I. Introduction
Stainless steel is an alloy that is made up of iron and chromium, whereby chromium makes it better in corrosion resistance. It may also contain nickel and a variety of metals in some instances. Other elements that might be found in it include; titanium, nickel, molybdenum, aluminum, phosphorus, copper, carbon, sulfur, nitrogen, selenium, or niobium. These are usually added so as to boost the resistance of stainless steel to corrosion and oxidation when exposed to certain kinds of conditions. Stainless steel is made up of at least one hundred grades whereby the larger part is divided into five groups which are; ferritic, austenitic, duplex, martensitic as well as precipitation-hardening.
- The stainless steel grades in the Austenitic category are considered to have the best resistance to corrosion, they are nonmagnetic and heat treatment does not harden them. They are used in various industries such as the aircraft and food processing industries.
- The ferritic steels usually contain a small amount of carbon which is usually below 0.2 percent and can also not be hardened through heat treatment.
- Martensitic steels on the other hand can be hardened through heat treatment and contain more carbon than ferritic steels, 1.2 percent. They are usually used in the manufacture of cutlery and surgical tools.
- Duplex steels are usually formed when the austenitic and ferritic steels are combined together in the same amounts. They resist corrosion more than the two and are stronger. This makes them efficient for constructing storage tanks and containers used for the transportation of chemicals.
- Precipitation-hardening steel is known for its strength which is a result of aluminum, copper, and niobium being added to it. They are used to construct long shafts.
The various properties of stainless steel such as high corrosion resistance and its ability to last long make it suitable for use in a number of applications. A large number of industries normally depend on stainless steel in the manufacturing process. They include the automotive industries, medical industries, architectural industries, among others.
II. Stainless Steel Grades
A. Brief overview of 300 series stainless steel
A numbering system was invented by the American Iron and Steel Institute(AISI) to categorize the various grades of stainless steel. These categories are the 200, 300 and 400 series of stainless steel. In this article, we will discuss some of the grades in the 300 series. The 300 series stainless steel is normally a part of the austenitic category. The grades in this series are made up of chromium and nickel as the crucial alloys, and can only be hardened through cold working procedures. They are good resistors to corrosion and do not weaken when exposed to high temperatures which makes them easy to maintain. These grades include; 301, 302, 303, 304, 304L, 304H, 316, 316L, 317/L, 317LMN, 321/321H, 347/347H, 309/309S, 310/S, 310H and 330. The 304 grade is considered to be the one used mostly compared to other stainless steels. The grades in the 300 series are commonly used in pipe, tube, sheet, bar, plate, and other structural products.
B. Introduction to 303 and 304 stainless steel
In this article, we are going to discuss 303 and 304 stainless steels, their properties and applications. Each of the two has areas where it is best suited due to its exceptional properties. 303 stainless steel is usually considered the most machinable while 304 stainless steel is the most frequently used type.
III. Composition
A. 303 Stainless Steel
303 is also referred to as type 1.4305 and is usually very machinable compared to other austenitic grades. Its machinability is high because it contains Sulphur in its composition, however, Sulphur also decreases its resistance to corrosion and its toughness. This makes it have a lower level of resistance to corrosion in comparison to 304 stainless steel. The chemical properties of 303 stainless steel include; Iron, 0.0 to 0. 10 percent of Carbon, 17 to 19 percent of Chromium, 0 to 2 percent of Manganese, 0 to 1 percent of Silicon, 0 to 0.05 percent of phosphorous, 0.15 to 0.35 percent of Sulphur, 8 to 10 percent of Nickel, 0 to 1 percent of copper and 0 to 0.11 percent of Nitrogen.
B. 304 Stainless Steel
On the other hand, 304 stainless steel which also consists of 304L can be referred to as 1.4301 and 1.4307 respectively. It is the most commonly used stainless steel grade and the most flexible one. Also, it is known as 18/8 since it is made up of 18% chromium and 8 % nickel. 304L is usually the type of 304 stainless steel that contains low carbon which helps boost its weldability. Stainless steel 304 also has 304H which, unlike 304L, consists of a high percentage of carbon making it suitable in cases where temperatures are high. 304 is made up of iron, 0 to 0.07 percent of carbon, 0 to 2 percent of manganese,0 to 1 percent of silicon, 0 to 0.05 percent of phosphorous, 0 to 0.03 percent of Sulphur, 17.50 to 19.50 percent of chromium and 8 to 10.50 percent of nickel. 304L is made up of 0.03 percent of carbon unlike stainless steel 304 while 304H is made up of 0.08 percent of carbon.
IV. Mechanical Properties
A. 303 Stainless Steel
303 stainless steel has an ultimate tensile strength of 500 to 750 MPa, a yield strength of 415 MPa, elongation at break of 35min%, modulus elasticity of 193 GPa, and comprehensive strength of 190 MPa. It also has physical properties of a density of 8.03kg/m3, a melting point of 14550C, and a thermal conductivity of 16.3W/m.K.
B. 304 Stainless Steel
304 stainless steel has a tensile strength of between 520 and 720 MPa, an elongation at break of 45min%, and a yield strength of 215 MPa. Its physical properties include a density of 8.00g/cm3, a melting point of 14500C, and a thermal conductivity of 16.2W/m.K.
V. Corrosion Resistance
A. 303 Stainless Steel
In 303 stainless steel, there is an addition of Sulphur which makes it less resistant to corrosion. This makes 303 only suitable for use in areas or sectors with mild conditions. For example, if 303 stainless steel is used in environments that contain chloride and above 600C, there will be pitting and crack corrosion. This makes 303 stainless steel unfit for usage in marine conditions. On the other hand, 303 stainless steel can work well and resist oxidation when in conditions of up to 7600C temperatures in irregular intervals. At regular intervals, it can resist oxidation up to 8700C even though it is not advisable due to the sensitivity of 303 to precipitation of carbide under a constant temperature of between 425 and 8600C.
