The conflicting nature of heat treating metal parts is to achieve proper metallurgical and mechanical properties while maintaining minimal part distortion. It often is a difficult balancing act to achieve the optimal cooling rate that produces and limits the inherent stresses within the material.
However, over the past 20 years, the advancement of vacuum heat treating has made great strides in balancing these conflicting dynamics to achieve desired results. This is possible within the vacuum furnace environment due to precise temperature processing and controlled gas quenching. How is gas quenching controlled? One must vary the gas pressure and velocity. The following examples demonstrate the advances of vacuum processing that balance these two conflicting heat treating goals.
Vacuum heat treating aircraft parts
At Solar Atmospheres of Western PA (Hermitage, Pa.), the challenge was to harden and temper two carrier parts for Triumph Gear, Macomb, Mich. After final machining at Triumph's Salt Lake City, Utah, plant, the parts were sent to Solar for vacuum heat treating. The challenge for Triumph and Boeing engineers was to minimize the distortion typically associated with oil quenching, yet attain BAC specification 5617. The carriers, made of 4340M alloy steel, were required for bench testing of an experimental helicopter gearbox design. Each carrier was 24 in. in diameter by 26 in. high with a maximum cross-sectional thickness of 1.562 in. (609 by 660 and 40 mm) and weighed 407 lb (185 kg).
Due to the complex geometry of the carriers, the material needed to be gas quenched. Therefore, heat treatment required an exception to the BAC specification for the quenching medium. Gas pressure quenching was allowed in lieu of oil or martempering per BAC 5617 Rev R. Solar is a Boeing-approved commercial heat treating facility with extensive vacuum heat treating capabilities. This includes its Advanced Quenching process that combines high pressure and high velocity gas quenching with helium.
To achieve part specification, the components were preheated at a temperature of 1400°F (760°C) and held for 30 minutes based on readings from work thermocouples placed inside the customer-supplied test pieces. The parts were austenitized at a temperature of 1600°F ±25°F (870°C ±14°C) and held for 105 minutes. The quench gas was circulated by means of a 300-hp motor at 5,000 rpm, and the load was pressure quenched at 10-bar helium. The hardness of the parts after heat treatment was 62 HRC. Dimensional inspection of the parts confirmed that there was minimal change in geometry. Subsequently, the components were double vacuum tempered and sent back to the customer for grinding.
The result achieved with vacuum heat treating was a step forward in achieving part specifications on a highly complex part while minimizing distortion. It was achieved through the advanced furnace capability to provide a controlled high-pressure gas quench as determined by the heat treating cycle. By comparison, the expected excessive distortion of the parts using oil quenching could result in not being able to salvage the expensive parts.
Case hardening with minimal distortion
A second example is the development of a new vacuum furnace and process to extend part wear life. Solar Atmospheres, Souderton, Pa., a commercial vacuum heat treater and Solar Manufacturing, a furnace manufacturer, worked together to offer a unique vacuum process and furnace for low-torr range vacuum carburizing. The furnace capability and vacuum process offers case hardening for parts while minimizing distortion.
Carburizing is carried out in the newly developed single chamber vacuum furnace with an in situ, pressure gas quenching system. Advantages of the system include precise temperature control, the use of acetylene gas mixtures to achieve optimal case depths and very fast gas cooling to minimize part distortion. Control of the caburizing gas provides a uniform case depth including improved root to pitch ratios for gears.
Metallurgically, low-torr range vacuum carburizing prevents microstructure surface intergranular oxidation (IGO) and decarburization. The avoidance of IGO provides high integrity to the case. The presence or absence of carbides can be controlled according to specification requirements. Additionally, the process produces compressive residual stress for improved fatigue life. Metallurgical properties attained provide optimal surface and core hardness extending part performance and wear life.
Operational advantages of the single-chamber carburizing furnace process compliment the goal of having minimal distortion. This includes efficient part handling for quality control and turnaround. Bright, clean parts eliminate the need for post-processing machining. Precision temperature control enables automation of specific cycles to ensure repeatability, which in turn enables the use of customized cycles for a range of alloys and applications, such as powder metallurgy parts. Secondary benefits include the ability to carburize blind holes.
Conclusion
Achieving material specifications often requires maintaining part geometry while achieving a desired hardness and metallurgical properties. These operations are being carried out increasingly in vacuum heat treating furnaces. The result compliments the trend for increased metal part value for aerospace and other industries that require demanding part specifications in critical applications. IH
