SAE 304 Stainless Steel
SAE 304 Stainless Steel
Steel is the backbone of modern industry, as it's used in almost any conceivable industry. This alloy is created by mixing alloying elements into the base metal — in this case; carbon is added to iron to enhance its properties. However, adding traces of other elements can grant steel different, unique properties.
One such type of steel is stainless steel, which uses chromium to lessen the typical corrosion that most iron-based materials suffer from and gain corrosion resistance. The most popular and most employed type of stainless steel is SAE 304 stainless steel. Its unique properties make it suitable for use in various industries, ranging from construction to surgical implants.
In this article, we'll talk about SAE 304 stainless steel and its varieties, the various properties of different types of stainless steel, and what exactly makes this steel so suitable for such a wide range of applications.
What is 304 Stainless Steel?
SAE 304 stainless steel, formerly known as 18/8 (which refers to its chemical composition), is the most versatile and widely used grade of stainless steel due to its outstanding drawing and welding properties.
304 is an austenitic stainless steel, meaning it's a non-magnetic type of steel with great mechanical properties, weldability, and tensile strength. Additionally, its structure allows for extreme drawing without needing intermediate annealing, making the steel easily shaped into various components for different industrial applications.
304 stainless steel got its name from the Society of Automotive Engineers (SAE) and the American Iron & Steel Institute (AISI), and though both organizations use their respective designations for this type of steel, they're both referring to the same chemical composition.
Type 304 and 304L Stainless Steel
Type 304 comes in three main variants: the typical 304, 304L, and 304H alloys, which chemically differ based on their carbon content. Type 304 is austenitic steel that contains 0.08% carbon, between 17.50% and 19.50% chromium, and anywhere between 8% and 10.5% nickel.
Depending on the actual alloy type, 304 can also contain manganese, silicon, phosphorous, sulfur, and nitrogen. Admittedly, those alloying elements are only found in traces. Chromium content gives 304 its stainless properties, while the nickel content increases temperature strength and allows for an easier polish.
SAE 304L is a low-carbon steel variant of SAE 304 stainless steel and contains 0.03% carbon. It's mostly used in heavy gauge components due to its improved weldability since it doesn't require post-welding annealing. SAE 304H has the highest carbon content of all, containing anywhere between 0.04% to 0.10% carbon. This increases its strength in high-temperature applications.
Possible Alternative Grades
There are several possible alternative steel grades to 304-grade stainless steel. For example, 302HQ has a lower hardening rate which is needed for cold-forging fasteners; 301 is great for producing kitchen equipment and cutlery, while 303 offers greater machinability properties.
SAE 316, which we'll mention later, has increased resistance to pitting and excellent corrosion resistance, especially in chlorine environments. Choosing an adequate grade greatly depends on your application, so it pays to have a greater understanding of the various characteristics of different steel grades.
304 Stainless Steel vs 304V Stainless Steel
As you might've guessed so far, the letter following the numerical code in SAE stainless steel designations has certain denotations. In the case of 304V, the "V" stands for "vacuum." Allow us to elaborate.
The 304V alloy is a double-melted type of austenitic stainless steel, which is initially melted using an electric arc, followed by a Vacuum Arc Re-melt (VAR). This type of steel is mostly used in high-value applications, as it minimizes the voids and contaminants in the microstructure, yielding a more uniform chemistry.
This type of steel is mostly used in surgical instruments and implants, catheters, springs, mandrels, needles, and high-performance non-medical uses, such as the aerospace industry.
The Difference Between 304 and 316 Stainless Steel
While there aren't any noticeable visual differences between SAE 304 and SAE 316, the two types of stainless steel differ chemically. For example, SAE 304 stainless steel has 8% nickel and 18% chromium content. SAE 316 stainless steel, on the other hand, has 16% chromium, 10% nickel, and 2% molybdenum.
When considering the two grades of stainless steel intended purposes, the chemical differences between them are very significant. The molybdenum within the compound increases corrosion resistance, making it more suitable for use in highly acidic environments. Molybdenum also prevents oxidation caused by salt and chlorides in marine environments.
Grade Specification Comparison
Despite SAE 316, SAE 304 shares many of its properties with numerous 3xx stainless steel varieties, with only subtle differences between them. However, these differences can make or break your project. Here's how SAE 304 grade compares with other steels.
- SAE 304 vs SAE 302 — SAE 302 has a significantly higher carbon content, which makes it tougher than 304, which is more ductile and has better machinability.
