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Stainless Steel Fasteners: Types 304 and 316

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One of the great things about stainless steel fasteners is the intrinsic protection within the makeup of the steel. The fastener maintains a shiny, clean finish (when electropolished) while maintaining corrosion resistance. Let’s get into the meaning of stainless steel, its characteristics, and the differences in Type 304 and 316.

What Makes Stainless Steel Fasteners “Stainless?”

The term stainless steel is used for a family of iron-based alloys that contain at minimum 10.5% chromium. This amount of chromium allows for corrosion and heat resistance due to the creation of a surface layer that prevents oxygen from penetrating the steel. Rust cannot form, thus making it “stainless.” Often, their composition also includes other alloys such as nickel, manganese, sulfur, or titanium. These alloys add extra corrosion resistance.

In the fastener world, most stainless steel is austenitic stainless steel, commonly referred to as the 300 series. While there are many compositions that fall under the 300 series, we mostly use types 304 and 316. While they both provide excellent corrosion resistance and have similar compositions, their differences lie in the ratio of chromium to nickel and the environments they are used in.

Stainless Steel: Type 304

You will often hear the term “18-8” when researching the 304 series of stainless steel fasteners. 18-8 refers to the 18% chromium and 8% nickel in the steel. While many stainless materials in the 300 series have this 18-8 mixture, their chemistries and corrosion resistance do not necessarily meet that of type 304. Type 304 is the most popular of the 300 series of stainless steel, as it provides some of the best corrosion resistance in the series.

Stainless Steel: Type 316

Type 316 stainless steel fasteners fall right behind Type 304 in terms of popularity and provides excellent corrosion resistance. Unlike 304, Type 316 does not follow the 18-8 makeup. The chromium is lowered to 16% and the nickel is raised to 10%. Type 316 also contains 2% of molybdenum. Adding molybdenum provides superior resistance to chlorides which can cause pitting.

Because of its composition, Type 316 is used in more corrosive environments where the fasteners will be exposed to chemicals, solvents, or salt water. Type 316 stainless steel fasteners are best used in applications with abrasive chemicals or direct salt spray.

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ASTM Designations – Interpreting the Standards

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In other blogs, we discuss the ASTM International, what it is, and how we use them in the fastener industry. Our sales team will often refer our customers to the ASTM standards, and our team uses and adheres to them each day. For those unfamiliar, the ASTM standards may seem unapproachable, but the standards are actually accessible once you understand their organization. Let’s go over the standards, their designations, what they contain, and how you can find the right standard.

ASTM Designations

There are over 12,000 ASTM standards covering a variety of fields and industries. For our purposes, we’ll focus on the standards that relate to fasteners. The ASTM designation is a mandatory title of each standard (example: F3125 or A325). The first letter is the classification of the standard:

A - Ferrous metals and products (steel)
B - Nonferrous metals and products (coatings)
C - Cementitious, ceramic, concrete, and masonry materials
F - end-use materials and products (bolts)
G - Corrosion, deterioration, weathering, durability, and degradation of materials and products

After the letter, there is a reference number given to the standard.  From there, a grade can be added. For example, F3125 is the standard for high strength bolts. A325 and A490 are grades that fall under that standard, so you’d call a bolt F3125 Grade A325. When reading an ASTM standard, you will usually see two numbers after the standard, like A449-14. The last two numbers signify the year the standard was updated.

Reading an ASTM Standard

ASTM standards are written in a specific, organized manner. Each standard begins with the scope, which informs the reader the purpose and limitations of the standard. It then has a bibliography of referenced documents within the standard and may also include a list of any terminology used that may be unfamiliar.

From there, in regards to fasteners, you will find details on everything needed to manufacture the fastener. This includes materials, mechanical properties, chemical compositions, dimensions, appearances, and testing methods. These details can also include fastener limitations, reference tables, supplementary requirements, and notes on packaging. Referring to an ASTM standard is a great way for manufacturers and customers alike to determine the right fastener for a particular job, as well recognize the advantages and limitations of a standard.

