Q: 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, 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.
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.
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