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Achieving Predictable Valve Performance for Safety Applications

Achieving Predictable Valve Performance for Safety Applications

A focus on having the proper specificati...

Putting Servo Valves Back to Work

Putting Servo Valves Back to Work

Industries as varied as paper production...

Corrosion and Fouling: Is There a Solution?

Corrosion and Fouling: Is There a Solution?

According to a 1998 study released by the ...

Valves with All the Trimmings

Valves with All the Trimmings

The term valve trim has been around for ...

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Industry Headlines

IHS Markit Manufacturing Index Rose in February

Thursday, 22 February 2018  |  Chris Guy

U.S. manufacturers reported a strong upturn in business conditions during February, which continued the positive trend seen at the start of 2018. At 5...

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Valve Positioners Offer Improved Control Valve Performance

Valve Positioners Offer Improved Control Valve Performance

Monday, 19 February 2018  |  David Matherly

From time to time, we are re-posting well-received or particularly valuable articles that have previously run on VALVEMagazine.com so that those who m...

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Industry Headlines

MRC Global Announces Fourth Quarter 2017 Results

1 DAY AGO

MRC Global’s sales were $903 million for the fourth quarter of 2017, which was 26% higher than the fourth quarter of 2016 and 6% lower than the third quarter of 2017 due to seasonality. As compared to 2016, all sectors increased driven primarily by midstream and upstream.

MRC Global's fourth qu...

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Curtiss-Wright Acquires Dresser-Rand Government Business

2 DAYS AGO

Curtiss-Wright has entered into an agreement to acquire the assets that comprise the Dresser-Rand Government Business (Dresser-Rand), a business unit of Siemens Government Technologies, for $212.5 million in cash.

Dresser-Rand designs and manufactures mission-critical, high-speed rotating equipment sol...

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U.S. Refiners Increasingly Turning to Export Markets

1 DAY AGO

“U.S. gasoline consumption has leveled off as the stimulus provided by low and falling oil prices between 2014 and 2016 has faded, so refiners are increasingly turning to diesel and customers in emerging markets,” Reuters  reports .

“In geographical terms, the fastest growth in co...

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$900M Power Plant Planned for Indiana

1 DAY AGO

Vectren Energy Delivery of Indiana is proposing to install an additional 50 megawatts (MW) of universal solar and build an 800 to 900-MW natural gas-fired generation facility in addition to other critical investments that will significantly change the way the company generates power for the region .

Th...

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IHS Markit Manufacturing Index Rose in February

5 HOURS AGO

U.S. manufacturers reported a strong upturn in business conditions during February, which continued the positive trend seen at the start of 2018. At 55.9, up from 55.5 in January, the seasonally adjusted IHS Markit Flash U.S. Manufacturing Purchasing Managers’ Index (PMI) pointed to the fastes...

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Survey Finds Surge in Confidence of Small Business Owners

1 DAY AGO

CNBC and SurveyMonkey recently announced the results of their quarterly Small Business Survey. According to the findings, 47% percent of small-business owners say overall business conditions are good , up from 44% in the fourth quarter of 2017. Also, 32% of small business owners expect to increase the...

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Checking Metal Materials With a Magnet

materials_q_and_a_graphicQ: I’ve seen people checking metal materials with a magnet. Is this a useful method of sorting materials, and if so, how does it work?

A: Valve companies deal with a large number of pure metals and alloys due to the variety of applications in the process industry. Occasionally, alloy identification must be performed on parts due to customer inquiries, mix-ups in bar stock, questionable machining characteristics or for some other reason. Although positive material identification (PMI) has become quite common, a PMI tester is not always available in a timely manner. One common identification/sorting technique that is often overlooked—and sometimes misapplied—is magnetic inspection. Magnetic inspection can save a lot of time by quickly proving a material is not what it is supposed to be.

Categorizing Materials by Magnetic Attraction

Magnetic inspection refers to categorization of a material by observation of its magnetic attraction force. Various alloy groups behave differently when exposed to a magnet. However, virtually all alloys fall into one of four behavior categories:

  • F - Fully magnetic:Materials such as carbon steels, alloy steels.
  • N - Never magnetic: Materials such as aluminum alloys, copper alloys, most nickel-base alloys, some stainless steels, etc. These materials exhibit no perceptible attraction to a magnet.
  • P - Partly magnetic: Materials such as some stainless-steel castings and wrought products. These materials exhibit some attraction to a magnet, but less than the fully magnetic materials.
  • V - Varying:Materials such as some stainless steels, nickel-copper alloys, etc. These materials may or may not be attracted to a magnet, and when attracted to a magnet, the attraction strength may vary significantly, depending upon the exact composition and processing history.

Information on magnetic characteristics can usually be found in material product literature.

An unknown material’s magnetic characteristic is determined by placing a magnet against the material and observing whether it is attracted or not. If there is no perceptible attraction, the material falls into category “N”. If there is attraction, decide whether it is full or partial. This is best done by placing the magnet against the unknown material and then bringing a piece of carbon steel into contact with the opposite end of the magnet. If the carbon steel easily removes the magnet from the unknown material, then the unknown falls into category “P”. If the magnet is attracted with approximately equivalent force by both materials, then the unknown falls into category “F”. In performing this comparative test, it is important that the surface contour and finish of the unknown piece and the carbon steel piece be the same (preferably flat). It is also important that both parts are more massive than the magnet or, in the case of sheet materials, that both parts have approximately the same thickness.

Limitations of Magnetic Inspection

The most important thing to keep in mind regarding magnetic inspection is that, although it can prove that a part is not a particular material, it cannot prove that a part is a particular material.

Here are some example applications of magnetic inspection:

  • Example 1: Records have been lost for a valve shaft that has been stored for several years. It is assumed the shaft is probably either S17400 or S20910, since these are the standard materials of construction for this part. Magnetic inspection determines that the shaft is fully magnetic. S17400 is fully magnetic, whereas S20910 is never magnetic. Therefore, the shaft is not S20910, and may be S17400. It could also be some other fully magnetic material.
  • Example 2: A customer orders a “316” valve body, but upon receipt of the body finds that it is slightly magnetic. The customer calls and complains that he did not receive a “316” body as ordered, because he knows that 316 stainless steel is never supposed to be magnetic. The problem with this logic is that the body is not 316, but rather is a CF8M casting, the equivalent of 316 wrought material. The chemistry of the cast material is adjusted to intentionally produce a small percentage of “ferrite,” which is a magnetic phase. This renders the casting partly magnetic, and often leads to this type of confusion. This highlights the importance of accuracy in material designation and product form (cast, wrought, etc.) when using magnetic inspection.
  • Example 3: A casting is sent to inspection for a material check because the machine operator noticed its machining characteristics were unusual. The casting is supposed to be CW2M. Magnetic inspection may save a great deal of time vs. performing a PMI. The casting is found to be partly magnetic. This proves that the casting is not CW2M, which is never magnetic.
  • Example 4: Same situation as example 3. This time, the material is found to be non-magnetic. This does not mean that the material is CW2M. This is one of the most important limitations in magnetic inspection. Magnetic inspection can prove that a part is not a particular material, but it cannot prove that a part is a particular material. In this case the casting should be further evaluated by PMI or some other method to determine if it is CW2M or some other non-magnetic material.

Remember that magnetic inspection can be a valuable, time-saving technique, but if used improperly it can produce erroneous identification of materials.

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