Stainless steel is a steel alloy with at least 10.5% chromium with or without other alloying elements and a maximum of 1.2% carbon by mass. Stainless steels, also known as inox steels or inox from French inoxydable (inoxidizable), are steel alloys, which are very well known for their corrosion resistance, which increases with increasing chromium content.
Stainless Steel Hardness is the ability of a material to resist localized plastic deformation. In most cases, localized deformation is caused by mechanical indentation or wear. Therefore, this property usually includes several meanings, such as the ability of a material to resist scratching, cutting, abrasion, indentation, penetration, etc. Intuitively, hardness is the ability of one metal to cut another.
A harder metal can usually cut a softer metal or can make indentations on the surface of a softer metal. For example, a tool is hard enough to cut a metallic material. If the material is very hard (hardened), it will need to be ground because the abrasive (grit) of the wheel is harder.
Typical Stainless steel hardness
Hardness of stainless steel – type 304 is max: Rockwell HRB 92, Brinell. HBW 201.
Hardness of ferritic stainless steel – grade 430 is max: Rockwell HRB 89, Brinell. HBW 183.
Hardness of martensitic stainless steel – grade 410 is max: Rockwell HRB 96, Brinell. HBW 217.
Hardness of duplex stainless steel – 2205 is max: Rockwell HRB 31, Brinell. HBW 293.
Hardness of precipitation hardening steel – 17-4 PH is max: Rockwell HRC 38, Brinell. HBW 363.
What is the Hardness of Stainless Steel?
Hardness is the ability to withstand surface indentation (localized plastic deformation) and scratching. Hardness is probably the most poorly defined material property because it may indicate resistance to scratching, resistance to abrasion, resistance to indentation, or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.
Hence, ascertaining its hardness value by carrying out hardness testing using some standard hardness testing methods often helps to determine its tensile strength. The hardness value of stainless steel will determine its suitability for an intended design or use. It can also indicate the durability of stainless steel components to overcome abrasion, wear, tear and deformations during use.
What Determines the Hardness of Stainless Steel?
Stainless steel contains carbon and chromium in varying percentages depending on the level of anti-corrosion and the hardness level required. It turns out that more carbon reduces its anti-corrosion properties but increases its hardness value, while more chromium reduces its hardness value but increases its anti-corrosion properties. Interestingly, the addition of other elements like Nickel, vanadium, titanium, and aluminum helps to increase its hardness and as such, it can resist more impact.
The hardness of the stainless steel (as measured by a hardness test) is determined primarily by the amount of ferrite that is present in the microstructure, and secondarily by composition and heat treatment conditions.
The composition effects are primarily due to the presence of interstitial elements. Interstitial elements are those that have small atoms and are generally found between the larger Cr, Ni, and Fe atoms in the crystal lattice. All other factors held constant, the higher the level of interstitial elements (C, N, etc.), the higher the hardness.
Heat Treatment
The effect of heat treatment is small and may not be measurable. It involves the higher residual stresses created when parts are water-quenched. Parts that are air-cooled or gas-quenched as in vacuum heat treating have less residual stress and therefore would be expected to have a slightly lower hardness.
Ferrite Content
The primary determinant of hardness in stainless steel is the ferrite content. Ferrite is harder (and stronger) than austenite. The amount of ferrite which is present in stainless steel depends primarily on the composition, and secondarily on the section thickness and the heat treatment the part received. Thicker sections generally have higher ferrite contents than thinner sections. The heat treatment effect is due to the temperature used; the precise effect is often not predictable since it also depends on the composition.
Chemical Elements
Some elements promote the formation of ferrite; others promote austenite. The relative amounts of these elements have a major effect on the ferrite content. Elements such as chromium, molybdenum, silicon, and columbium (niobium) promote ferrite. The higher their total content, the higher the ferrite content. Conversely, some elements (nickel, manganese, carbon, nitrogen, etc.) promote the formation of austenite: the higher their combined content, the less ferrite is formed.
These elements are not equal in their power to affect the ferrite content. ASTM A800 gives multipliers which are used to adjust the concentrations for estimating the ferrite level. This specification is based on research conducted by the Steel Founders’ Society of America. Other researchers have found different “multipliers.” The difference between the several researchers’ results is likely due to the casting methods used, the casting section sizes, and the Alloy compositions studied. For example, the Welding Research Council has reported that Manganese has no effect at all!
