A hardness test certificate records the measured hardness of a material or weld using one of the standardized indentation scales. Hardness is a proxy for tensile strength, wear resistance, and susceptibility to stress corrosion cracking — making it a key quality control parameter in fabrication, heat treatment verification, and sour service qualification.
Quick Answer
Quick Answer
A hardness test certificate reports hardness values measured using Brinell (HB/HBW), Vickers (HV), or Rockwell (HRC/HRB) scales on a specific material lot or weld. The certificate must identify the test scale, applied load, indenter type, individual readings, specimen location, and the applicable acceptance criterion. NACE MR0175 and ASME specifications define maximum hardness limits for sour and pressure service applications.
The Three Principal Hardness Scales
Brinell Hardness (HB / HBW)
Principle: A tungsten carbide ball (10 mm diameter for standard test) is pressed into the surface under a defined load (typically 3000 kgf for steel). The diameter of the indentation is measured optically and the HBW value is calculated from the load and indentation area.
Load options: 500 kgf (HBW 10/500), 1500 kgf (HBW 10/1500), 3000 kgf (HBW 10/3000)
Governing standards: ASTM E10, EN ISO 6506
Typical range: 80–650 HBW. Materials above ~450 HBW require a smaller ball or lower load.
When used:
- Mill product hardness reporting (plates, forgings, bars)
- Rough surfaces or coarse microstructures where averaging over a large indent is desirable
- Correlation with tensile strength (UTS ≈ 3.45 × HBW in MPa for steel)
Certificate field: State the full scale designation (HBW 10/3000), not just "Brinell." Report the indentation diameter(s) in addition to the calculated HBW value.
Vickers Hardness (HV)
Principle: A diamond pyramid indenter (136° face angle) is pressed into the surface under a defined load. The diagonal lengths of the square indentation are measured and the HV value is calculated.
Load range: 1 gf to 100 kgf. Micro-Vickers (HV 0.01–1.0 kgf) is used for thin coatings and individual microstructural constituents.
Governing standards: ASTM E92, EN ISO 6507
Typical range: 5–3000 HV (virtually unlimited upper range compared to Brinell)
When used:
- PWHT hardness surveys (most preferred method in ASME and EN standards)
- Weld cross-section traverses (HAZ, weld metal, base metal profiling)
- Case depth measurement on surface-hardened components
- Thin materials, coatings, and thin-walled tubing
Certificate requirement for PWHT surveys: Report individual HV readings at each location across the weld traverse (typically base metal → HAZ → weld metal → HAZ → base metal at minimum), not just the maximum value.
Rockwell Hardness (HRC / HRB)
Principle: A diamond cone (HRC) or hardened ball (HRB) is pressed in two stages; the hardness is determined from the increment of permanent depth of penetration.
Common scales:
- HRC (C scale): Diamond Brale indenter, 150 kgf total load. Range: 20–70 HRC. Used for hard steels and heat-treated components.
- HRB (B scale): 1/16" ball, 100 kgf. Range: 25–100 HRB. Used for soft steels, aluminum, brass.
- HRA: Diamond, 60 kgf. Used for cemented carbides.
Governing standards: ASTM E18, EN ISO 6508
When used:
- Incoming inspection of bar stock and forgings
- Hardened fastener verification (ISO 898 property class)
- Heat-treated component acceptance (e.g., hardened valve trim)
- Verification that hardness is below an upper limit (e.g., < 22 HRC per NACE MR0175)
Certificate note: Always state the full scale (HRC, HRB, etc.) — "Rockwell hardness: 20" is meaningless without the scale identifier.
