What Is Hardness Testing?
Hardness testing measures a material\u0027s resistance to permanent indentation under a controlled load. For piping materials, hardness verifies that base metal, weld metal, and heat-affected zones (HAZ) meet specification limits—particularly the maximum hardness requirements of NACE MR0175/ISO 15156 for sour service. The three primary methods are Brinell (HBW), Rockwell (HRC/HRB), and Vickers (HV), each using a different indenter and load combination.
Hardness Test Methods
| Method | Standard | Indenter | Load | Reading | Best For |
|---|---|---|---|---|---|
| Brinell (HBW) | ASTM E10 | 10 mm tungsten carbide ball | 3,000 kgf (steel) | Diameter of impression | Base metal (pipes, fittings, flanges, plates) |
| Rockwell C (HRC) | ASTM E18 | Diamond cone (120-degree) | 150 kgf | Depth of penetration | Hardened materials, valve trim, bolting |
| Rockwell B (HRB) | ASTM E18 | 1/16 in. steel ball | 100 kgf | Depth of penetration | Softer metals (aluminum, copper, low-carbon steel) |
| Vickers (HV) | ASTM E92 | Diamond pyramid (136-degree) | 1-120 kgf | Diagonal of impression | Welds, HAZ, thin sections, micro-hardness surveys |
| Portable/Leeb | ASTM A956 | Impact body (tungsten carbide tip) | Dynamic impact | Rebound velocity ratio | Field hardness on installed piping, in-service inspection |
Hardness Conversion (Approximate)
| HBW | HRC | HV | Application Reference |
|---|---|---|---|
| 121 | - | 128 | Typical ASTM A105 forging (low end) |
| 187 | - | 197 | Typical A106 Gr. B pipe |
| 200 | - | 210 | ASTM A350 LF2 impact-tested forging |
| 235 | 22 | 248 | Maximum for NACE MR0175 (carbon steel, Zone 1) |
| 248 | 24 | 261 | Typical A333 Gr. 6 low-temperature pipe |
| 321 | 34 | 340 | A193 B7 stud bolt (quenched + tempered) |
Conversions per ASTM E140. Exact equivalence depends on material; these are approximations for carbon and low-alloy steels.
NACE MR0175 Hardness Limits
NACE MR0175/ISO 15156 sets maximum hardness values for materials exposed to sour (H2S-containing) environments to prevent sulfide stress cracking (SSC):
| Material | Maximum Hardness | Test Location |
|---|---|---|
| Carbon steel (ASTM A106, A333, A516) | 22 HRC / 248 HV / 235 HBW | Base metal, weld, HAZ |
| Low-alloy steel (A335 P11, P22) | 22 HRC (P11), 22 HRC (P22) | Base metal, weld, HAZ |
| Austenitic stainless steel (304/316) | No limit (SSC resistant) | Not required unless cold worked >%strain limit |
| Duplex stainless steel (UNS S31803) | 28 HRC / 310 HV | Base metal, weld, HAZ |
| Nickel alloy 625 | 35 HRC | Base metal |
Exceeding these limits in any zone (base metal, weld deposit, or HAZ) results in rejection. Post-weld heat treatment (PWHT) is the standard remedy to reduce weld/HAZ hardness below the NACE limit.
Hardness Survey Locations
For production weld qualification and piping fabrication, hardness surveys follow a defined pattern across the weld cross-section:
| Location | Number of Indents | Purpose |
|---|---|---|
| Base metal (each side) | 2-3 per side | Confirm base material compliance |
| Weld metal (center) | 2-3 across weld deposit | Verify filler metal hardness |
| HAZ (each side) | 2-3 per side (within 2 mm of fusion line) | Detect hard zones from welding thermal cycle |
| Root, fill, cap | Traverse at each level for thick welds | Full through-thickness profile |
Key Standards
| Standard | Scope |
|---|---|
| ASTM E10 | Brinell hardness testing |
| ASTM E18 | Rockwell hardness testing |
| ASTM E92 | Vickers hardness testing |
| ASTM E384 | Microhardness testing (Knoop and Vickers) |
| ASTM A956 | Portable (Leeb) hardness testing |
| ASTM E140 | Hardness conversion tables |
| NACE MR0175/ISO 15156 | Maximum hardness for sour service |
| ASME B31.3 | Hardness requirements for process piping welds |
Hardness results are recorded on the weld procedure qualification record (PQR) and production weld test reports, filed alongside mill test certificates and pipe inspection documentation.
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