Articles

Hardness Conversions and Approximated Tensile Strength

  1. Abstract

    Hardness is in many cases a means to an end; measuring hardness provides insight on the materials strength. Two common material specifications, ASTM A370 and ISO 18265, offer guidance on converting hardness to ultimate tensile strength.

  2. Discussion

    ASTM A370 titled Standard Test Methods and Definitions for Mechanical Testing of Steel Products, and ISO 18265 titled Metallic Materials – Conversion of Hardness Values, curiously, do not always agree when discussing hardness to tensile strength conversion; they give slightly different values. So, which is right? Which do we use? The answer is if neither is specifically called out to be used, we use both and we understand these conversions were developed from empirical testing of various steels & chemistries. Nonetheless, the conversions between these specifications are usually quite similar.

    Different hardness scales exist to measure different types of materials; e.g., a very hard material such as 300M (~290 ksi, tensile) cannot be measured on the same scale as an elastomer (rubber) as the hardness disparity would simply be too great. Ductile cast irons do not have a homogenous microstructure (they have nodes, pearlite and ferrite) therefore using a small indenter, such as Vickers micro-hardness, is not indicative of the bulk hardness; this is why the 10 mm ball indenter of a Brinell hardness test is typically called out for measuring castings. In summary, indent geometry and the applied force vary to compensate for what type of material is being measured for hardness. The product specification or mechanical property specification should dictate the appropriate hardness scale for product testing.

    A brief overview of indent geometry:

    • Rockwell Hardness A (HRA), C (HRC) and D (HRD) scales have a 120° cone shaped indent.
    • Rockwell Hardness B (HRB) and F (HRF) scales have sphere shaped indent which have a 1/16" diameter.
    • Superficial Rockwell scales HR15N, HR30N, HR45N, HR15T, HR30T and HR45T are used for shallow indents such as measuring thin materials or surface hardness. The "N" and "T" stand for cone shaped and 1/16" diameter ball shaped indenters, respectively.
    • Vickers hardness (HV) scale has a pyramidal shaped indent.
    • Knoop hardness (Knoop) scale has a pyramidal diamond indent.
    • Brinell hardness (Brinell) scale, similar to HRB and HRF, has a sphere-shaped indent though the sphere diameter is 10 mm.

    Jump to: ASTM A370, ISO 18265.

