What is steel?
Steel is a broad term for iron and carbon alloys. The carbon content (0.05% - 2% by weight) and the addition of other elements determine the specific alloy of steel and its material properties. Other alloying elements include manganese, silicon, phosphorus, sulfur and oxygen. Carbon increases the hardness and strength of steel, while other elements can be added to improve corrosion resistance or machinability. The manganese content is also generally high (at least 0.30% to 1.5%) to reduce the brittleness of the steel and improve its strength.
The strength and hardness of steel is one of its most popular properties. It is they that make steel suitable for construction and transportation applications because this material can be used for a long time under heavy and repeated loads. Some steel alloys, i.e. stainless steel varieties, are corrosion resistant, which makes them the best choice for parts working in extreme environments.
However, this strength and hardness will also prolong machining time and increase tool wear. Steel is a high density material, which makes it too heavy in some applications. However, steel has a high strength to weight ratio, which is why it is one of the most commonly used metals in manufacturing.
Type of steel
Let's talk about many types of steel. As steel, carbon must be added to iron. However, the content of carbon will be different, resulting in great changes in its performance. Carbon steel generally refers to steel other than stainless steel and is identified by the 4-digit grade of steel. More broadly, it is low carbon steel, medium carbon steel or high carbon steel.
Low carbon steel: carbon content less than 0.30% (by weight)
Medium carbon steel: 0.3 – 0.5% carbon content
High carbon steel: 0.6% and above
The main alloying elements of steel are represented by the first digit of the four digit grade. For example, any 1xxx steel, such as 1018, will use carbon as the main alloying element. 1018 steel contains 0.14 – 0.20% carbon and a small amount of phosphorus, sulfur and manganese. This universal alloy is commonly used to machine gaskets, shafts, gears and pins.
Easy to process carbon steel is re phosphated and re phosphated to break the chips into smaller pieces. This prevents long or large chips from getting entangled with the tool during cutting. Easy to machine steel can speed up processing time, but may reduce ductility and impact resistance.
stainless steel
Stainless steel contains carbon, but it also contains about 11% chromium, which increases the corrosion resistance of the material. More chromium means less rust! The addition of nickel can also improve the rust resistance and tensile strength. In addition, stainless steel has good heat resistance and is suitable for aerospace and other applications in extreme environments.
According to the crystal structure of metal, stainless steel can be divided into five types. These five types are austenite, ferrite, martensite, duplex and precipitation hardening. Stainless steel grades are identified by three digits instead of four digits. The first number represents the crystal structure and major alloying elements.
For example, 300 series stainless steel is austenitic chromium nickel alloy. 304 stainless steel is the most common grade, also known as 18 / 8, because it has 18% chromium and 8% nickel. 303 stainless steel is a free machining version of 304 stainless steel. The addition of sulfur reduces its corrosion resistance, so type 303 stainless steel is more likely to rust than type 304 stainless steel.
Stainless steel can be used in a wide range of industries. Type 316 stainless steel can be used for valve components in medical equipment such as machines and pipelines after proper processing. 316 stainless steel is also used for processing nuts and bolts, many of which are used in the aerospace and automotive industries. 303 stainless steel is used for gears, shafts and other parts necessary for aircraft and automobiles.
chisel tool steel
Tool steel is used to manufacture tools for various manufacturing processes, including die casting, injection molding, stamping and cutting. There are many different tool steel alloys available for different applications, but they are all known for their hardness. Each of them can withstand the wear of multiple uses (the steel mold used for injection molding can withstand a million times or more of materials) and has high temperature resistance.
A common application of tool steel is injection molding, which is processed by hardened steel CNC to produce the highest quality production parts. H13 steel is usually selected because of its good thermal fatigue performance - its strength and hardness can withstand long-term exposure to extreme temperatures. H13 mold is very suitable for advanced injection molding materials with high melting temperature, because it provides longer mold life than other steels – 500000 to 1 million times. At the same time, S136 is stainless steel, and the die life exceeds one million times. This material can be polished to the highest level and used for special applications of parts requiring high optical clarity.
Steel treatment
Some of the most useful properties of steel come from additional processing and machining steps. These methods can be carried out before processing to change the properties of the steel and make the steel easier to process. Please remember that hardening materials before machining will prolong machining time and increase tool wear, but steel can be processed after machining to increase the strength or hardness of the finished product. That is to say, it is important to anticipate any planned treatment that you need to apply to achieve the necessary properties for your parts.
heat treatment
Heat treatment refers to several different processes involving manipulating the temperature of steel to change its material properties. An example is annealing, which is used to reduce hardness and increase ductility, making steel easier to process. 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 increasing carbon content. Finally, the metal is slowly cooled in the furnace or surrounded by insulating material.
Normalizing heat treatment can eliminate the internal stress in the steel while maintaining higher strength and hardness than annealed steel. During normalizing, 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 it, it hardens steel. It also increases strength, but also makes the material more brittle. The hardening process consists of slowly heating the steel, soaking it at high temperature, and then rapidly cooling the steel in water, oil or brine solution.
Finally, tempering heat treatment process is adopted to reduce the brittleness of quenched steel. Tempered steel is almost identical to normalizing: slowly heat it to a selected temperature, and then air cool the steel. The difference is that the tempering temperature is lower 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 values in their names, which means that they have precipitation hardening characteristics. The main difference between precipitation hardening steels is that they contain additional elements: copper, aluminum, phosphorus or titanium. There are many different alloys. In order to activate the precipitation hardening property, the steel is formed into the final shape and then subjected to age hardening treatment. The aging hardening process heats the material for a long time to precipitate the added elements and form solid particles with different sizes, thus improving the strength of the material.
17-4PH (also known as 630 steel) is a common example of stainless steel precipitation hardening grades. The alloy contains 17% chromium and 4% nickel, and 4% copper, which contributes to precipitation hardening. Due to 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 the steel can also be changed without applying a large amount of heat. For example, cold worked steel is made stronger by a work hardening process. Work hardening occurs when the metal is plastically deformed. This can be achieved by hammering, rolling or drawing the metal. During machining, if the tool or workpiece is overheated, work hardening may also occur accidentally. 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 characteristics of the material. Making it well suited to the characteristics of your application may require additional consideration of manufacturing design (DFM).
Due to the hardness of the material, it takes longer to process steel than other softer materials such as aluminum or brass. You need to use the correct machine settings to optimize machining quality and minimize tool wear. In fact, this means slower spindle speed and feed speed to protect your parts and molds.
Even if you don't do the processing itself, you should still evaluate the steel type suitable for your project, not only the hardness and strength, but also the difference in machinability. For example, the processing time of stainless steel is about twice that of carbon steel. When deciding on different grades, you should also consider which properties are the highest priority and which steel alloys are easy to obtain. Commonly used 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.