Can you have clear ideas when buying a vacuum furnace?
Of course yes and the subject, only apparently so complex, is addressed by reasoning
by points.
In the previous article we got to
know some of the elements that contribute to the realization and perfect functioning
of a vacuum oven, starting from the three which, over time, have
remained more constant. To them was added another trio, that become more popular
in recent years. Here they are in line:
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Today I'm going to complete the review of the
10 elements to evaluate before buying a new vacuum furnace.
I'm talking about:
The core of the activity of a high vacuum furnace is concentrated in two parts: a double wall vacuum seal chamber, water-cooled, and an insulated thermal chamber with low conductivity materials (graphite wafer), in which the resistor and the charge to be treated are positioned.
The operation of the vacuum heat treatment furnace provides two possible levels of vacuum:
In the first case the heat transfer, by the action of the molecules moving from the hot part to the cold one, takes place under a free molecular conduction regime: at more severe operating conditions, the exchanged energy has a value close to 0.025 kW/m2 ( therefore truly negligible). The heat exchanged in the transition regime is also negligible, as is the transmission heat in the viscous field (0.08 kW/cm2). It can be deduced that the energy exchanged by thermal conduction in a high vacuum furnace is, however, practically nil, irrespective of the value of the pressure (vacuum level).
In case of thermal exchange due to irradiation by partial gas the values are
very different. I also consider the most difficult operating conditions here: values
of 3÷4 kW/m2 are achieved. The practice confirms the scientific data,
bringing to light also an important aspect for the purposes of the thermal uniformity
of the steady-state system: that of the heat leaks from the chamber.
Some constituent elements of the system work, against our will, against its perfect
functioning. I would like to mention the heat loss caused by the load supports and/or
drafts of radiation, but above all those generated by power feedthroughs.
The power feedthroughs are of large cross-section graphite bars connected to water-cooled
copper clamps: three or four in number, these are one of the coldest areas in the
chamber too. Also the load support points are potential energy escape vehicles,
which we tend to obviate by inserting refractory materials with low conductivity
(alumina pads) between the graphite part and the steel mantle. In some cases they are added by calculating heating zones that have higher specific energy, near to the load support base.
I’ll move now on another element of the oven based on vacuum technology that has undergone major revisions in recent years. I'm talking about the resistor and, in particular, the sizing of its supply chain (static converter, transformer, bars, etc.). In fact, if this must be such as to quickly reach the set temperature to the heating elements, the same - however - must be large enough to reach the set temperature to the charge in the shortest time, compatibly with the geometries and arrangement of the pieces. The current trend is to provide a power supply to the resistor of about four times the power dissipated at full power by the system at maximum temperature (°C).
The resistor surface also plays a fundamental role in accelerating
the process under conditions of low temperature treatments (for example, in the
case of the austenitization temperature, at "only" 800°C), in which the
power that can be supplied by irradiation is always low.
The position of the resistor is also extremely important: today
it is established with certainty that in
horizontal geometry furnaces, in
the presence of charges consisting of several pieces, the optimal heating is that
achieved with the heating elements placed in the upper and lower surface of the
furnace. This leads to a similar thermal cycle both for the pieces placed at the
periphery of the charge and for those at the center of the charge.
The flow of gas is crucial in the process of turning off the
vacuum oven.
To obtain the same thermal history for all the pieces is one of the primary interests
of the vacuum furnace designer: for this reason, the procedure to start the charge
hardening phase in gas is carefully studied.
What does this procedure consist of?
At the beginning, pressurized gas is introduced into the vessel; when the set
pressure has been reached, the resistor is de-energized and the gas is recirculated
at low temperature through a large high-head impeller. The gas, circulating, takes
on heat and then transfer it along the path to a heat exchanger. The path of
the gas is important, because it must flow through the charge, so that even
the smallest and inner elements are cooled.