  3. Tables:

TABLE 1: ASTM A370 Standard Test Methods and Definitions for Mechanical Testing of Steel Products
Su
KSI
Su
MPa
HVHRCHRBBrinellKnoopHRDHRAHRFHR15NHR30NHR45NHR15THR30THR45T
...... 94068 ......920...85.6... 93.284.475.4... ... ...
...... 90067 ......895...85.0... 92.983.674.2... ... ...
...... 86566 ......870...84.5... 92.582.873.3... ... ...
...... 83265 ...739846...83.9... 92.281.972.0... ... ...
...... 80064 ...722822...83.4... 91.881.171.0... ... ...
...... 77263 ...706799...82.8... 91.480.169.9... ... ...
...... 74662 ...688776...82.3... 91.179.368.8... ... ...
...... 72061 ...670754...81.8... 90.778.467.7... ... ...
...... 69760 ...654732...81.2... 90.277.566.6... ... ...
351242067459 ...634710...80.7... 89.876.665.5... ... ...
338233065358 ...615690...80.1... 89.375.764.3... ... ...
325224063357 ...595670...79.6... 88.974.863.2... ... ...
313216061356 ...577650...79.0... 88.373.962.0... ... ...
301207059555 ...560630...78.5... 87.973.060.9... ... ...
292201057754 ...543612...78.0... 87.472.059.8... ... ...
283195056053 ...525594...77.4... 86.971.258.6... ... ...
273188054452 ...512576...76.8... 86.470.257.4... ... ...
264182052851 ...496558...76.3... 85.969.456.1... ... ...
255176051350 ...482542...75.9... 85.568.555.0... ... ...
246170049849 ...468526...75.2... 85.067.653.8... ... ...
238164048448 ...455510...74.7... 84.566.752.5... ... ...
229158047147 ...442495...74.1... 83.965.851.4... ... ...
221152045846 ...432480...73.6... 83.564.850.3... ... ...
215148044645 ...421466...73.1... 83.064.049.0... ... ...
208143043444 ...409452...72.5... 82.563.147.8... ... ...
201139042343 ...400438...72.0... 82.062.246.7... ... ...
194134041242 ...390426...71.5... 81.561.345.5... ... ...
188130040241 ...381414...70.9... 80.960.444.3... ... ...
182125039240 ...371402...70.4... 80.459.543.1... ... ...
177122038239 ...362391...69.9... 79.958.641.9... ... ...
171118037238 ...353380...69.4... 79.457.740.8... ... ...
166114036337 ...344370...68.9... 78.856.839.6... ... ...
161111035436 ...336360...68.4... 78.355.938.4... ... ...
156108034535 ...327351...67.9... 77.755.037.2... ... ...
152105033634 ...319342...67.4... 77.254.236.1... ... ...
149103032733 ...311334...66.8... 76.653.334.9... ... ...
146101031832 ...301326...66.3... 76.152.133.7... ... ...
141970 31031 ...294318...65.8... 75.651.332.5... ... ...
138950 30230 ...286311...65.3... 75.050.431.3... ... ...
135930 29429 ...279304...64.6... 74.549.530.1... ... ...
131900 28628 ...271297...64.3... 73.948.628.9... ... ...
128880 27927 ...264290...63.8... 73.347.727.8... ... ...
125860 27226 ...258284...63.3... 72.846.826.7... ... ...
123850 26625 ...253278...62.8... 72.245.925.5... ... ...
119820 26024 ...247272...62.4... 71.645.024.3... ... ...
117810 25423 ...243266...62.0... 71.044.023.1... ... ...
116800 240...100240251...61.5... ... ... ... 93.183.172.9
115790 24822 ...237261...61.5... 70.543.222.0... ... ...
114785 234...99 234246...60.9... ... ... ... 92.882.571.9
112770 24321 ...231256...61.0... 69.942.320.7... ... ...
110760 23820 ...226251...60.5... 69.441.519.6... ... ...
109750 228...98 228241...60.2... ... ... ... 92.581.870.9
104715 222...97 222236...59.5... ... ... ... 92.181.169.9
102705 216...96 216231...58.9... ... ... ... 91.880.468.9
100690 210...95 210226...58.3... ... ... ... 91.579.867.9
98 675 205...94 205221...57.6... ... ... ... 91.279.166.9
94 650 200...93 200216...57.0... ... ... ... 90.878.465.9
92 635 195...92 195211...56.4... ... ... ... 90.577.864.8
90 620 190...91 190206...55.8... ... ... ... 90.277.163.8
