How does steel hardness correlate with tensile strength?

Steel hardness and tensile strength have a strong empirical correlation because both measure resistance to deformation. For carbon and low-alloy steels, the approximate relationship is: Fu (ksi) ≈ 0.5 × HB (Brinell Hardness Number), or more precisely Fu (MPa) ≈ 3.45 × HB. This holds within ±10% for Brinell values up to 400 HB. Examples: HB 100 → Fu ≈ 50 ksi (A36 range), HB 200 → Fu ≈ 100 ksi (A514 range), HB 300 → Fu ≈ 150 ksi (high-strength Q&T steel), HB 400 → Fu ≈ 200 ksi. The ASTM A370 standard provides a recognized hardness-tensile strength conversion table. For Rockwell C: HRC 20 → Fu ≈ 105-115 ksi, HRC 25 → 120-130 ksi, HRC 30 → 140-150 ksi, HRC 35 → 165-175 ksi, HRC 40 → 190-200 ksi. The correlation breaks down above HRC 40 (HB 370+) where the relationship becomes material-dependent. Hardness testing is used in fabrication for: (1) verifying heat treatment (quenching/tempering) achieved target properties, (2) ensuring weld heat-affected zone hardness is below 22 HRC (200 HB) for sulfide stress cracking resistance per NACE MR0175, (3) verifying base metal hardness before welding to assess cold-cracking risk. For structural design: always use the certified mill test report (MTR) tensile values, not hardness-derived estimates. Hardness-derived Fu is for approximate material identification and quality control only.

What are the differences between Brinell, Rockwell, and Vickers hardness tests?

The three standard hardness tests differ in indenter type, load, and application per ASTM standards: (1) Brinell (ASTM E10) — uses a 10 mm tungsten carbide ball at 500-3,000 kgf load. The indentation diameter (measured optically at 10-20× magnification) converts to the Brinell Hardness Number (HB). Best for: castings, forgings, plate, and coarse-grained materials where a large indentation averages microstructure variations. Limitation: cannot test above ~450 HB (ball deforms), not suitable for thin specimens (indentation depth ~2-4 mm). (2) Rockwell (ASTM E18) — measures depth of penetration differential. Rockwell B (HRB): 1/16" steel ball, 100 kgf, for soft steel (annealed, A36, up to HRB 100 ≈ HB 220). Rockwell C (HRC): diamond cone, 150 kgf, for hardened steel (HRC 20-70). Direct reading on dial — fastest method, most common in fabrication quality control. (3) Vickers (ASTM E92) — diamond pyramid indenter at 1-100 kgf load. The diagonal of the square indentation (measured microscopically) converts to Vickers Hardness (HV). Advantages: one scale covers soft to very hard (HV 100-1000+), small indentation size suitable for thin specimens, coatings, and heat-affected zones. Correlation: HV ≈ HB for values below 300; above 300, HV reads slightly higher. For structural steel quality control (Fy 36-50 ksi), Brinell and Rockwell B are standard because they integrate over a larger area, reducing scatter from local microstructure variations.

What are typical hardness values for common structural steels?

Typical hardness ranges for structural steels: ASTM A36 — Brinell 120-160 HB, Rockwell B 67-83 HRB, Fu 58-80 ksi. The low hardness reflects the ferrite-pearlite microstructure of hot-rolled carbon steel. ASTM A992 (50 ksi W-shapes) — 150-170 HB, 80-87 HRB, Fu 65+ ksi. Slightly harder than A36 due to higher strength and microalloying (vanadium/columbium). ASTM A572 Gr 50 — 150-175 HB, similar to A992. ASTM A572 Gr 65 — 165-200 HB, 85-93 HRB, harder due to higher Mn (1.35-1.65%) and possible vanadium strengthening. ASTM A514 (Q&T, Fy 100 ksi) — 235-293 HB, 20-31 HRC. The martensitic microstructure from quenching and tempering produces the higher hardness; the tempering temperature (typically 1050-1200°F) controls the final hardness. ASTM A588 (weathering) — 145-170 HB, similar to A572 Gr 50 with slightly higher hardness from chromium addition. A913 Gr 50 (QST) — 150-180 HB, surface hardness slightly higher than core due to the quenching and self-tempering process. For welded structures, hardness in the heat-affected zone (HAZ) should be evaluated — martensite formation in the HAZ can produce local hardness 250-400 HB (25-43 HRC), increasing cold-cracking risk if hydrogen is present. Maximum HAZ hardness limits: typically 325 HV (≈300 HB) for standard structural steels per AWS D1.1, or 248 HV (≈235 HB) for sour service per NACE MR0175.

Why does hardness matter for weldability and fabrication?

