Stainless steel fasteners come in several grades and material compositions, so it can be difficult to know exactly what to buy when drawing up a schedule of materials. In this article, we explain stainless steel grades, where they are used and their respective strengths. Let’s begin with 316 stainless steel, what it is and why it is suitable.
316 stainless steel is an austenitic chromium-nickel alloy with a molybdenum content of 2.5% or more. This added content gives the alloy improved corrosion resistance against pitting and crevice corrosion in chloride environments, especially at high temperatures or pressures.
The higher molybdenum content also confers greater resistance to localised corrosion and prevents small holes from taking a foothold in the metal. That is why 316 stainless steel is primarily used in marine environments, the oil and gas industry, chemical processing plants, and food and beverage manufacturing.
There are two main types of 316: austenitic and martensitic. The austenitic form is non-magnetic and the martensitic is magnetic. Why is that important? Some applications require non-magnetic 316 (such as food processing) while others will only work with magnetic 316 (like some oil and gas industry applications).
316 also has high tensile strength, meaning it can withstand higher temperatures and pressures without deforming and is therefore the preferred choice in high-temperature or high-pressure environments.
Now let’s take a look at 304 stainless steel, another popular choice for fasteners. This is the most common grade of stainless steel and is also known as 18/8 stainless steel because it contains 18% chromium and 8% nickel.
304 is also non-magnetic but can become slightly magnetic when cold worked. This is why it is often used in appliances and kitchenware where a degree of corrosion resistance is required but the magnetism of 316 would be undesirable.
In the marine and offshore oil and gas environment, the lower molybdenum content in 304 stainless steel makes it more susceptible to pitting and crevice corrosion, while its lower tensile strength means it is not suitable for high-temperature or high-pressure applications.
The offshore industry also specifies 321 fasteners and fixings for certain applications. These fasteners are used for process piping and pressure vessels in sour service environments where the higher content of titanium in 321 helps to resist sensitisation and intergranular corrosion. However, its lower carbon content means it is not as resistant to pitting and the likelihood of corrosion, where surfaces join, compared to 316 stainless steel.
There are two other grades of stainless steel fasteners that are worth mentioning: duplex and super duplex. These are austenitic-ferritic alloys with a higher chromium and molybdenum content than both 316 and 304 grades. Duplex and super duplex fasteners are used in similar environments to 316 stainless steel but offer improved resistance to pitting and crevice corrosion.
Buyers will also come across classifications like B8, B8M, B8C and B8T. They refer to the different classes of austenitic stainless steel alloys, with B8 fasteners being the most common. Class B8C is an alloy with enhanced corrosion resistance specifically for use in salt-water environments, while B8T fasteners have a higher temperature rating than other austenitic grades and are therefore suitable for use in high-temperature applications.
In summary, there are several grades of stainless steel fasteners to choose from, each with their own benefits and drawbacks. The most important thing is to ensure you select the right grade for the environment in which the fastener will be used. For example, if you require stainless steel fasteners for a high-temperature or high-pressure environment, then 316 is the grade you need. Equally, if you are looking for a fastener that is less likely to corrode in salt water or other chloride-rich environments, then 316 is also the right choice.
So, when choosing stainless steel fasteners and fixings for your project, you need to consider the environment they will be used in, the temperature and pressure conditions, and whether magnetic properties are required.
Contact our expert sales team at Prosper Fasteners and Fixings for all your fastener and fixings needs and for advice on the best products for your application. Prosper Fasteners and Fixings – we help you make the right connections!