Carbon fibre-reinforced carbons (C/C for short) are used in modern vacuum or protective gas furnaces in the form of heating elements or charging systems. They are characterised by thermal shock resistance, absence of distortion, low mass and strength increase with rising temperature. These features enable users to operate their plants more effectively, minimise reject rates and therefore reduce the cost of production. C/C is thus a key element in many process optimisation steps and helps companies to improve their competitiveness. by Alexander Racek, SGL CARBON GmbH

The heat treatment of landing gear is a complex operation requiring precise control of time, temperature, and carbon control. Understanding the interaction of quenching, racking, and distortion contributes to reduced distortion and residual stress. Arguably, landing gear has perhaps the most stringent requirements for performance. They must perform under severe loading con­ditions and in many different envi­ronments. They have complex shapes and thick sections. Alloys used in these applications must have high strengths between 260 to 300 ksi (1,792 to 2,068 MPa) and excellent fracture toughness (up to100 ksi in.1/2, or 110 MPa×m0.5). To achieve these design and per­formance goals, heat treatments have been developed to extract the optimum performance for these alloys. By D. Scott MacKenzie, Houghton International Inc. Valley Forge, PA

The hot zone is perhaps the most critical feature of a vacuum furnace in terms of its affect on furnace performance and operating cost. There are a variety of hot zone designs and the choice of a design should be based on a careful analysis of specific processing applications. Most vacuum-furnace hot zones consist of four major components: the heating elements and the details on which they are mounted; the insulation package (or heat shields); a surrounding structure that supports the heating elements and insulation package; and a hearth that supports the load during processing. By Jeff Pritchard - VAC AERO International Inc., Ontario, Canada

High strength steels based on the 43XX series are commonly used for aircraft landing gear components because of their desirable mechanical properties; these alloys are usually heat treated by quenching in oil from the hardening temperature, then tempered. During hardening, internal stresses created from the thermal shock of direct oil quenching almost always cause some degree of distortion in the heat treated component. Interrupted quenching techniques - such as ausbay quenching offer good potential for reducing quench distortion. However, due to the difficulty of machining these alloys in the hardened condition, machining allowances must be minimized. By Jeff Pritchard of VAC AERO International Inc., Oakville, Ontario, Canada.

Vacuum heat treatment offers an alternative method to traditional salt-bath and controlledatmosphere furnace hardening techniques for high strength steels, such as AISI 4340M and 300M. However, heat treaters must be prudent when choosing between oil and gas quenching for vacuum hardening of high strength steels, because each process has its advantages and disadvantages. High strength steel alloys, such as 4340M, 300M, and others, are most commonly used in the manufacture of landing gear components. These alloys are hardened and tempered to produce ultimate tensile strengths exceeding 280 ksi (1,930 MPa). By Jeff Pritchard and Scott Rush - VAC AERO International Inc., Oakville, Ontario, Canada

The Thermal Processing Divisions of VAC AERO International have provided repair services for damaged components from land-based and aerospace gas turbine engines. Engine manufacturers, operators, overhaul centers and commercial airlines are just a few of the customers that depend on these services. Many hot section engine components are fabricated from nickel-based superalloys. These materials cannot be repaired by traditional techniques, such as welding, without causing significant reductions in mechanical properties. As a result, VAC AERO developed proprietary vacuum brazing techniques to repair cracks, wear and other service-induced damage. By Jeff Pritchard - Vac Aero International Inc.

Stretching the service life of a hot zone-one of the most expensive consumables in a vacuum furnace-is of prime importance to the furnace owner. Proper care and maintenance can slow down hot-zone deterioration, which directly translates to cost savings. Hot-zone service life is directly related to the type of heat treating process used in a vacuum furnace and to the severe thermal stresses imposed by continuous vacuum-furnace cycles. Distortion and contamination are the main factors causing a breakdown of hot-zone components. Distortion is inevitable because thermal cycling is inherent to the process. However, periodic inspection and remediation of hot-zone components together with thorough component cleaning can extend service life. By Bill Potts, Vac-Aero International Inc., Oakville, Ontario, Canada