Differences in Hardness HB, HR, and HV of Stainless Steel

Introduction to Hardness: Hardness indicates a material's ability to resist the penetration of hard objects into its surface. It is one of the important performance indicators of metal materials. Generally, the higher the hardness, the better the wear resistance. Commonly used hardness indicators include Brinell hardness, Rockwell hardness, and Vickers hardness.

Brinell Hardness (HB): A certain load (usually 3000 kg) is applied to press a hardened steel ball of a specific size (generally 10 mm in diameter) into the surface of the material for a period of time. After unloading, the ratio of the load to the indentation area is the Brinell hardness value (HB), with units in kgf/mm² (N/mm²).

Rockwell Hardness (HR): When HB > 450 or the sample is too small, the Brinell hardness test cannot be used, and the Rockwell hardness measurement is adopted instead. It uses a diamond cone with a 120° apex angle or a steel ball with a diameter of 1.59 mm or 3.18 mm, pressed into the surface of the material under a certain load. The hardness of the material is determined by the depth of the indentation.

Depending on the hardness of the test material, three different scales are used:

HRA: Measured using a 60 kg load and a diamond cone indenter, used for extremely hard materials (such as cemented carbide).

HRB: Measured using a 100 kg load and a hardened steel ball with a diameter of 1.58 mm, used for materials with lower hardness (such as annealed steel, cast iron, etc.).

HRC: Measured using a 150 kg load and a diamond cone indenter, used for materials with high hardness (such as quenched steel).

Vickers Hardness (HV): A load of up to 120 kg and a diamond square pyramid indenter with an apex angle of 136° are used to press into the material surface. The Vickers hardness value (HV) is obtained by dividing the surface area of the indentation pit by the load value, with units in kgf/mm².

Stainless steel involves many variable factors that characterize corrosive media, such as the type and concentration of chemicals, atmospheric conditions, temperature, and time. Therefore, it is difficult to select and use materials without understanding the exact nature of the medium. Below is a brief introduction to several common stainless steel materials:

304: A widely used material. It can withstand general rusting in construction and resists erosion from food processing media (though corrosion may occur under high temperatures with concentrated acids and chlorides). It resists organic compounds, dyes, and a wide range of inorganic compounds. Type 304L (low carbon) offers good resistance to nitric acid and withstands moderate temperatures and concentrations of sulfuric acid. It is widely used in liquid gas storage tanks, low-temperature equipment (304N), appliances, and other consumer products, such as kitchen equipment, medical devices, transportation tools, and wastewater treatment plants.

316: Contains slightly more nickel than Type 304 and 2%–3% molybdenum, providing better corrosion resistance than Type 304, particularly in chloride environments prone to pitting corrosion. Type 316 was developed for use in sulfite pulp machines due to its durability in sulfuric compounds. Its applications have expanded to handle many chemicals in the processing industry.

317: Contains 3%–4% molybdenum (the highest level in this series) and more chromium than Type 316, offering higher resistance to pitting and crevice corrosion.

430: Has lower alloy content than Type 304 and is used for highly polished decorative purposes in mild atmospheres. It can also be used in nitric acid and food processing equipment.

410: Has the lowest alloy content among the three general-purpose stainless steels. It is selected for high-strength components requiring a combination of strength and corrosion resistance, such as fasteners. Type 410 resists corrosion in mild atmospheres, water vapor, and many moderate chemical products.

2205: Superior to Type 304 and Type 316 due to its high resistance to chloride stress corrosion cracking and approximately double the strength.