For all of the known benefits of titanium alloys in all sorts of applications, from medical to aerospace to automotive, titanium is also known to exhibit poor tribological properties. That is, it has a high coefficient of friction (COF) when in moving contact with essentially all structural metals, resulting in poor sliding and adhesive wear resistance that leads to failure by galling (cold welding). Because of this, metal-to-metal applications encountering friction and wear considerations require a surface treatment for adequate serviceability. One such treatment is solution nitriding, which is performed in a vacuum furnace using partial pressure nitrogen gas at elevated temperatures in the annealing range. Solution nitriding is classified as a diffusion process where nitrogen gas dissociates and nascent nitrogen is adsorbed and diffused into the titanium matrix. Like other diffusion processes, the depth of the diffusion zone is dependent on the time of the treatment. For alloy Ti-6Al-4V with a core hardness of 30 HRC, Solar Atmospheres has generated hardnesses as high as the mid-60’s to upper-60’s HRC (converted from HV 25gf) at a depth of 0.0076mm (0.0003”), followed by a gradual decrease in hardness to the core over a distance of 0.25mm (0.01”). Shorter cycle times have produced hardnesses in the mid-50’s to high-50’s HRC and shallower total case depths.
A recent process development test relating to carburizing illustrated the need to better understand the effect of surface emissivity and the proper use of dummy thermocouple test blocks. The testing involved carburizing areas of a partially copper plated alloy steel part. The copper plating covered areas of the part that were not to be carburized. Since the configuration of the part made it impossible to place a thermocouple within the part, a dummy test block made of carbon steel with the approximate same cross-section was used for the process thermocouple without proper consideration of the surface condition of the test block. Using the test block as the control, carburizing was initiated at the proper temperature based on the test block having reached that temperature. At the completion of the test, the part was examined for carburizing results and found in the non-copper plated areas, the depth of the carburized case to be shallow. This indicated that the cycle performed did not initially hold the part long enough at the correct temperature prior to carburizing. This resulted in the conclusion that when using dummy test blocks for controlling process times and temperatures, many factors must be considered including surface emissivity.
Advanced material solutions for fixtures, grids and internal furnace components are available today. They are designed to allow for higher processing temperatures, larger loads, increased production rates, energy savings, and lower overall cycle costs.
Solar’s Souderton plant recently received a Nadcap accreditation in carburizing, allowing it to better serve the aerospace market. This accreditation joins Solar’s other Nadcap approvals for heat treating, brazing and fluorescent penetrant inspection. Additionally, earlier this year the company became an approved supplier for General Electric Aviation (GEA), UTC Aerospace Systems (UTAS) and Moog Corporation.
A vacuum-purge gas nitriding furnace was modified to develop a process and a furnace enhancement to produce a controlled in situ oxide layer on the surfaces of steel parts using various oxidation techniques. The process is an effective alternative to conventional grit blasting of materials as a means of surface preparation for uniform and consistent nitriding results. Pre-oxidation is known to enhance receptivity of steel part surfaces to the effects of nitriding, and in situ oxidation is inherently efficient and economical. Topics discussed include the type of oxidizing carrier used in the furnace, practical methods used to control the oxidation, and a gas delivery system developed to inject gases with an elevated dew point for the purpose of providing a controlled oxidizing atmosphere. Comparative tests with other activation techniques, and results with no activation, will be discussed along with approaches to technical process difficulties encountered.
A specific concern to a vacuum furnace user is processing critical work during summer months with high temperatures and high humidity. This same concern could also be a problem on rainy winter months. Under these difficult conditions, it is most important to understand the impact that humid conditions can have on the final surface condition and appearance of the processed parts. Work discolored or oxidized by residual water vapor is unacceptable and must be controlled for many critical components like aerospace parts or medical implants and instruments. This paper will try to explain factors relating to humidity, air temperature, and methods to improve final product appearance and minimize possibility of contamination.
Proportional-Integral-Derivative (PID) control is the most common control type algorithm used and accepted in the furnace industry. These popular controllers are used because of their robust performance in a wide range of operating conditions and because of their simplicity of function once understood by the processing operator. The purpose of this paper is to further define and thoroughly explain the basics of the PID controller. It should be noted that many current instruments incorporate what is called an “Autotune” feature which can automatically set the PID variables for a given temperature setting allowing the operator to bypass much of the initial manual requirements. However, Autotuning was not introduced until the late 1980’s and there still exists many instruments in use which do not have this tuning feature and must still be manually set-up. Also, Autotuning often requires additional tuning or tweaking to reach final acceptable results. By understanding fully the basics of the PID functions as described below, it is hoped that any final adjustments or tuning will be simplified. Further discussion of the Autotune feature follows below. As the name suggests, the PID algorithm consists of three basic components: proportional, integral and derivative which are varied to get optimal response. If we were to observe the temperature of the furnace during a heating cycle it would be rare to find the temperature reading to be exactly at set point temperature. The temperature would vary above and below the set point most of the time. What we are concerned about is the rate and amount of variation. This is where PID is applied.
Since the early development of the vacuum furnace, engineers and thermal experts have continually tried to improve the insulating characteristics of the furnace hot zone. Several materials have been used for different applications with varying success. However, all designs must still deal with the heat losses penetrating through the insulation materials and the need to minimize these losses. This is especially important today with the continual escalation of electrical power cost. This paper will review the different types of hot zone insulation materials used, the projected losses of the different designs, the impact relating to furnace cycle heating rates and cycle times, and the projected cost advantages of one design over another.