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Wastewater Treatment

Wastewater Treatment

Society’s desire for a clean envir...

Controlling Our Water Systems, Part II

Controlling Our Water Systems, Part II

To better understand the actuators and c...

Controlling Our Water Systems

Controlling Our Water Systems

Actuators and controls are a critical pr...

Hardfacing Alloys and Processes for Advanced Ultra-Supercritical

Hardfacing Alloys and Processes for Advanced Ultra-Supercritical

Cobalt-based Stellite 6 has been the wor...

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Basics of Elastomeric Seal Design

Basics of Elastomeric Seal Design

Wednesday, 20 July 2016  |  Kate Kunkel

Engineers need critical design information when choosing a seal for a particular valve application. This includes:

  • Operating temperature and pressure r...

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Industry Headlines

Pentair Valves & Controls Rolls Out Customer Education Program

2 DAYS AGO

Pentair Valves & Controls has introduced the Pentair University customer education program for past, present and prospective clients and industry leaders in various locations throughout the world. Pentair University’s invitation-only seminars are free to attend. In some regions of the worl...

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Watson Valve Names Bob White New President

2 DAYS AGO
Watson Valve Names Bob White New President

Watson Valve Services Inc. has named Robert “Bob” White as its new president. The current president, John M. Watson, will remain as the company’s CEO.

Bob has served as executive vice president, and ownership, of Watson Valve Services since its inception in 2002. He has continued to p...

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Canadian Oil & Gas Earnings Signal Industry Recovery

1 DAY AGO

The Canadian oil and gas earnings season began yesterday with “signs of an industry recovery as Encana Corp and Precision Drilling Corp outlined plans to boost activity,” Reuters reports .

Analysts say “the uptick in optimism might be mirrored by some U.S. shale companies like Pioneer ...

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U.S. Lower 48 Sustains $150B in Cuts by Upstream Developers

2 DAYS AGO

Out of the more than $370 billion in global capital expenditure cut by upstream developers across 2016 and 2017, $150 billion was slashed in the U.S. Lower 48 alone — more than three times any other single country. Largely due to responsiveness and flexibility in the unconventional space, spen...

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STEM Jobs in the U.S. Growing Faster Than Other Fields

2 DAYS AGO

The U.S. Department of Commerce estimates that jobs in STEM will grow 17% by 2018—that’s 55% faster than non-STEM jobs over the next decade. Several reports have linked STEM education to the continued scientific leadership and economic growth of the U.S. However, economic projections also ...

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IMF Cuts Global Growth Forecast After Brexit Vote

3 DAYS AGO

The International Monetary Fund (IMF) cut its forecasts for global economic growth this year and next as the unexpected U.K. vote to leave the European Union creates a wave of uncertainty amid already-fragile business and consumer confidence.

The global economy is projected to expand 3.1% this year 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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