89 615 185...90 185201...55.2... ... ... ... 89.976.462.8
88 605 180...89 180196...54.6... ... ... ... 89.575.861.8
86 590 176...88 176192...54.0... ... ... ... 89.275.160.8
84 580 172...87 172188...53.4... ... ... ... 88.974.459.8
83 570 169...86 169184...52.8... ... ... ... 88.673.858.8
82 565 165...85 165180...52.3... ... ... ... 88.273.157.8
81 560 162...84 162176...51.7... ... ... ... 87.972.456.8
80 550 159...83 159173...51.1... ... ... ... 87.671.855.8
77 530 156...82 156170...50.6... ... ... ... 87.371.154.8
73 505 153...81 153167...50.0... ... ... ... 86.970.453.8
72 495 150...80 150164...49.5... ... ... ... 86.669.752.8
70 485 147...79 147161...48.9... ... ... ... 86.369.151.8
69 475 144...78 144158...48.4... ... ... ... 86.068.450.8
68 470 141...77 141155...47.9... ... ... ... 85.667.749.8
67 460 139...76 139152...47.3... ... ... ... 85.367.148.8
66 455 137...75 137150...46.899.6... ... ... 85.066.447.8
65 450 135...74 135147...46.399.1... ... ... 84.765.746.8
64 440 132...73 132145...45.898.5... ... ... 84.365.145.8
63 435 130...72 130143...45.398.0... ... ... 84.064.444.8
62 425 127...71 127141...44.897.4... ... ... 83.763.743.8
61 420 125...70 125139...44.396.8... ... ... 83.463.142.8
60 415 123...69 123137...43.896.2... ... ... 83.062.441.8
59 405 121...68 121135...43.395.6... ... ... 82.761.740.8
58 400 119...67 119133...42.895.1... ... ... 82.461.039.8
57 395 117...66 117131...42.394.5... ... ... 82.160.438.7
56 385 116...65 116129...41.893.9... ... ... 81.859.737.7
...... 114...64 114127...41.493.4... ... ... 81.459.036.7
...... 112...63 112125...40.992.8... ... ... 81.158.435.7
...... 110...62 110124...40.492.2... ... ... 80.857.734.7
...... 108...61 108122...40.091.7... ... ... 80.557.033.7
...... 107...60 107120...39.591.1... ... ... 80.156.432.7
...... 106...59 106118...39.090.5... ... ... 79.855.731.7
...... 104...58 104117...38.690.0... ... ... 79.555.030.7
...... 103...57 103115...38.189.4... ... ... 79.254.429.7
...... 101...56 101114...37.788.8... ... ... 78.853.728.7
...... 100...55 100112...37.288.2... ... ... 78.553.027.7
...... ......54 ...111...36.887.7... ... ... 78.252.426.7
...... ......53 ...110...36.387.1... ... ... 77.951.725.7
...... ......52 ...109...35.986.5... ... ... 77.551.024.7
...... ......51 ...108...35.586.0... ... ... 77.250.323.7
...... ......50 ...107...35.085.4... ... ... 76.949.722.7
...... ......49 ...106...34.684.8... ... ... 76.649.021.7
...... ......48 ...105...34.184.3... ... ... 76.248.320.7
...... ......47 ...104...33.783.7... ... ... 75.947.719.7
...... ......46 ...103...33.383.1... ... ... 75.647.018.7
...... ......45 ...102...32.982.6... ... ... 75.346.317.7
...... ......44 ...101...32.482.0... ... ... 74.945.716.7
...... ......43 ...100...32.081.4... ... ... 74.645.015.7
...... ......42 ...99 ...31.680.8... ... ... 74.344.314.7
...... ......41 ...98 ...31.280.3... ... ... 74.043.713.6
...... ......40 ...97 ...30.779.7... ... ... 73.643.012.6
...... ......39 ...96 ...30.379.1... ... ... 73.342.311.6
...... ......38 ...95 ...29.978.6... ... ... 73.041.610.6
...... ......37 ...94 ...29.578.0... ... ... 72.741.09.6
...... ......36 ...93 ...29.177.4... ... ... 72.340.38.6
...... ......35 ...92 ...28.776.9... ... ... 72.039.67.6
...... ......34 ...91 ...28.276.3... ... ... 71.739.06.6
...... ......33 ...90 ...27.875.7... ... ... 71.438.35.6
...... ......32 ...89 ...27.475.2... ... ... 71.037.64.6
...... ......31 ...88 ...27.074.6... ... ... 70.737.03.6
...... ......30 ...87 ...26.674.0... ... ... 70.436.32.6

