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Why do fasteners need quench and temper? Heat treatment, grade mapping and acceptance essentials

2026-11-01

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

The strength grade is heat treated into the part - and buyers cannot see it

Cold-formed carbon-steel fasteners that skip heat treatment top out at grades 4.6 / 5.6; everything above grade 8.8 is created by quench and temper. GB/T 3098.1 / ISO 898-1 are explicit: grade 8.8 means a nominal tensile strength of 800 MPa with a yield ratio of 0.8, and these numbers are only reliably achieved in medium-carbon or alloy steels that have been quenched and tempered. Cheap parts sold as grade 8.8 typically share the same causes: heat treatment omitted, tempering temperature too low, or cold-drawing hardness passed off as tempering. A bolt whose yield ratio is out of spec breaks with little plastic deformation - the most dangerous failure mode in service. The practical difficulty is that heat treatment is a continuous batch process: a mesh-belt furnace passes thousands of parts per hour, and any drift in furnace temperature, carbon potential or quench-oil temperature affects the whole batch, while the buyer can only verify afterwards by sampling - so acceptance criteria must be written into the contract up front.

Matching materials and tempering temperatures across grades 8.8 / 10.9 / 12.9

Materials fall into three tiers: grade 8.8 uses AISI 1035/1045 equivalents or 40Cr (51xx/41xx class), quenched at roughly 830-850 degrees C in oil; grade 10.9 needs 40Cr or 35CrMo to guarantee hardenability; grade 12.9 uses 42CrMo with tighter sulfur and phosphorus limits. GB/T 3098.1 / ISO 898-1 set minimum tempering temperatures: 425 degrees C for grade 8.8, 410 for 9.8, and 425 for 10.9 and 12.9. Tempering below the limit gives a nice hardness number but leaves no toughness reserve, and raises hydrogen-embrittlement sensitivity in 12.9 parts. Hardenability is controlled by the Jominy end-quench test (GB/T 225) so the core is fully through-hardened at the specified diameter; otherwise the surface hardness looks fine while the core remains ferrite plus pearlite, and breaking load falls well short. For sizes above M24, ask the supplier for Jominy curves or measured core hardness rather than surface-only reports.

Three acceptance gates: hardness, decarburization and the re-tempering test

Gate one, hardness: test surface and core separately to GB/T 231.1 Brinell or GB/T 230.1 Rockwell - roughly 32-39 HRC for grade 10.9 and 39-44 HRC for 12.9; a large surface-to-core gap means the part was not through-hardened. Gate two, decarburization: GB/T 3098.1 limits complete decarburization to 0.015 mm, judged metallographically or by the hardness method; excessive decarburization causes thread stripping and early surface fatigue. Gate three, the re-tempering test: reheat a sample 10 degrees C below the minimum tempering temperature, hold, and re-measure - the hardness spread across four points of one part must not exceed 20 HV; a larger spread indicates insufficient original tempering or unstable microstructure. Electroplated batches additionally require hydrogen-relief baking per ISO 4042 / GB/T 5267.1 at 190-220 degrees C for 4-22 hours, with grade 12.9 entering the oven within 4 hours of plating. Sample per GB/T 2828.1 inspection level II with AQL 1.0-2.5, and request EN 10204 3.1 certificates tied to heat numbers. For grades 8.8-12.9 with heat-treatment reports and 3.1 certificates, contact Chiali Industrial (164.hk) for samples and a proposal.

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