The fastener industry continues to expand in scale, transitioning towards high-end, intelligent, and green products. Fasteners are widely used in various applications such as automobile manufacturing, construction and infrastructure, energy and power, machinery and equipment, and rail transportation. Consequently, there are numerous production enterprises, leading to fierce market competition. The future development trend is inevitably towards high-end, intelligent, green, and international products. Therefore, as practitioners, we should continuously deepen our understanding and application of these products. Bolts, as a major category of fasteners, can be judged based on the following aspects:
Quick inspection: 1. Head inspection: Prioritize observing the structural integrity, confirm that the head has no plastic deformation or radial cracks, and focus on inspecting the necking area where the head connects to the shaft. Ensure that cross, straight, and hexagonal recesses have no chipped or missing edges, and the groove walls are not excessively worn (to avoid tool slippage).
2. Surface treatment: The plating (such as galvanizing, Dacromet) should be uniform without exposed areas, blisters, or rust. The coating should not fall off, and the stainless steel surface should not be oxidized or blackened. Any hard damage will directly affect the assembly and service life.
3. Thread testing: Take a nut or threaded hole of matching specifications, and lightly tighten it by hand or with a tool for 1-2 turns. The core judgment is whether the resistance is uniform, without any jamming or thread skipping. At the same time, confirm that the thread profile is free of burrs and deformation to ensure smooth subsequent assembly. Avoid connection loosening or failure due to insufficient thread precision.
System: I. Appearance and Dimensions
The bolt head should bear clear strength grade markings (such as 8.8, 10.9, etc.), as bolts without markings or with illegible markings may be of poor quality. High-strength bolts are typically black, and their surfaces should be uniform and free from rust, oxidation, stains, peeling of plating, and uneven blackening; inspect for defects such as cracks, burrs, bumps, deformations, bends, incomplete thread forms, and damaged threads. Dimensional inspection (using professional tools): total length, nominal diameter, thread pitch, head thickness, width across flats, chamfer, and fillet.
II. Thread accuracy
Tolerances of thread pitch diameter, major diameter, and minor diameter; thread go/no-go gauge inspection: the go gauge can be smoothly screwed in, while the no-go gauge cannot be screwed in; thread profile angle, thread profile perpendicularity, and cumulative pitch error.
III. Mechanical properties (core)
Tensile strength test: Use a tensile machine to test the maximum tensile force that the bolt can withstand.
Yield strength test: To detect the critical point where the bolt begins to undergo plastic deformation, as well as the reduction of area and impact toughness (low temperature/room temperature impact).
Wedge load strength: Test the load-bearing capacity of threaded parts, torque coefficient, and stability of tightening preload.
IV. Material Metallography
Different materials are suitable for different occasions, withstanding varying loads and impacts. Chemical composition: content of alloy elements such as carbon, manganese, silicon, chromium, molybdenum, and boron; metallographic structure: grain size, pearlite/martensite/tempering structure, depth of decarburized layer; non-metallic inclusions, intergranular cracks, overburning/underburning defects.
V. Surface treatment
Coating/Plating Thickness: Thickness of galvanized, nickel-plated, Dacromet, blackened, and hot-dip galvanized layers; Adhesion: No peeling occurs during cross-cutting, knocking, or bending; Corrosion Resistance: Salt spray test (neutral/acidic salt spray), heat and humidity resistance, aging resistance; Surface Roughness, Coating Uniformity, No missed plating, No hydrogen embrittlement.
VI. Non-destructive testing
Magnetic particle inspection: detects surface and near-surface cracks; Penetrant inspection: detects fine cracks and open defects; Ultrasonic inspection: detects internal shrinkage cavities, inclusions, and internal cracks; Eddy current inspection: detects surface defects and screens for material mixing.
VII. Assembly and Adaptation
Coordinate clearance, coaxiality, and perpendicularity; ensure tightening repeatability, torque decay, and anti-loose performance (self-locking, gluing, washer matching); ensure compatibility with nuts and washers, without any clamping stagnation or looseness.
8. Craftsmanship and Durability
Heat treatment process compliance: compliance of quenching and tempering temperature processes; fatigue life: alternating load fatigue test; high/low temperature environmental resistance, vibration resistance and relaxation performance; batch consistency, dimensional and performance dispersion.
Post time: 9月-07-2026
