Rigid Carbon Felt for Sintering and Heat Treatment Furnaces: A Practical Guide to Insulation Hard Felt

Sintering and heat treatment look different on the shop floor, but they ask the same thing of a furnace: hold a set temperature, keep it even across the load, and do it cycle after cycle without contaminating the parts. The insulation package decides much of that. If it sags, sheds fibre, outgasses, or leaks heat at a joint, the process window narrows.

That is why engineers searching for "insulation hard felt" or "rigid carbon felt for vacuum furnace" usually want a material that insulates and also holds its own shape. Rigid carbon felt does both. This article covers what the material is, where it fits in sintering and heat treatment furnaces, which parameters to check when buying, and how to install and maintain it. Product data comes from the Overall Insulation Hard Felt product page.

What Is Rigid Carbon Felt?

Rigid carbon felt is a carbon-fibre insulation board or shell that has been bonded and cured so that it supports itself. Where soft felt is a blanket you wrap and pin, rigid felt is a part you fit.

Structure and Manufacturing Process

According to the product page, Overall Insulation Hard Felt starts with short-cut carbon fibres that are carded into a web. The mats are laid in specific layers and needled with 2.5D needling to make a thin felt. That felt is glued, hot-pressed and cured into a green body. Carbonization, high-temperature treatment, purification and machining follow.

Each stage affects what the furnace builder receives. Needling ties the layers together so the board resists delamination. Bonding and curing give it stiffness. High-temperature treatment and purification drive off volatile residues and lower ash, which matters in clean processes. Machining sets tolerances for assembly.

The manufacturer's quality inspection and technical capability pages explain how output is checked before shipment.

Rigid Carbon Felt vs. Soft Carbon Felt

Many furnaces use both: rigid felt as the structural inner layer and soft felt behind it to fill gaps and trim heat loss. Neither is better in the abstract; the design decides.

Key Properties for Furnace Insulation

The product page lists these advantages: high strength, low thermal conductivity, good thermal shock resistance, strong ablation and oxidation resistance, low expansion coefficient and low ash content. Here is what each means for design.

• Low thermal conductivity. Less heat crosses the wall, so shell temperatures and heater load are easier to manage. The page lists vertical-direction conductivity, which is the direction that matters for wall heat loss.
• Thermal shock resistance. Fast heating and cooling stress the lining. A material that tolerates this is less likely to crack at a ramp.
• Low expansion coefficient. Small dimensional change keeps joints closed and reduces stress on mounting hardware.
• Strength. Listed bending strength is 0.8 MPa for flat and 1.0 MPa for cylindrical felt, with compressive strength of 0.6 and 0.8 MPa. Those values suit a lining, not a load-bearing fixture.
• Low ash. Fewer mineral impurities means less risk of contaminating the work in clean processes.

For the wider product family, see the vacuum furnace field section and the full carbon fibre composite product range.

Applications in Sintering Furnaces

Insulation for Powder Metallurgy and Hard-Metal Sintering

Powder metallurgy and cemented-carbide (hard-metal) sintering run at high temperature for long holds, often in vacuum or controlled gas. The insulation must keep the hot zone stable and stay clean, because binder residues, vapours and powder can be present in the chamber. Rigid felt suits these furnaces as the inner thermal wall that surrounds the heating elements.

Supporting Temperature Uniformity During Sintering

Uniformity depends on more than conductivity. A panel with an open joint, a gap at a door, or a thin spot near a power feedthrough creates a local cold area. Parts nearby sinter differently from the rest.

Good practice is to stagger joints between layers, avoid straight-through gaps, and give the same attention to end plates and door seals as to the side walls. Those areas often lose more heat than the main wall.

Material Selection for Vacuum and Inert-Atmosphere Sintering

Atmosphere sets the rules. In vacuum or inert gas, carbon-based insulation is stable at high temperature. Oxygen is the problem: carbon oxidizes in air at elevated temperature, so leak tightness and gas purity matter. Volatiles released by the work can deposit on cooler felt surfaces, so surface treatment (see below) and bake-out routines are worth planning.

Applications in Heat Treatment Furnaces

Insulation for Vacuum Heat Treatment

Vacuum heat treatment furnaces often use a graphite or carbon-felt hot zone: heating elements inside, insulation outside, and a cooled vessel around it. Rigid felt forms the lining and keeps the heat inside the working volume. See our industry news section for other hot-zone topics.

