CFC Fixtures for Heat Treatment Applications

The emergence of CFC Fixtures within the Heat Treatment Industry

Heat treatment furnaces require significant capital investment; they take up much needed factory floor space and they are often a bottleneck process in the production workflow. Proper fixturing of components is essential when it comes to ensuring that all parts are subject to the correct heat transfer, have uniform surface and mechanical properties and maintain critical part dimensions.

Cast and wrought steels are the most traditional and widely used fixturing materials, although due to their high density, these fixtures take up a large portion of the furnace load capacity, which limits the overall batch component throughput. Steel also has a high co-efficient of thermal expansions (CTE) which means that the fixture tends to distort and lose shape over time, after exposure to many rapid heating and cooling cycles.

In recent years, CFC (commonly referred to as ‘Carbon Fiber Composites’) has been the preferred fixturing material for a range of heat treatment processes, offering a number of significant benefits over traditional cast alloy fixturing. The most noteworthy benefits of using CFC include a fixture weight reduction, which in turn allows for an increased furnace batch throughput, as well as reduced maintenance and upkeep costs as a result of CFC’s unique resistance to distortion and its ability to hold its shape throughout the fixtures entire lifecycle.

Perhaps a decade ago, CFC was considered to be an ‘exotic’ and expensive fixturing material and few companies had the know-how and practical experience to understand how CFC fixtures could be applied to improve processes and offer cost savings to their businesses. However, in recent years, CFC and raw material manufacturing processes have become more streamlined, whilst CFC fixture designs themselves have become more reliable and repeatable. For these reasons, CFC fixtures and materials have become much more affordable and now offer a commercially viable fixturing solution across a broad range of applications. Whilst improving, the introduction and uptake of CFC fixtures throughout industry has remained slow, mainly due to a common misconception that CFC fixtures lack strength and robustness during material handling, when compared to their steel counterparts.

Nowadays, CFC fixtures are not only limited to vacuum applications. The introduction of a broad range of CFC material grades has enabled the use of CFC fixtures within other heat treatment applications, including nitriding, gas carburising, LPC (low pressure carburising) and even processes which adopt oil quenching or air atmospheres at low temperatures (typically below 450c).

Why use CFC Fixtures over cast steel alloy fixtures?

The unique physical and chemical properties of CFC materials allows for significant benefits to be realised when using CFC fixtures in lieu of cast alloy fixtures – given the right process conditions. The main characteristics of CFC and their associated benefits versus cast steel fixtures are as follows:

• LIGHTWEIGHT – CFC fixtures are around 10-20% of the weight of alloy steel fixtures due to CFC’s lower material density. This allows for safer handling of the fixtures by heat treatment operatives and better overall ergonomics. Customers who charge alloy fixtures in to furnaces running at their maximum load capacity can increase their part throughout by 40-60% (typically) when switching to CFC fixturing, as a direct result of the lower overall fixture weight.
• HIGH STRENGTH – CFC is stronger at high temperature than cast alloy steels and is roughly 10x stronger at 1000°c. In fact, CFC is unique in that its strength actually increases with an increase in temperature. The material has much improved load bearing capacity at high temperatures and typically offers a net fixture load increase of up to 100% when compared to cast alloys.
• LOWER ENERGY COSTS – The lower thermal mass of CFC results in quicker heating and cooling times (typically 30% lower) when compared to cast steel fixtures, with CFC requiring only 1/4 of the electrical power input during heating. This results in shorter furnace cycle times, lower running costs, improved furnace capacity and reduced machine downtime. Although load and furnace conditions vary and each process is different, energy savings of up 65% can be achieved.
• LONGER FIXTURE LIFE CYCLE TIME – The life cycle time of CFC fixtures is in the order of 5x longer than cast steel alloys fixtures. This is mainly due to the fact that cast steel fixtures will distort, crack and become embrittled over time
• LITTLE / NO REWORK – CFC has a much lower thermal coefficient of expansion (CTE) than cast steel alloys, which means that it is resistant to creep and distortion during repeated heating and cooling cycles. A reduction in total fixture rework time of 90% can be expected when compared to cast steel fixtures, resulting in significantly lower maintenance and fixture upkeep costs. Given that CFC fixtures do not distort, automated handling of CFC fixtures is also possible. This can offer a huge advantage for large heat treatment facilities with high part throughputs and multiple batch furnaces installed in series.
• IMPROVED PROCESS RELIABILITY & QUALITY – CFC has exceptional dimensional accuracy and stability during heating and cooling which ensures that the fixture always retains its shape and allows for all parts to be fixtured correctly during processing. Properly designed fixtures which support and space out the components evenly will ensure that all components are processed in a reliable and repeatable manner and that component quality requirements are achieved and maintained at all times. In contrast, the instability and unpredictability of cast steel fixtures can occasionally result in missing components, or parts that have been improperly treated and require re-work or further processing.

