Annealing is a technique that craftsmen use. However, a particular method is extremely popular among shooters. I am writing about salt bath annealing. Experienced shooters use it to alter the mechanical properties of the bullet case. In this article, I will mention the benefits of this method. Also, I will offer a step-by-step method to perform it. Let’s get to it.
- 1 What is Salt Bath Annealing?
- 2 Why is Salt Bath Annealing Performed?
- 3 How to Perform Salt Bath Annealing?
- 4 What is the Mechanism behind the Process?
- 5 What are the Advantages and Disadvantages?
- 6 What Equipment Do I Need for Salt Bath Annealing?
- 7 7. What are the Alternatives to This Method?
- 8 Conclusion
What is Salt Bath Annealing?
Salt bath annealing is a heat treatment process that modifies the physical and chemical properties of the materials. Annealing increases the workability of material by increasing its ductility and reducing its hardness.
All of the annealing process consists of heating a material above its recrystallization temperature and stabilizing the material at a suitable temperature for a constant period and then decreasing the temperature at a specified cooling rate. The suitable temperature ranges from the recrystallization temperature of the material to the transformation temperature. The temperature should not cross the transformation temperature and it should be more than the recrystallization temperature.
In different annealing processes, induction rods and flames heat the working metal. In salt bath annealing, a molten salt bath serves the purpose of heating the metal. It’s definitely a cheap, easy, and reliable process.
With the help of digital thermometers, the temperature of the salt bath is controlled. Optimized conditions can only be achieved if the bath is at the right temperature. There are three major steps involved in salt bath annealing, i.e., recovery, recrystallization, and grain growth.
Why is Salt Bath Annealing Performed?
In different manufacturing processes such as rolling, materials undergo different steps and they develop internal stresses and residual stresses in their grains. The machining of different materials at higher temperatures introduces internal stresses into the materials. Salt bath annealing is a process that relieves these internal stresses and makes the material more ductile and flexible.
Moreover, different industries alter the microstructure properties of materials with the help of annealing. Alteration of different microstructure properties helps in preparing the materials for further machining processes. For instance, there are chances of crack during the cold working of the material. Salt bath annealing refines the internal structure of the materials and reduces the chances of crack during the cold working.
On a larger scale, annealing improves the machinability of a material. Brittle and hard materials cause excessive tool wear. Salt bath annealing reduces the hardenability of material and results in reducing the tool wear.
Why Shooters Use It
Shooters use salt bath annealing for annealing the brass cases. In the beginning, brass cartridge cases are very ductile and soft. They are very easy to mold. However, firing causes a decrease in the ductility and brass becomes springy. With time, the re-sizing of cartridges becomes difficult and it reduces the chances to chamber the cartridges. Moreover, it makes the neck-tension inconsistent. Serious shooters chose salt bath annealing over scrapping the brass.
Other annealing processes such as spinning the cases in the electric-drill are very dangerous and it is a hit and trial method. Similarly, AMP inductions annealing is a safe process for the brass cartridges but it does not offer accuracy and consistency. On the contrary, salt bath annealing offers an accurate, safe, and consistent process for annealing the brass cartridges. Moreover, the process is compatible with performing at domestic levels.
How to Perform Salt Bath Annealing?
This portion of the article will highlight the procedure of salt bath annealing. Various methods can produce similar results. However, AMP annealer, blow-torch annealing, and paint methods are very expensive. It ensures the same results with a very simple process. Since most readers are interested in using it on brass cases, I will focus on this process. However, the process will be similar to any other project. Understanding the following steps will help you perform the process correctly.
Preparing for the Process
Selecting the right equipment for obtaining accurate results is very important. They will be discussed in the next section. This section provides the general procedure for salt bath annealing. The process must take place in a well-ventilated area.
Salt bath annealing requires the following equipment:
- Salts (more on this later)
- Digital Thermometer
- Sample Material (Brass Cartridges)
- Smelting Lee pot
- Water Bucket
- Protective Gloves
- Protective Glasses
You will find links about the recommended equipment later in this article.
Mixing the Salts
Salt bath annealing of the material (brass) requires three different types of salts. Potassium Nitrate, Sodium Nitrate, and Sodium Nitrate are mixed. The mass/mass mixture of three salts are prepared by mixing 50% of potassium nitrate, 45% of sodium nitrite, and 5% of sodium nitrate. Melting points of the salts are given below:
(Note: Composition of the material decides the mixture of the salts). This process requires these three salts in the given composition).
- Potassium Nitrate = 334 ˚C
- Sodium Nitrite = 271 ˚C
- Sodium Nitrate= 308 ˚C
Mix all of the three salts in the right proportion in the open air. Do not mix the salts in a closed room. If you have to mix them in a closed room, then ensure the proper ventilation.
Preparing the Salt Mixture
After preparing the mixture of salts, place the salt in the smelting pot. From the melting points of the salts, it is obvious that heating the mixture at any temperature above 400 ˚C will melt it. Most shooters consider that the optimal temperature is around 500 ˚C (935 ˚ F). Don’t be stressed to achieve exactly this temperature. Any temperature from 450 ˚C to 530 ˚C is suitable for annealing brass cases. After increasing the temperature to 500 ˚C, wait for the stabilization of the mixture. Thermocouples help in monitoring monitor the temperature. Stabilization of the temperature is also important for maintaining the uniform composition of the mixture while melting. This step is the most time-taking step.
