Riza Hirfan’s Blog

There are several factors to be considered in preparing workpieces for vacuum heat treating or brazing. Cleanliness of the workpieces and baskets or fixtures is very important.

They must be free of oil, dirt, machining lubricants or other contaminants prior to being loaded into the furnace. Some lubricants contain sulphur compounds which can adversely affect the alloys being heat treated. Inadequate cleaning can also cause staining and discoloring of the end product or result in poor braze alloy flow. Contaminants with high vapour pressures will evaporate during heating causing loss of vacuum. The vapours may eventually condense on colder surfaces in the furnace only to re-vaporize to cause contamination problems in subsequent runs.

It is good practice to check all workpieces for cleanliness prior to loading for vacuum processing. Tubular assemblies or parts with deep holes or recessed passages should be inspected for entrapped lubricants, machining chips or residual casting core debris. Particular attention should be paid to castings. In some cases, they are cleaned in molten salt baths to remove core residues. These corrosive salts may remain entrapped after removal from the salt bath. If not cleaned out prior to vacuum processing they will vaporize during heating. Workpieces should also be inspected for metal identification tags. Both the tag and the wire that is used to attach it to the workpiece should be checked to ensure it is not made from a low melting point material such as aluminum.

Selection of the cleaning process depends on the prior processing history of the workpiece and the alloy from which it is fabricated. For parts contaminated with stamping lubricants or cutting oils, it is important to know the type of fluid that was used. Performance of some cleaning processes may be adversely affected depending on the nature of the contaminant (ie. mineral- versus water-based oils). When the composition of the contaminant is unknown, solvent or vapour degreasing is usually the first choice for cleaning. However, these cleaners do not work well on water-based contaminants. In these cases, an alkaline or emulsion cleaner should be used. It should also be noted that water can cause the deterioration of many fluids used in vapour degreasing, creating an acidic solution that can attack both the workpiece and degreasing equipment.

Certain materials may require specific cleaning precautions. Titanium, zirconium and their alloys should never be cleaned in chlorinated solvents such as trichlorethylene or methylchloride. Chloride residues can cause stress-corrosion cracking in titanium and zirconium alloys when they are heated above 280ºC (550ºF). Instead, these alloys should be cleaned in non-chlorinated solvents such as acetone or alcohol or in alkaline solutions. Many deep drawing lubricants contain sulphur and lead. Both elements can attack nickel alloy surfaces during heating, causing the formation of low-melting point eutectics that will severely embrittle the base metal. Nickel alloy workpieces must be carefully cleaned to ensure complete removal of the lubricants. After cleaning, workpieces must be thoroughly dried before loading into the furnace. Solvents and water residues will volatilize during heat treating and degrade pumping performance.

VAC AERO engineers can provide expert advice on both heat treating processes and equipment.

Good fixturing and loading practices are essential elements in achieving proper heat treating results and long equipment life.

Fixture materials and design must be appropriate for the processing application.  Maintenance of fixtures is equally important.  The possibility of reactions between the workpieces and baskets or fixtures must also be considered.  High temperature sintering of the workpieces to themselves or the fixtures can occur.  Eutectic melting can also occur when certain chemical compositions come into contact at high temperature.  Selection of a fixture material is influenced by cost, service environment and compatibility with the workpiece and furnace hearth.

For service temperatures up to about 980ºC (1800ºF), austenitic stainless steels such as Types 304, 309 and 310 are commonly used.  However, these alloys can become embrittled from long exposure to temperatures in the range of 595ºC to 815ºC (1100ºF to 1500ºF) due to carbide precipitation and sigma formation.  In this temperature range, more expensive alloys such as 35Ni-15Cr or Inconel 600 tend to be more stable and may justify the extra cost. 

For higher temperatures, alloys like Haynes 230, nickel-base oxide-dispersed alloys such as MA 956 or pure molybdenum provide good life.  Care must be taken in handling fixtures made from molybdenum because of their embrittled condition after high temperature exposure.  For some special applications, graphite fixtures are used.  The strength of graphite increases with temperature, it has good thermal shock resistance and conductivity and it is relatively inexpensive.  These properties make graphite an excellent fixture material, particularly in the form of a plate to support large workpieces where flatness must be maintained.  However, because graphite reacts with many alloys at heat treating temperatures, the furnace hearth and workpieces should insulated from graphite fixtures during processing.

Multi-layer fixture for bottom-loading vacuum furnace	Fixture designed for vacuum hardening die parts

The primary objectives in designing a fixture are to minimize thermal mass (weight), achieve a long service life and minimize cost.  A low thermal mass is important for efficient heating and cooling.  Service life is largely dictated by the fixture material.  However, certain fixture materials, such as molybdenum, are not weldable and must be fabricated using mechanical fastening.  This can add to fabrication and maintenance costs.

When designing fixtures that clamp or restrain the workpiece (such as hot straightening fixtures or some brazing fixtures), careful consideration must be given to differences in thermal expansion between the workpiece and fixture.  When possible, the coefficient of thermal expansion of the fixture material should closely match that of the workpiece.

Before being used in production, it is recommended that a new fixture be subject to a vacuum bake at a temperature at least 25ºC (45ºF) higher than its maximum operating temperature.  This will degas the fixture to rid it of any contaminants formed in fabrication, such as welding oxides.  Fixtures may also require periodic grit blasting if they become discoloured during service.  Regular inspections for cracking, distortion or other thermal cycling damage should be performed.  To avoid catastrophic failures, damaged fixtures should be repaired or replaced immediately.

VAC AERO engineers can assist you in fixture material selection and design for vacuum processing.