- SAE 304 vs SAE 303 — SAE 303 is very similar to SAE 304, with the addition of sulfur, which increases its machinability. However, it lowers the alloy's corrosion resistance by nearly negligible margins (depending on the application).
- SAE 304 vs SAE 310 — SAE 310 has very high concentrations of chromium and nickel, with additional alloying elements. It retains excellent oxidation and corrosion resistance at high temperatures, and its best suited for cryogenic applications, as it retains toughness at -268°C.
- SAE 304 vs SAE 321 — SAE 321 has traces of Titanium and higher carbon content which grants it greater yield strength. It's more ductile and resistant to stress fractures than SAE 304.
- SAE 304 vs SAE 409 — SAE 409 has a lower chromium and nickel content compared to SAE 304, which makes it more susceptible to corrosion. However, it has better corrosion resistance than aluminized steel.
- SAE 304 vs SAE 430 — SAE 430 is the second most commonly used stainless steel, with SAE 304 being the first. Basically, it's a more budget-friendly option compared to SAE 304 but features a slightly lower corrosion resistance.
Key Properties of 304 Stainless Steel
SAE 304 is part of the 3xx stainless steel alloys, which means that the iron-carbon alloy is further enhanced with chromium and nickel. Here are the key properties of 304 stainless steel:
Physical Properties
As previously stated, the SAE 304 alloy is austenitic stainless steel with a molecular structure that has relatively good corrosion resistance and magnetic non-permeability — for more information on the latter, please refer to our guide. Its chemical composition contains the following:
- Approximately 0.08% of carbon
- <=0.08% carbon
- 18-20% chromium
- 66.345-74% iron
- <= 2% manganese
- 8-10.5% nickel
- <=0.045% phosphorus
- <=0.03% sulfur
- <=1% silicon
Corrosion Resistance
The SAE 304 has good corrosion resistance making it the most widely used stainless steel. It can withstand corrosion in a variety of settings; chlorides are the sole main enemy. Warm temperatures (over 60°C) also cause more pitting, albeit the higher carbon grades (304H) significantly lessen this effect.
This means that 304 steel rusts mostly in aqueous solutions rather than at high temperatures when prolonged exposure to corrosive substances might weaken the alloy.
Heat Resistance and Heat Treatment
In intermittent service up to 870 °C and in continuous service up to 925 °C, grade 304 exhibits good oxidation resistance. If aqueous corrosion resistance is crucial, it is not advised to use 304 continuously in the 425-860 °C range. Grade 304L can be heated to these temperatures and is more resistant to carbide precipitation.
Grade 304H is frequently used for structural and pressure-containing applications at temperatures above 500 °C up to around 800 °C because it has greater strength at high temperatures. In the temperature range of 425-860 °C, 304H will become sensitive. While this won't be an issue for high-temperature applications, it will lower the resistance to aqueous corrosion.
Machinability
304 is easily machined; however, the sharp cutting edges must be maintained. Cutting with dull tools will accelerate work hardening. Light but sufficient cuts should be made to avoid the material's surface riding on the cut to prevent work hardening.
Chip breakers should be used to help ensure that the swarf is kept away from the job. Heat concentrates near the cutting edges due to the low thermal conductivity of austenitic metals. As a result, coolants and lubricants must be used frequently and in significant amounts.
Weldability
Grade 304 is very weldable using any conventional fusion technique, both with and without filler metals. For best corrosion resistance, heavy welded parts made of SAE 304 might need to be annealed after the welding process — this doesn't apply to 304L. If heavy section welding is necessary and post-weld heat treatment isn't feasible, SAE 321 may also be utilized as an alternative to grade 304.
Applications of 304 Steel
Since its non-reactive with most organic acids, 304 steel is frequently referred to as "food-grade" stainless steel, as it's mostly used in the food-processing industry. These stainless steels are ideal for applications that require both intricacy and a certain level of corrosion resistance due to their high weldability, machinability, and workability. As a result, 304 has a wide range of applications, including:
- Kitchen equipment
- Various Tubing
- Food Equipment
- Pharmaceutical processing equipment
- Hypodermic needles
Main Components Made of 304 Steel
Summary
It's evident why 304 excels in a wide range of applications. It's a fantastic first choice when picking a stainless steel alloy with good working qualities and great accessibility. If you want to learn more about SAE 304 stainless steel, or other types of surgical steels, visit Reid Supply, and refer to their Engineer Resource Guides or some of their guides on Surgical and Stainless steel.