“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
In the world of heat treatment, discoloration of the parts might have different sources; the most common one is the formation of surface oxides due to the reaction between metals and oxygen or moisture present in the furnace, at high temperatures. Additionally, other gases can lead to similar reaction, for example nitrogen can form nitrides layers having a distinguishable tint when encountering certain metals at specific temperatures.
To avoid the formation of surface oxides on metal parts, we “simply” need to carry out our heat treatment in conditions that are reducing in respect to the material, meaning that we need to create an environment in which metal oxides are more likely to lose oxygen atoms and convert back into their pure metallic form rather than the other way around.
We have deliberately put the word “simply” in quotes since, in industrial practice, knowing exactly how reducing or oxidizing the atmosphere in our furnace is, can be everything but an easy job; that means having a good knowledge about the chemical composition of the parts, a good understanding of the oxidation/reduction curves of their alloying elements and a good control on the relevant process parameters inside the furnace.
The oxidation behavior of metal parts is dictated, in relation to their chemical composition, by the Ellingham diagram; a graphical representation of the temperature dependence of the standard Gibbs free energy change (ΔG°) for the formation of oxides from their elements. In more practical terms, the Ellingham diagram indicates for which combination of partial pressure of oxygen and temperature an element will oxide or, vice versa, on oxide will reduce. Let’s have a look at the following picture.
Ellingham diagram
Each line on the Ellingham diagram represents the reaction for the formation of a specific metal oxide; if a specific combination of temperature (“T / °C” horizontal axis) and partial pressure of oxygen (“p02” vertical axis, right) puts us above the metal oxide line, it means that the conditions in our furnace are oxidizing for that metal. If that combination puts us below the metal oxide line, it means that the conditions in our furnace are reducing for that metal.
It is easy to see from the diagram that some metal oxides are far easier to reduce than others; copper oxide for example, at high temperature, will be reduced back to its metal form even at relatively high partial pressure of oxygen. Aluminum oxide, in contrast, will require significantly lower p02 to be reduced at a given temperature; in reality, reduction of aluminum oxide requires an oxygen partial pressure so low, that it is impossible to be reduced by thermal means before reaching the melting point of the material.
An important note is that Ellingham only provides information about the favorability of oxidation/reduction reactions from a thermodynamic point of view, while it does not take into account reaction kinetics, i.e. how fast those reactions will proceed. In practical term, that means that how much our parts will be oxidized or deoxidized during a heat treatment does not only depend on the favorability of the heat treatment conditions but also on its duration.
Finally, a crucial information contained in the diagram is how metal oxides will behave when paired with each other; a given metal can reduce the oxides of all other metals whose lines lie above his on the diagram. For example, calcium oxide will reduce titanium oxide to metallic titanium; for that very reason, special care should be exercised about which materials are placed in close contact during bright annealing cycles.
Vacuum Furnaces: The Ideal Environment for Bright Annealing
Vacuum furnaces are inherently able to create a very low partial pressure of oxygen and other residual gases (for instance, nitrogen); for that reason, they represent the ideal environment for bright annealing.
That said, there is no such thing such a perfect vacuum; even at the deepest vacuum level, infinitesimal quantities of oxygen, nitrogen, hydrogen and other species are still present.
Of course, with different vacuum levels are associated different impurity levels; vacuum furnaces can generally reach working vacuum as low as 1E-6 mbar, though deeper vacuums are achievable for more niche applications. If you want to know more about vacuum pumps models and their ultimate vacuum levels, check out our article Roughing pump in high-vacuum furnaces for beginners.
Even if measuring the vacuum level of a vacuum furnace is a relatively easy task, knowing the exact composition of the residual gases, hence the reducing power inside the furnace, is far from straightforward: dedicated sensors, such as residual gas analyzers (RGA), are available, but their applications are generally limited and interpreting the results typically requires experience.
For that reason, in first instance, the vacuum level inside a vacuum furnace may roughly be approximated with the partial pressure of water vapor. Keeping that in mind, the Ellingham diagram showed above, can be redesigned as follows.