What are some of the limitations of CFC Fixtures?

Whilst CFC is considered to be an excellent fixturing materials, there are certain process conditions which do not favor the use of CFC fixtures, or limit the use of CFC in general. These conditions are outlined below:

• EUTECTIC REACTIONS – Above 1000°c, CFC may undergo contact reaction with steel alloys and cause discoloration, unwanted carburization or in severe cases, local eutectic melting (welding of the parts to the fixture). To prevent this, ‘hybrid’ type fixtures are employed which combine CFC with ceramic components, typically made from sintered alumina or alumina fibre composites. The ceramic parts are used to create contact points for the metal parts to rest on, and can also be used to shroud the CFC entirely. The images above provide examples of hybrid type fixtures previously supplied by PSS TECHNOLOGIES. CFC fixtures can also be supplied with a thermal barrier coating such as an Yttria-stabilized zirconia plasma spray coating in order to prevent contact reactions. However, from our experience, coatings do wear over time and fixtures often need to be sent out for re-coating, which results in higher fixture maintenance costs. The use of ‘built-in’ ceramics is always the preferred choice when working on new fixture design concepts with a primary function to prevent eutectic reactions. The ceramic parts become a low cost consumable item and can easily be replaced in the event of damage.
• REACTION WITH OXYGEN (O2) – When CFC is exposed to air and oxygen containing atmospheres and heated above 350°c, the carbon contained within the CFC matrix undergoes an oxidizing reaction whereby it burns with oxygen (O2) in the atmosphere and sublimates to form carbon-dioxide (CO2). If water vapor (H2O) is also present in the atmosphere, carbon within the matrix oxidizes above 700°c to form carbon monoxide (CO) or carbon-dioxide (CO2) with the elimination of hydrogen (H2). To prevent oxidation of CFC, many continuous pusher type furnaces use pre-oxidation chambers to burn-off any excess oxygen prior to heating the parts and fixture. This is achieved by purging the chamber with nitrogen gas and employing a pilot light to ignite any discharged oxygen. With single chamber vacuum furnaces, removal of oxygen is achieved using vacuum pumps prior to furnace heating. When specifying CFC fixtures, it is important that customers consider their entire heat treatment process from start to finish to ensure that at no point the CFC fixtures will be exposed to air above 350°c. For processes with very low oxygen concentrations, impregnated or coated CFC’s are successfully used. Pre-production testing of CFC for non-vacuum applications is always recommended in order to assess the potential for oxidation. This is normally done by running small CFC samples through the full production process multiple times and weighing the samples after each furnace run. If there is no reduction in sample weight after many runs, then this confirms the absence of oxidation.
• REACTIONS WITH MEDIA – Untreated CFC does have some open porosity in its structure which can lead to adsorption of liquid media such as quench oil and and washing solutions. To counteract this, impregnated CFC can be used to close up the open porosity and prevent ingress of liquids. A chemical vapor infiltration (CVI) treatment is used to infiltrate the porous carbon fibre composite material with a phosphorous salt, and prevent adsorption of media in to the matrix. This impregnation also improves oxidation resistance. However, this material grade is limited to a temperature of 900°c as at higher temperatures the phosphorous can leach out and deposit on component surfaces, resulting in lower surface hardness. When selecting treated CFC material grades for oil quench applications, pre-production testing using small fixture samples and scrap components is always recommended, particularly for gas carburising processes with post quench washing and air tempering chambers. Careful consideration must be given to the effectiveness of the post quench washing operation as residual oil left on the CFC fixture can result in smoke generation inside the tempering chamber and in some cases, the oil can even ignite if passing through a flame curtain. Test fixtures must be passed through the furnace multiple times to check for any signs of oil transfer and smoke generation. Following multiple furnace cycles, cracked oil will eventually fill any remaining open porosity in the CFC and prevent further smoke generation.