Finding an ASTM Standard

ASTM International provides a comprehensive search engine on their website for over 12,000 standards. Standards are available for purchase in print or electronically. For specific questions on a standard, contact your project engineer or the Birmingham Fastener sales team, who are always available to answer specific questions about the products we sell.

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Electroplated Fasteners: The Process and its Benefits

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We’ve discussed a few different types of zinc coatings for corrosion resistance in the past. Electroplating is another way to coat fasteners to ensure corrosion resistance and improve the finish of the bolt. Electroplating, or electrodepositing, is covered in the ASTM standards F1941 and B633. Let’s go over the electroplating process, characteristics of an electroplated fastener, and the benefits of this type of coating.

The Electroplating Process

To begin the process, workers clean the fastener with alkaline detergent solutions before being treated with acid. This removes any rust, oils, or other impurities on the surface of the fastener. Once cleaned, the electroplating process begins through electro deposition, or submerging the fasteners into a zinc bath. There, the direct current is sent through the chemical bath, creating a chemical reaction on the fastener, thus electroplating it.

Types of Plating Processes

Most commonly, fasteners are rack plated or barrel plated. Each have advantages and disadvantages. In rack plating, the fasteners are left to dry on a rack after the chemical bath. This allows the threads of the fastener to remain untouched, but because the fasteners are set on a rack, the fastener ends up without the coating in certain, small areas. In barrel plating, the fasteners are dried in a turning barrel. While you get full coverage of the coating, there runs a risk of nicked threads. If the fasteners are made from steel and then heat treated to a hardness above 39 HRC, they are baked to minimize the risk of hydrogen embrittlement. Once this process is complete, the fasteners are tested for coating thickness, corrosion resistance, and embrittlement.

Electroplating Characteristics and Benefits

Electroplating allows for a thinner coating with a clean finish, particularly compared to the hot dipped galvanized fasteners where the coating is much thicker and requires overtapping of the nut. Electroplated fasteners are typically plated in yellow or clear zinc. The yellow zinc produces a yellow/muted gold finish, while the clear zinc produces a bright, silver finish on the bolt. The benefits are mostly aesthetic, as this finish is smooth and even while still providing corrosion resistance. It is also less expensive than some other types of coatings.

electroplated fasteners
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Electropolishing vs. Passivation – Choosing the Right Finish

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A lot can go into choosing a finish for your stainless steel fasteners. The two finishing operations most industries specify for their parts are electropolishing and passivation. Today, we’ll look at these two chemical processes, learn their differences, and discuss the advantages one may have over another.

Electropolishing

Electropolishing is a chemical process that can be performed on almost all metal to remove imperfections and contaminants on the surface of the metal. The process involves submerging the fastener or other metal product in a temperature-controlled chemical bath. A current is then passed through the bath, oxidizing the surface and causing it to shed and deburr the contaminants and smooth out any flaws. The fastener comes out to a very bright finish. Not only is the steel visibly shinier, but electropolishing has greater corrosion resistance than that of passivation.

Passivation

Like electropolishing, passivation is also a chemical process designed to remove imperfections from steel and provide a clean finish. In contrast to electropolishing, passivation does not produce a bright or shiny finish. Passivation is designed to improve corrosion resistance on stainless steel. Passivation is effective on most grades of stainless steel, but it is not recommended for certain types of alloys. One difference between electropolishing and passivation is the pre-cleaning step in passivation. Sometimes, a pre-cleaning operation is required prior to passivation, because the process alone is not completely effective. After pre-cleaning, the process of passivation involves placing the steel parts into a nitric or citric acid bath at a set temperature and time depending on the type of alloy. The free iron and other materials on the surface of the steel is then removed in the bath. The finished product has a dull, silver colored finish. On average, passivation is a third less expensive than electropolishing, making it a cost-effective option that produces good corrosion resistance.