When designing a vacuum heat treatment system, solutions with geometries that allow
gas to bypass the charge or only touch the outer surfaces must be avoided: the fluid
threads of gas that pass through the charge must not have components of speed perpendicular
to the pieces.
What to pay attention to?
The gas flow section must have a dimension equal to that of the piece
basket. The best channeling of the gas occurs with paths from the bottom to the
top, since a flow distribution congruent with the dislocation of the pieces
is realized, which present themselves to the gaseous streams as on a single surface.
Otherwise, with a path from top to bottom, the flow regularity would be disturbed
by the pieces of greater height, which act causing the so-called "umbrella
effect" on the underlying parts.
In the furnace for vacuum heat treatment, in any case, gas paths with axial and
central geometry must be avoided inside the thermal chamber: in fact, the pieces
would tend to assume different temperatures, the process would be badly controlled
and would run the risk of important deformations on the pieces themselves.
As the only alternative I would suggest splitting the flow: a contemporary jet from the top and from the bottom solves the problem of fast hardening on large pieces or on overlapping stacked baskets.
I arrived at the vibrant heart of vacuum technology treatments: the modernity
of this system lies above all in the fact that the cycle is completely automatic.
Once designed and assembled, the high vacuum furnace becomes manageable as a work
center: the active presence of operators during the process is neither foreseen
nor required.
Extremely accurate and safe, the microprocessor programming guarantees the repeatability
of the cycle and the start of automatic sequence checks.
These process controls diagnose the different states of activity of the furnace
for heat treating under vacuum. A series of indicators highlights
situations of major malfunction or disservice. The system is able to automatically
set itself in safety conditions in the face of anomalous events such as:
Within a short time the operation of a vacuum oven required less and less supervision and surveillance: a great advantage for the company, which finds itself a system capable of working even at night, offering the maximum economic yield. In a few years there has been a leap of generations in vacuum technology. Moreover, pressurization, in addition to allowing the heat treatment of more "difficult" materials, considerably reduces cooling times at lower temperatures.
This is how this presentation of the key elements of a vacuum heat treatment
furnace closes: I wanted to offer you a little review of their basic characteristics,
imagining what your questions might be and that this could give you a hand in the
approach with the designer and the price quotation. The best operating result is
obtained when every aspect has been adequately taken into account.
The specialist will find in you a prepared and aware interlocutor: the best conditions
to start a successful collaboration.
Non-evaporative getters (NEG) in vacuum heat treating are materials used to maintain or improve the vacuum quality by absorbing gas molecules without evaporating or changing their state. Unlike traditional getters that work by evaporating a material which reacts with and traps gas molecules, NEGs absorb gases through a different mechanism.
Here’s a breakdown of how they work and their benefits:
Absorption Mechanism: NEGs typically consist of materials like zirconium, vanadium, and iron. These materials can absorb gases like hydrogen, nitrogen, oxygen, carbon monoxide, and water vapor at the molecular level. The process usually involves chemical reactions where gas molecules are chemically bonded to the getter material.
Activation Process: NEGs often require an activation process to start working efficiently. This process usually involves heating the getter to a specific temperature, which allows it to react with the gases more effectively.
Long-term Stability: One of the key advantages of NEGs is their long-term stability and efficiency. Unlike evaporative getters that lose their effectiveness over time as the getter material is depleted, NEGs can maintain their gas-absorbing properties for a longer duration.
Cleanliness and Safety: Since NEGs do not evaporate or release any material into the vacuum, they are considered cleaner and safer compared to traditional getters. This is particularly important in processes where contamination needs to be minimized.
Applications: NEGs are widely used in various high-vacuum applications, including semiconductor manufacturing, vacuum tubes, particle accelerators, and vacuum furnaces used in heat treating processes. They are essential in environments where maintaining a high-quality vacuum is crucial for the process’s success.
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Limitations: While NEGs offer several advantages, they also have limitations. For instance, they have specific activation conditions and temperature ranges within which they operate effectively. Additionally, once saturated with gas, they need to be reactivated or replaced.
