Cast Steel Grit Testing

Cast steel shot inspection mainly conducts scientific testing and analysis on the chemical composition analysis, hardness testing, particle size distribution and sieving, metallographic structure inspection, density and apparent density determination and defect and cleanliness inspection, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Chemical composition analysis: Mainly testing the contents of the elements such as the carbon (C), silicon (Si), manganese (Mn), sulfur (S) and phosphorus (P) in the cast steel shot. The precise chemical composition ratios are the core of determining the hardness, toughness and service lives of the cast steel shot, and are the basic basis of judging whether its designation and quality meet the standards.

Hardness testing: The macro hardness of the cast steel shot is measured through the Rockwell hardness testers (HRC) or Brinell hardness testers (HB). During the testing, the representative particles need to be selected, and the indentation depths or diameters are measured under the specific loads. The hardness values directly affect its cutting forces and cleaning efficiencies, and are the key indicators of evaluating the wear resistance.

Particle size distribution and sieving: The standard sieve meshes are used for the machine sieving or hand sieving, analysing the weight proportions of the particles in the different mesh size ranges. The precise particle size distributions (such as G18-G120) are crucial for the uniformity of the blasting processes, surface roughness control and abrasive consumption calculation.

Metallographic structure inspection: The cast steel shot samples are prepared, and their micro-structures are observed through the metallographic microscopes, such as the morphologies, proportions and distributions of the martensite, bainite and retained austenite. The good metallographic structures are the internal factors of guaranteeing the high hardness and moderate toughness, and can effectively prevent the premature crushing during the use.

Density and apparent density determination: The true densities are measured using the density bottle method, and the bulk densities (apparent densities) are measured through the standard containers. The densities reflect the compactness of the material; the bulk densities directly affect the conveying efficiencies of the blasting equipment and the impact energies of the abrasives per unit volume.

Defect and cleanliness inspection: Whether the cast steel shot has the porosity, slag inclusions, cracks, severe scales and excessive dust impurities is checked through the visual inspection or with the help of the magnifying glasses. These defects will significantly reduce the durability of the abrasives, and may pollute the surfaces of the treated workpieces.

Crush resistance (durability) testing: Under the specific conditions (such as the standard blasting equipment and fixed parameters), the circulating blasting testing is carried out on the cast steel shot, and the durability is evaluated by measuring its crushing rates or circulation cycles. This is the core practical indicator of measuring the economy of the cast steel shot.

Testing Scope

New shot ex-factory inspection: The comprehensive testing is carried out on the newly produced cast steel shot, ensuring that the key indicators such as its chemical compositions, hardness and particle sizes meet the requirements of the purchase contracts or the relevant national standards (such as GB/T 18838.1). It is the first gate of the quality control.

Incoming material acceptance testing: The sampling re-inspection carried out by the users when receiving the cast steel shot, emphatically focusing on the items directly related to the process requirements, such as the particle size compositions, hardness and cleanliness, to verify the incoming material qualities and avoid the production losses caused by the unqualified abrasives.

In-use abrasive condition monitoring: The periodic testing is carried out on the circulating cast steel shot, mainly monitoring its particle size refinement, dust content increase, hardness changes and crushed particle proportions. According to the testing results, whether the new shot needs to be supplemented or completely replaced is determined, to maintain the stable process effects.

Process adaptability research: For the specific surface treatment processes (such as the steel plate pretreatment, casting cleaning and steel structure coating pretreatment), through the testing of the properties of the different specification cast steel shot, the models with the comprehensively optimal cleaning efficiencies, surface roughness and consumption costs are screened.

Failure analysis and quality arbitration: When the cast steel shot has the abnormal fast crushing, unqualified cleaning effects or causes the workpiece damages during the use, through the systematic testing and analysis of its chemical compositions and metallographic structures, etc., the failure roots are found, providing the technical basis for the quality responsibility determination.

Environmental protection and safety compliance inspection: Whether the contents of the harmful elements (such as the heavy metals such as the lead and chromium) in the cast steel shot exceed the environmental regulation limits, as well as the flammability and explosiveness characteristics of the dust are tested, ensuring that its use processes meet the occupational health and safe production requirements.

Research and development evaluation: In the evaluation of the cast steel shot products developed with the new materials and new processes, the testing scope covers all the routine and special performance indicators, such as the high-temperature stability and corrosion resistance, to comprehensively evaluate their innovation and application potentials.

Testing Methods and Standards

Chemical analysis method: Mainly according to the GB/T 223 standard series, the spark direct-reading spectrometers (OES) are used for the fast quantitative analysis, or the carbon-sulfur analysers and inductively coupled plasma optical emission spectrometers (ICP-OES) are used for the precise determination of the specific elements, ensuring the controllable compositions.

