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Last updateMon, 21 May 2018 7pm

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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...

Cavitation in Globe Valves—and Proposed Solutions

Cavitation in Globe Valves—and Proposed Solutions

Straight pattern globe valves are widely...

The VMA Knowledge Forum, Part Two: The Human Factor

The VMA Knowledge Forum, Part Two: The Human Factor

Finding and retaining the right talent i...

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Industry Headlines

Manufacturers Are Boosting Hiring, Wages and Investment

Monday, 21 May 2018  |  Chris Guy

A new survey from the National Association of Manufacturers (NAM) shows that manufacturers are keeping their promise after passage of the historic tax...

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Industry Headlines

Flowserve Appoints Lanesha Minnix as Chief Legal Officer

11 HOURS AGO
Flowserve Appoints Lanesha Minnix as Chief Legal Officer

Lanesha Minnix will join Flowserve as senior vice president and chief legal officer, beginning Monday, June 11. Minnix comes from BMC Stock Holdings, Inc., where she served as senior vice president, general counsel and corporate secretary, since 2017.

Prior to BMC, Minnix was vice president, deputy gen...

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Emerson Agrees to Buy Aventics

3 DAYS AGO

Emerson has agreed on terms to acquire Aventics from Triton for a cash purchase price of $620 million. Aventics deals in smart pneumatics technologies that power machine and factory automation applications. Aventics significantly expands Emerson’s reach in a growing $13 billion market.

With cen...

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Outside of U.S., Natural Gas Outlook Diminishing

4 DAYS AGO

The mood in the natural gas industry, at least outside the U.S., is not as optimistic as it once was. According to the International Energy Agency (IEA), demand has slowed considerably for most of the period since 2011, from an average of 2.8% per year between 2000 and 2010, to 1.4% per year from 2011...

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ExxonMobil, BASF Form Gas Treating Alliance

5 DAYS AGO

ExxonMobil Catalysts and Licensing LLC and BASF Corporation have signed an alliance agreement to jointly develop new gas treating solvents and process technologies for use in natural gas processing and petroleum refining. Under this new agreement , BASF will market and license technologies developed f...

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Manufacturers Are Boosting Hiring, Wages and Investment

5 HOURS 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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Leading Economic Indicators Up 0.4% in April

3 DAYS AGO

The Conference Board Leading Economic Index (LEI) for the U.S. increased 0.4% in April to 109.4, following a 0.4% increase in March, and a 0.7% increase in February.

April’s increase and continued uptrend in the U.S. LEI suggest solid growth should continue in the second half of 2018. How...

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Cobalt-based Alloy 6 Materials and Boiler Feedwater Service

materials_q_and_a_graphicQ: I’ve heard that cobalt-base Alloy 6 materials should not be used in boiler feedwater service. Is this true?

A: Cobalt-chromium Alloy 6 is a very popular material for hard valve trim in applications requiring resistance to sliding wear, erosion and/or cavitation. It’s even successfully used in applications that are somewhat corrosive. However, in some areas of boiler feedwater applications where it would seem Alloy 6 should perform well, problems have been encountered. Here are some theories and facts about the problems:

Alloy 6 is available in cast, wrought and weld overlay forms. The cast material designation is UNS R30006; wrought material is designated UNS R30016, but is commonly called Alloy 6B; the generic AWS designation for hardsurfacing material is CoCr-A (specific designations are ECoCr-A for SMAW electrodes and ERCoCr-A for bare electrodes).

Alloy 6 is a cobalt-chromium-tungsten alloy with approximately 1% carbon. The material consists of a soft, solid solution matrix of cobalt-chromium tungsten surrounding a small percentage of hard, brittle chromium carbides.

Although the carbide phase provides the high hardness (approximately 40 HRC), research has shown that the cobalt-chromium-tungsten matrix is responsible for the alloy’s excellent wear and cavitation resistance. Alloy 6 undergoes a phase transformation (i.e., a change in crystal structure) when highly stressed, such as in a wear or cavitation situation. This phase transformation absorbs some energy that would otherwise cause damage, in effect, reducing the overall level of damage compared with a material that does not exhibit this “shock absorption” property.

Like most corrosion-resistant alloys containing chromium as an alloying element, Alloy 6 achieves corrosion resistance from the formation of a stable chromium oxide passive layer. This passive layer protects the underlying material from reacting with the environment. Certain chemicals can weaken the passive layer, reducing its ability to protect the material from corrosion.

Performance problems with Alloy 6 have been experienced in boiler feedwater applications where the water is treated with hydrazine or some other amine derivative. The problems occur exclusively in regions where the flow velocity is high, indicating that the failure mode is actually erosion-corrosion. Two possible explanations for this phenomenon are that:

1) The amine compounds weaken the oxide passive layer so that it erodes easily. The passive layer is repeatedly eroded away and rebuilt, resulting in accelerated corrosion.

2) The amines prevent the oxide passive layer from reforming after it has eroded initially, thus leaving the alloy unprotected from corrosion. Further erosion-corrosion then occurs at accelerated rates.

Other possible mechanisms for this type of failure may also exist, but the point remains that the alloy is attacked at higher rates than would be expected in an equivalent water application without the presence of the amine compounds.

Studies of returned parts have demonstrated that these attacks definitely correlate to the presence of Alloy 6. The photographs in Figures 1 through 4 were obtained during evaluation of a valve plug with Alloy 6 seat and guide surfaces that suffered erosion- corrosion damage in boiler feedwater. It’s very clear that the damage occurred exclusively in the CoCr-A material, and that the adjacent S31600 material is relatively unaffected, even though it is much softer.

Many failures have occurred in feedwater regulating valves, too. Failures have been reported in special- and standard-trim valves operating at temperatures as low as 300° F (149° C) and pressure drops as low as 100 psi (7 bar). Similar failures have been experienced in tungsten carbide trim in amine-treated feedwater and in ammonia applications when a cobalt binder phase is used.

No amine content, temperature or velocity limits have been established for safe use of Alloy 6 materials in feedwater. Common practice is to avoid the use of cobalt-containing alloys in feedwater service unless the feedwater is known to be compatible with cobalt alloys.

Common alternatives to Alloy 6 include hardened stainless steels such as S41600 (Type 416 SST), S41000 (Type 410 SST), S42000 (Type 420 SST), S44004 (Type 440C SST), or S17400 (17-4 PH SST). In some severely erosive applications, nickel base or iron-base hard-surfacing materials have been used.

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