This article discusses stainless steel.

We often use the word "steel" to describe something strong, such as the Great Wall of Steel to describe military might, the Iron Will to describe ambition, Iron Man to describe a character, and some even name their children "Gangzi" (Steel), which sounds very cool.

In engineering, "steel" is even more important and an indispensable material. Therefore, those working in engineering must know about steel, understand steel, and know how to use steel.

What exactly is "steel", what are its properties, and how did the commonly used grades 304, 304L, 316, and 316L come about, and what are the differences between them?

Steel: A material with iron as the main element, a carbon content generally below 2%, and containing other elements.——GB/T 13304－91《Steel Classification》

Stainless steel: A type of steel that is resistant to weak corrosive media such as air, steam, and water, or has rust-resistant properties. Commonly used grades are 304, 304L, 316, and 316L, which are 300 series austenitic stainless steels.

   304 Stainless Steel   

    Performance Introduction

304 stainless steel is the most common grade. As a widely used steel, it has good corrosion resistance, heat resistance, low-temperature strength, and mechanical properties; good hot workability such as stamping and bending, no work hardening phenomenon (non-magnetic, usable temperature -196℃～800℃).

    Application Range

Household appliances (Class 1 and 2 tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs)

Automotive parts (windshield wipers, mufflers, molded products)

Medical instruments, building materials, chemicals, food industry, agriculture, ship components

304L Stainless Steel (L for low carbon)

    Performance Introduction

As a low-carbon 304 steel, under normal conditions, its corrosion resistance is similar to that of 304 steel, but after welding or stress relief, its resistance to intergranular corrosion is excellent; it also maintains good corrosion resistance without heat treatment, usable temperature -196℃～800℃.

    Application Range

Used in outdoor machinery in the chemical, coal, and petroleum industries with high requirements for intergranular corrosion resistance, heat-resistant parts for building materials, and parts that are difficult to heat treat.

316 Stainless Steel

    Performance Introduction

316 stainless steel, due to the addition of molybdenum, has particularly good corrosion resistance, atmospheric corrosion resistance, and high-temperature strength, and can be used under harsh conditions; excellent work hardening properties (non-magnetic).

    Application Range

Equipment used in seawater, chemical, dye, papermaking, oxalic acid, fertilizer, etc., production equipment; photography, food industry, coastal facilities, ropes, CD rods, bolts, nuts.

  316L Stainless Steel (L for low carbon)

    Performance Introduction

As a low-carbon series of 316 steel, in addition to having the same properties as 316 steel, it has superior intergranular corrosion resistance.

    Application Range

Products with special requirements for intergranular corrosion resistance.

 

Performance Comparison

    Chemical Composition

316 and 316L stainless steels are molybdenum-containing stainless steels. The molybdenum content in 316L stainless steel is slightly higher than that in 316 stainless steel. Due to the molybdenum in the steel, the overall performance of this steel grade is superior to 310 and 304 stainless steels. Under high-temperature conditions, when the concentration of sulfuric acid is below 15% and above 85%, 316 stainless steel has a wide range of applications. 316 stainless steel also has good resistance to chloride corrosion, so it is often used in marine environments. The maximum carbon content of 316L stainless steel is 0.03, and it can be used in applications where annealing cannot be performed after welding and where maximum corrosion resistance is required.

    Corrosion Resistance

316 stainless steel has better corrosion resistance than 304 stainless steel and has good corrosion resistance in the pulp and papermaking process. Moreover, 316 stainless steel is also resistant to marine and corrosive industrial atmospheric corrosion.

Generally speaking, the difference in chemical corrosion resistance between 304 and 316 stainless steel is not large, but there are differences under certain specific media.

The initially developed stainless steel was 304, and under certain conditions, this material is relatively sensitive to pitting corrosion. Adding an extra 2-3% molybdenum can reduce this sensitivity, thus creating 316. In addition, this extra molybdenum can also reduce corrosion by certain hot organic acids.

