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Applications in Water Harvesting Systems

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Safety Relief Valve FAQs

Safety Relief Valve FAQs

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An Update on U.S. Valve-Related Standards

An Update on U.S. Valve-Related Standards

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Industry Headlines

Leading Economic Indicators for the U.S. Increased in July

Wednesday, 23 August 2017  |  Chris Guy

The Conference Board Leading Economic Index (LEI) for the U.S. increased 0.3% in July to 128.3 (2010 = 100), following a 0.6% increase in June, and a ...

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Low Cracking Pressure in Chemical Processing Condensate Lines

Low Cracking Pressure in Chemical Processing Condensate Lines

Tuesday, 22 August 2017  |  Arie Bregman

Steam condensate is the liquid by-product formed when steam goes from the vapor state to the liquid state; this process occurs in a wide range of appl...

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Industry Headlines

Velan Names Duc Tran Vice-President, Engineering

14 HOURS AGO

Velan Inc. has appointed Duc Tran as its new vice-president, engineering. Tran will be a key member of the company's senior management team and is succeeding Gil Perez, who has assumed a new mandate as vice-president, product technology and strategic initiatives. He will report directly to president...

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AIV and Gulf Coast Modification Opening NW Houston Facility

1 DAY AGO

AIV, LP has announced plans for a new 320,000+ square-foot facility at NW Lake Drive, near Telge Road and Highway 290 in northwest Houston. The 32-acre construction site allows for the future expansion of up to 700,000 sq. ft. This facility will consolidate AIV and Gulf Coast Modification ’s curr...

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Group Asking Feds to Halt LNG Exports

12 HOURS AGO

The Industrial Energy Consumers of America recently sent a letter to the Secretary of Energy on two LNG export demand scenarios. They argue that the Obama Administration’s “public interest” LNG export studies, which are still being used to justify LNG export approvals to non-free tra...

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Chemical Activity Barometer Shows Modest Slowing

1 DAY AGO

The Chemical Activity Barometer (CAB) from the American Chemistry Council remained unchanged from July, continuing a modest deceleration of growth. The flat reading follows a 0.1% increase in July and a flat reading in June. Compared to a year earlier, the CAB is up 3.2% year-over-year, an easing fr...

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Leading Economic Indicators for the U.S. Increased in July

9 HOURS AGO

The Conference Board Leading Economic Index (LEI) for the U.S. increased 0.3% in July to 128.3 (2010 = 100), following a 0.6% increase in June, and a 0.3% increase in May.

“The U.S. LEI improved in July, suggesting the U.S. economy may experience further improvements in economic activity in the...

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U.S. Factory Output Declined in July

5 DAYS AGO

Manufacturing output edged down 0.1% in July. The index for durables decreased 0.5%. Among durable manufacturing industries, with the indexes for primary metals and for furniture and related products each dropped more than 1%. The index for other manufacturing (publishing and logging) moved down 0.4%....

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Conversion of Hardness

materials_q_and_a_graphicQ: Are there any issues regarding conversion of hardness from one method or scale to another?

A: In one simple word, yes. Hardness is not a fundamental property of a material. In other words, it is not a property like density or elastic modulus. In the case of fundamental properties, conversion factors from one scale to another (such as from pounds per cubic inch to grams per cubic centimeter for density, or pounds per square inch to megapascals for tensile strength) involve simple unit conversion that can be as accurate as necessary depending on the number of significant digits used in the conversion factor.

The word “hardness” is usually used in reference to indentation hardness, which is the resistance of metal to plastic deformation by indentation. Indentation hardness may be measured by a number of different hardness test methods, including Brinell, Rockwell, Vickers, comparison and ultrasonic contact impedance (UCI) testers, as discussed in the previous column (Spring 2008, page 60). Indentation hardness is also sometimes determined by using a rebound hardness method (such as a Leeb tester) and converting the value to one of the indentation hardness scales.

Unfortunately, these test methods produce and measure the indentations in a variety of different manners. For example, Brinell testing involves using a very high load (usually 3000 kgf) to load a 1 cm tungsten carbide ball into the part, measuring the indentation and calculating the hardness based on an equation. Vickers testing is similar, except it indents the specimen with a square-based diamond pyramid using loads usually ranging from 1 gf to 30 kgf. Rockwell testing uses a round-based conical diamond indenter (A, C and N scales) or a spherical tungsten carbide indenter (B, F and T scales), and loads the material in two stages (minor and major loads). The differential penetration of the indenter between the minor and major loads is measured and used to determine the Rockwell hardness.

Indentation hardness readings are affected to various degrees by the fundamental properties of the material being tested, such as the elastic modulus, the yield strength and the work-hardening coefficient. Since the indentation methods are different, the various methods are measuring different combinations of these factors. This makes correlation of hardness readings taken with various methods difficult, even when only one material is involved.

This fact does not seem to be well-recognized in industry, but is known among hardness testing experts. For example, the following paragraph, extracted from ASTM E140-07 (emphasis added), provides strong indications that hardness conversion is not as straightforward as one would like to believe. Paragraphs 6.1 through 6.3 also include a number of cautionary statements regarding conversions.

1.12 Conversion of hardness values should be used only when it is impossible to test the material under the conditions specified, and when conversion is made it should be done with discretion and under controlled conditions. Each type of hardness test is subject to certain errors, but if precautions are carefully observed, the reliability of hardness readings made on instruments of the indentation type will be found comparable. Differences in sensitivity within the range of a given hardness scale (for example, Rockwell B) may be greater than between two different scales or types of instruments. The conversion values, whether from the tables or calculated from the equations, are only approximate and may be inaccurate for specific application.1

The following examples using the tables in ASTM E140 show that hardness conversion is a very risky business:

  • In Table 1 (Approximate Hardness Conversion Numbers for Non-Austenitic Steels [Rockwell C Hardness Range]), 248 Vickers is “equivalent” to 61.5 Rockwell “A”. In Table 2 (Approximate Hardness Conversion Numbers for Non-Austenitic Steels [Rockwell B Hardness Range]), Rockwell A 61.5 is “equivalent” to 240 Vickers. Which is correct?
  • In Table 2, 240 Brinell is equal to 240 Vickers, but in Table 1, 240 Brinell is equal to 251 Vickers (by interpolation). Which is correct?

The conversion issue becomes even more problematic for materials that are not covered by the standard conversion tables. Many people use ASTM E140 Tables 1 and 2 for hardness conversions for materials that are not covered in any of the tables in E140. For example, assume a specification (such as one of the NACE sour service standards) calls for a particular maximum Rockwell C hardness for a duplex stainless steel (such as 28 Rockwell C), and the hardness for the part is reported in Brinell (e.g., 286 Brinell). The existing ASTM E140 Table 1 for non-austenitic steels would indicate a conversion of 286 Brinell = 30 Rockwell C, which would cause rejection of the material. However, some private testing indicates that 286 Brinell actually converts to less than 28 HRC in at least one duplex stainless-steel material. Unfortunately, verified and standardized tables of conversion values for duplex stainless steels do not exist. This results in false rejection of materials, leading to increased costs and equipment delivery delays.

In summary, hardness conversion is a very complex subject. Conversion of readings from one scale to another or one method to another should be performed only when absolutely necessary, and with great care and consideration. Furthermore, hardness requirements for materials should be specified using methods and scales that are most appropriate for the material (e.g., Brinell for large castings instead of Rockwell B or C). This approach eliminates the need for conversion and the issues that can result.


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 of Flowserve Corporation (www.flowserve.com).


References

 

1. ASTM E140-07 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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