Ferrite content can be determined by any of several methods. Metallography, calculation from chemical composition (as is done in A800), and correction from magnetic property measurements are the most common. Metallographic methods fail when the structure is very fine (as in welds, thus the use of ferrite numbers in the welding industry).
Hardness Test Methods for Stainless Steel Hardness
Not carrying out hardness tests on stainless steel can often box manufacturers into a tight corner when the material or component eventually fails. To avoid operational failure, material experts and quality assessment professionals turn to some standard hardness testing methods to determine the hardness value of stainless steel material.
ASTM test methods are ASTM E 10 for Brinell hardness, ASTM E 18 for Rockwell hardness, and ASTM E 384 for Microindentional testing.
Hardness testing measures the resistance of metals to plastic deformation by indentation. The table below lists the various tests, the indentor type, and the method of measurement.
| Test | Indentor | Measurement |
|---|---|---|
| Brinell hardness | 10 mm tungsten carbide ball | Indentation diameter |
| ASTM E 10 Rockwell Hardness | 1/16” or 1/8” steel ball | Penetration depth |
| Diamond pyramid | conical brale diamond indentor | Ave. indentation diagonals |
| Vickers Hardness | diamond pyramid | Ave. indentation diagonals |
| Knoop Hardness | elongated diamond pyramid | Ave. indentation diagonals |
Rockwell Hardness Test
Considered one of the most popular hardness tests, the Rockwell hardness test utilizes an indenter with a range of loads. These loads combine as a set of two loads where one is smaller and applied first, followed by the second load with a higher value applied next. The difference between the indentation depth caused by the set of two loads gives the Rockwell hardness value. Depending on the sets of two loads employed and the type of indenter, Rockwell can have different scales often designated as A, B, C, D, or E. These alphabets align with the Rockwell hardness value’s general HR designation to identify the different scales.
Normally the B and C scales are used on stainless steel, B for softened steel and C for hardened steel. The Rockwell B scale hardness uses the 1/16″ ball indenter and a 100 kilograms major load. The Rockwell C scale hardness uses the diamond “Brale” indenter and 150 kilograms major load for harder stainless steels.
Brinell Hardness Test
This hardness test is suitable for ferrous and non-ferrous metals, including hardened stainless steel materials. The Brinell hardness test uses a load range of between 500 to 3,000kgf to cause an indentation using a ball indenter on a smooth surface. Additionally, the ball indenter is often 10mm in diameter and made of hardened steel with the 3,000kgf used for very hard metals, while smaller weights deal with softer materials. The hardness value obtained using the Brinell hardness test is often called the Brinell hardness number designated by HB. This Brinell number is obtained after dividing the applied force by the area of the material where there is an indentation.
Brinell hardness test uses a 10 mm (0.39 in) diameter hardened steel ball as an indenter with a 3,000 kgf (29.42 kN; 6,614 lbf) force. The load is maintained constant for a specified time (between 10 and 30 s). For softer materials, a smaller force is used; for harder materials, a tungsten carbide ball is substituted for the steel ball.
The test provides numerical results to quantify the hardness of a material, which is expressed by the Brinell hardness number – HB. The Brinell hardness number is designated by the most commonly used test standards (ASTM E10-14[2] and ISO 6506–1:2005) as HBW (H from hardness, B from brinell and W from the material of the indenter, tungsten (wolfram) carbide). In former standards, HB or HBS were used to refer to measurements made with steel indenters.
The Brinell hardness number (HB) is the load divided by the surface area of the indentation. The diameter of the impression is measured with a microscope with a superimposed scale.
Vickers Hardness Test
This test is often suitable for hard materials. It utilizes a diamond-tipped indenter which is in the form of a cone. The cone-shaped indenter presses into the material surface to cause an indentation which gives the Vickers hardness value upon measuring the width of the indentation. The load often varies but can be as high as 120kg, which makes it suitable for hardened stainless steel.