Performing the Annealing Process
It is time to perform the annealing process. Pick up the sample (brass) with the help of the holder and dip the end that needs to be annealed in the salt bath. Keep it inside the bath for 6 to 8 seconds. However, time may vary with the other materials.
Cooling down the Material
Now, place the annealed case in the water bucket. The temperature of the water defines the cooling rate of the material. Different materials require different cooling rates. Leave the sample in the water bucket and anneal the next sample. It will help in annealing multiple numbers of samples at a time.
Leave the pot and wait for the cooling down and solidification of the salt. Knock out the salt from the tip and store it for the next use. You will see a slight shrinkage in the salt after the solidification.
What is the Mechanism behind the Process?
When the metal dips in the salt bath, the atoms of the material shape themselves up in a crystal lattice and reduce the dislocations. Dislocations are the abrupt changes in the arrangements of atoms that occur due to different manufacturing processes. Sometimes, the environment also affects the materials and create dislocations in the materials.
Dislocations affect the ductility and the hardness of the materials. When the material cools down, it recrystallizes itself with the new arrangements of the atoms. The composition may be referred to as the physical composition as well as the chemical composition. The grain size determines the material properties. The phase diagram and composition of the material affects the way this process is performed correctly.
What are the Advantages and Disadvantages?
- First of all, salt bath annealing is a cheap process. It has very minute start-up costs. You do not need any heating over or furnace for it. The only consumables are salt and water.
- It allows you to accurately monitor the temperature. It eradicates the chances of overheating and burning of the material. Moreover, it offers consistency and consistency improves the quality of the results.
- Salt bath annealing is a quick process. The soak time is different for different materials but it is no larger than a few minutes. The use of two holes baffles can add more speed to the process.
- Salt baths are made up of aluminum and corrosive-salts can corrode it. To avoid the corrosion of your salt bath, install high-temperature corrosive-resistance paint coatings before initiating the annealing process. Otherwise, you will end in corroding all of your precious tools.
- The temperature consistency of the process is not up to the mark. At higher temperatures, it flatters a lot that disturbs the consistency of the annealing process.
- Salt catches moisture from the environment and it becomes wet. It causes the salt to smoke during heating. The use of air evacuation mechanisms can solve this problem.
- It does not soften the material by more than 120 HV. This is much harder than the required softness of the material.
The exposure to the high temperature of the molten salt is very risky. Moreover, there are no signs for such a high temperature such as smoke or roar of the blow torch.
What Equipment Do I Need for Salt Bath Annealing?
In this section, I will talk mention everything you need to perform salt bath annealing.
LEE PRECISION manufactures the Pot IV (Grey). This pot can hold up to 10 pounds of molten salt. The base of the pot is very stable and safe. It can bear temperature up to 500˚C. There is a coil around the walls of a pot that provides heating to the salt mixture. You can check it out here on Amazon.
It is a digital thermometer with the attached thermocouple. It senses temperature from -50 ˚C to 1300 ˚C. The unit contains a 2 type K thermocouple. One is the 3 inch K type thermocouple that ranges from -50 ˚C to 400 ˚C and the second is the Kiln K type thermocouple that ranges from -50 ˚C to 1300 ˚C. You can check it out here on Amazon.
Potassium nitrate is an essential salt of the salt mixture. One point potassium nitrate is available here on Amazon.
Shor provides a bag of 1 pound sodium nitrate. You can check it out here on Amazon.
Loudwolf provides the fine quality of sodium nitrite. One bottle contains 4 Ounces of the product. Therefore, 4 bottles of this product can provide you with the required amount (1 lb) of sodium nitrite. You can check it out here on Amazon.
Safety is an important part of any process and many people overlook it. So far I have already mentioned that you have to perform this process in a very well ventilated area. However, apart from protecting yourself from the fumes during boiling, there some additional safety measures.
7. What are the Alternatives to This Method?
Salt bath annealing is an easy and cheap process but it is not the only process available for achieving the desired results. There are many processes available for improving the strain size and microstructure properties of the materials. Some of the alternatives are:
- Normalizing: It is a technique that improves the uniformity of the grain size over the entire composition of the ferrous alloy. It involves heating the material above its upper critical temperature.
- Stress Relieving: This technique removes the internal and residual stresses from the metal.
- Aging: Aging helps in increasing the life of the alloy by increasing the tensile strength of the materials. Aging prevents the individual compositions of the materials in an alloy.
- Quenching: Quenching involves the cooling of the metal at a rapid rate. This helps in increasing the hardness of the material. Quenching helps in reducing the chances of low-temperature processes and it prevents the occurrence of phase transformations.
- Tempering: Tempering is used for increasing the softness of a metal. It is usually performed for the brittle steel. Tempering reduces the hardness of the materials and introduces toughness. Reduction in hardness increases the ductility of the materials. Tempering is preferable for the alloys.
I hope that you have found helpful this article. If you have any recommendations about the process let me know in the comments. I would appreciate it if you could share this article with anyone that could benefit from it.