case Hardening by and Nitrocarburizing the success of the brazing operation. Parts that ramp too fast are Plasma (ion) nitriding (Fig. 6) using pulsed power generators is at risk for distortion and uneven temperature throughout the as- an option to the traditional gas nitriding process. Nitriding is sembly. Parts that are not stabilized will not see proper braze fl ow. used in many applica- If the assembly quenches too rapidly, there is risk for distortion, tions for increased wear quench cracking of the braze joint and splatter. resistance and improved Brazing of these high-temperature nickel alloys is typically per- sliding friction as well formed at 1040–1200°C (1900–2200°F) in a vacuum level of 10-4 to as in components where 10-5 mbar (10-4 to 10-5 torr). Brazing is performed 40–65°C (100– increased load-bearing 150°F) above the braze-alloy melting point. capacity, fatigue strength Common problems include splatter of the braze alloy, quench and corrosion resistance cracking and distortion. All of these problems can be prevented are important. Corrosion by controlling cleanliness of the part, using proper setup tech- resistance can be especial- nique, designing a proper brazing recipe and operating the fur- ly enhanced by a plasma nace properly. Multiple re-brazes can be performed using shorter post-oxidation treatment. brazing cycles at slightly higher temperatures. Stop-off paints such Dimensional changes are as alumina oxide (preferred) can be applied to reduce the risk of minimal, and the masking unwanted braze fl ow. process for selective nitrid- ing is simple and effective. Case Hardening by Carburizing and Plasma nitriding uses Low-pressure vacuum carburizing (LPC) or low-pressure vacuum gas at low pres- Fig. 6. Plasma nitriding of automo- carbonitriding (LPCN) combined with either high-pressure gas sures in the range of 1–10 tive crankshafts (Photograph Cour- quenching (HPGQ) or oil quenching (OQ) has become increas- mbar (0.75–7.5 torr) as tesy of Surface ) ingly popular over the past decade, with industries such as aero- space, automotive and commercial heat treating spearheading the use of this technology. It is generally agreed that low pressure can be defi ned as car- burizing less than 27 mbar (20 torr), typically at temperatures from 830–980°C (1525–1800°F) for carburizing and 800–900°C (1475–1650°F) for carbonitriding. In the past several years, higher carburizing temperatures – up to 1200°C (2200°F) in several in- stances – have been used for certain advanced materials.[1] There is also growing interest in Cr-Mn steels as alternatives to the more expensive conventional alloy grades.

Popular hydrocarbons include (C2H2) and acety- lene mixtures such as [acetylene + hydrogen] and [acetylene + ethylene (C2H4) + hydrogen] and cyclohexane (C6H12), a . The use of propane and methane, while still popular, is declining. can be added to the selected hydrocarbon gas usually during the steps for carbonitriding. Process control is achieved through the use of simulation pro- grams applied to the respective kinetic and diffusion models to determine the boost and diffuse times for a given case depth. transfer rates are now well established for a given tem- perature, gas type, gas pressure and fl ow rate. Material chemistry and surface area must be taken into consideration in these pro- grams, as well as initial and fi nal surface-carbon levels. Predic- tion of case depth and profi les are the most obvious out- put of these programs, with research continuing into prediction of microstructural results such as carbide size and distribution and retained levels.

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γ the source for nitrogen transfer. Above gamma prime ( ') (Fe4N) and the References ε 1000°C (1832°F), nitrogen becomes reac- nitrogen-rich epsilon ( ) phase (Fe2-3N). 1. Otto, Frederick J. and D. H. Herring, tive when an electric fi eld in the range of The temperature of the workpiece is “Advancements in Precision Carburiz- 300–1200 V is applied. The electrical fi eld another important control variable. The ing of New Aerospace and Motorsports is established in such a way that the work- depth of the diffusion layer also depends Materials,” Heat Treating Progress, May/ load is at the negative potential (cathode) strongly on the nitriding temperature, June 2007 and the furnace wall is at ground potential part uniformity and time. For a given (anode). The nitrogen transfer is caused temperature, the case depth increases ap- For more information: Dan Herring is presi- by the attraction of the positively charged proximately as the root of time. A dent of THE HERRING GROUP Inc., P.O. Box nitrogen ions to the cathode (workpieces) third process variable is the plasma power 884 Elmhurst, IL 60126; tel: 630-834-3017; fax: 630-834-3117; e-mail: heattreatdoctor@ with the ionization and excitation pro- or current density, which is a function of industrialheating.com; : www.heat-treat- cesses taking place in the glow discharge surface area and has an infl uence on the doctor.com. Dan’s Heat Treat Doctor columns near the cathode's surface. The rate of thickness of the compound layer. appear monthly in Industrial Heating, and he nitrogen transfer can be adjusted by dilut- Plasma nitrocarburizing is achieved by is also a research associate professor at the ing the nitrogen gas with hydrogen (above adding small amounts (1–3%) of methane Illinois Institute of Technology/Thermal Pro- 75%). The higher the nitrogen concentra- or gas to the nitrogen-hy- cessing Technology Center. tion, the thicker the compound layer. drogen gas mixture to produce a carbon- The compound layer consists of containing epsilon (ε) compound layer Additional related information may be found by searching for these (and other) and alloy that develop in the outer (Fe C N ). It is commonly used only for 2-3 X Y key words/terms via BNP Media SEARCH region of the diffusion layer after satura- unalloyed steels and cast irons. IH at www.industrialheating.com: vacuum tion with nitrogen. According to the iron- brazing, fi ler metal, , diffusion, nitrogen phase diagram, basically two iron Part Three of this series will talk about fu- retained austenite, low-pressure carbur- nitrides are possible – the nitrogen-poor ture trends in vacuum heat treating. izing, carbonitriding, plasma

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62 November 2008 - IndustrialHeating.com