Understanding Hard to Interpret Field Hardness Values for Common Pressure Vessel and Piping Steels

Introduction

Often when reviewing field hardness values, if the results are not consistent with expectations, the values measured are dismissed. However, field hardness tests can provide extremely valuable information when evaluating piping and pressure vessel anomalies. In the specific failures and equipment examinations discussed in this article, field hardness tests gave the most insightful information for assessing challenging material anomalies compared to other laboratory and field tests.

This article summarizes laboratory examinations designed to gain a better understanding of Ultrasonic Contact Impedance (UCI) hardness test results taken in the field. Moreover, examples of cases where anomalous hardness values of new steel parts were associated with metallurgical and physical irregularities are shown. In all the cases, the UCI values measured were relevant and accurate.

To understand the cases, first, laboratory examinations performed to compare field UCI and bench model hardness tests are presented. Furthermore, the standard surface finish specifications are summarized.

Comparing Field UCI and Bench Model Hardness Tests

Bench model 10 kg HV indenter hardness tester indentations were compared to those made by a Krautkramer MIC 10 Ultrasonic Contact Impedance (UCI) tester. The hardness tests/indentations were made on a 142 HV and a 302 HV calibration block. The calibration blocks had a mirror grit finish (finer than #400 grit).

The MIC 10 UCI indentations were made with the field hardness indenter positioned at various angles with respect to the calibration block. The indentations were imaged with a KEYENCE 3D microscope. Despite the various indentation angles, the indentations had similar diameters and depths to those obtained by the bench model hardness tester for the same calibration block. See Figures 1 to 8. A key finding through these examinations was that the UCI field hardness indentation depth for 142 HV steel is approximately 20 μm.