Technical Sheet: Controlling temperature and pressure during hot isostatic pressing (HIP)
Tight tolerances are often requested when processing a component since they are very desirable for quality control, considering mechanical and dimensional tolerances, as well as time or resource savings. However, before defining the desired tolerances or ranges within which the equipment should work, it is important to understand what are the limitations of it and which physical mechanisms will happen during the process that cannot be controlled. With that in mind, this article will be about some aspects of how stable a hot isostatic pressing process can be, focused upon temperature and pressure control.
The main process parameters of hot isostatic pressing are the temperature, pressure and the dwell time. Besides that, heating and cooling rates can play an important role in the material or part properties, depending on the rates’ order of magnitude.
Therefore, to maintain reliability and control to the system in terms of temperature and pressure, two main hardware features of the machine should be taken care, respectively: the thermocouples and the sealing surfaces.
Yet for industrial applications, tight tolerances are often not required during HIP processes [1], it depends highly on the parameters applied during the process, being more difficult to maintain it at high pressures and high temperatures.
In the next sections it will be described how the use of different thermocouples can influence the measurement of temperature and its assurance. Further, how maintenance of the sealing surfaces provides a good pressure stability, yet small instabilities in pressure are normal in HIP due to densification mechanisms, especially for encapsulated parts.
Temperature accuracy
Before talking about thermocouples, it is important to mention that HIP vessels are distributed in different thermal zones, like floors, as showed in Figure 1. Number of thermal zones will depend on the size of the machine and how it is constructed. Each zone presents more than one thermocouple for redundancy.
Further, heating in hot isostatic pressing occurs not only by radiation, but as well through circulation of hot gases inside the furnace.
Between the heating elements and the pressure vessel, there is the heat shield, which prevents heat losses and protects the external hardware from the high temperatures.
Thermocouples
Thermocouples usually used in HIP vessels are Type S and Type B for control of temperature in the zones, or the pieces being treated. Type S is normally only used when special kind of certifications are required, for example for aerospace industry. Other kinds of thermocouples, such as type K, can be found in the system to control temperature of hardware parts, such as the top cover.
Thermocouple Type K: these are nickel bases thermocouples usually with 10% of chromium (+) vs nickel with 5% of aluminum and silicon (-). Applicable for oxidizing or inert atmospheres with a temperature limit up to 1260 °C. Tolerance of reading is high in comparison with Type S and B, being almost +/- 2%, depending on the composition [2].
Thermocouple Type S: It has an operational temperature up to 1480 °C, and it is platinum (-) vs platinum-10%rhodium (+). It is very stable, but must be protected from metallic and non-metallic vapor, and it is suitable for air or inert atmospheres. Tolerance of this kind of thermocouple is very small compared to the other types, in the range of +/- 0.25% [2].
Thermocouple Type B: Made of platinum with addition of rhodium (30% (+) vs. 6% (-)) suitable for use until 1700 °C. Besides the working temperature, has the advantage over the thermocouple of Type S to be more resistance to grain growth, which could result into failure. However, requires the double of tolerance rate than type S (+/- 0.5%) [2].
Heat transfer inside the vessel
When the vessel is heating, it is very possible that the upper zone will be heated up faster than the others, since the density of heated gas decreases and it dislocates up within the furnace.
The effect of convection inside of the furnace will also depend on the pressure of the system, because with less gas the effect will be less pronounced. [3]
Pressure accuracy
To maintain the pressure of the system it is obviously important that it is leak checked. One way to avoid leakage is to perform inspection of the vessel at regular intervals to check for cracks, to prevent fatigue failure. Small cracks can already be identified in these inspections.
On a daily basis it is recommended that the furnace is maintained clean and without moisture. Both can not only damage the heating system, but also the pumping one.
By the processing of an encapsulated powder, a slight change of pressure can be seen during the dwell stage, due to the densification process of the capsule.
With the densification and subsequent reduction of volume of the piece inside of the HIP, it is possible to observe a slightly decrease in the system pressure, depending on the load volume and on how the parts have been filled and evacuated previous to the thermomechanical process, as represented in Image 2.
Overview
HIP is very reliable and has many advantages when applied for densification as a manufacturing or post-process. However, it is important to observe that even for calibrated thermocouples, there is a margin of measurement error from 0.25 to 0.5% of the temperature. Besides that, even with new vessels that have a very good control of temperature, it is possible, especially in heating up and cooling down, to see a difference between temperatures between the top and bottom zones, due to the convection mechanism.
Regarding pressure accuracy, small drops in pressure during dwell stage can help due to the densification of parts. This effect is more pronounced in PM cannisters, due to the low density previous HIP.
References
[1] ATKINSON, H. V.; RICKINSON, B. Hot Isostatic Pressing.(Book). Adam Hilger, 190, 1991, S. 1991.
[2] PARK, Richard M. Thermocouple fundamentals. Course Tech., Temp, 2010, S. 2-1.
[3] MEHRA, Raman K.; MAHMOOD, Shah; RUNKEL, Joe. Temperature control of hot isostatic pressing (HIP) furnace using model predictive and risk sensitive optimal controllers. In: Proceedings of International Conference on Control Applications . IEEE, 1995. S. 949-954.