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Last updateFri, 16 Feb 2018 8pm

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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 ...

VMA Chair Mark Nahorski: Industry Has Many Pathways to Success

VMA Chair Mark Nahorski: Industry Has Many Pathways to Success

Part of the discussion today about what ...

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Industry Headlines

ValvTechnologies ITC 2018 Recap

Friday, 16 February 2018  |  Chris Guy

The biennial ValvTechnologies’ International Technical Conference and Users Group (ITC) was held on February 8-9 in Houston. Seeking to educate,...

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Putting Servo Valves Back to Work

Putting Servo Valves Back to Work

Monday, 12 February 2018  |  Christopher Valiquette

Industries as varied as paper production, steelmaking and power generation rely on servo valves for precise motion control. A servo valve can last wel...

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Industry Headlines

ValvTechnologies ITC 2018 Recap

20 HOURS AGO

The biennial ValvTechnologies’ International Technical Conference and Users Group (ITC) was held on February 8-9 in Houston. Seeking to educate, inspire and engage attendees over a two-day period, the event brought together hundreds of attendees from across the globe to share valuable insight ...

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Matthew Davis Named Field Service Manager at Weir

2 DAYS AGO

Weir Valves & Controls USA (WVC USA) has appointed Matthew Davis to the position of field service manager. In his new role, Davis will be responsible for managing the WVC USA Field Service crew and organizing WVC Service jobs for power plants worldwide.

Davis joins WVC USA with over 20 years of ex...

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TransCanada Turns Attention to Natural Gas System

1 DAY AGO

TransCanada Corp. will move forward with a $1.9 billion expansion of its NGTL System to connect incremental supply and expand basin export capacity by one billion cubic feet of natural gas per day at the interconnection with its Canadian Mainline. NGTL expects to begin construction in 2019.

The increme...

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U.S. Oil Output Surge Reminiscent of Run-Up to 2014 Dive

3 DAYS AGO

According to the International Energy Agency (IEA), fast rising production in non-OPEC countries, led by the U.S., is likely to grow by more than demand in 2018. For now, the upward momentum that drove the price of Brent crude oil to $70/bbl has stalled; partly due to investors taking profits, but a...

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U.S. Industrial Production Fell 0.1% in January

1 DAY AGO

Industrial production edged down 0.1% in January following four consecutive monthly increases. At 107.2% of its 2012 average, total industrial production was 3.7% higher in January than it was a year earlier.

Manufacturing output was unchanged in January for a second consecutive month; the index has i...

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U.S Business Inventories Rose 0.4% in December

1 DAY AGO

Manufacturers’ and trade inventories, adjusted for seasonal variations but not for price changes, were estimated at an end-of-month level of $1,902.2 billion, up 0.4% from November 2017 and were up 3.2% from December 2016.

A poll of economists “had forecast inventories, a key component of...

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Portable Hardness Testers and NACE Sour Service Standards

materials_q_and_a_graphicQ: Are portable hardness testers adequate for verifying the hardness of parts for compliance to industry standards such as the NACE sour service standards?

A: There are a number of portable hardness testing devices on the market. These devices are attractive to manufacturers and repair companies for several reasons, such as:

  • Portability: The units can be easily used throughout the shop or in the field for hardness testing of components that cannot be easily brought to a bench-top hardness tester.
  • Large components: They can be used on components that are too large to test with a bench-top tester.
  • Cost: The devices typically cost less than a bench-top tester.

There are several popular portable devices on the market. Three common devices are described below:

Comparison Hardness Testers

Comparison hardness testers, covered by ASTM A833, feature a device that incorporates a Brinell ball and a square cross-section bar of known Brinell hardness.

A hammer is used to strike the device against the part to be measured. Indentations are produced in the part and in the known bar. An equation or slide-rule device is used to determine the Brinell hardness of the part based on the indentation diameters in the known bar and the specimen and hardness of the known bar.

