Technical information

Hardness testing on thermowells to prove their
NACE hardness values
WIKA data sheet IN 00.43

General information
Special material requirements for sour gas applications
are widely used in the oil and gas and also petrochemicalindustries. These include, among others, a hardness
limitation for the components used.
The hardness values of the primary material used aregenerally confirmed by the 3.1 acceptance test certificates inaccordance with EN 10204.
For more detailed information, see Technical information
IN 00.21.
Thermowells in different versions

Description
The proof of the maximum hardness values in Rockwell C(HRC), required by NACE MR 0175 (ISO 15156-3) andMR 0103, is not trivial, since thermowells are mainly
manufactured from stainless steels such as 316L or special
materials such as Alloy 400. Thermowells, due to theirgeometry, often cannot be tested on a stationary Rockwell
hardness tester, so portable hardness testing machines mustthen be used.
These work through the UCI method (Ultrasonic ContactImpedance). The test impression of a Vickers diamond ismeasured electronically and the values are output directly inHV (Vickers) or converted to Brinell or Rockwell.

Stationary Rockwell hardness tester (example)
 ATP Messtechnik + Waagen

Generally, several measurements are carried out per test andthe average value taken.

Portable hardness tester
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Falsification of the measured values through work-hardening
Illustration of the surface layer principle

The machining of the thermowells results in the effect ofwork-hardening the surface layer of the material.

Thermowell material with low
hardness of the base material

Depending on the test methods and test force used, therespective methods measure at different penetration depths.
Thus they deliver different hardness values.

work-hardened
surface layer
Portablehardness tester■■ Many portable hardness testers operate with low test

Stationary HRC
hardness tester

loads. These do not penetrate through the work-hardened
surface layer and do not deliver a hardness value that canbe compared with Rockwell.

■■ A stationary Rockwell hardness test rig, due to higher

test loads, penetrates through the work-hardened surface
layer and thus measures the hardness of the softer basematerial.

In cooperation with TÜV Hessen, WIKA has carried out numerous test series to obtain hardness comparison measurements
between the Rockwell, Brinell and Vickers test methods with different test loads on a special stepped-profile test specimen
with work-hardened surface layer.
The measured values have shown that it is not possible to compare the hardness readings of a portable measuring instrument(Brinell or Vickers) with those of a stationary Rockwell hardness tester. While the hardness values from the portable measuring
instrument increase with rising cutting depth, the Rockwell hardness values of the stationary measuring device are almost
constant.
Schematic representation of the influence of the cutting depth on the measured hardness values

Cutting depth

HBHRC

HRC

HB

HVHRC

HB

HVHRC

HB

HV

22 HRC

HVHardness

The maximum permissible hardness values are material-dependent. These correspond to the application of the respectively
applicable regulation e.g. NACE MR 01/75 or NACE MR 01/03.

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Hardness testing for thermowells with welded flanges
The surface of a welded flange thermowell consists eitherof work-hardened or solution-annealed zones. Thus, thebase material, through the manufacturing process, hasbeen brought to a state in which, in accordance with thestrictest specifications, a hardness test in accordance with
NACE is no longer possible. WIKA has therefore decided
to ensure NACE conformity through the sub-suppliercertificate and, optionally, through a hardness measurementin accordance with this technical information in theheat-affected zone (HAZ) next to the weld seam.

Testing range, flanged thermowell
Test surface,thermowell

Test surface,weld seam

12 mm (½")

Test surface,flange

12 mm(½")

In addition, this will prove that there has been no hardening ofthe materials due to the welding.
The proof of suitability in accordance with NACE MR 0175
(ISO 15156-3) and MR 0103 is provided through themeasurement in the HAZ.

Hardness testing for screw-in or weld-in thermowells
The hardness testing is carried out in an area of thethermowell with the lowest possible cutting depth.

Hardness testing on a screw-in thermowell with aportable hardness tester

A reliable statement about the core hardness of the material
cannot be made by a test with a portable tester.
However, the use of stationary test equipment is also
problematic here, since, due to the very high test loads, asecure support surface for the thermowell is rarely available.

10/2018 EN© 10/2018 WIKA Alexander Wiegand SE & Co. KG, all rights reserved.
The specifications given in this document represent the state of engineering at the time of publishing.
We reserve the right to make modifications to the specifications and materials.

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WIKA Alexander Wiegand SE & Co. KG
Alexander-Wiegand-Straße 30
63911 Klingenberg/GermanyTel.
+49 9372 132-0
Fax+49 9372 132-406
info@wika.de
www.wika.de