Hardness directly affects steel fabrication: (1) Weldability — high base metal hardness combined with rapid weld cooling can produce martensite in the HAZ, which is hard (350-450 HB) and brittle, creating a risk of hydrogen-induced cold cracking (HICC or hydrogen-assisted cracking). Carbon equivalent (CE) and hardness are correlated: CE > 0.45 typically corresponds to as-quenched HAZ hardness > 325 HV, which AWS D1.1 considers susceptible to cracking without preheat. (2) Preheat requirements per AWS D1.1 Table 3.2 are driven by carbon equivalent and section thickness, with the goal of slowing the cooling rate to avoid hard HAZ microstructures. For A36 (CE ~0.30-0.35): no preheat for thickness up to 3/4", 50°F minimum for 3/4-1-1/2", 150°F for 1-1/2-2-1/2". For A514 (CE ~0.50-0.60): preheat 150-200°F always, and careful interpass temperature control to avoid overtempering. (3) Machining — annealed steel (HB 120-160) machines easily; as-rolled structural steel (HB 150-180) is machinable but requires carbide tooling; Q&T steel above 300 HB requires rigid setups and reduced cutting speeds (50-60% of annealed steel speeds). (4) Cold forming — higher hardness reduces formability; A36 plates (HB 120-160) can be cold-bent to tighter radii than A514 plates (HB 235-293). The minimum bend radius for A36 is 1.5t-2t (t = thickness); for A514 it is 3t-5t.

How is hardness used in field inspection and quality control?

Hardness testing in field inspection and quality control serves: (1) Heat treatment verification — after post-weld heat treatment (PWHT, 600-650°C), hardness should drop from as-welded HAZ levels (250-400 HB) back to near base metal levels (150-180 HB for A36/A992). If hardness remains above 200-220 HB, PWHT was insufficient. (2) Material verification — a quick Rockwell B or portable Leeb/Dynamic hardness test can distinguish A36 (HRB 67-83) from A514 (HRC 20-31) when MTRs are lost. (3) Weld cap pass hardness — the final weld pass tempers the HAZ below it; the cap pass HAZ may be harder (no tempering from subsequent passes). Specifying that the cap pass be located away from stress concentrations mitigates this. (4) Portable hardness testers: Leeb (Equotip) rebound method per ASTM A956 provides ±5% accuracy vs benchtop Brinell when properly calibrated. Microdur instruments use ultrasonic contact impedance (UCI) for thin sections and HAZ testing. (5) Acceptance criteria per AWS D1.1: maximum 325 HV (≈300 HB, ≈32 HRC) for HAZ of standard structural steels. Per NACE MR0175/ISO 15156 for sour service: maximum 22 HRC (≈235 HB) for carbon and low-alloy steels, verified by hardness traverse across weld, HAZ, and base metal. Hardness testing is the most common NDE supplement to visual inspection because it can detect incorrect heat treatment that visual examination cannot.

Steel Hardness Chart — Brinell, Rockwell, Vickers Values

Hardness is a measure of steel's resistance to localized plastic deformation (indentation). It correlates closely with tensile strength and is used for quality control, heat treatment verification, and material identification. This page provides hardness values for common structural and industrial steels.

Hardness Testing Methods

Method	Standard	Indenter	Load Range	Typical Use
Brinell (HB)	ASTM E10	10 mm steel or tungsten ball	500-3,000 kgf	Castings, forgings, plate
Rockwell B (HRB)	ASTM E18	1/16 in steel ball	100 kgf	Soft steel, annealed
Rockwell C (HRC)	ASTM E18	Diamond cone	150 kgf	Hardened steel, tools
Vickers (HV)	ASTM E92	Diamond pyramid	1-100 kgf	Thin specimens, coatings
Knoop (HK)	ASTM E92	Diamond rhombus	0.01-1 kgf	Very thin sections
Shore (HS)	—	Spring-loaded hammer	Dynamic	Field testing

Hardness Conversion Table

Approximate conversions between hardness scales for steel:

Brinell (HB)	Rockwell B (HRB)	Rockwell C (HRC)	Vickers (HV)	Approx. Fu (ksi)
100	56	—	100	50
120	67	—	120	60
140	76	—	140	70
160	83	—	160	80
180	88	—	180	90
200	93	—	200	100
217	96	17	220	108
229	99	20	230	114
241	—	23	245	120
255	—	25	258	127
269	—	28	272	134
285	—	30	288	142
302	—	32	305	151
321	—	34	325	160
341	—	36	345	170
363	—	39	368	180
388	—	42	392	193
415	—	44	420	207
444	—	46	450	220
477	—	48	485	235
514	—	51	520	255
555	—	54	565	275
600	—	57	610	300
650	—	60	660	325

Values are approximate. Actual conversion depends on steel composition and heat treatment.

