What exactly is the heat treatment of metal?
Heat treatment is a process that combines heating and cooling at a specific rate, with or without changing the metal composition. The heat treatment process is applied to solid metals or alloys in order to obtain particular properties.
The heat treatment process is divided into several stages. Begin by heating to a specific temperature, then hold the temperature for a few moments before cooling at a particular rate.
Heating Time (Holding Time)
The time it takes to reach the desired temperature is determined by several factors, including the type of furnace, heating element, and steel. In general, the highest setting temperature from a predetermined hardening temperature range is chosen.
Process of Rapid Cooling (Quenching)
The process of rapidly cooling metal from the austenite temperature, which is typically 815°C – 870°C for steel, is known as quenching. The quenching medium used is determined by the alloy’s hardenability, product thickness, and the amount of cooling required to achieve the desired microstructure. Oil, water, and gas are common media (helium, argon, nitrogen).
The Heat Treatment process is divided into two stages:
Softening
Softening is the process of reducing mechanical properties in order to make something soft. Usually to make the machining process more accessible.
Hardening
Hardening is the process of increasing a material’s properties, particularly its hardness. Typically found in high-hardness products such as stone chopping tools, excavator bucket nails, and so on.
Types of Heat Treatment Processes on Metals and Alloys
Annealing
Annealing is a heat treatment that involves heating and holding at a suitable temperature before cooling at a suitable rate, primarily to soften metallic materials. In general, this process produces a ferrite-pearlite microstructure in ordinary carbon steels. Carbon steel is annealed to improve mechanical or electrical properties, as well as to improve dimensional stability.
Hardening
Hardening is a type of heat treatment used to increase the hardness and strength of the metal material. The steel is heated to between 30°C and 50°C during this process. It is held at this temperature for a short time before being rapidly cooled to form a hard microstructure. The harder the product, the higher the carbon content. This hardening process is suitable for carbon steel, high and low alloy steel. The elements Mn, Ni, Cr, and Mo increase hardness but have a slow cooling rate.
Tempering
Tempering is a process in which previously hardened or normalized steel is heated to a temperature below a lower critical temperature and cooled at an appropriate rate. It is used to increase ductility, toughness, matrix grain size, relieve stress, and reduce the hardness that increases during welding.
Normalizing
Normalizing is a heat treatment process that involves heating to a specific temperature and then slowly cooling it with air cooling media. The goal is to achieve a uniform structure with fine crystalline grains. Depending on the heat treatment and mechanical properties of the steel prior to the normalizing process, this process can also increase or decrease the strength and hardness of the steel.
Nitriding
Nitriding is a heat treatment process in which the workpiece is heated to 500°C for 40-100 hours as desired. This heating takes place in a dense nitrogen gas chamber with circulating ammonia. Ammonia gas decomposes, releasing nitrogen atoms that are absorbed by the steel surface. The object will change after the nitriding process, such as:
- Very hard surfaces
- Corrosion resistance
- Remains firm up to 500°C
Carburizing
Carburizing is the process of adding carbon to the surface of steel with a carbon content of less than 0.3% in order to harden the product’s surface. This process can be done in three ways: solid media carburizing, liquid media carburizing, and gas media carburizing. See the carburizing method explanation below.
Solid Media Carburizing
How does solid media carburizing work?
- The carburized product is placed in a closed steel box containing carburizing material, such as durferrit-coconut shell or charcoal, which is added to a catalyst. This catalyst is typically sodium carbonate (soda ash) mixed with barium carbonate, and it is useful for speeding up chemical reaction processes.
- Slowly heat until it reaches a temperature of 900°C-950°C and maintain that temperature for up to 6 hours or more, depending on the desired hardness,
- Cooling is done in a heating furnace until the temperature drops to 350°C, at which point it is cooled in free air.
- If there are parts that do not need to be carbonized, they can be coated with copper as thick as 0.07-0.1 mm because carbon does not dissolve in solid copper.
- Small products can be protected by using a special fire-resistant paste.
Liquid Media Carburizing
Liquid media carburizing is used in the following ways:
- The carburizing process takes place in a liquid mixture of 20% to 25% sodium cyanide (NaCN), 40% sodium carbonate (Na2Co3), and 40% sodium chloride (NaCl).
- The decomposition of sodium cyanide compounds on the steel surface is the working concept of Carburizing liquid media. Where the carbon atoms and nitrogen atoms separate and increase the carbon content of the steel material product’s surface,
- The chemical liquid mixture is heated in an iron box to 870°C-950°C, then the carburized product is placed in a wire basket and immersed in the chemical liquid for approximately 5 minutes before being removed and cleaned.
Gas Media Carburizing
Gas media carburizing is accomplished by heating the product at 900°C for 3 hours or more in a special furnace containing the hydrocarbon gases methane and propane so that the product’s surface is coated with carbon atoms.
To improve gas distribution, the hydrocarbon is usually mixed with a gas mixture of nitrogen, hydrogen, and carbon monoxide.
Carburizing of gas media has several advantages over other carburizing processes, including:
- keeping the metal product‘s surface clean
- allowing for mass production
- controlling the carbon content of the coating