What Is Hardness?

Hardness is a material’s resistance to localized permanent deformation; specifically, resistance to indentation by a harder object. In piping and pressure equipment, hardness testing serves two critical purposes: verifying material conformance to ASTM specifications and confirming compliance with NACE MR0175 maximum hardness limits for sour (H2S) service.

Three Main Hardness Test Methods

Feature	Brinell (HB/HBW)	Rockwell C (HRC)	Vickers (HV)
Standard	ASTM E10	ASTM E18	ASTM E92 / E384
Indenter	10 mm tungsten carbide ball	120° diamond cone	Diamond pyramid (136°)
Load	3,000 kgf (for steel)	150 kgf	1-120 kgf (variable)
Measurement	Diameter of indentation	Depth of indentation	Diagonal of indentation
Range (steel)	80-450 HBW	20-70 HRC	80-900+ HV
Best for	Bulk hardness of castings, forgings, pipes	Hard materials (>20 HRC): heat-treated, hardfacing	All materials; weld HAZ; thin sections
Surface prep	Ground flat, smooth surface	Ground flat, smooth surface	Polished surface (micro-hardness)
Portability	Lab (bench) or portable	Lab or portable	Lab only (standard); portable available

Hardness Conversion Table (Approximate)

Conversions between scales are approximate and vary by material. The following table is based on ASTM E140 for carbon and alloy steels:

Brinell (HBW)	Rockwell C (HRC)	Vickers (HV)	Approx. UTS (MPa)
120	-	120	410
150	-	150	515
180	-	180	615
200	(16)	200	690
220	20	220	760
250	25	250	860
280	29	280	965
300	32	300	1,035
320	34	320	1,105
350	37	350	1,205
400	43	400	-

Below 20 HRC, the Rockwell C scale is unreliable. For softer materials, Rockwell B (HRB, 1/16” ball, 100 kgf) is used instead. Most carbon steel piping falls in the HRB range (typically 75-95 HRB).

NACE MR0175 Hardness Limits

For sour service (H2S-containing environments), NACE MR0175/ISO 15156 imposes maximum hardness limits to prevent sulfide stress cracking (SSC):

Material	Maximum Hardness
Carbon steel (A106 Gr. B, A105)	22 HRC (237 HBW)
Low-alloy steel (P11, P22)	22 HRC (237 HBW)
Austenitic SS (304L, 316L); base metal	No HRC limit (generally exempt from SSC)
Duplex 2205 (S31803)	28 HRC (280 HBW)
Super Duplex 2507 (S32750)	32 HRC (310 HBW)
Weld and HAZ (carbon/low-alloy)	22 HRC (237 HBW); must be tested

Pro Tip

For sour service, hardness testing must be performed on the production weld and heat-affected zone, not just the base metal. The HAZ of carbon steel welds often exceeds 22 HRC without proper PWHT. NACE requires hardness surveys per NACE SP0472 (formerly RP0472) on production welds.

When Each Method Is Used in Piping

Application	Preferred Method
Mill test certificate (base metal)	Brinell (HBW); reported per ASTM product specs
Weld procedure qualification (WPQR)	Vickers HV10 traverse across weld, HAZ, and base metal
Production weld hardness	Portable Brinell (Equotip/Leeb) or portable Vickers
Valve trim and hardfacing	Rockwell C (HRC); Stellite overlays typically 38-50 HRC
NACE sour service verification	Vickers HV10 or Rockwell C, per NACE SP0472

Hardness testing is a non-destructive or semi-destructive test (small indentation) that provides a quick indication of material strength, heat treatment condition, and sour service suitability. It does not replace tensile testing for material acceptance but is a standard field verification tool during pipe inspections.

Read the full guide to steel corrosion