COMMENTS:
"..." indicates undefined values.
Columns for KSI and MPa are the approximate ultimate tensile strength (Su).
Note: Table 1, above, combines two tables from ASTM A370. Between 116 to 110 ksi (790 to 760 MPa) are the beginning and end, respectively, of the integration of these two ASTM A370 tables. Notice how tensile strengths remain linear through this range while HV, Brinell, Knoop and HRA are non-linear. When it comes to these regions, the user of these tables will need to determine how to best utilize this information.

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TABLE 2: ISO 18265 Metallic Materials - Conversion of Hardness Values
Su
KSI
Su
MPa
HVHRCHRBBrinellKnoopHRDHRAHRFHR15NHR30NHR45NHR15THR30THR45T
...... 94068.0... ... ...76.985.6... 93.284.475.4.........
...... 92067.5... ... ...76.585.3... 93.084.074.8.........
...... 90067.0... ... ...76.185.0... 92.983.674.2.........
...... 88066.4... ... ...75.784.7... 92.783.173.6.........
...... 86065.9... ... ...75.384.4... 92.582.773.1.........
...... 84065.3... ... ...74.884.1... 92.382.272.2.........
...... 82064.7... ... ...74.383.8... 92.181.771.8.........
...... 80064.0... ... ...73.883.4... 91.881.171.0.........
...... 78063.3... ... ...73.383.0... 91.580.470.2.........
...... 76062.5... ... ...72.682.6... 91.279.769.4.........
...... 74061.8... ... ...72.182.2... 91.079.168.6.........
...... 72061.0... ... ...71.581.8... 90.778.467.7.........
...... 70060.1... ... ...70.881.3... 90.377.666.7.........
...... 69059.7... ... ...70.581.1... 90.177.266.2.........
...... 68059.2... ... ...70.180.8... 89.876.865.7.........
...... 67058.8... ... ...69.880.6... 89.776.465.3.........
...... 66058.3... ... ...69.480.3... 89.575.964.7.........
316218065057.8... 618 ...69.080.0... 89.275.564.1.........
311214564057.3... 608 ...68.779.8... 89.075.163.5.........
305210563056.8... 599 ...68.379.5... 88.874.663.0.........
300207062056.3... 589 ...67.979.2... 88.574.262.4.........
294203061055.7... 580 ...67.578.9... 88.273.761.7.........
289199560055.2... 570 ...67.078.6... 88.073.261.2.........
284195559054.7... 561 ...66.778.4... 87.872.760.5.........
278192058054.1... 551 ...66.278.0... 87.572.159.9.........
273188057053.6... 542 ...65.877.8... 87.271.759.3.........
268184556053.0... 532 ...65.477.4... 86.971.258.6.........
263181055052.3... 523 ...64.877.0... 86.670.557.8.........
257177554051.7... 513 ...64.476.7... 86.370.057.0.........
252174053051.1... 504 ...63.976.4... 86.069.556.2.........
247170052050.5... 494 ...63.576.1... 85.769.055.6.........
241166551049.8... 485 ...62.975.7... 85.468.354.7.........
236163050049.1... 475 ...62.275.3... 85.067.753.9.........
231159549048.4... 466 ...61.674.9... 84.767.153.1.........
226155548047.7... 456 ...61.374.5... 84.366.452.2.........
220152047046.9... 447 ...60.774.1... 83.965.751.3.........
215148546046.1... 437 ...60.173.6... 83.664.950.4.........
211145545045.3... 428 ...59.473.3... 83.264.349.4.........
206142044044.5... 418 ...58.872.8... 82.863.548.4.........
201138543043.6... 409 ...58.272.3... 82.362.747.4.........
196135042042.7... 399 ...57.571.8... 81.861.946.4.........
191132041041.8... 390 ...56.871.4... 81.461.145.3.........
187129040040.8... 380 ...56.070.8... 80.860.244.1.........
182125539039.8... 371 ...55.370.3... 80.359.342.9.........
177122038038.8... 361 ...54.469.8... 79.858.441.7.........
