In the manufacturing industry, steel shot is a commonly used surface treatment medium, and its hardness grade plays a key role in the treatment effect of the workpiece. Different application scenarios have different requirements for the hardness of steel shots, and an accurate understanding of the classification of steel shot hardness grades will help us make appropriate choices in shot blasting, sandblasting and other processes.

Hardness test method and steel shot hardness indication

The hardness of steel shot is usually determined by two methods: Rockwell hardness (HRC) and Vickers hardness (HV). The Rockwell hardness test is simple and fast, and the hardness value is determined by measuring the depth of the indenter pressing into the surface of the specimen under a certain test force; Vickers hardness is expressed by measuring the diagonal length of the indentation and then calculating the test force per unit of indentation surface area. In practice, these two hardness values can be converted to each other by conversion tables to meet the needs of different standards and scenarios.

Classification and characteristics of common steel shot hardness grades

 •  Low hardness steel shots: The hardness is generally around HRC 30 - 40 (or HV 300 - 400). This kind of steel shot material is relatively soft, and the impact force on the surface of the workpiece is small during the shot blasting process, and it is not easy to cause damage to the surface of the workpiece. It is suitable for some materials with delicate surfaces and easy to deform, such as aluminum alloy, copper alloy and other non-ferrous metal workpieces. It can remove slight dirt and oxide film on the surface, and at the same time make the surface achieve a certain finish, and will not change the original dimensional accuracy and surface roughness requirements of the workpiece.

 •  Medium hardness steel shots: The hardness is between HRC 40 - 50 (approx. HV 400 - 500). This is the most widely used type of steel shot. Its hardness is moderate, with good wear resistance and impact toughness. It has excellent performance in shot blasting and rust removal and surface strengthening of ordinary steels, such as common carbon steel and alloy steel. It can not only effectively remove the rust and oxide scale on the surface of the steel, but also form a certain depth of residual compressive stress on the surface of the steel through impact, so as to improve the fatigue strength and stress corrosion resistance of the steel. In construction steel structure, machinery manufacturing and other industries, medium-hardness steel shot is widely used for surface pretreatment of various metal components.

 •  High hardness steel shots: hardness above HRC 50 (HV 500 or more). These steel shots are extremely hard and wear-resistant, capable of producing strong impact forces. It is often used to treat workpieces with serious surface rust and hard oxide scale, or special materials with high hardness requirements, such as high-strength alloy steel, stainless steel, etc. In the field of shipbuilding, marine steel has been in the harsh marine environment for a long time, and a large number of marine organisms and thick rust layers are often attached to the surface.

The application of steel shot with different hardness in a specific industry

 •  Automobile manufacturing industry: In the production process of auto parts, in order to improve the surface fatigue strength and wear resistance of key components such as engine blocks and crankshafts, medium hardness steel shots are usually used for shot blasting and strengthening. For components with high surface quality requirements such as aluminum alloy wheels, low-hardness steel shots will be selected for surface cleaning to avoid damaging the surface finish and accuracy of the wheel hub.

 •  Aerospace field: aero engine blades, landing gear and other parts have extremely high requirements for material performance and surface quality. High-hardness steel shot can be used to remove machining allowance and oxide scale on the surface of the blade, and at the same time, the residual compressive stress is introduced on the surface through shot blasting to improve the fatigue resistance of the blade. For some high-precision aluminum alloy structural parts, low-hardness steel shots are used to remove oil stains and slight oxide film on the surface to ensure that the surface quality meets aerospace standards.

The classification of steel shot hardness grades is based on different hardness test methods, and different hardness grades of steel shot have their own unique performance characteristics and scope of application. In actual production, we need to reasonably select the hardness grade of the steel shot according to the material of the workpiece, the surface treatment requirements and the purpose of the process, so as to achieve the best surface treatment effect and production benefits.

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