Drawbacks of Comparison Testers: Comparison hardness testing requires the use of calibrated, consumable bars of known hardness (i.e., the bar must be replaced after a certain number of readings have been made). Indentations should be 3-4 mm in diameter (although ASTM A833 only specifies that the indentations not exceed 4.2 mm), and such large indentations may not be acceptable on many parts, especially smaller ones. Finally, the resulting Brinell hardness value must often be converted to another scale, likely Rockwell B or C, which can introduce uncertainty in some situations. However, since ASTM E140 includes some conversion tables that include Brinell and Rockwell values, there is at least a standardized basis for those conversions for some materials.

Leeb Testers

Leeb testers, covered by ASTM A956, bounce an indenter off of a part and use the difference in the inbound and outbound velocities to determine the hardness. The value measured is a Leeb hardness number. Generally, this Leeb hardness number is then converted to some other scale (such as Rockwell or Brinell) using conversion charts to determine if the material meets some specified requirement.

Drawbacks of Leeb Testers: Leeb testing requires that the part have a certain minimum weight, ranging from 1.5 kg (4 lbs) to 15 kg (40 lbs) depending on the device type and the thickness of the part. There are minimum surface finish requirements, ranging from 0.4 µm (16 µin) Ra (corresponding to 500 grit emery paper) to 7 µm (250 µin) (corresponding to 65 grit emery paper) depending upon the impact device. The hardness measured is superficial, and therefore may not be representative of the bulk hardness. Finally, since virtually no specifications reference Leeb hardness requirements, the resulting hardness values must be converted to another scale. ASTM E140 includes no hardness conversion tables that include Leeb values, so one must rely on tables provided by the vendor for conversion, which introduces more uncertainty than when using tables published in ASTM E140.

Ultrasonic Contact Impedance (UCI) Testers

Ultrasonic contact impedance testers, covered by ASTM A1038, utilize an ultrasonically-excited, spring-loaded Vickers diamond indenter and a device to measure the frequency shift that occurs when the indenter penetrates the specimen. The change in frequency is used to determine the size of the indentation, which is then used to determine the Vickers hardness.Drawbacks of UCI Testers: UCI testing requires the part have a minimum weight of at least 300 g (0.7 lbs). There are restrictions regarding minimum wall thickness and part stiffness to ensure the part itself is not excited by the ultrasonic vibration. There are minimum surface finish requirements that are dependent upon the test load and the hardness of the specimen. The accuracy of the measurement is dependent upon the surface finish. For example, calibration blocks are required to be finished to 0.4 µm (16 µin) or better for accuracy. The hardness measured is superficial, and may not be representative of the bulk hardness. Since most specifications do not list Vickers hardness requirements, the resulting Vickers hardness values must often be converted to another scale. ASTM E140 includes some hardness conversion tables that include Vickers values, but conversion tables do not exist for all materials and hardness ranges.

Summary

Hopefully, this information is not taken to indicate these testers should never be used. All of these devices have their place in production and field applications, as well as in laboratory environments. However, like all hardness testers, they need to be used with careful consideration of their capabilities and limitations. They really cannot be used to replace all of the bench-top hardness test devices. The main reason for this is that hardness conversion to the scales called out in specifications is not a simple and straightforward matter. A future column will cover the topic of hardness conversion issues in more detail.


Don Bush is a Principal Materials Engineer at Emerson Process Management-Fisher Valve Division (www.emersonprocess.com). Reach him at This email address is being protected from spambots. You need JavaScript enabled to view it.. The author wishes to acknowledge the assistance of Thomas Spence, Director of Materials Engineering, Flowserve Corporation (www.flowserve.com).

REFERENCES:

1. ASTM A833 Standard Practice for Indentation Hardness of Metallic Materials by Comparison Hardness, ASTM International, West Conshohocken, PA

2. ASTM A956 Standard Test Method for Leeb Hardness Testing of Steel Products, ASTM International, West Conshohocken, PA

3. ASTM A1038 Standard Practice for Portable Hardness Testing by the Ultrasonic Contact Impedance Method, ASTM International, West Conshohocken, PA

4. ASTM E140 Standard Hardness Conversion Tables for Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness, Superficial Hardness, Knoop Hardness, and Scleroscope Hardness, ASTM International, West Conshohocken, PA

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