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Food and Beverage Processing

Food and Beverage Processing

When wandering the grocery store aisles,...

Variable Frequency Drives in Electric Actuators

Variable Frequency Drives in Electric Actuators

Electric actuators are vital for operati...

Braided Packing: An Old Technology with a Modern Twist

Braided Packing: An Old Technology with a Modern Twist

Compression packing is the primary metho...

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Industry Headlines

Rotork Acquires Mastergear from Regal Beloit

1 DAY AGO  |  Chris Guy

Rotork has acquired the entirety of Mastergear’s business and assets from Regal Beloit Corporation for $25 million on a cash-free and debt-free ...

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Industry Headlines

Rotork Acquires Mastergear from Regal Beloit

1 DAY AGO

Rotork has acquired the entirety of Mastergear’s business and assets from Regal Beloit Corporation for $25 million on a cash-free and debt-free basis and a value of inventory that will be finalized at the end of a transition service agreement.

Mastergear will become part of Rotork's Gears divis...

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Emerson Opens New Helix Innovation Center

2 DAYS AGO

Emerson announced the launch of its “Helix” initiative, including the opening of The Helix innovation center, an industry-first, $35 million hub dedicated to advancing research and education for the global heating, ventilation, air conditioning and refrigeration (HVACR) industry. The new ...

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Energy Sector Weighs on Construction Starts

2 DAYS AGO

At a seasonally adjusted annual rate of $660.5 billion, new construction starts in March receded 1% from February’s pace, according to Dodge Data & Analytics. Total construction starts had jumped 13% in February, led by a huge gain for the electric utility and gas plant category. The dolla...

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Chevron Investing Billions in The Permian Basin

3 DAYS AGO

At Chevron’s annual security analyst meeting last month, chairman and CEO John Watson said that his company plans on investing billions into oil & gas projects in the Permian Basin.

According to the Hobbs News-Sun , Watson predicted they could be pumping up to 350,000 barrels a day out of West ...

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House Passes Bill To Lower Import Tariffs On Manufacturers

2 DAYS AGO

This week, the House voted 415-2 for the American Manufacturing Competitiveness Act of 2016. The bipartisan, bicameral bill reforms the way Congress considers legislation for suspending tariffs on products not made in the U.S., what some are calling, “manufacturing tax breaks.” The legis...

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Texas Manufacturing Index Shrinks More Than Forecast

4 DAYS AGO

Texas factory activity increased for a second month in a row in April, according to business executives responding to the Texas Manufacturing Outlook Survey. The production index, a key measure of state manufacturing conditions, rose from 3.3 to 5.8, suggesting a slight pickup in output growth.

Perce...

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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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