Process Annealing versus Spheroidising Anneal
Characteristic, Precautions, Limitation
Figure 1: Heat treatment process
Annealing is a heat treatment technique to alter the microstructure and properties of a metal. The properties that will be affected are ductility, electrical conductivity, toughness and machinability. Typically, annealing reverse the changes made by cold working on a metal. Different metal require different time and temperature in order to carry out annealing. It is basic heat treatment process. In a mass production industry, almost every piece of metal experience once or more annealing in the transition from raw material to end product. A suitable temperature that higher than critical temperature is chosen to carry out annealing. Another reason why annealing is preventing crack as annealing increase the ductility of a metal. Annealing is mainly used for steel. However, other metal like aluminium, copper or brass can be subjected to a process called solution annealed. In industry, large oven is used for annealing of steel. The oven that used must be large enough to allow air flow circulation inside the oven for slow cooling.
Process annealing is a type of annealing and also called intermediate annealing. Process annealing mostly used for low carbon steel (carbon <0.25% carbon). The main idea of process annealing is to restore the ductility of the steel. By increase the ductility of the steel, the low carbon steel is allowed to undergo cold working without fracture. Process annealing require raise temperature up to somewhere just below the Ferrite-Austenite region. The temperature is held for a sufficient time to allow ferrite phase recrystallization. Then, cool down to room temperature. Process annealing is done to reduce hardness and increase ductility. Process annealing is mostly used in sheet and wire industries.
Spheroidising is a type of annealing process mainly used for iron-based alloys. Spheroidising commonly used for high carbon steel (carbon > 0.6%). To carry out spheroidising annealing, the metal is heated at slightly below its eutectoid temperature (temperature at which the solution is a solid solution rather than liquid) then cool slowly. The microstructure of the metal will change during spheroidising annealing. After spheroidising annealing, microstructure of the metal will contain sphere-like cementite particles. Spherodising annealing is normally carried out under a protective (endothermic) atmosphere to prevent oxidation and decarburization. After spheroidising annealing, the ductility of high carbon steel will be increased, spheroidite structure formed reduce energy for following processes and machinability will be increased.
Characteristic of process annealing
Figure 2: Phase diagram of steel
Working procedure of process annealing
Steel is selected to be the example of metal because annealing is mainly focus on steel. In process annealing, low carbon steel should heated to around 550 ºC to 650 ºC which in the ferrite region. In other word, the temperature should be held at just below the A1 line on figure 2. Then, the low carbon steel is held in particular temperature for a sufficient time to allow recrystallization of ferrite phase. Lastly, the steel is cooled in air to allow grain growth. From figure 2, for low carbon percent steel and below A1 line are ferrite phase. Therefore, there is no phase changes in process annealing.
Effect of process annealing
After process annealing, the major changes are on size of the low carbon steel, shape of the low carbon steel and distribution of the grain structure as the recrystallization occur. The low carbon steel that experience process annealing will reduce the hardness and increase ductility of the steel. Normally, process annealing will be done after cold working on low carbon steel. Process annealing allow the cold worked steel to perform further cold working without fracture.
Application of process annealing
Since this process is heated to below 727 ºC, the energy consumption for process annealing is very low. Hence, the low carbon steel does not require to cool down in furnace. Therefore, process annealing is cheaper than either full annealing or normalizing in a steel industry. Process annealing has been widely used in the treatment of sheets and wires. Part which are fabricated by cold forming such as stamping, extrusion, upsetting and drawing are frequently given this treatment as an intermediate step.
Precaution and limitation on process annealing
Cooling rate is not important in process annealing as it is done at sub critical temperatures. However, scaling or oxidation can be prevented or minimized by process annealing. Specifically when annealed at lower temperatures or in non-oxidizing areas. Process annealing is very sensitive on temperature. The temperature should not exceed 727 ºC to prevent phase changes. Process annealing are mainly used for low carbon steel. When the carbon percent in steel increase, the effect will be different even using the same temperature.
