The iron-based alloys known as Steel are arguably the most widely used materials in construction and manufacturing worldwide. A wide variety of Steel grades, each with unique chemical compositions, have been created, based on the requirements of the end use a steel component must serve.
Carbon steel, alloy steel, and stainless steel are all metals with a wide range of consumer and commercial applications. The grades known as Stainless Steels offer advantages in creation of a wide range of products. One of these applications is the production of stainless steel forged fittings, typically used in fluid and gas transport, chemical and food processing, and any location or environment that requires the properties that Stainless Steel proves the benefit justifies the cost.
About Stainless Steel
Like all other kinds of steel, stainless steel primarily consists of iron. What makes stainless steel different from other grades of steel, is addition of the element chromium, in contents that range from 11% to 30%. Suggestions of potential for the element chromium, added to iron can be traced to scientific experiments as early as 1800. An excellent book describing the discovery and invention of the alloy, The History of Stainless Steel, by Harold M. Cobb, is available from ASM International. Additions of other elements, most commonly Nickel in volumes of 8% to 11%, provide enhanced properties and broaden the service range of the alloy group.
With the addition of chromium, the Steel we now call stainless was first termed “Rust Proof” steel, one of the early brands being called “Staybright”. Chromium oxide forms a protective surface, limiting the ability of iron, water, and atmospheric oxygen to form iron oxide, what we call rust. In addition to rust resistance, stainless steel alloys also provide the following advantages over other steel types:
- Resistance to high and low temperatures – allowing stainless to serve both as furnace and cryogenic components
- Strength and durability – resistance not only to rusting but to chemical attack
- Long-lasting functionality with a low lifecycle cost – steels that survive far longer in hostile environments in critical use applications
- A glossy and aesthetic appearance – many alloys in the group are used where a desirable surface finish must be achieved and sustained
Stainless Steel Forgings
Stainless Steel Forged Pipe Fittings
Stainless Steel Rebar Couplers
The characteristics of stainless steel make it ideal as a material for many components that must meet the requirements of ASTM A182 or ASME SA182, frequently used specifications for all types forged fittings.
Stainless Steel Types
Variations of stainless steel in the alloy group generally fall into several major classifications. Four of the major categories of stainless steel variations:
Martensitic
Martensitic stainless steels, the initial commercially produced grade, has relatively low corrosion-resistance generally due to lower chromium content. With levels of carbon that provide the capability to be heat treated, these alloys may achieve high hardness and high strengths. In particular, this group includes several of the precipitation hardening alloys (“PH”), often designated for structural components in aircraft and medical end uses. Initial alloy inventions in this category were known for wear resistance. Later additions of other elements have provided versions with improved weldability, toughness, and resistance to creep deformation. Additional alloy inventions substitute nitrogen for some carbon content, improving hardness, ultimate strength and corrosion resistance.
Ferritic
Ferritic stainless steels have microstructure most like carbon steels. Chromium-based with a low carbon and little to no nickel content, theses alloys are magnetic. Lacking the addition of nickel, ferritic stainless steels are less formable and less corrosion resistant, however they are less costly making them desirable for applications where product life may be limited by other considerations or environments are not as corrosive. The 400 series grades in this alloy group are, generally, heat treatable and do not strengthen with cold working.
Austenitic
Likely used in greater volume of applications than any other of the stainless steels, within this non-magnetic group is alloy 304, often referred to as “18-8” stainless. Austenitic stainless steels include the alloys in the series numbered 200 and 300, known for use in food service, chemical processing and for a wide variety of components due the relative ease of formability and performance at reasonable cost. These alloys cannot be heat treated but will harden and strengthen with cold working. Highly resistant to rust and chemical attack, Austenitic stainless steel properties make this alloy ideal when used to produce forged fittings to meet ASTM A182 or ASME SA182.
Duplex
This type has a microstructure mixing the characteristics of ferritic and austenitic stainless steels providing improved strength and resistance to stress corrosion than either austenitic or ferritic steels. Steels in this category are moderately formable and less magnetic than ferritic steels. Seeking improvements in strength and corrosion resistance, additions to these alloys have produced costly grades called “super duplex” and “hyper duplex” stainless steels to meet specific end use demands.
Precipitation Hardening
Although the initial PH grades invented fall within the group known as Martensitic, additional versions of PH grades have been invented that extend PH stainless steels into unique categories. In particular, there are grades that, through cryogenic treatment or cold working may transform martensite into austenite in the microstructure. This alloy group also contains some of the first ferrous “superalloys” with austenitic structure that could sustain mechanical properties and creep strength at temperatures in excess of 1000F (540C) used in high temperature turbomachinery.
Common Stainless Steel Specifications
A range of national and international standards govern the mechanical properties, grade compositions, and production specifications of stainless steel. These specifications indicate the physical and chemical characteristics of the steel and are critical to determining if a material or product is suitable for a specific application.
The American Society for Testing and Materials (ASTM) is one of the most prominent standards developing organization. There are ASTM standards for all engineering materials, however, the prefix letter that designates iron and steel material specifications is “A”, for example, ASTM A182.
Specifications for forged fittings indicate requirements for chemical compositions, forging temperatures, heat treatment, and conformance to mechanical properties, such as hardness, yield strength, and ultimate tensile strength.
Within a specification, many different grades and reference to other specifications may be indicated. It is critical to select the stainless steel specification standard and grade that meets the application end use requirements.