Hardness Conversion Table
Approximate conversions (carbon steel only — not valid for stainless, non-ferrous, or case-hardened materials):
| HBW | HV | HRC | HRB | UTS approx. (MPa) |
|---|---|---|---|---|
| 85 | 87 | — | 43 | 290 |
| 120 | 124 | — | 70 | 415 |
| 180 | 185 | — | 88 | 620 |
| 200 | 207 | 14 | 93 | 690 |
| 250 | 263 | 24 | — | 860 |
| 300 | 319 | 31 | — | 1035 |
| 350 | 374 | 37 | — | 1210 |
| 400 | 423 | 42 | — | 1380 |
| 450 | 475 | 46 | — | 1555 |
Reference: ASTM E140. Do not apply conversions across metal families.
PWHT Hardness Limits
Post-weld heat treatment hardness verification is one of the most common applications for a hardness test certificate in pressure equipment fabrication.
ASME BPVC requirements (ASME VIII-1 UHA-51 for P4/P5 materials):
- After PWHT: maximum 225 HBW (or per the material specification)
- Survey method: Vickers preferred; Brinell or Rockwell acceptable
NACE MR0175 / ISO 15156 (sour service):
- Carbon and low-alloy steel base metal and weld HAZ: ≤ 22 HRC (equivalent to ~248 HBW / 265 HV)
- Exception: some grades allow higher hardness with documented SSC (sulfide stress cracking) resistance tests
EN 13445-4 (unfired pressure vessels, European):
- Maximum HV10 values specified by material group; typically 350 HV for carbon steel HAZ, 200 HV for post-PWHT
Required Fields on a Hardness Test Certificate
- Material / component identification — heat number, drawing number, weld ID
- Test date and location
- Test method and scale — HBW 10/3000, HV10, HRC, etc.
- Equipment identification — hardness tester make, model, serial, calibration date
- Reference block calibration — hardness value verified on certified reference block before and after testing
- Test locations — sketch or description showing where each reading was taken
- Individual readings — every measurement, not just average
- Average value (if acceptance is based on average)
- Acceptance criterion — specification reference and maximum/minimum limit
- Result — Pass / Fail
- Technician name, qualification, signature
Which hardness scale does NACE MR0175 use for sour service limits?
NACE MR0175 / ISO 15156 expresses hardness limits primarily in HRC (Rockwell C scale). The maximum is 22 HRC for carbon and low-alloy steels in H₂S environments. Equivalent limits in other scales are approximately 248 HBW or 265 HV. When converting, always verify using ASTM E140 conversion tables rather than rule-of-thumb approximations.
Can hardness results be used to estimate tensile strength?
Yes, approximately. For carbon steels, UTS (MPa) ≈ 3.45 × HBW. However, this correlation is less reliable for alloy steels, stainless steels, and non-ferrous materials. ASTM E140 and ASME Section II provide correlation tables with stated uncertainty ranges. Hardness-derived tensile estimates are acceptable for screening but not as a substitute for tensile testing in code construction.
What is the difference between a Vickers macro-hardness test and a micro-hardness test?
Macro-hardness (HV1 and above, with loads ≥ 1 kgf) measures the bulk hardness of a material or weld zone. Micro-hardness (HV 0.01–0.5 kgf) is used to measure the hardness of individual microstructural features — specific phases, precipitates, or very thin coatings — where a large indent would average across multiple constituents. Weld hardness surveys typically use HV5 or HV10 as a standard macro-hardness method.
How many hardness readings are required for a PWHT survey?
A minimum of three readings per zone (base metal, HAZ, weld metal) on each face of the weld cross-section is common practice. Specific codes define more detailed requirements: ASME VIII-1 UHA-51 requires hardness readings on production welds in P4 and P5 base metals; BS PD 5500 Annex D specifies minimum traverse patterns. The certificate must show all individual readings, not just the maximum.
Does annealing or PWHT always reduce hardness?
PWHT (stress relief) reduces residual stresses and tempers hard microstructures in carbon and low-alloy steels, typically reducing HAZ hardness significantly. Full annealing (heating to above the upper critical temperature and slow cooling) also reduces hardness. However, solution annealing of austenitic stainless steel does not reduce hardness in the same way — it dissolves precipitates and homogenizes the microstructure, which may slightly increase hardness in heavily cold-worked material.
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