High-strength bolts, nuts, springs and chains obtain their property classes through cold heading, thread rolling and quenching-and-tempering or carburizing treatments. Decarburized and recarburized layers at the thread root and on the shank surface directly reduce fatigue life and are the core items of fastener metallographic inspection, while stamped hardware parts focus on grain size, banded structure and surface defects. Fasteners are mass-produced at a fast pace, so preparation and testing efficiency also determines whether quality control can be implemented in practice.
Typical test objects
- 42CrMoA and B7 high-strength bolts of property classes 8.8/10.9/12.9
- 35CrMo and 40Cr high-strength nuts
- 60Si2MnA and 50CrVA spring wire and leaf springs
- 20CrMnTi and 20MnCr5 carburized chain pins and bushings
- SCM435 wind-turbine and automotive wheel bolts
- 08Al and SPCC stamped hardware parts
Inspection challenges
- Normal-direction measurement of decarburization at the thread root radius is extremely sensitive to section orientation, and a tilted cut distorts depth readings;
- A plastically flowed layer exists on rolled threads; when decarburization and recarburization coexist the structural contrast is faint and etching depth directly affects interpretation;
- Spring wire can be thinner than 2 mm, making it hard to hold a true transverse section during mounting and grinding, while edge rounding distorts decarburization measurement;
- Carburized pins show a large hardness gap between case and core, so polishing relief interferes with case-depth determination;
- High-strength bolts are hard, and preparation heating can cause local tempering artifacts, so cutting must be amply cooled.
Recommended preparation and inspection route
Take a longitudinal section along the thread axis according to GB/T 3098.1, using a low-speed precision cut-off machine with a resin-bonded diamond wheel and ample cooling; cold mount small-diameter samples. Grind with SiC papers from P240 to P2000, then polish with 9, 3 and 1 μm diamond. Etch with 4% nital and measure complete and total decarburization by the microscopic method of GB/T 224 at 100–400×, arbitrated by the microhardness method (HV0.3) when disputed; determine the effective carburized case depth by the hardness method of GB/T 9450 at the 550 HV limit. Rate the quenched-and-tempered martensite level and core free ferrite according to GB/T 13298, and verify the property class with Rockwell HRC hardness.
Common standards
- GB/T 3098.1 Mechanical properties of fasteners — Bolts, screws and studs
- GB/T 224 Determination of depth of decarburization of steels
- GB/T 9450 Determination and verification of carburized and hardened case depth of steel parts
- GB/T 13298 Metallographic examination methods for metals
- GB/T 13299 Steel — Determination of microstructure
The above methods and parameters are application examples for technical communication rather than fixed process recipes; the actual preparation route, etchants and acceptance criteria should be confirmed against the material, dimensions and inspection purpose of each specific specimen.
Typical inspection scenarios
Decarburization measurement at the thread root of a 42CrMoA class-10.9 high-strength bolt
- Challenge
- Decarburization inspection requires a longitudinal section along the thread axis with the measurement direction normal to the surface, and a tilted cut systematically inflates depth readings; a plastically flowed layer exists on rolled threads and structural contrast is faint when decarburization and recarburization coexist; the high-strength quenched-and-tempered matrix is hard, so polishing heat and uneven etching interfere with judging the boundaries of complete and total decarburization.
- Preparation & method
- Take a longitudinal section along the thread axis per GB/T 3098.1 on a low-speed precision saw with a resin-bonded diamond wheel and ample cooling, ensuring the cut passes through the thread roots; cold mount upright. Grind with SiC papers from P240 to P2000 and polish with 9, 3 and 1 μm diamond to a mirror finish without heating. Etch with 4% nital and measure the complete and total decarburized depth at the thread root in bright field at 100–400× by the microscopic method of GB/T 224, arbitrating doubtful boundaries with the HV0.3 microhardness profile method; rate the core tempered sorbite per GB/T 13298 on the same section, verify the property class by Rockwell HRC, and issue a combined verdict.
