It is sometimes necessary to meet hardness limits for duplex and super duplex stainless steels, and it is convenient to be able to convert between the Rockwell C (HRC) and Vickers (HV) hardness scales. A conversion exists for carbon steel in ASTM E140 but there is no standard conversion for duplex and super duplex grades and the carbon steel conversion has been shown to be inappropriate for these high alloy steels. A correlation between the two hardness scales for duplex and super duplex grades has been developed at TWI from a compilation of data supplied by various organizations. There was considerable scatter in the data, indicating that the correlation between the two scales was not well defined, but the best-fit line was:

HRC = 0.091HV – 2.4

The 28HRC limit previously given in MR 0175-99 (now superseded) for wrought UNS S31803 austenitic/ferritic stainless steel is calculated to be equivalent to 334HV using this equation and the previous 32HRC limit for UNS S32750 steel (25%Cr super duplex) becomes 378HV.

Reference:

NACE MR 0175/ISO 15156-3 – Petroleum and natural gas industries – Materials for use in H ₂ S – ‘containing environments in oil and gas production, Part 3: Cracking-resistant CRAs (corrosion-resistant alloys) and other alloys’.

ASTM E140-97 – ‘Standard hardness conversion tables for metals’

Key notes:

There is no direct, universally used correlation between the different hardness scales. However, a few national standards do list hardness conversion tables, and related equations. These have arisen from many hardness readings, the values have been plotted and a conversion established. Two such standards are BS EN ISO 18265:2013^[1], and ASTM E140: 2012^[2]. The conversions for these standards are not exactly the same, but are similar, incorporating Vickers, Brinell, Rockwell B, and Rockwell C hardness scales, and conversions for a limited number of material types. Some other hardness scales are included in the standards, but these are used less often.

A useful conversion used extensively in applications involving H₂S is that 22 HRC (Rockwell C) is equivalent to 248 HV (Vickers), which is often rounded to 250 HV.

An earlier standard, BS 860:1967^[3], which has been superseded by BS EN ISO 18265:2003 gives the following equation for hardness conversion between Brinell and Vickers scales.

HB = 0.95 HV

As with all conversions, it must be appreciated that the conversion between hardness scales is approximate, not absolute.

References

1. BS EN ISO 18265:2013’Metallic materials – Conversion of hardness values’. BSI, London.
2. ASTM E140: 2012’Standard hardness conversion tables for metals relationship among Brinell hardness, Vickers hardness, Rockwell hardness, Superficial hardness, Knoop hardness Scleroscope hardness and Leeb hardness’. ASTM International, West Conshohocken, PA, www.astm.org.
3. BS 860:1967’Tables for comparison of hardness scales’ BSI, London.