Process Conditions and CFC Fixture Design Criteria

When specifying CFC fixtures for use on a new furnace or when replacing existing cast steel fixtures , careful consideration must be given to the process conditions as well as the fixture design criteria. By doing so, this ensures that the fixture will be fit for purpose and meet the full demands of the application.

The lists below provides an overview of the process conditions and fixture design criteria that must be collected and passed on to our design team so that an initial fixturing design concept can be drawn up.

Fixture Design Criteria & Component Details

• Name or part reference for each component to be processed
• CAD model of each part to be processed (we are happy to sign an NDA) or main dimensions in the event that a CAD model cannot be provided
• Desired component orientation for each component (e.g. horizontal, vertical, hanging etc)
• Component dimensional and shape tolerances
• Load density (component spacing, positional accuracy)
• Desired load capacity (parts per fixture)
• Component fixturing support (e.g. point contacts, full contact positions

Process Conditions

• Material grade for each component processed (so we can check for chemical composition)
• Weight of each component processed
• Type of furnace (vacuum furnace, pusher type continuous, integral batch furnace etc)
• Furnace loading (front or bottom loading furnace? manual or automatic?)
• Furnace atmosphere (vacuum, LPC, gas) – Does the atmosphere contain Oxygen(O2)?
• Furnace temperature profile (e.g. raise temp. x°C per minute, hold at 1050c and quench to RT)
• Quenching medium (nitrogen/argon gas or oil quenching?)
• Furnace maximum working zone dimensions (L x W x H in mm)
• Furnace maximum load capacity (Kg)
• Hearth rail details
  • Hearth rail material (graphite, molybdenum, steel)
  • No. of hearth rails
  • Hearth rail width, length and height (mm)
  • Spacing between hearth rail centers (mm)
• Fixture handling method (e.g. Fixtures lifted by forklifts or pushed along)
• If forklifts are used, the following details would be helpful:
  • No. of forklift arms
  • Arm length (mm)
  • Arm width (mm)
  • Fixed or moveable arms (if moveable, what is the maximum separation)
• Existing no. of parts per fixture (for cast steel fixtures if applicable)
• For brazing processes (if applicable)
  • Braze material
  • Brazing line and position on part/assembly
  • If braze weights are applied, weight dimensions and position

How can PSS TECHNOLOGIES support?

PSS Technologies specialize in the design and supply of bespoke Carbon Fibre Composite (CFC) fixtures used for high temperature applications. Our team of materials engineers have a wealth of experience working within the heat treatment industries, and have first hand experience working with a broad range of thermal processes including vacuum heat treatment, gas and low pressure carburising (LPC), nitriding and ferritic nitrocarburizing.

We offer all fixture design work up-front and free of charge to all customers without obligation to purchase.

When designing fixtures, our primary focus is always to offer added value to the customer, achieve cost savings and ultmately, reduce the return on investment (ROI) time. That is achieved by:

• Increasing part loading per fixture
• Reducing furnace running (energy) costs
• Improving furnace capacity by reducing process cycle times
• Reducing fixture maintenance and upkeep costs
• Offering fixtures with extended product life cycles
• Reducing component re-work costs

Our materials engineers work hand in hand with our design team and together, they will be able to guide you through the material selection and fixture design process in order to offer a customised and commercially viable fixturing solution.

To speak to one of our expert advisors and discuss your fixturing requirements, please use our LIVE CHAT feature at the bottom right of the screen. Or alternatively, please email sales@psstech.co.uk and we will be in touch within 24 hours.