When you perform a Brinell hardness test with a well maintained, calibrated testing device, you can learn:

  • The charted Brinell hardness number (symbolized by HBW, per ASTM E10)1 specified for the size of impression made by the Indenter
  • The Tensile Strength of the material tested, in the location tested
  • That the Brinell Hardness falls within or outside of specified limits

You do not learn:

  • The Chemistry of the material tested
  • The Thermal History of the material tested
  • The Yield Strength of the material tested
  • The suitability of the material tested for any particular Service

Often inspectors engaged by operators use the results of Brinell Hardness tests on existing equipment to satisfy questions of compliance with API and NACE Standards requirements. However, the results of these hardness tests prove inconclusive without the original documented thermal history of the material and a documented history of any secondary or repair thermal processing the equipment may have experienced. The steels used to make oil tools for pressure control can have measured hardness in compliance with both API and NACE requirements, but not meet any other API requirements for chemistry or mechanical properties.

API Spec 6A specifies connectors, in 45K, 60K, AND 75K Yield Strength material. Among other requirements API specifies minimum Brinell Hardness for these materials. Commonly, manufacturers use commercial steels with chemistry acceptable to NACE to make flanges or hubbed equipment and connectors. NACE specifies maximum hardness requirements for these materials for Hydrogen Sulfide Service (H2S). MR0175 specifies that the hardness shall not exceed 22 Rockwell C (22 RC), equivalent to 237 Brinell Hardness (237 HBW) per ASTM E140, see table below.

API Minimum
NACE Maximum
6B Weld Neck Flanges45K140237
Integral Flanges and Hubs, and 6BX Weld Neck Flanges.60K*174*237
*75K material can substitute for 60K material.

API Spec 16A for Drill Through Equipment specifies the same yield strength material as API 6A, these specifications allow manufacturers to choose among 4 categories of material strength for bodies and bonnets based on calculated stress levels within the assembled equipment at test pressure. In addition to the material strengths shown in the table above for connectors, manufacturers may use 36K Yield Strength Material (minimum Brinell Hardness 140 HBW) for bodies when stress levels allow. The manufacturer may join one strength of material to another by welding as determined by design requirements.

Material that requires heat treating may receive heat treating in one of two configurations;

  1. In “bulk form”, as a large piece of bar stock or a large ring or barrel shaped forging.
  2. In “near net shape” as any material shape machined or forged to dimensions that leaves only a small amount of material for finish machine removal.

A Brinell hardness taken in selected areas, where an operator believes significant stock removal may have occurred, may reveal high stress areas with less than the required hardness and other mechanical properties.

Operators should reference the original documentation of that equipment or consult with the equipment manufacturer when questions arise concerning justification of acceptance of the Brinell Hardness of a body that falls below that required for the lowest pressure end or outlet connector integral to that body. 

Caution: Hardness tests taken on equipment without known process history do not confirm anything but hardness and approximate tensile strength. Additionally, Brinell hardness tests cannot confirm NACE or API requirements for welding or welding consumables. The extremely small width of welding Heat Affected Zones (HAZ) makes Brinell Hardness testing of these areas difficult and probably meaningless.

Some operators request a Brinell Hardness test on the base material HAZ within 1/2″ or 1″ of the weld. In this case the operator redefines “heat affected zone” to mean: That material which may have lost some of its properties as a result of tempering due to poor control of heat input during a welding process. This method has some value, as acceptable Brinell Hardness in these areas does confirm that welding heat inputs did not reduce the properties of the base metal near the weld.

Brinell Hardness has no bearing on material suitability for “low temperature” service.

Conclusion: operators can reasonably reject equipment not complying with specified hardness requirements, but acceptance on hardness alone contains high risk. Performing a Hydrotest at Test Pressure on equipment with acceptable hardness numbers should give reasonable comfort 2 to operators using equipment on “Sweet” wells. Currently no non-destructive method exists to confirm the acceptability of welded equipment to NACE “Sour” well requirements without the manufacturers’ original documentation and a documented history of any later repairs that may have included welding.

To learn more about material requirements for API pressure control equipment. see Material Requirements for End and Outlet Connectors, Bodies and Bonnets.

For a chart of Brinell hardness impression sizes, their respective Brinell hardness numbers, the approximate tensile strength corresponding to those numbers, and conversion between Brinell and Rockwell hardness numbers, see Hardness Number Conversion Chart.

1API Spec 6A now abbreviates Brinell Hardness Number with HBW to designate the test device utilized a tungsten carbide indenter.
2Words in bold italic indicate they have subjective meaning and persons using this information must use experience to improve the reliability of their judgment when the meaning of these words can have impact on performance.

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