B.304 Stainless Steel
304 stainless steel is more resistant to corrosion than 303 stainless steel. However, pitting and cracking can take place when 304 is exposed to conditions with chlorides. 304 is a good resistor to oxidation in irregular intervals involving up to 8600C and in constant temperatures of up to 9250C. In cases where resistance to corrosion in water is needed, 304L is usually preferred due to its ability to resist the precipitation of carbide. This makes it suitable for marine environments. In some instances where there is a requirement of high strength and in conditions of high temperatures between 500 to 8000C, 304H grade is the best to use. This is because it contains more carbon which makes it withstand high temperatures and remain flexible.
VI. Machinability and Weldability
A. 303 Stainless Steel
303 stainless steel is considered to have the best machinability compared to other grades of stainless steel in the austenitic group. This is because of the availability of Sulphur as one of the elements composing it. Engineers can increase machinability in a variety of ways. These ways include:
- Ensuring that edges used for cutting are sharp to work easier.
- The cuts made should be made lightly but deep enough to ensure efficiency.
- One must make use of cooling aids and lubricants to reduce the concentration of heat on the edges used for cutting.
The Sulphur present in 303 stainless steel however, causes poor weldability. Even though Sulphur boosts the machinability of 303, it also decreases its toughness slightly. In cases where welding is necessary for this grade, the engineers are advised to use grades 308L and 309 as filler rods and electrodes to boost its weldability. It is also advisable for the welds to be annealed to boost the resistance to corrosion.
B. 304 Stainless Steel
The 304 steel has a version of 304L and the two are considered to have the same level of machinability which is usually 70. Despite the fact that it is not as machinable as grade 303, it still has good machinability. One can also use the same ways to increase its machinability as stated in grade 303 stainless steel. Welding in 304 stainless steel can be done with fillers or without. 308 stainless steel is recommended in cases where fillers are needed, to be used as electrodes and filler rods. When using 304L, 308L is preferred to be used as the fillers. Post-weld annealing may be required in areas that are heavily welded but not in processes involving 304L.
VII. Applications
A. 303 Stainless Steel
303 stainless steel is in many cases used in instances that require high strength and good resistance to corrosion. Some common uses of 303 stainless steel include:
- This grade is used in the manufacture of medical equipment such as the ones used for surgeries. It is non-toxic and impenetrable hence ensuring that no bacteria are growing on its surfaces hence ensuring cleanliness or leaking of dangerous chemicals into one’s body.
- It is also used in the manufacture of equipment used in food processing because they are easy to keep clean hence ensuring good hygiene and perfect for the job.
- 303 is also used in the architecture industry when constructing various equipment since it contains high strength suitable for the role and lasts long. Its ability to also resist heat plays a big role in the industry.
- It is nonmagnetic which makes it efficient in the manufacture of various electronics without any issues from other metals that might be nearby during the process.
- It is also used in the manufacture of shafts due to its strength and machinability.
- It is used in the making of nuts, bolts, gears, and screws due to its high machinability feature.
B. 304 Stainless Steel
304 stainless steel is also on the other hand, applicable in many industries and fields. These include:
- It is used in the chemical industry to store and transport chemicals. This is possible due to its high resistance to corrosion.
- It is also used in the food industry to make equipment used in food processing. Its high level of nickel makes it able to resist corrosion and oxidation hence no rusting. Food processed is stored in equipment made of 304 stainless steel.
- It is used in the automotive industry as well as the aerospace industry to make various equipment such as fuel tanks. Its corrosion resistance makes it suitable for the role.
- 304 stainless steel is also used in hospitals to make surgical tools.
- It is also used in the manufacture of domestic equipment such as sinks, tabletops, refrigerators, and utensils among other items due to its ability to resist reactions with various acids that may be found in milk, fruits, and other foodstuffs.
- In architecture, it is used in molding design and mould manufacturing which is a method used in the production of various items.
- It is also used in the manufacture of various fasteners such as screws, bolts, and nuts.
- 304 can also be used in the manufacture of tubes and pipes that are likely to come into contact with chemicals due to their ability to resist corrosion.
VIII. Conclusion
A. Summary of key differences between 303 and 304 stainless steel
303 and 304 stainless steel vary in their levels of corrosiveness, machinability, and weldability. This is because of the various percentages of different elements in each of them. This makes them suitable for different roles and applications. By using normal eyes to look at them, one might fail to distinguish between the two. However, it is important to note that their differences lie mainly in their elements and characteristics. It is evident that the flexibility of 304 stainless steel makes it to be the most popular and used stainless steel grade. 303 stainless steel is better than 304 stainless steel because of its high machinability which also comes with a disadvantage which is low weldability.
B. Guidance on selecting the appropriate grade for specific applications
It is important to study all grades of stainless steel well before using them to manufacture certain equipment since, for the equipment to be efficient for its roles, the right grades have to be used. Each grade has properties that make it suitable for applications. Most industries may decide to use rapid prototyping to find out the efficiency of different grades in various applications. This happens when the industry does not know which grade of stainless steel to use. Rapid prototyping involves creating a prototype before mass production to test its efficiency and make corrections where necessary. CNC machining can also be used to test the machinability of different prototypes made from each grade of steel. It is very crucial to have every detail about the grade to use before producing products for the public.
