“Bright annealing” is a general term referring to several heat treatment processes performed on metal alloys to remove residual stresses, modify their microstructure and, generally speaking, improve their mechanical properties. Regardless of the specific material and heat treatment recipe applied, the aim of bright annealing is always to obtain surfaces that are shiny, reflective, and oxide-free, in one word: bright.

For that reason, bright annealing needs to be performed in a controlled environment, such as a vacuum, inert gas or a reducing atmosphere, to prevent oxidation and contamination, ensuring the metal retains its original luster.

Vacuum furnaces are of course the perfect machines to perform bright annealing treatments: by effectively eliminating air and other gases that could cause oxidation or contamination of the metal surface, we can always be sure to get our heat-treated parts as bright as possible, right?

Well, yes and no. In reality, several factors are influencing the surface appearance of metal parts after the heat treatment; only a careful optimization of the process parameters in relation to the material can ensure its success.

Let’s have a look more in detail at what are the key factors in that respect and how they will influence the outcome of our bright annealing treatment!

Principles of Bright Annealing

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On April 17-19 2024, TAV VACUUM FURNACES participated as a speaker at the 4th MCHTSE, Mediterranean Conference on Heat Treatment and Surface Engineering.

Our speech focused on the energy aspects of vacuum heat treatment, a subject towards which all of us within the industry need to pay attention for reducing the carbon emissions aiming at a zero net emissions future.

We have already analyzed the essential role that vacuum furnaces will play in this transition, with a focus on the optimization of energy consumption in our previous article; with this new presentation instead, we wanted to emphasize how selecting the right vacuum furnace configuration for specific processes may impact the energy required to perform such process. For doing so, we compared two different furnace designs, i.e. single chamber VS dual chamber vacuum furnaces, detailing all of the components energy consumption for a specific process.

As a sneak peek into our presentation, we will summarize below how the main features of the two vacuum furnaces design are affecting their energy performance.

Let’s start by introducing the protagonist of our comparison: a single chamber, graphite insulated vacuum furnace, model TAV H4, and a dual chamber furnace TAV DC4, both having useful volume 400-400-600mm / / 16”x16”x24” (WxHxD). 

TAV DC4, dual chamber vacuum furnace for low pressure carburizing and gas quenching

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In the first part of the article we discussed about powders and the classifications adopted for cemented carbide, as well as the importance of using a proper dewaxing cycle.

In this second part we will go more in depth about the sintering process, discussing the difference between vacuum sintering and sinter-HIP for cemented carbide and the equipment.

Sintering

Vacuum furnaces can be designed to carry out both thermal dewaxing and sintering on the same piece of equipment. In that case, furnaces are equipped with a wax trapping system and a box with calibrated gas nozzles for an homogeneous gas flow distribution. The gas flow distribution is critical to efficiently remove wax vapors from the parts and ensure an optimal temperature uniformity.

TAV VACUUM FURNACES HM Series - Sinter-HIP furnace equipped for hydrogen overpressure operation. 
The electrically fired H2 burner is visible on top of the furnace.

However, separated dewaxing furnaces are sometimes used to improve productivity and cleanliness on the sintering furnace. In the that case, even if dewaxing is generally completed at temperatures below 500°C, parts are heated to higher temperatures approaching 1000°C as a pre-sintering step, to ensure sufficient strength on the parts to be moved to the sintering furnace.

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