STAINLESS STEEL 300 SERIES

by Royce Lowe

The stainless 300 series might be described as the workhorses of the greater family of stainless steels. As a group, they show better corrosion resistance, toughness, high-temperature strength, and low-temperature properties. They are based on what was called the 18/8 steel, or type 302, but developments dating from their discovery period led to the evolution of a group of steels that could handle a whole host of corrosive conditions, together with a wide range of elevated temperatures. With exceptions of course, such as seawater.

The 300 series, for the most part, are known as the austenitic grades, because they retain the phase austenite to room temperature.

Today the base stainless 300 grade is type 304, 18% chromium, 10% nickel, often produced as 304L, or 0.03% max carbon. The low-carbon grade will not show carbide precipitation, as does the normal grade. As such, there is no need for post-weld anneal. The grade is resistant to mild to medium corrosive environments, but will not resist corrosive media that might cause pitting. Grades that show much-improved resistance to pitting are types 316 and 317, often produced in their low-carbon form. These grades contain molybdenum, around 2.5% in type 316, around 3.5% in type 317. 

Type 300 series stainless steels are not that easy to machine, but if a little sulfur is added to the 304 base, the machinability of the steel (type 303) is greatly improved. For good machinability and a better machined finish, selenium may be added instead of sulfur, (type 303Se). However, these additions will reduce the ductility and toughness of the steel. 

Type 321, which is basically type 304 with a titanium addition, is not susceptible to carbide precipitation during welding, or in service use between 800 and 1650ºF (430 and 900ºC), nor is type 347, basically 304 with a columbium addition. These two additions prevent the formation of chromium carbides, which migrate to grain boundaries and render the 304 type not at all stainless. These grain-boundary carbides can be removed by an annealing treatment at around 1950ºF, but the use of “stabilized” grades such as 321 or 347, or low carbon grades, 304L, 316L etc., is a much more efficient way to fabricate stainless steels.

Increasing chromium and nickel beyond the basic 18/10 grades sees the formation of types 309 (nominally 23Cr/13.5 Ni) and 310 (nominally 25Cr/20Ni) that show good oxidizing resistance to 2000ºF and 2100ºF respectively. Additions of a relatively small percentage of silicon, some 1.5/3.0%, will give even further scaling resistance, in type 314. Nitrogen is added as a strengthener, nominally to 0.2-0.4%. Certain of these high chromium/nickel alloys will show resistance to oxidizing, carburizing, and sulfur-bearing atmospheres.

All Metals & Forge Group takes great pride in maintaining one of the most stringent ISO9001, AS9100 quality systems in the open die forging and seamless rolled ring industry.  It begins with learning the customer’s end use, required forging surface condition, mechanical properties of the specified material and alloy, forged shape, heat treatment, delivery need, and competitive price, coupled with forging soundness proven by ultrasonic testing, and care in packing goods to arrive in pristine condition at the customer’s desired location.  Every step is monitored by the quality system at AMFG and continuously improved.

AMFG performs rough machining to within 3mm of finish dimensions to reduce the CNC cost at the customer’s machine shop for all the forged shapes the company produces.  The rough machine surface condition is 250 RMS so that proper quality testing can be performed on each part – the AMFG standard.  

The company can perform finish machining to within .001 of an inch for final tolerance and 64 or 32 RMS surface finish.  This level of quality and precision is unique in an industry where other company’s parts are often delivered black, as forged, or with a rough machined surface of 500 RMS without the internal or external steps and dimensions to reduce machining costs.

From inquiry to invoice, All Metals & Forge Group quality is managed and not assumed. Whether the need is for one part, an entire project, or a production run, AMFG delivers.