Supporting Annealing, Hardening, and Brazing Processes

• Annealing: long, steady holds; stability and uniformity matter most.
• Hardening: fast heating and then gas or oil quench; thermal shock and dimensional stability matter.
• Brazing: clean surfaces are critical, so low ash and low outgassing are key. Filler metal vapour can deposit on the lining.

Thermal Cycling and Dimensional Stability

Every ramp expands and contracts the lining. Over hundreds of cycles, weak material cracks, erodes or loses fibre. Low expansion and good shock resistance slow that wear. Different furnace types (horizontal, bottom-loading, gas quench) apply different gas-flow and mechanical loads, so a grade that works in one may not suit another.

How to Select Rigid Carbon Felt for Industrial Furnaces

Operating Temperature and Furnace Atmosphere

Start with the real maximum temperature and the atmosphere: vacuum level, inert gas type, or partial pressure. Check the oxidation risk, including leaks and air exposure during cooling. Graphitization temperature of the felt should sit well above your working temperature; the product page lists ≥2000 ℃.

Density and Thermal Conductivity

Density and conductivity are linked, but the number alone is not enough. Ask for the test temperature, test direction and method. Standard methods such as ASTM C177 (guarded hot plate) or ASTM E1461 (laser flash diffusivity) are common references; confirm which one the supplier used. Conductivity changes with temperature, so a value at room temperature does not predict performance at 1500 ℃.

Mechanical Strength and Dimensional Requirements

Choose the form by how the lining is supported, the space available and the machined size needed. Flat felt is listed up to length 2600 mm, and cylindrical felt up to diameter 2000 mm. Larger shells are normally built from segments.

Ash Content and Material Purity

For clean sintering or brazing, ask about ash content, volatiles and the purification step. Ash analysis is commonly run to ASTM C561 or an equivalent method. Our semiconductor field products show how purity-sensitive applications are handled.

Flat vs. Cylindrical Insulation Hard Felt

Flat Felt for Panels and Furnace Sections

Flat felt fits walls, partitions, end plates, doors and rectangular chambers. Panels are easy to cut, replace in sections and stack in layers. The product page lists flat felt density of 0.16–0.20 g/cm³.

Cylindrical Felt for Circular Hot Zones

Cylindrical felt matches round chambers, so it has fewer seams and a more even wall. It speeds assembly and cuts down on joint losses. Listed density is 0.16–0.25 g/cm³.

Surface Treatments and Customized Fabrication

• Bare felt: the base option for backup layers.
• Graphite paper laminated: smoother face that helps limit fibre shedding and is easier to wipe clean.
• Carbon cloth laminated: added surface toughness.
• Low-porosity plate laminated: stronger hot face and lower gas permeation.
• Coating: a sealed surface where erosion or deposits are a concern.

For custom shapes, browse the preform field, or contact the engineering team with a drawing.

Installation and Maintenance Considerations

Proper Fit and Joint Design

Check dimensions on receipt. Dry-fit the assembly before final install, keep gaps small and consistent, and stagger joints across layers. Handle with clean gloves and avoid crushing edges. Because carbon dust is conductive, keep it away from heater insulators and electrical feedthroughs.

Inspection After Repeated Thermal Cycles

At scheduled shutdowns, look for:

• cracks or open joints,
• worn, eroded or flaking surfaces,
• permanent deformation or warped panels,
• deposits or discoloration near gas inlets and heaters.

When to Repair or Replace Furnace Insulation

Plan replacement when you see rising power use at the same setpoint, new cold spots, shell hot spots, or visible fibre release in clean processes. Small damage may be repaired by replacing a panel. Widespread cracking or contamination risk is a signal to re-line during a planned outage rather than wait for an unplanned one.

Why Choose Overall Insulation Hard Felt from Dehong?

Zhejiang Dehong Carbon Fiber Composite Material Co., Ltd. makes Overall Insulation Hard Felt in flat and cylindrical forms, with five surface options: bare felt, graphite paper laminated, carbon cloth laminated, low-porosity plate laminated and coating. Learn more on the company profile page, or see related photovoltaic field products.

Important: the values above are representative values from the product page, not guaranteed values. Final specifications must be confirmed by the supplier for your specific order.