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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

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Industry Headlines

IHS: Manufacturing at Strongest Level Since Sept. 2014

Thursday, 24 May 2018  |  Chris Guy

The seasonally adjusted IHS Markit Flash U.S. Manufacturing PMI registered 56.6 in May, up fractionally from 56.5 in April, to signal the strongest im...

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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

17 HOURS 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

18 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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IHS: Manufacturing at Strongest Level Since Sept. 2014

-1 DAYS AGO

The seasonally adjusted IHS Markit Flash U.S. Manufacturing PMI registered 56.6 in May, up fractionally from 56.5 in April, to signal the strongest improvement in business conditions since September 2014 . May data revealed relatively strong rises in both manufacturing production and incoming new busi...

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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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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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