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The Different Types of Heat Treatment

December 1st, 2021

You may have seen the words “heat treated” in relation to steel products like knives, hand tools, and other works of steel. But what exactly does the term “heat treated” really mean? Heat Treatment consists of heating up metal to at or above a predetermined critical temperature and then cooling it down. This extreme heating will change certain chemistry within the alloy and the cooling down process will set those changes within the alloy permanently. We’ll look at the different types of heat treatment for metal alloys below.

Annealing

Annealing is widely used and one of the most popular heat treatments around for metal alloys. The typical procedure for annealing is to put the metal into a walk-in furnace or hot room in the plant. The metal’s temperature will rise to a reading around 50°C above critical at a steady rate. Once the metal has reached this point for a set period of time (2-3 hours) the metal will then cool slowly back to its normal range.

This cooling down procedure is done by stepping the hot room temperature down little by little over a range of time. Think of a heater’s thermostat that kicks on and off due to the air temperature. In the case of annealing, this thermostat will slowly drop in temperature until the final cooldown temperature is reached. This process will “soften” the steel to make it more malleable for machine work.

This process will also remove any gases that remain in the steel’s composition, thereby strengthening the steel. Another benefit to annealing is that it will magnify the metal’s magnetic and electrical properties which is ideal for powder coat and other electrical paint mediums.

Normalizing

Normalizing has some similarities with Annealing as they both rise to the same 50°C above critical threshold. The major difference with Normalizing is that the holding time at that temperature is much less than with Annealing. The cool down process is also much quicker as the metal is transferred out of the hot room back into the open plant to air dry at normal ambient temperatures (room temperature).

This cool down process (or air quenching) will forge the changes made at the heat point made earlier. Normalizing relieves any stresses within the metal and is typically done to cold-worked steel such as stamped, hammered, or rolled steel.

Hardening

Hardening is the process of toughing up the steel to make it harder and less brittle. If Annealing is the “Yin”, Hardening is the “Yang” as it’s the exact opposite of Annealing in terms of heat treatment. If you’ve ever seen a blacksmith make a sword or a knife, you’ve seen Hardening in action as it’s an important part of the process.

The metal is heated to 40°C above critical temperature and then rapidly cooled by placing it in oil or water. Sometimes, a brine solution is used in place of water as water can sometimes cause bubbles to form on the metal’s surface. You can see the end results of hardening in objects such as the aforementioned knives and swords and also gears, taps, dies, and chisels.

Tempering

Hardening can cause metal to become brittle in places depending on the thickness of the metal being hardened. The typical next step when using the Hardening method to heat treat thin metals such as blades is to temper it. Tempering will reduce the brittleness of the metal by removing internal stresses on the metal that developed during the Hardening process.

The usual process for tempering metal is to take the freshly hardened piece and reheat the metal back up. This heating point is usually at a range between ambient temperature and critical point temperature. This temperature could reach as high as 700°C and once this temperature is reached, the metal will stay ensconced for a period of time. Tempering is typically done in an oil bath or molten lead bath and the holding time at top heat is based upon the type of metal used and the desire of hardness in the metal.

Nitriding

Nitriding is the process of forming a thick “skin” on the surface of the metal and this is typically done by utilizing nitrogen gas, hence the name “nitrding”. This type of heat treatment is perfect for those who don’t want the entire metal piece to be hardened and just want the surface toughened up. This method is sometimes used for tools that typically see a lot of dings and scratches over time such as wrenches.

The process for nitriding is to place the metal in an airtight chamber of which there are tubes feeding into the chamber. The tubes are inlet and outlet for ammonia gas which will circulate inside the chamber. The chamber is heated up in a furnace reaching 500°C which will cause nitrogen gas to be released from the circulating ammonia inside the chamber. The nitrogen gas will form atomic nitrogen due to the heat and this will cause a chemical reaction with the metal, forming iron nitrate on the surface.

The longer the metal is at this iron nitrate state at elevated temperatures, the deeper the iron nitrate will penetrate the metal. Once the process is completed, there is no cool down or quenching needed as the iron nitrate will be permanently bonded to the metal.

Cyaniding

Cyaniding is a heat treatment method for small to medium-sized pieces of metal which will strengthen the metal’s fatigue point and limit. This heat treatment method will also raise the surface hardness of the metal. The name “cyaniding” is derived from the sodium cyanide that is used in the heat treatment process.

The process for cyaniding starts with the metal being placed in a molten cyanide salt bath at a temperature of 950°C. The molten salt bath consists of sodium cyanide, soda ash, sodium carbonate, and sodium chloride. The metal is kept in this molten bath for around 20 minutes as the sodium cyanide will break down in the heated environment. This chemical breakdown will cause nitrogen to form as well as carbon from the carbon monoxide. The nitrogen and carbon will seep into the metal’s surface causing it to harden during the process.

Once this process is completed after 20 minutes, the metal is taken out of the molten bath and then quenched in either an oil or water bath to cool the metal down and completing the hardening process.

Induction Hardening

You may have used an induction burner in a kitchen before and wondered how your steel pan heated up with no visible glow from the burner. This is due to the chemical reaction from the metal in the pot or pan used along with the burner’s internal AC current. This is very much the same chemical reaction that is used in induction hardening for heat treatment of steel.

The process typically uses a large copper coil in which the metal is placed inside. AC or alternating current is then applied to the copper coil which causes a chemical reaction within the metal to heat it up beyond critical temperature. The metal is then placed into a quench bath to complete the heat treatment process.

Flaming Hardening

Unlike other methods so far, flame hardening is done by actual flame and not hot rooms or furnaces or molten baths. The process forms a hard surface on the metal and improves wear resistance. The process time to heat treat in this way is also much faster than other methods. An oil or water quench is immediately used once the metal is off of the flame. This method works best on machined parts such as crane wheels and rollers.

SST for Heat Treating

Specialty Steel Treating Inc. has been in the steel heat treatment game for over 60 years. We have provided steel heat treatment services for industries such as industrial, rail, heavy equipment, automotive, tool & die, and aerospace. If you’d like to know more about our steel heat treatment services, please contact SST today!

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