In order to change the properties of steel and make it easier to process, some additional treatment and process are usually done before the mechanical processing is completed. Hardening the material before processing will prolong processing time and increase tool wear, but the steel can be treated after processing to increase the strength or hardness of the finished product. The following are some common processing technologies of steel.
heat treatment
Heat treatment refers to several different processes that involve manipulating the temperature of steel to change its material properties. One example is annealing, which is used to reduce hardness and increase ductility, making steel easier to machine. The annealing process slowly heats the steel to the desired temperature and holds it for a period of time. The time and temperature required depend on the specific alloy and decrease with the increase of carbon content. Finally, the metal is cooled slowly in the furnace or surrounded by insulating materials.
Normalizing heat treatment can reduce the internal stress in steel, while maintaining higher strength and hardness than annealing steel. In the normalizing process, the steel is heated to a high temperature and then air cooled to obtain higher hardness.
Quenched steel is another heat treatment process. You guessed right, it can make steel hard. It also increases strength, but also makes the material more brittle. The hardening process consists of slowly heating the steel, soaking it at high temperatures, and then rapidly cooling it by immersing it in liquids such as water, oil, or saline solutions.
Finally, the tempering heat treatment process is used to reduce some brittleness caused by steel hardening. Tempering and normalizing of steel are almost identical: you heat it slowly, keep it at the selected temperature, and then air cool the steel. The difference is that tempering is carried out at a lower temperature than other processes, which reduces the brittleness and hardness of tempered steel.
Precipitation hardening
Precipitation hardening improves the yield strength of steel. Some grades of stainless steel may contain PH in the designation, which means that they have precipitation hardening properties. The main difference between precipitation hardening steels is that they have additional elements: copper, aluminum, phosphorus, or titanium. Many different alloys are possible here. In order to activate the precipitation hardening characteristics, the steel is formed into the final shape and then the age hardening process is carried out. The aging hardening process will heat the material for a long time, making the added elements precipitate - forming solid particles of different sizes - to increase the strength of the material.
17-4PH (also known as 630 steel) is a common example of a precipitation hardening grade of stainless steel. This alloy contains 17% chromium, 4% nickel, and 4% copper, which is helpful for precipitation hardening. Due to the increased hardness, strength and high corrosion resistance, 17-4PH is used for helideck platforms, turbine blades and nuclear waste drums.
Cold working
The properties of steel can also be changed without applying a lot of heat. For example, cold worked steel is made stronger by a work hardening process. Work hardening occurs when a metal undergoes plastic deformation. This can be done intentionally by hammering, rolling, or drawing the metal. If the cutting tool or workpiece becomes too hot, work hardening can also occur unintentionally during machining. Cold working can also improve the machinability of steel. Low carbon steel is very suitable for cold working.
Precautions for steel structure design
When designing steel parts, it is important to remember the unique properties of materials. The features that make it very suitable for your application may require some additional consideration of design for manufacturing (DFM).
Because of the hardness of materials, it takes longer to process steel than other softer materials such as aluminum or brass. You can protect your parts and tools by reducing the spindle speed and feed rate.
Even if you do not machine yourself, you still need to select the steel type suitable for the project, not only considering hardness and strength, but also considering the difference in machinability. For example, the processing time of stainless steel is about twice that of carbon steel. When determining the different grades, it is also necessary to consider which properties are most important and which steel alloys are readily available. Common grades, such as 304 or 316 stainless steel, have a wider range of stock sizes to choose from, and require less time to find and purchase.
Due to the wide application of CNC processing steel and its strong physical properties, steel has become the preferred material for parts manufacturing. When designing your CNC processing steel parts, please remember to balance the properties you need according to the machinability of the materials.