For example, the standard specification for threaded couplings and galvanized steel for use in forged steel pipe fittings is ASTM A865. The standard specification for seamless, forged, welded, and heavily cold worked austenitic stainless steel pipe fittings, and threaded fasteners is ASTM A312. ASTM A182, covers a variety of stainless steel pipe, pipe fittings, and forged low alloy components for use in pressure systems, including flanges, fittings, valves, and similar components. ASTM A182 is used internationally and industries in many other countries use this standard in specifications for their products.
What are Stainless Steel Forged Fittings?
Stainless steel forged fittings are essential components of some industrial or commercial systems, making the joining of two or more pipes, possible. Piping systems less than 2 or 4 inches in diameter generally use forged fittings and differ from buttweld fittings used for piping systems with larger diameters.
The manufacturing process of stainless steel forged fittings involves the application of thermal and mechanical energy to shape solid stainless steel blanks into functional shapes within the tolerances established by the applicable specifications. These forged fittings are available in a wide range of sizes and diameters, and there are several shapes of forged fittings to cater to any type of pipe joint.
Steel forged fittings have to meet several requirements to ensure optimal functionality as socket weld or threaded fittings.
These steel forged fittings have to be corrosion-resistant. If a socket weld or threaded steel forged fitting is susceptible to rust, premature deterioration and leaks can result. Corrosion can also make it difficult to install or remove a threaded steel forged fitting.
Stainless steel forged fittings should also be lightweight, but durable and abrasion-resistant during high-temperature services or strenuous weather conditions. Additionally, stainless steel forged fittings should be suitable for installing high-pressure (3,000lb, 6,000lb, and 9,000lb) piping systems.
Each fitting has a class grading of 3000, 6000, or 9000, representing the maximum pressure that the stainless steel forged fitting can withstand. Pipes in schedule 80/XS require class 3000 fittings, pipes in schedule 160 require class 6000 fittings, and XXS pipes with thicker walls require class 9000 fittings.
Stainless steel grades that generally meet these requirements are in the 300 series, including, among others, 304, 304L, 309, 310, 316, 316L, 317L, 321, and 347. The L-grades offer additional corrosion resistance after welding. Some socket weld steel forged fittings are 304L stainless steel.
Careful application of ASME B16.11 and ASTM A182 standards will help users comply with all the applicable regulations while achieving optimal cost, operational, and safety benefits from steel forged fittings. The most common material grades are ASTM A105 and ASTM A350 LF/1/2/3/6 for low-temperature applications and ASTM A182 for high-temperature, corrosive applications.
Stainless Steel Forged Fittings Types
There are two basic types of stainless steel forged fittings, namely:
- Threaded stainless steel forged fittings
- Socket weld stainless steel forged fittings
With threaded stainless steel fittings, pipe and fitting join to each other using threaded sections. One or both components must be rotated to engage the threads and fully close any gap between them. The strength of the joint is dependent upon the strength of the engaged threads. With socket weld fittings, the fitting connects to the pipe with pipe entering a shallow depression in the fitting. The gap is then closed with a “fillet weld” which involves applying heat and filler metal to provide a seal, metal melted and re-solidified to close the gap.
Stainless steel forged fittings are available in a variety of shapes and sizes including 45-degree elbows, 90-degree elbows, tees – which can be equal (straight) or reducing, lateral or Y-shaped pipe fittings to branch a pipe at 30 degrees, cross fittings to connect four pipes, plugs or caps to blind pipes, half or full couplings to join pipes, and reducer inserts to reduce the bore size
Threaded Stainless Steel Forged Fittings
In common use in are several types of thread forms including British Standard Pipe (BSP), National Pipe Tapered (NPT), and ISO Pipe and Port Size Metric sizes. These threads are not compatible with each other.
Threaded pipe forged fittings are suitable for applications such as cooling, fire protection, and water distribution. If the pipe system manages fluids with continually changing temperatures, threaded pipe fittings are not ideal as temperature fluctuations can cause the fitting to crack in the thread forms.
Socket Weld Stainless Steel Forged Fittings
Socket weld forged fittings may be considered more permanent than threaded forged fittings. Welded joints are considered more relaiable if the welding process is performed correctly, providing a stronger joint attachment. The drawback of fittings of this type is the time and labor for installation. Typical applications for socket weld forged fittings include steam and other gases, toxic fluids and acids, and explosive liquids and gases where highest connection strength and leak prevention are critical.
Stainless Steel Forged Fittings Specifications
Several specifications apply to stainless steel forged fittings. The American Society of Mechanical Engineers (ASME) B16.11 specifications cover forged fittings to fit ASME B36.10 and ASME B36.19 pipes.
This specification covers the material requirements, dimensions, ratings, marking, and tolerances for socket weld forged fittings and threaded forged fittings. Users should apply B16.11 specifications in conjunction with equipment that other volumes of the ASME B16 series of standards describe.
ASTM A182 is another specification that applies to forged pipe fittings. Specifically, ASTM A182 covers forged fittings, forged- or rolled alloy pipe flanges, stainless steel pipe flanges, valves, and other components for high-temperature service.
ASTM A 182 also applies to forged fittings for use in pressure systems. ASTM A182 include forged fittings of ferritic, martensitic, austenitic, and duplex stainless steels.
Queen City Forging Co
When it comes to stainless steel forged fittings, compliance with specifications, such as ASTM A182 and ASME B16.11, is paramount. Queen City Forging is a leading provider of high-end forged fittings to ensure optimal compatibility, functionality, and safety across the piping system.