Carbon, stainless and alloy steel fittings: choosing the right grade
A234 WPB, A403 WP316L, A420 WPL6 and A234 WP11 or WP22: what each grade is for, and how to choose between them for temperature, corrosion and cost.
Fortune Technical Desk4 min readOn this page
The material grade decides how long a fitting survives in its service. Carbon steel is the workhorse, stainless steel handles corrosion, low-temperature carbon steel avoids brittle fracture, and chrome-moly alloy steel keeps strength in hot lines. The grades below cover most Indian process piping.
Carbon steel: ASTM A234 WPB
ASTM A234 is the standard specification for piping fittings of wrought carbon steel and alloy steel for moderate and high temperature service. Grade WPB is the everyday carbon steel grade used for water, oil, gas, air and steam within its temperature limits. It is weldable, widely available and the lowest in cost. The matching forging grade for forged fittings and flanges is ASTM A105, and the matching pipe is typically ASTM A106 grade B or API 5L.
Stainless steel: ASTM A403
ASTM A403 covers wrought austenitic stainless steel fittings. The most common grades are WP304, WP304L, WP316 and WP316L, with WP321 and WP347 used for higher-temperature service where intergranular corrosion is a concern. Grade 316 adds molybdenum, which improves resistance to chlorides and to pitting compared with 304.
Why the L grades matter
The L stands for low carbon. When ordinary 304 or 316 is welded or held in a certain temperature range, carbide can form at grain boundaries and reduce corrosion resistance in the heat-affected zone. Low carbon grades such as 304L and 316L avoid most of this, which is why they are the usual choice for welded pipework. Many mills dual-certify material as 316/316L.
Low-temperature carbon steel: ASTM A420 WPL6
Ordinary carbon steel loses toughness as temperature falls and can fracture in a brittle manner. ASTM A420 covers fittings made from steels that are impact tested at low temperature. Grade WPL6 is the common one, impact tested at minus 46 degrees Celsius, and it is used on LPG, ethylene, cold ammonia and similar lines. The forging equivalent is ASTM A350 LF2.
Chrome-moly alloy steel: A234 WP11 and WP22
Chromium and molybdenum improve strength and creep resistance at high temperature and resist hydrogen and oxidation attack. Grade WP11 is about 1.25 percent chromium with 0.5 percent molybdenum, and WP22 is about 2.25 percent chromium with 1 percent molybdenum. They are used in boiler piping, steam headers and refinery lines. These alloys usually require preheat and post-weld heat treatment, so check the welding procedure before specifying them.
Choosing between them
Questions to ask before choosing
- What is the fluid, and is it corrosive, including trace chlorides?
- What are the design and minimum metal temperatures?
- Will the line be welded on site, and is heat treatment practical?
- Does the project require impact testing or hydrogen service compliance?
- What does the piping class or client specification already dictate?
Share your service conditions and grade list with Fortune Fittings Private Limited and we will quote fittings in carbon, stainless or alloy steel, with test certificates on request.
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Request a QuoteFrequently asked questions
Is 316L always better than 304L?
It resists chlorides and pitting better, so it is better for those environments. For clean water or mild service 304L can be perfectly adequate and costs less.
What is the difference between WP and F grades?
WP prefixes denote wrought fittings made from pipe or plate, for example WP316L. F prefixes denote forgings, for example F316L, used for forged fittings and flanges.
Can I use WPB in a cold climate?
It depends on the minimum design metal temperature and the code. Below roughly the limits in your design code, impact-tested material such as WPL6 is used instead.
