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Last updateMon, 22 May 2017 6pm

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The Future of Coal: Efficiency Over Politics?

The Future of Coal: Efficiency Over Politics?

Many changes in the power industry have ...

Valve World Americas Event Set for June 20-21

Valve World Americas Event Set for June 20-21

End users, distributors, EPC/AEC personn...

Water Hammer

Water Hammer

Water hammer is a shock wave transmitted...

Turning the Tables on Valve Corrosion

Turning the Tables on Valve Corrosion

Multiple valve manufacturers and users w...

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Industry Headlines

MRC Global Opening Gulf Coast Regional Distribution Center

1 HOUR AGO

MRC Global (US) Inc. is opening a new regional distribution center (RDC) and operational hub for the Gulf Coast area in La Porte, TX. The new facility will house over 400,000 square feet of warehouse space, have over 75,000 square feet of office space and an extensive pipe yard. MRC Global expects t...

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United Valve Puts Huge New Machine Tool in Service

2 DAYS AGO
United Valve Puts Huge New Machine Tool in Service

United Valve has just finished the first four weeks of operation of its huge new vertical boring Lathe (VTL). The 38’ tall machine, which weighs nearly 350,000 pounds, has the capacity to swing components up to 24 feet tall and 20 feet wide. Situated under a 40-ton capacity crane, the machine i...

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Pulp & Paper Waste Could Lead to New Manufacturing Industry

5 DAYS AGO

About 50 million tons of lignin -- or structural part of a plant -- piles up each year as waste from the U.S. pulp & paper industry. Additional lignin could come from biorefineries that use plants to produce ethanol, yielding another 100 million to 200 million tons of lignin waste each year. Yet o...

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Mining Companies Increasingly Using Renewable Power

5 DAYS AGO

Some of the world's largest mining companies "are seeking to use more renewable energy themselves as they strive to drive down costs and curb emissions,” Reuters reports .

Miners “say that coal still can be the cheapest, most reliable baseload power depending on circumstances, but the sector...

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Labor Secretary Pushes Public-Private Apprenticeships

6 HOURS AGO

Speaking at the G-20 labor and employment ministers' meeting on May 19, U.S. Secretary of Labor Alexander Acosta emphasized his support for fostering the growth of apprenticeships, a major priority for President Trump and the Department of Labor. The secretary's remarks reinforce the U.S.’s comm...

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Mid-Atlantic Region Factory Activity Accelerates in May

3 DAYS AGO

The index for current manufacturing activity in the mid-Atlantic region increased from a reading of 22.0 in April to 38.8 this month. The index has been positive for 10 consecutive months. This month, the index recovered some of the declines of the previous two months, but it still remains slightly ...

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Wrong Alloy?

materials_q_and_a_graphicQ: My 316 stainless-steel valve is rusty and attracts a magnet. Did I get the wrong alloy?

 

A: Let's answer the second part of the question first. Forged 300-series stainless steels should be non-magnetic. However, cast versions of the 300-series stainless steels-such as CF8 (304), CF3 (304L), CF8M (316), CF3M (316L), CG8M (317), CG3M (317L), CF8C (347), etc.-are all formulated to contain some ferrite. The presence of ferrite makes the alloy less prone to cracking as the hot casting cools in the mold. Weld filler materials are also formulated to contain some ferrite for the same reason. The ferrite makes the material attract a magnet. The exact amount of ferrite, which influences the strength of magnetic attraction, is dependent on the exact chemical composition and the thermal history of the casting. In any event, CF3, CF8, CF3M, CF8M, CG3M, CG8M, and CF8C should all attract a magnet to some degree. In fact, if you have a casting in one of these alloys that doesn't attract a magnet, you should wonder whether something is wrong.

 

Now let's discuss the rusting portion of the question. While the term stainless steel implies that such an alloy is "stainless" or will not rust, the reality is that stainless-steel castings can exhibit a rusty appearance if not processed correctly. The rusty appearance is only superficial surface rust, i.e. iron oxide, and for most applications it is neither a problem nor an indication as to how the equipment will perform in service. However, there are some services where this surface rust is objectionable and special processing is necessary to make sure the stainless-steel castings are truly "stainless." Some of the industries where any rust is undesirable are food and beverage, pharmaceutical, and electronics.

Shot blasting is routinely used by foundries and forge shops to remove burned-in sand, core material, and scale that are formed during casting, forging, and/or heat-treating processes. Steel shot is normally used because it is the quickest and most economical cleaning abrasive. A slight amount of iron contamination remains on the surface of the stainless steel, and may later cause surface discoloration or rusting.

Sand blasting or grit blasting with various abrasives may also be used to clean castings and forgings. If the sand or abrasive was previously used on steel, it may contaminate the stainless-steel surface, which can result in rusting. Therefore, if sand or grit blasting is utilized, the sand or grit should either be new or only used previously for cleaning stainless-steel items.

Pickling, or the more technically correct term acid cleaning, is used to remove iron oxide that formed during the heat treatment of stainless steel or to remove free iron contamination that may have occurred from shot blasting, grit blasting, and/or grinding of stainless- steel castings or forgings. Normally, acid cleaning is only necessary for alloys that do not have sufficient chromium content to prevent oxidation of the surface during heat treatment, such as the 300-series stainless steels. The higher-chromium-content duplex stainless steels do not require acid cleaning unless they are to be used in one of the rust-sensitive industries mentioned above. Pickling is normally done as a last step before machining.

Passivation is the process by which a stainless steel will form a thin, invisible, chemically inactive surface when exposed to air or other oxygen-containing environments. At one time it was considered necessary to apply an oxidizing treatment to stainless steels in order to establish this protective oxide film. It is now accepted that this oxide film will form spontaneously in any oxygen-containing environment provided the surface has been thoroughly acid cleaned, i.e. pickled or descaled. This oxide film is the mechanism by which corrosion-resistant alloys achieve their corrosion resistance. Although a stainless steel is basically self passivating in any oxygen-containing environment, even air, some users of stainless-steel equipment still feel it is necessary to require a separate passivation treatment. If passivation is performed, it is generally done after finish machining.

Acid cleaning and passivation are covered by ASTM specifications A380 and A967. These specifications provide complete details on the acid cleaning and passivation of stainless steels as well as a variety of tests to measure the effectiveness of these processes, including simple wet-dry immersion tests to chemical testing for the detection of free iron. As with any special order requirement, there should be discussions between the manufacturer and customer up front so that everyone is in complete agreement on the processes to be used and how the results will be measured.

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