316 stainless steel has become almost the standard material in the food and beverage industry. Due to the global shortage of molybdenum and the higher nickel content in 316 stainless steel, 316 stainless steel is more expensive than 304 stainless steel.

Pitting corrosion is a phenomenon mainly caused by corrosion deposits on the surface of stainless steel, which is due to the lack of oxygen and the inability to form a chromium oxide protective layer.

Especially in small valves, the possibility of deposits on the valve plate is small, so corrosion rarely occurs.

In various types of water media (distilled water, drinking water, river water, boiler water, seawater, etc.), the corrosion resistance of 304 stainless steel and 316 stainless steel is almost the same, unless the chloride ion content in the medium is very high, in which case 316 stainless steel is more suitable.

In most cases, the corrosion resistance of 304 and 316 stainless steel is not much different, but in some cases, the difference can be very large, and it needs to be analyzed on a case-by-case basis. Generally speaking, valve users should know this, because they will choose the material of the container and pipeline according to the medium, and we do not recommend materials to users.

    Heat Resistance

316 stainless steel has good oxidation resistance in intermittent use below 1600 degrees and continuous use below 1700 degrees. In the range of 800-1575 degrees, it is best not to continuously use 316 stainless steel, but 316 stainless steel has good heat resistance when used continuously outside this temperature range. 316L stainless steel has better resistance to carbide precipitation than 316 stainless steel and can be used in the above temperature range.

    Heat Treatment

Anneal in the temperature range of 1850-2050 degrees, then quickly anneal, then quickly cool. 316 stainless steel cannot be hardened by overheating.

    Welding

316 stainless steel has good weldability. All standard welding methods can be used. During welding, 316Cb, 316L, or 309Cb stainless steel filler rods or welding rods can be used according to the application. For optimal corrosion resistance, the welded section of 316 stainless steel needs post-weld annealing. If 316L stainless steel is used, post-weld annealing is not required.

    Mechanical Properties

 

Among all steels, austenitic stainless steel has the lowest yield point. Therefore, from the perspective of mechanical properties, austenitic stainless steel is not the best material for valve stems, because to ensure a certain strength, the diameter of the valve stem will increase. The yield point cannot be improved by heat treatment, but it can be improved by cold forming.

    Magnetic Properties

Due to the widespread use of austenitic stainless steel, people have the wrong impression that all stainless steel is non-magnetic. For austenitic stainless steel, it can be basically understood as non-magnetic, and this is true for quenched forged steel. However, 304 processed by cold forming will have some magnetism. For cast steel, if it is 100% austenitic stainless steel, it is non-magnetic.

Low-Carbon Stainless Steel

The corrosion resistance of austenitic stainless steel comes from the chromium oxide protective layer formed on the metal surface. If the material is heated to a high temperature of 450℃ to 900℃, the structure of the material will change, and chromium carbide will be formed along the grain boundaries. In this way, a chromium oxide protective layer cannot be formed at the grain boundaries, resulting in reduced corrosion resistance. This corrosion is called "intergranular corrosion".

Therefore, 304L stainless steel and 316L stainless steel were developed to combat this corrosion. 304L stainless steel and 316L stainless steel both have lower carbon content, so chromium carbide will not be produced due to the reduced carbon content, and intergranular corrosion will not be generated.

It should be noted that higher intergranular corrosion sensitivity does not mean that non-low-carbon materials are easier to corrode. In high-chlorine environments, this sensitivity is also higher.

Please note that this phenomenon is due to high temperature (450℃-900℃). Welding is usually the direct cause of reaching this temperature. For soft seat butterfly valves, since we do not perform welding operations on the valve plate, using low-carbon stainless steel does not make much sense, but most specifications require 304L stainless steel or 316L stainless steel.

Disclaimer: This article is compiled for the purpose of conveying more information. If there are any errors in the source annotation or infringement of your legitimate rights, please contact this website with proof of ownership, and we will correct and delete it in time, thank you.