Hardness by Steel Grade

Structural Steels (As-Rolled)

ASTM Spec	Grade	Typical HB	HRB	HRC	Notes
A36	—	110-150	62-80	—	Soft, easily welded
A992	50	150-180	80-88	—	Standard W-shapes
A572	50	150-180	80-88	—	Similar to A992
A572	65	180-210	88-95	—	Higher carbon
A588	—	150-185	80-89	—	Weathering steel
A514	—	260-320	—	25-33	Quenched & tempered

Heat-Treated Steels

Condition	HB Range	HRC Range	Typical Application
Annealed	120-170	—	Machining, forming
Normalized	140-200	—	Uniform grain structure
Quenched	400-600	40-58	Wear surfaces, tools
Quenched & tempered	250-400	24-43	Structural, springs
Case-hardened surface	550-650	52-60	Gears, shafts
Through-hardened	400-550	40-52	Tooling, dies

Tool Steels

Tool Steel	HB (annealed)	HRC (hardened)	Use
O1	201-229	57-62	Oil-hardening, general tools
O2	201-229	57-62	Oil-hardening, punches
A2	201-241	57-62	Air-hardening, forming dies
A6	217-248	57-60	Air-hardening, heavy dies
D2	217-255	58-64	High chromium, blanking dies
D3	217-255	58-64	High carbon, wear plates
H13	192-229	44-54	Hot work, die casting
H21	217-241	38-50	Hot work, forging dies
M2	212-241	63-65	High-speed cutting
M4	223-255	64-66	High-speed, heavy cutting
S7	187-223	54-58	Shock-resistant, chisels
P20	170-220	28-36	Mold steel (pre-hardened)

Hardness vs Tensile Strength

For carbon and low-alloy steels, tensile strength correlates linearly with Brinell hardness:

Approximate formula: Fu (ksi) ≈ HB × 0.5

More precisely: Fu (MPa) ≈ 3.45 × HB

HB	Fu (ksi) Approx.	Typical Steel Condition
100	50	Annealed mild steel
150	75	As-rolled A36/A992
200	100	Normalized medium carbon
250	125	Quenched & tempered
300	150	Hardened alloy steel
350	175	High-strength quenched
400	200	Heavily hardened
500	250	Tool steel range
600	300	Hardened tool steel

Hardness Testing Guidelines

Surface Preparation

Method	Surface Finish	Minimum Thickness	Notes
Brinell	Ground flat	10× indentation depth	Large indentation, good for rough surfaces
Rockwell	Smooth filing or grinding	10× indentation depth	Most common shop-floor test
Vickers	Polished	Test-dependent	Lab testing, thin specimens

Minimum Spacing Requirements

Method	Between Indentations	From Edge
Brinell	3× diameter	2.5× diameter
Rockwell	3× diameter	2.5× diameter
Vickers	2.5× diagonal	2.5× diagonal

Frequently Asked Questions

What is the hardness of A36 steel? A36 typically has a Brinell hardness of 110-150 HB (approximately HRB 62-80). It is relatively soft and easily machined, welded, and formed.

What is Rockwell C hardness? Rockwell C (HRC) is measured using a diamond cone indenter under 150 kgf load. It is used for hardened steels (typically above 20 HRC). Below 20 HRC, Rockwell B (HRB) is used instead.

Does higher hardness mean stronger steel? Generally yes. For carbon and low-alloy steels, tensile strength correlates approximately as Fu (ksi) ≈ 0.5 × HB. However, hardness and strength are different from toughness (resistance to impact) and ductility (ability to deform before fracture). Very hard steel can be brittle.

What hardness is considered "hard" for structural steel? As-rolled structural steel (A36, A992) is 110-180 HB. Anything above 250 HB is considered hard for structural applications. Quenched and tempered A514 reaches 260-320 HB.

Can I use hardness to estimate steel grade? Hardness testing can help identify whether steel is in the annealed, normalized, or hardened condition, but it cannot uniquely identify the grade. Multiple steel grades can have the same hardness but different compositions.

Steel Yield Strength — Fy values by grade
Steel Tensile Strength — Fu values by grade
Fracture Toughness — Charpy V-notch data
Steel Grades — ASTM specification guide
Steel Stress-Strain Curve — Complete material behavior

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.

Brinell (HB)	Rockwell B (HRB)	Rockwell C (HRC)	Vickers (HV)	Approx. Fu (ksi)
100	56	—	100	50
120	67	—	120	60
140	76	—	140	70
160	83	—	160	80
180	88	—	180	90
200	93	—	200	100
217	96	17	220	108
229	99	20	230	114
241	—	23	245	120
255	—	25	258	127
269	—	28	272	134
285	—	30	288	142
302	—	32	305	151
321	—	34	325	160
341	—	36	345	170
363	—	39	368	180
388	—	42	392	193
415	—	44	420	207
444	—	46	450	220
477	—	48	485	235
514	—	51	520	255
555	—	54	565	275
600	—	57	610	300
650	—	60	660	325

Brinell (HB)	Rockwell B (HRB)	Rockwell C (HRC)	Vickers (HV)	Approx. Fu (ksi)
100	56	—	100	50
120	67	—	120	60
140	76	—	140	70
160	83	—	160	80
180	88	—	180	90
200	93	—	200	100
217	96	17	220	108
229	99	20	230	114
241	—	23	245	120
255	—	25	258	127
269	—	28	272	134
285	—	30	288	142
302	—	32	305	151
321	—	34	325	160
341	—	36	345	170
363	—	39	368	180
388	—	42	392	193
415	—	44	420	207
444	—	46	450	220
477	—	48	485	235
514	—	51	520	255
555	—	54	565	275
600	—	57	610	300
650	—	60	660	325