Choosing the Best Hot Zone for Your Needs

Over the past 40 years, the heat-treating industry has seen a notable shift in the type of hot zones commonly used in furnaces. In the 1970s, all-metal hot zones predominated, but today, the majority are graphite.

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Shifting to Graphite

Two major factors contributed to this shift:

1. Enhanced design and quality of graphite hot zones
   • The quality of graphite hot zones has greatly improved as manufacturers began using felt to prevent heat loss. Contemporary designs typically feature 2" of graphite felt insulation backed by a Carbon/Carbon flex shield. Previously, they comprised just 1" graphite felt and 1" graphite board insulation.
2. Increased education about the benefits of graphite
   • Predictive maintenance and data analysis have made furnace users more knowledgeable about their equipment. This research has highlighted the benefits of graphite hot zones, which are more energy-efficient, less costly to replace, and can have a longer lifespan with proper maintenance.

Choosing Your Hot Zone

Despite the improvements and growing popularity of graphite hot zones, all-metal hot zones continue to be in demand. Compared to their graphite counterparts, all-metal hot zones use multi-layered, metallic shields made of molybdenum and stainless steel. These hot zones offer better leakage rates, improved pumpdown capabilities, a cleaner working environment, and lower chances of part contamination.

Given the unique advantages of each type, how do you choose the best hot zone for your needs? Here are three essential questions to guide your decision:

1. What are your process and material requirements?

All-Metal

If your process cannot tolerate incidental dust or dirt, an all-metal hot zone is better suited for your needs. Sensitive processes like diffusion bonding and aluminum brazing benefit from all-metal hot zones. They are designed to handle materials such as superalloys (e.g., Titanium, Hastelloy, and Tungsten), resulting in parts that are bright and clean—crucial for the medical industry.

Also, consider how your materials might react with carbons in a graphite furnace. Graphite dust can lower melting temperatures and adversely affect certain materials. In such cases, an all-metal hot zone ensures optimal results.

Graphite or All-Metal

Graphite hot zones can also produce clean parts, but incidental carbon dust might occur. If your material does not react with carbon, you can choose either hot zone. Metals like carbon or alloyed steels and processes like tempering, aging, carburizing, hardening, and solution annealing work well in either type.

2. What are your desired temperatures and ramp rates?

All-Metal

If your cycle requires high temperatures and ramp rates, an all-metal hot zone is suitable. These can reach temperatures above 2,400 °F (1,371 °C) with a maximum ramp rate of 75 °F (41 °C) per minute.

Graphite or All-Metal

If your temperatures are below 2,400 °F (1,371 °C) and you need a maximum ramp rate of 45 °F (25 °C) per minute, either option is viable.

3. What is your expected range of temperature uniformity?

All-Metal

A metallic hot zone with end elements can achieve a temperature uniformity range of +/- 5 °F (3 °C). According to AMS 2750F standards, these furnaces are categorized as Class 1, offering the least temperature variation in the work zone.

Graphite or All-Metal

For parts and processes allowing a wider uniformity range of +/- 10 °F (6 °C) or more, a graphite hot zone without end elements can achieve the desired results. By AMS 2750F standards, this is classified as a Class 2 furnace. Heat-treating furnaces should always meet the narrowest temperature uniformity range required, so either hot zone can be suitable depending on this criterion.

Conclusions

Clearly, each hot zone has unique capabilities and advantages. If your answers to the above questions point to an all-metal hot zone, your decision is straightforward. But if you can choose between the two, consider the earlier discussed benefits to determine the best hot zone for your needs.

Welcome back! This is the second part of the article dedicated to vacuum furnace hot zones, providing the necessary information to make an informed choice on the most economical and best-performing hot zone concerning losses and overall power costs. In the first part, we discussed the graphite-based hot zone design, delving into its specific characteristics and challenges. Now, we focus on the all-metal design, highlighting its strengths, especially regarding energy consumption.

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All-metal hot zones: an energy, cost saving solution

In an all-metal hot zone, the shielding consists of molybdenum, tungsten, or stainless steel. Molybdenum (Mo) is typically used in conventional all-metal hot zones for vacuum furnaces. To simplify, molybdenum alloys are commonly used up to 1600 °C for both the resistor and the insulation. Tungsten alloys are used for higher temperatures in commercial installations.

All-metal hot zones cater to high-demand industries where sensitive materials are processed, such as aerospace, electronics, and medical fields. Some heat treatments require particularly clean environments or extreme vacuum levels. In some cases, graphite from the chamber could interfere with the process, causing unwanted carburization of the pieces treated. In other cases, the load could be sensitive to residues in oxygen or hydrogen atmospheres (which could lead to embrittlement of the pieces); thus, graphite wafer degassing during the cycle could be harmful. In such scenarios, users should opt for all-metal heating chambers (shields and resistor).

Despite a slightly higher initial investment, all-metal hot zones offer the most economical solution due to the following properties:

• Rapid heating and cooling
• Shorter pump-down times
• Higher ultimate vacuums
• Uniform heat distribution

How do reflecting shields work in a metal hot zone

In a vacuum, heat transfer can be significantly reduced by multiple reflecting shields.

A shield is a surface that blocks the transmission of radiation when it has high thermal conductivity and low emissivity. The barrier effect is enhanced if shielding is provided by a set of minimal-thickness molybdenum sheets. The innermost sheet faces the hot zone and is paired with parallel sheets, while the outermost sheet faces the cold wall of the vacuum vessel. Minimal thickness is necessary to reduce the heated metallic mass without altering the shielding effect.

Higher temperatures require more metallic sheets. The lower the material's emissivity, the more effective the shield, leading to lower energy loss. Molybdenum sheets possess the fortunate property of very low emissivity. As a result, manufacturers use full molybdenum shielding for outer and less hot surfaces to minimize energy loss.

Metallic furnaces also exhibit interesting characteristics in terms of load cooling rates.

The ultimate shield of the heating chamber—in contact with the vessel’s water-cooled wall—maintains the same temperature as the hot zone, albeit higher than the equivalent surface of a graphite chamber.

The presence of refractory insulation material and significant resistor mass (both in graphite) tends to make graphite furnace hot zones slower during cooling. In contrast, the cooling rates in all-metal hot zones are greater, especially at high temperatures, due to the shields' higher temperatures but smaller masses. This feature makes metallic hot zones more appealing for certain applications, not necessarily because of the final vacuum but due to the faster cooling.

Now, let's talk about the downsides. What are the disadvantages of an all-metal hot zone lined with molybdenum sheets?

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All-metal hot zones: the drawback of molybdenum sheets

Molybdenum has several properties users should consider. Upon reaching operating temperatures, the material becomes brittle and can no longer be dismantled after initial heatings without breaking. Any attempt to handle the material will inevitably cause it to crumble.

Molybdenum also forms oxides in the presence of oxygen, even at low temperatures, and such oxides have greater emissive power. Any vacuum loss causes this unwanted effect. Stringent procedures are essential to ensure no leaks before starting the heat cycle.

Significant, non-repairable damage can occur if the load strikes the shield.

Furthermore, "colouration" of the material due to oxygen traces or evaporating substances from treated pieces can alter and reduce shielding conditions, compromising thermal uniformity as required by specifications.

Installing an all-metal hot zone demands a higher level of skill and care.

By contrast, these issues can be resolved in graphite hot zones with simple maintenance or cleaning cycles for deposits on the wafer surfaces.

So, the moment of choice has come!

Graphite-based or all-metal hot zone design for your next vacuum furnace? If you need more information, feel free to contact us using the form below.

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