- Reference standards
- GB/T 3098.1 Mechanical properties of fasteners — Bolts, screws and studs; GB/T 224 Determination of depth of decarburization of steels; GB/T 13298 Metallographic examination methods for metals
Complete decarburization and quenched-and-tempered structure inspection of 60Si2MnA spring wire
- Challenge
- The wire is small (under 3 mm) and a true transverse section is hard to keep during mounting and grinding, while edge rounding distorts complete-decarburization readings; silicon-manganese spring steel decarburizes readily, the fully decarburized ferrite layer is thin with a gradual transition into the partial decarburization zone, and etching depth directly changes the reading; the quenched-and-tempered structure is fine and requires high magnification to resolve lath morphology.
- Preparation & method
- Cut the wire normal to its axis on a low-speed precision saw with a resin-bonded diamond wheel and ample cooling; cold mount with same-grade fine wires as auxiliary support to keep the section upright. Grind with SiC papers from P400 to P2000, polish with 3 μm and 1 μm diamond, and finish with 0.05 μm colloidal silica if needed. Etch with 4% nital, measure complete and total decarburized depth at four evenly spaced circumferential positions in bright field at 200–500× per GB/T 224 and take the maximum; observe the tempered troostite/sorbite morphology above 500× and rate it per GB/T 13298; measure the surface-to-core hardness gradient by micro-Vickers at HV0.3, and judge the wire surface quality from structure, layer depth and hardness.
- Reference standards
- GB/T 224 Determination of depth of decarburization of steels; GB/T 13298 Metallographic examination methods for metals; GB/T 13299 Steel — Determination of microstructure
Class 10.9 42CrMoA high-strength bolt for wind tower flanges
- Challenge
- Decarburization must be measured normal to the flank at the thread root fillet, making section orientation critical — a skewed cut distorts the depth reading; the rolled-thread flow layer contains both decarburization and carburization with weak structural contrast, so etch depth strongly affects interpretation; the bolt is very hard and preparation heat can create local tempering artefacts.
- Preparation & method
- Section the full thread profile longitudinally along the bolt axis on a low-speed precision saw with a resin-bonded diamond wheel and ample coolant; cold mount the section. Grind with SiC papers from P240 to P2000 and polish with 9 μm, 3 μm then 1 μm diamond. Etch with 4% nital and measure total and complete decarburization at the thread root and flanks at 100–400× per GB/T 224; in case of dispute, arbitrate with an HV0.3 microhardness profile; verify the strength class with Rockwell testing and rate the tempered sorbite core per GB/T 13298.
- Reference standards
- GB/T 3098.1 Mechanical properties of fasteners — Bolts, screws and studs; GB/T 224 Steel — Determination of depth of decarburization; GB/T 4340.1 Metallic materials — Vickers hardness test
20CrMnTi carburized and hardened chain pin
- Challenge
- The pin is small in diameter with a large case-to-core hardness difference, so polishing readily produces relief and edge damage; the effective case depth must be defined by the hardness method, with the first indentation row close to the surface but clear of any edge rounding; high-volume production demands a fast, stable preparation routine.
- Preparation & method
- Cut a small transverse section across the pin axis at low speed with ample coolant; cold mount to protect the edges. Grind from P400 to P2000, polish with 3 μm then 1 μm diamond and finish with 0.05 μm colloidal silica if needed. Lightly etch with 4% nital and examine the case martensite, retained austenite, carbides and core structure; run a radial HV0.5 microhardness profile with the first point about 0.05 mm from the surface and a 0.1 mm step, and interpolate the effective case depth at the 550 HV1 limit per GB/T 9450.
- Reference standards
- GB/T 9450 Determination and verification of the depth of carburized and hardened cases of steel parts; GB/T 13298 Metallographic method for microstructure inspection of metals; GB/T 4340.1 Metallic materials — Vickers hardness test
Scenarios are method examples, not customer test reports; confirm the process per specimen.
MICROCRE learning notes · confirm the method for the specimen.