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Metal Additive Manufacturing in the Valve Industry

Metal Additive Manufacturing in the Valve Industry

Metal Additive Manufacturing (AM) is a f...

The Past, Present and Future of Fire Testing

The Past, Present and Future of Fire Testing

Because so many of the applications wher...

Advancements in Blue Laser Scanning

Advancements in Blue Laser Scanning

As the industrial world continues to exp...

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Industry Headlines

LNG, Ethanol Sellers Buoyed by China Trade Talk

Wednesday, 23 May 2018  |  Chris Guy

“China's interest in reducing its trade surplus with the United States through increased energy imports could advance plans for U.S. liquefied n...

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Industry Headlines

Curtiss-Wright Supporting Ford-Class Aircraft Carrier Program

1 DAY AGO

Curtiss-Wright has been awarded a contract valued in excess of $85 million to provide main propulsion steam turbines and auxiliary equipment for the U.S. Navy’s Ford-class aircraft carrier Enterprise (CVN 80). The award was received from Huntington Ingalls, Newport News Shipbuilding (HII-NNS) ...

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Crane Celebrates Opening of New Plant in India

1 DAY AGO

Crane ChemPharma & Energy celebrated the inauguration of its newest location, an aseptic diaphragm valve factory in Satara, India. Crane invited hundreds of customers and industry stakeholders to participate in a day-long event complete with a tour of the new plant and luncheon held on May 10, 201...

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LNG, Ethanol Sellers Buoyed by China Trade Talk

1 HOUR AGO

“China's interest in reducing its trade surplus with the United States through increased energy imports could advance plans for U.S. liquefied natural gas (LNG) plants and ethanol sales, said analysts and energy executives involved in developing new LNG facilities.”

Reuters  reports th...

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Chemical Activity Up as Pace of Growth Slows

2 HOURS AGO

The Chemical Activity Barometer (CAB) from the American Chemistry Council (ACC), rose 0.1% on a three-month moving average (3MMA) basis in May to 121.9. The barometer is up 3.9% on a 3MMA compared to a year earlier. The unadjusted CAB showed a second consecutive month of gains, up 0.2% in May and also...

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Philly Fed: Manufacturing Activity Up in May Survey

1 DAY AGO

The Philadelphia Fed’s monthly survey indicate a pickup in growth for the region’s manufacturing sector. The indexes for general activity, new orders, shipments, and employment all improved from their readings last month. The indexes for prices paid and received continued to suggest price ...

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Manufacturers Are Boosting Hiring, Wages and Investment

1 DAY AGO

A new survey from the National Association of Manufacturers (NAM) shows that manufacturers are keeping their promise after passage of the historic tax reform legislation by boosting wages, hiring and investment. The survey , released by Emerson chairman and CEO and NAM board chair David Farr during a ...

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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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