Metal / Metal oxide equilibrium curves in a vacuum furnace
Additionally, vacuum furnaces allow the flexibility of using partial pressures (i.e. pressures below atmospheric) of inert gases or hydrogen, which is a common practice while annealing at high temperatures materials containing volatile alloying elements; that is case, for example, of solution annealing for austenitic stainless steel. When partial gas pressures are used, obviously, it is not only the initial vacuum level that influences the reducing power of the vacuum furnace, but also the composition and dew point of the gas itself (the dew point of a gas is inversely proportional to its moisture content, hence to its purity). Still, the gas consumption required to operate vacuum furnaces under partial pressures is significantly lower compared to traditional controlled atmosphere or hydrogen furnaces, so, in this respect, vacuum furnaces also offer a significant advantage in terms of gas consumption.
Practical cases
At this point, we have understood the physical principles that govern the bright annealing process in a vacuum furnace, but how should we apply them in practice?
Let’s have a look at a couple of examples:
- Austenitic Stainless Steel: Austenitic Stainless steel, such as 304/304L or 316/316L, is one of the most commonly vacuum bright-annealed class of materials. The process enhances its corrosion resistance, mechanical properties and removes residual magnetism; it typically involves heating the steel to approximately 1060-1070°C, soaking for a sufficient amount of time, then cooling to room temperature. To avoid carbide precipitation, the heating and cooling rates are generally fast and pressurized inert gas is introduced in the vacuum furnace to speed up the cooling.
The main alloying elements of such steels are chromium and nickel. From the diagrams presented above, we can easily see that at 1060°C even a medium vacuum level, in a range around 0.1 mbar, is enough to place us well below the equilibrium curves for nickel and chromium oxides. Since heating and cooling are typically fast, our part will go through the less favorable, low temperature range quickly, preventing the parts from reoxidizing.
In industrial practice, the vacuum furnace is first evacuated below 0.1 mbar, then a partial pressure of inert gas, with a dew point at least below -50°C, is introduced to avoid chromium depletion at high temperatures due to an excessively high vacuum level.
- Precipitation Hardening Stainless Steel: The heat treatment of precipitation hardening stainless steel, such as 17-4 PH or 15-5 PH, involves a first solution annealing treatment at temperatures above 1000°C, with rapid cooling to room temperature, followed by an aging treatment to form evenly distributed, copper-rich precipitates that enhances its hardness and strength. The aging treatment is carried out in the 480°C-620°C range.
The main alloying elements of such steels are, again, chromium, nickel and copper. In that case, however, the temperature range used for the aging treatment places us in a much less favorable condition; nickel and copper oxides are generally easily reduced in those conditions even in a medium vacuum, however, to stay in a reducing condition in respect to the chromium oxide curve, the vacuum level required is significantly higher, in the 1E-5 mbar range. For that reason, whenever a bright finish is required for precipitation hardening stainless steel, the aging heat treatment should be carried out in a suitably designed vacuum furnace, able to comply with the above specifications.
Conclusions
Vacuum furnaces are ideal for bright annealing processes; by creating a vacuum environment, vacuum furnaces eliminate the risk of oxidation and contamination, ensuring that the annealed materials exhibit a bright, metallic finish, while minimizing gas consumption in respect to controlled atmosphere or hydrogen furnaces.
The success of bright annealing on vacuum furnaces, depends on a thorough understanding of the material's chemical composition and its oxidation/reduction dynamics; optimal results require a precise control of the vacuum level in respect to the time-temperature profile to ensure the metal remains in a reducing environment during the heat treatment.
Choosing the correct configuration of vacuum furnace, depending on the treatment, is crucial to ensure that you can operate within the correct parameter range for your material; with forty years of experience in vacuum heat treatments, TAV VACUUM FURNACES is the perfect partner to help you choose the best solution for you.
If you’re interested in the topic, get in touch with us to learn more about bright annealing and discover what is the perfect solution for your needs!