Bottom Line

While both methods of finishing provides increased corrosion resistance, electropolishing produces a brighter finish with better corrosion resistance than passivation. That said, passivation provides a clean finish with corrosion resistance at a third of the cost.

electropolish-passivation
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Hot Heading vs. Cold Heading – What’s the Difference?

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Hot heading and cold heading are the two ways to form the head of a bolt. Each process has its own advantages, though there are a few important considerations to make when choosing one over another. Let’s take a look at both processes, how they are used, and what makes them different.

Hot Heading

At Birmingham Fastener, hot headed bolts generally range from 3/8” diameter to 2” diameter. Any material grade we supply can be hot headed, including stainless steel. The process of hot heading a bolt involved heating round bar in a furnace, then forming the head of the bolt in a header (like a die form). Hot heading allows for better formability with less “die blows” on certain head styles. Another advantage to hot heading is that tooling cost is lower than cold head tooling. That said, there are some other points to consider before choosing to hot head your bolts. This process is both much slower and more costly than cold heading when it comes to large quantity orders in diameters under 1 1 /8”. Because of the time and money it takes, hot heading is better for low quantity orders and orders over 1 1/8” diameter.

Cold Heading

Birmingham Fastener cold heads bolts ranging in size from ½” diameter to 1 1/8” diameter. Most material grade we supply can be cold headed. As its name suggests, cold head forming is performed at room temperature. The round bar is shaped into a head by being squeezed between two die forms. The cold heading process allows for very tight dimensional specification to be achieved. It also allows for complex head shapes to be forged with multiple “die blows.” Cold heading is a very fast and very cost effective process. Keep in mind that while this is a better option for large quantity orders, tooling cost is very high.

Bottom Line

While hot heading and cold heading are similar processes, there are a few key differences your company needs to consider before purchasing. While cold heading is a much more timely and cost effective option, we can head a wider range and much larger diameter part through hot heading.

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Waterworks Fastener Standards: AWWA, ASTM, and ASME

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When we talk about the specifications on our waterworks products, we often reference the ASTM, AWWA, and ASME. These organizations provide the standards over many things including all fasteners used in waterworks applications. It’s important to keep up with these organizations to ensure that all standards are being followed. Here’s an overview of the organizations and their responsibilities.

American Water Works Association (AWWA)

The American Water Works Association (AWWA) is an international non-profit association founded to manage water quality and supply. It is the largest organization of water supply professionals in the world, including scientists, public water and waste system professionals, manufacturing companies, regulators, environmentalists, and others working to improve water systems around the world. The AWWA develops industry standards for products and processes related to water. There are over 150 AWWA standards covering filtration systems, utility management practices, pumps, tanks, steel, pipes, and fittings. At Birmingham Fastener, we follow AWWA standards for all of our water works products including t-head bolts, mechanical joint gasket and bolt packs, and other products. To learn more about the AWWA, visit their website.

ASTM International

ASTM International is an all-encompassing organization that produces over 12,000 standards for many different types of products globally. In relation to structural bolt specification, the ASTM provides the standards for how the bolts are made and what raw materials can be used to create them. Each ASTM standard is reviewed at a minimum of every three years, meaning standards can change. For example, prior to 2015, ASTM A325 and A490 were separate specifications. After a review of the standards, these were replaced by F3125, making A325 and A490 grades under the F3125 specification. ASTM International also provides the standards on testing methods and what tests have to be done on all structural bolts prior to installation. To learn more about ASTM International, visit their website.

ASME

The American Society of Mechanical Engineers (ASME) is a not-for-profit organization that works towards collaboration and skill development for engineering disciplines. They produce codes and standards for many different technical fields, including the fastener field. In regards to fasteners, the ASME provides manufacturers with dimensional standards and thread lengths for all types of bolts, including bolts used in waterworks. For more information on the ASME, visit their website.

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