Characteristic of spheroidising annealing
Annealed hypereutectoid steel with a microstructure of pearlite and cementite network generally gives poor machinability. Since cementite is hard and brittle, the cutting tool cannot cut through these plates. Instead, the plates have to be broken. Therefore, the tool is subjected to continual shock load by the cementite plates and results in a ragged surface finish. A heat-treating process which will improve the machinability is known as spheroidise annealing. This process will produce a spheroidal or globular form of carbide in a ferritic matrix as shown in the figure given below. Spheroidising annealing is mainly used for high carbon steel (carbon > 0.6%).
Figure 3: Spheroidised cementite in a ferrite matrix
Working procedure of spheroidising annealing
There are three ways to carry out spheroidising annealing. The first way is start with heat the high carbon steel to a temperature just below the Ferrite-Austenite line which is A1 line or below the Austenite-Cementite line. In short, heat at below 727 ºC. Then, hold the temperature for a period of time. At this moment, pearlite (the lowest energy arrangement of steel) transform to ferrite and cementite. Lastly, high carbon steel is cooled slowly. The graphite content of steel assumes a spheroidal shape after spheroidizing and after prolonged heating the pearlite layers are broken down and spherical lumps of cementite, or spheroidite, are formed. The second way is by cycle few times between temperature slightly above and slightly below the 727 ºC line. After that, high carbon steel is slow cooled. The third way is mainly for alloy and tool steels. Alloy or tool steels is heated to 750 to 800 ºC and held for several hours followed by slow cooling.
Effect of spheroidising annealing
Figure 4: SpheroiditeAll these method mentioned above will cause the microstructure whereby all the cementite is in the form of small globules (spheroids) dispersed throughout the ferrite matrix. In short, spheroidising annealing gets globular perlite. In the globular pearlite, the carburized body appears as a spherical particle. Spherical pearlite has low hardness so it is convenient for machining. Furthermore, the austenite grain is not easy to be coarse when it is heated and the deformation and cracking tendency is small when cooling. The structures in spheroidite are one thousand times larger than pearlite. Spherodite are spaced further apart. Therefore, spheroidising annealing make the steel extremely ductile. After spheroidising annealing, high carbon steel will decrease in hardness. The structure will be uniform. The machinability is improved and quenching can be done afterwards.
Application of spheroidising annealing
Spheroidite structure lessens energy needed for subsequent process. Therefore, the production cost will be cheaper compared to full annealing. Thus, spheroidising annealing has been widely used in industry. High carbon steel grade SAE 52100 was widely used in the manufacture of rolling element bearings. Steel AISI 1045, AISI 4135 are the typical steels spheroidised for cold forging. It has long been the practice to anneal tool steel and high carbon steel to produce a structure of dispersed carbide spheroids in a ferrite matrix. The heat treating process of spheroidising steel produces a structure which is soft and much easier to machine than any other structure obtainable with these steels. Some of the industrial application are rail road tracks, tyre cords, bridge cables and machinable steel.
Precaution and limitation of spheroidising annealing
Ideal when subsequent machining and/or hardening is required. The spheroidized condition is the equilibrium state of steal in its softest condition. Spherodising annealing is normally carried out under a protective atmosphere to prevent oxidation and decaburization. Spherodising annealing is not capable for low carbon percent steel. Low carbon steels are seldom spheroidised for machining, because they are excessively soft and gummy in the spheoridised conditions. The cutting tool will tend to push the material rather than cut it, causing excessive heat and wear on the cutting tip.
Table 1: Table of comparison between process annealing and spheroidising annealing
Process annealing Spheroidisng annealing
Low carbon steel High carbon steel
No phase changes Pearlite brake down and form spheroidite after prolonged heating
Heating temperature around 550 ºC to 650 ºC Heating temperature slightly below 727 ºC
One method to achieve Several method to achieve
Low energy consumption High energy consumption
Application: treatment on sheet or wire Application: rail road tracks, machinable steel