In vacuum heat treating, NEGs play a crucial role in ensuring that the vacuum remains free of unwanted gases, which can otherwise impact the quality of the heat treatment process.
See lessThe changes from AMSF to AMSG include revisions and updates to the pyrometric requirements for equipment used in the thermal processing of metallic materials. The AMSG specification covers temperature sensors, instrumentation, thermal processing equipment, correction factors and instrument offsets, system accuracy tests, and temperature uniformity surveys. These changes are necessary to ensure that parts or raw materials are heat treated in accordance with the applicable specifications. The purpose of AMSG is to ensure that parts or raw materials are heat treated in accordance with the applicable specifications.
The revision from AMSF to AMSG mainly consists of administrative changes rather than significant technical changes. One notable change is that digital recording instruments are not required to read to the tenth of a degree until one year after the release of AMSG. Additionally, over temperature controls have been given a permitted error within AMSG, which was not present in revision F.
To comply with AMSG, suppliers who already conform to AMSF should find the transition relatively straightforward. It is recommended to consult experts or training courses that specialize in AMS standards and Nadcap requirements to ensure compliance. Calibration services and process certification services can also be helpful in meeting industry standards.
Some specific changes between AMSF and AMSG include:
Revision G marking: Historically, AMS did not mark the revision when released using change bars, unlike other AMS specifications. However, when AMSF was revised to revision G, change bars were included. This makes it easier for suppliers to understand the modifications and account for those changes internally.
Extensive changes: AMSG is considered a complete rewrite of the specification, and there are no change bars to indicate specific changes. The changes are extensive and cover various aspects of pyrometry requirements.
It is important to refer to the official AMSG specification for a comprehensive understanding of all the changes and updates.
For more detailed information and updates on AMSG, you can visit the SAE International website or refer to the Conrad Kacsik blog post on the differences between AMSF and AMSG.
SOURCES:
Pyrometry AMSG – SAE International
Differences Between AMSF and AMSG – Conrad Kacsik
See lessUsing a clean and controlled environment in vacuum heat treating is important for several reasons:
1. Contamination prevention:
Vacuum heat treating involves subjecting materials to high temperatures in a controlled atmosphere. Any contaminants present in the environment can react with the material or form unwanted surface layers, leading to discoloration or other quality issues. By maintaining a clean environment, the risk of contamination is minimized, ensuring the integrity of the heat-treated parts.
2. Consistent results:
A controlled environment helps in achieving consistent and repeatable results during the heat-treating process. Contaminants or impurities can introduce variability in the heat treatment process, leading to inconsistent outcomes. By eliminating or minimizing the presence of contaminants, the heat-treating process can be more predictable, resulting in consistent and reliable results.
3. Surface quality:
Cleanliness of the environment is crucial for achieving desired surface quality in heat-treated parts. Contaminants can cause surface imperfections, such as stains, discoloration, or pitting, which can affect the appearance and functionality of the parts. A clean environment helps maintain the desired surface finish and appearance of the heat-treated parts.
4. Material integrity:
Some materials are sensitive to contamination and can react with certain elements present in the environment. For example, reactive metals like titanium can easily react with oxygen, nitrogen, or carbon, leading to surface discoloration or degradation. By using a clean and controlled environment, the risk of such reactions can be minimized, preserving the integrity of the material.
5. Process efficiency:
A clean environment can contribute to the overall efficiency of the vacuum heat-treating process. Contaminants can interfere with heat transfer, affecting the uniformity of temperature distribution within the vacuum chamber. By ensuring a clean environment, heat can be transferred more efficiently, leading to faster and more effective heat treatment cycles.
In summary, using a clean and controlled environment in vacuum heat treating is essential to prevent contamination, achieve consistent results, maintain surface quality, preserve material integrity, and enhance process efficiency.
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