Hardness testing method: According to GB/T 230.1 (Rockwell hardness) or GB/T 231.1 (Brinell hardness). For the small particles, the mounted sample blocks can be made for the testing on the cross-sections; the micro hardness testers (HV) can also be used to measure the micro hardness of the single particles, with the more precise results.

Particle size sieving method: Strictly implementing the sieving procedures specified in GB/T 18838.1, Preparation of steel substrates before application of paints and related products — Specifications for metallic blast-cleaning abrasives — Part 1: General introduction and classification, or SAE J444, using the standard vibrating sieve machines, ensuring the comparability and reliability of the particle size distribution data.

Metallographic inspection method: According to GB/T 13298, Inspection method of microstructure for metals, after the mounting, grinding, polishing and etching (commonly the 4% nital solution) of the cast steel shot samples, the structure types and grades are observed, photographed and rated under the metallographic microscopes.

Density determination method: The true density determination can refer to the density bottle method principle in GB/T 1423, Test method for density of precious metals and their alloys. The bulk density determination usually follows SAE J444 or the internal specifications, using the funnels and measuring cups of the specific volumes for the standardized operations.

Crush resistance testing method: The common methods include the "drop weight testing" and "blasting circulation testing". The latter is closer to the actual working conditions, usually referring to the SAE J444 appendices or the internal enterprise standards. Under the controlled parameters such as the pressures, angles and distances, the loss rates of the abrasives after a certain number of circulations are determined.

Defect and cleanliness evaluation method: Mostly adopting the manual visual sorting and weighing methods, or combined with the image analysis systems. The dust contents can be determined through the sieving and weighing with the standard sieves (such as the bottom sieve of 75μm), and the specific methods can refer to the relevant industry standards or customer agreements.

Testing Standards

Domestic Standards:

1. GB/T 18838.1, Preparation of steel substrates before application of paints and related products — Specifications for metallic blast-cleaning abrasives — Part 1: General introduction and classification

2. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

3. GB/T 231.1-2018, Metallic materials — Brinell hardness test — Part 1: Test method

4. GB/T 13298, Inspection method of microstructure for metals

5. GB/T 1423, Test method for density of precious metals and their alloys

International Standards:

1. SAE J444, Cast shot and grit size specifications for cleaning and peening

2. ISO 3310-1, Test sieves — Technical requirements and testing — Part 1: Test sieves of metal wire cloth

3. ISO 6506-1, Metallic materials — Brinell hardness test — Part 1: Test method

4. ISO 6508-1, Metallic materials — Rockwell hardness test — Part 1: Test method

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Direct-reading spectrometer (OES): Used for the fast, non-destructive and multi-element simultaneous analysis of the chemical compositions of the cast steel shot. With the fast analysis speed and high precision, it is the indispensable equipment in the production and incoming material inspection, and needs to be regularly calibrated with the standard samples to ensure the accuracy.

Hardness tester series: Including the Rockwell hardness testers, Brinell hardness testers and micro Vickers hardness testers. The Rockwell and Brinell hardness testers are used for the macro hardness testing with the simple operations; the micro hardness testers are used for analysing the hardness of the single particles or specific phase structures, with the higher precision.

Standard test sieves and sieve shaker: Composed of a set of the sieve meshes of the different openings conforming to the ISO 3310-1 standard and the automatic timed sieve shaking machines, it is the core equipment of determining the particle size distributions of the cast steel shot. The sieve mesh ranges need to cover all the specifications of the tested abrasives.

Metallographic microscope and sample preparation equipment: Including the mounting machines, pre-grinding machines, polishing machines, etching devices and metallographic microscopes with the image acquisition systems. The complete set of equipment is used to prepare the observation samples and reveal the internal micro-structure morphologies of the cast steel shot, and is the key tool for the quality analysis and failure diagnosis.

Density measurement devices: The true density measurement needs to use the precision analytical balances and density bottles (pycnometers). The bulk density measurement needs the standard funnels, measuring cups (usually 1 litre or 0.5 litre) and balances, and the device structures need to meet the provisions of the relevant testing standards.

Wear testing machine/blasting testing bench: Used to test the crush resistance of the cast steel shot by simulating the actual working conditions. The testing bench usually includes the compressed air systems, spray guns, standard target plates and closed recovery circulation systems, which can precisely control the process parameters and quantify the abrasive losses.

Auxiliary and general equipment: Including the precision electronic balances (for the weighing), drying ovens (for the sample drying), stereomicroscopes (for the surface defect observation), magnetic separators (for the magnetic content testing), as well as the sample crushing and splitting tools, etc., jointly supporting the complete testing processes.