173119037037.7... 352 ...53.669.2... 79.257.440.4.........
168115536036.6... 342 ...52.868.7... 78.656.439.1.........
163112535035.5... 333 ...51.968.1... 78.055.437.8.........
159109534034.4... 323 ...51.167.6... 77.454.436.5.........
154106033033.3... 314 ...50.267.0... 76.853.635.2.........
149103032032.2... 304 ...49.466.4... 76.252.333.9.........
144995 31031.0... 295 ...48.465.8... 75.651.332.5.........
140965 30029.8... 285 ...47.565.2... 74.950.231.1.........
138950 29529.2... 280 ...47.164.8... 74.649.730.4.........
135930 29028.5105 276 ...46.564.5... 74.249.029.5.........
133915 28527.8... 271 ...46.064.2... 73.848.428.7.........
131900 28027.1104 266 ...45.363.8... 73.447.827.9.........
128880 27526.4102 261 ...44.963.5... 73.047.227.1.........
125865 27025.6102 257 ...44.363.1... 72.646.426.2.........
123850 26524.8101 252 ...43.762.7... 72.145.725.2.........
121835 26024.0101 247 ...43.162.4... 71.645.024.3.........
119820 25523.1... 242 ...42.262.0... 71.144.223.2.........
116800 25022.299.5238 ...41.761.6115.170.643.422.2.........
114785 24521.3... 233 ...41.161.2... 70.142.521.1.........
112770 24020.398.1228 ...40.360.7114.369.641.719.9.........
110755 235... ... 223 ...... ... ... ... ... ... .........
107740 230... 96.7219 ...... ... 113.4... ... ... .........
104720 225... 96.0214 ...... ... ... ... ... ... .........
102705 220... 95.0209 ...... ... 112.4... ... ... .........
100690 215... 94.0204 ...... ... ... ... ... ... .........
98 675 210... 93.5199 ...... ... 111.3... ... ... .........
96 660 205... 92.5195 ...... ... ... ... ... ... .........
93 640 200... 91.5190 ...... ... 110.1... ... ... .........
91 625 195... ... 185 ...... ... ... ... ... ... .........
88 610 190... 89.5181 ...... ... 108.7... ... ... .........
86 595 185... ... 176 ...... ... ... ... ... ... .........
83 575 180... 87.1171 ...... ... 107.2... ... ... .........
81 560 175... 85.0166 ...... ... 105.5... ... ... .........
79 545 170... 85.0162 ...... ... 105.5... ... ... .........
77 530 165... 81.7156 ...... ... 103.6... ... ... .........
74 510 160... 81.7152 ...... ... 103.6... ... ... .........
72 495 155... ... 147 ...... ... ... ... ... ... .........
70 480 150... 78.7143 ...... ... 101.4... ... ... .........
67 465 145... ... 138 ...... ... ... ... ... ... .........
65 450 140... 75.0133 ...... ... 99.0 ... ... ... .........
62 430 135... ... 128 ...... ... ... ... ... ... .........
60 415 130... 71.2124 ...... ... 96.4 ... ... ... .........
58 400 125... 66.7119 ...... ... 93.6 ... ... ... .........
56 385 120... 66.7114 ...... ... 93.6 ... ... ... .........
54 370 115... 62.3109 ...... ... 90.5 ... ... ... .........
51 350 110... 62.3105 ...... ... 90.5 ... ... ... .........
49 335 105... ... 99.8...... ... ... ... ... ... .........
46 320 100... 56.295.0...... ... 87.0 ... ... ... .........
44 305 95 ... 52.090.2...... ... ... ... ... ... .........
41 285 90 ... 48.085.5...... ... 82.6 ... ... ... .........
39 270 85 ... 41.080.7...... ... ... ... ... ... .........
37 255 80 ... ... 76.0...... ... ... ... ... ... .........

COMMENTS:
"..." indicates undefined values.
Columns for KSI and MPa are the approximate ultimate tensile strength (Su).

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Respectfully,
Tyler Olson
Fastener Enthusiast
Polycrew Contributor
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