Stainless Steel Hardness Table Chart, HRB | HRC
The values are according to ASTM stainless steel tube and pipe standards.
| Grade | Hardness (Max) Brinell | Hardness (Max) Rockwell |
|---|---|---|
| 201 | 219 HBW | 95 HRB |
| 304 | 192 HBW | 90 HRB |
| 304L | 192 HBW | 90 HRB |
| 304H | 192 HBW | 90 HRB |
| 304N | 192 HBW | 90 HRB |
| 309S | 192 HBW | 90 HRB |
| 309H | 192 HBW | 90 HRB |
| 310S | 192 HBW | 90 HRB |
| 310H | 192 HBW | 90 HRB |
| 316 | 192 HBW | 90 HRB |
| 316L | 192 HBW | 90 HRB |
| 316H | 192 HBW | 90 HRB |
| 316Ti | 192 HBW | 90 HRB |
| 317 | 192 HBW | 90 HRB |
| 317L | 192 HBW | 90 HRB |
| 321 | 192 HBW | 90 HRB |
| 321H | 192 HBW | 90 HRB |
| 347 | 192 HBW | 90 HRB |
| TP347H | 192 HBW | 90 HRB |
| N08904 | 192 HBW | 90 HRB |
| N08020 | 217 HBW | 95 HRB |
| 800 N08800 Cold Work | 192 HBW | 90 HRB |
| 800H N08810 | 192 HBW | 90 HRB |
| 800HT N08811 | 192 HBW | 90 HRB |
| S31803 | 290 HBW | 30 HRC |
| S31803 | 290 HBW | 30 HRC |
| S32101 | 290 HBW | 30 HRC |
| S32205 | 290 HBW | 30 HRC |
| S32550 | 297 HBW | 31 HRC |
| S32304 | 290 HBW | 30 HRC |
| S32750 | 300 HBW | 32 HRC |
| S32760 | 300 HBW | … |
Hardness of the stainless steel material is also of utmost importance to ensure that it can resist the impact of external force and maintain its durability. More importantly, the hardness testing for determining the hardness of stainless steel material is crucial since the different hardness tests have their merits and demerits and are also most suitable for different ranges of material hardness.
Hardness Conversion Chart, HB | HV | HRB | HRC
| Brinell Hardness (HB) | Vickers Hardness (HV) | Rockwell (HRB) | Rockwell(HRC) | Ultimate Tensile Strength (N/mm2) |
|---|---|---|---|---|
| – | 640 | – | 57 | – |
| – | 615 | – | 56 | – |
| – | 591 | – | 54.5 | – |
| – | 569 | – | 53.5 | – |
| – | 547 | – | 52 | – |
| – | 528 | – | 51 | – |
| – | 508 | – | 49.5 | – |
| – | 491 | – | 48.5 | 1539 |
| 444 | 474 | – | 47 | 1520 |
| 429 | 455 | – | 45.5 | 1471 |
| 415 | 440 | – | 44.5 | 1422 |
| 401 | 425 | – | 43 | 1363 |
| 388 | 410 | – | 42 | 1314 |
| 375 | 396 | – | 40.5 | 1265 |
| 363 | 383 | – | 39 | 1236 |
| 352 | 372 | – | 38 | 1187 |
| 341 | 360 | – | 36.5 | 1157 |
| 331 | 350 | – | 35.5 | 1118 |
| 321 | 339 | – | 34.5 | 1089 |
| 311 | 328 | – | 33 | 1049 |
| 302 | 319 | – | 32 | 1020 |
| 293 | 309 | – | 31 | 990 |
| 285 | 301 | – | 30 | 971 |
| 277 | 292 | – | 29 | 941 |
| 269 | 284 | – | 27.5 | 912 |
| 262 | 276 | – | 26.5 | 892 |
| 255 | 269 | 100 | 25.5 | 873 |
| 248 | 261 | 99 | 24 | 853 |
| 241 | 253 | 98 | 23 | 824 |
| 235 | 247 | 97 | 22 | 794 |
| 229 | 241 | 96 | 20.5 | 775 |
| 223 | 235 | – | – | 755 |
| 217 | 228 | 95 | – | 745 |
| 212 | 223 | 94 | – | 716 |
| 207 | 218 | 93 | – | 696 |
| 197 | 208 | 91 | – | 667 |
| 187 | 197 | 89 | – | 637 |
| 179 | 189 | 87 | – | 608 |
| 170 | 179 | 85 | – | 559 |
| 163 | 172 | 83 | – | 539 |
| 156 | 165 | 81 | – | 530 |
| 149 | 157 | 79 | – | 500 |
| 143 | 150 | 77 | – | 481 |
| 137 | 144 | 74 | – | 471 |
| 131 | 138 | 72 | – | 461 |
| 126 | 133 | 69 | – | 451 |
| 121 | 127 | 67 | – | 431 |
| 116 | 122 | 64 | – | 422 |
| 111 | 117 | 61 | – | 402 |
| 107 | 113 | – | – | 382 |
| 103 | 108 | – | – | 373 |